Preparation method, product and application of nitrogen-sulfur co-doped carbon quantum dots

Abstract

The application relates to the field of visual detection of metals, and particularly relates to a preparation method, product and application of nitrogen-sulfur co-doped carbon quantum dots. The preparation method of the nitrogen-sulfur co-doped carbon quantum dots comprises the following steps: phenolphthalein, m-phenylenediamine and glutathione are dissolved in an alkali solution to perform a hydrothermal reaction; after the hydrothermal reaction is completed, filtration, dialysis and freeze-drying are sequentially performed to obtain the nitrogen-sulfur co-doped carbon quantum dots. Phenolphthalein is used as a precursor, m-phenylenediamine and glutathione are respectively used as a nitrogen source and a sulfur source to synthesize nitrogen-sulfur co-doped carbon quantum dots (NS-CDs). The preparation method is simple, the quantum yield of the NS-CDs prepared by using the method reaches 71.16%, and the result is higher than that of most reported blue fluorescent carbon dots. The NS-CDs prepared by using the method can be successfully applied in the visual detection of Cr 6+ and Mn 7+ .

Description

TECHNICAL FIELD

[0001] The application relates to the field of visual detection of metals, in particular to a preparation method of nitrogen-sulfur co-doped carbon quantum dots, a product and application thereof. BACKGROUND

[0002] Cr 6+ is a highly toxic heavy metal pollutant, most of which exists in industrial wastewater and waste residues and is finally discharged into the biological environment. Excessive Cr 6+ has carcinogenic and mutagenic properties to the respiratory tract, internal organs and neural tissue due to its high oxidation potential and strong penetration of biological membranes. Permanganate (MnO4 - is commonly used as a strong oxidizing agent in the laboratory and as a preservative and disinfectant material in industry. Excessive contact with permanganate can have adverse effects on human health, such as causing deformities and respiratory system damage, and severely damaging the central nervous system. Therefore, it is urgent to develop reliable, sensitive, highly selective and cost-effective detection techniques to monitor the concentration of Cr 6+ and Mn 7+ in aqueous media and reduce their adverse effects.

[0003] So far, various analytical methods for monitoring Cr 6+ and Mn 7+ have been reported. For example, patent CN106544008B shows a method for detecting Cr 6+ , but the preparation of the fluorescent probe is complex. As shown in patent CN118580854A, only a method for fluorescent detection of Mn 7+ is shown, and the method is not applied to practice. Moreover, the fluorescent probe prepared by the method has a low quantum yield (QY) and insufficient fluorescence intensity, and therefore is not the optimal means for detecting the ions. SUMMARY

[0004] Based on the above, the application provides a preparation method of nitrogen-sulfur co-doped carbon quantum dots, a product and application thereof in visual detection of Cr 6+ and Mn 7+ .

[0005] To achieve the above-mentioned purposes, the application provides the following solutions.

[0006] One of the technical solutions of the application is a preparation method of nitrogen-sulfur co-doped carbon quantum dots, comprising the following steps:

[0007] dissolving phenolphthalein, m-phenylenediamine and glutathione in an alkali solution and performing a hydrothermal reaction;

[0008] After the hydrothermal reaction, the nitrogen and sulfur co-doped carbon quantum dots (NS-CDs) are obtained by filtration, dialysis and freeze-drying in sequence.

[0009] The second technical solution of the present application is the nitrogen and sulfur co-doped carbon quantum dots prepared by the above preparation method.

[0010] The third technical solution of the present application is a fluorescent filter paper strip, wherein the filter paper is soaked in a 1-2 mg / mL NS-CD solution for 4-6 min, and then dried at 50-60°C to obtain the fluorescent filter paper strip; the NS-CD solution is prepared by dissolving the above nitrogen and sulfur co-doped carbon quantum dots in water.

[0011] The fourth technical solution of the present application is a fluorescent hydrogel and a kit, wherein agarose is dissolved in ultrapure water, and then mixed with an NS-CD solution and solidified to obtain the fluorescent hydrogel.

[0012] The mass-volume ratio of the agarose to the ultrapure water is 0.5 g:50 mL; the volume ratio of the ultrapure water to the NS-CD solution is 50:3; the concentration of the NS-CD solution is 3 mg / mL; and the NS-CD solution is prepared by dissolving the above nitrogen and sulfur co-doped carbon quantum dots in water.

[0013] The fifth technical solution of the present application is the application of the above nitrogen and sulfur co-doped carbon quantum dots, the above fluorescent filter paper strip or the above fluorescent hydrogel in detecting Cr 6+ and Mn 7+ in a water environment.

[0014] The sixth technical solution of the present application is a visual detection method for Cr 6+ and Mn 7+ in a water environment, comprising the following steps:

[0015] The above fluorescent filter paper strip or the above fluorescent hydrogel is soaked in a water sample for 20 min, and then observed under the irradiation of a 365 nm ultraviolet lamp to observe whether the blue fluorescence is darkened or quenched; if the blue fluorescence is observed to be darkened or quenched, it indicates that Cr 6+ or Mn 7+ exists in the water environment. In a preferred embodiment of the present application, the pH of the water sample is 4-6.

[0016] The present application discloses the following technical effects:

[0017] The NS-CDs are synthesized by taking phenolphthalein as a precursor, m-phenylenediamine and glutathione as a nitrogen source and a sulfur source respectively. The preparation method is simple, and the quantum yield of the NS-CDs prepared by the method reaches 71.16%, which is higher than the quantum yield of most of the reported blue fluorescent carbon dots. As shown in patent CN116285975A, the quantum yield of the carbon dots synthesized from biomass is only 9.03%. As shown in patent CN116622368B, the blue carbon quantum dots are synthesized from the wind blade material by using the solid waste resource recycling concept, and the QY is 28%-35%. As shown in patent CN116367572A, the QY of the synthesized blue carbon quantum dots is 59.75%, which is higher than the yield of the general synthesis method, but still lower than the quantum yield of the present application.

[0018] The NS-CDs prepared by the method can be successfully applied in the visual detection of Cr 6+ and Mn 7+ .

[0019] The NS-CDs prepared by the method as a fluorescent probe show excellent selectivity for Cr 6+ and Mn 7+ under acidic conditions, and the fluorescence change amount of the NS-CDs has a good linear relationship with the concentration of Cr 6+ and Mn 7+ . It is calculated that the linear range for detecting Cr 6+ is 0-60 μM, and the detection limit is 0.48 μM, and the linear range for detecting Mn 7+ is 0-70 μM, and the detection limit is 0.14 μM. Compared with other reported Cr 6+ and Mn 7+ sensors, the NS-CDs of the present application have a wider detection range.

[0020] The NS-CDs prepared by the method as a fluorescent probe can be successfully applied to the detection of Cr 6+ and Mn 7+ in real water samples, and has excellent recovery rate (the recovery rate of Cr 6+ is 96%-106%, and the recovery rate of Mn 7+ is 94%-104%). BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0022] Figure 1 The effects of precursor ratio (a), reaction temperature (b), and reaction time (c) on the quantum yield of NS-CDs under the given synthesis conditions.

[0023] Figure 2 In the figure, a is the TEM image of NS-CDs, the inset is the HR-TEM image, b is the particle size distribution map, c is the XRD pattern of NS-CDs, and d is the Raman spectrum.

[0024] Figure 3 In the image, a is the infrared spectrum of NS-CDs, b is the XPS full spectrum, c is the high-resolution C1s spectrum, d is the N1s spectrum, e is the O1s spectrum, and f is the S2p spectrum.

[0025] Figure 4 In the figure, a represents the UV-Vis absorption and fluorescence spectra of NS-CDs, b represents the emission spectra under different excitations, c represents the xenon lamp stability of NS-CDs, d represents the storage time stability, e represents the salt concentration stability, and f represents the pH stability.

[0026] Figure 5 In Figure a, 'a' represents the effect of metal ions on the fluorescence intensity of NS-CDs; 'b' represents the selectivity of NS-CDs for metal ions; 'c' represents the absorption spectra of NS-CDs after adding different metal ions; 'd' represents the selectivity of NS-CDs for metal ions under different pH conditions; and 'e' represents the effect of pH on Cr. 6+ With Mn 7+ The effect of quenching efficiency, where f is the effect of time on Cr 6+ With Mn 7+ The impact of quenching efficiency.

[0027] Figure 6 For Cr 6+ (a) and Mn 7+ (d) Effect of concentration (0-500 μM) on the fluorescence intensity of NS-CDs, and the degree of fluorescence quenching of NS-CDs and Cr 6+ (b,c) and Mn 7+ Linear and nonlinear fitting curves for (e,f) concentrations.

[0028] Figure 7 In the figure, a represents the excitation and emission spectra of NS-CDs and Cr. 6+ and Mn 7+ The absorption spectrum, b represents the presence or absence of Cr in NS-CDs. 6+ and Mn 7+ The fluorescence lifetime, c is the UV absorption spectrum, and d is the Cr. 6+ The ultraviolet titration spectrum, e is Mn 7+ The ultraviolet titration spectrum.

[0029] Figure 8 a is the selective detection of metal ions by NS-CDs fluorescent test strips under 365 nm UV light, b is the visual detection of different concentrations of Cr 6+ c is the visual detection of different concentrations of Mn 7+ d is the linear relationship between (G+B) / R and Cr 6+ e is the linear relationship between (G+B) / R and Mn 7+ f is the linear relationship between (G+B) / R and Cr+Mn.

[0030] Figure 9 a is the photos of hydrogel with and without NS-CDs under daylight and 365 nm UV light, b is the time stability of NS-CDs fluorescent hydrogel, c is the optimal time of NS-CDs fluorescent hydrogel to detect Cr 6+

[0031] Figure 10 a is the color change of NS-CDs fluorescent hydrogel with the increase of Mn 7+ (a) and Cr 6+ (b) concentration under daylight and 365 nm UV light, (c), (d) the selectivity of NS-CDs fluorescent hydrogel to metal ions. DETAILED DESCRIPTION

[0032] Various exemplary embodiments of the present application will now be described in detail, with reference to the figures. The detailed description is not to be regarded as limiting the application, but rather as a description of certain aspects, features and embodiments of the application.

[0033] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Additionally, the use of the term "about" in relation to a value or a range of values is to be understood as encompassing each individual value or range of values within the range of the stated value or range of values. In any statement of a value or a range of values, the intermediate values and the smaller ranges within the range of the stated value or range of values are also included in the application. The upper and lower limits of these smaller ranges can independently be included or excluded from the range.

[0034] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although preferred methods and materials are described, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In case of conflict, the content of the present specification will control.

[0035] ​Many modifications and variations to the illustrative embodiments described herein will be apparent to those of ordinary skill in the art from the foregoing description. Such variations may not depart from the scope or spirit of the present disclosure. Other embodiments of the disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure disclosed herein. The specification and examples given are exemplary only.

[0036] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having", and the like are open-ended terms that are intended to permit but not limit the inclusion of elements or the number of elements, as the case can be.

[0037] Fluorescence spectroscopy, especially fluorescent probes, has attracted much attention due to its high sensitivity, cost-effectiveness, practicality, fast response, and simplicity. CDs, as an environmentally friendly fluorescent nanomaterial, have low toxicity, good water solubility, and excellent optical properties compared to traditional quantum dots. In particular, moderate doping of heteroatoms (such as N, S, and P) can change the surface structure and electron distribution, leading to changes in the physical and chemical properties of CDs, thereby improving quantum yield and fluorescence performance. Therefore, CDs are widely used in the detection of heavy metal ions in the environment.

[0038] In addition, the visual analysis based on the photoluminescence of CDs is an important feature of CDs in sensing applications. The carbon quantum dots reported so far can be used to qualitatively analyze target species by using a fluorescence spectrometer, but solution-based fluorescent sensors still have to rely on the help of sophisticated fluorescence spectrometers, which hinders their application for real-time and on-site detection of target substances. Inspired by the easy recognition of color change of fluorescent CDs, visual fluorescence test paper can be developed to realize naked-eye qualitative and semi-quantitative determination. This paper-based sensing system demonstrates its advantages of low cost, portability, and versatility.

[0039] The first aspect of the present application provides a preparation method of nitrogen and sulfur co-doped carbon quantum dots, comprising the following steps:

[0040] Dissolve phenolphthalein, m-phenylenediamine and glutathione in an alkaline solution and perform a hydrothermal reaction;

[0041] After the hydrothermal reaction, sequentially filter, dialyze, and freeze-dry to obtain the nitrogen and sulfur co-doped carbon quantum dots.

[0042] In a preferred embodiment of the present application, the molar ratio of phenolphthalein, m-phenylenediamine and glutathione is 1:20:1.

[0043] In a preferred embodiment of the present application, the alkaline solution is a 0.01M (pH=12) NaOH solution.

[0044] In a preferred embodiment of the present application, the temperature of the hydrothermal reaction is 160°C and the time is 8h.

[0045] The precursor (phenolphthalein) species selection, the molar ratio of phenolphthalein, m-phenylenediamine and glutathione, and the temperature and time of hydrothermal reaction are the keys to realize the preparation of high quantum yield NS-CDs in the application, in order to obtain the NS-CDs with the best fluorescence performance, the effects of precursor molar ratio, reaction temperature and reaction time on the quantum yield of the synthesized material are investigated. Figure 1 It can be seen that when the molar ratio is 1:20:1, the quantum yield of the synthesized NS-CDs is as high as 71.16%. This result is higher than the quantum yield of most of the blue fluorescent carbon dots reported so far. In the application, if the molar ratio is further increased, the quantum yield will decrease, and the reason may be that the raw materials are too much, thereby hindering carbonization. When the temperature is changed, both too low and too high temperature will lead to the decrease of the quantum yield of NS-CDs, and similarly, when the reaction time is changed by the single factor variable method, the quantum yield will also be affected. The best reaction condition is 160℃, 8h. This result shows that the mass ratio of phenolphthalein, m-phenylenediamine and glutathione and the reaction temperature and time are crucial for obtaining NS-CDs with high quantum yield.

[0046] The filtration is specifically filtering with a 0.22μm filter membrane; the dialysis is specifically dialysis with a 500-1000Da dialysis membrane for 48h; and the freeze-drying time is 2d. The application does not particularly limit the vacuum degree and temperature of freeze-drying, and the freeze-drying parameter settings well known to those skilled in the art can be adopted.

[0047] The application provides a nitrogen and sulfur co-doped carbon quantum dot prepared by the preparation method.

[0048] The NS-CDs provided by the application have excellent salt resistance, acid-base resistance (such as Figure 4 obvious stability at pH 5-11, and light bleaching resistance.

[0049] The third aspect of the application provides a fluorescent filter paper strip, the filter paper is soaked in a NS-CD solution of 1-2mg / mL for 4-6min, and then dried at 50℃-60℃ to obtain the fluorescent filter paper strip; the NS-CD solution is prepared by dissolving the nitrogen and sulfur co-doped carbon quantum dot in water.

[0050] The fourth aspect of the application provides a fluorescent hydrogel, agarose is dissolved in ultrapure water, then the NS-CD solution is added and mixed uniformly, and then solidified to obtain the fluorescent hydrogel.

[0051] The mass-volume ratio of the agarose and the ultrapure water is 0.5 g:50 mL; the volume ratio of the ultrapure water and the NS-CD solution is 50:3; the concentration of the NS-CD solution is 3 mg / mL; and the NS-CD solution is prepared by dissolving the nitrogen-sulfur co-doped carbon quantum dots described above in water.

[0052] The fifth aspect of the present application provides the application of the nitrogen-sulfur co-doped carbon quantum dots described above, the fluorescent filter paper strip described above, or the fluorescent hydrogel described above in the detection of Cr 6+ and Mn 7+ in a water environment.

[0053] Under the irradiation of a 365 nm ultraviolet lamp, the bright blue fluorescence of the NS-CDs of the present application gradually weakens with the increase of the concentration of Cr 6+ and Mn 7+ in an aqueous solution, and is completely quenched, which proves that the NS-CDs fluorescent probe of the present application has the potential for visual detection of Cr 6+ and Mn 7+ .

[0054] The sixth aspect of the present application provides a method for visual detection of Cr 6+ and Mn 7+ in a water environment, which comprises the following steps:

[0055] The fluorescent filter paper strip described above or the fluorescent hydrogel kit described above is soaked in a water sample for 20 min, and then it is observed whether the blue fluorescence is darkened or quenched under the irradiation of a 365 nm ultraviolet lamp; if the blue fluorescence is observed to be darkened or quenched, it indicates that Cr 6+ or Mn 7+ exists in the water environment.

[0056] The technical solutions described in the present application are conventional solutions in the art if not specifically described, and the reagents or raw materials used are purchased from commercial channels or are already disclosed if not specifically described.

[0057] The technical solutions provided by the present application will be described in detail below in combination with examples, but they should not be understood as limiting the scope of protection of the present application.

[0058] Example 1

[0059] 0.0318 g of phenolphthalein, 0.216 g of m-phenylenediamine and 0.0307 g of glutathione were weighed out in a ratio of 1:20:1, dissolved in 20 mL of 0.01 M (pH = 12) NaOH solution, and ultrasonically treated at room temperature for 10 min to fully dissolve, transferred into a 50 mL polytetrafluoroethylene liner, sealed in a stainless steel high-pressure hydrothermal reaction kettle, and placed in an oven at 160°C for 8 h. After the reaction was completed, the obtained solution was naturally cooled to room temperature, filtered with a 0.22 μm filter membrane, then dialyzed with a 500-1000 Da dialysis membrane for 48 h, and then freeze-dried for 2 d to obtain NS-CDs powder.

[0060] The NS-CDs prepared in Example 1 were characterized and performance tested, as follows:

[0061] 1. Morphology characterization of NS-CDs

[0062] The morphology of NS-CDs was characterized by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM). As can be seen from Figure 2 Fig. a, the prepared NS-CDs were quasi-spherical nanoparticles with uniform dispersion. The inserted HRTEM showed clear lattice fringes with a spacing of 0.21 nm, corresponding to the (100) plane of graphite, indicating the successful synthesis of NS-CDs. Figure 2 Fig. b shows that the particle size distribution of NS-CDs was between 0.5 and 5 nm, with an average particle size of 2.54 ± 0.5 nm. The crystallinity of NS-CDs was further analyzed by XRD spectrum, Figure 2 Fig. c shows a typical broad peak at about 2θ = 25°, indicating that NS-CDs had good crystallinity, which corresponded to the result obtained in Figure 2 Fig. a HRTEM image. The Raman spectrum of NS-CDs is shown in Figure 2 Fig. d, and two characteristic peaks D peak and G peak of NS-CDs were observed at 1392 cm -1 and 1584 cm -1 , respectively. The D peak is related to sp 3 hybridized carbon atoms, and the G peak is from the planar vibration of sp 2 carbon atoms, and ID / IG = 0.95, reflecting the tendency of carbon atoms in NS-CDs to be ordered sp 2 carbon, indicating the formation of carbon nuclei.

[0063] 2. Structural characterization of carbon dots

[0064] The surface functional groups and elemental composition of NS-CDs were analyzed by FT-IR spectrum and XPS. As shown in Figure 3 Fig. a, NS-CDs had absorption peaks at 3218-3391 cm -1The absorption band in this range corresponds to the stretching vibrations of -OH and NH, at 1607 cm⁻¹. -1 The strong absorption peak at 1490 cm⁻¹ is attributed to the stretching vibration of C=O and the bending vibration of NH₄⁺; -1 This is a C=C stretching vibration, 1314 cm. -1 The stretching vibration belongs to CN, 1165cm -1 The stretching vibration belongs to the COC type. 953.1 cm -1 The peak at 684.4 cm⁻¹ belongs to the CS bending vibration. -1 The weak absorption peak at that point is also attributed to the stretching vibration of CS, which shows that N and S elements have been successfully doped into the framework of NS-CDs.

[0065] Figure 3 Figure b shows the XPS full spectrum of NS-CDs, which shows four characteristic peaks at 285.25 eV, 399.92 eV, 532.10 eV, and 169.29 eV, corresponding to C1s, N1s, O1s, and S2p, respectively. Figure 3 In the image, c represents the C1s spectrum, with peaks at 284.8 eV, 286.05 eV, and 288.43 eV corresponding to the CC / C=C, CS, and C=N / CO functional groups, respectively. The N1s spectrum (…) Figure 3 In (d), it can be decomposed into two peaks, located at 399.47 eV (C=N) and 400.87 eV (CN). Figure 3 In the spectrum of e O 1s, the peaks at 531.81, 532.96 and 536.38 eV are attributed to C=O, C–O and O–H functional groups. Figure 3 The characteristic peaks of f for S2p are located at 163.88 and 169.43 eV, respectively, representing the two forms of S2p in -C-SOx-. These results indicate that the surface of NS-CDs is rich in functional groups.

[0066] 3. Optical properties of carbon dots

[0067] The optical properties of NS-CDs were analyzed using ultraviolet-visible spectroscopy and fluorescence spectroscopy. For example... Figure 4 As shown in Figure a, the UV-Vis absorption spectrum exhibits a distinct absorption band at 289 nm, attributed to the n→π* transition in the C=O bond, which is typically caused by the internal chemical and band structure of the carbon dots. The optimal excitation and emission wavelengths for NS-CDs are 345 nm and 438 nm, respectively. Furthermore, they exhibit strong blue fluorescence under 365 nm UV illumination (inset). Figure 4The results in Fig. 3c show that the emission wavelength of NS-CDs does not change with the excitation wavelength, which is a typical excitation-independent emission. The reason for this can be attributed to the good size uniformity of the as-synthesized NS-CDs, which leads to similar energy band structures under different excitation wavelengths. Thus, the emission wavelength does not change with the excitation wavelength. Moreover, the UV-Vis spectrum has only one obvious absorption peak, leading to only one main n→π* transition energy level, thus showing single-wavelength luminescence. The quantum yield of NS-CDs was calculated to be 71.16% with quinine sulfate as the reference. This result is higher than most of the quantum yields of previously reported blue fluorescent carbon dots. Figure 4 The results in Fig. 3c show that the emission wavelength of NS-CDs does not change with the excitation wavelength, which is a typical excitation-independent emission. The reason for this can be attributed to the good size uniformity of the as-synthesized NS-CDs, which leads to similar energy band structures under different excitation wavelengths. Thus, the emission wavelength does not change with the excitation wavelength. Moreover, the UV-Vis spectrum has only one obvious absorption peak, leading to only one main n→π* transition energy level, thus showing single-wavelength luminescence. The quantum yield of NS-CDs was calculated to be 71.16% with quinine sulfate as the reference. This result is higher than most of the quantum yields of previously reported blue fluorescent carbon dots. Figure 4 The results in Fig. 3c show that the emission wavelength of NS-CDs does not change with the excitation wavelength, which is a typical excitation-independent emission. The reason for this can be attributed to the good size uniformity of the as-synthesized NS-CDs, which leads to similar energy band structures under different excitation wavelengths. Thus, the emission wavelength does not change with the excitation wavelength. Moreover, the UV-Vis spectrum has only one obvious absorption peak, leading to only one main n→π* transition energy level, thus showing single-wavelength luminescence. The quantum yield of NS-CDs was calculated to be 71.16% with quinine sulfate as the reference. This result is higher than most of the quantum yields of previously reported blue fluorescent carbon dots. Figure 4 The results in Fig. 3c show that the emission wavelength of NS-CDs does not change with the excitation wavelength, which is a typical excitation-independent emission. The reason for this can be attributed to the good size uniformity of the as-synthesized NS-CDs, which leads to similar energy band structures under different excitation wavelengths. Thus, the emission wavelength does not change with the excitation wavelength. Moreover, the UV-Vis spectrum has only one obvious absorption peak, leading to only one main n→π* transition energy level, thus showing single-wavelength luminescence. The quantum yield of NS-CDs was calculated to be 71.16% with quinine sulfate as the reference. This result is higher than most of the quantum yields of previously reported blue fluorescent carbon dots. Figure 4 The results in Fig. 3c show that the emission wavelength of NS-CDs does not change with the excitation wavelength, which is a typical excitation-independent emission. The reason for this can be attributed to the good size uniformity of the as-synthesized NS-CDs, which leads to similar energy band structures under different excitation wavelengths. Thus, the emission wavelength does not change with the excitation wavelength. Moreover, the UV-Vis spectrum has only one obvious absorption peak, leading to only one main n→π* transition energy level, thus showing single-wavelength luminescence. The quantum yield of NS-CDs was calculated to be 71.16% with quinine sulfate as the reference. This result is higher than most of the quantum yields of previously reported blue fluorescent carbon dots.

[0068] 4. Selectivity of NS-CDs for Cr 6+ , Mn 7+ and condition optimization

[0069] To study the selectivity of NS-CDs for metal ions in aqueous environments, K + , Na + , Ag + , Mg 2+ , Zn 2+ , Ca 2+ , Hg 2+ , Ba 2+ , Mn 2+ , Fe 2+ , Cu 2+ , Cd 2+ , Co2+ Fe 3+ Cr 6+ Mn 7+ Al 3+ and Cr 3+ Different metal ions were added to NS-CDs solution, and the changes in fluorescence intensity were analyzed (the concentration of all metal ions was set to 500 μM). Figure 5 As shown in Figure a, Hg 2+ Fe 3+ Cr 6+ Mn 7+ All of them can reduce the fluorescence of NS-CDs, but only Cr can. 6+ and Mn 7+ It can quench the fluorescence of NS-CDs, while the effect of other metal ions on the fluorescence of NS-CDs is negligible. Figure 5 In the figure, b is the F / F0 bar chart after adding different metal ions. The figure shows the fluorescence sensor of NS-CDs for Cr. 6+ and Mn 7+ It exhibits higher selectivity than other metal ions, but Fe 3+ For Cr 6+ and Mn 7+ The detection process also has a certain degree of interference. Furthermore, Figure 5 Figure c shows the absorption spectra of NS-CDs in the presence of different metal ions. The results confirm that in Hg 2+ Fe 3+ Cr 6+ and Mn 7+ In the presence of these metal ions, the absorption peak of NS-CDs shifted, and the degree of shift corresponded to the degree of fluorescence quenching. Other metal ions did not affect their absorption peaks, consistent with fluorescence detection results. Changes in pH significantly affect the sensor's sensitivity and selectivity to metal ions in the environment or biological matrix, particularly Fe. 3+ The different binding capacities of NS-CDs in acidic and alkaline environments often alter the sensor's response. Therefore, the selectivity of NS-CDs for common interfering ions at different pH values ​​was evaluated to reduce Fe... 3+ For Cr 6+ and Mn 7+ Interference. For example... Figure 5 As shown in Figure d, when the pH is adjusted to acidic (2-6), the fluorescence quenching effect induced by NS-CD increases, but it does not change the responsiveness of NS-CDs to other metal ions. NS-CDs respond to Cr 6+ and Mn 7+ It still exhibits high selectivity and sensitivity. However, when the pH is adjusted to alkaline, in alkaline solutions with pH 8 and 10, various metal ions such as Ag... + Hg2+ Fe 2+ Fe 3+ All of these factors lead to a decrease in the fluorescence emission intensity of NS-CDs, indicating that multiple interfering ions affect the fluorescence emission intensity of NS-CDs for Cr. 6+ and Mn 7+ The detection. In summary, an acidic environment can mask Fe. 3+ Interference, high sensitivity and high selectivity for Cr detection 6+ and Mn 7+ .

[0070] To obtain optimal sensing performance, NS-CDs were used to detect Cr. 6+ and Mn 7+ The conditions were optimized, including pH and incubation time. The effects of adding 500 μM Cr at pH 2-6 were investigated. 6+ and Mn 7+ The fluorescence quenching rate of NS-CDs after quenching. For example... Figure 5 As shown in Figure e, when the pH is 2-6, Cr 6+ The quenching rate of NS-CDs can reach over 90%, and the quenching efficiency remains almost stable within this pH range. The addition of Mn... 7+ After that, Mn 7+ The quenching efficiency for NS-CDs gradually increased with increasing pH, reaching its maximum at pH 5, and then decreased with further increases in pH. Therefore, pH 5 was chosen as the optimal pH for Cr detection. 6+ and Mn 7+ The optimal pH value was determined. Incubation time also plays a crucial role in the accuracy of the detection. Therefore, the addition of Cr was investigated. 6+ and Mn 7+ The effect of different incubation times on the fluorescence quenching rate of NS-CDs. For example... Figure 5 As shown in f, Cr is added 6+ and Mn 7+ The maximum quenching effect was reached after 30 seconds and remained stable for up to 5 minutes. Ultimately, 30 seconds was taken as the optimal reaction time. This demonstrates that the NS-CDs prepared in Example 1 are effective for detecting Cr. 6+ and Mn 7+ It is highly efficient.

[0071] 5. NS-CDs on Cr 6+ and Mn 7+ Sensing

[0072] Under optimal detection conditions, Cr at different concentrations (0-500 μM) was detected. 6+ and Mn 7+The fluorescence intensity of the NS-CDs solution after addition was used to evaluate the sensitivity of the NS-CDs sensor. As shown in Figure 6 Fig. 2a and Figure 6 Fig. 2c, the fluorescence intensity of the NS-CDs continuously decreased with the increase of the Cr 6+ and Mn 7+ concentrations, and the inset shows that the blue fluorescence of the NS-CDs aqueous solution gradually darkened under a 365 nm ultraviolet lamp with the increase of the Cr 6+ and Mn 7+ concentrations, which corresponded to the decrease of the intensity. The degree of quenching F0-F was nonlinearly fitted with the added Cr 6+ and Mn 7+ concentrations by using the Boltzmann equation, as shown in Figure 6 Fig. 2b and Figure 6 Fig. 2c. In the low concentration range, a rapid decrease in the fluorescence quenching value was observed, and the value gradually stabilized when the concentration approached 500 μM. The fitting equations were Y = 8386.644-2004991.103 / (1+exp((x+594.052) / 108.587)), R 2 = 0.998 Figure 6 (b) and Y = 7421.469-7422.497 / (1+exp((x+588.319) / 80.763)), R 2 = 0.988 Figure 6 (d). Wherein Y and X represent F0-F and the Cr 6+ / Mn 7+ concentration, respectively. Here, F0and F are the fluorescence intensities of the NS-CDs at 438 nm in the absence and presence of Cr 6+ / Mn 7+ . By fitting the linear relationship between F0-F and the Cr 6+ concentration in the range of 0-60 μM, Y = 65.94X + 22.60, R 2 = 0.996, and the detection limit (LOD) was 0.48 μM (LOD = 3SD / s). The linear fitting of Mn 7+ can be divided into two segments between 0-7.5 μM and 7.5-70 μM, and the fitting equations were Y = 226.55X + 35.39, R 2 = 0.991 and Y = 48.77X + 1353.78, R 2 = 0.990. The detection limit was 0.14 μM. Compared with other reported sensors for detecting Cr 6+ and Mn 7+ , the sensing performance of the NS-CDs prepared in the present application was better, which indicated that the NS-CDs were superior in detecting Cr 6+ and Mn7+ This aspect has potential application value.

[0073] 6. NS-CDs on Cr 6+ and Mn 7+ Sensing mechanism research

[0074] The study investigated the effects of Cr addition using ultraviolet spectroscopy, fluorescence lifetime, and changes in Zeta potential. 6+ and Mn 7+ The fluorescence quenching mechanism of post-NS-CDs. For example... Figure 7 As shown in Figure a, Cr 6+ and Mn 7+ They exhibit different absorption spectra, and Cr 6+ The absorption spectrum of Cr significantly overlaps with the excitation spectrum of NS-CDs, therefore it is inferred that Cr 6+ Fluorescence quenching is caused by the internal filtration effect. Further research into the quenching mechanism will be conducted using fluorescence lifetime data. Figure 7 As shown in Figure b, the fluorescence lifetime of NS-CDs is 6.38 ns, and the addition of Cr... 6+ The post-fluorescence lifetime is 6.08 ns, Cr 6+ The presence of [a certain substance] did not significantly alter the fluorescence lifetime of NS-CDs, suggesting a static quenching mechanism. Furthermore, UV absorption spectroscopy was used to confirm the quenching mechanism, which was determined by [a specific mechanism]. Figure 7 From c, we know that adding Cr 6+ Subsequently, the absorption band position of NS-CDs at 289 nm did not change, further confirming the internal filtration effect, i.e., Cr 6+ It absorbed the energy emitted from NS-CDs. Furthermore, the absorption band intensity in the range of 250 to 400 nm increased with Cr. 6+ The concentration increases gradually (and the effect gradually increases). Figure 7 As shown in d), this indicates the surface functional groups and Cr of NS-CDs. 6+ Non-fluorescent complexes can form between them, leading to static quenching. Mn... 7+ The absorption spectrum of Mn does not significantly overlap with the excitation and emission spectra of NS-CDs, thus ruling out the internal filtering effect and fluorescence resonance energy transfer mechanism. The fluorescence lifetime can be determined by detecting the Mn... 7+ After addition, the fluorescence lifetime decreased to 5.65 ns. Figure 7 As shown in b), it is therefore speculated that the dynamic quenching is caused by the interaction with the excited state of NS-CDs. Figure 7 c and Figure 7 The text appears to be a mix of Chinese characters and symbols, possibly related to a computer program or a document. A direct translation wouldn't be meaningful without further context or clarification. 7+ After addition, the exposed Mn 7+ The absorption band disappears near 520 nm, which may be due to Mn. 7+ (MnO 4- Redox transformation to Mn4+ (MnO 2 ), while the absorption peak of NS-CDs did not change significantly. Due to the photo-induced oxidation electron transfer reaction, Mn 7+ forms a non-radiative complex with NS-CDs.

[0075] 7. Cr 6+ detection

[0076] As shown in Table 1 and Table 2, in order to detect the practical applicability and feasibility of NS-CDs fluorescent probe, it was applied to the analysis of actual water samples, and tap water and lake water from Shanxi University were collected to analyze Cr 6+ and Mn 7+ spiked recovery experiments. Different concentrations of Cr 6+ (Table 1) and Mn 7+ (Table 2) were added to the water samples and the fluorescence was recorded. As a fluorescent probe, the recovery rate of Cr 6+ was 96% to 106%, and the recovery rate of Mn 7+ was 94% to 104%, with a low relative standard deviation (RSD). The results showed that NS-CDs can be used for the analysis of actual water samples.

[0077] Table 1

[0078]

[0079] Table 2

[0080]

[0081]

[0082] 8. Paper-based sensing platform for detecting Cr 6+ and Mn 7+

[0083] Based on the response of NS-CDs to Cr 6+ and Mn 7+ , the detection ability of NS-CDs as a paper-based sensor for Cr 6+ and Mn 7+ was further studied. The NS-CDs-based fluorescent filter paper strips (the preparation steps of the fluorescent filter paper strips are as follows: cut the filter paper into 1 cm x 4 cm rectangular strips, then immerse them in NS-CD solution (1 mg / mL) for about 5 minutes. Then dry them in an oven at 55°C for 30 minutes to obtain the fluorescent filter paper strips) were soaked in 500 μM different ion solutions, and after 30 min, they were dried, and the fluorescence color change was observed, as shown in Figure 8As shown in Figure a, the NS-CDs test strip exhibits significant blue fluorescence under 365nm ultraviolet light, and in Cr 6+ and Mn 7+ After immersion in the solution (pH 5), the fluorescence was significantly quenched, while other ions did not affect the fluorescence of the NS-CDs strip, indicating that the prepared NS-CDs fluorescent test strip has good selectivity for metal ions. Through this simple filter paper sensing, obvious fluorescence changes can be clearly observed with the naked eye, demonstrating its effectiveness against Cr in the environment. 6+ and Mn 7+ Perform rapid visual inspection.

[0084] For semi-quantitative detection, fluorescent filter paper strips of NS-CDs were soaked in Cr at different concentrations. 6+ and Mn 7+ In solution. After 30 minutes, it was dried and the fluorescence color change was observed under 365nm ultraviolet light. It could be clearly observed with the naked eye that the fluorescence color changed with Cr. 6+ and Mn 7+ As the concentration increases, the blue fluorescence of the NS-CDs fluorescent filter paper strip gradually dims until it is quenched. Figure 8 b, Figure 8 (c) To further achieve quantitative detection, images were captured using a smartphone under 365nm ultraviolet light, and color recognition software was used to identify the RGB values ​​of filter paper strips of corresponding concentrations. The resulting (G+B) / R value was then compared with the Cr value. 6+ and Mn 7+ The concentration was fitted to confirm its linear relationship. For example... Figure 8 d, Figure 8 As shown in Figure e, (G+B) / R changes with Cr 6+ and Mn 7+ The increase in concentration showed a good linear relationship, with the linear equations being Y = -0.0106X + 16.27(R). 2 =0.9998) and Y = -0.00098X + 21.19(R) 2 =0.9904). The linear range is 0-500 μM, indicating that the smartphone-assisted paper-based fluorescence sensing platform can quickly and accurately measure Cr in water. 6+ and Mn 7+ It is feasible.

[0085] 9. Fluorescent hydrogels based on NS-CDs for visual inspection of Cr 6+ and Mn 7+

[0086] Solid-state fluorescence of carbon dots is particularly important for their application in sensing. Therefore, based on the excellent luminescent properties of NS-CDs, an NS-CDs-based fluorescent hydrogel was prepared using agarose (preparation method as follows: 0.5 g of agarose was added to 50 mL of ultrapure water and stirred vigorously at 200 °C until completely dissolved into a transparent liquid. Then, 3 mL of a 3 mg / mL NS-CDs solution was added and stirred for 3 min to ensure thorough mixing. The solution was then rapidly transferred to a silicone mold or small reagent bottle and allowed to solidify. When the mixture was completely solidified, it was removed from the mold, forming an NS-CDs-based fluorescent hydrogel). Its performance as a solid-state fluorescent material in Cr... 6+ and Mn 7+ Applications in sensing. For example... Figure 9 As shown in Figure a, two patterns of hydrogels were fabricated using a silicone model. The hydrogel synthesized with agarose was colorless under 365nm ultraviolet light, while the hydrogel with added NS-CDs exhibited bright blue fluorescence under the same conditions, and as shown in Figure a... Figure 9 As shown in Figure b, when the NS-CDs hydrogel is exposed to air, its morphology and blue fluorescence remain unchanged for 3 days, indicating that it has strong stability as a solid-state fluorescence sensor and can be used for on-site visual monitoring of analytes.

[0087] To investigate the application of NS-CDs hydrogel as a solid sensor in the detection of Cr 6+ and Mn 7+ In its application, it is soaked in Cr at different concentrations. 6+ and Mn 7+ In the solution, the fluorescence color change was observed under 365nm UV light irradiation. First, the effect of immersion time on the degree of quenching was investigated by immersing the NS-CDs hydrogel in 500µM Cr solution. 6+ In the solution, observe the change in its fluorescence color every 5 minutes, such as Figure 9 As shown in Figure c, after immersion for 5 minutes, the fluorescence color of the hydrogel was significantly quenched, while after immersion for 20 minutes, the fluorescence was completely quenched. Furthermore, under fluorescent light, the hydrogel was observed to have been quenched by Cr. 6+ Upon immersion in the solution, the color turned pale yellow. Ultimately, a 20-minute immersion time was chosen as the final determination for Cr. 6+ and Mn 7+ The best time.

[0088] Secondly, the NS-CDs hydrogel was soaked in Cr at different concentrations. 6+ and Mn 7+ In a (0-500 μM) solution, after 20 minutes, observe the change in fluorescence color under a 365 nm UV lamp. Figure 10 a, Figure 10 As shown in b, with Cr6+ and Mn 7+ With the increase of concentration, the blue fluorescence of NS-CDs hydrogel under 365 nm ultraviolet light irradiation showed a gradual lightening trend, and when the concentration increased to 500 uM, the fluorescence was completely quenched, which corresponded to the detection results of the fluorescence spectrometer. In addition, the present application used a kit (pH = 5) made of NS-CDs fluorescent hydrogel to detect the selectivity of the NS-CDs hydrogel to metal ions. Some potential interfering ions were added to the hydrogel kit, and the fluorescence intensity of the hydrogel was detected by the fluorescence spectrometer. As shown in Table 1, the hydrogel showed a significant fluorescence quenching effect on Cr Figure 10 As can be seen from Table 1, the hydrogel containing Cr 6+ and Mn 7+ showed a significant fluorescence quenching effect, while the addition of other ions hardly affected the fluorescence of the hydrogel, indicating that it had excellent selectivity. Based on the above analysis, the NS-CDs fluorescent hydrogel has great application prospects as a solid sensor in realizing the on-site detection and visual semi-quantitative analysis of Cr 6+ and Mn 7+ .

[0089] On the basis of Example 1, the quantum yield of NS-CDs under different molar ratios (i.e., the only difference from Example 1 is that the molar ratio of phenolphthalein, m-phenylenediamine and glutathione is 1:5:1, 1:10:1, 1:15:1, 1:25:1, 1:30:1, 1:20:5, 1:20:10, respectively), different reaction temperatures (i.e., the only difference from Example 1 is that the reaction temperature in the oven is 140℃, 180℃, respectively), and different reaction times (i.e., the only difference from Example 1 is that the reaction time in the oven is 6h, 10h, respectively) was also verified, and the results are shown in Table 2. Figure 1 Figure 1 As shown in Table 2, the quantum yield of NS-CDs under the conditions of Example 1 is the highest.

[0090] The above-described examples only describe the preferred modes of the present application and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.​

Claims

1. A method for preparing nitrogen-sulfur co-doped carbon quantum dots, characterized in that, Includes the following steps: Phenolphthalein, m-phenylenediamine, and glutathione were dissolved in an alkaline solution and subjected to a hydrothermal reaction. After the hydrothermal reaction is completed, the mixture is filtered, dialyzed, and freeze-dried sequentially to obtain the nitrogen-sulfur co-doped carbon quantum dots.

2. The method for preparing nitrogen-sulfur co-doped carbon quantum dots according to claim 1, characterized in that, The molar ratio of phenolphthalein, m-phenylenediamine and glutathione is 1:(5-30):(1-10).

3. The method for preparing nitrogen-sulfur co-doped carbon quantum dots according to claim 1, characterized in that, The alkaline solution is a 0.01M NaOH solution or a KOH solution.

4. The method for preparing nitrogen-sulfur co-doped carbon quantum dots according to claim 1, characterized in that, The hydrothermal reaction is carried out at a temperature of 140℃ to 180℃ for a duration of 6 to 10 hours.

5. Nitrogen-sulfur co-doped carbon quantum dots prepared by the preparation method according to any one of claims 1 to 4.

6. A fluorescent filter paper strip, characterized in that, The filter paper was immersed in 1-2 mg / mL NS-CD solution for 4-6 min, and then dried at 50℃~60℃ to obtain the fluorescent filter paper strip; the NS-CD solution was prepared by dissolving the nitrogen-sulfur co-doped carbon quantum dots of claim 5 in water.

7. A fluorescent hydrogel reagent kit, characterized in that, Agarose was dissolved in ultrapure water, then NS-CD solution was added, mixed well, and solidified to obtain the fluorescent hydrogel. The mass-to-volume ratio of agarose to ultrapure water is 0.5 g: 50 mL; the volume ratio of ultrapure water to NS-CD solution is 50: 3; the concentration of NS-CD solution is 3 mg / mL; the NS-CD solution is prepared by dissolving the nitrogen-sulfur co-doped carbon quantum dots of claim 5 in water.

8. The nitrogen-sulfur co-doped carbon quantum dots as described in claim 5, the fluorescent filter paper strip as described in claim 6, or the fluorescent hydrogel kit as described in claim 7, for detecting Cr in an aqueous environment. 6+ and Mn 7+ Applications.

9. A type of Cr in an aquatic environment 6+ and Mn 7+ The visualization detection method is characterized by, Includes the following steps: Immerse the fluorescent filter paper strip of claim 6 or the fluorescent hydrogel of claim 7 in a water sample for 20 minutes, and then observe whether the blue fluorescence dims or quenches under 365 nm ultraviolet light. If the blue fluorescence dims or quenches, it indicates the presence of Cr in the aquatic environment. 6+ or Mn 7+ .

10. The Cr in the water environment according to claim 9 6+ and Mn 7+ The visualization detection method is characterized by, The pH of the water sample was 4–6.

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