A disposable photoelectrochemical sensor based on magnetic photocatalyst BiOI/NiFe@N-CNTs for the assay of hexavalent chromium

A disposable photoelectrochemical sensor based on magnetic photocatalyst BiOI/NiFe@N-CNTs for the assay of hexavalent chromium | 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 Research Article A disposable photoelectrochemical sensor based on magnetic photocatalyst BiOI/NiFe@N-CNTs for the assay of hexavalent chromium Sushuang Xia, Mingyu Zheng, Chunxiang Li, Xinmei Qian, Keqin Deng This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8513040/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 18 You are reading this latest preprint version Abstract Chromium is widely used in different industries and can be found in wastewater. Hexavalent chromium (Cr(VI)) is considered life-threatening pollutant and shows adverse effects on environment and human health. Therefore, an effective method is demanded for reliable detection of Cr(VI). In this work, a magnetic photocatalyst (BiOI/NiFe@N-CNTs) composed of bismuth oxyiodide (BiOI) and magnetic bamboo-like carbon nanotubes capsulated with nickel-iron (NiFe) particles (FeNi@N-CNTs) was prepared by one-step hydrothermal method. Various characterization techniques revealed that BiOI/NiFe@N-CNTs possessed abundant pores and larger surface area because of its interlaced nanotubes morphology. It showed excellent cathodic photoelectrochemical (PEC) activity and good photoelectron transfer capacity with [Fe(CN) 6 ] 3− . Based on these features, a disposable magnetic PEC sensor for Cr(VI) assay was constructed by modifying screen-printed electrode (SPE) with BiOI/NiFe@N-CNTs via magnetic control. The PEC sensor presented a broad detection range of 0.2 nM−10.0 µM and a low detection limit (LOD) of 65 pM (S/N = 3). The practical application of the sensor in real water samples obtained acceptable results. The low cost, low consumption volume in samples, and disposability make the magnetic PEC sensor a potential tool for monitoring Cr(VI). Magnetic photocatalyst ⋅ BiOI/NiFe@N-CNTs composite ⋅ Disposable sensor ⋅ Hexavalent chromium assay ⋅ Photoelectrochemical technique Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Chromium (Cr) is a typical heavy metal pollutant in industrial wastewater, and Its toxicity is significantly related to its chemical valence [ 1 ]. Hexavalent chromium (Cr(VI)) exhibits conspicuous biological permeability in aquatic environments, posing serious hazards to ecosystems and human health because of its bioaccumulation through the food chain. U.S. Environmental Protection Agency (EPA) has strictly regulated its total chromium content in drinking water (≤ 100 µg/L) [ 2 ]. Therefore, the development of diverse and convenient techniques for Cr(VI) analysis is necessary. Traditional methods, such as ion chromatography, atomic absorption spectrometry, inductively coupled plasma mass spectrometry, X-ray fluorescence spectrophotometry have been employed for Cr(VI) detection [ 3 , 4 ]. Owing to their high-price instruments, some new methods including colorimetric assay, microfluidic paper-based monitoring, dual-mode sensors, electrochemical and photoelectrochemical (PEC) sensors have emerged for on-site application [ 5 – 9 ]. Among them, PEC sensors are regarded to be rapid and high-sensitive because they combine photocatalytic and electrochemical technology [ 10 ], and PEC signals commonly depend on particular semiconductor materials [ 11 , 12 ]. Magnetic photocatalysts are a type of composite that can be efficiently recycled through magnetic control or magnetic separation [ 13 ]. They have been widely applied in environmental purification and energy conversion [ 14 , 15 ]. For example, some magnetic iron oxide-integrated photocatalysts, such as ZnFe 2 O 4 /CeO 2 , CoFe 2 O 4 /AgBr, Ag 2 WO 4 /BiOI/CoFe 2 O 4 , Fe 3 O 4 /ZnO/BiOI, exhibited high activities for degradation of organic pollutants or antibiotics [ 16 – 19 ]. Ag-AgCl/ZnFe 2 O 4 and AgI/CuFe 2 O 4 accelerated bacterial inactivation to clean environment [ 14 , 20 ]. C₃N₄-based magnetic photocatalysts produced hydrogen under visible light to obtain clean energy [ 21 , 22 ]. Among these magnetic semiconductors, BiOI coupled/doped photocatalysts emerged as one of the most interesting semiconductors because of their efficient electron-hole pairs separation and remarkable photocatalytic activity [ 18 , 19 , 23 ]. However, as so far, few magnetic BiOI-based photocatalysts were reported for PEC applications. Our previous work prepared a magnetic bamboo-like nitrogen-doped carbon nanotube sealed with NiFe nanoparticles (NiFe@N-CNTs) [ 24 ]. The NiFe@N-CNTs presented high superparamagnetic property, good electrical conductivity, large surface area, adequate active sites, periodic bamboo-like nodes, and nanoconfined structure. Multiwall carbon nanotubes/BiOI has been well used as photocatalysts for degradation of antipyrine [ 25 ]. AgI-carboxylated multiwalled carbon nanotubes-BiOI Z-scheme heterojunction material was applied for photoelectrochemical aptasensing of lincomycin [ 26 ]. These semiconductor photocatalysts coupled with carbon nanotubes showed significant electron-acceptor/transport matrix in photocatalysis. Inspired by the above research, we tried to prepare a new magnetic cathodic photoelectrochemical material BiOI/NiFe@N-CNTs. The BiOI/NiFe@N-CNTs displayed good magnetism, abundant pores, high photoelectrochemical activity, and remarkable conductivity. Then, BiOI/NiFe@N-CNTs was controlled on screen-printed electrode (SPE) by a designed magnetic sticker to develop a disposable magnetic photoelectrochemical (PEC) sensor for Cr(VI) detection. With [Fe(CN) 6 ] 3− as photoelectron acceptor and the photoreduction product of Cr(OH) 3 deposit as photocurrent quencher, the proposed sensor displayed a broad detection range and a low detection limit. Experimental procedure Apparatus and Materials X-ray photoelectron spectroscopy (XPS, thermo Scientific Escalab Xi+, USA) and X-ray power diffraction (XRD-6000, Shimadzu) were utilized to determine the chemical composition and the crystal structure of nanomaterials. Scanning electron microscopy (FESEM, JEOL-7800F, Japan) and transmission electron microscopy (TEM, JEOL-JEM 2100F, Japan) were applied to investigate the morphology of nanomaterials. The electrochemical station (CHI 760C, Shanghai Chenhua Corp., China) matched with a photoelectrochemical system (PEAC 200A, Ida) was used to perform all electrochemical and photoelectrochemical experiments. White LED light (20 mW/cm 2 ) acted as irradiation source. Nickel acetate, K 2 Cr 2 O 7 , iron acetate, Bi(NO 3 ) 3 ⋅5H 2 O, KI, ethylene glycol, polyvinyl pyrrolidone, dicyandiamide (DCD), KCl, soluble starch, and other chemical reagent were bought from Sinopharm Chemical Reagent Co., Ltd. (China). All reagents were of analytical grade without further purification. KCl (0.1 M) was applied for supporting electrolyte of photoelectrochemical measurement. All aqueous solutions were prepared with ultrapure water (≥ 18.3 M, Milli-Q, Millipore). Synthesis of magnetic NiFe@N-CNTs and BiOI/NiFe@N-CNTs Magnetic NiFe@N-CNTs was synthesized according to our previous work [ 31 ]. Briefly, soluble starch (0.5 g) was dissolved in boiling water. Then, nickel acetate (1.0 g), iron acetate (0.5 g), and N-cyanoguanidine (5.0 g) were added to the cooled starch solution and thoroughly stirred. After being dried, the solid mixture was then placed in an alumina crucible and heated up to 500°C in a tube furnace with a heating rate of 4°C ⋅ min − 1 , and then heated to 800°C and controlled for 2 h. Finally, the cooled product was collected. For further use, NiFe@N-CNTs was screened to obtain high dispersed magnetic carbon nanotubes by the following steps. Firstly, the material was dispersed in an aqueous solution containing polyvinylpyrrolidone (PVP, 0.1 wt%) and sonicated for 10 minutes. After standing for 5 minutes, graded centrifugation method was employed to isolate large agglomerates by centrifugation at 1,000 rpm. The collected supernatant was subsequently centrifuged at 8,000 rpm for 10 minutes. The obtained sediment was then washed three times and dried at 60°C. For the preparation of BiOI/NiFe@N-CNTs, 0.5 g of PVP and 1.0 g of Bi(NO 3 ) 3 ·5H 2 O were added into 25 mL of ethylene glycol to form a homogeneous solution. 40 mg of FeNi@N-CNTs was ultrasonically dispersed in EG (25 mL). The former was then slowly added dropwise to the latter under stirring. After 1 hour, 1 g of KI was added into the mixture, followed by an additional hour of stirring. Then, the dispersion liquid was transferred into an autoclave and heated at 140 ◦C for 12 h. Finally, the product was dried at 60°C for 12 h after washing several times and centrifugation. For comparison, BiOI was prepared by similar steps without addition of FeNi@N-CNTs. Fabrication of BiOI/NiFe@N-CNTs/SPE and detection of hexavalent chromium Screen-printed electrode (SPE) from Haoyang Tech. Co. (Shenzhen, China) has been printed carbon circle (d = 5 mm), carbon strip, and silver/silver chloride stripe as photocathode, counter electrodes, and reference electrode, respectively. A magnetic sticker was attached to the backside of the carbon circle. For modification, 10 µl of BiOI/NiFe@N-CNTs aqueous dispersion (2.0 mg mL − 1 ) was added on the surface of carbon circle and water was removed by filter paper. BiOI/NiFe@N-CNTs was immobilized on SPE by the strong magnetic sticker. For Cr (VI) detection, BiOI/NiFe@N-CNTs/SPE was dropped with 20 µl of Cr (VI) species containing KCl (0.1 M) and K 3 [Fe(CN) 6 ] (20 µM). The photocurrent was then recorded at 0 V bias potential after photo-reduction of Cr(VI) for 300 s. Results and discussion Physicochemical characteristics of magnetic materials Figure 1 exhibits the SEM and TEM images of magnetic NiFe@N-CNTs (Fig. 1 A, B) and BiOI/NiFe@N-CNTs (Fig. 1 C, D). NiFe@N-CNTs shows a hollow bamboo-like nanotube with the diameter range of 20 ~ 100 nm and the length < 3.0 µm. NiFe nanoparticles embed inside nanotubes, which contribute to the magnetism of NiFe@N-CNTs and BiOI/NiFe@N-CNTs. For BiOI/NiFe@N-CNTs, it displays a 3D aggregated structure with the size < 3.0 µm. The BiOI/NiFe@N-CNTs is assembled by interlaced nanotubes, which generates abundant pores and provides larger surface area. These nanotubes exhibit a rougher surface than NiFe@N-CNTs. The magnetic behavior of BiOI/NiFe@N-CNTs was then demonstrated by using an external magnetic field. It was found that the prepared material rapidly gathered towards the magnet and easily returned to a dispersed state after removing the magnet and shaking, Figure 2 A displays the XRD results of NiFe@N-CNTs, BiOI, and BiOI/NiFe@N-CNTs. It reveals that NiFe@N-CNTs has two low diffraction peaks at 41.2° and 48.0°, assigning to Fe 4 N from the coordination of metal iron and nitrogen [ 24 ]. The sharp and strong peaks at 43.7° and 50.6° corresponded to the (111) and (200) reflection of NiFe (JCPDS#47-1405), implying that NiFe nanoparticles has crystallized well. The broad peak at about 26.1° is indexed to amorphous carbon (002) [ 27 ]. Pure BiOI shows several characteristic peaks, which are related to the tetragonal crystal phase of BiOI [JCPDS No. 73-2062]. Similar peaks appear at the pattern of BiOI/NiFe@N-CNTs, indicating that NiFe@N-CNTs has no obvious influence upon the crystal phase of BiOI. However, the peaks for NiFe@N-CNTs can’t be found, which is ascribed to the low content of NiFe@N-CNTs in BiOI/NiFe@N-CNTs. The composition and chemical state of BiOI/NiFe@N-CNTs is further analyzed by XPS (Fig. 2 B–I). Seven elements, including Bi, I, Fe, Ni, C, O, and N, are found in the full survey spectrum. For element Bi, the high-resolution spectrum displays two peaks at 158.9 eV and 164.2 eV, corresponding to Bi 4f 7/2 and Bi 4f 5/2 in the form of Bi 3+ (Fig. 2 C). For I, it shows 3d 5/2 and 3d 3/2 of I − 1 at 618.8 eV and 630.3 eV, respectively (Fig. 2 D). In the spectrum of Fe 2p, four subpeaks are fitted for the multivalent state (Fig. 2 E). The binding energy at 710.7 eV and 712.5 eV are indexed to 2p 3/2 of Fe 2+ and Fe 3+ for iron oxides, respectively [ 28 , 29 ]. The peaks at 724.0 eV and 725.9 eV are assigned to 2p 1/2 of Fe 2+ and Fe 3+ . Ni 2p spectrum is deconvoluted into five peaks (Fig. 2 F), including three peaks for 2p 3/2 and 2p 1/2 of Ni 2+ at 854.9 and 872.5 eV, 2p 3/2 of Ni 0 at 853.1 eV, and two satellite peaks at 860.8 and 879.9 eV, respectively [ 24 , 30 ]. Then, C 1 s spectrum in Fig. 2 G is fitted into three peaks at 285.0, 286.1, and 287.9 eV, relating to C–C, C–N, and C–O bonds, respectively [ 31 ]. For the region of O 1s in Fig. 2 H, it shows that the peaks at 529.5, 531.3, 533.2 eV belong to Bi‒O bond, I‒O bond, and C‒O bond [ 32 ]. The N 1s spectrum is separated into five peaks (Fig. 2 I), relating to M‒N units, pyridinic N, pyrrolic N graphite N, and oxidized N with the binding energy of 397.6, 398.3, 399.6, 401.2 eV, and 402.5 eV, respectively [ 33 ]. From the discussions, we can see that BiOI and NiFe@N-CNTs coexist in prepared magnetic BiOI/NiFe@N-CNTs. Photoelectrochemical features of sensors The photoelectrochemical behaviors of BiOI/NiFe@N-CNTs/SPE were first investigated by cyclic voltammetry (CV). As shown in Fig. 3 A, the current intensity is affected by the light illumination and [Fe(CN) 6 ] 3− ions. Comparing the current under darkness (curve a, b) and light irradiation (curve a’, b’), BiOI/NiFe@N-CNTs shows a photocurrent of 1.1 µA at 0 V in 0.1 M KCl. After adding [Fe(CN) 6 ] 3− , it displays a pair of redox peaks at + 0.33/+0.25 V in darkness. While it is exposed to visible light, the peaks appear at + 0.33/+0.16 V and the cathodic photocurrent is greatly improved under the potential of < + 0.3 V. The photocurrent is about 8.4 µA at 0 V. We deduced that the negative shift of reduction peak is caused by the transfer of photogenerated electrons (e-) from BiOI/NiFe@N-CNTs to [Fe(CN) 6 ] 3− . Furthermore, the transient photocurrent of BiOI/SPE and BiOI/NiFe@N-CNTs/SPE is tested at the applied of 0 V (Fig. 3 B). Both electrodes display low photocurrent in 0.1 M KCl solution, about 0.42 and 0.78 µA for BiOI/SPE and BiOI/NiFe@N-CNTs/SPE, respectively. Obviously, BiOI/NiFe@N-CNTs possesses better photoelectrochemical activity. With the addition of [Fe(CN) 6 ] 3− , the photocurrent increases up to 4.5 and 8.3 µA, respectively. These results indicates that NiFe@N-CNTs can accelerate the photoelectron transfer and [Fe(CN) 6 ] 3− can efficiently accept photoelectrons from BiOI/NiFe@N-CNTs. The mechanism and feasibility for photoelectrochemical sensing of Cr(VI) Possible sensing mechanism of BiOI/NiFe@N-CNTs for Cr(VI) is described in Fig. 4 . Under light illumination, BiOI generates electron/hole pairs (e − /h + ). Part of photogenerated electrons (e − ) flow to NiFe@N-CNTs for the reduction of [Fe(CN) 6 ] 3− , while the rest are directly accepted by [Fe(CN) 6 ] 3− to form [Fe(CN) 6 ] 4− [ 34 , 35 ]. Furthermore, NiFe nanoparticles and bamboo-like N-CNTs can improve the conductivity of composite and facilitate electron transfer. Therefore, the e − /h + recombination is effectively suppressed, and the lifetime of charge carriers is extended. For these reasons, more [Fe(CN) 6 ] 3− ions are reduced and the higher photocurrent can be observed. With the presence of Cr (VI) anions, BiOI/NiFe@N-CNTs accumulates them for its high active surface area and reduces them to produce Cr(OH) 3 deposit under light irradiation at pH = 7 [ 34 ]. The low-conductivity deposit formed on BiOI/NiFe@N-CNTs not only blocks light irradiation to generate e − but also shields the active sites of BiOI-NiFe@N-CNTs to transfer electron towards [Fe(CN) 6 ] 3− . In addition, the adsorbed Cr (VI) anions hinder the diffusion of [Fe(CN) 6 ] 3− to SPE surface because of the electrostatic repulsion. Thus, photocurrents decrease, enabling ultra-sensitive determination of Cr (VI). The feasibility for Cr(VI) detection is confirmed by measuring transient photocurrent and interfacial resistance (Fig. 5 ). BiOI/SPE and BiOI/NiFe@N-CNTs showed the photocurrent of 4.5 and 8.3 µA, respectively. After photoreaction with Cr(VI) anions, the transient current decreased to 2.2 and 4.1 µA with the declining percentage of 51.1% and 50.6% (Fig. 5 A). Electrochemical impedance spectroscopy displayed a high Ret value of 2740 Ω for BiOI/SPE and a greatly decreased Ret of 512 Ω for BiOI/NiFe@N-CNTs/SPE (Fig. 5 B), also proving that FeNi@N-CNTs possesses excellent conductivity and high electron transfer efficiency. After photoreduction of Cr(VI), the resistance increased to 3085 Ω and 1108 Ω, respectively, indicating that photoelectron transfer from BiOI to [Fe(CN) 6 ] 3− is seriously blocked, and Cr(VI) anions indirectly affect photocurrent. Optimization of experimental conditions The mass ratio of FeNi@N-CNTs: Bi(NO 3 ) 3 used in the preparation of BiOI/NiFe@N-CNTs is a crucial parameter for achieving excellent photoelectrochemical performance. As shown in Fig. 6 A, the cathodic photocurrent increases rapidly with the ratio and reaches a plateau within the range of 0.04 ~ 0.06. However, further increasing the ratio to 0.08 results in a decline in photocurrent. Although FeNi@N-CNTs can promote electron transfer and facilitate the photoreduction of [Fe(CN) 6 ] 3− , excessive use can shield light irradiation, leading to lower light-induced electron efficiency. Therefore, a ratio of 0.05 is selected. Then, the influence of [Fe(CN) 6 ] 3− concentration is investigated (Fig. 6 B). The results indicate that the photocurrent enhances significantly with concentration, confirming that [Fe(CN) 6 ] 3− can effectively capture photogenerated electrons and inhibit electron-hole recombination. However, when the concentration exceeds 40 µM, the growth of photocurrent tends to plateau, indicating that the gain of the electron acceptor is approaching saturation [ 12 ]. Additionally, excessively high concentrations lead to an increase in background current, and the yellow color of the solution will mask light illumination. Based on these facts, 20 µM is chosen as the optimal working concentration. The applied potential also plays an important role in promoting photocurrent. We found that the current increased as the potential dropped from 0.3 V to 0.0 V and then remained stable (data not shown). Considering that high potential brought about high background current, 0.0 V was therefore applied for measurement. The performance of BiOI/NiFe@N-CNTs/SPE for analysis of Cr(VI) anions The photocurrent intensity of BiOI/NiFe@N-CNTs/SPE after photoreduction of Cr(VI) anions is examined under optimal conditions. Figure 7 A reveals the PEC signals of BiOI-FeNi@N-CNTs/SPE. Clearly, the photocurrent progressively decreases while increasing Cr(VI). Figure 7 B displays that the changed photocurrent (Δ i = i - i 0 ) is directly proportional to log C (Cr) ), where i and i 0 represent the photocurrent in Cr(VI) solution and the blank, respectively. Within the range of 0.2 nM−10.0 µM, the corresponding calibration curve shows a linear relationship of Δ i = 0.9769 + 0.9162 log C ( R 2 = 0.990). The detection limit (LOD) is 65 pM (S/N = 3), lower than the permissible limit of 0.96 µM specified by World Health Organization (WHO). By comparison with other methods listed in Table 1 , the disposable magnetic photoelectrochemical sensor presents comparable performance for Cr(VI) analysis. The stability of BiOI/NiFe@N-CNTs/SPE was tested. We found that photocurrent showed minimal changes after 15 consecutive “on/off” light treatments (Fig. 7 C). When it was exposed to light continuously for 300 s, the current decreased by only 3.8%, indicating BiOI-FeNi@N-CNTs/SPE can be stably used for the detection of Cr(VI) ions. Then, storage stability was regularly monitored over four weeks, where the magnetic sensors were kept at 4°C. Impressively, the current responses decreased by only 5.64%, demonstrating that BiOI/NiFe@N-CNTs/SPE is stable under low-temperature storage conditions. The reproducibility was also assessed by measuring the photocurrent response of six identical sensors. The relative standard deviation (RSD) was 6.2%, suggesting the acceptable reproducibility. Table 1 Performance comparison of proposed magnetic PEC sensor with previous reports Materials Linear range (M) LOD Ref. MoS 2 /BiOI CuO Film CuPc/TiO 2 NiCo-LDHs/TiO 2 NTAs/Ti PbS Au-TiO 2 BiVO 4 -7 BiOI/CN-34% CuS/Bi 2 MoO 6 TiO 2 @C/Au/BiOI BiOI/NiFe@N-CNTs 5×10 − 8 – 1.6×10 − 4 8×10 − 8 – 2×10 − 5 10×10 − 8 – 8×10 − 5 5×10 − 8 –1.8×10 − 3 2×10 − 11 – 2×10 − 6 1×10 − 8 – 5×10 − 5 2×10 − 6 − 2.1×10 − 4 5×10 − 7 − 1.9×10 − 4 5×10 − 7 − 2.3×10 − 4 1×10 − 8 – 2×10 − 4 2.0×10 − 10 – 1.0×10 − 5 10 nM 2.8 nM 5.6 nM 0.12 µM 10 pM 6 nM 10 nM 0.1 µM 0.12 µM 6 nM 65 pM [ 36 ] [ 37 ] [ 38 ] [ 39 ] [ 40 ] [ 41 ] [ 42 ] [ 43 ] [ 44 ] [ 45 ] This work The selectivity was studied by measuring the photocurrent in presence of interference ions. By comparison with the blank, Fig. 7 D shows inconspicuous photocurrent changes in 20.0 µM of ions including NO₂⁻, I⁻, NO₃⁻, Fe³⁺, Hg²⁺, Cu²⁺, Mn²⁺, Co²⁺, PO₄³⁻, CO₃²⁻, Zn²⁺, Fe³⁺+EDTA, Mg²⁺, Cr³⁺. But Fe 3+ and PO 4 3− generate unexpected photocurrent changes. Since EDTA can mask Fe 3+ interference (column 14) and ZnCl 2 can precipitate PO 4 3− [ 34 ], we performed the interference assay in the mixed solution with 20.0 µM different ions and 1.0 µM Cr(VI) anions. After the solution was treated by ZnCl 2 and EDTA, the photocurrent was tested. Similar signals in column 2 and 17 confirmed that the interference was insignificant. To evaluate the practical reliability of BiOI/NiFe@N-CNTs/SPE for the analysis of water samples, various contents of Cr(VI) was spiked into real samples. As shown in Table 2 , where the relative standard deviation (RSD) values are smaller than 7.6% ( n = 4), and the recoveries ranged from 96.1 to 106.0%. These results manifest that the disposable magnetic sensor based on BiOI/NiFe@N-CNTs is feasible for Cr (VI) detection in real water samples. Table 2 The detection results of real samples spiked with Cr(VI) ions (n = 4) Samples Added (nM) Found (nM) Recovery (%) RSD (%) Lake water River water 0.00 20.00 500.00 10000.00 0.00 20.00 500.00 10000.00 ND a 19.21 487.88 10295.42 ND 21.2 514.39 10277.38 / 96.1 97.9 103.0 / 106.0 102.9 102.8 / 4.4 6.0 5.7 / 4.6 7.6 3.4 a Not detected Conclusions In this work, a disposable photoelectrochemical sensor utilizing magnetic BiOI/NiFe@N-CNTs and screen-printed electrode (SPE) was proposed for Cr(VI) assay in water samples. The magnetic photocatalyst of BiOI/NiFe@N-CNTs was first synthesized through hydrothermal method. It displayed good magnetic property, excellent photoelectrochemical activity, and brilliant photoelectron transfer capacity because of the N-doped bamboo-like carbon nanotubes, encapsulated magnetic NiFe components, and hybrid photocatalytic semiconductor constituent. With [Fe(CN) 6 ] 3− serving as photoelectron acceptor and signal amplifier, the disposable magnetic PEC sensor exhibited good performance for Cr(VI) ions detection. Notably, the BiOI/NiFe@N-CNTs/SPE is applicable in real water samples with acceptable recoveries, suggesting this work can present an effective strategy for the development of on-site and real-time analysis. Declarations Competing interests The authors declare no competing interests. Author Contribution Sushuang Xia and Mingyu Zheng: Investigation, Software, Data curation, Conceptualization, Writing – original draft. Xinmei Qian: Writing – review & editing. Chunxiang Li and Keqin Deng: Funding, Supervision, Writing – review & editing. 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Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 08 Feb, 2026 Reviews received at journal 04 Feb, 2026 Reviews received at journal 02 Feb, 2026 Reviews received at journal 24 Jan, 2026 Reviews received at journal 20 Jan, 2026 Reviews received at journal 20 Jan, 2026 Reviewers agreed at journal 18 Jan, 2026 Reviewers agreed at journal 18 Jan, 2026 Reviewers agreed at journal 18 Jan, 2026 Reviewers agreed at journal 14 Jan, 2026 Reviews received at journal 12 Jan, 2026 Reviewers agreed at journal 12 Jan, 2026 Reviewers agreed at journal 12 Jan, 2026 Reviewers agreed at journal 12 Jan, 2026 Reviewers invited by journal 12 Jan, 2026 Editor assigned by journal 11 Jan, 2026 Submission checks completed at journal 11 Jan, 2026 First submitted to journal 04 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8513040","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":573589062,"identity":"b7932b25-b0cd-40e3-a5eb-9763bfa37c14","order_by":0,"name":"Sushuang Xia","email":"","orcid":"","institution":"Hunan University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Sushuang","middleName":"","lastName":"Xia","suffix":""},{"id":573589063,"identity":"02e7e6b2-ab3a-4b32-b1f3-7df670cbe289","order_by":1,"name":"Mingyu Zheng","email":"","orcid":"","institution":"Hunan University of Science and 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1","display":"","copyAsset":false,"role":"figure","size":2378791,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image (A) and TEM image (B) of NiFe@N-CNTs. SEM images of BiOI/NiFe@N-CNTs (C, D)\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8513040/v1/a2c89ec4e8a5c1f73898b7b5.png"},{"id":100224836,"identity":"5873c586-21cd-4cde-a535-4cfd5cb56145","added_by":"auto","created_at":"2026-01-14 10:11:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":722085,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns (A), XPS spectrum (B), and high-resolution XPS (C-I) of BiOI/NiFe@N-CNTs\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8513040/v1/cbbfab885febc19630e40ca4.png"},{"id":100224860,"identity":"7f7b7d74-deb7-433f-8058-25292984f40f","added_by":"auto","created_at":"2026-01-14 10:11:44","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":138125,"visible":true,"origin":"","legend":"\u003cp\u003e(A) CVs of BiOI/NiFe@N-CNTs/SPE in 0.1 M KCl solution (curve a, a’) and 0.1 M KCl mixed with 20 μM K\u003csub\u003e3\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e] solution (curve b, b’) under darkness (curve a, b) and light irradiation (curve a’, b’) at a scan rate of 100 mV s\u003csup\u003e-1\u003c/sup\u003e, (B) The photocurrent of BiOI/SPE (curve c, d) and BiOI/NiFe@N-CNTs/SPE (curve c’, d’) in 0.1 M KCl solution (curve c, c’) and 0.1 M KCl mixed with 20 μM K\u003csub\u003e3\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e] solution (curve d, d’)\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8513040/v1/98e1230154cca4803ba7cdf9.png"},{"id":100224816,"identity":"80c67515-3841-4946-9c46-99a802c8b408","added_by":"auto","created_at":"2026-01-14 10:11:40","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":728899,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of BiOI/NiFe@N-CNTs/SPE for Cr(VI) determination\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8513040/v1/f0745f8deddc0111f49f853d.png"},{"id":100224852,"identity":"66e6530e-95dc-4cbf-be06-dbccf6b2ff8d","added_by":"auto","created_at":"2026-01-14 10:11:43","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":122137,"visible":true,"origin":"","legend":"\u003cp\u003eThe photocurrent (A) and EIS (B) of BiOI/SPE (curve a, a’) and BiOI/NiFe@N-CNTs/SPE (curve b, b’) before/after reaction with 35 μM Cr(VI) ions\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8513040/v1/66a6f9cde1c966aa42a715c0.png"},{"id":100224811,"identity":"04913799-25e3-4d84-aec0-a9bf3bf98a7a","added_by":"auto","created_at":"2026-01-14 10:11:38","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":89404,"visible":true,"origin":"","legend":"\u003cp\u003eThe optimization experiments of BiOI/NiFe@N-CNTs/SPE. (A) The mass ratio FeNi@N-CNTs: Bi(NO₃)₃, (B) different concentrations of K\u003csub\u003e3\u003c/sub\u003eFe(CN)\u003csub\u003e6\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8513040/v1/f3e7e8036235587437d42d5c.png"},{"id":100224875,"identity":"059408fc-6e67-4dee-bb24-709ec38372a4","added_by":"auto","created_at":"2026-01-14 10:11:45","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":249835,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Photocurrent of BiOI/NiFe@N-CNTs/SPE after reaction in various concentrations of Cr(VI) ions, (B) the corresponding calibration curve, (C) Photocurrent of BiOI/NiFe@N-CNTs/SPE under continuous on/off light irradiation in 0.1 M KCl, (D) Interference assay. From 1 to 17 correspond to: blank, Cr₂O₇²⁻, NO₂⁻, I⁻, NO₃⁻, Fe³⁺, Hg²⁺, Cu²⁺, Mn²⁺, Co²⁺, PO₄³⁻, CO₃²⁻, Zn²⁺, Fe³⁺+EDTA, Mg²⁺, Cr³⁺, all ions, respectively.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8513040/v1/86fdb770892c9b661f8a8069.png"},{"id":100383111,"identity":"700bddeb-e07e-4ec3-985c-0b4655531dd5","added_by":"auto","created_at":"2026-01-16 10:46:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6584695,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8513040/v1/586e3f1c-a138-4002-b5c6-a51c76863ede.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"A disposable photoelectrochemical sensor based on magnetic photocatalyst BiOI/NiFe@N-CNTs for the assay of hexavalent chromium","fulltext":[{"header":"Introduction","content":"\u003cp\u003eChromium (Cr) is a typical heavy metal pollutant in industrial wastewater, and Its toxicity is significantly related to its chemical valence [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Hexavalent chromium (Cr(VI)) exhibits conspicuous biological permeability in aquatic environments, posing serious hazards to ecosystems and human health because of its bioaccumulation through the food chain. U.S. Environmental Protection Agency (EPA) has strictly regulated its total chromium content in drinking water (\u0026le;\u0026thinsp;100 \u0026micro;g/L) [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Therefore, the development of diverse and convenient techniques for Cr(VI) analysis is necessary. Traditional methods, such as ion chromatography, atomic absorption spectrometry, inductively coupled plasma mass spectrometry, X-ray fluorescence spectrophotometry have been employed for Cr(VI) detection [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Owing to their high-price instruments, some new methods including colorimetric assay, microfluidic paper-based monitoring, dual-mode sensors, electrochemical and photoelectrochemical (PEC) sensors have emerged for on-site application [\u003cspan additionalcitationids=\"CR6 CR7 CR8\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Among them, PEC sensors are regarded to be rapid and high-sensitive because they combine photocatalytic and electrochemical technology [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], and PEC signals commonly depend on particular semiconductor materials [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMagnetic photocatalysts are a type of composite that can be efficiently recycled through magnetic control or magnetic separation [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. They have been widely applied in environmental purification and energy conversion [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. For example, some magnetic iron oxide-integrated photocatalysts, such as ZnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/CeO\u003csub\u003e2\u003c/sub\u003e, CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/AgBr, Ag\u003csub\u003e2\u003c/sub\u003eWO\u003csub\u003e4\u003c/sub\u003e/BiOI/CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/ZnO/BiOI, exhibited high activities for degradation of organic pollutants or antibiotics [\u003cspan additionalcitationids=\"CR17 CR18\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Ag-AgCl/ZnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and AgI/CuFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e accelerated bacterial inactivation to clean environment [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. C₃N₄-based magnetic photocatalysts produced hydrogen under visible light to obtain clean energy [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Among these magnetic semiconductors, BiOI coupled/doped photocatalysts emerged as one of the most interesting semiconductors because of their efficient electron-hole pairs separation and remarkable photocatalytic activity [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. However, as so far, few magnetic BiOI-based photocatalysts were reported for PEC applications.\u003c/p\u003e \u003cp\u003eOur previous work prepared a magnetic bamboo-like nitrogen-doped carbon nanotube sealed with NiFe nanoparticles (NiFe@N-CNTs) [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. The NiFe@N-CNTs presented high superparamagnetic property, good electrical conductivity, large surface area, adequate active sites, periodic bamboo-like nodes, and nanoconfined structure. Multiwall carbon nanotubes/BiOI has been well used as photocatalysts for degradation of antipyrine [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. AgI-carboxylated multiwalled carbon nanotubes-BiOI Z-scheme heterojunction material was applied for photoelectrochemical aptasensing of lincomycin [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. These semiconductor photocatalysts coupled with carbon nanotubes showed significant electron-acceptor/transport matrix in photocatalysis. Inspired by the above research, we tried to prepare a new magnetic cathodic photoelectrochemical material BiOI/NiFe@N-CNTs. The BiOI/NiFe@N-CNTs displayed good magnetism, abundant pores, high photoelectrochemical activity, and remarkable conductivity. Then, BiOI/NiFe@N-CNTs was controlled on screen-printed electrode (SPE) by a designed magnetic sticker to develop a disposable magnetic photoelectrochemical (PEC) sensor for Cr(VI) detection. With [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e as photoelectron acceptor and the photoreduction product of Cr(OH)\u003csub\u003e3\u003c/sub\u003e deposit as photocurrent quencher, the proposed sensor displayed a broad detection range and a low detection limit.\u003c/p\u003e"},{"header":"Experimental procedure","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eApparatus and Materials\u003c/h2\u003e \u003cp\u003eX-ray photoelectron spectroscopy (XPS, thermo Scientific Escalab Xi+, USA) and X-ray power diffraction (XRD-6000, Shimadzu) were utilized to determine the chemical composition and the crystal structure of nanomaterials. Scanning electron microscopy (FESEM, JEOL-7800F, Japan) and transmission electron microscopy (TEM, JEOL-JEM 2100F, Japan) were applied to investigate the morphology of nanomaterials. The electrochemical station (CHI 760C, Shanghai Chenhua Corp., China) matched with a photoelectrochemical system (PEAC 200A, Ida) was used to perform all electrochemical and photoelectrochemical experiments. White LED light (20 mW/cm\u003csup\u003e2\u003c/sup\u003e) acted as irradiation source.\u003c/p\u003e \u003cp\u003eNickel acetate, K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e, iron acetate, Bi(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026sdot;5H\u003csub\u003e2\u003c/sub\u003eO, KI, ethylene glycol, polyvinyl pyrrolidone, dicyandiamide (DCD), KCl, soluble starch, and other chemical reagent were bought from Sinopharm Chemical Reagent Co., Ltd. (China). All reagents were of analytical grade without further purification. KCl (0.1 M) was applied for supporting electrolyte of photoelectrochemical measurement. All aqueous solutions were prepared with ultrapure water (\u0026ge;\u0026thinsp;18.3 M, Milli-Q, Millipore).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSynthesis of magnetic NiFe@N-CNTs and BiOI/NiFe@N-CNTs\u003c/h3\u003e\n\u003cp\u003eMagnetic NiFe@N-CNTs was synthesized according to our previous work [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Briefly, soluble starch (0.5 g) was dissolved in boiling water. Then, nickel acetate (1.0 g), iron acetate (0.5 g), and N-cyanoguanidine (5.0 g) were added to the cooled starch solution and thoroughly stirred. After being dried, the solid mixture was then placed in an alumina crucible and heated up to 500\u0026deg;C in a tube furnace with a heating rate of 4\u0026deg;C \u0026sdot; min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and then heated to 800\u0026deg;C and controlled for 2 h. Finally, the cooled product was collected.\u003c/p\u003e \u003cp\u003eFor further use, NiFe@N-CNTs was screened to obtain high dispersed magnetic carbon nanotubes by the following steps. Firstly, the material was dispersed in an aqueous solution containing polyvinylpyrrolidone (PVP, 0.1 wt%) and sonicated for 10 minutes. After standing for 5 minutes, graded centrifugation method was employed to isolate large agglomerates by centrifugation at 1,000 rpm. The collected supernatant was subsequently centrifuged at 8,000 rpm for 10 minutes. The obtained sediment was then washed three times and dried at 60\u0026deg;C. For the preparation of BiOI/NiFe@N-CNTs, 0.5 g of PVP and 1.0 g of Bi(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;5H\u003csub\u003e2\u003c/sub\u003eO were added into 25 mL of ethylene glycol to form a homogeneous solution. 40 mg of FeNi@N-CNTs was ultrasonically dispersed in EG (25 mL). The former was then slowly added dropwise to the latter under stirring. After 1 hour, 1 g of KI was added into the mixture, followed by an additional hour of stirring. Then, the dispersion liquid was transferred into an autoclave and heated at 140 ◦C for 12 h. Finally, the product was dried at 60\u0026deg;C for 12 h after washing several times and centrifugation. For comparison, BiOI was prepared by similar steps without addition of FeNi@N-CNTs.\u003c/p\u003e\n\u003ch3\u003eFabrication of BiOI/NiFe@N-CNTs/SPE and detection of hexavalent chromium\u003c/h3\u003e\n\u003cp\u003eScreen-printed electrode (SPE) from Haoyang Tech. Co. (Shenzhen, China) has been printed carbon circle (d\u0026thinsp;=\u0026thinsp;5 mm), carbon strip, and silver/silver chloride stripe as photocathode, counter electrodes, and reference electrode, respectively. A magnetic sticker was attached to the backside of the carbon circle. For modification, 10 \u0026micro;l of BiOI/NiFe@N-CNTs aqueous dispersion (2.0 mg mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was added on the surface of carbon circle and water was removed by filter paper. BiOI/NiFe@N-CNTs was immobilized on SPE by the strong magnetic sticker. For Cr (VI) detection, BiOI/NiFe@N-CNTs/SPE was dropped with 20 \u0026micro;l of Cr (VI) species containing KCl (0.1 M) and K\u003csub\u003e3\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e] (20 \u0026micro;M). The photocurrent was then recorded at 0 V bias potential after photo-reduction of Cr(VI) for 300 s.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003ePhysicochemical characteristics of magnetic materials\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e exhibits the SEM and TEM images of magnetic NiFe@N-CNTs (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA, B) and BiOI/NiFe@N-CNTs (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC, D). NiFe@N-CNTs shows a hollow bamboo-like nanotube with the diameter range of 20\u0026thinsp;~\u0026thinsp;100 nm and the length\u0026thinsp;\u0026lt;\u0026thinsp;3.0 \u0026micro;m. NiFe nanoparticles embed inside nanotubes, which contribute to the magnetism of NiFe@N-CNTs and BiOI/NiFe@N-CNTs. For BiOI/NiFe@N-CNTs, it displays a 3D aggregated structure with the size\u0026thinsp;\u0026lt;\u0026thinsp;3.0 \u0026micro;m. The BiOI/NiFe@N-CNTs is assembled by interlaced nanotubes, which generates abundant pores and provides larger surface area. These nanotubes exhibit a rougher surface than NiFe@N-CNTs. The magnetic behavior of BiOI/NiFe@N-CNTs was then demonstrated by using an external magnetic field. It was found that the prepared material rapidly gathered towards the magnet and easily returned to a dispersed state after removing the magnet and shaking,\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA displays the XRD results of NiFe@N-CNTs, BiOI, and BiOI/NiFe@N-CNTs. It reveals that NiFe@N-CNTs has two low diffraction peaks at 41.2\u0026deg; and 48.0\u0026deg;, assigning to Fe\u003csub\u003e4\u003c/sub\u003eN from the coordination of metal iron and nitrogen [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. The sharp and strong peaks at 43.7\u0026deg; and 50.6\u0026deg; corresponded to the (111) and (200) reflection of NiFe (JCPDS#47-1405), implying that NiFe nanoparticles has crystallized well. The broad peak at about 26.1\u0026deg; is indexed to amorphous carbon (002) [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Pure BiOI shows several characteristic peaks, which are related to the tetragonal crystal phase of BiOI [JCPDS No. 73-2062]. Similar peaks appear at the pattern of BiOI/NiFe@N-CNTs, indicating that NiFe@N-CNTs has no obvious influence upon the crystal phase of BiOI. However, the peaks for NiFe@N-CNTs can\u0026rsquo;t be found, which is ascribed to the low content of NiFe@N-CNTs in BiOI/NiFe@N-CNTs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe composition and chemical state of BiOI/NiFe@N-CNTs is further analyzed by XPS (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB\u0026ndash;I). Seven elements, including Bi, I, Fe, Ni, C, O, and N, are found in the full survey spectrum. For element Bi, the high-resolution spectrum displays two peaks at 158.9 eV and 164.2 eV, corresponding to Bi 4f\u003csub\u003e7/2\u003c/sub\u003e and Bi 4f\u003csub\u003e5/2\u003c/sub\u003e in the form of Bi\u003csup\u003e3+\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC). For I, it shows 3d\u003csup\u003e5/2\u003c/sup\u003e and 3d\u003csup\u003e3/2\u003c/sup\u003e of I\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 618.8 eV and 630.3 eV, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD). In the spectrum of Fe 2p, four subpeaks are fitted for the multivalent state (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eE). The binding energy at 710.7 eV and 712.5 eV are indexed to 2p\u003csup\u003e3/2\u003c/sup\u003e of Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e for iron oxides, respectively [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The peaks at 724.0 eV and 725.9 eV are assigned to 2p\u003csup\u003e1/2\u003c/sup\u003e of Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e. Ni 2p spectrum is deconvoluted into five peaks (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eF), including three peaks for 2p\u003csup\u003e3/2\u003c/sup\u003e and 2p\u003csup\u003e1/2\u003c/sup\u003e of Ni\u003csup\u003e2+\u003c/sup\u003eat 854.9 and 872.5 eV, 2p\u003csup\u003e3/2\u003c/sup\u003e of Ni\u003csup\u003e0\u003c/sup\u003e at 853.1 eV, and two satellite peaks at 860.8 and 879.9 eV, respectively [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Then, C 1 s spectrum in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eG is fitted into three peaks at 285.0, 286.1, and 287.9 eV, relating to C\u0026ndash;C, C\u0026ndash;N, and C\u0026ndash;O bonds, respectively [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. For the region of O 1s in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eH, it shows that the peaks at 529.5, 531.3, 533.2 eV belong to Bi‒O bond, I‒O bond, and C‒O bond [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The N 1s spectrum is separated into five peaks (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eI), relating to M‒N units, pyridinic N, pyrrolic N graphite N, and oxidized N with the binding energy of 397.6, 398.3, 399.6, 401.2 eV, and 402.5 eV, respectively [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. From the discussions, we can see that BiOI and NiFe@N-CNTs coexist in prepared magnetic BiOI/NiFe@N-CNTs.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003ePhotoelectrochemical features of sensors\u003c/h2\u003e \u003cp\u003eThe photoelectrochemical behaviors of BiOI/NiFe@N-CNTs/SPE were first investigated by cyclic voltammetry (CV). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA, the current intensity is affected by the light illumination and [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e ions. Comparing the current under darkness (curve a, b) and light irradiation (curve a\u0026rsquo;, b\u0026rsquo;), BiOI/NiFe@N-CNTs shows a photocurrent of 1.1 \u0026micro;A at 0 V in 0.1 M KCl. After adding [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e, it displays a pair of redox peaks at +\u0026thinsp;0.33/+0.25 V in darkness. While it is exposed to visible light, the peaks appear at +\u0026thinsp;0.33/+0.16 V and the cathodic photocurrent is greatly improved under the potential of \u003cem\u003e\u0026lt;\u0026thinsp;+\u003c/em\u003e\u0026thinsp;0.3 V. The photocurrent is about 8.4 \u0026micro;A at 0 V. We deduced that the negative shift of reduction peak is caused by the transfer of photogenerated electrons (e-) from BiOI/NiFe@N-CNTs to [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e. Furthermore, the transient photocurrent of BiOI/SPE and BiOI/NiFe@N-CNTs/SPE is tested at the applied of 0 V (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB). Both electrodes display low photocurrent in 0.1 M KCl solution, about 0.42 and 0.78 \u0026micro;A for BiOI/SPE and BiOI/NiFe@N-CNTs/SPE, respectively. Obviously, BiOI/NiFe@N-CNTs possesses better photoelectrochemical activity. With the addition of [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e, the photocurrent increases up to 4.5 and 8.3 \u0026micro;A, respectively. These results indicates that NiFe@N-CNTs can accelerate the photoelectron transfer and [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e can efficiently accept photoelectrons from BiOI/NiFe@N-CNTs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eThe mechanism and feasibility for photoelectrochemical sensing of Cr(VI)\u003c/h3\u003e\n\u003cp\u003ePossible sensing mechanism of BiOI/NiFe@N-CNTs for Cr(VI) is described in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Under light illumination, BiOI generates electron/hole pairs (e\u003csup\u003e\u0026minus;\u003c/sup\u003e/h\u003csup\u003e+\u003c/sup\u003e). Part of photogenerated electrons (e\u003csup\u003e\u0026minus;\u003c/sup\u003e) flow to NiFe@N-CNTs for the reduction of [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e, while the rest are directly accepted by [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e to form [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Furthermore, NiFe nanoparticles and bamboo-like N-CNTs can improve the conductivity of composite and facilitate electron transfer. Therefore, the e\u003csup\u003e\u0026minus;\u003c/sup\u003e/h\u003csup\u003e+\u003c/sup\u003e recombination is effectively suppressed, and the lifetime of charge carriers is extended. For these reasons, more [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e ions are reduced and the higher photocurrent can be observed. With the presence of Cr (VI) anions, BiOI/NiFe@N-CNTs accumulates them for its high active surface area and reduces them to produce Cr(OH)\u003csub\u003e3\u003c/sub\u003e deposit under light irradiation at pH\u0026thinsp;=\u0026thinsp;7 [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The low-conductivity deposit formed on BiOI/NiFe@N-CNTs not only blocks light irradiation to generate e\u003csup\u003e\u0026minus;\u003c/sup\u003e but also shields the active sites of BiOI-NiFe@N-CNTs to transfer electron towards [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e. In addition, the adsorbed Cr (VI) anions hinder the diffusion of [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e to SPE surface because of the electrostatic repulsion. Thus, photocurrents decrease, enabling ultra-sensitive determination of Cr (VI).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe feasibility for Cr(VI) detection is confirmed by measuring transient photocurrent and interfacial resistance (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). BiOI/SPE and BiOI/NiFe@N-CNTs showed the photocurrent of 4.5 and 8.3 \u0026micro;A, respectively. After photoreaction with Cr(VI) anions, the transient current decreased to 2.2 and 4.1 \u0026micro;A with the declining percentage of 51.1% and 50.6% (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). Electrochemical impedance spectroscopy displayed a high Ret value of 2740 Ω for BiOI/SPE and a greatly decreased Ret of 512 Ω for BiOI/NiFe@N-CNTs/SPE (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB), also proving that FeNi@N-CNTs possesses excellent conductivity and high electron transfer efficiency. After photoreduction of Cr(VI), the resistance increased to 3085 Ω and 1108 Ω, respectively, indicating that photoelectron transfer from BiOI to [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e is seriously blocked, and Cr(VI) anions indirectly affect photocurrent.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eOptimization of experimental conditions\u003c/h3\u003e\n\u003cp\u003eThe mass ratio of FeNi@N-CNTs: Bi(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e used in the preparation of BiOI/NiFe@N-CNTs is a crucial parameter for achieving excellent photoelectrochemical performance. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eA, the cathodic photocurrent increases rapidly with the ratio and reaches a plateau within the range of 0.04\u0026thinsp;~\u0026thinsp;0.06. However, further increasing the ratio to 0.08 results in a decline in photocurrent. Although FeNi@N-CNTs can promote electron transfer and facilitate the photoreduction of [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e, excessive use can shield light irradiation, leading to lower light-induced electron efficiency. Therefore, a ratio of 0.05 is selected. Then, the influence of [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e concentration is investigated (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eB). The results indicate that the photocurrent enhances significantly with concentration, confirming that [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e can effectively capture photogenerated electrons and inhibit electron-hole recombination. However, when the concentration exceeds 40 \u0026micro;M, the growth of photocurrent tends to plateau, indicating that the gain of the electron acceptor is approaching saturation [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Additionally, excessively high concentrations lead to an increase in background current, and the yellow color of the solution will mask light illumination. Based on these facts, 20 \u0026micro;M is chosen as the optimal working concentration. The applied potential also plays an important role in promoting photocurrent. We found that the current increased as the potential dropped from 0.3 V to 0.0 V and then remained stable (data not shown). Considering that high potential brought about high background current, 0.0 V was therefore applied for measurement.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eThe performance of BiOI/NiFe@N-CNTs/SPE for analysis of Cr(VI) anions\u003c/h2\u003e \u003cp\u003eThe photocurrent intensity of BiOI/NiFe@N-CNTs/SPE after photoreduction of Cr(VI) anions is examined under optimal conditions. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eA reveals the PEC signals of BiOI-FeNi@N-CNTs/SPE. Clearly, the photocurrent progressively decreases while increasing Cr(VI). Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eB displays that the changed photocurrent (Δ\u003cem\u003ei\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003ei\u003c/em\u003e-\u003cem\u003ei\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e) is directly proportional to log\u003cem\u003eC\u003c/em\u003e\u003csub\u003e(Cr)\u003c/sub\u003e), where \u003cem\u003ei\u003c/em\u003e and \u003cem\u003ei\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e represent the photocurrent in Cr(VI) solution and the blank, respectively. Within the range of 0.2 nM\u0026minus;10.0 \u0026micro;M, the corresponding calibration curve shows a linear relationship of Δ\u003cem\u003ei\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.9769\u0026thinsp;+\u0026thinsp;0.9162 log C (\u003cem\u003eR\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.990). The detection limit (LOD) is 65 pM (S/N\u0026thinsp;=\u0026thinsp;3), lower than the permissible limit of 0.96 \u0026micro;M specified by World Health Organization (WHO). By comparison with other methods listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the disposable magnetic photoelectrochemical sensor presents comparable performance for Cr(VI) analysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe stability of BiOI/NiFe@N-CNTs/SPE was tested. We found that photocurrent showed minimal changes after 15 consecutive \u0026ldquo;on/off\u0026rdquo; light treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eC). When it was exposed to light continuously for 300 s, the current decreased by only 3.8%, indicating BiOI-FeNi@N-CNTs/SPE can be stably used for the detection of Cr(VI) ions. Then, storage stability was regularly monitored over four weeks, where the magnetic sensors were kept at 4\u0026deg;C. Impressively, the current responses decreased by only 5.64%, demonstrating that BiOI/NiFe@N-CNTs/SPE is stable under low-temperature storage conditions. The reproducibility was also assessed by measuring the photocurrent response of six identical sensors. The relative standard deviation (RSD) was 6.2%, suggesting the acceptable reproducibility.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePerformance comparison of proposed magnetic PEC sensor with previous reports\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMaterials\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLinear range (M)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLOD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRef.\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMoS\u003csub\u003e2\u003c/sub\u003e/BiOI\u003c/p\u003e \u003cp\u003eCuO Film\u003c/p\u003e \u003cp\u003eCuPc/TiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003cp\u003eNiCo-LDHs/TiO\u003csub\u003e2\u003c/sub\u003eNTAs/Ti\u003c/p\u003e \u003cp\u003ePbS\u003c/p\u003e \u003cp\u003eAu-TiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003cp\u003eBiVO\u003csub\u003e4\u003c/sub\u003e-7\u003c/p\u003e \u003cp\u003eBiOI/CN-34%\u003c/p\u003e \u003cp\u003eCuS/Bi\u003csub\u003e2\u003c/sub\u003eMoO\u003csub\u003e6\u003c/sub\u003e\u003c/p\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e@C/Au/BiOI\u003c/p\u003e \u003cp\u003eBiOI/NiFe@N-CNTs\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e\u0026ndash; 1.6\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e8\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e \u0026ndash; 2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e10\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e \u0026ndash; 8\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e\u0026ndash;1.8\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;11\u003c/sup\u003e \u0026ndash; 2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e \u0026ndash; 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e \u0026minus;\u0026thinsp;2.1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e \u0026minus;\u0026thinsp;1.9\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e \u0026minus;\u0026thinsp;2.3\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e \u0026ndash; 2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e2.0\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003e \u0026ndash; 1.0\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10 nM\u003c/p\u003e \u003cp\u003e2.8 nM\u003c/p\u003e \u003cp\u003e5.6 nM\u003c/p\u003e \u003cp\u003e0.12 \u0026micro;M\u003c/p\u003e \u003cp\u003e10 pM\u003c/p\u003e \u003cp\u003e6 nM\u003c/p\u003e \u003cp\u003e10 nM\u003c/p\u003e \u003cp\u003e0.1 \u0026micro;M\u003c/p\u003e \u003cp\u003e0.12 \u0026micro;M\u003c/p\u003e \u003cp\u003e6 nM\u003c/p\u003e \u003cp\u003e65 pM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eThis work\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe selectivity was studied by measuring the photocurrent in presence of interference ions. By comparison with the blank, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eD shows inconspicuous photocurrent changes in 20.0 \u0026micro;M of ions including NO₂⁻, I⁻, NO₃⁻, Fe\u0026sup3;⁺, Hg\u0026sup2;⁺, Cu\u0026sup2;⁺, Mn\u0026sup2;⁺, Co\u0026sup2;⁺, PO₄\u0026sup3;⁻, CO₃\u0026sup2;⁻, Zn\u0026sup2;⁺, Fe\u0026sup3;⁺+EDTA, Mg\u0026sup2;⁺, Cr\u0026sup3;⁺. But Fe\u003csup\u003e3+\u003c/sup\u003e and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e generate unexpected photocurrent changes. Since EDTA can mask Fe\u003csup\u003e3+\u003c/sup\u003e interference (column 14) and ZnCl\u003csub\u003e2\u003c/sub\u003e can precipitate PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], we performed the interference assay in the mixed solution with 20.0 \u0026micro;M different ions and 1.0 \u0026micro;M Cr(VI) anions. After the solution was treated by ZnCl\u003csub\u003e2\u003c/sub\u003e and EDTA, the photocurrent was tested. Similar signals in column 2 and 17 confirmed that the interference was insignificant.\u003c/p\u003e \u003cp\u003eTo evaluate the practical reliability of BiOI/NiFe@N-CNTs/SPE for the analysis of water samples, various contents of Cr(VI) was spiked into real samples. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, where the relative standard deviation (RSD) values are smaller than 7.6% (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4), and the recoveries ranged from 96.1 to 106.0%. These results manifest that the disposable magnetic sensor based on BiOI/NiFe@N-CNTs is feasible for Cr (VI) detection in real water samples.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThe detection results of real samples spiked with Cr(VI) ions (n\u0026thinsp;=\u0026thinsp;4)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSamples\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAdded (nM)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eFound (nM)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRecovery (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eRSD (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLake water\u003c/p\u003e \u003cp\u003eRiver water\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003cp\u003e20.00\u003c/p\u003e \u003cp\u003e500.00\u003c/p\u003e \u003cp\u003e10000.00\u003c/p\u003e \u003cp\u003e0.00\u003c/p\u003e \u003cp\u003e20.00\u003c/p\u003e \u003cp\u003e500.00\u003c/p\u003e \u003cp\u003e10000.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eND \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e19.21\u003c/p\u003e \u003cp\u003e487.88\u003c/p\u003e \u003cp\u003e10295.42\u003c/p\u003e \u003cp\u003eND\u003c/p\u003e \u003cp\u003e21.2\u003c/p\u003e \u003cp\u003e514.39\u003c/p\u003e \u003cp\u003e10277.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e/\u003c/p\u003e \u003cp\u003e96.1\u003c/p\u003e \u003cp\u003e97.9\u003c/p\u003e \u003cp\u003e103.0\u003c/p\u003e \u003cp\u003e/\u003c/p\u003e \u003cp\u003e106.0\u003c/p\u003e \u003cp\u003e102.9\u003c/p\u003e \u003cp\u003e102.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e/\u003c/p\u003e \u003cp\u003e4.4\u003c/p\u003e \u003cp\u003e6.0\u003c/p\u003e \u003cp\u003e5.7\u003c/p\u003e \u003cp\u003e/\u003c/p\u003e \u003cp\u003e4.6\u003c/p\u003e \u003cp\u003e7.6\u003c/p\u003e \u003cp\u003e3.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e\u003csup\u003ea\u003c/sup\u003e Not detected\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn this work, a disposable photoelectrochemical sensor utilizing magnetic BiOI/NiFe@N-CNTs and screen-printed electrode (SPE) was proposed for Cr(VI) assay in water samples. The magnetic photocatalyst of BiOI/NiFe@N-CNTs was first synthesized through hydrothermal method. It displayed good magnetic property, excellent photoelectrochemical activity, and brilliant photoelectron transfer capacity because of the N-doped bamboo-like carbon nanotubes, encapsulated magnetic NiFe components, and hybrid photocatalytic semiconductor constituent. With [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e serving as photoelectron acceptor and signal amplifier, the disposable magnetic PEC sensor exhibited good performance for Cr(VI) ions detection. Notably, the BiOI/NiFe@N-CNTs/SPE is applicable in real water samples with acceptable recoveries, suggesting this work can present an effective strategy for the development of on-site and real-time analysis.\u003c/p\u003e"},{"header":"Declarations","content":" \u003cp\u003e \u003cstrong\u003eCompeting interests\u003c/strong\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eSushuang Xia and Mingyu Zheng: Investigation, Software, Data curation, Conceptualization, Writing \u0026ndash; original draft. Xinmei Qian: Writing \u0026ndash; review \u0026amp; editing. Chunxiang Li and Keqin Deng: Funding, Supervision, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e \u003cp\u003eWe are grateful for the Hunan Provincial Natural Science Foundation of China (Grant No. 2023JJ50229).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eKieber RJ, Willey JD, Zvalaren SD (2002) Chromium speciation in rainwater: temporal variability and atmospheric deposition. Environ Sci Technol 36:5321\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBell J, Ma X, McDonald TJ, Huang CH, Sharma VK (2022) Overlooked role of chromium(V) and chromium(IV) in chromium redox reactions of environmental importance. 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J Alloys Compd 882:160690\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang G, Cheng D, Li M, Feng C, Wu H, Mei H (2021) Enhanced the photoelectrochemical performance of Bi\u003csub\u003e2\u003c/sub\u003eXO\u003csub\u003e6\u003c/sub\u003e (X\u0026thinsp;=\u0026thinsp;W, Mo) for detecting hexavalent chromium by modification of CuS. J Environ Sci 103:185\u0026ndash;195\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMeng L, Zhang Y, Wang J, Zhou B, Xu Z, Shi J (2023) Construction of MIL-125 derived 2D TiO\u003csub\u003e2\u003c/sub\u003e@C nanocake/Au/3D peony-like BiOI Z-scheme heterojunction for sensitive and facile photoelectrochemical sensing assay of Cr(VI) based on competitive consumption of surface sacrificial agent. Sens Actuators B- Chem 396:134578\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Magnetic photocatalyst ⋅ BiOI/NiFe@N-CNTs composite ⋅ Disposable sensor ⋅ Hexavalent chromium assay ⋅ Photoelectrochemical technique","lastPublishedDoi":"10.21203/rs.3.rs-8513040/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8513040/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eChromium is widely used in different industries and can be found in wastewater. Hexavalent chromium (Cr(VI)) is considered life-threatening pollutant and shows adverse effects on environment and human health. Therefore, an effective method is demanded for reliable detection of Cr(VI). In this work, a magnetic photocatalyst (BiOI/NiFe@N-CNTs) composed of bismuth oxyiodide (BiOI) and magnetic bamboo-like carbon nanotubes capsulated with nickel-iron (NiFe) particles (FeNi@N-CNTs) was prepared by one-step hydrothermal method. Various characterization techniques revealed that BiOI/NiFe@N-CNTs possessed abundant pores and larger surface area because of its interlaced nanotubes morphology. It showed excellent cathodic photoelectrochemical (PEC) activity and good photoelectron transfer capacity with [Fe(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e. Based on these features, a disposable magnetic PEC sensor for Cr(VI) assay was constructed by modifying screen-printed electrode (SPE) with BiOI/NiFe@N-CNTs via magnetic control. The PEC sensor presented a broad detection range of 0.2 nM\u0026minus;10.0 \u0026micro;M and a low detection limit (LOD) of 65 pM (S/N\u0026thinsp;=\u0026thinsp;3). The practical application of the sensor in real water samples obtained acceptable results. The low cost, low consumption volume in samples, and disposability make the magnetic PEC sensor a potential tool for monitoring Cr(VI).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e","manuscriptTitle":"A disposable photoelectrochemical sensor based on magnetic photocatalyst BiOI/NiFe@N-CNTs for the assay of hexavalent chromium","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-14 10:11:17","doi":"10.21203/rs.3.rs-8513040/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-08T11:55:18+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-05T00:57:09+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-03T02:41:08+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-24T18:44:15+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-21T02:47:57+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-20T06:45:14+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"127319386913617260447853569896309673303","date":"2026-01-19T01:32:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"44416587343258549428184396392962589323","date":"2026-01-19T00:55:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"225418155366164619918096199482479554594","date":"2026-01-18T11:53:49+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"249809183069280535613142534553212115513","date":"2026-01-14T14:12:59+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-13T01:47:52+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"11602060889474736524766196951941613282","date":"2026-01-13T00:48:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"227812387445216602475760853154210598123","date":"2026-01-12T15:15:16+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"136232610466129960482370084712628569330","date":"2026-01-12T14:08:33+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-01-12T13:27:09+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-12T03:23:44+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-12T03:23:34+00:00","index":"","fulltext":""},{"type":"submitted","content":"Ionics","date":"2026-01-04T12:28:51+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"7bc4fe9c-26af-418b-a5fd-1f1c3b29a72a","owner":[],"postedDate":"January 14th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-20T21:53:41+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-14 10:11:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8513040","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8513040","identity":"rs-8513040","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}