Preparation method of concentration-dependent red light sulfur quantum dots and sulfur quantum dots

Abstract

The application discloses a preparation method of concentration-dependent red light sulfur quantum dots and the red light sulfur quantum dots. The preparation method comprises the following steps: grinding, and reacting with sublimed sulfur, inorganic alkali and a heterocyclic functional reagent as raw materials and hydrogen peroxide as an etching agent; and then performing dialysis and freeze-drying to obtain the red light sulfur quantum dots. The heterocyclic functional reagent is a nitrogen-containing heterocyclic compound, and is at least one of 2-aminothiazole, imidazole-2-formaldehyde and 2-aminooxazole. The three SQDs all have concentration dependence, and the optimal emission wavelength is gradually red-shifted with the increase of the SQD concentration, and the maximum red shift is 130 nm. The maximum emission wavelength of the prepared SQDs can reach 690 nm, which indicates that the SQDs are red light emission fluorescent nanomaterials, and the fluorescence wavelength red shift can be attributed to non-radiative transition due to molecular assembly at a high concentration, i.e. fluorescence resonance energy transfer, and production of new emission centers. The application provides a new strategy for the research of concentration-dependent nanomaterials and red light emission SQDs.

Description

Technical Field

[0001] This invention belongs to the field of fluorescent nanomaterials technology, specifically, it relates to a method for preparing concentration-dependent red sulfur quantum dots and the sulfur quantum dots themselves. Background Technology

[0002] Fluorescent nanomaterials, with their unique optical properties, have shown broad application prospects in fields such as bioimaging, sensing and detection, optoelectronic devices, and anti-counterfeiting technology. In recent years, sulfur quantum dots (SQDs), as an emerging class of zero-dimensional nanomaterials, have gradually become another research hotspot after carbon quantum dots and metal quantum dots, thanks to their excellent biocompatibility, good chemical stability, low toxicity, and low preparation cost. Compared to traditional cadmium- and lead-containing heavy metal quantum dots, SQDs completely break free from the toxicity of heavy metals. Their green and environmentally friendly properties highly align with the core concept of sustainable development, making them irreplaceable in the biomedical field.

[0003] Currently, research on SQDs mainly focuses on two directions: optimization of synthesis methods and regulation of fluorescence properties. Existing literature shows that the emission wavelength of SQDs in the blue to yellow-green light band can be regulated by adjusting conditions such as the type of passivating agent, reaction temperature, pH value, and precursor ratio. However, how to prepare long-wavelength, highly stable red-emitting SQDs with low biological background interference remains a key challenge that urgently needs to be overcome in this field. Red fluorescent materials have advantages such as strong penetration into biological tissues, weak light scattering effect, and low interference from biological autofluorescence, giving them significant advantages in in vivo imaging and deep tissue detection. Therefore, developing red-emitting SQDs has important scientific significance and practical application value for expanding their application scope in the biomedical field.

[0004] It is noteworthy that concentration-dependent fluorescence behavior is a remarkable yet often overlooked physicochemical phenomenon in the study of the fluorescence properties of quantum dots (SQDs). Concentration dependence refers to the regular evolution of the fluorescence intensity, emission peak position, and even fluorescence lifetime of quantum dots as their concentration changes. This phenomenon not only reveals the complex coupling relationship between the internal energy level structure, surface state distribution, and aggregation state of quantum dots, but also provides a novel approach to achieving precise control of fluorescence performance through simple concentration adjustment.

[0005] Therefore, developing a sulfur quantum dot preparation technology that can achieve stable red light emission through concentration control has become an urgent technical problem to be solved in this field. Summary of the Invention

[0006] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a method for preparing concentration-dependent red-light sulfur quantum dots and sulfur quantum dots. Heterocyclic functionalized sulfur quantum dots are synthesized in one step by grinding, and the fluorescence wavelength is red-shifted to the red light region by concentration regulation, thus solving the problems of short emission wavelength and large interference in biological applications of traditional sulfur quantum dots.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] This invention provides a method for preparing concentration-dependent red-light sulfur quantum dots, comprising:

[0009] The product is obtained by grinding, using sublimed sulfur, inorganic base and heterocyclic functionalized reagent as raw materials, and hydrogen peroxide as etching agent. After dialysis and freeze drying, the product is obtained.

[0010] The heterocyclic functionalizing agent is a nitrogen-containing heterocyclic compound selected from at least one of 2-aminothiazole, imidazole-2-carboxaldehyde, and 2-aminooxazole.

[0011] Specifically, the inorganic base is sodium hydroxide.

[0012] Specifically, the molar ratio of sublimed sulfur, inorganic base and heterocyclic functionalizing agent is (12~36):(12~36):(1~5).

[0013] Specifically, the heterocyclic functionalizing agent is 2-aminothiazole, and the molar ratio of sublimed sulfur, inorganic base and 2-aminothiazole is 24:24:3.

[0014] Specifically, the preparation method involves mixing and grinding the raw materials for 10 minutes, slowly adding hydrogen peroxide and continuing the reaction for 15 minutes, dialyzing the resulting solution with a dialysis bag for 24 hours, and then freeze-drying to obtain the target sulfur quantum dots.

[0015] The present invention also provides a concentration-dependent red-light sulfur quantum dot prepared by the above preparation method.

[0016] Specifically, the sulfur quantum dots are monodisperse spherical nanoparticles with concentration-dependent fluorescence redshift characteristics, and stably emit red light at high concentrations.

[0017] Specifically,

[0018] When the heterocyclic functionalizing agent is 2-aminothiazole, the average particle size of sulfur quantum dots is approximately 1.9 nm.

[0019] When the heterocyclic functionalizing agent is imidazole-2-carboxaldehyde, the average particle size of sulfur quantum dots is approximately 3.5 nm.

[0020] When the heterocyclic functionalizing agent is 2-aminooxazole, the average particle size of sulfur quantum dots is approximately 2.2 nm.

[0021] After adopting the above technical solution, the method for preparing concentration-dependent red sulfur quantum dots and the sulfur quantum dots provided by the present invention have the following beneficial effects compared with the prior art.

[0022] (1) This invention uses sublimed sulfur, inorganic base and nitrogen-containing heterocyclic functionalized reagent as raw materials and hydrogen peroxide as etching agent to prepare concentration-dependent red sulfur quantum dots in one step by grinding method. The process is simple and easy to operate, without the need for complex equipment and harsh reaction conditions. The raw materials are green and non-toxic and have no heavy metal residues. It breaks through the wavelength limitation of traditional sulfur quantum dots that can only emit blue to yellow-green light from the source, and provides a new technical path for the efficient synthesis of red sulfur quantum dots.

[0023] (2) The preparation method provided by the present invention provides optimized raw material molar ratio and specific process parameters, which can precisely control the reaction process and quantum dot nucleation and growth. The efficient reaction can be completed under mild conditions. The purification methods of dialysis and freeze drying are simple and efficient, which can retain the fluorescence performance of quantum dots to the greatest extent and ensure that the concentration-dependent fluorescence characteristics of the obtained sulfur quantum dots are stable and the red light emission performance is excellent.

[0024] (3) The concentration-dependent red sulfur quantum dots provided by the present invention are monodisperse spherical nanoparticles with precise controllable particle size with functionalized reagents. They have stable concentration-dependent fluorescence redshift characteristics and can stably emit red light at high concentrations. They also have advantages such as uniform particle size, excellent optical performance and good biocompatibility. They can be directly adapted to multiple application scenarios such as bioimaging and sensing detection, greatly expanding the practical value of sulfur quantum dots. Attached Figure Description

[0025] The accompanying drawings, as part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention, but do not constitute an undue limitation of the invention. Obviously, the drawings described below are merely some embodiments, and those skilled in the art can obtain other drawings based on these drawings without creative effort. In the drawings:

[0026] Figure 1 These are TEM images (a) and particle size distribution diagrams (b) of the AT-SQDs of this invention.

[0027] Figure 2 These are TEM images (a) and particle size distribution diagrams (b) of the IC-SQDs of this invention.

[0028] Figure 3 These are TEM images (a) and particle size distribution diagrams (b) of the OA-SQDs of this invention.

[0029] Figure 4The following are three-dimensional fluorescence spectra of the AT-SQDs of this invention at different concentrations: (a) 1 mg / mL, (b) 5 mg / mL and (c) 10 mg / mL; (d) comparison of normalized maximum emission wavelengths at different concentrations.

[0030] Figure 5 The following are three-dimensional fluorescence spectra of the IC-SQDs of this invention at different concentrations: (a) 1 mg / mL, (b) 5 mg / mL and (c) 10 mg / mL; (d) comparison of normalized maximum emission wavelengths at different concentrations.

[0031] Figure 6 The following are three-dimensional fluorescence spectra of the OA-SQDs of this invention at different concentrations: (a) 1 mg / mL, (b) 5 mg / mL and (c) 10 mg / mL; (d) comparison of normalized maximum emission wavelengths at different concentrations.

[0032] Figure 7 This is a comparison of the three-dimensional fluorescence images of (a) AT-Dots, (b) IC-Dots, and (c) OA-Dots of the present invention with the maximum fluorescence emission wavelengths of (d) AT-SQDs, (e) IC-SQDs, and (f) OA-SQDs.

[0033] Figure 8 These are the UV-Vis absorption spectra of different concentrations of (a) AT-SQDs, (b) IC-SQDs, and (c) OA-SQDs of the present invention.

[0034] Figure 9 This invention relates to the mechanism study of concentration-dependent fluorescence. (a) Zeta potentials of SQDs at different concentrations. TEM images of SQDs at low concentration (b) and high concentration (c). (d) Fluorescence lifetime decay curves of SQDs at different concentrations. (e) Fluorescence lifetime of SQDs at different concentrations.

[0035] It should be noted that these accompanying drawings and textual descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art by referring to specific embodiments. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0037] [Example 1]

[0038] Preparation of aminothiazole-functionalized sulfur quantum dots (AT-SQDs)

[0039] 2-Aminothiazole-functionalized sulfur quantum dots (AT-SQDs) were synthesized via a grinding method using hydrogen peroxide as an etchant. Sublimed sulfur powder, sodium hydroxide, and 2-aminothiazole were mixed in a specific ratio and ground in an agate mortar for 10 minutes. H₂O₂ was slowly added, and the reaction was continued with stirring for another 15 minutes. The resulting solution was dialyzed for 24 hours using a dialysis bag, and then AT-SQDs were obtained by freeze-drying. The ratio of sublimed sulfur powder:sodium hydroxide:2-aminothiazole was (12–36 mmol):(12–36 mmol):(1–5 mmol). Those without sublimed sulfur powder were named AT-Dots.

[0040] As an example, the specific amounts of raw materials added in this embodiment are: 24 mmol of sublimed sulfur powder, 24 mmol of sodium hydroxide, 3 mmol of 2-aminothiazole, 10 mL of H2O2, and 7.75%.

[0041] [Example 2]

[0042] Preparation of imidazole-2-formaldehyde functionalized sulfur quantum dots (IC-SQDs)

[0043] Imidazole-2-carboxaldehyde-functionalized sulfur quantum dots (IC-SQDs) were synthesized via a grinding method using hydrogen peroxide as an etchant. Sublimed sulfur powder, sodium hydroxide, and imidazole-2-carboxaldehyde were mixed thoroughly and ground in an agate mortar for 10 minutes. H₂O₂ was slowly added, and the reaction was continued with stirring for 15 minutes. The resulting solution was dialyzed for 24 hours using a dialysis bag, and then freeze-dried to obtain IC-SQDs. The ratio of sublimed sulfur powder:sodium hydroxide:imidazole-2-carboxaldehyde was (12–36 mmol):(12–36 mmol):(1–5 mmol). Those without sublimed sulfur powder were named IC-Dots.

[0044] As an example, the specific amounts of raw materials added in this embodiment are: 24 mmol of sublimed sulfur powder, 24 mmol of sodium hydroxide, 3 mmol of imidazole-2-carboxaldehyde, 10 mL of H2O2, and 7.75%.

[0045] [Example 3]

[0046] Preparation of aminooxazole-functionalized sulfur quantum dots (OA-SQDs)

[0047] 2-Aminooxazole-functionalized sulfur quantum dots (OA-SQDs) were synthesized via a grinding method using hydrogen peroxide as an etchant. Sublimed sulfur powder, sodium hydroxide, and 2-aminooxazole were mixed thoroughly and ground in an agate mortar for 10 minutes. H₂O₂ was slowly added, and the reaction was continued with stirring for 15 minutes. The resulting solution was dialyzed for 24 hours using a dialysis bag, and then OA-SQDs were obtained by freeze-drying. The ratio of sublimed sulfur powder:sodium hydroxide:2-aminooxazole was (12–36 mmol):(12–36 mmol):(1–5 mmol). Those without sublimed sulfur powder were named OA-Dots.

[0048] As an example, the specific amounts of raw materials added in this embodiment are: 24 mmol of sublimed sulfur powder, 24 mmol of sodium hydroxide, 3 mmol of 2-aminooxazole, 10 mL of H2O2, and 7.75%.

[0049] The morphology of the three synthesized quantum dots was characterized using transmission electron microscopy, and the analysis is as follows:

[0050] from Figure 1 As can be seen from a, the synthesized AT-SQDs exhibit a clear dispersed morphology and present as small, round particles; particle size statistical analysis was performed on them ( Figure 1 (b) It can be seen that the average particle size of the synthesized AT-SQDs is about 1.9 nm.

[0051] from Figure 2 As can be seen from a, the synthesized IC-SQDs exhibit a clear dispersed morphology and present as small, round particles; particle size statistical analysis was performed on them ( Figure 2 (b) It can be seen that the average particle size of the synthesized IC-SQDs is about 3.5 nm.

[0052] from Figure 3 As can be seen from a, the synthesized OA-SQDs exhibit a clear dispersed morphology and present as small, round particles; particle size statistical analysis was performed on them ( Figure 3 (b) It can be seen that the average particle size of the synthesized OA-SQDs is about 2.2 nm.

[0053] Fluorescence properties of the three synthesized quantum dots were analyzed, and the conclusions are as follows:

[0054] from Figure 4 As can be seen from a, the optimal emission wavelength for low concentrations of AT-SQDs (1 mg / mL) is approximately 560 nm; however, as the concentration of AT-SQDs increases to 5 mg / mL, the optimal emission wavelength redshifts to approximately 650 nm. Figure 4 b); Further increasing its concentration to 10 mg / mL, its emission wavelength continues to redshift to approximately 690 nm. Figure 4 c). From Figure 4 The normalized wavelength of d clearly shows a significant redshift in its optimal emission wavelength, reaching approximately 130 nm.

[0055] from Figure 5 As can be seen from a, the optimal emission wavelength for low concentrations of IC-SQDs (1 mg / mL) is approximately 500 nm; however, as the concentration of IC-SQDs increases to 5 mg / mL, the optimal emission wavelength redshifts to approximately 590 nm. Figure 5 b); Further increasing its concentration to 10 mg / mL, its emission wavelength continues to redshift to approximately 630 nm. Figure 5 c). From Figure 5 The normalized wavelength of d clearly shows a significant redshift in its optimal emission wavelength, reaching approximately 130 nm.

[0056] from Figure 6 As can be seen from a, the optimal emission wavelength for low-concentration OA-SQDs (1 mg / mL) is approximately 530 nm; while as the concentration of IC-SQDs increases to 5 mg / mL, its optimal emission wavelength redshifts to approximately 580 nm. Figure 6 b); Further increasing its concentration to 10 mg / mL, its emission wavelength continues to redshift to approximately 620 nm. Figure 6 c). From Figure 6 The normalized wavelength of d clearly shows a significant redshift in its optimal emission wavelength, reaching approximately 90 nm.

[0057] This application further investigates the fluorescence properties of AT-Dots, IC-Dots, and OA-Dots synthesized without the addition of sublimed sulfur, and compares them with AT-SQDs, IC-SQDs, and OA-SQDs. Figure 7 As can be seen, the fluorescence emission wavelengths of nanomaterials synthesized without the addition of sublimed sulfur (AT-Dots, IC-Dots, and OA-Dots) are significantly shorter than those of quantum dots synthesized with the addition of sublimed sulfur (AT-SQDs, IC-SQDs, and OA-SQDs). This indicates that sublimed sulfur plays a significant role in red luminescence.

[0058] The above results indicate that three SQDs with red light emission were successfully synthesized, and all three SQDs exhibited fluorescence emission that was concentration-dependent.

[0059] To investigate the mechanism of its wavelength redshift, we conducted UV-Vis absorption spectroscopy studies on these SQDs and analyzed their electronic transition behavior. For example... Figure 8As shown, a broadband absorption at approximately 345 nm can be observed in the spectrum of SQDs. This absorption mainly originates from the n-π* transition of non-bonding electrons in sulfur atoms and the band gap transition of zero-valent sulfur. Interestingly, the absorption spectrum changes with the gradual increase of SQDs concentration, with a new absorption band appearing in the 400-600 nm range, which is likely due to the π-π* transition. This phenomenon is consistent with the redshift trend of the emission wavelength. UV-Vis absorption spectroscopy results indicate that a new absorption band appears in the system with increasing concentration.

[0060] To further investigate the reason for the redshift in luminescence with increasing concentration, we used one type of SQDs as the research object and systematically studied the potential, TEM morphology, and fluorescence lifetime of SQDs at different concentrations. First, we studied the Zeta potential. As the concentration increased, the Zeta potential of the SQDs increased from -24.3 mV to -8.0 mV. The absolute value of the Zeta potential decreased, indicating a weakening of electrostatic stabilization and an increased tendency for particle aggregation. Figure 9 a). It is noteworthy that SQDs exhibit monodispersity in dilute solutions, while at high concentrations they display distinct particle aggregates. This is clearly due to the self-assembly of large-sized metal-free clusters by the monodisperse SQDs. Figure 9 (bc), this result is consistent with the Zeta potential result. Furthermore, combined with TEM and UV-Vis absorption spectroscopy data, it is shown that the aggregation of SQDs generates new emission centers, leading to a redshift in fluorescence. Subsequently, the effect of SQD concentration on fluorescence lifetime was further investigated. Figure 9 As shown in d, the fluorescence lifetime decreases with increasing concentration. The fluorescence lifetime of the 20 mg / mL solution (3.00 ns) is shorter compared to the 5 mg / mL solution (4.32 ns). Figure 9 e). There is also greater overlap between the UV absorption and emission spectra at low concentrations, indicating the possible presence of FRET.

[0061] The above studies indicate that the redshift of the fluorescence wavelength of SQDs can be attributed to nonradiative transitions, namely fluorescence resonance energy transfer, that occur due to molecular assembly at high concentrations, and the production of new emission centers.

[0062] [Comparative Example]

[0063] Taking one type of SQDs as an example, the effects of different precursors on the performance of the synthesized SQDs were investigated in detail. This study used AT-SQDs as an example to investigate the effects of the amounts of sublimed sulfur powder, sodium hydroxide, and 2-aminothiazole on the luminescence properties of the synthesized SQDs. Detailed experimental results are shown in Tables 1, 2, and 3. It can be seen from the tables that different proportions of precursors have a significant impact on the experiments. This indicates that the precursors seriously affect the luminescence properties of the synthesized SQDs. Therefore, the final SQDs were synthesized using a ratio of sublimed sulfur powder: sodium hydroxide: 2-aminothiazole = 24:24:3.

[0064] Table 1. Effect of different amounts of 2-aminothiazole on fluorescence properties

[0065] Serial Number Sublimed sulfur Sodium hydroxide 2-Aminothiazole EX / EM (nm) 1 24 24 1 522 / 665 2 24 24 3 560 / 690 3 24 24 5 562 / 693

[0066] Table 2 Effect of different amounts of sodium hydroxide on fluorescence properties

[0067] Serial Number Sublimed sulfur Sodium hydroxide 2-Aminothiazole EX / EM (nm) 1 24 12 3 510 / 618 2 24 24 3 560 / 690 3 24 36 3 535 / 655

[0068] Table 3. Effect of different amounts of sublimed sulfur on fluorescence properties

[0069] Serial Number Sublimed sulfur Sodium hydroxide 2-Aminothiazole EX / EM (nm) 1 12 24 3 533 / 674 2 24 24 3 560 / 690 3 36 24 3 558 / 682

[0070] This application prepared 2-aminothiazole-functionalized sulfur quantum dots (AT-SQDs), imidazole-2-formaldehyde-functionalized sulfur quantum dots (IC-SQDs), and 2-aminooxazole-functionalized sulfur quantum dots (OA-SQDs) via a grinding method. The three types of SQDs were uniformly dispersed with particle sizes of approximately 1.9 nm, 3.5 nm, and 2.2 nm, respectively. All three SQDs exhibited concentration-dependent emission; their optimal emission wavelength gradually red-shifted with increasing SQD concentration, reaching a maximum red-shift of 130 nm. The maximum emission wavelength of the prepared SQDs reached 690 nm, indicating that they are red-emitting fluorescent nanomaterials. Furthermore, the mechanism of the red-shift in SQD fluorescence was investigated in detail using various techniques. The red-shift can be attributed to non-radiative transitions, i.e., fluorescence resonance energy transfer, that occur due to molecular assembly at high concentrations, resulting in the production of new emission centers. This application provides a new strategy for the study of concentration-dependent nanomaterials and red-emitting SQDs.

[0071] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0072] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. The implementation schemes in the above embodiments can also be further combined or replaced. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing concentration-dependent red-light sulfur quantum dots, characterized in that: The product is obtained by grinding, using sublimed sulfur, inorganic base and heterocyclic functionalized reagent as raw materials, and hydrogen peroxide as etching agent. After dialysis and freeze drying, the product is obtained. The heterocyclic functionalizing agent is a nitrogen-containing heterocyclic compound selected from at least one of 2-aminothiazole, imidazole-2-carboxaldehyde, and 2-aminooxazole.

2. The preparation method according to claim 1, characterized in that: The inorganic base is sodium hydroxide.

3. The preparation method according to claim 1, characterized in that: The molar ratio of sublimed sulfur, inorganic base and heterocyclic functionalizing agent is (12~36):(12~36):(1~5).

4. The preparation method according to claim 3, characterized in that: The molar ratio of sublimed sulfur, inorganic base and 2-aminothiazole is 24:24:

3.

5. The preparation method according to claim 1, characterized in that, The specific process of the preparation method is as follows: the raw materials are mixed and ground for 10 min, hydrogen peroxide is slowly added and the reaction continues for 15 min, the resulting solution is dialyzed with a dialysis bag for 24 h, and then freeze-dried to obtain the target sulfur quantum dots.

6. A concentration-dependent red-light sulfur quantum dot, characterized in that: It is prepared by the preparation method described in any one of claims 1 to 5.

7. The concentration-dependent red-light sulfur quantum dot as described in claim 6, characterized in that: The sulfur quantum dots are monodisperse spherical nanoparticles with concentration-dependent fluorescence redshift characteristics.

8. The concentration-dependent red-light sulfur quantum dot as described in claim 6, characterized in that: When the heterocyclic functionalizing agent is 2-aminothiazole, the average particle size of sulfur quantum dots is 1.9 nm; When the heterocyclic functionalizing agent is imidazole-2-carboxaldehyde, the average particle size of sulfur quantum dots is 3.5 nm; When the heterocyclic functionalizing agent is 2-aminooxazole, the average particle size of sulfur quantum dots is 2.2 nm.

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