Synthesis of carbon dots via selective microwave heating of reverse micelles in a microwave-transparent solvent

Synthesis of carbon dots via selective microwave heating of reverse micelles in a microwave-transparent solvent | 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 Synthesis of carbon dots via selective microwave heating of reverse micelles in a microwave-transparent solvent Srikrishna Tummala, Yen-Peng Ho This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8963582/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 4 You are reading this latest preprint version Abstract We present a novel approach for the rapid synthesis of carbon dots using a microwave-assisted reverse micelle (MARM) method. Bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) was used to form reverse micelles in a nonpolar medium, dodecane, which does not absorb microwave radiation. Reverse micelles containing an aqueous phase were dispersed in the oil phase, and these small polar droplets acted as nanoreactors. The aqueous phase contained citric acid and Tris base as reactants. Under microwave heating, only these microreactors absorbed microwave energy and were heated efficiently and homogeneously, facilitating the formation of carbon dots. The micelle size could be tuned by adjusting the water-to-surfactant molar ratio, producing carbon dots with uniform sizes ranging from 1.8 to 2.6 nm, depending on the AOT concentration. Transmission electron microscopy and photospectroscopy data were compared for carbon dots prepared using hydrothermal, domestic microwave-assisted, solvothermal reverse micelle, hexanoic-acid-based MARM, and dodecane-based MARM approaches. The characteristics of carbon dots produced using the proposed method were similar to those obtained using hydrothermal and domestic microwave-assisted methods. However, the proposed method is more rapid than the hydrothermal method and may produce more homogeneous carbon dots than the domestic microwave-assisted method. carbon dot reverse micelle microwave dodecane selective heating Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Carbon quantum dots (CDs) have garnered significant attention in recent years across various fields owing to their exceptional physical, chemical, and optical properties. Their unique fluorescence characteristics and nontoxic nature have made CDs a preferred alternative to conventional quantum dots [1-3]. CDs are promising substitutes for organic fluorescent dyes [4] and metal-based quantum dots in applications such as fluorescence sensing [5], bio-imaging [6] drug delivery [7], catalysis [8], and optoelectronic devices [9]. In this context, most research has focused on biological applications, particularly bioimaging and biosensing. Bioimaging applications, in particular, require precise tuning of optical and surface properties, which depends on careful control of particle shape and size distribution [10]. Strategies for synthesizing carbon dots are broadly categorized into top-down and bottom-up approaches [11, 12]. The bottom-up approach involves the polymerization and carbonization of small organic molecules using methods such as hydrothermal [13], solvothermal [14], microwave-assisted [15], reverse micelles [16], and pyrolysis [17] techniques. These synthesis methods have been continuously improved and tailored to tune the optical, physical, and chemical properties of carbon dots[18]. Several strategies have been reported to control particle size and modify the surface chemistry of CDs [10]. Rhee et al. introduced a synthetic approach using reverse micelles as nanoreactors to produce highly luminescent graphene quantum dots. The process involved the carbonization of glucose within reverse micelles, followed by in-situ surface passivation, offering advantages such as size tunability and narrow size distribution without the need for impractical post-synthesis size separation processes [16]. Zhang et al. reported the formation of uniform CDs by loading glucose into metal–organic frameworks followed by carbonization [19]. Bishnu et al. demonstrated that encapsulating luminescent graphene quantum dots without a capping agent within zeolitic imidazolate framework (ZIF-8) nanocrystals results in well-confined and ordered dispersion[20]. This encapsulation significantly influences the growth, shape, and size of the ZIF-8 nanocrystals. Microwave-assisted synthesis of CDs offers several advantages, including significantly accelerated reaction rates, reduced energy consumption, and increased yields [21]. Microwave heating efficiency depends on factors such as the dielectric constant, dipole moment, dielectric loss, and dielectric relaxation time of the solvent. The dielectric constant reflects a solvent’s ability to store electric charge, and molecules with large dipole moments typically exhibit high dielectric constants. Polar molecules can readily align with a rapidly oscillating microwave field, leading to enhanced polarization and heating efficiency. Overall, the efficiency of microwave heating is governed by the loss tangent, defined as the ratio of dielectric loss to dielectric constant. A high loss tangent corresponds to efficient microwave absorption. Solvents with high loss tangents, such as ethylene glycol and water, absorb microwave energy efficiently, whereas solvents such as toluene and n -hexane, which have low loss tangents, do not [22]. Here, we present a novel approach for the synthesis of carbon dots using a microwave-assisted reverse micelle (MARM) method. Reverse micelles, also known as water-in-oil microemulsions, were formed using bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT), a widely used surfactant for reverse micelle formation in nonpolar media. In this work, dodecane was selected as the oil phase because it does not absorb microwave radiation under the experimental conditions. Reverse micelles containing an aqueous solution of reactants were dispersed in the oil phase, where the confined polar droplets acted as nanoreactors. Under microwave irradiation, only these microreactors absorbed microwave energy and were heated efficiently and homogeneously, thereby facilitating carbon dot formation. The micelle size was tuned by adjusting the water-to-surfactant molar ratio [23], and the concentration of AOT was used to control the size of the resulting CDs. The size distribution and optical properties of the synthesized CDs were systematically investigated and compared with those obtained using conventional synthesis methods. Keywords: reverse micelle, carbon dot, bis(2-ethylhexyl) sulfosuccinate sodium salt, quantum yield, microwave. Materials and methods Materials and instruments Citric acid (CA, anhydrous) was purchased from J. T. Baker (NJ, USA). Bis(2-ethylhexyl) sulfosuccinate sodium salt and polyethyleneimine (PEI, MW ≈ 600) were obtained from Sigma-Aldrich (MA, USA). Tris(hydroxymethyl)aminomethane (Tris) was purchased from MD Bio, Inc. Ultrapure water (18.2 MΩ·cm) was produced using a Milli-Q water purification system (Millipore, MD, USA). All chemicals were of analytical grade and used as received. UV–visible absorption and fluorescence spectra were recorded using a SpectraMax® ID3 multimode microplate reader (Molecular Devices, CA, USA). High-resolution transmission electron microscopy (HR-TEM) images were obtained using a JEM-2001F microscope (JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI 500 VersaProbe system (ULVAC-PHI, Japan). Quantum yield measurements were carried out using a fluorescence spectrometer (FLS920, Edinburgh Instruments, USA). Fourier-transform infrared (FT-IR) spectra were recorded on a Spectrum One spectrometer (PerkinElmer, MA, USA). Synthesis of CDs using the microwave-assisted reverse micelle method Carbon dots synthesized via the microwave-assisted reverse micelle method are denoted as MARM-CDs. Reverse micelle reactions were conducted under microwave irradiation (2.45 GHz) using a Discover focused microwave reactor (CEM, NC, USA). To optimize precursor composition, various weight ratios of CA and Tris dissolved in 1 mL of water were investigated. The CA:Tris ratios examined were 0:100 mg, 100:0 mg, 30:70 mg, 50:50 mg, and 70:30 mg. To optimize surfactant concentration, AOT solutions of different concentrations (30, 40, 50, and 60 mM) were prepared in n -dodecane. Subsequently, 10 µL of the CA/Tris aqueous solution was mixed with 1 mL of the AOT solution and sonicated for 30 min to form a reverse micelle system. The resulting micellar solutions were transferred into Teflon tubes and subjected to microwave irradiation for 8 min at a maximum temperature of 300°C and a microwave power of 300 W. To optimize microwave irradiation time and power, a solution containing 70 mg of CA and 30 mg of Tris in 1 mL of water (10 µL aliquot) was mixed with 1 mL of 40 mM AOT in n -dodecane and sonicated for 30 min to form a transparent water-in-oil microemulsion. The micellar solutions were transferred into Teflon tubes and subjected to varying microwave powers or reaction times, with the maximum temperature maintained at 300°C. After the reaction, the products were filtered through a PVDF membrane and purified by dialysis (MWCO 1000 Da) for 2 days. For the hexanoic acid–based MARM approach, n -dodecane was replaced with hexanoic acid while maintaining identical reactants and optimized conditions. To synthesize PEI-functionalized CDs (PEI-CDs), a 10 µL aqueous solution containing 70 mg of CA and 30 mg of PEI in 1 mL of water was mixed with 1 mL of 40 mM AOT in n -dodecane under sonication for 30 min, followed by microwave irradiation under identical conditions. Temperature monitoring The temperature profile of the reverse micelle system during microwave heating was monitored using a Discover focused microwave reactor operating at 300 W for 8 min, with a maximum temperature setting of 300°C. The temperature was measured by inserting a thermometer (Taylor’s Eye Witness) directly into the Teflon tube containing the reaction mixture. Synthesis of carbon dots using conventional hydrothermal, microwave, and solvothermal-assisted reverse micelle approaches The optimization of synthetic conditions for CDs prepared via conventional hydrothermal, domestic microwave-assisted, and solvothermal reverse micelle approaches is described in the Supporting Information (Experimental section and Figures S1 and S2). For the hydrothermal synthesis, 700 mg of CA and 300 mg of Tris were dissolved in 10 mL of deionized water. The resulting solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 180°C for 2 h. After cooling, the reaction mixture was centrifuged at 14,000 rpm for 10 min to remove large particles, and the supernatant was purified by dialysis (MWCO 1000 Da) for 2 days. For domestic microwave synthesis, a beaker containing 700 mg of CA and 300 mg of Tris dissolved in 5 mL of deionized water was placed in a domestic microwave oven (Panasonic NN-SF564, Japan) and irradiated at 1000 W for 3 min. The resulting solution was diluted with 5 mL of water and centrifuged at 14,000 rpm for 10 min prior to dialysis. For the solvothermal reverse micelle approach, 100 µL of an aqueous solution containing 70 mg of CA and 30 mg of Tris in 1 mL of water was added to 10 mL of 40 mM AOT in n -dodecane and sonicated for 30 min to form a homogeneous water-in-oil emulsion. The emulsion was transferred into a Teflon-lined stainless-steel autoclave and heated at 180°C for 4 h. The resulting solutions were filtered through a PVDF membrane and purified by dialysis (MWCO 1000 Da) for 2 days. Quantum yield calculation The quantum yield of CDs was determined using quinine sulfate as a reference standard according to the following equation: Φ x = Φ ST (m x /m ST ) (η 2 x / η 2 ST ) where Φ x and Φ ST are the quantum yields of the sample and standard, respectively; m x and m ST are the slopes of the integrated intensity vs absorption plots for the sample and standard, respectively; and η x and η ST are the refractive indexes of the solvents used for the sample and standard, respectively. Results and discussion Microwave-assisted reverse micelle approach The synthesis procedure for carbon dots is illustrated in Scheme 1 . We developed a rapid method for synthesizing carbon dots with high quantum yield and narrow size distribution using a microwave-assisted reverse micelle approach. In this method, an aqueous solution of citric acid and Tris base was emulsified in a microwave-transparent nonpolar medium, n -dodecane, containing AOT surfactant to form a water-in-oil microemulsion. The reverse micelles containing the aqueous phase were dispersed within the oil phase, where the confined polar droplets acted as nanoreactors that facilitated carbon dot formation. To achieve a high quantum yield, key reaction parameters—including precursor ratio, surfactant concentration, reaction temperature, microwave irradiation time, and microwave power—were systematically optimized. As shown in Fig. 1 a, the quantum yield increased with increasing citric acid–to–Tris weight ratio, reaching a maximum at a precursor ratio of 7:3. In contrast, reactions conducted using either citric acid or Tris base alone resulted in relatively low quantum yields. Figure 1 b shows that optimal quantum yield was obtained when the AOT concentration was in the range of 40–50 mM. Microwave irradiation time played a critical role in determining the quantum yield of the synthesized carbon dots. As illustrated in Fig. 1 c, the quantum yield increased with increasing irradiation time; however, when the reaction time exceeded 8 min, the quantum yield began to decrease. Prolonged microwave irradiation likely led to excessive carbonization, resulting in changes to surface functional groups and a consequent reduction in quantum yield. The quantum yield also increased with increasing microwave power. Because the maximum operational power of the microwave reactor was 300 W, the highest quantum yield was achieved at this power, as shown in Fig. 1 d. Carbon dots were synthesized under optimized conditions using an AOT concentration of 40 mM and microwave irradiation at 300 W for 8 min. For comparison, carbon dots were also synthesized using several conventional methods. The quantum yields of CDs prepared by the different approaches are summarized in Table 1 . The quantum yield obtained using the proposed dodecane-based MARM method (method 5) is comparable to those achieved using the hydrothermal (method 1) and domestic microwave-assisted (method 2) methods. In contrast, the quantum yield is significantly higher than those obtained using the solvothermal reverse micelle approach in AOT/ n -dodecane (method 3) and the hexanoic acid–based MARM method (AOT/hexanoic acid, method 4). Hexanoic acid, used as the reaction medium in method 4, possesses a substantial dipole moment and can absorb microwave radiation, which may influence the heating profile. The pronounced differences between the proposed method and methods 3 and 4 can be primarily attributed to differences in microwave heating behavior. In the dodecane-based MARM approach (method 5), microwave energy selectively heats only the aqueous phase confined within the reverse micelles. In contrast, in methods 3 and 4, both the micellar interior and the surrounding organic solvent are significantly heated during the reaction, leading to the participation of both phases in carbon dot formation and potentially altering the resulting quantum yield. In method 5, the confined aqueous phase within the micelles is selectively heated, closely resembling the heating conditions in hydrothermal and domestic microwave-assisted methods, where only the aqueous phase undergoes effective microwave heating. Table 1 Comparison of the quantum yields (QY) of MARM-CDS with those obtained using conventional synthesis methods. Method Composition Reaction time QY ± RSD % (n = 3) 1 (Hydrothermal) 700 mg CA* and 300 mg Tris (in 10 mL H 2 O) 2 h 95 ± 4 2 (Domestic microwave) 700 mg CA and 300 mg Tris (in 5 mL H 2 O) 3 min 91 ± 7 3 (Solvothermal–reverse micelle) 100 µL CA/Tris (7/3) in 10 mL 40 mM AOT/dodecane 4 h 21 ± 5 4 [Microwave assisted-reverse micelle (hexanoic acid)] 10 µL CA/Tris (7/3) in 1 mL 40 mM AOT/hexanoic acid 8 min 9 ± 3 5 [Microwave assisted-reverse micelle (dodecane)] 10 µL CA/Tris (7/3) in 1 mL 40 mM AOT/dodecane 8 min 96 ± 3 * CA: citric acid To investigate the heating behavior in the microwave-assisted reverse micelle approach, the temperature of the reaction mixture was monitored throughout the reaction. Figure 2 shows the temperature profiles of different solvents and solutions under microwave irradiation. The temperature of n -dodecane reached 45°C after 4 min of microwave exposure. As a nonpolar solvent, dodecane is transparent to microwave radiation and absorbs very little energy [ 22 ]. The slight temperature increase in pure dodecane is attributed to heat transfer from the heated Teflon container. In contrast, the temperature of dodecane containing 40 mM polar AOT surfactant increased to 69°C under microwave irradiation. Hexanoic acid, which contains a polar carboxylic group, can absorb microwave energy via dielectric heating; as shown in Fig. 2 , its temperature increased to 159°C after 8 min of microwave exposure. When hexanoic acid containing 40 mM AOT was mixed with 10 µL of an aqueous CA/Tris solution, the temperature of the resulting water-in-oil micelle solution increased further, reaching a plateau of 210°C within 4 min, close to the boiling point of hexanoic acid (205°C). For dodecane containing AOT mixed with 10 µL of the aqueous CA/Tris solution, the temperature of the water-in-oil micelle solution was significantly higher than that of the dodecane/AOT mixture without water. These results indicate that the aqueous phase is a much more efficient microwave absorber. Increasing the AOT concentration slightly enhanced the heating of the water-in-dodecane micelle solution, likely because more AOT molecules can absorb microwave energy. After 8 min of microwave irradiation, the temperatures of micelle solutions containing 30, 40, and 50 mM AOT reached 177, 183, and 190°C, respectively. These observations suggest that the temperature increase in the dodecane/AOT/H₂O system is predominantly driven by selective heating of the aqueous phase, followed by heat transfer to the surrounding dodecane. For example, the temperature of the dodecane/40 mM AOT/H₂O system gradually increased from 120°C to 183°C over 8 min of microwave heating. In contrast, the hexanoic acid/40 mM AOT/H₂O system reached 201°C within 2 min. The hexanoic acid reverse micelle solution heats more rapidly to high temperatures compared with the dodecane-based system, which likely contributes to the observed differences in quantum yields of CDs synthesized in these two solvent systems. In the dodecane/AOT/H₂O system, microwave energy is primarily absorbed by the confined aqueous phase within the micelles, whereas in the hexanoic acid/AOT/H₂O system, both the micelle interior and the surrounding solvent are simultaneously heated. Characterization of CDs using XPS and FT-IR We employed XPS and FT-IR to investigate the surface functionalization and elemental compositions of the CDs obtained using various approaches. The XPS spectrum shown in Fig. 3 a reveals that the surface of CDs prepared using the dodecane based-MARM approach is mainly composed of C, O, S, and Na, with elemental composition of C 59.4%, O 29.6%, S 5.0%, and Na 6.0%. In Fig. 3 b, the high-resolution S 2p spectrum contains two peaks at 167.9 and 169.1 eV, which are associated with the structures of oxidized sulfur. In Fig. 3 c, the high-resolution spectrum of C 1s is differentiated into four bands: C = C/C-C (283.9 eV), C-S (284.8 eV), C-O (286.0 eV), and HO-C = O (288.9 eV) [ 24 ]. The high-resolution O 1s spectrum displayed in Fig. 3 d shows two peaks at 531.7 and 533.4 eV, which are attributed to O–C = O and C–O, respectively. The data correspond to the structure of surfactant AOT, suggesting that the dodecane-based MARM CDs were coated with AOT. Figure S3 shows the XPS data for CDs prepared using hydrothermal and domestic microwave methods. The elemental compositions were similar for hydrothermal (C 65.0%, N 4.8%, and O 30.2%), and domestic microwave (C 58.6%, N 5.0%, and O 36.4%) samples. The synthesis of these two CDs did not involve AOT and the compositions suggest the formation of N doped-CDs. The XPS data shown in Figure S4 reveal the elemental compositions of CDs prepared by the solvothermal method (C 81.1%, N 1.7%, O 14.6%, S 0.9%, and Na 1.8%) and the decanoic acid–based MARM method (C 63.3%, N 1.8%, O 27.8%, S 4.4%, and Na 2.7%). Both methods involved AOT reverse micelles. The elemental composition suggests that the CDs may be coated with the AOT surfactant. Because AOT does not contain nitrogen atoms, the presence of nitrogen indicates that the surfaces of these two CDs exhibit structures other than the AOT surfactant. In contrast, the dodecane-based MARM CDs do not show any nitrogen content, even though reverse micelles were involved. This difference may be attributable to whether the surfactant and the organic solvent were significantly heated and participated in the CD formation reaction. As shown in Figure S5, hydrothermal- and domestic microwave–synthesized CDs exhibited broad absorption bands at 3405 cm⁻¹, corresponding to O–H and N–H stretching vibrations, indicative of surface hydroxyl and amino groups. Carbonyl (C = O) stretching was observed at 1715 cm⁻¹ for hydrothermal-CDs and 1728 cm⁻¹ for domestic microwave-CDs, while N–H bending/C = C aromatic stretching appeared in the range of 1630–1650 cm⁻¹. Moreover, in microwave-synthesized CDs, the peak at 1540 cm⁻¹ (amide II) arises from N–H bending coupled with C–N stretching. Additional bands at 1400 and 1215 cm⁻¹ were attributed to C–N and C–O stretching vibrations, and the signal at 1060 cm⁻¹ was assigned to C–O–C vibrations. Collectively, these features confirm the formation of functionalized CDs with abundant surface functionalities. The FTIR spectra of the solvothermal CDs and the two types of MARM-CDs closely resemble that of pure AOT (Figure S5). In the AOT spectrum, the peaks at 1053 cm⁻¹ and 1162 cm⁻¹ were assigned to the symmetric and asymmetric stretching vibrations of sulfonate (SO₃⁻) groups. Bands at 1237 cm⁻¹ and 1381 cm⁻¹ were attributed to ester C(= O)–O–C stretching and CH₂ wagging vibrations, respectively. The absorption at 1460 cm⁻¹ corresponds to methylene (CH₂) scissoring, while the peak at 1740 cm⁻¹ indicates C = O stretching of ester groups. Additionally, aliphatic CH₂ and CH₃ stretching vibrations were observed at 2870, 2932, and 2964 cm⁻¹. Overall, the FTIR profiles suggest that hydrothermal and microwave CDs retain surface functional groups derived directly from citric acid and Tris precursors, whereas the MARM and solvothermal methods yield CDs encapsulated within an AOT surfactant environment. TEM images The morphology and size of dodecane-based MARM-CDs were examined using TEM. As shown in Fig. 4 , TEM images and the corresponding size distribution histograms were obtained for CDs prepared at AOT concentrations of 30, 40, and 50 mM. The average particle sizes were approximately 2.55, 2.10, and 1.82 nm, respectively, indicating that the size of MARM-CDs decreases with increasing AOT concentration. This demonstrates that the particle size can be controlled by adjusting the surfactant concentration. AOT forms stable reverse micelles in nonpolar solvents such as n -dodecane, where the size of the aqueous core is primarily determined by the molar water-to-surfactant ratio. At a fixed water content, increasing the AOT concentration decreases the water-to-AOT molar ratio, resulting in smaller micellar water cores and a higher number density of micelles [ 28 ]. Reverse micelles act as nanoreactors, where the size of the confined aqueous pool sets an upper limit for particle growth. The size of carbon dots synthesized in such systems has been shown to correlate with the average micelle size [ 16 ]. Larger micelles produce larger carbon dots, whereas smaller micelles lead to reduced particle sizes. However, the final CD diameter is generally smaller than the micellar water core and is determined primarily by nucleation–growth kinetics, surface passivation, and precursor availability, rather than by strict geometric confinement [ 29 ]. These results are consistent with the established role of AOT reverse micelles as tunable nanoreactors for the controlled synthesis of nanoscale materials. TEM measurements were also performed on carbon dots prepared using hydrothermal, domestic microwave heating, solvothermal reverse micelle, and hexanoic acid–based MARM approaches (Fig. 5 ). The TEM images of hydrothermal CDs show a well-dispersed morphology with an average size of 1.88 ± 0.40 nm (Figs. 5 a and 5 d). In contrast, the TEM images of domestic microwave CDs exhibit a relatively broad size distribution ranging from 1 to 4.5 nm, with an average of 2.10 ± 0.63 nm (Figs. 5 b and 5 e). The results shown in Fig. 5 confirm the successful synthesis of carbon dots across all methods. Variations in particle size among the different CDs may arise from multiple factors, including the chemical nature of the precursors and the specific reaction conditions employed. High-resolution TEM analysis (Figure S6) reveals that all CDs possess a graphitic core structure. For domestic microwave CDs, the observed d-spacing of 0.22 nm corresponds to the (100) plane (in-plane lattice spacing). For CDs prepared via dodecane-based MARM, hydrothermal, solvothermal reverse micelle, and hexanoic acid–based MARM approaches, the observed d-spacings range from 0.28 to 0.32 nm, consistent with the (002) plane (interlayer spacing). Absorption and fluorescence spectroscopy The optical properties of MARM-CDs in n -dodecane were investigated using UV–visible absorption and fluorescence spectroscopy. As shown in Fig. 6 a, the absorption spectrum exhibits a peak at 287 nm, attributed to the π–π* transition of C = C, and a prominent peak at 330 nm, corresponding to the n–π* transition of C = O. MARM-CDs display excitation-independent emission, with a maximum at approximately 415 nm under 340 nm excitation and only a minor red shift of 5 nm at excitation wavelengths longer than 380 nm (Fig. 6 b). Figures 6 c and 6 d compare the normalized fluorescence emission spectra of MARM-CDs prepared at different AOT concentrations and via various synthesis methods, respectively. Increasing the AOT concentration results in a slight red shift in the emission maximum, consistent with the increase in particle size. Such red shifts are commonly observed in carbon dots as particle size increases, primarily due to weakened quantum confinement and the enlargement of sp²-conjugated domains, which reduce the effective band gap [ 30 ]. MARM-CDs prepared using dodecane exhibit optical properties similar to hydrothermal and domestic microwave-synthesized CDs. Despite the presence of a micellar template, the chemical environment is dominated by water and simple precursors, promoting the formation of surface states that emit in the 410–420 nm range. In contrast, hexanoic acid–based MARM-CDs and solvothermal reverse micelle CDs show red-shifted emission with maxima around 450 nm under 340 nm excitation. This shift likely arises from the involvement of micelles and organic solvents during carbonization, which generates multiple surface states distinct from those formed in purely aqueous systems. Notably, AOT does not fluoresce (Figure S7), confirming that the observed differences originate from intrinsic properties of the CDs rather than surface coating effects. Figure 7 further illustrates that hydrothermal and domestic microwave -CDs exhibit excitation-independent emission, whereas solvothermal reverse micelle and hexanoic acid–based MARM-CDs display excitation-dependent emission. The excitation-independent emission at 415 nm across the 300–400 nm excitation range indicates that fluorescence in these CDs originates from a single dominant emissive state. Rapid internal conversion and effective surface passivation contribute to the high quantum yield. Although FTIR analysis reveals a variety of surface functional groups, many moieties, such as –OH and –NH₂, primarily act as passivating groups and do not introduce radiative electronic states. Efficient energy relaxation funnels excited carriers to the lowest-energy emissive state, likely associated with C = O-related surface states, resulting in excitation-wavelength-independent emission. In contrast, CDs emitting at 450 nm might exhibit more heterogeneous and highly oxidized surface structures, which introduce multiple surface trap states and lead to excitation-dependent emission [ 31 ]. Furthermore, the microwave-assisted reverse micelle method was applied to synthesize carbon dots using polyethyleneimine and citric acid as precursors. The optical properties of CDs prepared using methods 1–5 were compared (Figure S8). Dodecane-based MARM, hydrothermal, and domestic microwave CDs exhibited excitation-independent emission, whereas solvothermal reverse micelle and hexanoic acid–based MARM-CDs displayed excitation-dependent emission. These trends are consistent with those observed for citric acid/Tris CDs synthesized via the same methods, indicating that the observed excitation behaviour is robust across different precursor systems. Conclusion We have developed a rapid microwave-assisted reverse micelle (MARM) approach for the synthesis of carbon dots. The selected nonpolar solvent does not absorb microwave energy, allowing efficient and selective heating within the reverse micelle system. Carbon dots produced using this method exhibit characteristics similar to those obtained via hydrothermal and domestic microwave-assisted approaches. However, the MARM method is faster than the hydrothermal process and can produce more homogeneous CDs compared with the domestic microwave method. Furthermore, by adjusting the concentration of the surfactant AOT, the size of the carbon dots can be precisely controlled. 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Nanoscale 13:16662–16671 Scheme Scheme 1 is available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files CDsReverseMicellesubmitsupportingInfo.docx Scheme1.docx Cite Share Download PDF Status: Under Review Version 1 posted Reviewers invited by journal 27 Apr, 2026 Editor assigned by journal 28 Feb, 2026 Submission checks completed at journal 25 Feb, 2026 First submitted to journal 25 Feb, 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-8963582","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":630500888,"identity":"2028976b-e269-4e6b-a8d0-db30947fbd2f","order_by":0,"name":"Srikrishna Tummala","email":"","orcid":"","institution":"National Dong Hwa University","correspondingAuthor":false,"prefix":"","firstName":"Srikrishna","middleName":"","lastName":"Tummala","suffix":""},{"id":630500890,"identity":"1264a09b-5fd8-4b29-8cbb-654e4e90c6d8","order_by":1,"name":"Yen-Peng Ho","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA10lEQVRIiWNgGAWjYFACHgaGDxCWGYhgbCBGC+MMVC3MhLUw85CkxeB278HHNr+22TNIN297zMNgI7vhAP8xCbxa7pxLNs7tu53YIHOs3JiHIc14wwFmNvxabuSYSef23E5gkAAyeBgOJ4K03CCoxbLntj1Uy38itTD8uM3YANFygLAWyTtnjA17G24ntkmklUnOMUg2nnmY2fwHPi18t3sMH/z4c9ueXyJ5m8SbCjvZvuONjw3waVEAuYGxjYGBDeJOICYUk/LgmP9DQNUoGAWjYBSMbAAAuyRJEnTo9pgAAAAASUVORK5CYII=","orcid":"","institution":"National Dong Hwa University","correspondingAuthor":true,"prefix":"","firstName":"Yen-Peng","middleName":"","lastName":"Ho","suffix":""}],"badges":[],"createdAt":"2026-02-25 05:54:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8963582/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8963582/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108601078,"identity":"9e79d3e1-2da9-4b6f-866b-00b745dad8d6","added_by":"auto","created_at":"2026-05-06 11:28:04","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":165859,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOptimization of MARM-CD quantum yield as a function of: \u003c/strong\u003e(a) citric acid (CA)–to–Tris base ratio, (b) AOT concentration, (c) reaction temperature, (d) reaction time, and (e) microwave power.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/53c7d5f3b5c0bfa859b8f5b6.png"},{"id":108601079,"identity":"bdae7fef-d30f-4c2f-a06b-d6f5c25f3d35","added_by":"auto","created_at":"2026-05-06 11:28:04","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":173073,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTemperature profiles of dodecane, hexanoic acid, and water-in-oil micelle solutions under 300 W microwave irradiation.\u003c/strong\u003e The 10 µL of aqueous solution comprised 0.7 mg citric acid and 0.3 mg Tris base.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/cc30e658820cc86a0c88546f.png"},{"id":108601030,"identity":"3aacecf5-cdcd-45c9-95c6-5be1145518a7","added_by":"auto","created_at":"2026-05-06 11:27:59","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":243307,"visible":true,"origin":"","legend":"\u003cp\u003eXPS spectra of dodecane-based MARM-CDs: a) full-scan, b) S\u003csub\u003e2p\u003c/sub\u003e spectra, c) C\u003csub\u003e1s\u003c/sub\u003e spectra, and d) O\u003csub\u003e1s\u003c/sub\u003e spectra.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/14cb01a38b593aaaddce71bc.png"},{"id":108601099,"identity":"bd256fea-2613-4353-b5d4-c05fd35ca505","added_by":"auto","created_at":"2026-05-06 11:28:17","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":491162,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTEM images and size distribution histograms of MARM-CDs prepared at varying AOT concentrations. \u003c/strong\u003eTEM images of CDs obtained at AOT concentrations of (a) 30 mM, (b) 40 mM, and (c) 50 mM. Corresponding size distribution histograms are shown for (d) 30 mM (2.55 ± 0.46 nm, n = 33), (e) 40 mM (2.10 ± 0.41 nm, n = 48), and (f) 50 mM (1.82 ± 0.30 nm, n = 64).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/4efc42278ff30c83ee104e0f.png"},{"id":108600959,"identity":"edf0648a-e5a4-4173-9890-291a9a1768fd","added_by":"auto","created_at":"2026-05-06 11:27:50","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":470681,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTEM images and size distribution histograms of CDs synthesized via different methods. \u003c/strong\u003eTEM images of CDs obtained from (a) hydrothermal, (b) domestic microwave, (c) solvothermal reverse micelle, and (d) hexanoic acid–based MARM approaches. Corresponding size distribution histograms are shown for (e) hydrothermal (1.88 ± 0.40 nm, n = 87), (f) domestic microwave (2.10 ± 0.63 nm, n = 51), (g) solvothermal (2.37 ± 0.44 nm, n = 65), and (h) hexanoic acid–based MARM (1.89 ± 0.40 nm, n = 65).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/dca52da85a7b5d1a5f3aadaa.png"},{"id":108805030,"identity":"6a19c77a-958a-4b9f-8357-08b5a85dc333","added_by":"auto","created_at":"2026-05-08 15:24:34","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":228063,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOptical characterization of MARM-CDs prepared in dodecane.\u003c/strong\u003e (a) UV–Vis absorption spectrum and (b) fluorescence spectra at various excitation wavelengths. Comparison of normalized fluorescence emission spectra (excitation at 340 nm) of MARM-CDs prepared (c) at different AOT concentrations and (d) using different synthesis methods.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/049fd9664b495e928a947154.png"},{"id":108600960,"identity":"09d0a7e2-39a9-4cbd-b864-846e8ff43e30","added_by":"auto","created_at":"2026-05-06 11:27:50","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":285349,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFluorescence spectra of carbon dots synthesized from citric acid and Tris using different methods. \u003c/strong\u003eCDs were prepared via (a) hydrothermal, (b) domestic microwave-assisted, (c) solvothermal reverse micelle in dodecane, and (d) microwave-assisted reverse micelle in hexanoic acid.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/8f68bff63ecab2fa95155f99.png"},{"id":108809417,"identity":"a6ecb014-fd80-452d-8ca1-7e15c62b38e9","added_by":"auto","created_at":"2026-05-08 15:52:57","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1872157,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/e943bdc9-d148-4cb1-987e-850394c6f38b.pdf"},{"id":108601100,"identity":"e8f50ee1-9085-454e-a0ab-e66f51f3e042","added_by":"auto","created_at":"2026-05-06 11:28:18","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":23621504,"visible":true,"origin":"","legend":"","description":"","filename":"CDsReverseMicellesubmitsupportingInfo.docx","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/22823d2681f82418c83d968b.docx"},{"id":108600957,"identity":"fd6b1999-94a0-42b1-8c48-b5b9167429b5","added_by":"auto","created_at":"2026-05-06 11:27:49","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":1071716,"visible":true,"origin":"","legend":"","description":"","filename":"Scheme1.docx","url":"https://assets-eu.researchsquare.com/files/rs-8963582/v1/b9bac6060a8d33dd1ca9f233.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Synthesis of carbon dots via selective microwave heating of reverse micelles in a microwave-transparent solvent","fulltext":[{"header":"Introduction","content":"\u003cp\u003eCarbon quantum dots (CDs) have garnered significant attention in recent years across various fields owing to their exceptional physical, chemical, and optical properties. Their unique fluorescence characteristics and nontoxic nature have made CDs a preferred alternative to conventional quantum dots [1-3].\u0026nbsp;CDs are promising\u0026nbsp;substitutes for\u0026nbsp;organic fluorescent dyes [4] and metal-based quantum dots in applications such as fluorescence sensing [5], bio-imaging [6] drug delivery [7], catalysis [8], and optoelectronic devices [9]. In this context, most research has focused on biological applications, particularly bioimaging and biosensing. Bioimaging applications, in particular, require precise tuning of optical and surface properties, which depends on careful control of particle shape and size distribution [10].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eStrategies for synthesizing carbon dots are broadly categorized into top-down and bottom-up approaches [11, 12]. The bottom-up approach involves the polymerization and carbonization of small organic molecules using methods such as hydrothermal\u0026nbsp;[13], solvothermal [14], microwave-assisted [15], reverse micelles [16], and pyrolysis [17] techniques. \u0026nbsp;These synthesis methods have been continuously improved and tailored to tune the optical, physical, and chemical properties of carbon dots[18]. Several strategies have been reported to control particle size and modify the surface chemistry of CDs [10]. Rhee et al. introduced a synthetic approach using reverse micelles as nanoreactors to produce highly luminescent graphene quantum dots. The process involved the carbonization of glucose within reverse micelles, followed by in-situ surface passivation, offering advantages such as size tunability and narrow size distribution without the need for impractical post-synthesis size separation processes [16]. Zhang et al. reported the formation of uniform CDs by loading glucose into metal\u0026ndash;organic frameworks followed by carbonization [19]. Bishnu et al. demonstrated that encapsulating luminescent graphene quantum dots without a capping agent within zeolitic imidazolate framework (ZIF-8) nanocrystals results in well-confined and ordered dispersion[20]. \u0026nbsp;This encapsulation significantly influences the growth, shape, and size of the ZIF-8 nanocrystals.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eMicrowave-assisted synthesis of CDs offers several advantages, including significantly accelerated reaction rates, reduced energy consumption, and increased yields [21]. Microwave heating efficiency depends on factors such as the dielectric constant, dipole moment, dielectric loss, and dielectric relaxation time of the solvent. The dielectric constant reflects a solvent\u0026rsquo;s ability to store electric charge, and molecules with large dipole moments typically exhibit high dielectric constants. Polar molecules can readily align with a rapidly oscillating microwave field, leading to enhanced polarization and heating efficiency. Overall, the efficiency of microwave heating is governed by the loss tangent, defined as the ratio of dielectric loss to dielectric constant. A high loss tangent corresponds to efficient microwave absorption. Solvents with high loss tangents, such as ethylene glycol and water, absorb microwave energy efficiently, whereas solvents such as toluene and \u003cem\u003en\u003c/em\u003e-hexane, which have low loss tangents, do not\u0026nbsp;[22].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHere, we present a novel approach for the synthesis of carbon dots using a microwave-assisted reverse micelle (MARM) method. Reverse micelles, also known as water-in-oil microemulsions, were formed using bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT), a widely used surfactant for reverse micelle formation in nonpolar media. In this work, dodecane was selected as the oil phase because it does not absorb microwave radiation under the experimental conditions. Reverse micelles containing an aqueous solution of reactants were dispersed in the oil phase, where the confined polar droplets acted as nanoreactors. Under microwave irradiation, only these microreactors absorbed microwave energy and were heated efficiently and homogeneously, thereby facilitating carbon dot formation. The micelle size was tuned by adjusting the water-to-surfactant molar ratio [23], and the concentration of AOT was used to control the size of the resulting CDs. The size distribution and optical properties of the synthesized CDs were systematically investigated and compared with those obtained using conventional synthesis methods.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eKeywords:\u003c/strong\u003e reverse micelle, carbon dot, bis(2-ethylhexyl) sulfosuccinate sodium salt, quantum yield, microwave.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterials and instruments\u003c/h2\u003e \u003cp\u003eCitric acid (CA, anhydrous) was purchased from J. T. Baker (NJ, USA). Bis(2-ethylhexyl) sulfosuccinate sodium salt and polyethyleneimine (PEI, MW\u0026thinsp;\u0026asymp;\u0026thinsp;600) were obtained from Sigma-Aldrich (MA, USA). Tris(hydroxymethyl)aminomethane (Tris) was purchased from MD Bio, Inc. Ultrapure water (18.2 MΩ\u0026middot;cm) was produced using a Milli-Q water purification system (Millipore, MD, USA). All chemicals were of analytical grade and used as received.\u003c/p\u003e \u003cp\u003eUV\u0026ndash;visible absorption and fluorescence spectra were recorded using a SpectraMax\u0026reg; ID3 multimode microplate reader (Molecular Devices, CA, USA). High-resolution transmission electron microscopy (HR-TEM) images were obtained using a JEM-2001F microscope (JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI 500 VersaProbe system (ULVAC-PHI, Japan). Quantum yield measurements were carried out using a fluorescence spectrometer (FLS920, Edinburgh Instruments, USA). Fourier-transform infrared (FT-IR) spectra were recorded on a Spectrum One spectrometer (PerkinElmer, MA, USA).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSynthesis of CDs using the microwave-assisted reverse micelle method\u003c/h3\u003e\n\u003cp\u003eCarbon dots synthesized via the microwave-assisted reverse micelle method are denoted as MARM-CDs. Reverse micelle reactions were conducted under microwave irradiation (2.45 GHz) using a Discover focused microwave reactor (CEM, NC, USA). To optimize precursor composition, various weight ratios of CA and Tris dissolved in 1 mL of water were investigated. The CA:Tris ratios examined were 0:100 mg, 100:0 mg, 30:70 mg, 50:50 mg, and 70:30 mg. To optimize surfactant concentration, AOT solutions of different concentrations (30, 40, 50, and 60 mM) were prepared in \u003cem\u003en\u003c/em\u003e-dodecane. Subsequently, 10 \u0026micro;L of the CA/Tris aqueous solution was mixed with 1 mL of the AOT solution and sonicated for 30 min to form a reverse micelle system. The resulting micellar solutions were transferred into Teflon tubes and subjected to microwave irradiation for 8 min at a maximum temperature of 300\u0026deg;C and a microwave power of 300 W.\u003c/p\u003e \u003cp\u003eTo optimize microwave irradiation time and power, a solution containing 70 mg of CA and 30 mg of Tris in 1 mL of water (10 \u0026micro;L aliquot) was mixed with 1 mL of 40 mM AOT in \u003cem\u003en\u003c/em\u003e-dodecane and sonicated for 30 min to form a transparent water-in-oil microemulsion. The micellar solutions were transferred into Teflon tubes and subjected to varying microwave powers or reaction times, with the maximum temperature maintained at 300\u0026deg;C. After the reaction, the products were filtered through a PVDF membrane and purified by dialysis (MWCO 1000 Da) for 2 days.\u003c/p\u003e \u003cp\u003eFor the hexanoic acid\u0026ndash;based MARM approach, \u003cem\u003en\u003c/em\u003e-dodecane was replaced with hexanoic acid while maintaining identical reactants and optimized conditions. To synthesize PEI-functionalized CDs (PEI-CDs), a 10 \u0026micro;L aqueous solution containing 70 mg of CA and 30 mg of PEI in 1 mL of water was mixed with 1 mL of 40 mM AOT in \u003cem\u003en\u003c/em\u003e-dodecane under sonication for 30 min, followed by microwave irradiation under identical conditions.\u003c/p\u003e\n\u003ch3\u003eTemperature monitoring\u003c/h3\u003e\n\u003cp\u003eThe temperature profile of the reverse micelle system during microwave heating was monitored using a Discover focused microwave reactor operating at 300 W for 8 min, with a maximum temperature setting of 300\u0026deg;C. The temperature was measured by inserting a thermometer (Taylor\u0026rsquo;s Eye Witness) directly into the Teflon tube containing the reaction mixture.\u003c/p\u003e\n\u003ch3\u003eSynthesis of carbon dots using conventional hydrothermal, microwave, and solvothermal-assisted reverse micelle approaches\u003c/h3\u003e\n\u003cp\u003eThe optimization of synthetic conditions for CDs prepared via conventional hydrothermal, domestic microwave-assisted, and solvothermal reverse micelle approaches is described in the Supporting Information (Experimental section and Figures \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and S2).\u003c/p\u003e \u003cp\u003eFor the hydrothermal synthesis, 700 mg of CA and 300 mg of Tris were dissolved in 10 mL of deionized water. The resulting solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 180\u0026deg;C for 2 h. After cooling, the reaction mixture was centrifuged at 14,000 rpm for 10 min to remove large particles, and the supernatant was purified by dialysis (MWCO 1000 Da) for 2 days.\u003c/p\u003e \u003cp\u003eFor domestic microwave synthesis, a beaker containing 700 mg of CA and 300 mg of Tris dissolved in 5 mL of deionized water was placed in a domestic microwave oven (Panasonic NN-SF564, Japan) and irradiated at 1000 W for 3 min. The resulting solution was diluted with 5 mL of water and centrifuged at 14,000 rpm for 10 min prior to dialysis.\u003c/p\u003e \u003cp\u003eFor the solvothermal reverse micelle approach, 100 \u0026micro;L of an aqueous solution containing 70 mg of CA and 30 mg of Tris in 1 mL of water was added to 10 mL of 40 mM AOT in \u003cem\u003en\u003c/em\u003e-dodecane and sonicated for 30 min to form a homogeneous water-in-oil emulsion. The emulsion was transferred into a Teflon-lined stainless-steel autoclave and heated at 180\u0026deg;C for 4 h. The resulting solutions were filtered through a PVDF membrane and purified by dialysis (MWCO 1000 Da) for 2 days.\u003c/p\u003e\n\u003ch3\u003eQuantum yield calculation\u003c/h3\u003e\n\u003cp\u003eThe quantum yield of CDs was determined using quinine sulfate as a reference standard according to the following equation:\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eΦ\u003csub\u003ex\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;Φ\u003csub\u003eST\u003c/sub\u003e (m\u003csub\u003ex\u003c/sub\u003e/m\u003csub\u003eST\u003c/sub\u003e) (η\u003csup\u003e2\u003c/sup\u003e\u003csub\u003ex\u003c/sub\u003e/ η\u003csup\u003e2\u003c/sup\u003e\u003csub\u003eST\u003c/sub\u003e)\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere Φ\u003csub\u003ex\u003c/sub\u003e and Φ\u003csub\u003eST\u003c/sub\u003e are the quantum yields of the sample and standard, respectively; m\u003csub\u003ex\u003c/sub\u003e and m\u003csub\u003eST\u003c/sub\u003e are the slopes of the integrated intensity vs absorption plots for the sample and standard, respectively; and η\u003csub\u003ex\u003c/sub\u003e and η\u003csub\u003eST\u003c/sub\u003e are the refractive indexes of the solvents used for the sample and standard, respectively.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eMicrowave-assisted reverse micelle approach\u003c/h2\u003e \u003cp\u003eThe synthesis procedure for carbon dots is illustrated in Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. We developed a rapid method for synthesizing carbon dots with high quantum yield and narrow size distribution using a microwave-assisted reverse micelle approach. In this method, an aqueous solution of citric acid and Tris base was emulsified in a microwave-transparent nonpolar medium, \u003cem\u003en\u003c/em\u003e-dodecane, containing AOT surfactant to form a water-in-oil microemulsion. The reverse micelles containing the aqueous phase were dispersed within the oil phase, where the confined polar droplets acted as nanoreactors that facilitated carbon dot formation. To achieve a high quantum yield, key reaction parameters\u0026mdash;including precursor ratio, surfactant concentration, reaction temperature, microwave irradiation time, and microwave power\u0026mdash;were systematically optimized. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, the quantum yield increased with increasing citric acid\u0026ndash;to\u0026ndash;Tris weight ratio, reaching a maximum at a precursor ratio of 7:3. In contrast, reactions conducted using either citric acid or Tris base alone resulted in relatively low quantum yields. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003eb shows that optimal quantum yield was obtained when the AOT concentration was in the range of 40\u0026ndash;50 mM.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMicrowave irradiation time played a critical role in determining the quantum yield of the synthesized carbon dots. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003ec, the quantum yield increased with increasing irradiation time; however, when the reaction time exceeded 8 min, the quantum yield began to decrease. Prolonged microwave irradiation likely led to excessive carbonization, resulting in changes to surface functional groups and a consequent reduction in quantum yield. The quantum yield also increased with increasing microwave power. Because the maximum operational power of the microwave reactor was 300 W, the highest quantum yield was achieved at this power, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003ed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCarbon dots were synthesized under optimized conditions using an AOT concentration of 40 mM and microwave irradiation at 300 W for 8 min. For comparison, carbon dots were also synthesized using several conventional methods. The quantum yields of CDs prepared by the different approaches are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The quantum yield obtained using the proposed dodecane-based MARM method (method 5) is comparable to those achieved using the hydrothermal (method 1) and domestic microwave-assisted (method 2) methods. In contrast, the quantum yield is significantly higher than those obtained using the solvothermal reverse micelle approach in AOT/\u003cem\u003en\u003c/em\u003e-dodecane (method 3) and the hexanoic acid\u0026ndash;based MARM method (AOT/hexanoic acid, method 4). Hexanoic acid, used as the reaction medium in method 4, possesses a substantial dipole moment and can absorb microwave radiation, which may influence the heating profile.\u003c/p\u003e \u003cp\u003eThe pronounced differences between the proposed method and methods 3 and 4 can be primarily attributed to differences in microwave heating behavior. In the dodecane-based MARM approach (method 5), microwave energy selectively heats only the aqueous phase confined within the reverse micelles. In contrast, in methods 3 and 4, both the micellar interior and the surrounding organic solvent are significantly heated during the reaction, leading to the participation of both phases in carbon dot formation and potentially altering the resulting quantum yield. In method 5, the confined aqueous phase within the micelles is selectively heated, closely resembling the heating conditions in hydrothermal and domestic microwave-assisted methods, where only the aqueous phase undergoes effective microwave heating.\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\u003eComparison of the quantum yields (QY) of MARM-CDS with those obtained using conventional synthesis methods.\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=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMethod\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eComposition\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eReaction time\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eQY \u0026plusmn; RSD % (n\u0026thinsp;=\u0026thinsp;3)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003e1\u003c/b\u003e (Hydrothermal)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e700 mg CA* and 300 mg Tris (in 10 mL H\u003csub\u003e2\u003c/sub\u003eO)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2 h\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e95\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003e2\u003c/b\u003e (Domestic microwave)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e700 mg CA and 300 mg Tris (in 5 mL H\u003csub\u003e2\u003c/sub\u003eO)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3 min\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e91\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003e3\u003c/b\u003e (Solvothermal\u0026ndash;reverse micelle)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e100 \u0026micro;L CA/Tris (7/3) in 10 mL 40 mM AOT/dodecane\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4 h\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e21\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003e4\u003c/b\u003e [Microwave assisted-reverse micelle (hexanoic acid)]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10 \u0026micro;L CA/Tris (7/3) in 1 mL 40 mM AOT/hexanoic acid\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8 min\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e9\u0026thinsp;\u0026plusmn;\u0026thinsp;3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003e5\u003c/b\u003e [Microwave assisted-reverse micelle (dodecane)]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10 \u0026micro;L CA/Tris (7/3) in 1 mL 40 mM AOT/dodecane\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8 min\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e96\u0026thinsp;\u0026plusmn;\u0026thinsp;3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003e*\u003c/b\u003eCA: citric acid\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTo investigate the heating behavior in the microwave-assisted reverse micelle approach, the temperature of the reaction mixture was monitored throughout the reaction. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the temperature profiles of different solvents and solutions under microwave irradiation. The temperature of \u003cem\u003en\u003c/em\u003e-dodecane reached 45\u0026deg;C after 4 min of microwave exposure. As a nonpolar solvent, dodecane is transparent to microwave radiation and absorbs very little energy [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The slight temperature increase in pure dodecane is attributed to heat transfer from the heated Teflon container. In contrast, the temperature of dodecane containing 40 mM polar AOT surfactant increased to 69\u0026deg;C under microwave irradiation. Hexanoic acid, which contains a polar carboxylic group, can absorb microwave energy via dielectric heating; as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e2\u003c/span\u003e, its temperature increased to 159\u0026deg;C after 8 min of microwave exposure. When hexanoic acid containing 40 mM AOT was mixed with 10 \u0026micro;L of an aqueous CA/Tris solution, the temperature of the resulting water-in-oil micelle solution increased further, reaching a plateau of 210\u0026deg;C within 4 min, close to the boiling point of hexanoic acid (205\u0026deg;C).\u003c/p\u003e \u003cp\u003eFor dodecane containing AOT mixed with 10 \u0026micro;L of the aqueous CA/Tris solution, the temperature of the water-in-oil micelle solution was significantly higher than that of the dodecane/AOT mixture without water. These results indicate that the aqueous phase is a much more efficient microwave absorber. Increasing the AOT concentration slightly enhanced the heating of the water-in-dodecane micelle solution, likely because more AOT molecules can absorb microwave energy. After 8 min of microwave irradiation, the temperatures of micelle solutions containing 30, 40, and 50 mM AOT reached 177, 183, and 190\u0026deg;C, respectively.\u003c/p\u003e \u003cp\u003eThese observations suggest that the temperature increase in the dodecane/AOT/H₂O system is predominantly driven by selective heating of the aqueous phase, followed by heat transfer to the surrounding dodecane. For example, the temperature of the dodecane/40 mM AOT/H₂O system gradually increased from 120\u0026deg;C to 183\u0026deg;C over 8 min of microwave heating. In contrast, the hexanoic acid/40 mM AOT/H₂O system reached 201\u0026deg;C within 2 min. The hexanoic acid reverse micelle solution heats more rapidly to high temperatures compared with the dodecane-based system, which likely contributes to the observed differences in quantum yields of CDs synthesized in these two solvent systems. In the dodecane/AOT/H₂O system, microwave energy is primarily absorbed by the confined aqueous phase within the micelles, whereas in the hexanoic acid/AOT/H₂O system, both the micelle interior and the surrounding solvent are simultaneously heated.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCharacterization of CDs using XPS and FT-IR\u003c/h3\u003e\n\u003cp\u003eWe employed XPS and FT-IR to investigate the surface functionalization and elemental compositions of the CDs obtained using various approaches. The XPS spectrum shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003ea reveals that the surface of CDs prepared using the dodecane based-MARM approach is mainly composed of C, O, S, and Na, with elemental composition of C 59.4%, O 29.6%, S 5.0%, and Na 6.0%. In Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, the high-resolution S\u003csub\u003e2p\u003c/sub\u003e spectrum contains two peaks at 167.9 and 169.1 eV, which are associated with the structures of oxidized sulfur. In Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, the high-resolution spectrum of C\u003csub\u003e1s\u003c/sub\u003e is differentiated into four bands: C\u0026thinsp;=\u0026thinsp;C/C-C (283.9 eV), C-S (284.8 eV), C-O (286.0 eV), and HO-C\u0026thinsp;=\u0026thinsp;O (288.9 eV) [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. The high-resolution O\u003csub\u003e1s\u003c/sub\u003e spectrum displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003ed shows two peaks at 531.7 and 533.4 eV, which are attributed to O\u0026ndash;C\u0026thinsp;=\u0026thinsp;O and C\u0026ndash;O, respectively. The data correspond to the structure of surfactant AOT, suggesting that the dodecane-based MARM CDs were coated with AOT. Figure S3 shows the XPS data for CDs prepared using hydrothermal and domestic microwave methods. The elemental compositions were similar for hydrothermal (C 65.0%, N 4.8%, and O 30.2%), and domestic microwave (C 58.6%, N 5.0%, and O 36.4%) samples. The synthesis of these two CDs did not involve AOT and the compositions suggest the formation of N doped-CDs. The XPS data shown in Figure S4 reveal the elemental compositions of CDs prepared by the solvothermal method (C 81.1%, N 1.7%, O 14.6%, S 0.9%, and Na 1.8%) and the decanoic acid\u0026ndash;based MARM method (C 63.3%, N 1.8%, O 27.8%, S 4.4%, and Na 2.7%). Both methods involved AOT reverse micelles. The elemental composition suggests that the CDs may be coated with the AOT surfactant. Because AOT does not contain nitrogen atoms, the presence of nitrogen indicates that the surfaces of these two CDs exhibit structures other than the AOT surfactant. In contrast, the dodecane-based MARM CDs do not show any nitrogen content, even though reverse micelles were involved. This difference may be attributable to whether the surfactant and the organic solvent were significantly heated and participated in the CD formation reaction.\u003c/p\u003e \u003cp\u003eAs shown in Figure S5, hydrothermal- and domestic microwave\u0026ndash;synthesized CDs exhibited broad absorption bands at 3405 cm⁻\u0026sup1;, corresponding to O\u0026ndash;H and N\u0026ndash;H stretching vibrations, indicative of surface hydroxyl and amino groups. Carbonyl (C\u0026thinsp;=\u0026thinsp;O) stretching was observed at 1715 cm⁻\u0026sup1; for hydrothermal-CDs and 1728 cm⁻\u0026sup1; for domestic microwave-CDs, while N\u0026ndash;H bending/C\u0026thinsp;=\u0026thinsp;C aromatic stretching appeared in the range of 1630\u0026ndash;1650 cm⁻\u0026sup1;. Moreover, in microwave-synthesized CDs, the peak at 1540 cm⁻\u0026sup1; (amide II) arises from N\u0026ndash;H bending coupled with C\u0026ndash;N stretching. Additional bands at 1400 and 1215 cm⁻\u0026sup1; were attributed to C\u0026ndash;N and C\u0026ndash;O stretching vibrations, and the signal at 1060 cm⁻\u0026sup1; was assigned to C\u0026ndash;O\u0026ndash;C vibrations. Collectively, these features confirm the formation of functionalized CDs with abundant surface functionalities.\u003c/p\u003e \u003cp\u003eThe FTIR spectra of the solvothermal CDs and the two types of MARM-CDs closely resemble that of pure AOT (Figure S5). In the AOT spectrum, the peaks at 1053 cm⁻\u0026sup1; and 1162 cm⁻\u0026sup1; were assigned to the symmetric and asymmetric stretching vibrations of sulfonate (SO₃⁻) groups. Bands at 1237 cm⁻\u0026sup1; and 1381 cm⁻\u0026sup1; were attributed to ester C(=\u0026thinsp;O)\u0026ndash;O\u0026ndash;C stretching and CH₂ wagging vibrations, respectively. The absorption at 1460 cm⁻\u0026sup1; corresponds to methylene (CH₂) scissoring, while the peak at 1740 cm⁻\u0026sup1; indicates C\u0026thinsp;=\u0026thinsp;O stretching of ester groups. Additionally, aliphatic CH₂ and CH₃ stretching vibrations were observed at 2870, 2932, and 2964 cm⁻\u0026sup1;. Overall, the FTIR profiles suggest that hydrothermal and microwave CDs retain surface functional groups derived directly from citric acid and Tris precursors, whereas the MARM and solvothermal methods yield CDs encapsulated within an AOT surfactant environment.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eTEM images\u003c/h2\u003e \u003cp\u003eThe morphology and size of dodecane-based MARM-CDs were examined using TEM. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003e, TEM images and the corresponding size distribution histograms were obtained for CDs prepared at AOT concentrations of 30, 40, and 50 mM. The average particle sizes were approximately 2.55, 2.10, and 1.82 nm, respectively, indicating that the size of MARM-CDs decreases with increasing AOT concentration. This demonstrates that the particle size can be controlled by adjusting the surfactant concentration. AOT forms stable reverse micelles in nonpolar solvents such as \u003cem\u003en\u003c/em\u003e-dodecane, where the size of the aqueous core is primarily determined by the molar water-to-surfactant ratio. At a fixed water content, increasing the AOT concentration decreases the water-to-AOT molar ratio, resulting in smaller micellar water cores and a higher number density of micelles [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Reverse micelles act as nanoreactors, where the size of the confined aqueous pool sets an upper limit for particle growth. The size of carbon dots synthesized in such systems has been shown to correlate with the average micelle size [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Larger micelles produce larger carbon dots, whereas smaller micelles lead to reduced particle sizes. However, the final CD diameter is generally smaller than the micellar water core and is determined primarily by nucleation\u0026ndash;growth kinetics, surface passivation, and precursor availability, rather than by strict geometric confinement [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. These results are consistent with the established role of AOT reverse micelles as tunable nanoreactors for the controlled synthesis of nanoscale materials.\u003c/p\u003e \u003cp\u003eTEM measurements were also performed on carbon dots prepared using hydrothermal, domestic microwave heating, solvothermal reverse micelle, and hexanoic acid\u0026ndash;based MARM approaches (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The TEM images of hydrothermal CDs show a well-dispersed morphology with an average size of 1.88\u0026thinsp;\u0026plusmn;\u0026thinsp;0.40 nm (Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). In contrast, the TEM images of domestic microwave CDs exhibit a relatively broad size distribution ranging from 1 to 4.5 nm, with an average of 2.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.63 nm (Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003ee). The results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003e confirm the successful synthesis of carbon dots across all methods. Variations in particle size among the different CDs may arise from multiple factors, including the chemical nature of the precursors and the specific reaction conditions employed. High-resolution TEM analysis (Figure S6) reveals that all CDs possess a graphitic core structure. For domestic microwave CDs, the observed d-spacing of 0.22 nm corresponds to the (100) plane (in-plane lattice spacing). For CDs prepared via dodecane-based MARM, hydrothermal, solvothermal reverse micelle, and hexanoic acid\u0026ndash;based MARM approaches, the observed d-spacings range from 0.28 to 0.32 nm, consistent with the (002) plane (interlayer spacing).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eAbsorption and fluorescence spectroscopy\u003c/h2\u003e \u003cp\u003eThe optical properties of MARM-CDs in \u003cem\u003en\u003c/em\u003e-dodecane were investigated using UV\u0026ndash;visible absorption and fluorescence spectroscopy. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, the absorption spectrum exhibits a peak at 287 nm, attributed to the π\u0026ndash;π* transition of C\u0026thinsp;=\u0026thinsp;C, and a prominent peak at 330 nm, corresponding to the n\u0026ndash;π* transition of C\u0026thinsp;=\u0026thinsp;O. MARM-CDs display excitation-independent emission, with a maximum at approximately 415 nm under 340 nm excitation and only a minor red shift of 5 nm at excitation wavelengths longer than 380 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003ed compare the normalized fluorescence emission spectra of MARM-CDs prepared at different AOT concentrations and via various synthesis methods, respectively. Increasing the AOT concentration results in a slight red shift in the emission maximum, consistent with the increase in particle size. Such red shifts are commonly observed in carbon dots as particle size increases, primarily due to weakened quantum confinement and the enlargement of sp\u0026sup2;-conjugated domains, which reduce the effective band gap [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. MARM-CDs prepared using dodecane exhibit optical properties similar to hydrothermal and domestic microwave-synthesized CDs. Despite the presence of a micellar template, the chemical environment is dominated by water and simple precursors, promoting the formation of surface states that emit in the 410\u0026ndash;420 nm range. In contrast, hexanoic acid\u0026ndash;based MARM-CDs and solvothermal reverse micelle CDs show red-shifted emission with maxima around 450 nm under 340 nm excitation. This shift likely arises from the involvement of micelles and organic solvents during carbonization, which generates multiple surface states distinct from those formed in purely aqueous systems. Notably, AOT does not fluoresce (Figure S7), confirming that the observed differences originate from intrinsic properties of the CDs rather than surface coating effects.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e7\u003c/span\u003e further illustrates that hydrothermal and domestic microwave -CDs exhibit excitation-independent emission, whereas solvothermal reverse micelle and hexanoic acid\u0026ndash;based MARM-CDs display excitation-dependent emission. The excitation-independent emission at 415 nm across the 300\u0026ndash;400 nm excitation range indicates that fluorescence in these CDs originates from a single dominant emissive state. Rapid internal conversion and effective surface passivation contribute to the high quantum yield. Although FTIR analysis reveals a variety of surface functional groups, many moieties, such as \u0026ndash;OH and \u0026ndash;NH₂, primarily act as passivating groups and do not introduce radiative electronic states. Efficient energy relaxation funnels excited carriers to the lowest-energy emissive state, likely associated with C\u0026thinsp;=\u0026thinsp;O-related surface states, resulting in excitation-wavelength-independent emission. In contrast, CDs emitting at 450 nm might exhibit more heterogeneous and highly oxidized surface structures, which introduce multiple surface trap states and lead to excitation-dependent emission [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFurthermore, the microwave-assisted reverse micelle method was applied to synthesize carbon dots using polyethyleneimine and citric acid as precursors. The optical properties of CDs prepared using methods 1\u0026ndash;5 were compared (Figure S8). Dodecane-based MARM, hydrothermal, and domestic microwave CDs exhibited excitation-independent emission, whereas solvothermal reverse micelle and hexanoic acid\u0026ndash;based MARM-CDs displayed excitation-dependent emission. These trends are consistent with those observed for citric acid/Tris CDs synthesized via the same methods, indicating that the observed excitation behaviour is robust across different precursor systems.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eWe have developed a rapid microwave-assisted reverse micelle (MARM) approach for the synthesis of carbon dots. The selected nonpolar solvent does not absorb microwave energy, allowing efficient and selective heating within the reverse micelle system. Carbon dots produced using this method exhibit characteristics similar to those obtained via hydrothermal and domestic microwave-assisted approaches. However, the MARM method is faster than the hydrothermal process and can produce more homogeneous CDs compared with the domestic microwave method. Furthermore, by adjusting the concentration of the surfactant AOT, the size of the carbon dots can be precisely controlled.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors thank the National Science and Technology Council of the Republic of China for financially supporting this work.\u0026nbsp;We thank Mao-Jung Huang and Chia-Ying Chien from the Instrumentation Center, National Taiwan University, for their assistance with TEM measurements.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLiu Y, Huang Z, Wang X, Hao Y, Yang J, Wang H, Qu S (2025) Recent Advances in Highly Luminescent Carbon Dots. 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Nanoscale 13:16662\u0026ndash;16671\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Scheme","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"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":"journal-of-nanoparticle-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nano","sideBox":"Learn more about [Journal of Nanoparticle Research](http://link.springer.com/journal/11051)","snPcode":"11051","submissionUrl":"https://submission.nature.com/new-submission/11051/3","title":"Journal of Nanoparticle Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"carbon dot, reverse micelle, microwave, dodecane, selective heating","lastPublishedDoi":"10.21203/rs.3.rs-8963582/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8963582/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe present a novel approach for the rapid synthesis of carbon dots using a microwave-assisted reverse micelle (MARM) method. Bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) was used to form reverse micelles in a nonpolar medium, dodecane, which does not absorb microwave radiation. Reverse micelles containing an aqueous phase were dispersed in the oil phase, and these small polar droplets acted as nanoreactors. The aqueous phase contained citric acid and Tris base as reactants. Under microwave heating, only these microreactors absorbed microwave energy and were heated efficiently and homogeneously, facilitating the formation of carbon dots. The micelle size could be tuned by adjusting the water-to-surfactant molar ratio, producing carbon dots with uniform sizes ranging from 1.8 to 2.6 nm, depending on the AOT concentration. Transmission electron microscopy and photospectroscopy data were compared for carbon dots prepared using hydrothermal, domestic microwave-assisted, solvothermal reverse micelle, hexanoic-acid-based MARM, and dodecane-based MARM approaches. The characteristics of carbon dots produced using the proposed method were similar to those obtained using hydrothermal and domestic microwave-assisted methods. However, the proposed method is more rapid than the hydrothermal method and may produce more homogeneous carbon dots than the domestic microwave-assisted method.\u003c/p\u003e","manuscriptTitle":"Synthesis of carbon dots via selective microwave heating of reverse micelles in a microwave-transparent solvent","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-06 11:27:25","doi":"10.21203/rs.3.rs-8963582/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewersInvited","content":"","date":"2026-04-27T16:10:48+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-28T09:50:10+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-25T23:56:32+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Nanoparticle Research","date":"2026-02-25T05:47:38+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-nanoparticle-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nano","sideBox":"Learn more about [Journal of Nanoparticle Research](http://link.springer.com/journal/11051)","snPcode":"11051","submissionUrl":"https://submission.nature.com/new-submission/11051/3","title":"Journal of Nanoparticle Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"01b052a3-6542-40ff-a77d-3ff1daeb4004","owner":[],"postedDate":"May 6th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-06T11:27:26+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-06 11:27:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8963582","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8963582","identity":"rs-8963582","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}