Ultrasensitive SERS detection and efficient flotation removal of nanoplastics from water using bubble-spouting micromotor swarms

Ultrasensitive SERS detection and efficient flotation removal of nanoplastics from water using bubble-spouting micromotor swarms | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Ultrasensitive SERS detection and efficient flotation removal of nanoplastics from water using bubble-spouting micromotor swarms Shikuan Yang, Ning An, Jintao Li, Liyan Zhao, Shaojing Su, Qundong Xia, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4730825/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Nanoplastics are ubiquitous in aquatic environments. Most of the nanoplastics suspend in the water column, facilitating their transportation and increasing their influence on the ecosystems. Owing to their small size and good dispersion in water, detection and separation of the nanoplastics from an extremely large volume of water are very challenging. Here, we demonstrate a concept to employ carefully engineered microbubble-spouting magnetic Ag/Co micromotors to sensitively detect the nanoplastics by the surface-enhanced Raman spectroscopy (SERS) technique, as well as remove the nanoplastics from a large volume of water with the “microbubble armies” to attract, capture, and transport the nanoplastics to the water surface ( i.e. , flotation method) resembling the white blood cells chasing and swallowing nanointruders in biology. The SERS detection sensitivity reaches single nanoplastic debris level, enabled by the microscale cavities on the micromotor surface and the slippery substrate facilitating nanoplastic enrichment during water evaporation. The removal efficiency of nanoplastics from water reaches 94.3% arising from the strong interactions between the “microbubble armies” spouted from the Ag/Co micromotor swarms and the nanoplastics via the hydrophobic interactions. The Ag/Co micromotors can be separated from water after nanoplastics removal by a magnet for recycling usage. The practical applicability of the flotation method was proved by the high flotation removal efficiency of the PS nanospheres spiked into the lake and tap water using the Ag/Co micromotors. The high SERS sensitivity and the high nanoplastic removal efficiency, as well as the high throughput production and the recyclability of the Ag/Co micromotors provide valuable multifunctional materials for simultaneous detection and treatment of nanoplastic pollution in contaminated water. Physical sciences/Optics and photonics/Optical techniques/Optical spectroscopy/Raman spectroscopy Physical sciences/Materials science/Nanoscale materials/Synthesis and processing Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Main Plastic pollution has become a pervasive problem and even a planetary threat 1 – 3 . Plastic debris is now a ubiquitous pollutant in aquatic and terrestrial environments 4 , 5 . Microplastics (1 µm-5 mm) generated by weathering larger plastic waste have attracted much concern, due to their risks to ecosystems and human health 6 . As much as around 8 million metric tons of macroplastics and 1.5 million metric tons of microplastics migrate to the ocean every year 7 . At least 710 million metric tons of plastic contaminants have entered aquatic and terrestrial ecosystems 3 . Considering the plastic-carbon cycle, microplastics will eventually break down to produce nanoplastics (< 1 µm) as they age 4 . Theoretically, one microplastic can be fragmented into 10 14 nanoplastics after long-term environmental exposure 6 . However, nanoplastics with much smaller size and greatly enhanced activity, mobility, environmental exposure, and toxicity have not received the same attention as microplastics 8 , 9 . Even worse, the quantity, the exact interactions with organisms, the fate, and the transport pathways of nanoplastics are still unclear due to the lack of analytical tools and skill sets to detect and characterize plastics in the nanoscale size range. Adequate analytical techniques capable of sampling, isolating, detecting, quantifying, and characterizing nanoplastics are urgently required 9 . In addition, different from microplastics that are expected to eventually reach the bottom of water, the nanoplastics tend to suspend in the water column owing to their small size 10 . The long residence times in the water column enable the nanoplastics to transport globally and make them have more severe impact on the ecosystems than microplastics and extremely difficult to remove. Techniques capable of removing the suspended nanoplastics from water are urgently required. The most critical step to the analytical and the separation techniques of the nanoplastics is to concentrate them from the water column, while their outstanding dispersion capability and nanometer size indicate a great challenge for enrichment. Membrane ultrafiltration process shows high efficiency in micro/nanoplastic interception 11 – 13 . Biochar adsorption is promising in micro/nanoplastics removal, however the water environment should be appropriate to obtain high removal efficiency and the polycyclic aromatic hydrocarbons within biochar tend to release into water 14 . The flocculation method is generally ineffective for the small-size nanoplastics removal 15 . The flotation process employs bubbles to collect surfactant molecules, biospecies, ions, or mineral particles dispersed in water and drag them to the water surface during the bubble ascent to realize separation 16 – 19 . The strong interactions between the microbubbles and the nanoplastics via hydrophobic interactions and the easy scalability 20 make the flotation process a promising candidate technology to capture and enrich the nanoplastics from water. Compared with the conventional bubble-generator system needing electric power and complex periphery apparatus, chemically active micromotors can decompose H 2 O 2 to continuously generate microbubbles. We envision that the the micromotors may be able to produce “microbubble armies” to chase, capture, and efficiently separate nanoplastics from the water column. Further considering the capability of providing fingerprint signals and the single-molecule detection sensitivity of the surface-enhanced Raman spectroscopy (SERS) sensing technique 21 – 24 , as well as the recyclability, we designed chemically active and magnetically responsive Ag/Co micromotors via a chemical redox potential-driven ion exchange and shape-preserving chemical reduction method (Fig. 1 a) capable of continuously ejecting oxygen microbubbles to capture, SERS detect, and efficiently remove the nanoplastics from the water column (Fig. 1 b and Supplementary Video 1 ). The microbubbles automatically chase, capture, and transport the nanoplastics “on-the-fly” from the bulk water to the water surface (Fig. 1 c), resembling the white blood cells autonomously searching and removing the nanointruders in biology. The magnetic field could control the moving trajectories of the Ag/Co micromotor swarms and in turn the microbubbles to cruise the whole water volume and separate the micromotors from water for recycling usage. The dense Ag nanoparticles on the Ag/Co micromotors endowed outstanding SERS performance to the micromotors, enabling single nanoplastic particle-level SERS detection. Design principle and characterization of bubble-spouting Ag/Co micromotors The working mechanism of bubble-propelled micromotors requires them to have asymmetric morphologies to achieve directional bubble ejection, typically requiring torpedo-like morphology 25 . Considering the easy setup and simple processing, we tried to directly electrodeposit chemically active Ag 7 O 8 NO 3 asymmetric microstructures as the starting templates to construct the magnetic Ag/Co micromotors (Fig. 1 a). We first investigated the electrochemical nucleation and growth mechanism of the Ag 7 O 8 NO 3 crystals to realize electrodeposition of torpedo-like Ag 7 O 8 NO 3 microparticles. The chronoamperometric curve indicated that at a potential of 1.3 V, the current decay followed the Cottrell equation, ( i.e. , i ∝ t − 1/2 , Supplementary Fig. 1 ), suggesting a rapid electron transfer rate under this potential with the electrodeposition being controlled by the diffusion process 26 . From the equilibrium potential measured in the Tafel plot (1.24 V vs. Ag/AgCl, Supplementary Fig. 2) , it could be deduced that only about 60 mV overpotential was required for the oxidation of Ag + ions to be diffusion-controlled, suggesting that the electrochemical reaction was relatively reversible 26 . However, this potential did not trigger the nucleation process of Ag 7 O 8 NO 3 . As the potential increased further towards higher overpotential reaching 1.6 V, the chronoamperometric curve indicated that the electrodeposition of Ag 7 O 8 NO 3 followed the mechanism of three-dimensional nucleation with diffusion-controlled growth mechanism ( Supplementary Fig. 3 ) 27 . The dimensionless form of the chronoamperometric curve showed the nature of instantaneous nucleation for the electrodeposition of Ag 7 O 8 NO 3 ( Supplementary Fig. 3 ). Overpotential significantly affects the electron transfer rate, the mass transport rate, and the nucleation density during the electrodeposition process 28 . We employed finite element analysis to investigate the crystal morphology evolution of Ag 7 O 8 NO 3 and the concentration of Ag + ions in the electrolyte at different overpotentials. In the model, a single micropyramid and two closely spaced micropyramids represented discrete nuclei formed at low overpotential and dense nuclei formed at high overpotential, respectively ( Supplementary Fig. 4 ). At low potentials ( e.g. , 0.2 V), the discrete micropyramid formed a hemispherical diffusion layer of Ag + ions around it. Although the electrodeposition was diffusion-controlled, the concentration gradient of Ag + ions at the apexes and facets of the micropyramid was minimal, leading to the thermodynamically dominated crystal growth. Consequently, the Ag 7 O 8 NO 3 crystal evolved into a well-defined micropyramid according to Wulff’s theorem (Fig. 2 a). In contrast, at high potentials ( e.g. , 0.7 V), Ag + ion depletion zones formed around the nuclei, and the diffusion layers between adjacent nuclei rapidly overlapped. In this case, the concentration gradient of Ag + ions was parallel to the electrode surface, suppressing growth along this direction. The diffusion of Ag + ions primarily occurred perpendicular to the electrode surface. Furthermore, since the top apexes of micropyramids were spatially closer to regions of high Ag + ion concentration, the concentration gradient at these apexes was higher, triggering the growth of nanorods perpendicular to the electrode surface (Fig. 2 b). The electrodeposition experiments revealed that Ag 7 O 8 NO 3 formed highly symmetric micropyramids at low potentials, while Ag 7 O 8 NO 3 tended to form nanorods with uniform diameters at higher potentials, which was consistent with the above simulation results ( Supplementary Fig. 5 ). However, neither microstructure met the asymmetric morphology requirements for constructing bubble-propelled micromotors. To fabricate micromotors with asymmetric morphology that could be easily released from the electrode surface and be dispersed into the water (as discussed below), we designed a potential waveform combining gradually increasing and gradually decreasing potential between 10 V and 2 V directed by the above simulation results, resulting in the formation of asymmetric Ag 7 O 8 NO 3 microscale bipyramids (MBPs) (Fig. 2 c, d and Supplementary Fig. 6 ). When the potential gradually increased from 2 V to 4 V, a faceted micropyramid was formed on the apex of a microscale torpedo. Then, high potentials ( i.e. , >4 V) created an elongated micropyramid at the top because of the pronounced vertical diffusion of Ag + ions to the top apexes as discussed above ( Supplementary Fig. 7 ). The Ag 7 O 8 NO 3 MBPs with uniform morphologies were released from the electrode surface via sonication treatment in water (Fig. 2 e, f). The length and the width of the MBPs were 23.2 ± 2.6 µm and 4.39 ± 0.4 µm, respectively (Fig. 2 g ) . We demonstrated the scalable production of gram level of Ag 7 O 8 NO 3 MBP powders through the sonication-regrowth cycles (Fig. 2 h and Supplementary Fig. 8 ), suggesting their potential for being scaled up to kilogram level production for treating nanoplatic pollution in millions of tons of water. Ag 2+ and Ag 3+ ions within the Ag 7 O 8 NO 3 have very strong oxidization capability with redox potentials of almost 2.0 V. Therefore, we expect that the Ag 7 O 8 NO 3 MBPs can oxidize Co 2+ ions. The occurrence of the redox reactions between Ag 7 O 8 NO 3 and Co 2+ was verified from a thermodynamic perspective by comparing the potential–pH diagrams of Co-H 2 O and Ag-H 2 O systems ( Supplementary Fig. 9 ). The electrode potential of high-valent silver ions remains higher than that of Co 2+ at room temperature within the pH range of 0 ≤ pH ≤ 7.1. The measured pH of a 50 mM Co(NO 3 ) 2 solution was 5.4 and high-valent silver ions in Ag 7 O 8 NO 3 can oxidize Co 2+ to Co 3+ thermodynamically. Experimentally, introduction of Ag 7 O 8 NO 3 MBPs into aqueous solutions containing Co 2+ ions triggered the galvanic replacement reactions. The reactions resulted in the diffusion out of silver ions from the inner Ag 7 O 8 NO 3 core and the gradual adherence of Co 3+ ions to the surface of the Ag 7 O 8 NO 3 MBPs, giving rise to the formation of continuously growing AgCoO 2 shell (Fig. 2 i and Supplementary Fig. 10 ). Immersing the electrode covered by the AgCoO 2 BMPs slowly into water could peel off uniformly structured AgCoO 2 BMPs ( Supplementary Fig. 11 and Supplementary Fig. 12 ). The AgCoO 2 BMPs were hollow reflected from the TEM images ( Supplementary Fig. 13 ). The transformation process from Ag 7 O 8 NO 3 MBPs to AgCoO 2 MBPs was monitored by the EDX element measurements (Fig. 2 j). To create Ag nanostructures as the SERS sensing substrates, the AgCoO 2 MBPs were reduced to Ag/Co MBPs with the morphology maintained using NaBH 4 as the reducing agent without introducing any organic pollutants that might influence the SERS enhancement. X-ray diffraction (XRD) patterns of the chemically reduced shape-preserving Ag/Co MBPs demonstrated sharp peaks indexed to crystalline Ag (PDF #04-0783) and showed no obvious peak of Co owing to its amorphous structure (Fig. 3 a). The annealed Ag/Co MBPs exhibited peaks indexed to the crystalline Co (PDF #15–0806). The magnified SEM image (Fig. 3 b, c) revealed that dense Co nanosheets (~ 50 nm in thickness) decorated with Ag nanoparticles (~ 8 nm in diameter) covered the surface of the Ag/Co MBPs. The atomic ratio between Ag and Co within the Ag/Co MBPs could be adjusted by varying the reaction time between Ag 7 O 8 NO 3 MBPs and Co 2+ ions, which was decreased from about 3 to 0.2 when the reaction time was prolonged from 1 min to 60 min ( Supplementary Fig. 14) . EDX mapping results demonstrated the uniform distribution of Ag and Co elements within a single Ag/Co MBP (Fig. 3 d). To further elucidate the distribution of Ag nanoparticles on the Co nanosheets and their different crystallinity, we broke the structures of Ag/Co MBPs in ethanol through sonication treatment for 1 h. We could easily distinguish the Ag nanoparticles on an individual Co nanosheet with a lateral size of tens of nanometers under transmission electron microscope (TEM) (Fig. 3 e). The high-resolution TEM image indicated the high crystallinity of the Ag nanoparticles with lattice fringes of 0.118 nm and 0.102 nm corresponding to the (222) and (400) planes, respectively (Fig. 3 f). In contrast, the Co nanosheet zone exhibited an amorphous feature (Fig. 3 g). The selected area electron diffraction (SAED) from the nanosheets with nanoparticles showed distinct diffraction rings of Ag (Fig. 3 h), suggesting the structure of amorphous Co and crystalline Ag. Moreover, EDX mapping of Ag and Co elements on a single nanosheet further verified the distribution of Ag nanoparticles on Co nanosheets (Fig. 3 i-k). X-ray photoelectron spectroscopy (XPS) further confirmed the coexistence of Ag and Co on the surface of MBPs (Fig. 3 l). Moreover, high-resolution XPS spectra were used to characterize the valence change of Ag and Co during the reduction process. In the Co 2p spectrum (Fig. 3 m), the peaks at 780.2 eV and 781.7 eV were assigned to Co 3+ and Co 2+ , respectively. The satellite peaks at 785.0 eV were due to the shake-up excitation of the high-spin Co 2+ ions 29–31 . After NaBH 4 reduction, the majority of Co 3+ ions were reduced to Co 0 (778.2 eV) and the Co 2+ state was due to oxidation under ambient conditions 32 . The more prominent satellite peak further confirmed the predominance of Co 2+ ions after reduction. Additionally, the Ag 3d spectra exhibited characteristic peaks at 368.1 and 374.1 eV, corresponding to Ag 3d 5/2 and Ag 3d 3/2 of Ag + , respectively. After NaBH 4 reduction, most of the Ag + ions were reduced into zero-valence state Ag (3d 5/2 368.4 eV and 3d 3/2 374.4 eV) 29 . The small amount of Ag + after reduction originated from the oxidation of Ag under ambient conditions (Fig. 3 n). The magnetic hysteresis loop revealed that the fabricated Ag/Co MBPs had a saturation magnetization ( M s ) of 8.19 emu/g as well as a negligible coercivity ( H c ) and remanent magnetization ( M r ) at 300 K (Fig. 3 o and Supplementary Fig. 15 ). These results indicated a superparamagnetic property of the Ag/Co MBPs, which is desirable for the magnetically controllable movement and separation. The superparamagnetic property enabled the MBPs to be easily separated from water by a NdFeB magnet within 20 s ( Inset in Fig. 3 o and Supplementary Fig. 16 ). Moving mechanism of bubble-spouting Ag/Co micromotors The Ag/Co MBPs could be used as micromotors due to both their unique chemical activeness and appropriate torpedo-mimicking asymmetric morphology. The catalytic properties of the Ag/Co micromotors facilitated the rapid decomposition of hydrogen peroxide (H 2 O 2 ) to produce oxygen microbubbles 33 , 34 . Oxygen microbubbles nucleated on the surface of Ag/Co MBPs and grew until they detached due to buoyancy and fluid shear forces. The momentum released at the moment of microbubble detachment created a recoil force, propelling the Ag/Co micromotors in the opposite direction of the bubble’s detachment ( Supplementary Fig. 17 and Supplementary Fig. 18 ). Understanding the moving mechanism of the micromotors needs determination of the bubble formation sites on the micromotors. Finite element analysis revealed that oxygen tended to be concentrated at three different kinds of tips of the MBPs (Fig. 4 a and Supplementary Fig. 19 ) capable of serving as nucleation sites for bubbles after reaching the maximum supersaturation concentration necessary for oxygen bubble nucleation (68 mM) ( Supplementary Video 2) 35 . To enhance the visibility of the position of bubbles and the moving trajectory of the Ag/Co micromotors, propylene carbonate (PC) was added to the H 2 O 2 solution ( m PC : m water = 90:7) to reduce the bubble size and longevity 36 . We experimentally identified three distinct positions prone to form bubbles on the tips of Ag/Co micromotors, consistent with the COMSOL simulation results ( Supplementary Video 3 ). The observed trajectories were all spiral because the micromotors experienced both the recoil force from bubbles and the fluid drag force 37 ( Supplementary Video 4 ). The drag force was along the central axis, while the bubbles were not strictly released along the central axis, indicating that the recoil force deviated from the central axis and causing the Ag/Co micromotor to move spirally (Fig. 4 b and Supplementary Fig. 20 ). The moving mechanism of the bubble-spouting Ag/Co micromotors was quantitatively studied. Obviously, the generation and the detaching process of the microbubbles from the micromotors determine their moving speed 38 , 39 . We tried to calculate the moving speed by the bubble generation frequency and the moving step length propelled by one bubble ejection. The bubble production rate can be experimentally measured from the videos. We roughly assume the micromotor (simplified as MM ) to be an ideal cone with a surface area \(\:S=\pi\:{R}_{MMs}({L}_{MMs}+{R}_{MMs})\) , where \(\:{R}_{MMs}\) is the radius of the thick side of the cone and \(\:\:{L}_{MMs}\) is the length of the cone. The oxygen production rate k can be expressed as 38 : $$\:\begin{array}{c}k=\frac{d{V}_{{O}_{2}}}{dt}=n{C}_{{H}_{2}{O}_{2}}S=n\pi\:{C}_{{H}_{2}{O}_{2}}{R}_{MMs}\left({L}_{MMs}+{R}_{MMs}\right) \left(1\right)\end{array}$$ where n is related to the specific experiment condition and \(\:{C}_{{H}_{2}{O}_{2}}\) is the concentration of H 2 O 2 . Using the average bubble radius \(\:{R}_{b}\) , we could calculate the microbubble formation frequency f : $$\:\begin{array}{c}f=\frac{n{C}_{{H}_{2}{O}_{2}}{R}_{MMs}\left({L}_{MM}+{R}_{MMs}\right)}{{V}_{bubble}}=\:\frac{3n{C}_{{H}_{2}{O}_{2}}{R}_{MMs}\left({L}_{MMs}+{R}_{MMs}\right)}{4{R}_{b}^{3}}\:\:\:\:\:\:\:\:\:\:\left(2\right)\end{array}$$ The cone and the microbubble follow the momentum conservation principle. \(\:\:\) The force exterted on the bubble and the cone was represented as \(\:{F}_{bubble}\) and \(\:{F}_{MMs}\) , respectively. The cone-bubble system has two critical states, that it, before ( \(\:{t}_{0}\) ) and after ( \(\:{t}_{1}\) ) the bubble ejection. The entire momentum of the cone-bubble system is: $$\:\begin{array}{c}{\int\:}_{{t}_{0}}^{{t}_{1}}{F}_{bubble}dt+{\int\:}_{{t}_{0}}^{{t}_{1}}{F}_{MMs}dt=\\\:{m}_{b}\left({v}_{b}\left({t}_{1}\right)-{v}_{b}\left({t}_{0}\right)\right)+{m}_{MMs}\left({v}_{MTs}\left({t}_{1}\right)-{v}_{MMs}\left({t}_{0}\right)\right) \left(3\right)\end{array}$$ where \(\:{m}_{b}\) and \(\:{m}_{MMs}\) are the mass of the bubble and the cone, respectively. \(\:{v}_{b}\) and \(\:{v}_{MTs}\) are the velocity of the bubble and the cone, respectively. The \(\:{F}_{bubble}\) can be estimated by the Stokes’s law: $$\:\begin{array}{c}{F}_{bubble}=-6\pi\:\mu\:{R}_{b}{v}_{b}\left(t\right) \left(4\right)\end{array}$$ where \(\:{R}_{b}\) represents the bubble radius; The speed of the bubble at time t is \(\:{v}_{b}\left(t\right)\) and \(\:{\mu\:}\) is the fluid viscosity. \(\:{F}_{MMs}\) can be written as 40 : $$\:\begin{array}{c}{F}_{MMs}=-\frac{2\pi\:\mu\:{L}_{MMs}{v}_{MMs}\left(t\right)}{\text{ln}\left(\frac{{L}_{MMs}}{{R}_{MMs}}\right)-0.72} \left(5\right)\end{array}$$ where \(\:{L}_{MMs}\) is the length and \(\:{R}_{MMs}\) is the radius of the cylinder. \(\:{v}_{b}\left({t}_{0}\right)\) and \(\:{v}_{MMs}\left({t}_{0}\right)\) are the starting velocity of the bubble and the micromotor, respectively, both of which equal zero. The bubble and the micromotor all rest with a speed of zero after the bubble leaves. The momentum equation is simplified to be: $$\:\begin{array}{c}{\int\:}_{{t}_{0}}^{{t}_{1}}{F}_{bubble}dt+{\int\:}_{{t}_{0}}^{{t}_{1}}{F}_{MMs}dt=0 \left(6\right)\end{array}$$ The bubble and the micromotor are separated by a distance of 2 \(\:{R}_{b}\) , which is the sum of the displacement of the micromotor and the bubble: $$\:\begin{array}{c}2{R}_{b}={\int\:}_{{t}_{0}}^{{t}_{1}}{v}_{bubble}\left(t\right)dt+{\int\:}_{{t}_{0}}^{{t}_{1}}{v}_{MMs}\left(t\right)dt \left(7\right)\end{array}$$ The one step length pushed by a single bubble release event can be described by: $$\:\begin{array}{c}l={\int\:}_{{t}_{0}}^{{t}_{1}}{v}_{MMs}\left(t\right)dt=\frac{6{R}_{b}^{2}}{3{R}_{b}-\frac{{L}_{MMs}}{\text{ln}\left(\frac{{L}_{MMs}}{{R}_{MMs}}\right)-0.72}} \left(8\right)\end{array}$$ The average velocity of the micromotor \(\:{v}_{MMs}^{ave}\) was simplified to be: $$\:\begin{array}{c}{v}_{MMs}^{a\text{v}\text{e}}=f\times\:l=\frac{9n{C}_{{H}_{2}{O}_{2}}{R}_{MMs}\left({L}_{MMs}+{R}_{MMs}\right)}{6{R}_{b}^{2}-\frac{2{L}_{MMs}{R}_{b}}{\text{ln}\left(\frac{{L}_{MMs}}{{R}_{MMs}}\right)-0.72}} \left(9\right)\end{array}$$ In our case, the R MMs , L MMs , and R b equal 4.4 µm, 23.2 µm, and 23.0 µm, respectively. Eq. 9 is simplified to be: $$\:\begin{array}{c}{v}_{MMs}^{a\text{v}\text{e}}=0.53n{C}_{{H}_{2}{O}_{2}}\: \left(10\right)\:\end{array}$$ Equation 10 indicates that the average velocity of the micromotor is linearly proportional to \(\:n{C}_{{H}_{2}{O}_{2}}\) , which is related to the oxygen production rate. Based on the experimentally measured moving speed of the micromotors in H 2 O 2 at different concentrations ( Supplementary Fig. 21 and Supplementary Video 5 ), we calculated n values in H 2 O 2 at different concentrations according to Eq. 10. The value of n varied from 0.056 to 0.224, to 0.162, and further to 0.103 m − 2 kg − 1 s − 1 when the concentration of H 2 O 2 increased from 1 wt.%, to 3 wt.%, to 5 wt.%, and further to 10 wt.%, instead of a constant. We fitted the relationship between the experimentally measured moving speed v of the micromotors and \(\:{C}_{{H}_{2}{O}_{2}}\) by applying a modified Michaelis-Menten Eq. 4 1,42 : $$\:v\left(\text{y}\right)=\frac{550\times\:{\text{C}}_{{H}_{2}{O}_{2}}^{2.28}}{{2.58}^{2.28}+{\text{C}}_{{H}_{2}{O}_{2}}^{2.28}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(11\right)$$ Combining Eq. 10 with Eq. 11, we obtained the relationship between n and \(\:{C}_{{H}_{2}{O}_{2}}\) : $$\:n=\frac{1037.76\times\:{\text{C}}_{{H}_{2}{O}_{2}}^{1.28}}{{2.58}^{2.28}+{\text{C}}_{{H}_{2}{O}_{2}}^{2.28}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(12\right)$$ Based on Eq. 10 and Eq. 12, we can predict the moving speed of the Ag/Co micromotors in H 2 O 2 aqueous solutions with different concentrations. In addition to the concentration of H 2 O 2 , the moving speed of the micromotors was also influenced by the weight ratio of Ag and Co within the micromotors with different catalytic performance. As the weight ratio between Ag and Co was decreased by prolonging the reaction time between Ag 7 O 8 NO 3 MBPs with Co 2+ ions, the catalytic performance was gradually decreased reflected by the slower concentration decrease of the H 2 O 2 ( Supplementary Fig. 22 ). Therefore, we need to balance the catalytic performance and the magnetic properties by designing the Ag and Co ratio within the micromotors. The superparamagnetic properties enabled precise manipulation of the moving direction of the micromotors in water using a magnet (Fig. 4 c and Supplementary Video 6 ). The micromotors showed spiral trajectories towards random directions in H 2 O 2 aqueous solutions (Fig. 4 b). A magnetic field could be used to rationally steer the moving direction of the bubble-propelled Ag/Co micromotors to cruise the whole water volume, facilitating capturing nanoplastics well-dispersed in water (Fig. 4 d and Supplementary Video 7 ). Ultrasensitive SERS detection of nanoplastics by Ag/Co micromotors Among various types of nanoplastics, polystyrene (PS) is widely used in daily products and cannot be biodegraded. Previous studies have shown that PS nanoplastics have detrimental effects on nerve systems 43 , 44 . Therefore, PS nanospheres were selected as a model nanoplastic to evaluate the SERS sensing performance of the Ag/Co micromotors. The Raman spectrum of the Ag/Co micromotors showed peaks at 476 cm − 1 , 530 cm − 1 , and 680 cm − 1 corresponding to the E g , F 2g , and A 1g modes of the cubic phase of cobalt oxides 45 , respectively ( Supplementary Fig. 23 ). These Raman peaks did not overlap with the Raman peaks of PS nanoplastics, ensuring a clean background for detecting the weak Raman signals of PS nanoplastics. First, we studied the necessity of PS nanosphere detection using SERS. We dispersed isolated PS nanospheres with a size of 5 µm, 2 µm, 1 µm, and 200 nm on a piece of silicon wafer. Strong Raman signals were observed from a single PS sphere with a size larger than 1 µm without using SERS ( Supplementary Fig. 24 ). However, no Raman signals were observed from PS nanospheres with a size of 200 nm on the silicon wafer. This means that it is necessary to employ SERS technique to detect single nanoplastics. Only about two times of enhancement of the single PS sphere Raman signals was observed on the conventional Au nanosphere array SERS substrate ( Supplementary Fig. 24 ), because the large PS spheres could not enter the < 10 nm crevices between neighboring Au nanoparticles where strong electromagnetic fields located (known as “hot spots”) 21,22,23,24,46 . Therefore, SERS substrates with volumetric hot spots are desired for detection of nanoplastics. Volumetric hot spots were formed between the interlaced nanoplates covered by densely packed Ag nanoparticles within the Ag/Co MBPs prepared by reacting for 3 min between Ag 7 O 8 NO 3 MBPs and Co 2+ ions ( Supplementary Fig. 25 ), enabling them to detect single PS nanospheres with a diameter of 200 nm. Although the Ag/Co micromotors can detect single PS nanospheres, it is necessary to increase the number density of the PS nanospheres on the Ag/Co micromotors to make sure that shining the laser at a randomly spot on the Ag/Co micromotors covers at least one PS nanosphere. A slippery polydimethylsiloxane (PDMS) layer-covered silicon substrate 47 , 48 was used to force the very few amounts of PS nanospheres to attach the Ag/Co micromotors during water evaporation. Previous reports have shown that strong π-metal interactions exist between Ag nanomaterials and aromatic hydrocarbons under ambient conditions, despite the lack of conventional metal-binding functional groups. This interaction is essentially a type of van der Waals force originated from the dispersive interactions between the π-system of aromatic hydrocarbons and the silver surface 49 . After water was completely evaporated, most of the PS nanoplastics were adsorbed onto the surface of Ag/Co micromotors (Fig. 5 a, b). Uniform SERS signals of PS nanoplastics were clearly observed from an individual Ag/Co micromotor, reflected by the uniform color in the SERS mapping results. These results proved that the PDMS-Ag/Co micromotors could be used as an integrated platform to sensitively detect nanoplastics (Fig. 5 c, d). The relative standard deviation (RSD) of the 1001 cm − 1 SERS peak of the PS nanoplastics assigned to C–C ring breathing mode was only 10.8% by calculating its intensity distribution, indicating a remarkable detection reliability of the integrated SERS platform (Fig. 5 d, e). Additionally, the SERS spectra measured on ten randomly selected Ag/Co micromotors showed negligible intensity variation, suggesting the high SERS stability and repeatability of the Ag/Co micromotors (Fig. 5 f). The Au/Co micromotors were also used to detect polyethylene terephthalate (PET) nanoparticles with a size of 120 nm with a detection limit of < 20 mg/ml ( Supplementary Fig. 26 ). We further evaluated the SERS sensitivity of the Ag/Co micromotors. An obvious SERS peak at 1001 cm − 1 was still distinguishable even at a PS nanoplastics concentration as low as 5 µg/mL (Fig. 5 g). SERS signals of PS nanoplastics at a concentration of 5 µg/mL were consistently detectable at five random positions ( Supplementary Fig. 27 ). This concentration level of nanoplastic detection is superior to previous studies regarding the sensitivity, reliability, and practicability ( Supplementary Table 1 ). The relationship between the SERS intensity at 1001 cm − 1 and the concentration of PS nanoplastics could be described by I = 10.85 C + 78.16 with a correlation coefficient R 2 = 0.9919 (Fig. 5 h), manifesting the good quantification capability of the Ag/Co micromotors as single-particle SERS substrates. The magnetically controllable moving Ag/Co micromotors were expected to accelerate and enhance the PS nanoplastics adsorption process by effectively mixing the suspensions, thereby increasing the chance of contact between the volumetric hot spots and the PS nanoplastics. To observe the effect of magnetic mixing, 1 µL of fluorescent PS nanosphere dispersions were added to a static droplet of water (10 µL) containing Ag/Co micromotors. A rotary magnetic field generated by a magnetic stirring plate was applied to rotate the Ag/Co micromotors (Fig. 5 i). The bright PS nanospheres could be clearly observed under UV irradiation (λ = 365 nm). The passive diffusion of PS nanospheres was slow when the stirring process was off, resulting in an inhomogeneous distribution after 260 s. In contrast, when the stirring was on, it could be observed that PS nanospheres swirled around in the droplet along the direction of stirring and the homogenization of PS nanospheres was accelerated ( Supplementary Fig. 28 ). To further verify this, the 200-nm red fluorescent PS spheres on the Ag/Co micromotors were observed via confocal fluorescence microscopy. Magnetic strring effectively overcame the diffusion limit, enabling active capture of the spheres and resulting in significantly higher accumulation on the micromotors compared to non-stirred conditions (Fig. 5 j and Supplementary Video 8 ). The intensity of the SERS signals from PS nanoplastics under magnetic stirring was much higher than that of the control group without stirring (Fig. 5 k and Supplementary Fig. 29 ). Therefore, the magnetically controllable mobile Ag/Co micromotors showed more sensitive detection performance of the PS nanoplastics than that of the static ones ( Supplementary Fig. 30 ). Recyclable flotation separation of nanoplastics by “microbubble armies” generated by the Ag/Co micromotor swarms The working mechanism of the flotation process relies on the difference in the surface hydrophobicity of different components. The flotation technique has been widely used in the mining industry to collect valuable ores 50 where hydrophobic particles are separated from the liquid phase as bubbles adhere to them, causing the particles to ascend to the foam layer on the water surface 51 . The flotation method seemingly a promising way to remove nanoplastics from a large volume of water, but has not been reported in nanoplastics removal. The interaction between the bubbles and the nanoplastics is critical for successful nanoplastic flotation removal, which consists of collision, attachment, and detachment process. These processes work together to govern the flotation kinetics and influence the separation efficiency. The bubble-nanoplastics collision efficiency is determined by the fluid hydrodynamics and the size of the nanoplastics and the bubble 52 . Sutherland, et al. built a collision model for a bubble-particle system 53 with the derivation of an expression for E c (the number ratio of the particles encountering a bubble per unit time to the number of the particles approaching the bubble in a flow tube with a cross-sectional area equal to the projected area of the bubble) from fluid stream functions. They assumed that the particles within the collision radius would attach onto the bubble and in turn the collision efficiency was determined by the ratio of the cross-sectional area of the stream tube ( \(\:\pi\:{R}_{c}^{2}=3\pi\:{d}_{p}{d}_{b}/4\) ) to the projected area of the bubble ( \(\:\pi\:{d}_{b}^{2}/4\) ): $$\:{E}_{c}=3{d}_{p}/{d}_{b}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(13\right)$$ This simple model is a close approximation to the interceptional effect in the case of high flow velocity around the bubble, particularly for the small nanoplastic particles in our case. According to the equation, it is necessary to decrease the size of the bubbles to improve the bubble-nanoplastics interactions. The calculated E c for our system was about 0.6 supposing that the diameter of the nanoplastics was 1 µm and the average bubble diameter was 5 µm as observed from the microscope, which was one order of magnitude higher than that of the conventional flotation method using millimeter sized bubbles. This was the reason why high nanoplastic removal efficiency was achieved using the “microbubble armies” produced by Ag/Co micromotors. We experimentally confirmed that microbubbles could efficiently capture PS nanospheres at their surfaces ( Supplementary Video 9 ). Therefore, we tried to employ Ag/Co micromotors to generate “microbubble armies” to chase, capture, and transport the nanoplastics to the water surface as microbubbles ascending to the water surface and eventually to completely remove the nanoplastics floating at the water surface from the water column. We designed a recyclable and easily scalable process to treat nanoplastic contaminated water using the bubble-spouting Ag/Co micromotors (Fig. 6 a ) . A little amount of H 2 O 2 acting as fuels and cetyltriethylammnonium bromide (CTAB) as a microbubble stabilizer were simultaneously introduced into water containing PS nanoplastics. Microbubbles were vigorously produced through the decomposition of H 2 O 2 . These microbubbles cruised to capture and transport the nanoplastics to the water surface, concentrating them into the froth layer (Fig. 6 b and Supplementary Video 1 ). Removing the foam layer could easily separate the PS nanoplastics from the water column. The Ag/Co micromotors were completely extracted from water after transporting all of the nanoplastics to the water surface simply using a magnet for recycling usage ( Supplementary Fig. 31 ). The used Ag/Co micromotors were rinsed by water and ethanol to remove absorbed nanoplastics, and then reborn by treating with NaBH 4 solutions to completely restore their catalytic activity and in turn the bubble-spouting capability. The removal efficiency of PS nanoplastics by the Ag/Co micromotors was evaluated using 200 nm-sized red PS nanospheres as the nanoplastic model. The content of Ag within the Ag/Co micromotors prominently influenced the nanoplastic removal efficiency. Prolonged reaction time between Ag 7 O 8 NO 3 MBPs and Co(NO 3 ) 2 gave rise to Ag/Co micromotors with a less amount of Ag, while with better removal efficiency of PS nanoplastics (Fig. 2 j and Supplementary Fig. 32 ). This was probably due to the quick consumption of H 2 O 2 in the presence of high amount of Ag, limiting the longevity of the microbubbles. Therefore, the reaction time between Ag 7 O 8 NO 3 MBPs and Co(NO 3 ) 2 was set to 60 min in the following experiments. The absorbance peak of the red PS nanospheres at 527 nm almost disappeared after treatment with Ag/Co micromotors, indicating a high removal efficiency. The removal efficiency was estimated to be 94.3% based on the correlation between the absorbance and the amount of the PS nanospheres (Fig. 6 c and Supplementary Fig. 33 ). The concentration of CTAB critically affected the removal efficiency of PS nanoplastics (Fig. 6 d and Supplementary Fig. 34 ). When the concentration of CTAB was below 0.0007 wt.%, the removal efficiency dropped lower than 50% due to the instability of the bubble foam layer. As a result, the PS nanoplastics delivered to the water surface re-entered the water column. When the CTAB concentration exceeded 0.0007 wt.%, the removal efficiency also declined because CTAB molecules occupied most of the adsorption sites of the microbubbles and prevented PS nanospheres from attaching to the CTAB-surrounded bubble surface 54 . To remove 95% nanoplastics from 1 ton of water one time, only 7 g of CTAB was needed. Additionally, the removal efficiency was greatly improved when the concentration of H 2 O 2 increased. The removal efficiency reached 94.3% when the concentration of H 2 O 2 was 4.8 wt.% (Fig. 6 d and Supplementary Fig. 35 ). This is because the vigorous decomposition of H 2 O 2 at high concentrations created tremendous tiny microbubbles efficient in nanoplastics trapping and removing. The removal efficiency first increased rapidly with the concentration of Ag/Co micromotors, due to the increased microbubble generation rate. When the concentration of Ag/Co micromotors reached 3.75 mg/mL, the removal efficiency was saturated at 94.3% (Fig. 6 e and Supplementary Fig. 36 ). The removal efficiency remained above 70% regardless of the concentration of PS nanospheres (Fig. 6 e). The kinetic characteristics of the flotation process showed rapid PS nanoplastic separation in the first two minutes and the removal efficiency rose steadily until it stabilized at > 90% within 40 min (Fig. 6 f). The outstanding SERS performance of the Ag/Co micromotors enabled us to monitor the removal process of the nanoplastics using the SERS technique. Before the flotation treatment, strong SERS signals of PS nanoplastics at 1001 cm − 1 were observed. After the flotation removal, no SERS peaks of PS nanoplastics were observed (Fig. 6 g and Supplementary Fig. 37 ). The SERS spectra only showed the peak of Ag/Co micromotors at 680 cm − 1 and no SERS peaks of CTAB were observed, indicating that CTAB molecules were simultaneously separated into the foam layer. To further confirm that the CTAB molecules were separated into the foam layer with the nanoplastics after the flotation process, we utilized methylene blue (MB) as a model dye molecule because of the easy observation to demonstrate the capability of the flotation method to separate small molecules. The flotation process using the bubble-spouting Ag/Co micromotors removed 98.9% of MB molecules within 40 min from the water column. The solution became completely colorless and transparent after removing the MB molecules ( Supplementary Fig. 38 ). This result confirmed that small molecules, such as CTAB, were effectively separated together with the nanoplastics after the flotation process, and further demonstrated that the flotation method using the Ag/Co micromotors was applicable to the separation of harmful molecules. The Ag/Co micromotors were slowly oxidized during usage ( Supplementary Fig. 39 ). Therefore, the removal efficiency of the PS nanoplastics was decreased from ~ 95% with fresh Ag/Co micromotors to 77% in the second time use, and further to 63% in the third use, and eventually maintained at ~ 60% for the following repeatable usage (Fig. 6 h). Even after usage for six times, the Ag/Co micromotors still contained Ag and Co elements according to the EDX results ( Supplementary Fig. 40 ), indicating their negligible material loss and stable structure during the flotation process. Simple treatment of the used Ag/Co micromotors with NaBH 4 could easily reduce the oxidized Ag to metallic Ag, thereby increasing the removal efficiency of the PS nanoplastics to ~ 90% (Fig. 6 h). The overall nanoplastic removal performance (including time, cost, simplicity, durability, etc. ) using the bubble-spouting micromotor swarms outperforms previous methods ( Supplementary Table 2 ). After 40 min nanoplastic removal process, the concentration of the remaining H 2 O 2 in water was ~ 1 mM. Keeping the micromotors in water after the nanoplastic removal for 2 h reduced the H 2 O 2 concentration to 0.1 mM ( Supplementary Fig. 41 ). During the decomposition of the H 2 O 2 to generate oxygen microbubbles, it is inevitable to release Co nanoparticles to water after long-time processing. We dissolved these nanoparticles using HNO 3 to determine the concentration of Co 2+ ions using the inductively coupled plasma (ICP) method. The Co 2+ concentration was around 1 mg/L, which met the requirements of international standards for Co 2+ ions for water treatment. Replacing the Co component with other magnetic responsive but more friendly elements can avoid the Co left concern. Natural water bodies are complex systems with many influencing components, such as diverse contaminants, organic matter, and varying pH. The spiked samples were treated by Ag/Co micromotors to assess the practical applicability of the flotation method. In real-world water conditions, the Ag/Co micromotors displayed comparable nanoplastics removal performance to that in deionized water. As an example, red PS nanoplastics with a concentration of 4.7 × 10 − 5 g/mL spiked in real lake water (obtained from a lake on the campus) were separated into the foam layer after the flotation process. The removal efficiency was calculated to be 97.7% (Fig. 6 i, Supplementary Fig. 42 ). Similarly, PS nanoplastics spiked in drinking tap water were also almost completely removed by the flotation method ( Supplementary Fig. 43 ), further demonstrating the broad and real applicability of the flotation method using the microbubble-spouting Ag/Co micromotors across real-world waters. These results indicated that the complex water matrices did not affect the catalytic activity, stability, and nanoplastic removal efficiency of the Ag/Co micromotors. Conclusion In summary, we realized sensitive SERS detection and efficient flotation removal of nanoplastics from the water column using bubble-spouting Ag/Co micromotors. The Ag/Co micromotors were transformed from the electrochemically engineered Ag 7 O 8 NO 3 microparticles using a redox potential-driven ion exchange and shape-preserving reduction process with controllable Ag and Co ratios, which could be easily scaled up to kilogram-level production. The nanoplastics were concentrated onto the Ag/Co micromotors with volumetric hot spots after water evaporated on the PDMS slippery surface, achieving SERS detection of nanoplastics detection at a concentration of microgram per milliliter level. The Ag/Co micromotors continuously eject “microbubble armies” to capture the nanoplastics via hydrophobic interactions and transport them to the foam layer during the bubble ascending process. The removal efficiency of the nanoplastics reached 94.3% within 40 min. The nanoplastic removal performance of the used Ag/Co micromotors could be restored to the level of the fresh ones simply by NaBH 4 aqueous solution treatment. The easy regeneration of the Ag/Co micromotors and the simple magnetic separation process of the micromotors from water readily by a magnet make the micromotors recyclable. PS nanospheres spiked into the lake and tap water could also be removed by the flotation method using the Ag/Co micromotors, proving the practical applicability of the flotation method. We proved the possibility to detect and separate the nanoplastics using bubble-spouting Ag/Co micromotors, providing promising materials and concepts to solve the notorious nanoplastic pollution crisis. Methods Materials and reagents All the experimental chemicals were used as received without further purification. Silver nitrate (AgNO 3 , 99.9%), sodium borohydride (NaBH 4 , 96%), boric acid (H 3 BO 3 , 99%), and hydrogen peroxide (H 2 O 2 ) were purchased from Sinopharm Chemical Reagent. Cobalt nitrate hexahydrate [Co(NO 3 ) 2 ·6H 2 O, 99%] and cetyltrimethylammonium bromide (CTAB, 99%) were purchased from Aladdin. Methylene blue (MB) and sodium dodecyl sulfate (SDS, 99.0%) were purchased from Sigma-Aldrich. Propylene carbonate (PC, 99.5%) was purchased from J&K Scientific. Polystyrene (PS) nanosphere suspensions were purchased from Huge Biotechnology. Electrochemical tests All electrochemical tests were performed in a three-electrode system. The working electrode was prepared by thermally evaporating a 2 nm-thick layer of titanium followed by a 50 nm-thick layer of gold onto a piece of silicon wafer. A graphite rod (about 5 mm in diameter) and the Ag/AgCl were used as the counter electrode and the reference electrode, respectively. The electrolyte consisted of 0.06 M AgNO 3 , 0.16 M H 3 BO 3 , and 0.1 M KNO 3 . For chronoamperometry tests, a step potential was applied after the open-circuit potential stabilized. The potential range for the Tafel test was ± 200 mV from the open-circuit potential, with a scan rate of 1 mV/s. Preparation of AgONO MBPs The three-electrode system used for electrodeposition was the same as above. The Au electrode was immersed into an electrolyte solution comprising 0.06 M AgNO 3 and 0.16 M H 3 BO 3 . The anodic electrodeposition was conducted by applying a potential waveform first gradually increasing from 10 V to 2 V and then gradually increasing to 10 V. The potential increasing and decreasing rate was set to 0.1 V/s. Subsequently, the ultrasonication was employed to peel off the Ag 7 O 8 NO 3 MBPs from the electrode surface. The obtained black powder was dried at 60 ℃ in an oven. Preparation of Ag/Co micromotors To transform the Ag 7 O 8 NO 3 MBPs into Ag/Co micromotors, the synthesized Ag 7 O 8 NO 3 were immersed in 50 mM Co(NO 3 ) 2 solutions ( V water : V ethanol = 9:1) for different times to vary the Ag and Co ratios. Then, the micromotors were collected via centrifugation at 9000 rpm for 10 min. The micromotors were washed by deionized water before reducing by 50 mM NaBH 4 aqueous solutions for 50 min at room temperature. The Ag/Co micromotors were collected by centrifugation at 9000 rpm for 10 min. After rinsing with deionized water and ethanol, the micromotors were dried under vacuum at 60 ℃ for 1 h. For the annealed samples, the as-obtained Ag/Co micromotors were placed in a tube furnace and heated to 500°C with a temperature increasing rate of 5 ℃/min for 3 h in a stream of Ar. Moving trajectory tracking of Ag/Co micromotors 5 wt.% H 2 O 2 aqueous or PC solutions were introduced into the wells of a 96-well plate. The Ag/Co micromotors used for motion analysis were prepared as follows: First, Ag 7 O 8 NO 3 MBPs on the electrode were converted to AgCoO 2 MBPs by directly immersing them into 50 mM Co(NO 3 ) 2 solutions ( V water : V ethanol = 9:1) for 1 h; Second, they were subsequently reduced by 50 mM NaBH 4 aqueous solutions for 1 h at room temperature; Eventually, the Ag/Co micromotors were released from the gold substrates by ultrasonic treatment. These Ag/Co micromotors were dispersed into the solution within the 96-well plate. A NdFeB magnet (grade N35) was used at a distance of ~ 5 cm to control the moving behavior of the Ag/Co micromotors. An optical microscope (RX50M SOPTOP) was immediately used to track and record the movement of the Ag/Co micromotors at a rate of 60 frames per second. The trajectory was analyzed using the Tracker software. SERS measurements Ag/Co micromotors were added to PS nanosphere suspensions at different concentrations. The Ag/Co micromotors were maintained at a concentration of 0.5 mg/mL. 20 µL of the mixture was dropped onto a slippery PDMS-functionalized silicon surface, which was prepared according to our previous publication 34 . The mixture solution droplet resting at the slippery surface was dried at 40 ℃. The SERS spectra were recorded with a confocal microscopic Raman system (Renishaw Invia Reflex). The excitation laser wavelength was 532 nm with a power of 0.25 mW. The laser was focused on the samples through a 50× objective lens. All spectra were collected with 20 s of integration time and three acquisitions. SERS mapping images were conducted with 1 s integration time for each point with a step length of 1 µm for the laser spot movement. Catalytic performance of the Ag/Co micromotors The UV-Vis absorption spectra of potassium titanium oxalate solutions with different concentrations of H 2 O 2 were measured to build the relationship between the absorption intensity at 400 nm and the concentration of H 2 O 2 . Ag/Co micromotors with different Ag and Co ratios were introduced into the potassium titanium oxalate solutions composed of 1.47 M H 2 O 2 . The absorption intensity decrease at 400 nm was monitored to predict the concentration decrease of H 2 O 2 , reflecting the catalytic performance. The remaining concentration of the H 2 O 2 after nanoplastic removal was also measured using this method. Nanoplastic removal efficiency estimation Ag/Co micromotor suspensions, CTAB aqueous solutions, and H 2 O 2 aqueous solutions were sequentially injected into PS nanosphere suspensions in a 5 mL glass vial. After the flotation process, 1 mL of the solution in the bottom layer was extracted for immediate recording of the absorption spectra. The removal efficiency (ƞ) was calculated according to the following equation: $$\:\left(\text{\%}\right)=\frac{{C}_{0}-{C}_{\text{f}}}{{C}_{0}}\times\:100\%$$ 1 where C 0 and C f are the initial and final concentration of the PS nanoplastic suspensions, respectively. Ag/Co micromotors were washed thoroughly with deionized water and absolute ethanol after the nanoplastic removal process. The used Ag/Co micromotors were reacted with 50 mM NaBH 4 solutions for 10 min followed by washing with deionized water to regenerate the flotation function. A NdFeB magnet was used to separate the Ag/Co micromotors from the liquid phase. For the nanoplastics removal in the lake water, Ag/Co micromotors suspensions, PS nanosphere suspensions and CTAB aqueous solutions were all prepared using the lake water collected from the Qizhen lake (Hangzhou, China). Characterization X-ray diffraction (XRD) patterns were obtained using a Rigaku D/MAX 2550 diffractometer with Cu K α radiation (λ = 1.5418 Å) as the light source. The morphology and the element information were investigated using a field-emission SEM (Zeiss Supra55) operated at an accelerating voltage of 15 kV equipped with an X-ray energy-dispersive spectroscopy. Transmission electron microscopy (TEM) images of samples were investigated by FEI Talos F200x. X-ray photoelectron spectroscopy (XPS) spectra were acquired using an X-ray photoelectron spectroscopy (XPS) spectra were acquired using an EscaLab 250Xi photoelectron spectrometer (Thermo Scientific). The incident radiation was 50 W. The C 1s peak at 284.8 eV served as a reference for the position of all of the XPS peaks. The absorption spectra were measured using an ultraviolet–visible (UV–Vis) absorption spectrometer (Lambda 950, PerkinElmer). The magnetic hysteresis loops of the samples were obtained from the physical property measurement system (PPMS-9). The distribution of red PS fluorescent nanospheres on the Ag/Co micromotors was observed using a confocal laser scanning microscope (Zeiss LSM 980 with Airyscan). Finite element simulation We used the Tertiary Current Distribution and Deformed Geometry modules in COMSOL Multiphysics to simulate the distribution of Ag + ions concentration and track the interface deformation during the electrodeposition process of Ag 7 O 8 NO 3 . Electrode kinetics were described using the Butler-Volmer equation. The initial concentration of Ag + ions was set to 0.06 mol/L. The diffusion coefficient of Ag + ions was set to 8.51 × 10 − 6 cm 2 /s 55 . Based on the Tafel plot (Supplementary Fig. 2), the anode transfer coefficient was set to 0.82, and the exchange current density was set to 9 × 10 − 6 A/cm 2 . The applied high and low voltages were 0.7 V and 0.2 V, respectively. We exploited the Transport of Diluted Species module in COMSOL Multiphysics to simulate oxygen concentration distribution around the Ag/Co micromotors. The initial concentration of H 2 O 2 was set to 1.41 × 10 3 mol/m 3 . The constant of reaction velocity k cat was set to 7.0 × 10 − 3 cm/s. The diffusion coefficient of oxygen and H 2 O 2 was set to 2.10 × 10 − 6 cm 2 /s 56 and 1.35 × 10 − 5 cm 2 /s 57 , respectively. Declarations Competing interests Authors declare that they have no competing interests. Author contributions S. Y. and N. A. conceived the idea and designed the study. N. A., J. L., L. Z. and Z. Z. carried out the materials synthesis and characterizations. N.A., L.Q., S.S., H.Z., Y.L., M.Y. and S.Y. analyzed the data. N.A., L.Q, and S.Y. wrote the manuscript. All authors contributed to the revision of the manuscript. 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02:46:05","extension":"html","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":173140,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/8a65d6574b0935e1ef579075.html"},{"id":96455513,"identity":"61de2b82-ff5b-44c3-a897-f4f2ef2cd10b","added_by":"auto","created_at":"2025-11-21 10:04:14","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":8421722,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFabrication of the Ag/Co micromotors and their applications in sensitive SERS detection and efficient flotation removal of the nanoplastics from water using the “microbubble armies” generated by the Ag/Co micromortors.\u003c/strong\u003e (a) Schematic of the fabrication process of the Ag/Co micromotors, including electrochemical design of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs, peeling off them from the electrode surface by the ultrasonic treatment, and transforming them into AgCoO\u003csub\u003e2\u003c/sub\u003e MBPs by reacting with Co\u003csup\u003e2+\u003c/sup\u003e ions. The AgCoO\u003csub\u003e2\u003c/sub\u003e MBPs were reduced by NaBH\u003csub\u003e4\u003c/sub\u003e aqueous solutions maintaining the morphology unchanged to form Ag/Co micromotors. (b) Nanoplastics were concentrated onto the Ag/Co micromotors during water evaporation on a slipper surface and tightly attracted onto the micromotors by van der Waals force, facilitating SERS detection. (c) Nanoplastics were captured by the “microbubble armies” ejected from the Ag/Co micromotors and were transported to the foam layer at the water surface during the microbubble ascending process, realizing nanoplastic removal by the flotation method. Inset: More than ten PS microspheres with a size of 5 mm captured by a single microbubble captured from a Supplementary video.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/1f09e1c1bac8f3ef7fd80a8f.png"},{"id":96427315,"identity":"4f02dac9-0219-4ef3-9858-d5ba1a77c56c","added_by":"auto","created_at":"2025-11-21 02:46:05","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":5060159,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGrowth mechanism of Ag\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e7\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e8\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eNO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e MBPs and their transformation into AgCoO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e MBPs.\u003c/strong\u003e (a, b) The electrodeposited crystal morphology and the corresponding Ag\u003csup\u003e+\u003c/sup\u003e ion concentration distribution during electrodeposition at different times under 0.2 V and 0.7 V, respectively. Black lines represent concentration contours. Scale bar: 2 mm. (c) Potential waveform employed to synthesize Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3 \u003c/sub\u003eMBPs. (d) SEM image of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3 \u003c/sub\u003eMBPs electrodeposited at the electrode surface. (e) Uniform Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3 \u003c/sub\u003eMBPs selectively peeled off from the electrode surface by ultrasonic treatment. (f) A single Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3 \u003c/sub\u003eMBP. (g) Size distribution of the length (\u003cem\u003eL\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e) and the width (\u003cem\u003eL\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e) of the Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3 \u003c/sub\u003eMBPs. (h) Gram-level production of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3 \u003c/sub\u003eMBPs fabricated by the electrodeposition-ultrasonic treatment-regrowth method. (i) AgCoO\u003csub\u003e2\u003c/sub\u003e MBPs transformed from the Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3 \u003c/sub\u003eMBPs by reacting with Co\u003csup\u003e2+\u003c/sup\u003e ions for 60 min. Inset: enlarged observation. Scale bar: 500 nm. (j) EDX spectra of Ag/Co MBPs under different reaction times with Co\u003csup\u003e2+\u003c/sup\u003e ions.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/7e29de4334a94b01a6d19195.png"},{"id":96427313,"identity":"859327b6-c31a-4981-9712-c0774da84df7","added_by":"auto","created_at":"2025-11-21 02:46:05","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":6845052,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFabrication and characterization of Ag/Co micromotors.\u003c/strong\u003e (a) XRD pattern of the Ag/Co micromotors. (b) Reducing the AgCoO\u003csub\u003e2\u003c/sub\u003e MBPs to form Ag/Co micromotors. Inset: Enlarged observation of the surface structure. (c) Enlarged observation of the dense Ag nanoparticles on Co nanosheets. These Ag nanoparticles-covered nanosheets interlaced to form volumetric SERS hot spots accommodating PS nanospheres. (d) Element mapping results of a single Ag/Co micromotor. (e) TEM image of the Ag nanoparticles on a Co nanosheet. (f, g) High resolution TEM image of the Ag nanoparticles and Co nanosheets in the red and cyan rectangles in (e), respectively. (h) SAED pattern of the Ag/Co micromotors. (i-k) High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images of the Ag nanoparticles on Co nanosheets and the corresponding elemental mapping results. (l) X-ray photoelectron spectroscopy (XPS) spectrum of the Ag/Co micromotors. (m) High-resolution XPS spectra of Co 2p for the AgCoO\u003csub\u003e2\u003c/sub\u003e BMPs before and after reduction. (n) High-resolution XPS spectra of Ag 3d for the AgCoO\u003csub\u003e2\u003c/sub\u003e MBPs before and after reduction. (o) Magnetic hysteresis loop of Ag/Co micromotors. Inset: the digital image of the Ag/Co micromotors exhibiting a magnetic response when approximating a NdFeB magnet.\u003cbr\u003e\n\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/27488930d70a454abc704f1c.png"},{"id":96427312,"identity":"12f2577b-c719-4653-aee9-e3782ba35faf","added_by":"auto","created_at":"2025-11-21 02:46:05","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2358912,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMoving mechanism and behavior of Ag/Co micromotors.\u003c/strong\u003e (a) Simulated oxygen concentration profiles over the initial 500 ms captured from a Supplementary video. High oxygen concentration region located at position A, B, and C. Scale bar: 2 mm. (b) Representative trajectories of Ag/Co micromotors in 5 wt.% H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e PC solutions (\u003cem\u003em\u003c/em\u003e\u003csub\u003ePC\u003c/sub\u003e:\u003cem\u003em\u003c/em\u003e\u003csub\u003ewater\u003c/sub\u003e = 90:7) corresponding to the bubble formation positions at A, B, and C as shown in (a). (c) Time-resolved images and trajectories illustrating the controllable moving direction of Ag/Co micromotors in deionized water by a magnet captured from a Supplementary Video. Scale bar: 100 mm. (d) Time-resolved images and trajectories illustrating the controllable moving direction of Ag/Co micromotors in 5 wt.% H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e solution (\u003cem\u003em\u003c/em\u003e\u003csub\u003ePC\u003c/sub\u003e:\u003cem\u003em\u003c/em\u003e\u003csub\u003ewater\u003c/sub\u003e = 90:7) captured from a Supplementary video. Scale bar: 20 mm.\u0026nbsp;\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/e44a17ed6a86a291ec926ba6.png"},{"id":96455045,"identity":"54077d49-d440-47e6-99de-1238453144f9","added_by":"auto","created_at":"2025-11-21 10:03:27","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":4521307,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSERS detection of nanoplastics by Ag/Co micromotors. \u003c/strong\u003e(a) Schematic of the enrichment and delivery of the nanoplastics onto the volumetric SERS hot spots on the Ag/Co micromotors during water evaporation on a slippery PDMS surface. (b) SEM image of nanoplastics adsorbed on the surface of a Ag/Co micromotor via the van der Waals attraction. \u003cem\u003eC\u003c/em\u003e\u003csub\u003ePS\u003c/sub\u003e\u003csub\u003e\u003cem\u003e \u003c/em\u003e\u003c/sub\u003e= 20 mg/mL. (c) SERS mapping result of the 1001 cm\u003csup\u003e−1\u003c/sup\u003e peak of PS nanoplastics on a single Ag/Co micromotor. Inset: Optical micrograph of the SERS mapping area. \u003cem\u003eC\u003c/em\u003e\u003csub\u003ePS\u003c/sub\u003e\u003csub\u003e\u003cem\u003e \u003c/em\u003e\u003c/sub\u003e= 100 mg/mL. Scale bar: 2 mm. (d) SERS mapping result of the 1001 cm\u003csup\u003e−1\u003c/sup\u003e peak on a Ag/Co micromotor. \u003cem\u003eC\u003c/em\u003e\u003csub\u003ePS\u003c/sub\u003e\u003csub\u003e\u003cem\u003e \u003c/em\u003e\u003c/sub\u003e= 100 mg/mL. (e) Intensity variation of the 1001 cm\u003csup\u003e−1\u003c/sup\u003e SERS peak at 40 randomly chosen sites on a single Ag/Co micromotor. (f) SERS spectra obtained from 10 randomly chosen Ag/Co micromotors. \u003cem\u003eC\u003c/em\u003e\u003csub\u003ePS\u003c/sub\u003e\u003csub\u003e\u003cem\u003e \u003c/em\u003e\u003c/sub\u003e= 100 mg/mL. (g) SERS spectra of PS nanospheres at different concentrations on Ag/Co micromotors. (h) Relationship between the SERS intensity at 1001 cm\u003csup\u003e−1\u003c/sup\u003e and the concentration of the PS nanospheres. Error bars show the mean ± SD (n = 5). (i) Schematic showing magnetic stirring increased the density of the PS nanospheres on the Ag/Co micromotors on the PDMS slippery surface after water evaporation. (j) Magnetic stirring of the Ag/Co micromotors could greatly increase the density of the PS nanospheres on the Ag/Co micromotors. (k) Comparison of the intensity of the SERS peak at 1001 cm\u003csup\u003e−1 \u003c/sup\u003eunder the magnetically stirred (1000 rpm) and non-stirred conditions. Error bars show the mean ± SD (n = 5). \u003cem\u003eC\u003c/em\u003e\u003csub\u003ePS\u003c/sub\u003e\u003csub\u003e\u003cem\u003e \u003c/em\u003e\u003c/sub\u003e= 10 mg/mL. \u0026nbsp;\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/611b4f1794a9993886fb468c.png"},{"id":96427332,"identity":"2ee7aca0-dbc0-4d14-8b9b-cf56d08fccd7","added_by":"auto","created_at":"2025-11-21 02:46:05","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":3850330,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFlotation removal of the nanoplastics from water using the “microbubble armies” generated by the magnetically recyclable Ag/Co micromotors.\u003c/strong\u003e (a) Schematic of the nanoplastic removal process using the bubble-spouting Ag/Co micromotors and the recyclability of the micromotors. (b) “Microbubble armies” ejected from the Ag/Co micromotors trapping 5 mm PS spheres captured from a Supplementary video. (c) Absorption spectra of the PS nanoplastics within water before and after the treatment by Ag/Co micromotors. Inset: the digital photographs of the PS nanoplastic suspension before (left) and after treatment (right). (d) The variation of the removal efficiency of the nanoplastics with different concentrations of CTAB and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. (e) The variation of the removal efficiency of the nanoplastics with different concentrations of Ag/Co micromotors and PS nanoplastics. (f) Flotation nanoplastic removal kinetics. The removal efficiency gradually increased as a function of time. Inset: digital photographs of the PS nanoplastic suspensions after treatment for different times. (g) SERS spectra of PS nanosphere suspensions before (i) and after (ii) the flotation separation process. (h) Comparison of the removal efficiency of the Ag/Co micromotors before and after restoring using NaBH\u003csub\u003e4\u003c/sub\u003e. Even after using for six times, the removal efficiency was readily restored to ~ 90%. All the error bars show the mean ± SD (n = 3). (i) Absorption spectra of the PS nanoplastics spiked into the natural lake water before and after the treatment by bubble-spouting Ag/Co micromotors. Inset: digital photograph of the PS nanoplastic suspension in the real lake water after the flotation treatment.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/d30cb628c23198fdd82c1f37.png"},{"id":96602965,"identity":"a1ef497a-60d8-44df-bfac-14b6299debec","added_by":"auto","created_at":"2025-11-24 09:05:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":39462895,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/14fef9f9-c077-4694-8a55-9f5344c3f638.pdf"},{"id":96455442,"identity":"20126426-ff3e-4474-80d5-7d99981d0cef","added_by":"auto","created_at":"2025-11-21 10:04:08","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":32197934,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/fdab2fe2254722955fdd7945.docx"},{"id":96455602,"identity":"0b7d8ea8-3713-4bc9-a3dd-b3d3f1ee2cdc","added_by":"auto","created_at":"2025-11-21 10:04:23","extension":"mp4","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2857748,"visible":true,"origin":"","legend":"Supplementary Video 1","description":"","filename":"SupplementaryVideo1.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/597c85163b8396b688f11acb.mp4"},{"id":96455257,"identity":"d81130ad-0a47-40a2-a676-bd116f5844cb","added_by":"auto","created_at":"2025-11-21 10:03:51","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":94396,"visible":true,"origin":"","legend":"Supplementary Video 2","description":"","filename":"SupplementaryVideo2.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/c41f2069459ada38447bde8c.mp4"},{"id":96427319,"identity":"6fad5bd0-a3c0-42b7-bffa-53844ed68450","added_by":"auto","created_at":"2025-11-21 02:46:05","extension":"mp4","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":614439,"visible":true,"origin":"","legend":"Supplementary Video 3","description":"","filename":"SupplementaryVideo3.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/abc88aea4413c81222f1d199.mp4"},{"id":96455055,"identity":"38838734-1e73-4d0c-ac77-2cdbe53bee90","added_by":"auto","created_at":"2025-11-21 10:03:28","extension":"mp4","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":5253070,"visible":true,"origin":"","legend":"Supplementary Video 4","description":"","filename":"SupplementaryVideo4.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/0e0d4cf7d0aed05bc24702b3.mp4"},{"id":96455594,"identity":"36831e1e-f406-4e87-89a1-e97b164707d3","added_by":"auto","created_at":"2025-11-21 10:04:22","extension":"mp4","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":1708109,"visible":true,"origin":"","legend":"Supplementary Video 5","description":"","filename":"SupplementaryVideo5.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/6e1daf14c26bfc100ce19a07.mp4"},{"id":96455269,"identity":"b60f4f12-71ad-4c31-b2c7-d666053ade75","added_by":"auto","created_at":"2025-11-21 10:03:53","extension":"mp4","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":6923412,"visible":true,"origin":"","legend":"Supplementary Video 6","description":"","filename":"SupplementaryVideo6.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/161aaeb25e480baf3f4380f6.mp4"},{"id":96454769,"identity":"e9711690-2ac8-4948-b43f-c0238f094952","added_by":"auto","created_at":"2025-11-21 10:03:07","extension":"mp4","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":6015335,"visible":true,"origin":"","legend":"Supplementary Video 7","description":"","filename":"SupplementaryVideo7.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/198e51433399b57c1f6d0f1d.mp4"},{"id":96427344,"identity":"f02e4fae-4a8f-4762-9b06-e7b2e2450467","added_by":"auto","created_at":"2025-11-21 02:46:06","extension":"mp4","order_by":8,"title":"","display":"","copyAsset":false,"role":"supplement","size":19672854,"visible":true,"origin":"","legend":"Supplementary Video 8","description":"","filename":"SupplementaryVideo8.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/d010ecf02ba266794c14eb5d.mp4"},{"id":96427336,"identity":"631664f6-94a7-4943-b80c-3806f94b3bdd","added_by":"auto","created_at":"2025-11-21 02:46:05","extension":"mp4","order_by":9,"title":"","display":"","copyAsset":false,"role":"supplement","size":3068780,"visible":true,"origin":"","legend":"Supplementary Video 9","description":"","filename":"SupplementaryVideo9.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4730825/v1/dc6af070994308d07362665f.mp4"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Ultrasensitive SERS detection and efficient flotation removal of nanoplastics from water using bubble-spouting micromotor swarms","fulltext":[{"header":"Main","content":"\u003cp\u003ePlastic pollution has become a pervasive problem and even a planetary threat\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Plastic debris is now a ubiquitous pollutant in aquatic and terrestrial environments\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Microplastics (1 \u0026micro;m-5 mm) generated by weathering larger plastic waste have attracted much concern, due to their risks to ecosystems and human health\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. As much as around 8\u0026nbsp;million metric tons of macroplastics and 1.5\u0026nbsp;million metric tons of microplastics migrate to the ocean every year\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. At least 710\u0026nbsp;million metric tons of plastic contaminants have entered aquatic and terrestrial ecosystems\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Considering the plastic-carbon cycle, microplastics will eventually break down to produce nanoplastics (\u0026lt;\u0026thinsp;1 \u0026micro;m) as they age\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Theoretically, one microplastic can be fragmented into 10\u003csup\u003e14\u003c/sup\u003e nanoplastics after long-term environmental exposure\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. However, nanoplastics with much smaller size and greatly enhanced activity, mobility, environmental exposure, and toxicity have not received the same attention as microplastics\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Even worse, the quantity, the exact interactions with organisms, the fate, and the transport pathways of nanoplastics are still unclear due to the lack of analytical tools and skill sets to detect and characterize plastics in the nanoscale size range. Adequate analytical techniques capable of sampling, isolating, detecting, quantifying, and characterizing nanoplastics are urgently required\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. In addition, different from microplastics that are expected to eventually reach the bottom of water, the nanoplastics tend to suspend in the water column owing to their small size\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. The long residence times in the water column enable the nanoplastics to transport globally and make them have more severe impact on the ecosystems than microplastics and extremely difficult to remove. Techniques capable of removing the suspended nanoplastics from water are urgently required. The most critical step to the analytical and the separation techniques of the nanoplastics is to concentrate them from the water column, while their outstanding dispersion capability and nanometer size indicate a great challenge for enrichment. Membrane ultrafiltration process shows high efficiency in micro/nanoplastic interception\u003csup\u003e\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. Biochar adsorption is promising in micro/nanoplastics removal, however the water environment should be appropriate to obtain high removal efficiency and the polycyclic aromatic hydrocarbons within biochar tend to release into water\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. The flocculation method is generally ineffective for the small-size nanoplastics removal\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe flotation process employs bubbles to collect surfactant molecules, biospecies, ions, or mineral particles dispersed in water and drag them to the water surface during the bubble ascent to realize separation\u003csup\u003e\u003cspan additionalcitationids=\"CR17 CR18\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. The strong interactions between the microbubbles and the nanoplastics via hydrophobic interactions and the easy scalability\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e make the flotation process a promising candidate technology to capture and enrich the nanoplastics from water. Compared with the conventional bubble-generator system needing electric power and complex periphery apparatus, chemically active micromotors can decompose H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e to continuously generate microbubbles. We envision that the the micromotors may be able to produce \u0026ldquo;microbubble armies\u0026rdquo; to chase, capture, and efficiently separate nanoplastics from the water column. Further considering the capability of providing fingerprint signals and the single-molecule detection sensitivity of the surface-enhanced Raman spectroscopy (SERS) sensing technique\u003csup\u003e\u003cspan additionalcitationids=\"CR22 CR23\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, as well as the recyclability, we designed chemically active and magnetically responsive Ag/Co micromotors via a chemical redox potential-driven ion exchange and shape-preserving chemical reduction method (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea) capable of continuously ejecting oxygen microbubbles to capture, SERS detect, and efficiently remove the nanoplastics from the water column (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb \u003cb\u003eand Supplementary Video 1\u003c/b\u003e). The microbubbles automatically chase, capture, and transport the nanoplastics \u0026ldquo;on-the-fly\u0026rdquo; from the bulk water to the water surface (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec), resembling the white blood cells autonomously searching and removing the nanointruders in biology. The magnetic field could control the moving trajectories of the Ag/Co micromotor swarms and in turn the microbubbles to cruise the whole water volume and separate the micromotors from water for recycling usage. The dense Ag nanoparticles on the Ag/Co micromotors endowed outstanding SERS performance to the micromotors, enabling single nanoplastic particle-level SERS detection.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eDesign principle and characterization of bubble-spouting Ag/Co micromotors\u003c/h3\u003e\n\u003cp\u003eThe working mechanism of bubble-propelled micromotors requires them to have asymmetric morphologies to achieve directional bubble ejection, typically requiring torpedo-like morphology\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. Considering the easy setup and simple processing, we tried to directly electrodeposit chemically active Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e asymmetric microstructures as the starting templates to construct the magnetic Ag/Co micromotors (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea).\u003c/p\u003e\n\u003cp\u003eWe first investigated the electrochemical nucleation and growth mechanism of the Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e crystals to realize electrodeposition of torpedo-like Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e microparticles. The chronoamperometric curve indicated that at a potential of 1.3 V, the current decay followed the Cottrell equation, (\u003cem\u003ei.e.\u003c/em\u003e, \u003cem\u003ei\u003c/em\u003e\u0026prop;\u003cem\u003et\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026thinsp;1/2\u003c/sup\u003e, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;1\u003c/strong\u003e), suggesting a rapid electron transfer rate under this potential with the electrodeposition being controlled by the diffusion process\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. From the equilibrium potential measured in the Tafel plot (1.24 V vs. Ag/AgCl, \u003cstrong\u003eSupplementary Fig.\u0026nbsp;2)\u003c/strong\u003e, it could be deduced that only about 60 mV overpotential was required for the oxidation of Ag\u003csup\u003e+\u003c/sup\u003e ions to be diffusion-controlled, suggesting that the electrochemical reaction was relatively reversible\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. However, this potential did not trigger the nucleation process of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e. As the potential increased further towards higher overpotential reaching 1.6 V, the chronoamperometric curve indicated that the electrodeposition of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e followed the mechanism of three-dimensional nucleation with diffusion-controlled growth mechanism (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;3\u003c/strong\u003e)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. The dimensionless form of the chronoamperometric curve showed the nature of instantaneous nucleation for the electrodeposition of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;3\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eOverpotential significantly affects the electron transfer rate, the mass transport rate, and the nucleation density during the electrodeposition process\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. We employed finite element analysis to investigate the crystal morphology evolution of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e and the concentration of Ag\u003csup\u003e+\u003c/sup\u003e ions in the electrolyte at different overpotentials. In the model, a single micropyramid and two closely spaced micropyramids represented discrete nuclei formed at low overpotential and dense nuclei formed at high overpotential, respectively (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;4\u003c/strong\u003e). At low potentials (\u003cem\u003ee.g.\u003c/em\u003e, 0.2 V), the discrete micropyramid formed a hemispherical diffusion layer of Ag\u003csup\u003e+\u003c/sup\u003e ions around it. Although the electrodeposition was diffusion-controlled, the concentration gradient of Ag\u003csup\u003e+\u003c/sup\u003e ions at the apexes and facets of the micropyramid was minimal, leading to the thermodynamically dominated crystal growth. Consequently, the Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e crystal evolved into a well-defined micropyramid according to Wulff\u0026rsquo;s theorem (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea). In contrast, at high potentials (\u003cem\u003ee.g.\u003c/em\u003e, 0.7 V), Ag\u003csup\u003e+\u003c/sup\u003e ion depletion zones formed around the nuclei, and the diffusion layers between adjacent nuclei rapidly overlapped. In this case, the concentration gradient of Ag\u003csup\u003e+\u003c/sup\u003e ions was parallel to the electrode surface, suppressing growth along this direction. The diffusion of Ag\u003csup\u003e+\u003c/sup\u003e ions primarily occurred perpendicular to the electrode surface. Furthermore, since the top apexes of micropyramids were spatially closer to regions of high Ag\u003csup\u003e+\u003c/sup\u003e ion concentration, the concentration gradient at these apexes was higher, triggering the growth of nanorods perpendicular to the electrode surface (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe electrodeposition experiments revealed that Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e formed highly symmetric micropyramids at low potentials, while Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e tended to form nanorods with uniform diameters at higher potentials, which was consistent with the above simulation results (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;5\u003c/strong\u003e). However, neither microstructure met the asymmetric morphology requirements for constructing bubble-propelled micromotors. To fabricate micromotors with asymmetric morphology that could be easily released from the electrode surface and be dispersed into the water (as discussed below), we designed a potential waveform combining gradually increasing and gradually decreasing potential between 10 V and 2 V directed by the above simulation results, resulting in the formation of asymmetric Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e microscale bipyramids (MBPs) (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec, d \u003cstrong\u003eand Supplementary Fig.\u0026nbsp;6\u003c/strong\u003e). When the potential gradually increased from 2 V to 4 V, a faceted micropyramid was formed on the apex of a microscale torpedo. Then, high potentials (\u003cem\u003ei.e.\u003c/em\u003e, \u0026gt;4 V) created an elongated micropyramid at the top because of the pronounced vertical diffusion of Ag\u003csup\u003e+\u003c/sup\u003e ions to the top apexes as discussed above (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;7\u003c/strong\u003e). The Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs with uniform morphologies were released from the electrode surface via sonication treatment in water (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee, f). The length and the width of the MBPs were 23.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.6 \u0026micro;m and 4.39\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 \u0026micro;m, respectively (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eg\u003cstrong\u003e)\u003c/strong\u003e. We demonstrated the scalable production of gram level of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBP powders through the sonication-regrowth cycles (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eh \u003cstrong\u003eand Supplementary Fig.\u0026nbsp;8\u003c/strong\u003e), suggesting their potential for being scaled up to kilogram level production for treating nanoplatic pollution in millions of tons of water.\u003c/p\u003e\n\u003cp\u003eAg\u003csup\u003e2+\u003c/sup\u003e and Ag\u003csup\u003e3+\u003c/sup\u003e ions within the Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e have very strong oxidization capability with redox potentials of almost 2.0 V. Therefore, we expect that the Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs can oxidize Co\u003csup\u003e2+\u003c/sup\u003e ions. The occurrence of the redox reactions between Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e and Co\u003csup\u003e2+\u003c/sup\u003e was verified from a thermodynamic perspective by comparing the potential\u0026ndash;pH diagrams of Co-H\u003csub\u003e2\u003c/sub\u003eO and Ag-H\u003csub\u003e2\u003c/sub\u003eO systems (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;9\u003c/strong\u003e). The electrode potential of high-valent silver ions remains higher than that of Co\u003csup\u003e2+\u003c/sup\u003e at room temperature within the pH range of 0\u0026thinsp;\u0026le;\u0026thinsp;pH\u0026thinsp;\u0026le;\u0026thinsp;7.1. The measured pH of a 50 mM Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e solution was 5.4 and high-valent silver ions in Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e can oxidize Co\u003csup\u003e2+\u003c/sup\u003e to Co\u003csup\u003e3+\u003c/sup\u003e thermodynamically. Experimentally, introduction of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs into aqueous solutions containing Co\u003csup\u003e2+\u003c/sup\u003e ions triggered the galvanic replacement reactions. The reactions resulted in the diffusion out of silver ions from the inner Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e core and the gradual adherence of Co\u003csup\u003e3+\u003c/sup\u003e ions to the surface of the Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs, giving rise to the formation of continuously growing AgCoO\u003csub\u003e2\u003c/sub\u003e shell (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ei \u003cstrong\u003eand Supplementary Fig.\u0026nbsp;10\u003c/strong\u003e). Immersing the electrode covered by the AgCoO\u003csub\u003e2\u003c/sub\u003e BMPs slowly into water could peel off uniformly structured AgCoO\u003csub\u003e2\u003c/sub\u003e BMPs (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;11\u003c/strong\u003e and \u003cstrong\u003eSupplementary Fig.\u0026nbsp;12\u003c/strong\u003e). The AgCoO\u003csub\u003e2\u003c/sub\u003e BMPs were hollow reflected from the TEM images (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;13\u003c/strong\u003e). The transformation process from Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs to AgCoO\u003csub\u003e2\u003c/sub\u003e MBPs was monitored by the EDX element measurements (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ej).\u003c/p\u003e\n\u003cp\u003eTo create Ag nanostructures as the SERS sensing substrates, the AgCoO\u003csub\u003e2\u003c/sub\u003e MBPs were reduced to Ag/Co MBPs with the morphology maintained using NaBH\u003csub\u003e4\u003c/sub\u003e as the reducing agent without introducing any organic pollutants that might influence the SERS enhancement. X-ray diffraction (XRD) patterns of the chemically reduced shape-preserving Ag/Co MBPs demonstrated sharp peaks indexed to crystalline Ag (PDF #04-0783) and showed no obvious peak of Co owing to its amorphous structure (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea). The annealed Ag/Co MBPs exhibited peaks indexed to the crystalline Co (PDF #15\u0026ndash;0806). The magnified SEM image (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb, c) revealed that dense Co nanosheets (~\u0026thinsp;50 nm in thickness) decorated with Ag nanoparticles (~\u0026thinsp;8 nm in diameter) covered the surface of the Ag/Co MBPs. The atomic ratio between Ag and Co within the Ag/Co MBPs could be adjusted by varying the reaction time between Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs and Co\u003csup\u003e2+\u003c/sup\u003e ions, which was decreased from about 3 to 0.2 when the reaction time was prolonged from 1 min to 60 min (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;14)\u003c/strong\u003e.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eEDX mapping results demonstrated the uniform distribution of Ag and Co elements within a single Ag/Co MBP (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed). To further elucidate the distribution of Ag nanoparticles on the Co nanosheets and their different crystallinity, we broke the structures of Ag/Co MBPs in ethanol through sonication treatment for 1 h. We could easily distinguish the Ag nanoparticles on an individual Co nanosheet with a lateral size of tens of nanometers under transmission electron microscope (TEM) (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ee). The high-resolution TEM image indicated the high crystallinity of the Ag nanoparticles with lattice fringes of 0.118 nm and 0.102 nm corresponding to the (222) and (400) planes, respectively (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ef). In contrast, the Co nanosheet zone exhibited an amorphous feature (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eg). The selected area electron diffraction (SAED) from the nanosheets with nanoparticles showed distinct diffraction rings of Ag (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eh), suggesting the structure of amorphous Co and crystalline Ag. Moreover, EDX mapping of Ag and Co elements on a single nanosheet further verified the distribution of Ag nanoparticles on Co nanosheets (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ei-k).\u003c/p\u003e\n\u003cp\u003eX-ray photoelectron spectroscopy (XPS) further confirmed the coexistence of Ag and Co on the surface of MBPs (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003el). Moreover, high-resolution XPS spectra were used to characterize the valence change of Ag and Co during the reduction process. In the Co 2p spectrum (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003em), the peaks at 780.2 eV and 781.7 eV were assigned to Co\u003csup\u003e3+\u003c/sup\u003e and Co\u003csup\u003e2+\u003c/sup\u003e, respectively. The satellite peaks at 785.0 eV were due to the shake-up excitation of the high-spin Co\u003csup\u003e2+\u003c/sup\u003e ions\u003csup\u003e29\u0026ndash;31\u003c/sup\u003e. After NaBH\u003csub\u003e4\u003c/sub\u003e reduction, the majority of Co\u003csup\u003e3+\u003c/sup\u003e ions were reduced to Co\u003csup\u003e0\u003c/sup\u003e (778.2 eV) and the Co\u003csup\u003e2+\u003c/sup\u003e state was due to oxidation under ambient conditions\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. The more prominent satellite peak further confirmed the predominance of Co\u003csup\u003e2+\u003c/sup\u003e ions after reduction. Additionally, the Ag 3d spectra exhibited characteristic peaks at 368.1 and 374.1 eV, corresponding to Ag 3d\u003csub\u003e5/2\u003c/sub\u003e and Ag 3d\u003csub\u003e3/2\u003c/sub\u003e of Ag\u003csup\u003e+\u003c/sup\u003e, respectively. After NaBH\u003csub\u003e4\u003c/sub\u003e reduction, most of the Ag\u003csup\u003e+\u003c/sup\u003e ions were reduced into zero-valence state Ag (3d\u003csub\u003e5/2\u003c/sub\u003e 368.4 eV and 3d\u003csub\u003e3/2\u003c/sub\u003e 374.4 eV)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. The small amount of Ag\u003csup\u003e+\u003c/sup\u003e after reduction originated from the oxidation of Ag under ambient conditions (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003en).\u003c/p\u003e\n\u003cp\u003eThe magnetic hysteresis loop revealed that the fabricated Ag/Co MBPs had a saturation magnetization (\u003cem\u003eM\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e) of 8.19 emu/g as well as a negligible coercivity (\u003cem\u003eH\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e) and remanent magnetization (\u003cem\u003eM\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e) at 300 K (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eo \u003cstrong\u003eand Supplementary Fig.\u0026nbsp;15\u003c/strong\u003e). These results indicated a superparamagnetic property of the Ag/Co MBPs, which is desirable for the magnetically controllable movement and separation. The superparamagnetic property enabled the MBPs to be easily separated from water by a NdFeB magnet within 20 s (\u003cstrong\u003eInset in\u003c/strong\u003e Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eo \u003cstrong\u003eand Supplementary Fig.\u0026nbsp;16\u003c/strong\u003e).\u003c/p\u003e\n\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003eMoving mechanism of bubble-spouting Ag/Co micromotors\u003c/h2\u003e\n\u003cp\u003eThe Ag/Co MBPs could be used as micromotors due to both their unique chemical activeness and appropriate torpedo-mimicking asymmetric morphology. The catalytic properties of the Ag/Co micromotors facilitated the rapid decomposition of hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) to produce oxygen microbubbles\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. Oxygen microbubbles nucleated on the surface of Ag/Co MBPs and grew until they detached due to buoyancy and fluid shear forces. The momentum released at the moment of microbubble detachment created a recoil force, propelling the Ag/Co micromotors in the opposite direction of the bubble\u0026rsquo;s detachment (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;17\u003c/strong\u003e and \u003cstrong\u003eSupplementary Fig.\u0026nbsp;18\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eUnderstanding the moving mechanism of the micromotors needs determination of the bubble formation sites on the micromotors. Finite element analysis revealed that oxygen tended to be concentrated at three different kinds of tips of the MBPs (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea and \u003cstrong\u003eSupplementary Fig.\u0026nbsp;19\u003c/strong\u003e) capable of serving as nucleation sites for bubbles after reaching the maximum supersaturation concentration necessary for oxygen bubble nucleation (68 mM) (\u003cstrong\u003eSupplementary Video 2)\u003c/strong\u003e \u003csup\u003e35\u003c/sup\u003e. To enhance the visibility of the position of bubbles and the moving trajectory of the Ag/Co micromotors, propylene carbonate (PC) was added to the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e solution (\u003cem\u003em\u003c/em\u003e\u003csub\u003ePC\u003c/sub\u003e:\u003cem\u003em\u003c/em\u003e\u003csub\u003ewater\u003c/sub\u003e = 90:7) to reduce the bubble size and longevity\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. We experimentally identified three distinct positions prone to form bubbles on the tips of Ag/Co micromotors, consistent with the COMSOL simulation results (\u003cstrong\u003eSupplementary Video 3\u003c/strong\u003e). The observed trajectories were all spiral because the micromotors experienced both the recoil force from bubbles and the fluid drag force\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e (\u003cstrong\u003eSupplementary Video 4\u003c/strong\u003e). The drag force was along the central axis, while the bubbles were not strictly released along the central axis, indicating that the recoil force deviated from the central axis and causing the Ag/Co micromotor to move spirally (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb \u003cstrong\u003eand Supplementary Fig.\u0026nbsp;20\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe moving mechanism of the bubble-spouting Ag/Co micromotors was quantitatively studied. Obviously, the generation and the detaching process of the microbubbles from the micromotors determine their moving speed\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. We tried to calculate the moving speed by the bubble generation frequency and the moving step length propelled by one bubble ejection. The bubble production rate can be experimentally measured from the videos. We roughly assume the micromotor (simplified as \u003cem\u003eMM\u003c/em\u003e) to be an ideal cone with a surface area \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:S=\\pi\\:{R}_{MMs}({L}_{MMs}+{R}_{MMs})\\)\u003c/span\u003e\u003c/span\u003e, where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{MMs}\\)\u003c/span\u003e\u003c/span\u003e is the radius of the thick side of the cone and\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:{L}_{MMs}\\)\u003c/span\u003e\u003c/span\u003e is the length of the cone. The oxygen production rate \u003cem\u003ek\u003c/em\u003e can be expressed as\u003csup\u003e38\u003c/sup\u003e:\u003c/p\u003e\n\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equa\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}k=\\frac{d{V}_{{O}_{2}}}{dt}=n{C}_{{H}_{2}{O}_{2}}S=n\\pi\\:{C}_{{H}_{2}{O}_{2}}{R}_{MMs}\\left({L}_{MMs}+{R}_{MMs}\\right) \\left(1\\right)\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003ewhere \u003cem\u003en\u003c/em\u003e is related to the specific experiment condition and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{{H}_{2}{O}_{2}}\\)\u003c/span\u003e\u003c/span\u003e is the concentration of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eUsing the average bubble radius\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{b}\\)\u003c/span\u003e\u003c/span\u003e, we could calculate the microbubble formation frequency \u003cem\u003ef\u003c/em\u003e:\u003c/p\u003e\n\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equb\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}f=\\frac{n{C}_{{H}_{2}{O}_{2}}{R}_{MMs}\\left({L}_{MM}+{R}_{MMs}\\right)}{{V}_{bubble}}=\\:\\frac{3n{C}_{{H}_{2}{O}_{2}}{R}_{MMs}\\left({L}_{MMs}+{R}_{MMs}\\right)}{4{R}_{b}^{3}}\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(2\\right)\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eThe cone and the microbubble follow the momentum conservation principle.\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:\\)\u003c/span\u003e\u003c/span\u003eThe force exterted on the bubble and the cone was represented as \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{bubble}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{MMs}\\)\u003c/span\u003e\u003c/span\u003e, respectively. The cone-bubble system has two critical states, that it, before (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{t}_{0}\\)\u003c/span\u003e\u003c/span\u003e) and after (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{t}_{1}\\)\u003c/span\u003e\u003c/span\u003e) the bubble ejection. The entire momentum of the cone-bubble system is:\u003c/p\u003e\n\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equc\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}{\\int\\:}_{{t}_{0}}^{{t}_{1}}{F}_{bubble}dt+{\\int\\:}_{{t}_{0}}^{{t}_{1}}{F}_{MMs}dt=\\\\\\:{m}_{b}\\left({v}_{b}\\left({t}_{1}\\right)-{v}_{b}\\left({t}_{0}\\right)\\right)+{m}_{MMs}\\left({v}_{MTs}\\left({t}_{1}\\right)-{v}_{MMs}\\left({t}_{0}\\right)\\right) \\left(3\\right)\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{m}_{b}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{m}_{MMs}\\)\u003c/span\u003e\u003c/span\u003e are the mass of the bubble and the cone, respectively. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{b}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{MTs}\\)\u003c/span\u003e\u003c/span\u003e are the velocity of the bubble and the cone, respectively. The \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{bubble}\\)\u003c/span\u003e\u003c/span\u003e can be estimated by the Stokes\u0026rsquo;s law:\u003c/p\u003e\n\u003cdiv id=\"Equd\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equd\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}{F}_{bubble}=-6\\pi\\:\\mu\\:{R}_{b}{v}_{b}\\left(t\\right) \\left(4\\right)\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{b}\\)\u003c/span\u003e\u003c/span\u003e represents the bubble radius; The speed of the bubble at time \u003cem\u003et\u003c/em\u003e is \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{b}\\left(t\\right)\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\mu\\:}\\)\u003c/span\u003e\u003c/span\u003e is the fluid viscosity.\u003c/p\u003e\n\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{MMs}\\)\u003c/span\u003e\u003c/span\u003e can be written as\u003csup\u003e40\u003c/sup\u003e:\u003c/p\u003e\n\u003cdiv id=\"Eque\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Eque\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}{F}_{MMs}=-\\frac{2\\pi\\:\\mu\\:{L}_{MMs}{v}_{MMs}\\left(t\\right)}{\\text{ln}\\left(\\frac{{L}_{MMs}}{{R}_{MMs}}\\right)-0.72} \\left(5\\right)\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{L}_{MMs}\\)\u003c/span\u003e\u003c/span\u003e is the length and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{MMs}\\)\u003c/span\u003e\u003c/span\u003e is the radius of the cylinder.\u003c/p\u003e\n\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{b}\\left({t}_{0}\\right)\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{MMs}\\left({t}_{0}\\right)\\)\u003c/span\u003e\u003c/span\u003e are the starting velocity of the bubble and the micromotor, respectively, both of which equal zero. The bubble and the micromotor all rest with a speed of zero after the bubble leaves. The momentum equation is simplified to be:\u003c/p\u003e\n\u003cdiv id=\"Equf\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equf\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}{\\int\\:}_{{t}_{0}}^{{t}_{1}}{F}_{bubble}dt+{\\int\\:}_{{t}_{0}}^{{t}_{1}}{F}_{MMs}dt=0 \\left(6\\right)\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eThe bubble and the micromotor are separated by a distance of 2\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{b}\\)\u003c/span\u003e\u003c/span\u003e, which is the sum of the displacement of the micromotor and the bubble:\u003c/p\u003e\n\u003cdiv id=\"Equg\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equg\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}2{R}_{b}={\\int\\:}_{{t}_{0}}^{{t}_{1}}{v}_{bubble}\\left(t\\right)dt+{\\int\\:}_{{t}_{0}}^{{t}_{1}}{v}_{MMs}\\left(t\\right)dt \\left(7\\right)\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eThe one step length pushed by a single bubble release event can be described by:\u003c/p\u003e\n\u003cdiv id=\"Equh\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equh\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}l={\\int\\:}_{{t}_{0}}^{{t}_{1}}{v}_{MMs}\\left(t\\right)dt=\\frac{6{R}_{b}^{2}}{3{R}_{b}-\\frac{{L}_{MMs}}{\\text{ln}\\left(\\frac{{L}_{MMs}}{{R}_{MMs}}\\right)-0.72}} \\left(8\\right)\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eThe average velocity of the micromotor \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{MMs}^{ave}\\)\u003c/span\u003e\u003c/span\u003e was simplified to be:\u003c/p\u003e\n\u003cdiv id=\"Equi\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equi\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}{v}_{MMs}^{a\\text{v}\\text{e}}=f\\times\\:l=\\frac{9n{C}_{{H}_{2}{O}_{2}}{R}_{MMs}\\left({L}_{MMs}+{R}_{MMs}\\right)}{6{R}_{b}^{2}-\\frac{2{L}_{MMs}{R}_{b}}{\\text{ln}\\left(\\frac{{L}_{MMs}}{{R}_{MMs}}\\right)-0.72}} \\left(9\\right)\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eIn our case, the \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eMMs\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eL\u003c/em\u003e\u003csub\u003e\u003cem\u003eMMs\u003c/em\u003e\u003c/sub\u003e, and \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003eb\u003c/em\u003e\u003c/sub\u003e equal 4.4 \u0026micro;m, 23.2 \u0026micro;m, and 23.0 \u0026micro;m, respectively. Eq.\u0026nbsp;9 is simplified to be:\u003c/p\u003e\n\u003cdiv id=\"Equj\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equj\" class=\"mathdisplay\"\u003e$$\\:\\begin{array}{c}{v}_{MMs}^{a\\text{v}\\text{e}}=0.53n{C}_{{H}_{2}{O}_{2}}\\: \\left(10\\right)\\:\\end{array}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eEquation 10 indicates that the average velocity of the micromotor is linearly proportional to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:n{C}_{{H}_{2}{O}_{2}}\\)\u003c/span\u003e\u003c/span\u003e, which is related to the oxygen production rate. Based on the experimentally measured moving speed of the micromotors in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e at different concentrations (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;21\u003c/strong\u003e and \u003cstrong\u003eSupplementary Video 5\u003c/strong\u003e), we calculated \u003cem\u003en\u003c/em\u003e values in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e at different concentrations according to Eq.\u0026nbsp;10. The value of \u003cem\u003en\u003c/em\u003e varied from 0.056 to 0.224, to 0.162, and further to 0.103 m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003ekg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e when the concentration of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e increased from 1 wt.%, to 3 wt.%, to 5 wt.%, and further to 10 wt.%, instead of a constant.\u003c/p\u003e\n\u003cp\u003eWe fitted the relationship between the experimentally measured moving speed \u003cem\u003ev\u003c/em\u003e of the micromotors and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{{H}_{2}{O}_{2}}\\)\u003c/span\u003e\u003c/span\u003e by applying a modified Michaelis-Menten Eq.\u0026nbsp;4\u003csup\u003e1,42\u003c/sup\u003e:\u003c/p\u003e\n\u003cdiv id=\"Equk\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equk\" class=\"mathdisplay\"\u003e$$\\:v\\left(\\text{y}\\right)=\\frac{550\\times\\:{\\text{C}}_{{H}_{2}{O}_{2}}^{2.28}}{{2.58}^{2.28}+{\\text{C}}_{{H}_{2}{O}_{2}}^{2.28}}\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(11\\right)$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eCombining Eq.\u0026nbsp;10 with Eq.\u0026nbsp;11, we obtained the relationship between \u003cem\u003en\u003c/em\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{{H}_{2}{O}_{2}}\\)\u003c/span\u003e\u003c/span\u003e:\u003c/p\u003e\n\u003cdiv id=\"Equl\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equl\" class=\"mathdisplay\"\u003e$$\\:n=\\frac{1037.76\\times\\:{\\text{C}}_{{H}_{2}{O}_{2}}^{1.28}}{{2.58}^{2.28}+{\\text{C}}_{{H}_{2}{O}_{2}}^{2.28}}\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(12\\right)$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eBased on Eq.\u0026nbsp;10 and Eq.\u0026nbsp;12, we can predict the moving speed of the Ag/Co micromotors in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e aqueous solutions with different concentrations. In addition to the concentration of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, the moving speed of the micromotors was also influenced by the weight ratio of Ag and Co within the micromotors with different catalytic performance. As the weight ratio between Ag and Co was decreased by prolonging the reaction time between Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs with Co\u003csup\u003e2+\u003c/sup\u003e ions, the catalytic performance was gradually decreased reflected by the slower concentration decrease of the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (\u003cstrong\u003eSupplementary Fig.\u0026nbsp;22\u003c/strong\u003e). Therefore, we need to balance the catalytic performance and the magnetic properties by designing the Ag and Co ratio within the micromotors.\u003c/p\u003e\n\u003cp\u003eThe superparamagnetic properties enabled precise manipulation of the moving direction of the micromotors in water using a magnet (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec \u003cstrong\u003eand Supplementary Video 6\u003c/strong\u003e). The micromotors showed spiral trajectories towards random directions in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e aqueous solutions (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb). A magnetic field could be used to rationally steer the moving direction of the bubble-propelled Ag/Co micromotors to cruise the whole water volume, facilitating capturing nanoplastics well-dispersed in water (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed \u003cstrong\u003eand Supplementary Video 7\u003c/strong\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eUltrasensitive SERS detection of nanoplastics by Ag/Co micromotors\u003c/h3\u003e\n\u003cp\u003eAmong various types of nanoplastics, polystyrene (PS) is widely used in daily products and cannot be biodegraded. Previous studies have shown that PS nanoplastics have detrimental effects on nerve systems\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Therefore, PS nanospheres were selected as a model nanoplastic to evaluate the SERS sensing performance of the Ag/Co micromotors. The Raman spectrum of the Ag/Co micromotors showed peaks at 476 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 530 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and 680 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponding to the \u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e, \u003cem\u003eF\u003c/em\u003e\u003csub\u003e2g\u003c/sub\u003e, and \u003cem\u003eA\u003c/em\u003e\u003csub\u003e1g\u003c/sub\u003e modes of the cubic phase of cobalt oxides\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e, respectively (\u003cb\u003eSupplementary Fig.\u0026nbsp;23\u003c/b\u003e). These Raman peaks did not overlap with the Raman peaks of PS nanoplastics, ensuring a clean background for detecting the weak Raman signals of PS nanoplastics.\u003c/p\u003e\u003cp\u003eFirst, we studied the necessity of PS nanosphere detection using SERS. We dispersed isolated PS nanospheres with a size of 5 \u0026micro;m, 2 \u0026micro;m, 1 \u0026micro;m, and 200 nm on a piece of silicon wafer. Strong Raman signals were observed from a single PS sphere with a size larger than 1 \u0026micro;m without using SERS (\u003cb\u003eSupplementary Fig.\u0026nbsp;24\u003c/b\u003e). However, no Raman signals were observed from PS nanospheres with a size of 200 nm on the silicon wafer. This means that it is necessary to employ SERS technique to detect single nanoplastics. Only about two times of enhancement of the single PS sphere Raman signals was observed on the conventional Au nanosphere array SERS substrate (\u003cb\u003eSupplementary Fig.\u0026nbsp;24\u003c/b\u003e), because the large PS spheres could not enter the \u0026lt;\u0026thinsp;10 nm crevices between neighboring Au nanoparticles where strong electromagnetic fields located (known as \u0026ldquo;hot spots\u0026rdquo;) \u003csup\u003e21,22,23,24,46\u003c/sup\u003e. Therefore, SERS substrates with volumetric hot spots are desired for detection of nanoplastics. Volumetric hot spots were formed between the interlaced nanoplates covered by densely packed Ag nanoparticles within the Ag/Co MBPs prepared by reacting for 3 min between Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs and Co\u003csup\u003e2+\u003c/sup\u003e ions (\u003cb\u003eSupplementary Fig.\u0026nbsp;25\u003c/b\u003e), enabling them to detect single PS nanospheres with a diameter of 200 nm.\u003c/p\u003e\u003cp\u003eAlthough the Ag/Co micromotors can detect single PS nanospheres, it is necessary to increase the number density of the PS nanospheres on the Ag/Co micromotors to make sure that shining the laser at a randomly spot on the Ag/Co micromotors covers at least one PS nanosphere. A slippery polydimethylsiloxane (PDMS) layer-covered silicon substrate\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e,\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e was used to force the very few amounts of PS nanospheres to attach the Ag/Co micromotors during water evaporation. Previous reports have shown that strong π-metal interactions exist between Ag nanomaterials and aromatic hydrocarbons under ambient conditions, despite the lack of conventional metal-binding functional groups. This interaction is essentially a type of van der Waals force originated from the dispersive interactions between the π-system of aromatic hydrocarbons and the silver surface\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. After water was completely evaporated, most of the PS nanoplastics were adsorbed onto the surface of Ag/Co micromotors (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, b). Uniform SERS signals of PS nanoplastics were clearly observed from an individual Ag/Co micromotor, reflected by the uniform color in the SERS mapping results. These results proved that the PDMS-Ag/Co micromotors could be used as an integrated platform to sensitively detect nanoplastics (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, d). The relative standard deviation (RSD) of the 1001 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e SERS peak of the PS nanoplastics assigned to C\u0026ndash;C ring breathing mode was only 10.8% by calculating its intensity distribution, indicating a remarkable detection reliability of the integrated SERS platform (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed, e). Additionally, the SERS spectra measured on ten randomly selected Ag/Co micromotors showed negligible intensity variation, suggesting the high SERS stability and repeatability of the Ag/Co micromotors (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef). The Au/Co micromotors were also used to detect polyethylene terephthalate (PET) nanoparticles with a size of 120 nm with a detection limit of \u0026lt;\u0026thinsp;20 mg/ml (\u003cb\u003eSupplementary Fig.\u0026nbsp;26\u003c/b\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eWe further evaluated the SERS sensitivity of the Ag/Co micromotors. An obvious SERS peak at 1001 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was still distinguishable even at a PS nanoplastics concentration as low as 5 \u0026micro;g/mL (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg). SERS signals of PS nanoplastics at a concentration of 5 \u0026micro;g/mL were consistently detectable at five random positions (\u003cb\u003eSupplementary Fig.\u0026nbsp;27\u003c/b\u003e). This concentration level of nanoplastic detection is superior to previous studies regarding the sensitivity, reliability, and practicability (\u003cb\u003eSupplementary Table\u0026nbsp;1\u003c/b\u003e). The relationship between the SERS intensity at 1001 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the concentration of PS nanoplastics could be described by \u003cem\u003eI\u003c/em\u003e\u0026thinsp;=\u0026thinsp;10.85 \u003cem\u003eC\u003c/em\u003e\u0026thinsp;+\u0026thinsp;78.16 with a correlation coefficient \u003cem\u003eR\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.9919 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eh), manifesting the good quantification capability of the Ag/Co micromotors as single-particle SERS substrates.\u003c/p\u003e\u003cp\u003eThe magnetically controllable moving Ag/Co micromotors were expected to accelerate and enhance the PS nanoplastics adsorption process by effectively mixing the suspensions, thereby increasing the chance of contact between the volumetric hot spots and the PS nanoplastics. To observe the effect of magnetic mixing, 1 \u0026micro;L of fluorescent PS nanosphere dispersions were added to a static droplet of water (10 \u0026micro;L) containing Ag/Co micromotors. A rotary magnetic field generated by a magnetic stirring plate was applied to rotate the Ag/Co micromotors (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ei). The bright PS nanospheres could be clearly observed under UV irradiation (λ =\u0026thinsp;365 nm). The passive diffusion of PS nanospheres was slow when the stirring process was off, resulting in an inhomogeneous distribution after 260 s. In contrast, when the stirring was on, it could be observed that PS nanospheres swirled around in the droplet along the direction of stirring and the homogenization of PS nanospheres was accelerated (\u003cb\u003eSupplementary Fig.\u0026nbsp;28\u003c/b\u003e). To further verify this, the 200-nm red fluorescent PS spheres on the Ag/Co micromotors were observed via confocal fluorescence microscopy. Magnetic strring effectively overcame the diffusion limit, enabling active capture of the spheres and resulting in significantly higher accumulation on the micromotors compared to non-stirred conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ej \u003cb\u003eand Supplementary Video 8\u003c/b\u003e). The intensity of the SERS signals from PS nanoplastics under magnetic stirring was much higher than that of the control group without stirring (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ek \u003cb\u003eand Supplementary Fig.\u0026nbsp;29\u003c/b\u003e). Therefore, the magnetically controllable mobile Ag/Co micromotors showed more sensitive detection performance of the PS nanoplastics than that of the static ones (\u003cb\u003eSupplementary Fig.\u0026nbsp;30\u003c/b\u003e).\u003c/p\u003e\n\u003ch3\u003eRecyclable flotation separation of nanoplastics by “microbubble armies” generated by the Ag/Co micromotor swarms\u003c/h3\u003e\n\u003cp\u003eThe working mechanism of the flotation process relies on the difference in the surface hydrophobicity of different components. The flotation technique has been widely used in the mining industry to collect valuable ores\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e where hydrophobic particles are separated from the liquid phase as bubbles adhere to them, causing the particles to ascend to the foam layer on the water surface\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. The flotation method seemingly a promising way to remove nanoplastics from a large volume of water, but has not been reported in nanoplastics removal.\u003c/p\u003e\u003cp\u003eThe interaction between the bubbles and the nanoplastics is critical for successful nanoplastic flotation removal, which consists of collision, attachment, and detachment process. These processes work together to govern the flotation kinetics and influence the separation efficiency. The bubble-nanoplastics collision efficiency is determined by the fluid hydrodynamics and the size of the nanoplastics and the bubble\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. Sutherland, et al. built a collision model for a bubble-particle system\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003ewith the derivation of an expression for \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e (the number ratio of the particles encountering a bubble per unit time to the number of the particles approaching the bubble in a flow tube with a cross-sectional area equal to the projected area of the bubble) from fluid stream functions. They assumed that the particles within the collision radius would attach onto the bubble and in turn the collision efficiency was determined by the ratio of the cross-sectional area of the stream tube (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pi\\:{R}_{c}^{2}=3\\pi\\:{d}_{p}{d}_{b}/4\\)\u003c/span\u003e\u003c/span\u003e) to the projected area of the bubble (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pi\\:{d}_{b}^{2}/4\\)\u003c/span\u003e\u003c/span\u003e):\u003cdiv id=\"Equm\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equm\" name=\"EquationSource\"\u003e\n$$\\:{E}_{c}=3{d}_{p}/{d}_{b}\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(13\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThis simple model is a close approximation to the interceptional effect in the case of high flow velocity around the bubble, particularly for the small nanoplastic particles in our case. According to the equation, it is necessary to decrease the size of the bubbles to improve the bubble-nanoplastics interactions. The calculated \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e for our system was about 0.6 supposing that the diameter of the nanoplastics was 1 \u0026micro;m and the average bubble diameter was 5 \u0026micro;m as observed from the microscope, which was one order of magnitude higher than that of the conventional flotation method using millimeter sized bubbles. This was the reason why high nanoplastic removal efficiency was achieved using the \u0026ldquo;microbubble armies\u0026rdquo; produced by Ag/Co micromotors.\u003c/p\u003e\u003cp\u003eWe experimentally confirmed that microbubbles could efficiently capture PS nanospheres at their surfaces (\u003cb\u003eSupplementary Video 9\u003c/b\u003e). Therefore, we tried to employ Ag/Co micromotors to generate \u0026ldquo;microbubble armies\u0026rdquo; to chase, capture, and transport the nanoplastics to the water surface as microbubbles ascending to the water surface and eventually to completely remove the nanoplastics floating at the water surface from the water column. We designed a recyclable and easily scalable process to treat nanoplastic contaminated water using the bubble-spouting Ag/Co micromotors (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea\u003cb\u003e)\u003c/b\u003e. A little amount of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e acting as fuels and cetyltriethylammnonium bromide (CTAB) as a microbubble stabilizer were simultaneously introduced into water containing PS nanoplastics. Microbubbles were vigorously produced through the decomposition of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. These microbubbles cruised to capture and transport the nanoplastics to the water surface, concentrating them into the froth layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb \u003cb\u003eand Supplementary Video 1\u003c/b\u003e). Removing the foam layer could easily separate the PS nanoplastics from the water column. The Ag/Co micromotors were completely extracted from water after transporting all of the nanoplastics to the water surface simply using a magnet for recycling usage (\u003cb\u003eSupplementary Fig.\u0026nbsp;31\u003c/b\u003e). The used Ag/Co micromotors were rinsed by water and ethanol to remove absorbed nanoplastics, and then reborn by treating with NaBH\u003csub\u003e4\u003c/sub\u003e solutions to completely restore their catalytic activity and in turn the bubble-spouting capability.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe removal efficiency of PS nanoplastics by the Ag/Co micromotors was evaluated using 200 nm-sized red PS nanospheres as the nanoplastic model. The content of Ag within the Ag/Co micromotors prominently influenced the nanoplastic removal efficiency. Prolonged reaction time between Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs and Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e gave rise to Ag/Co micromotors with a less amount of Ag, while with better removal efficiency of PS nanoplastics (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ej \u003cb\u003eand Supplementary Fig.\u0026nbsp;32\u003c/b\u003e). This was probably due to the quick consumption of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e in the presence of high amount of Ag, limiting the longevity of the microbubbles. Therefore, the reaction time between Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs and Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e was set to 60 min in the following experiments. The absorbance peak of the red PS nanospheres at 527 nm almost disappeared after treatment with Ag/Co micromotors, indicating a high removal efficiency. The removal efficiency was estimated to be 94.3% based on the correlation between the absorbance and the amount of the PS nanospheres (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec \u003cb\u003eand Supplementary Fig.\u0026nbsp;33\u003c/b\u003e).\u003c/p\u003e\u003cp\u003eThe concentration of CTAB critically affected the removal efficiency of PS nanoplastics (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed \u003cb\u003eand Supplementary Fig.\u0026nbsp;34\u003c/b\u003e). When the concentration of CTAB was below 0.0007 wt.%, the removal efficiency dropped lower than 50% due to the instability of the bubble foam layer. As a result, the PS nanoplastics delivered to the water surface re-entered the water column. When the CTAB concentration exceeded 0.0007 wt.%, the removal efficiency also declined because CTAB molecules occupied most of the adsorption sites of the microbubbles and prevented PS nanospheres from attaching to the CTAB-surrounded bubble surface\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e. To remove 95% nanoplastics from 1 ton of water one time, only 7 g of CTAB was needed.\u003c/p\u003e\u003cp\u003eAdditionally, the removal efficiency was greatly improved when the concentration of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e increased. The removal efficiency reached 94.3% when the concentration of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e was 4.8 wt.% (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed \u003cb\u003eand Supplementary Fig.\u0026nbsp;35\u003c/b\u003e). This is because the vigorous decomposition of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e at high concentrations created tremendous tiny microbubbles efficient in nanoplastics trapping and removing.\u003c/p\u003e\u003cp\u003eThe removal efficiency first increased rapidly with the concentration of Ag/Co micromotors, due to the increased microbubble generation rate. When the concentration of Ag/Co micromotors reached 3.75 mg/mL, the removal efficiency was saturated at 94.3% (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee \u003cb\u003eand Supplementary Fig.\u0026nbsp;36\u003c/b\u003e).\u003c/p\u003e\u003cp\u003eThe removal efficiency remained above 70% regardless of the concentration of PS nanospheres (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee). The kinetic characteristics of the flotation process showed rapid PS nanoplastic separation in the first two minutes and the removal efficiency rose steadily until it stabilized at \u0026gt;\u0026thinsp;90% within 40 min (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef).\u003c/p\u003e\u003cp\u003eThe outstanding SERS performance of the Ag/Co micromotors enabled us to monitor the removal process of the nanoplastics using the SERS technique. Before the flotation treatment, strong SERS signals of PS nanoplastics at 1001 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were observed. After the flotation removal, no SERS peaks of PS nanoplastics were observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eg \u003cb\u003eand Supplementary Fig.\u0026nbsp;37\u003c/b\u003e). The SERS spectra only showed the peak of Ag/Co micromotors at 680 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and no SERS peaks of CTAB were observed, indicating that CTAB molecules were simultaneously separated into the foam layer. To further confirm that the CTAB molecules were separated into the foam layer with the nanoplastics after the flotation process, we utilized methylene blue (MB) as a model dye molecule because of the easy observation to demonstrate the capability of the flotation method to separate small molecules. The flotation process using the bubble-spouting Ag/Co micromotors removed 98.9% of MB molecules within 40 min from the water column. The solution became completely colorless and transparent after removing the MB molecules (\u003cb\u003eSupplementary Fig.\u0026nbsp;38\u003c/b\u003e). This result confirmed that small molecules, such as CTAB, were effectively separated together with the nanoplastics after the flotation process, and further demonstrated that the flotation method using the Ag/Co micromotors was applicable to the separation of harmful molecules.\u003c/p\u003e\u003cp\u003eThe Ag/Co micromotors were slowly oxidized during usage (\u003cb\u003eSupplementary Fig.\u0026nbsp;39\u003c/b\u003e). Therefore, the removal efficiency of the PS nanoplastics was decreased from ~\u0026thinsp;95% with fresh Ag/Co micromotors to 77% in the second time use, and further to 63% in the third use, and eventually maintained at ~\u0026thinsp;60% for the following repeatable usage (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eh). Even after usage for six times, the Ag/Co micromotors still contained Ag and Co elements according to the EDX results (\u003cb\u003eSupplementary Fig.\u0026nbsp;40\u003c/b\u003e), indicating their negligible material loss and stable structure during the flotation process. Simple treatment of the used Ag/Co micromotors with NaBH\u003csub\u003e4\u003c/sub\u003e could easily reduce the oxidized Ag to metallic Ag, thereby increasing the removal efficiency of the PS nanoplastics to ~\u0026thinsp;90% (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eh). The overall nanoplastic removal performance (including time, cost, simplicity, durability, \u003cem\u003eetc.\u003c/em\u003e) using the bubble-spouting micromotor swarms outperforms previous methods (\u003cb\u003eSupplementary Table\u0026nbsp;2\u003c/b\u003e).\u003c/p\u003e\u003cp\u003eAfter 40 min nanoplastic removal process, the concentration of the remaining H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e in water was ~\u0026thinsp;1 mM. Keeping the micromotors in water after the nanoplastic removal for 2 h reduced the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e concentration to 0.1 mM (\u003cb\u003eSupplementary Fig.\u0026nbsp;41\u003c/b\u003e). During the decomposition of the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e to generate oxygen microbubbles, it is inevitable to release Co nanoparticles to water after long-time processing. We dissolved these nanoparticles using HNO\u003csub\u003e3\u003c/sub\u003e to determine the concentration of Co\u003csup\u003e2+\u003c/sup\u003e ions using the inductively coupled plasma (ICP) method. The Co\u003csup\u003e2+\u003c/sup\u003e concentration was around 1 mg/L, which met the requirements of international standards for Co\u003csup\u003e2+\u003c/sup\u003e ions for water treatment. Replacing the Co component with other magnetic responsive but more friendly elements can avoid the Co left concern.\u003c/p\u003e\u003cp\u003eNatural water bodies are complex systems with many influencing components, such as diverse contaminants, organic matter, and varying pH. The spiked samples were treated by Ag/Co micromotors to assess the practical applicability of the flotation method. In real-world water conditions, the Ag/Co micromotors displayed comparable nanoplastics removal performance to that in deionized water. As an example, red PS nanoplastics with a concentration of 4.7 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e g/mL spiked in real lake water (obtained from a lake on the campus) were separated into the foam layer after the flotation process. The removal efficiency was calculated to be 97.7% (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ei, \u003cb\u003eSupplementary Fig.\u0026nbsp;42\u003c/b\u003e). Similarly, PS nanoplastics spiked in drinking tap water were also almost completely removed by the flotation method (\u003cb\u003eSupplementary Fig.\u0026nbsp;43\u003c/b\u003e), further demonstrating the broad and real applicability of the flotation method using the microbubble-spouting Ag/Co micromotors across real-world waters. These results indicated that the complex water matrices did not affect the catalytic activity, stability, and nanoplastic removal efficiency of the Ag/Co micromotors.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we realized sensitive SERS detection and efficient flotation removal of nanoplastics from the water column using bubble-spouting Ag/Co micromotors. The Ag/Co micromotors were transformed from the electrochemically engineered Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e microparticles using a redox potential-driven ion exchange and shape-preserving reduction process with controllable Ag and Co ratios, which could be easily scaled up to kilogram-level production. The nanoplastics were concentrated onto the Ag/Co micromotors with volumetric hot spots after water evaporated on the PDMS slippery surface, achieving SERS detection of nanoplastics detection at a concentration of microgram per milliliter level. The Ag/Co micromotors continuously eject \u0026ldquo;microbubble armies\u0026rdquo; to capture the nanoplastics via hydrophobic interactions and transport them to the foam layer during the bubble ascending process. The removal efficiency of the nanoplastics reached 94.3% within 40 min. The nanoplastic removal performance of the used Ag/Co micromotors could be restored to the level of the fresh ones simply by NaBH\u003csub\u003e4\u003c/sub\u003e aqueous solution treatment. The easy regeneration of the Ag/Co micromotors and the simple magnetic separation process of the micromotors from water readily by a magnet make the micromotors recyclable. PS nanospheres spiked into the lake and tap water could also be removed by the flotation method using the Ag/Co micromotors, proving the practical applicability of the flotation method. We proved the possibility to detect and separate the nanoplastics using bubble-spouting Ag/Co micromotors, providing promising materials and concepts to solve the notorious nanoplastic pollution crisis.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eMaterials and reagents\u003c/h2\u003e\u003cp\u003eAll the experimental chemicals were used as received without further purification. Silver nitrate (AgNO\u003csub\u003e3\u003c/sub\u003e, 99.9%), sodium borohydride (NaBH\u003csub\u003e4\u003c/sub\u003e, 96%), boric acid (H\u003csub\u003e3\u003c/sub\u003eBO\u003csub\u003e3\u003c/sub\u003e, 99%), and hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) were purchased from Sinopharm Chemical Reagent. Cobalt nitrate hexahydrate [Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO, 99%] and cetyltrimethylammonium bromide (CTAB, 99%) were purchased from Aladdin. Methylene blue (MB) and sodium dodecyl sulfate (SDS, 99.0%) were purchased from Sigma-Aldrich. Propylene carbonate (PC, 99.5%) was purchased from J\u0026amp;K Scientific. Polystyrene (PS) nanosphere suspensions were purchased from Huge Biotechnology.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eElectrochemical tests\u003c/h3\u003e\n\u003cp\u003eAll electrochemical tests were performed in a three-electrode system. The working electrode was prepared by thermally evaporating a 2 nm-thick layer of titanium followed by a 50 nm-thick layer of gold onto a piece of silicon wafer. A graphite rod (about 5 mm in diameter) and the Ag/AgCl were used as the counter electrode and the reference electrode, respectively. The electrolyte consisted of 0.06 M AgNO\u003csub\u003e3\u003c/sub\u003e, 0.16 M H\u003csub\u003e3\u003c/sub\u003eBO\u003csub\u003e3\u003c/sub\u003e, and 0.1 M KNO\u003csub\u003e3\u003c/sub\u003e. For chronoamperometry tests, a step potential was applied after the open-circuit potential stabilized. The potential range for the Tafel test was \u0026plusmn;\u0026thinsp;200 mV from the open-circuit potential, with a scan rate of 1 mV/s.\u003c/p\u003e\n\u003ch3\u003ePreparation of AgONO MBPs\u003c/h3\u003e\n\u003cp\u003eThe three-electrode system used for electrodeposition was the same as above. The Au electrode was immersed into an electrolyte solution comprising 0.06 M AgNO\u003csub\u003e3\u003c/sub\u003e and 0.16 M H\u003csub\u003e3\u003c/sub\u003eBO\u003csub\u003e3\u003c/sub\u003e. The anodic electrodeposition was conducted by applying a potential waveform first gradually increasing from 10 V to 2 V and then gradually increasing to 10 V. The potential increasing and decreasing rate was set to 0.1 V/s. Subsequently, the ultrasonication was employed to peel off the Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs from the electrode surface. The obtained black powder was dried at 60 ℃ in an oven.\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003ePreparation of Ag/Co micromotors\u003c/h2\u003e\u003cp\u003eTo transform the Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs into Ag/Co micromotors, the synthesized Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e were immersed in 50 mM Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e solutions (\u003cem\u003eV\u003c/em\u003e\u003csub\u003ewater\u003c/sub\u003e:\u003cem\u003eV\u003c/em\u003e\u003csub\u003eethanol\u003c/sub\u003e = 9:1) for different times to vary the Ag and Co ratios. Then, the micromotors were collected via centrifugation at 9000 rpm for 10 min. The micromotors were washed by deionized water before reducing by 50 mM NaBH\u003csub\u003e4\u003c/sub\u003e aqueous solutions for 50 min at room temperature. The Ag/Co micromotors were collected by centrifugation at 9000 rpm for 10 min. After rinsing with deionized water and ethanol, the micromotors were dried under vacuum at 60 ℃ for 1 h. For the annealed samples, the as-obtained Ag/Co micromotors were placed in a tube furnace and heated to 500\u0026deg;C with a temperature increasing rate of 5 ℃/min for 3 h in a stream of Ar.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003eMoving trajectory tracking of Ag/Co micromotors\u003c/h2\u003e\u003cp\u003e5 wt.% H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e aqueous or PC solutions were introduced into the wells of a 96-well plate. The Ag/Co micromotors used for motion analysis were prepared as follows: First, Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e MBPs on the electrode were converted to AgCoO\u003csub\u003e2\u003c/sub\u003e MBPs by directly immersing them into 50 mM Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e solutions (\u003cem\u003eV\u003c/em\u003e\u003csub\u003ewater\u003c/sub\u003e:\u003cem\u003eV\u003c/em\u003e\u003csub\u003eethanol\u003c/sub\u003e = 9:1) for 1 h; Second, they were subsequently reduced by 50 mM NaBH\u003csub\u003e4\u003c/sub\u003e aqueous solutions for 1 h at room temperature; Eventually, the Ag/Co micromotors were released from the gold substrates by ultrasonic treatment. These Ag/Co micromotors were dispersed into the solution within the 96-well plate. A NdFeB magnet (grade N35) was used at a distance of ~\u0026thinsp;5 cm to control the moving behavior of the Ag/Co micromotors. An optical microscope (RX50M SOPTOP) was immediately used to track and record the movement of the Ag/Co micromotors at a rate of 60 frames per second. The trajectory was analyzed using the Tracker software.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003eSERS measurements\u003c/h2\u003e\u003cp\u003eAg/Co micromotors were added to PS nanosphere suspensions at different concentrations. The Ag/Co micromotors were maintained at a concentration of 0.5 mg/mL. 20 \u0026micro;L of the mixture was dropped onto a slippery PDMS-functionalized silicon surface, which was prepared according to our previous publication\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. The mixture solution droplet resting at the slippery surface was dried at 40 ℃. The SERS spectra were recorded with a confocal microscopic Raman system (Renishaw Invia Reflex). The excitation laser wavelength was 532 nm with a power of 0.25 mW. The laser was focused on the samples through a 50\u0026times; objective lens. All spectra were collected with 20 s of integration time and three acquisitions. SERS mapping images were conducted with 1 s integration time for each point with a step length of 1 \u0026micro;m for the laser spot movement.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003eCatalytic performance of the Ag/Co micromotors\u003c/h2\u003e\u003cp\u003eThe UV-Vis absorption spectra of potassium titanium oxalate solutions with different concentrations of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e were measured to build the relationship between the absorption intensity at 400 nm and the concentration of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Ag/Co micromotors with different Ag and Co ratios were introduced into the potassium titanium oxalate solutions composed of 1.47 M H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. The absorption intensity decrease at 400 nm was monitored to predict the concentration decrease of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, reflecting the catalytic performance. The remaining concentration of the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e after nanoplastic removal was also measured using this method.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003eNanoplastic removal efficiency estimation\u003c/h2\u003e\u003cp\u003eAg/Co micromotor suspensions, CTAB aqueous solutions, and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e aqueous solutions were sequentially injected into PS nanosphere suspensions in a 5 mL glass vial. After the flotation process, 1 mL of the solution in the bottom layer was extracted for immediate recording of the absorption spectra. The removal efficiency (ƞ) was calculated according to the following equation:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\left(\\text{\\%}\\right)=\\frac{{C}_{0}-{C}_{\\text{f}}}{{C}_{0}}\\times\\:100\\%$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e and \u003cem\u003eC\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e are the initial and final concentration of the PS nanoplastic suspensions, respectively. Ag/Co micromotors were washed thoroughly with deionized water and absolute ethanol after the nanoplastic removal process. The used Ag/Co micromotors were reacted with 50 mM NaBH\u003csub\u003e4\u003c/sub\u003e solutions for 10 min followed by washing with deionized water to regenerate the flotation function. A NdFeB magnet was used to separate the Ag/Co micromotors from the liquid phase. For the nanoplastics removal in the lake water, Ag/Co micromotors suspensions, PS nanosphere suspensions and CTAB aqueous solutions were all prepared using the lake water collected from the Qizhen lake (Hangzhou, China).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003eCharacterization\u003c/h2\u003e\u003cp\u003eX-ray diffraction (XRD) patterns were obtained using a Rigaku D/MAX 2550 diffractometer with Cu K\u003csub\u003eα\u003c/sub\u003e radiation (λ\u0026thinsp;=\u0026thinsp;1.5418 \u0026Aring;) as the light source. The morphology and the element information were investigated using a field-emission SEM (Zeiss Supra55) operated at an accelerating voltage of 15 kV equipped with an X-ray energy-dispersive spectroscopy. Transmission electron microscopy (TEM) images of samples were investigated by FEI Talos F200x. X-ray photoelectron spectroscopy (XPS) spectra were acquired using an X-ray photoelectron spectroscopy (XPS) spectra were acquired using an EscaLab 250Xi photoelectron spectrometer (Thermo Scientific). The incident radiation was 50 W. The C 1s peak at 284.8 eV served as a reference for the position of all of the XPS peaks. The absorption spectra were measured using an ultraviolet\u0026ndash;visible (UV\u0026ndash;Vis) absorption spectrometer (Lambda 950, PerkinElmer). The magnetic hysteresis loops of the samples were obtained from the physical property measurement system (PPMS-9). The distribution of red PS fluorescent nanospheres on the Ag/Co micromotors was observed using a confocal laser scanning microscope (Zeiss LSM 980 with Airyscan).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003eFinite element simulation\u003c/h2\u003e\u003cp\u003eWe used the Tertiary Current Distribution and Deformed Geometry modules in COMSOL Multiphysics to simulate the distribution of Ag\u003csup\u003e+\u003c/sup\u003e ions concentration and track the interface deformation during the electrodeposition process of Ag\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e. Electrode kinetics were described using the Butler-Volmer equation. The initial concentration of Ag\u003csup\u003e+\u003c/sup\u003e ions was set to 0.06 mol/L. The diffusion coefficient of Ag\u003csup\u003e+\u003c/sup\u003e ions was set to 8.51 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e/s \u003csup\u003e55\u003c/sup\u003e. Based on the Tafel plot (Supplementary Fig.\u0026nbsp;2), the anode transfer coefficient was set to 0.82, and the exchange current density was set to 9 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e A/cm\u003csup\u003e2\u003c/sup\u003e. The applied high and low voltages were 0.7 V and 0.2 V, respectively.\u003c/p\u003e\u003cp\u003eWe exploited the Transport of Diluted Species module in COMSOL Multiphysics to simulate oxygen concentration distribution around the Ag/Co micromotors. The initial concentration of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e was set to 1.41 \u0026times; 10\u003csup\u003e3\u003c/sup\u003e mol/m\u003csup\u003e3\u003c/sup\u003e. The constant of reaction velocity \u003cem\u003ek\u003c/em\u003e\u003csub\u003ecat\u003c/sub\u003e was set to 7.0 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e cm/s. The diffusion coefficient of oxygen and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e was set to 2.10 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e/s \u003csup\u003e56\u003c/sup\u003e and 1.35 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e/s \u003csup\u003e57\u003c/sup\u003e, respectively.\u003c/p\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eCompeting interests\u003c/h2\u003e\u003cp\u003eAuthors declare that they have no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e\u003cp\u003eS. Y. and N. A. conceived the idea and designed the study. N. A., J. L., L. Z. and Z. Z. carried out the materials synthesis and characterizations. N.A., L.Q., S.S., H.Z., Y.L., M.Y. and S.Y. analyzed the data. N.A., L.Q, and S.Y. wrote the manuscript. All authors contributed to the revision of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eWe acknowledge funding support from Key R\u0026amp;D Program of Zhejiang Province (2023C01088), National Natural Science Foundation of China (52273233 and 52471211), and the Open Research Program of Key Laboratory of 3D Micro/Nano Fabrication and Characterization of Zhejiang Province, Westlake University. Part of the work was conducted in the ZJU micro-nanofabrication center.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBorrelle SB et al (2020) Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution. 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Acta Chem Scand 26:3393\u0026ndash;3394\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4730825/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4730825/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNanoplastics are ubiquitous in aquatic environments. Most of the nanoplastics suspend in the water column, facilitating their transportation and increasing their influence on the ecosystems. Owing to their small size and good dispersion in water, detection and separation of the nanoplastics from an extremely large volume of water are very challenging. Here, we demonstrate a concept to employ carefully engineered microbubble-spouting magnetic Ag/Co micromotors to sensitively detect the nanoplastics by the surface-enhanced Raman spectroscopy (SERS) technique, as well as remove the nanoplastics from a large volume of water with the \u0026ldquo;microbubble armies\u0026rdquo; to attract, capture, and transport the nanoplastics to the water surface (\u003cem\u003ei.e.\u003c/em\u003e, flotation method) resembling the white blood cells chasing and swallowing nanointruders in biology. The SERS detection sensitivity reaches single nanoplastic debris level, enabled by the microscale cavities on the micromotor surface and the slippery substrate facilitating nanoplastic enrichment during water evaporation. The removal efficiency of nanoplastics from water reaches 94.3% arising from the strong interactions between the \u0026ldquo;microbubble armies\u0026rdquo; spouted from the Ag/Co micromotor swarms and the nanoplastics via the hydrophobic interactions. The Ag/Co micromotors can be separated from water after nanoplastics removal by a magnet for recycling usage. The practical applicability of the flotation method was proved by the high flotation removal efficiency of the PS nanospheres spiked into the lake and tap water using the Ag/Co micromotors. The high SERS sensitivity and the high nanoplastic removal efficiency, as well as the high throughput production and the recyclability of the Ag/Co micromotors provide valuable multifunctional materials for simultaneous detection and treatment of nanoplastic pollution in contaminated water.\u003c/p\u003e","manuscriptTitle":"Ultrasensitive SERS detection and efficient flotation removal of nanoplastics from water using bubble-spouting micromotor swarms","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-21 02:46:00","doi":"10.21203/rs.3.rs-4730825/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"38f615bb-e2a4-4343-8745-b6a3ac2f0d94","owner":[],"postedDate":"November 21st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":58322827,"name":"Physical sciences/Optics and photonics/Optical techniques/Optical spectroscopy/Raman spectroscopy"},{"id":58322828,"name":"Physical sciences/Materials science/Nanoscale materials/Synthesis and processing"}],"tags":[],"updatedAt":"2026-03-04T08:46:13+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-21 02:46:00","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4730825","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4730825","identity":"rs-4730825","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}