Nitrogen-sulfur-doped carbon dots with aggregation-induced emission and their applications in anti-counterfeiting, encryption inks, and test papers

Abstract

The present invention relates to the field of fluorescent nanomaterials and anti-counterfeiting technology, specifically nitrogen-sulfur-doped carbon dots with aggregation-induced luminescence properties and their use in anti-counterfeiting, encryption inks, and test papers. The preparation method of nitrogen-sulfur-doped carbon dots comprises: dissolving dithiosalicylic acid and formamide in ethanol or acetic acid, and ultrasonically obtaining a mixed solution; reacting the mixed solution at 180-230°C for 8-15 hours; and cooling the solution to room temperature after the reaction, drying, to obtain solid carbon dots. The anti-counterfeiting and dual-encryption fluorescent ink of the present invention comprises the prepared carbon dots. The carbon dots R-CDs or Y-CDs of the present invention are dissolved in ethanol or acetic acid solution and used as ink, and have anti-counterfeiting and encryption functions by displaying different colors under ultraviolet irradiation.

Description

Technical Field

[0001] The present invention relates to fluorescent nanomaterials and anti-counterfeiting technologies, specifically to the synthesis of nitrogen-sulfur-doped carbon dots (Cdots) exhibiting aggregation-induced emission (AIEL) using dithiosalicylic acid and formamide as raw materials and acetic acid and ethanol as solvents. In particular, the invention relates to the application of these Cdots in anti-counterfeiting and encryption fluorescent inks and fluorescent test strips. Background Art

[0002] Carbon dots (CDs) are a new type of zero-dimensional carbon-based nanomaterials, mainly composed of sp 2 Composed of a hybrid carbon core and surface functional groups such as hydroxyl, carbonyl, carboxyl, and amino groups, carbon dots typically have a particle size of less than 10 nm. Carbon dots possess unique properties such as fluorescence, photoelectricity, chemical stability, and low toxicity. Furthermore, their surfaces can be functionalized through chemical modification methods such as the introduction of functional groups, thereby endowing them with more specific functions and applications. These carbon dots have shown widespread application in many applications, including bioimaging, fluorescent labeling, environmental monitoring, drug delivery, and photodynamic therapy.

[0003] A significant feature of carbon dots compared to other materials is that their synthesis methods are extremely simple and diverse. Currently, there are many methods for preparing carbon dots, which can be roughly divided into two categories: "top-down" and "bottom-up". "Top-down" refers to cutting or peeling large molecules or complex carbon structures (such as carbon nanotubes, graphene, carbon fibers) into small-sized carbon dots through a series of physical or chemical processing methods. These mainly include arc discharge, laser ablation, electrochemical treatment and other methods. Although this method can control the structure and morphology of carbon dots to a certain extent, the synthesis steps are usually complicated and cumbersome, the cost is high and the yield is low, making it difficult to prepare on a large scale.

[0004] The "bottom-up" approach is the opposite of the "top-down" approach. It is a method that uses smaller carbon materials to gradually assemble, dehydrate, and carbonize to form large molecular weight carbon nanoparticles. The synthesis methods mainly include hydrothermal method, solvothermal method, microwave method, template method, etc. Among them, hydrothermal method and solvothermal method are deeply favored by experimentalists because of their convenient operation, low cost, and controllable parameters. In addition to using small organic molecules or oligomers as carbon sources, hydrothermal / solvothermal methods can also use biomass macromolecules as precursors. Biomass has a wide range of sources, abundant raw materials, and is easy to synthesize on a large scale, thereby accelerating the progress of carbon dots from laboratory preparation to industrial preparation to meet the needs of large-scale industrial applications.

[0005] Organic light-emitting materials have shown broad application prospects in the field of luminescence applications due to their advantages such as low cost, low toxicity, structural diversity, and easy modification. However, traditional organic light-emitting materials are prone to aggregation-induced quenching (ACQ) in the solid state or aggregated state. Aggregation-induced emission materials mainly include various tetraphenylethylene, silole, and anthracene compounds such as tetraphenylethylene (TPE), tetraphenylpyrazine (TPB), and distyrylanthracene (DSA), and their derivatives, which significantly enhance the luminescence in the solid state or aggregated state, and are named aggregation-induced emission (AIE). These compounds have a helical structure and have excellent thermal stability, modifiability, mechanical properties, signal amplification and other characteristics. At present, aggregation-induced emission materials have overcome the aggregation-induced quenching effect of traditional fluorescent molecules and have advantages such as photoelectric stability, high luminescence efficiency, biocompatibility, and high spatiotemporal resolution. They have been widely used in technology transformation in many aspects and platforms such as photoelectric sensing, bioimaging, medical diagnosis, and anti-counterfeiting labels. However, the synthesis of many AIE molecules currently requires multi-step reactions involving the separation and purification of multiple intermediates, which increases the complexity of the synthesis and may also reduce the overall yield. Furthermore, precisely controlling the emission color of AIE materials (such as red- or blue-shifting) may require complex molecular design and regulation, increasing the difficulty of material design. Summary of the Invention

[0006] The present invention aims to address the shortcomings of the prior art and to provide nitrogen-sulfur dual-doped carbon dots, their preparation methods, and applications. This research developed nitrogen-sulfur dual-doped carbon dots (Cdots) exhibiting aggregation-induced emission (AIEL) using dithiosalicylic acid and formamide as raw materials and acetic acid and ethanol as solvents. These Cdots were used to prepare a fluorescent ink for advanced anti-counterfeiting and dual encryption applications. These Cdots can be used as anti-counterfeiting inks and test strips, offering manufacturing advantages such as low cost, strong resistance to counterfeiting, high stability, and a wide range of applications, significantly enhancing product safety and reliability.

[0007] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0008] The present invention provides nitrogen-sulfur doped carbon dots, characterized in that the preparation method of the nitrogen-sulfur doped carbon dots comprises the following steps:

[0009] (1) Dissolve dithiosalicylic acid and formamide in ethanol or acetic acid and ultrasonicate to obtain a mixed solution;

[0010] (2) The mixed solution obtained in step (1) is placed in a polytetrafluoroethylene-lined autoclave and reacted in an oven at 180-230° C. for 8-15 hours; preferably at 200° C. for 10 hours;

[0011] (3) After the reaction, the solution was cooled to room temperature and placed in a water bath for evaporation and drying to obtain solid carbon dots.

[0012] In the above technical solution, further, the molar ratio of dithiosalicylic acid to formamide is 1:3-5; preferably 1:4.

[0013] In the above technical solution, further, the ultrasonic time in step (1) is 8-15 minutes, preferably 10 minutes.

[0014] In the above technical solution, further, the drying in step (3) is volatilization drying at 85-95°C; the preferred temperature is 90°C.

[0015] In the above technical solution, further, when step (1) is dissolved in ethanol, a red solid R-CDs is prepared; when step (1) is dissolved in acetic acid, a yellow solid Y-CDs is prepared.

[0016] In the above technical solution, further, the carbon dots have an AIE optical effect, and the AIE effect comes from disulfide bonds.

[0017] The present invention provides an anti-counterfeiting and double-encryption fluorescent ink, which includes the aforementioned carbon dots; preferably, the ink is prepared from an ethanol-water solution or an acetic acid-water solution of the carbon dots; the volume ratio of ethanol to the mixed solution is greater than 40%, and the volume ratio of acetic acid to the mixed solution is greater than 30%.

[0018] The present invention provides an anti-counterfeiting and double-encrypted fluorescent test paper, wherein the test paper is prepared by soaking filter paper in the aforementioned ink and drying, or spraying the aforementioned ink on the filter paper and drying; preferably, the test paper is prepared by soaking the filter paper in an R-CDs solution to obtain a red test paper.

[0019] The present invention provides applications of the aforementioned ink or test paper in the fields of anti-counterfeiting detection of commodities or certificates, information hiding and re-hiding, and information integrity verification; preferably, applications in anti-counterfeiting and encryption materials.

[0020] In the above technical solution, further, the application of ink is: using ink to write on white paper, after the ink dries, no color is displayed, and the written content can be identified by displaying the ink color under ultraviolet irradiation, after spraying ethanol, the ink displays different colors under ultraviolet irradiation, and after the ethanol evaporates, the aqueous solution is evenly sprayed to restore the color when no ethanol is sprayed; the application of test paper is: on the test paper, ethanol or water is used to write, no color is displayed after drying, and different colors are displayed under ultraviolet irradiation.

[0021] The parameters of the fluorescence spectrophotometer in this study can be set as follows: scanning speed (1000 nm / min), excitation bandwidth (10 nm), emission bandwidth (10 nm), and gain (medium, 650 V).

[0022] The carbon dots prepared above can be used to make fluorescent ink and fluorescent test paper for advanced anti-counterfeiting and double encryption. The process consists of the following steps:

[0023] Using R-CDs and Y-CDs solutions as fluorescent inks, simple letters were written on ordinary filter paper using a brush and their fluorescence was observed under UV light. Deionized water and ethanol solutions were simultaneously placed in a spray bottle for anti-counterfeiting testing. Furthermore, red fluorescent test paper was prepared by soaking ordinary filter paper in R-CDs solution. The paper was then written on with ethanol and water, respectively, to observe the fluorescence changes.

[0024] In this study, hydrophobic R-CDs and Y-CDs were synthesized using dithiosalicylic acid and formamide as raw materials and ethanol and acetic acid as solvents. The carbon dots undergo reversible fluorescence color transitions in both aqueous and ethanol / acetic acid solutions, enabling the preparation of fluorescent inks and fluorescent test papers for anti-counterfeiting and encryption applications. The carbon dots exhibited excellent selectivity and sensitivity, as well as good stability and durability. This expands the application of carbon dots in anti-counterfeiting markings and opens up new avenues for their further development.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] This invention provides novel nitrogen-sulfur-doped carbon dots and their preparation methods. The resulting R-CDs and Y-CDs carbon dots exhibit an AIE effect, which originates from the disulfide bonds of the carbon dots. These R-CDs or Y-CDs, when dissolved in ethanol or acetic acid, can be used as ink, displaying different colors under ultraviolet light, providing anti-counterfeiting and encryption functions.

[0027] The carbon quantum dot fluorescent test paper prepared from the carbon dots of the present invention has a wide range of raw material sources, is easily available, and is green and economical; the preparation process of the carbon quantum dot fluorescent test paper is simple, does not require other toxic and complex chemical reagents, has a certain sensitivity, and has high fluorescence intensity and quantum yield.

[0028] The carbon quantum dot fluorescent test paper prepared by the present invention can not only change color under ultraviolet light, but also change color through simple and easily available solvents such as water and ethanol, thereby achieving efficient information verification and hiding of key information, effectively ensuring the authenticity and traceability of the product.

[0029] The carbon quantum dot fluorescent test paper of the present invention has a short detection time and can be operated at room temperature.

[0030] After the carbon quantum dot fluorescent test paper prepared by the present invention is exposed to a normal environment for one month, the carbon dots still remain stably on the filter paper surface and exhibit a strong fluorescence effect under ultraviolet excitation. The R-CDs and Y-CDs placed in the filter paper carrier show good stability and durability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the preparation, dispersion and aggregation states of R-CDs and Y-CDs involved in the present invention and the anti-counterfeiting application of fluorescent ink;

[0032] Figure 2 Optical characterization results of R-CDs; (A) TEM image of R-CDs dispersed in ethanol; (B) HRTEM image of R-CDs dispersed in ethanol (inset: particle size distribution); (C) TEM image of R-CDs dispersed in water; (D) XRD image of R-CDs dispersed in water;

[0033] Figure 3 R-CDs spectra; (A) is the FT-IR spectrum of R-CDs; (B) is the XPS spectrum of R-CDs; (C) is the high-resolution O1s spectrum of R-CDs; (D) is the high-resolution N1s spectrum of R-CDs; (E) is the high-resolution C1s spectrum of R-CDs; (F) is the high-resolution S2p spectrum of R-CDs;

[0034] Figure 4 Optical characterization of Y-CDs; (A) TEM image of Y-CDs dispersed in acetic acid; (B) HRTEM image of Y-CDs dispersed in acetic acid (inset: particle size distribution); (C) TEM image of Y-CDs dispersed in water; (D) XRD pattern of Y-CDs dispersed in water;

[0035] Figure 5 Y-CDs spectra; (A) FT-IR spectrum of Y-CDs; (B) XPS spectrum of Y-CDs; (C) high-resolution O1s spectrum of Y-CDs; (D) high-resolution N1s spectrum of Y-CDs; (E) high-resolution C1s spectrum of Y-CDs; (F) high-resolution S2p spectrum of Y-CDs;

[0036] Figure 6UV-visible absorption, fluorescence excitation, and emission spectra of R-CDs; (A) UV-visible absorption, fluorescence excitation, and emission spectra of R-CDs solution (illustrated: the color of the solution under UV and sunlight); (B) emission spectra of R-CDs solution at different excitation wavelengths; (C) UV-visible absorption, fluorescence excitation, and emission spectra of R-CDs powder (illustrated: the color of the powder under UV and sunlight); (D) emission spectra of R-CDs powder at different excitation wavelengths;

[0037] Figure 7 UV-visible absorption, fluorescence excitation, and emission spectra of Y-CDs; (A) UV-visible absorption, fluorescence excitation, and emission spectra of a Y-CDs solution (illustrated: the color of the solution under UV and sunlight); (B) emission spectra of a Y-CDs solution at different excitation wavelengths; (C) UV-visible absorption, fluorescence excitation, and emission spectra of a Y-CDs powder (illustrated: the color of the powder under UV and sunlight); (D) emission spectra of a Y-CDs powder at different excitation wavelengths;

[0038] Figure 8 Example 4: Experimental results of R-CDs; (A) shows images of R-CDs in water-ethanol solutions at different ratios under UV radiation (top) and sunlight (bottom); (B) and (C) show spectra (λex = 358 nm) and UV-visible absorption spectra of R-CDs in water-ethanol solutions at different ratios; (D) shows emission spectra of R-CDs in different solvents.

[0039] Figure 9 Example 4 Y-CDs experimental results; (A) is an image of Y-CDs in water-ethanol solutions at different ratios under ultraviolet radiation (top) and sunlight (bottom); (B) and (C) are the optical spectra (λex = 358 nm) and UV-visible absorption spectra of Y-CDs in water-ethanol solutions at different ratios; (D) is the emission spectra of Y-CDs in different solvents;

[0040] Figure 10 Comparison of P-CDs and R-CDs; (A) Comparison of P-CDs (top) and R-CDs (bottom) in solution and solid state; (B) Fourier transform infrared spectra of P-CDs and R-CDs;

[0041] Figure 11 Figure 2 shows the fluorescence lifetime results; (A) Fluorescence lifetime of R-CDs in solution and solid state; (B) Fluorescence lifetime of Y-CDs in solution and solid state;

[0042] Figure 12Color development results of R-CDs and Y-CDs solutions; (A) Images of filter paper treated with ethanol and water under fluorescent light and ultraviolet light; (B) Multiple test cycles of the anti-counterfeiting ink; (C) Time stability of the anti-counterfeiting ink;

[0043] Figure 13 Experimental results of fluorescent ink and test paper; (A) R-CDs and Y-CDs are used as fluorescent ink for simple writing; (B) R-CDs and Y-CDs are used as fluorescent test paper for simple writing. DETAILED DESCRIPTION

[0044] The present invention is further described below with reference to specific examples, but is not intended to limit the present invention in any way.

[0045] Example 1

[0046] Preparation of red solid R-CDs.

[0047] Weigh 0.306 g of dithiosalicylic acid and 160 μL of formamide, respectively, in a molar ratio of 1:4, and dissolve them in 30 mL of ethanol. Ultrasonicate for 10 minutes to fully dissolve the mixture. Place the resulting mixture in a 200°C oven for 10 hours. After cooling to room temperature, transfer the reaction mixture to a water bath for evaporation and drying to obtain red solid R-CDs, which is then stored for later use.

[0048] Various optical characterization results of R-CDs are as follows:

[0049] Transmission electron microscopy characterization: Figure 2 The results of (A) show that the prepared R-CDs are spherical in ethanol solution, evenly dispersed, and have no obvious agglomeration. Figure 2 (C) shows that R-CDs dispersed in deionized water are black clumps, proving that they are aggregated in water. Figure 2 (A) The distribution histogram in the upper left corner shows that the particle size range is 2.20-7.25nm, with an average particle size of 4.21nm, showing a good normal distribution. Figure 2 (B) HRTEM shows that R-CDs have obvious lattice fringes with a spacing of 0.19 nm, which corresponds to the lattice spacing of graphite structure. Figure 2 (D) is the XRD analysis spectrum, which shows that there is a significant diffraction peak near 26°, corresponding to the (002) plane of graphite. The results of HRTEM and XRD both prove that R-CDs has a crystal structure similar to graphite.

[0050] Figure 3 The result of (A) shows 3068cm -1 Attributable to the stretching vibration of CH, 2674 cm -1 and 2561cm-1 The two strong absorption bands at 1687 cm-1 can be attributed to the stretching vibration of SH. -1 The significant absorption peak at 1423 cm is attributed to the stretching vibration of C=O. -1 and 1292cm -1 The sharp characteristic peak appears due to the stretching vibration of CN, 708cm -1 and 550cm -1 The nearby absorption peaks belong to CS and SS, respectively. Figure 3 (B) XPS analysis of the elemental composition and chemical bonding of R-CDs. Figure 3 The results of (A) are basically consistent. The four characteristic peaks centered at 163.72, 284.76, 399.94, and 532.13 are the binding energies of S2p, C 1s, N1s, and O1s, respectively, indicating the successful doping of nitrogen and sulfur elements. Figure 3 The results of (C) show that the two characteristic peaks at 531.72 and 533.28 eV can be attributed to C=O and CO. Figure 3 The results of (D) show the main peaks of pyrrolic N and pyridinic N, corresponding to 400.3 and 399.24 eV, respectively. Figure 3 The results of (E) show that the characteristic peaks at 284.77, 286.2, and 288.65 eV are attributed to CC / C=C, CN, and C=O, respectively. Figure 3 (F) shows that the two peaks at 163.52 and 164.45 eV correspond to SC and SS, respectively.

[0051] Example 2

[0052] Preparation of yellow solid Y-CDs.

[0053] Weigh 0.306 g of dithiosalicylic acid and 160 μL of formamide in a 1:4 molar ratio, dissolve in 30 mL of acetic acid, and sonicate for 10 minutes to fully dissolve. Place the resulting mixture in a 200°C oven for 10 hours. After cooling to room temperature, transfer the reaction mixture to a water bath for evaporation and drying to obtain yellow solid Y-CDs, which is stored for later use.

[0054] Various optical characterization results of Y-CDs are as follows:

[0055] Transmission electron microscopy characterization: Figure 4 (A) with Figure 4 (C) TEM images of Y-CDs dispersed in acetic acid and deionized water, respectively. It can be clearly observed that Y-CDs are dispersed in acetic acid solution and aggregated in deionized water. Figure 4(A) The distribution histogram in the upper right corner shows that the particle size of Y-CDs ranges from 2.4 to 7.01 nm, with an average particle size of 4.08 nm. Figure 4 HRTEM (B) shows that the nanoparticles have high crystallinity and a lattice spacing of 0.20 nm, corresponding to the (002) plane of graphene. Figure 4 (D) is the XRD analysis spectrum, which shows a significant diffraction peak near 2θ=26°, which is also associated with the (002) plane of graphite. The characterization results of lattice spacing and diffraction peaks confirm that Y-CDs is a graphite structure with high crystallinity.

[0056] The functional groups of Y-CDs are similar to those of R-CDs. Figure 5 The results of (A) show that the infrared wavelengths at 3009, 1680, 739 and 553 cm -1 The characteristic peaks at 2654 and 2518 cm-1 are attributed to CH, C=O, CS and SS respectively. -1 There are SH stretching vibrations at 1415 and 1262 cm -1 There is stretching vibration of CN. Figure 5 As shown in (B), Y-CDs are composed of C, O, S, and N elements, with the proportions of each element being 70.43%, 15.84%, 10.82%, and 2.92%, respectively. Compared with R-CDs, the C element is slightly decreased, and the O element is slightly increased, but the overall difference is not large. Figure 5 The results of (C) show that there are two fitting peaks at 531.63 and 532.89 eV, which represent C=O and CO, respectively. Figure 5 The results in (D) show that the peaks at 400.7 and 401.7 eV are attributed to graphitic N and NH, respectively. Figure 5 The results of (E) show that two characteristic peaks were resolved at 284.78 and 288.9 eV, representing CC / C=C and C=O, respectively. Figure 5 (F) shows that the two peaks at 163.61 and 164.78 eV correspond to SC and SS, respectively.

[0057] Comparing the TEM, FT-IR, and XPS spectra of R-CDs and Y-CDs reveals that both carbon dots appear aggregated in water and dispersed in acetic acid or ethanol. Given that acetic acid is rich in carboxyl groups and ethanol contains more hydroxyl groups, the functional groups and chemical bonds between the two are largely similar, with only slight variations in the C and O content.

[0058] Example 3

[0059] The optical properties of R-CDs and Y-CDs were investigated, and the UV-visible absorption spectra and fluorescence excitation and emission spectra of the solutions and powders of the two substances were tested respectively. Figure 6 (A) shows that the significant absorption peaks of R-CDs in ethanol solution at around 220nm and 306nm are caused by π-π* and n-π* transitions. Figure 6 The solid powder of R-CDs (C) exhibits distinct UV absorption at 560 nm, in addition to the same absorption peak at 306 nm, which can be attributed to n-π* transitions of C=O, CO, and CS structures. Ethanol solutions of R-CDs exhibit blue fluorescence, which appears pale pink under sunlight. The optimal emission peak is at 382 nm under excitation at 358 nm. Figure 6 (B) shows that when the excitation wavelength of the ethanol solution of R-CDs increases from 320 nm to 440 nm, the optimal emission peak gradually shifts, indicating an excitation-dependent phenomenon caused by the uneven surface defects of the carbon dots. Figure 6 (C) Powdered R-CDs exhibit red fluorescence with an emission peak at 645 nm and an optimal excitation wavelength of 560 nm. Figure 6 (D) shows that after changing the excitation wavelength, the emission peak of R-CDs powder always remains around 645nm, with no excitation dependence, exhibiting excitation-independent characteristics.

[0060] Figure 7 (A) The acetic acid solution of Y-CDs shows blue fluorescence, which is yellow under sunlight. The UV absorption peaks at 254 and 316 nm are attributed to the π-π* and n-π* transitions. Figure 7 (C) The solid-state Y-CDs exhibit yellow fluorescence. In addition to absorption at 316 nm, a shoulder peak at 450 nm arises from the n-π* transitions of C=O, CN, and CS. To investigate whether the fluorescence emission wavelength of Y-CDs in both solution and solid states depends on the excitation wavelength, their fluorescence properties were tested over an excitation wavelength range of 320–440 nm. Figure 7 (B) The results show that the maximum emission of Y-CDs solution under 370nm excitation is about 464nm, showing excitation wavelength dependence. Figure 7 (D) shows that the Y-CDs solid state obtains the optimal emission peak of 605nm under an excitation wavelength of 550nm, with no excitation dependence and the characteristic of being relatively independent of the excitation wavelength.

[0061] Comparing the optical properties of R-CDs and Y-CDs, both solutions exhibit blue fluorescence under ultraviolet light, which is excitation-dependent, while the solids exhibit red and yellow fluorescence, respectively, which do not have excitation-dependent characteristics. This means that the luminescence center of R-CDs and Y-CDs in the solid state is single.

[0062] Example 4

[0063] The study investigated the AIE optical effect of the carbon dots by uniformly dispersing R-CDs and Y-CDs powders in various ratios of ethanol-water, acetic acid-water, and solvents of varying polarity. The color changes under UV light and sunlight were observed, and UV and fluorescence emission spectra were measured. The ethanol used was anhydrous ethanol, and the acetic acid was anhydrous acetic acid.

[0064] Take 3mg of R-CDs powder and disperse it in 5mL of ethanol-water mixed solution. The fluorescence color changes to Figure 8 (A) When the volume ratio of ethanol in the mixed solution is ≤40%, the solution is suspended and exhibits red fluorescence. When the volume ratio of ethanol in the mixed solution is greater than 40%, the solution becomes clear and transparent and exhibits blue fluorescence. Figure 8 (B) and Figure 8 (C) shows that as the ethanol content decreases, the emission spectrum of the solution red-shifts from 382 nm to 450 nm, and the fluorescence gradually changes from blue to red. This is due to the limited solubility of R-CDs in ethanol. Powder that is not completely dissolved in ethanol is difficult to dissolve in water, forming aggregates and producing red fluorescence. This speculation is also supported by the increasing absorbance at 306 nm in the UV-visible spectrum with increasing ethanol content.

[0065] 3 mg of R-CDs powder was dissolved in 5 mL of solvents of different polarities, which were xylene, acetone, ethanol, N,N-dimethylformamide, dimethyl sulfoxide and water in order of polarity. Figure 8 As shown in (D), although the emission peak does not undergo a regular red shift with increasing solution polarity, it corresponds to the dispersion of R-CDs in the solvent. When the R-CDs powder is uniformly dispersed, the solution basically exhibits blue fluorescence with an emission peak near 382nm. When in an aggregated state, the emission peak exists at 450nm, and the solution exhibits red fluorescence. The above results show that R-CDs are insensitive to solution polarity and exhibit different emission wavelengths and fluorescence color changes in different dispersion media. However, due to its hydrophobicity, as the water content of R-CDs increases, the blue fluorescence of the dispersed state is turned off, and the red fluorescence of the aggregated state is turned on, resulting in a red-shifted fluorescence emission peak and a decreased absorbance. This trend provides strong evidence for the existence of π-π stacking in carbon dots.

[0066] When Y-CDs powder was dispersed in acetic acid-water solutions with different ratios and solvents with different polarities, Y-CDs exhibited similar AIE luminescence effects. Figure 9(A) shows that when the acetic acid content in the mixed system is ≥30%, the Y-CDs solution is a clear and transparent liquid with blue fluorescence. When the acetic acid content in the system is <30% and the water content gradually increases, the Y-CDs solution begins to separate the yellow powder, forming a turbid liquid with a suspension, which exhibits yellow fluorescence. Figure 9 (B) with Figure 9 (C) shows that as the acetic acid content decreases, the emission spectrum significantly red-shifts and the absorbance at 316 nm decreases. Figure 9 (D) The results show that Y-CDs in solutions of different polarities also prove that carbon dots in the dispersed state exhibit blue fluorescence, while carbon dots in the aggregated state exhibit yellow fluorescence.

[0067] In summary, both R-CDs and Y-CDs exhibit AIE effect and are dispersed in ethanol or acetic acid solution. As the water content increases, they change from a dispersed state to an aggregated state, emitting red and yellow fluorescence, respectively. Figure 2 and Figure 3 The TEM images of carbon dots dissolved in ethanol / acetic acid and water can further confirm this conclusion.

[0068] Example 5

[0069] Preparation of brown solid P-CDs.

[0070] Benzoic acid and formamide (0.122 g and 160 μL, respectively) were accurately weighed in a 1:4 molar ratio and dissolved in 30 mL of ethanol. Ultrasonication was performed for 10 minutes to fully dissolve the mixture. The resulting mixture was placed in a 200°C oven for 10 hours. After cooling to room temperature, the reaction solution was transferred to a water bath for evaporation and drying to obtain brown solid P-CDs, which was stored for later use.

[0071] Furthermore, the mechanism of the AIE effect of R-CDs and Y-CDs was studied. Generally speaking, the graphitized carbon core is composed of some conjugated molecules. The presence of the carbon core causes a wide range of emission and contributes to the formation of excitation-dependent fluorescence properties. Figure 8 and Figure 9 The typical excitation dependence exhibited by R-CDs and Y-CDs in solution suggests that the blue emission from the graphitized carbon core arises from the carbon dots. Due to the hydrophobic nature of carbon dots, they aggregate and precipitate upon addition of water. The graphitized carbon core undergoes aggregation-induced fluorescence quenching (ACQ) due to the π-π stacking of its conjugated system, resulting in the absence of blue emission. Comparative experiments suggest that the π-π stacking arises from disulfide bonds. P-CDs, which lack disulfide bonds, were prepared using benzoic acid instead of dithiosalicylic acid in ethanol as the solvent. Figure 10(A) The results show that P-CDs exhibit blue fluorescence in ethanol solvent, but no obvious fluorescence in the solid state. P-CDs are easily soluble in ethanol and aqueous solution. R-CDs containing disulfide bonds exhibit blue fluorescence in solution and significant red fluorescence in the solid state. In addition, Figure 10 As shown in (B), there is a significant difference between the infrared spectra of P-CDs and R-CDs at 550 nm, which proves that the AIE effect of R-CDs comes from the disulfide bond.

[0072] In addition, according to K r (radiation rate constant) and K nr (Non-radiative rate constant) calculation formula K r =QY / τ and K nr =(1-QY) / τ can also prove this conclusion. In the formula, QY is the photoluminescence quantum yield of carbon dots, τ is the fluorescence lifetime of carbon dots, Figure 11 The fluorescence lifetime diagrams of the carbon dots after fitting are shown in Tables 1 and 2. The calculated values ​​of R-CDs and Y-CDs in solution and solid states are shown in Tables 1 and 2. Tables 1 and 2 show that the Kr of R-CDs and Y-CDs in the solid state is slightly higher, while the Knr is significantly lower. This is consistent with the mechanism of restricted molecular motion in the AIE phenomenon. Specifically, in the aggregated state, the vibration and rotation of the chemical bonds within the AIE molecule are greatly restricted by the interactions of surrounding molecules or natural physical constraints, resulting in the obstruction of non-radiative transition pathways. The emission channel is mainly concentrated on radiative transitions, leading to enhanced luminescence efficiency. These results indicate that the appearance of blue fluorescence is due to π-π* transitions. The presence of uneven defects on the surface of the carbon dots in solution leads to different energy levels in the π-π* energy gap. Each energy level emits different fluorescence, resulting in excitation-dependent blue light emission. At this time, the self-rotation of the disulfide bonds is unhindered, and energy is exchanged through intramolecular rotation. The appearance of red or yellow fluorescence is due to the accumulation of conjugated structures inside the carbon dots in the aggregated state, which inhibits the occurrence of π-π* transitions and limits the blue fluorescence emission. At the same time, since the stacked conjugated structures limit the rotation of disulfide bonds, their energy is difficult to release through their own rotation and is radiated in a divergent form, thus forming red and yellow fluorescence.

[0073] Table 1 shows the QY, τ, Kr, and Knr parameters of R-CDs solutions and powders;

[0074]

[0075] Table 2 shows the QY, τ, Kr and Knr parameters of Y-CDs solution and powder

[0076]

[0077] Example 6

[0078] Based on the above characterization results, the reasons why R-CDs and Y-CDs produce different colors of solid-state luminescence under the same raw materials, temperature and reaction time but different reaction solvents are discussed. Figure 2 and Figure 3 TEM spectra of R-CDs prepared in ethanol as the solvent show an increase in average particle size and a decrease in the lattice spacing of the graphite (002) plane compared to Y-CDs prepared in acetic acid. This indicates that the carbonization degree increases and the graphitization effect becomes more pronounced when the solvent changes from acetic acid to ethanol, leading to a quantum size effect. A comparison of the optical properties of the two carbon dots reveals that the excitation / emission wavelengths of R-CDs in the solid state are significantly red-shifted at 560 / 645 nm compared to those of Y-CDs. The peaks of the n-π* transitions associated with C=O, CN, and CS also exhibit a red-shift. This red-shift is thought to cause a decrease in the band gap energy, and the band gap transitions caused by the red-shift in emission wavelength theoretically demonstrate the quantum size effect. These results suggest that, because ethanol has a lower boiling point than acetic acid, it exhibits a higher degree of carbonization at the same temperature. The size effect caused by the different degrees of carbonization is the primary factor in the modulation of fluorescence wavelengths and the generation of different solid-state luminescence colors.

[0079] Example 7

[0080] Due to the excellent AIE properties of R-CDs and Y-CDs, they exhibit different fluorescent colors in the dispersed and aggregated states, so they are used as dual-switch mode inks for anti-counterfeiting technology applications. Figure 12 As shown in Figure (A), an ethanol solution of R-CDs (0.2g R-CDs in 30ml anhydrous ethanol) was dispersed on the left side of a butterfly-shaped filter paper, while an acetic acid solution of Y-CDs (0.2g Y-CDs in 30ml anhydrous acetic acid) was dispersed on the right side. After drying, the butterfly exhibited no noticeable color change under sunlight. Under ultraviolet light, the left portion of the butterfly exhibited red fluorescence, while the right portion exhibited yellow fluorescence (due to the blue color of the filter paper under UV light, the colors appear superimposed in the image). After evenly spraying the butterfly with a 75% ethanol solution, both exhibited blue fluorescence. After drying, evenly spraying with an aqueous solution returned the butterfly's fluorescence to its initial red and yellow state. Furthermore, even after alternating spraying with the ethanol and aqueous solutions, the butterfly exhibited no color change under sunlight. This is believed to be due to the fact that spraying with the ethanol solution caused the carbon dots to transition from an aggregated state to a dispersed state, resulting in blue fluorescence. Further spraying with the aqueous solution caused the carbon dots to revert to an aggregated state due to their hydrophobic aggregation.

[0081] Figure 12As shown in (B), the stability of the anti-counterfeiting mark created using R-CDs and Y-CDs solutions was analyzed for multiple cycles. After evenly spraying ethanol and water on the butterfly filter paper surface four times, a significant color change effect was still observed. Although the fluorescent color was slightly weakened, this change was still within a controllable range considering the loss of carbon dots during the spraying process and the limited adhesion between the carbon dots and the filter paper. In addition, the time stability of the filter paper after four cycles was investigated. After one month of exposure in a normal environment, the carbon dots still remained stably on the filter paper surface and exhibited a strong fluorescence effect under ultraviolet excitation. The above results show that the R-CDs and Y-CDs placed in the filter paper carrier exhibited good stability and durability, providing a reliable foundation for the anti-counterfeiting field and demonstrating potential application prospects in anti-counterfeiting marks.

[0082] Figure 13 As shown in (A), R-CDs and Y-CDs solutions were used as fluorescent inks to write on blank filter paper. "S" and "U" were written with R-CDs, and "P" was written with Y-CDs. After the ink dried, red fluorescence was observed for "S" and "U," and yellow fluorescence was observed for "P" under UV light. (Since filter paper appears blue under UV light, the colors appear superimposed in the image.) After ethanol was sprayed on the test paper, "S" and "PU" both fluoresced blue. After the ethanol evaporated, the aqueous solution was evenly sprayed, and the red and yellow fluorescence reappeared.

[0083] Soaking ordinary filter paper in an R-CDs ethanol solution produces R-CDs fluorescent test paper, which fluoresces light red under ultraviolet light. Writing "SPU" on the red fluorescent test paper with ethanol and water, respectively, will result in "S, U" written with ethanol fluorescing blue, while "P" written with water will fluoresce red.

[0084] According to the above results, these two types of carbon dots can not only be used as invisible colorless fluorescent anti-counterfeiting inks, but can also be composited with filter paper to form fluorescent test paper. Not only can they change color under ultraviolet light, but they can also change color through simple and easily available solvents such as water and ethanol, achieving efficient information verification and hiding of key information, effectively ensuring the authenticity and traceability of the product.

[0085] The above embodiments are only used to illustrate the technical solutions of the present invention and are only intended to facilitate the understanding and use of the invention by those skilled in the art, but are not intended to limit the invention. Those skilled in the art can still modify the specific implementations of the present invention or replace some of the technical features with equivalents; and apply the general principles described herein to other embodiments without having to engage in creative work. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A nitrogen-sulfur doped carbon dot, characterized in that: The method for preparing nitrogen-sulfur doped carbon dots comprises the following steps: (1) Dissolve dithiosalicylic acid and formamide in ethanol or acetic acid and ultrasonicate to obtain a mixed solution; (2) reacting the mixed solution obtained in step (1) at 180-230° C. for 8-15 hours; (3) After the reaction, the solution is cooled to room temperature and dried to obtain solid carbon dots.

2. The nitrogen-sulfur doped carbon dots according to claim 1, characterized in that: The molar ratio of dithiosalicylic acid to formamide is 1:3-5.

3. The nitrogen-sulfur-doped carbon dots according to claim 1, characterized in that: The ultrasonic time in step (1) is 8-15 minutes.

4. The nitrogen-sulfur doped carbon dots according to claim 1, characterized in that: The drying in step (3) is performed by volatilization drying at 85-95°C.

5. The nitrogen-sulfur doped carbon dots according to claim 1, characterized in that: When step (1) is dissolved in ethanol, a red solid R-CDs is prepared; when step (1) is dissolved in acetic acid, a yellow solid Y-CDs is prepared.

6. The nitrogen-sulfur doped carbon dots according to claim 1, characterized in that The carbon dots have an AIE optical effect, and the AIE effect comes from disulfide bonds. 7.An anti-counterfeiting and double-encryption fluorescent ink, characterized in that: The ink comprises the carbon dots according to any one of claims 1 to 6; the ink is prepared from an ethanol-water solution or an acetic acid-water solution of the carbon dots; the volume ratio of ethanol to the mixed solution is greater than 40%, and the volume ratio of acetic acid to the mixed solution is greater than 30%.

8. An anti-counterfeiting and double-encrypted fluorescent test paper, characterized in that: The test paper is prepared by soaking filter paper in the ink of claim 7 and drying, or spraying the ink of claim 7 on the filter paper and covering it with the ink and drying; the test paper is prepared by soaking the filter paper in the R-CDs solution to obtain a red test paper.

9. Use of the ink according to claim 7 or the test paper according to claim 8 in the fields of anti-counterfeiting detection of goods or certificates, information hiding and re-hiding, information integrity verification, and encryption materials.

10. The use according to claim 9, characterized in that The application of ink is as follows: use ink to write on white paper. After the ink dries, it does not show any color. Under ultraviolet light, the written content can be identified by displaying the ink color. After spraying ethanol, the ink will show different colors under ultraviolet light. After the ethanol evaporates, evenly spray the aqueous solution to restore the color before spraying ethanol. The application of test paper is as follows: write on the test paper with ethanol or water. After drying, it does not show any color. Under ultraviolet light, it will show different colors.

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