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

Abstract

The invention relates to the field of visual metal detection, in particular to a preparation method, a product and application of a nitrogen-sulfur co-doped carbon quantum dot. The preparation method of the nitrogen-sulfur co-doped carbon quantum dots comprises the following steps: dissolving phenolphthalein, m-phenylenediamine and glutathione in an alkali solution, and carrying out a hydrothermal reaction; after the hydrothermal reaction is finished, sequentially filtering, dialyzing and freeze-drying to obtain the nitrogen and sulfur co-doped carbon quantum dots. The nitrogen-sulfur co-doped carbon quantum dots (NS-CDs) are synthesized by taking phenolphthalein as a precursor and taking m-phenylenediamine and glutathione as a nitrogen source and a sulfur source respectively. The preparation method disclosed by the invention is simple, the quantum yield of the NS-CDs prepared by utilizing the method disclosed by the invention is up to 71.16%, and the result is higher than the quantum yield of most blue fluorescent carbon dots reported at present. The NS-CDs prepared by the method disclosed by the invention can be successfully applied to the visual detection application aspect of Cr < 6 + > and Mn < 7 + >.

Description

Technical Field

[0001] The present invention relates to the field of visual metal detection, and in particular to a preparation method, product and application of nitrogen-sulfur co-doped carbon quantum dots. Background Art

[0002] Cr 6+ It is a highly toxic heavy metal pollutant, mostly found in industrial wastewater and waste residue, and eventually discharged into the biological environment. Excessive Cr 6+ Due to its high oxidation potential and strong ability to penetrate biological membranes, it is carcinogenic and mutagenic to the respiratory tract, internal organs and nervous tissues. - ) is widely used as a strong oxidant in laboratories and as a preservative and disinfectant in industry. Excessive exposure to permanganate may have adverse effects on human health, such as causing deformities and respiratory damage, and severely damaging the central nervous system. Therefore, there is an urgent need to develop reliable, sensitive, highly selective and cost-effective detection technologies to monitor Cr 6+ and Mn 7+ concentration in aqueous media and mitigate its adverse effects.

[0003] So far, various methods for monitoring Cr have been reported. 6+ and Mn 7+ The analysis method of Cr is shown in patent CN106544008B. 6+ However, this method is more complicated for the preparation of fluorescent probes. As shown in patent CN118580854A, only Mn 7+ The fluorescence detection method has not been applied in practice, and the quantum yield (QY) of the fluorescent probe prepared by this method is low and the fluorescence intensity is insufficient, so it is not the optimal means to detect the ion. Summary of the Invention

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

[0005] To achieve the above object, the present invention provides the following solutions:

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

[0007] Phenolphthalein, m-phenylenediamine and glutathione are dissolved in an alkaline solution and subjected to a hydrothermal reaction;

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

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

[0010] The third technical solution of the present invention is a fluorescent filter paper strip, which is obtained by soaking the filter paper in a 1-2 mg / mL NS-CD solution for 4-6 minutes and then drying it at 50°C to 60°C; the NS-CD solution is prepared by dissolving the nitrogen-sulfur co-doped carbon quantum dots according to claim 5 in water.

[0011] A fourth technical solution of the present invention is a fluorescent hydrogel and a kit, wherein agarose is dissolved in ultrapure water, and then an NS-CD solution is added, mixed, 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; the NS-CD solution is prepared by dissolving the nitrogen-sulfur co-doped carbon quantum dots according to claim 5 in water.

[0013] The fifth technical solution of the present invention is that the nitrogen-sulfur co-doped carbon quantum dots, the fluorescent filter paper strips or the fluorescent hydrogel are used to detect Cr in water environment. 6+ and Mn 7+ application.

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

[0015] Soak the fluorescent filter paper strips or the fluorescent hydrogel in the water sample for 20 minutes, and then observe whether the blue fluorescence becomes darker or quenched under 365nm ultraviolet light; if the blue fluorescence becomes darker or quenched, it indicates that Cr is present in the water environment. 6+ or Mn 7+ In a preferred embodiment of the present invention, the pH of the water sample is 4-6.

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

[0017] The present invention uses phenolphthalein as a precursor, m-phenylenediamine and glutathione as nitrogen and sulfur sources, respectively, to synthesize NS-CDs. The preparation method is simple, and the quantum yield of the NS-CDs prepared using the method is as high as 71.16%, which is higher than the quantum yields of most currently reported blue fluorescent carbon dots. As shown in patent CN116285975A, the quantum yield of carbon dots synthesized from biomass is only 9.03%. As shown in patent CN116622368B, using the concept of solid waste resource recycling, blue carbon quantum dots are synthesized from wind blade materials through secondary synthesis, with a QY of 28% to 35%. As shown in patent CN116367572A, the blue carbon quantum dots synthesized have a QY of 59.75%. While higher than the yield of conventional synthesis methods, it is still lower than the quantum yield of the present invention.

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

[0019] The NS-CDs prepared by the method of the present invention were used as fluorescent probes to detect Cr in acidic conditions. 6+ and Mn 7+ It showed excellent selectivity, and the fluorescence change of NS-CDs was similar to that of Cr 6+ and Mn 7+ There is a good linear relationship between the concentration of Cr 6+ The linear range of detection was 0–60 μM, and the detection limit was 0.48 μM for Mn 7+ The detection linear range is 0–70 μM, and the detection limit is 0.14 μM. 6+ and Mn 7+ Compared with the sensors, the NS-CDs of the present invention have a wider detection range.

[0020] The NS-CDs prepared by the present invention can be successfully used as fluorescent probes to detect Cr in real water samples. 6+ With Mn 7+ Detection with excellent recovery (Cr 6+ The recovery rate is 96% to 106%, Mn 7+ The recovery rate is between 94% and 104%). BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

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

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

[0024] Figure 3 a is the infrared spectrum of NS-CDs, b is the full XPS 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 a is the UV-visible absorption spectrum and fluorescence spectrum of NS-CDs, b is the emission spectrum under different excitations, c is the xenon lamp stability of NS-CDs, d is the storage time stability, e is the salt concentration stability, and f is the pH stability.

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

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

[0028] Figure 7 a is the excitation and emission spectra of NS-CDs and Cr 6+ and Mn 7+ Absorption spectra of NS-CDs with and without Cr. 6+ and Mn 7+ The fluorescence lifetime of Cr 6+ UV titration spectrum of Mn 7+ UV titration spectrum.

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

[0030] Figure 9 a is a photo of hydrogels with and without NS-CDs under sunlight and 365nm UV light, b is the time stability of NS-CDs fluorescent hydrogel, and c is the detection of Cr by NS-CDs fluorescent hydrogel. 6+ The best time.

[0031] Figure 10 The color of NS-CDs fluorescent hydrogel changes with the Mn 7+ (a) With Cr 6+ (b) Changes with increasing concentration, (c), (d) selectivity of NS-CDs fluorescent hydrogel for metal ions. DETAILED DESCRIPTION

[0032] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0033] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0034] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0035] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0036] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0037] Fluorescence spectrophotometry, particularly fluorescent probes, has attracted widespread attention due to its high sensitivity, cost-effectiveness, practicality, rapid response, and simplicity. CDs, as environmentally friendly fluorescent nanomaterials, possess low toxicity, good water solubility, and superior optical properties compared to traditional quantum dots. In particular, moderate doping with heteroatoms (such as nitrogen, sulfur, and phosphorus) can alter their surface structure and electron distribution, leading to changes in the physicochemical properties of CDs, thereby increasing quantum yield and improving fluorescence properties. Consequently, they are widely used for the detection of heavy metal ions in the environment.

[0038] Furthermore, visual analysis based on the photoluminescence of CDs is an important feature of CDs in sensing applications. Currently reported carbon quantum dots can be used to qualitatively analyze target species using fluorescence spectrometers, but solution-based fluorescent sensors still rely on sophisticated fluorescence spectrometers, which hinders their application in real-time and on-site detection of targets. Inspired by the easy recognition of the color changes of fluorescent CDs, visual fluorescent test strips can be developed to achieve qualitative and semi-quantitative determination by the naked eye. This paper-based sensing system demonstrates its advantages of low cost, portability, and versatility.

[0039] A first aspect of the present invention provides a method for preparing nitrogen-sulfur co-doped carbon quantum dots, comprising the following steps:

[0040] Phenolphthalein, m-phenylenediamine and glutathione are dissolved in an alkaline solution and subjected to a hydrothermal reaction;

[0041] After the hydrothermal reaction is completed, the nitrogen-sulfur co-doped carbon quantum dots are obtained by filtration, dialysis and freeze-drying in sequence.

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

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

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

[0045] The selection of the precursor (phenolphthalein), the molar ratio of phenolphthalein, m-phenylenediamine and glutathione, and the temperature and time of the hydrothermal reaction are the key to the preparation of high quantum yield NS-CDs in this invention. In order to obtain NS-CDs with the best fluorescence performance, the effects of the precursor molar ratio, reaction temperature and reaction time on the quantum yield of the synthesized material were 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 blue fluorescent carbon dots reported so far. In the present invention, if the molar ratio is further increased, the quantum yield will decrease. The reason may be that the raw materials accumulate too much, thereby hindering carbonization. When the temperature is changed, both too low and too high temperatures will lead to a decrease in the quantum yield of NS-CDs. Similarly, changing the reaction time through the single-factor variable method will also affect the quantum yield. The optimal reaction conditions are 160°C and 8h. This result shows that the mass ratio of phenolphthalein, m-phenylenediamine and glutathione, as well as the reaction temperature and time are crucial for obtaining NS-CDs with high quantum yield.

[0046] The filtration is specifically performed using a 0.22 μm filter membrane; the dialysis is specifically performed using a 500-1000 Da dialysis membrane for 48 hours; and the freeze-drying time is 2 days. The present invention does not impose any particular restrictions on the vacuum degree and temperature of freeze-drying, and freeze-drying parameters well known to those skilled in the art can be used.

[0047] A second aspect of the present invention provides nitrogen-sulfur co-doped carbon quantum dots prepared according to the above preparation method.

[0048] The NS-CDs provided by the present invention have excellent salt resistance, acid and alkali resistance (such as Figure 4 As shown in f, it has obvious stability at pH 5-11) and resistance to photobleaching.

[0049] A third aspect of the present invention provides a fluorescent filter paper strip, which is obtained by soaking the filter paper in a 1-2 mg / mL NS-CD solution for 4-6 minutes and then drying it at 50°C to 60°C; the NS-CD solution is prepared by dissolving the above-mentioned nitrogen-sulfur co-doped carbon quantum dots in water.

[0050] A fourth aspect of the present invention provides a fluorescent hydrogel, which is obtained by dissolving agarose in ultrapure water, then adding an NS-CD solution, mixing, and solidifying.

[0051] 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; the NS-CD solution is prepared by dissolving the above-mentioned nitrogen-sulfur co-doped carbon quantum dots in water.

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

[0053] When the NS-CDs of the present invention are irradiated by 365nm ultraviolet light, the bright blue fluorescence of the NS-CDs increases with the Cr 6+ and Mn 7+ The NS-CDs fluorescent probe of the present invention has the ability to visually detect Cr. 6+ and Mn 7+ potential.

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

[0055] Soak the fluorescent filter paper strips or the fluorescent hydrogel kit in the water sample for 20 minutes, and then observe whether the blue fluorescence becomes darker or quenched under 365nm ultraviolet light; if the blue fluorescence becomes darker or quenched, it indicates that Cr is present in the water environment. 6+ or Mn 7+ .

[0056] Unless otherwise specified, the technical solutions described in the present invention are all conventional solutions in the field, and the reagents or raw materials used, unless otherwise specified, are purchased from commercial channels or have been disclosed.

[0057] The technical solutions provided by the present invention are described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0058] Example 1

[0059] 0.0318g of phenolphthalein, 0.216g of m-phenylenediamine, and 0.0307g of glutathione were weighed and dissolved in 20mL of 0.01M (pH=12) NaOH solution at a molar ratio of 1:20:1. The solution was sonicated at room temperature for 10 minutes to fully dissolve the solution. The solution was then transferred to a 50mL Teflon-lined, sealed stainless steel high-pressure hydrothermal reactor and placed in a 160°C oven for 8 hours. After the reaction, the resulting solution was cooled to room temperature and filtered through a 0.22μm filter membrane. The solution was then dialyzed through a 500-1000Da dialysis membrane for 48 hours and freeze-dried for 2 days to obtain NS-CDs powder.

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

[0061] 1. Morphological 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). Figure 2 As can be seen in (a), the prepared NS-CDs are quasi-spherical nanoparticles with uniform dispersion. The inset HRTEM shows obvious 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 Figure b shows that the particle size distribution of NS-CDs is 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 spectroscopy. Figure 2 Figure c shows a typical broad peak around 2θ = 25°, indicating that NS-CDs have good crystalline properties, which corresponds to the results obtained in HRTEM images ( Figure 2 (a) Raman spectra of NS-CDs are shown in Figure 2. Figure 2 As shown in d, at 1392cm -1 and 1584cm -1 Two characteristic peaks of NS-CDs, D peak and G peak, were observed at the 3 Hybridized carbon atoms, G peak comes from sp 2 The plane vibration of carbon atoms and ID / IG = 0.95 reflect that the carbon atoms in NS-CDs tend 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 spectroscopy and XPS. Figure 3 As shown in a, NS-CDs are at 3218-3391 cm -1The absorption band in the range corresponds to the stretching vibration of -OH and NH, 1607cm -1 The strong absorption peak at 1490 cm is attributed to the stretching vibration of C=O and the bending vibration of NH; -1 Belongs to the stretching vibration of C=C, 1314cm -1 Belongs to the stretching vibration of CN, 1165cm -1 Belongs to the stretching vibration of COC. 953.1cm -1 The peak at 684.4 cm belongs to CS bending vibration. -1 The weak absorption peak at is also attributed to the stretching vibration of CS, which shows that N and S elements have been successfully doped into the skeleton of NS-CDs.

[0065] Figure 3 Figure b is the full XPS spectrum of NS-CDs, showing four characteristic peaks at 285.25 eV, 399.92 eV, 532.10 eV, and 169.29 eV, corresponding to C1s, N 1s, O 1s, and S2p, respectively. Figure 3 In the figure c is the C1s spectrum, and the peaks at 284.8eV, 286.05eV, and 288.43eV correspond to CC / C=C, CS, and C=N / CO functional groups, respectively. The spectrum of N1s ( Figure 3 In d) it can be decomposed into two peaks, located at 399.47eV (C=N) and 400.87eV (CN). Figure 3 In the e O 1s spectrum, the peaks at 531.81, 532.96, and 536.38 eV are attributed to the C=O, C–O, and O–H functional groups. Figure 3 The characteristic peaks of 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 has abundant functional groups.

[0066] 3. Optical properties of carbon dots

[0067] The optical properties of NS-CDs were analyzed by UV-visible spectroscopy and fluorescence. Figure 4 As shown in (a), the UV-visible absorption spectrum reveals a distinct absorption band at 289 nm, attributed to the n→π* transition within the C=O bond. This is typically caused by the internal chemical structure and energy band structure of the carbon dots. The optimal excitation and emission wavelengths of NS-CDs are 345 nm and 438 nm, respectively. Under 365 nm UV illumination, they exhibit strong blue fluorescence (inset). Figure 4Figure b shows that when the excitation wavelength of NS-CDs increases from 305nm to 385nm, NS-CDs exhibit typical excitation-independent emission. The reason for this can be attributed to the good size uniformity of the synthesized NS-CDs, which enables them to produce similar band structures under different excitation wavelengths. As a result, the emission wavelength does not change with the change of the excitation wavelength. And there is only one obvious absorption peak in the ultraviolet spectrum, resulting in only one main n→π* transition energy level, thus exhibiting a single wavelength of luminescence. Calculations show that the quantum yield of NS-CDs is 71.16% using quinine sulfate as a reference. This result is higher than the quantum yield of most blue fluorescent carbon dots reported previously. The stability of NS-CDs was studied by detecting the fluorescence intensity of the NS-CDs solution at different xenon lamp irradiation times, different storage times, different salt concentrations (KCl) and different pH. As Figure 4 As shown in c, the intensity of NS-CDs hardly changed after 60 min of xenon lamp irradiation, and after 60 days of storage ( Figure 4 In (d), the fluorescence intensity is almost unaffected, indicating that NS-CDs have good photostability. Figure 4 Figure e shows that the fluorescence intensity of NS-CDs remains stable in different concentrations of KCl solution (0-2 mol / L), indicating that it has good salt tolerance. The stability and fluorescence properties of NS-CDs at different pH values ​​were studied using BR buffer. Figure 4 As shown in Figure (f), in a strongly acidic environment (pH 2-5), the fluorescence intensity of NS-CDs is very low and increases with increasing pH. When the pH value increases from 5 to 10, the fluorescence intensity remains almost stable and reaches a maximum value. However, when the pH value is greater than 10, the fluorescence intensity of NS-CDs begins to decrease. The change in fluorescence intensity of NS-CDs with pH value may be caused by the protonation and deprotonation of nitrogen-containing functional groups in different pH environments.

[0068] 4. NS-CDs on Cr 6+ 、Mn 7+ Selectivity and condition optimization

[0069] In order to study the metal ion selectivity of NS-CDs in aqueous environment, 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 the NS-CDs solution and the changes in the fluorescence intensity of the solution were analyzed (the concentration of the metal ions was set to 500 μM). Figure 5 As shown in a, Hg 2+ 、Fe 3+ Cr 6+ 、Mn 7+ All of them can reduce the fluorescence of NS-CDs, but only Cr 6+ and Mn 7+ It can quench the fluorescence of NS-CDs, while the effects of other metal ions on the fluorescence of NS-CDs are negligible. Figure 5 The F / F0 histogram after adding different metal ions is shown in b, which shows that the fluorescence sensor of NS-CDs is sensitive to Cr. 6+ and Mn 7+ showed higher selectivity than other metal ions, but Fe 3+ Cr 6+ and Mn 7+ Detection is also somewhat disruptive. Figure 5 The absorption spectra of NS-CDs in the presence of different metal ions are shown in Figure c. 2+ 、Fe 3+ Cr 6+ and Mn 7+ In the presence of ions, the absorption peak of NS-CDs shifted, and the degree of shift corresponded to the degree of fluorescence quenching, while other metal ions did not affect its absorption peak, which was consistent with the fluorescence detection results. Changes in pH value can significantly affect the sensitivity and selectivity of the sensor to metal ions in the environment or biological matrix, among which Fe 3+ The sensor's response to the interfering ions is often affected by their different binding abilities in acidic and alkaline environments. Therefore, the selectivity of NS-CDs for common interfering ions at different pH values ​​was evaluated to reduce the Fe 3+ Cr 6+ and Mn 7+ Interference. Figure 5 As shown in Figure d, when the pH value is adjusted to acidic (2-6), the fluorescence quenching effect caused by NS-CD increases, but it does not change the response ability of NS-CDs to other metal ions. 6+ and Mn 7+ It still has high selectivity and sensitivity. When the pH is adjusted to alkaline, in alkaline solutions with pH of 8 and 10, various metal ions such as Ag + 、Hg2+ 、Fe 2+ 、Fe 3+ All of these will lead to a decrease in the fluorescence emission intensity of NS-CDs, that is, there are multiple interfering ions that affect the fluorescence of NS-CDs to Cr 6+ and Mn 7+ In summary, the acidic environment can mask Fe 3+ interference, highly sensitive and selective detection of Cr 6+ and Mn 7+ .

[0070] In order to obtain the best sensing performance, NS-CDs were used to detect Cr 6+ and Mn 7+ The conditions were optimized, including pH value and incubation time. 6+ and Mn 7+ The fluorescence quenching rate of NS-CDs is Figure 5 As shown in Figure e, when the pH is 2-6, Cr 6+ The quenching efficiency of NS-CDs can reach more than 90%, and the quenching efficiency remains almost stable within this pH range. 7+ After that, Mn 7+ The quenching efficiency of NS-CDs gradually increases with the increase of pH. When the pH increases to 5, the quenching effect reaches the maximum, and then decreases with the increase of pH. Therefore, pH 5 is selected as the detection temperature of Cr 6+ and Mn 7+ The incubation time also plays a crucial role in the accuracy of the test. 6+ and Mn 7+ The effect of different incubation times on the fluorescence quenching rate of NS-CDs. Figure 5 As shown in f, adding Cr 6+ and Mn 7+ The maximum quenching effect was achieved after 30 seconds and remained stable for up to 5 minutes. Finally, 30 seconds was selected as the optimal reaction time. This proves that the NS-CDs prepared in Example 1 are suitable for detecting Cr 6+ and Mn 7+ It is highly efficient.

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

[0072] Under the optimal detection conditions, the Cr 6+ and Mn 7+The sensitivity of the NS-CDs sensor was evaluated by measuring the fluorescence intensity of the NS-CDs solution after addition. Figure 6 A and Figure 6 As shown in c, with Cr 6+ and Mn 7+ With the increase of Cr concentration, the fluorescence intensity of NS-CDs continued to decrease, and the inset shows that under 365 nm UV light, the blue fluorescence of NS-CDs aqueous solution increased with the increase of Cr 6+ and Mn 7+ The Boltzmann equation was used to analyze the relationship between the quenching degree F0-F and the addition of Cr. 6+ and Mn 7+ The concentration was fitted nonlinearly, such as Figure 6 Zhongb and Figure 6 As shown in Figure c, the fluorescence quenching value decreased rapidly in the low concentration range and 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 Cr, respectively. 6+ / Mn 7+ Here, F0 and F are in the absence and presence of Cr 6+ / Mn 7+ The fluorescence intensity of NS-CDs at 438 nm was calculated by fitting F0-F and Cr in the range of 0–60 μM. 6+ The linear relationship between the concentrations is Y = 65.94X + 22.60, R 2 =0.996, the limit of detection (LOD) was 0.48 μM (LOD=3SD / s). 7+ The linear fitting can be divided into two sections between 0-7.5μM and 7.5-70μM, and the fitting equations are 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. 6+ and Mn 7+ Compared with the sensor prepared by the present invention, the sensing performance of NS-CDs is better, which shows that NS-CDs is more effective in detecting Cr 6+ and Mn7+ It has potential application value.

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

[0074] The addition of Cr was studied by UV spectrum, fluorescence lifetime and Zeta potential changes. 6+ and Mn 7+ The fluorescence quenching mechanism of NS-CDs. Figure 7 As shown in a, Cr 6+ and Mn 7+ Shows different absorption spectra, and Cr 6+ The absorption spectrum of Cr has obvious overlap with the excitation spectrum of NS-CDs, so it is speculated that 6+ The fluorescence quenching is caused by the inner filter effect. The quenching mechanism is further studied using the fluorescence lifetime. Figure 7 The fluorescence lifetime of NS-CDs is 6.38 ns. 6+ The fluorescence lifetime is 6.08 ns, and the Cr 6+ The presence of did not cause much change in the fluorescence lifetime of NS-CDs, which was considered to be a static quenching mechanism. In addition, the quenching mechanism was confirmed by UV absorption spectroscopy. Figure 7 From c, we can see that adding Cr 6+ After the filtration, the absorption band position of NS-CDs at 289 nm did not change, further confirming the inner filter effect, that is, Cr 6+ The absorption band intensity from 250 to 400 nm increases with the increase of Cr 6+ The concentration gradually increases ( Figure 7 (d), indicating that the surface functional groups of NS-CDs and Cr 6+ A non-fluorescent complex will form between them, leading to static quenching. 7+ The absorption spectrum of Mn has no obvious overlap with the excitation and emission spectra of NS-CDs, thus eliminating the inner filter effect and fluorescence resonance energy transfer mechanism. 7+ After adding Figure 7 Therefore, it is speculated that the dynamic quenching is caused by the interaction with the excited state of NS-CDs. Figure 7 Middle C and Figure 7 The middle e shows that in Mn 7+ After addition, the exposed Mn 7+ The absorption band around 520 nm disappears, which may be due to the fact that Mn 7+ (MnO 4- ) redox conversion to Mn4+ (MnO 2 ), while the absorption peak of NS-CDs did not change significantly. Due to the photoinduced oxidative electron transfer reaction, Mn 7+ Forms non-radiative complexes with NS-CDs.

[0075] 7. Cr in actual samples 6+ Detection

[0076] As shown in Tables 1 and 2, in order to test the practical applicability and feasibility of the NS-CDs fluorescent probe, it was applied to the actual water sample analysis. Tap water and lake water from Shanxi University were collected for Cr 6+ With Mn 7+ The spike recovery experiment was carried out to test the different concentrations of Cr 6+ (Table 1) and Mn 7+ (Table 2) was spiked into water samples and fluorescence was recorded. NS-CDs was used as a fluorescent probe to detect Cr 6+ The recovery rate is 96% to 106%, Mn 7+ The recoveries were between 94% and 104% with low relative standard deviations (RSDs). 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 NS-CDs 6+ and Mn 7+ The response of NS-CDs to Cr 6+ and Mn 7+ The detection ability of NS-CDs was improved. The fluorescent filter paper strips based on NS-CDs (the preparation steps of the fluorescent filter paper strips are as follows: the filter paper was cut into 1 cm × 4 cm rectangular strips, and then immersed in NS-CD solution (1 mg / mL) for about 5 minutes. Then, the strips were dried in an oven at 55°C for 30 minutes to obtain fluorescent filter paper strips) were immersed in 500 μM solutions of different ions, dried after 30 minutes, and the fluorescence color changes were observed, as shown in FIG. Figure 8As shown in a, the NS-CDs test strip has obvious blue fluorescence under 365nm ultraviolet light. 6+ and Mn 7+ After soaking in the solution (pH 5), the fluorescence was significantly quenched, while other ions had no effect on the fluorescence of the NS-CDs paper strips, indicating that the prepared NS-CDs fluorescent test strips have good selectivity for metal ions. Through this simple filter paper sensor, obvious fluorescence changes can be clearly observed with the naked eye, which can be used to detect Cr in the environment. 6+ and Mn 7+ Perform a quick visual inspection.

[0084] For semi-quantitative detection, fluorescent filter paper strips of NS-CDs were soaked in different concentrations of Cr 6+ and Mn 7+ After 30 minutes, it was dried and the fluorescence color change was observed under 365nm ultraviolet light. It can be clearly observed with the naked eye that as 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 Middle b, Figure 8 In order to further achieve quantitative detection, a smartphone was used to capture images under 365nm UV light, and color recognition software was used to identify the RGB values ​​of the filter paper strips of the corresponding concentrations. The obtained (G+B) / R values ​​were compared with the Cr 6+ and Mn 7+ The concentration was fitted to confirm the linear relationship. Figure 8 Middle d, Figure 8 As shown in Figure e, (G+B) / R increases with Cr 6+ and Mn 7+ The increase of concentration has a good linear relationship, and the linear equations are 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, which indicates that the smartphone-assisted paper-based fluorescence sensing platform is effective in quickly and accurately measuring Cr in water. 6+ and Mn 7+ It is feasible.

[0085] 9. NS-CDs-based fluorescent hydrogel for visual detection of Cr 6+ and Mn 7+

[0086] Solid-state fluorescence of carbon dots is particularly important for its application in the field of sensing. Therefore, based on the good luminescence properties of NS-CDs, a fluorescent hydrogel based on NS-CDs was prepared using agarose (the preparation method is as follows: add 0.5g agarose to 50mL ultrapure water and stir vigorously at 200℃ until it is completely dissolved into a transparent liquid, then add 3mL of 3mg / mL NS-CDs solution and stir for 3min to mix it thoroughly. The above solution is then quickly transferred to a silicone mold or a small reagent bottle and waited for it to solidify. When the mixture is completely solidified, it is removed from the mold to form a fluorescent hydrogel based on NS-CDs). Its use as a solid fluorescent in Cr 6+ and Mn 7+ Applications in sensing. Figure 9 As shown in Figure a, two types of hydrogels were made using a silica gel model. The hydrogel synthesized with agarose was colorless under 365nm ultraviolet light, while the hydrogel with NS-CDs had bright blue fluorescence under the same conditions. Figure 9 As shown in Figure b, when the NS-CDs hydrogel was exposed to air, the morphology and blue fluorescence of the hydrogel remained unchanged within 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] In order to study the application of NS-CDs hydrogel as a solid sensor in the detection of Cr 6+ and Mn 7+ In the application, it was immersed in different concentrations of Cr 6+ and Mn 7+ The fluorescence color change was observed under 365nm UV light. First, the effect of immersion time on the quenching degree was investigated. NS-CDs hydrogel was immersed in 500uM Cr 6+ The fluorescence color change of the solution was observed every 5 minutes. Figure 9 As shown in c, after immersion for 5 minutes, the fluorescence color of the hydrogel has been significantly quenched, and after immersion for 20 minutes, the fluorescence has been completely quenched, and the hydrogel has been observed to be Cr under fluorescent light. 6+ The solution was immersed and the color turned to light yellow. Finally, immersion for 20 minutes was selected as the final detection of Cr 6+ and Mn 7+ The best time.

[0088] Secondly, the NS-CDs hydrogel was immersed in different concentrations of Cr 6+ and Mn 7+ (0-500uM) solution, wait for 20 minutes, and observe the fluorescence color change under 365nm ultraviolet light, such as Figure 10 Middle a, Figure 10 As shown in b, with Cr6+ and Mn 7+ As the concentration increases, the blue fluorescence of the NS-CDs hydrogel shows a trend of gradually becoming lighter under 365nm ultraviolet light irradiation, until the concentration increases to 500uM, the fluorescence is completely quenched, which is consistent with the detection results of the fluorescence spectrometer. In addition, the present invention uses the fluorescent hydrogel based on NS-CDs to prepare a kit (pH=5) for detecting the selectivity of the NS-CDs hydrogel for metal ions. Some potential interfering ions are added dropwise to the hydrogel kit. Figure 10 As can be seen from c and d, it contains Cr 6+ and Mn 7+ The hydrogel showed obvious fluorescence quenching, while the addition of other ions had little effect on the fluorescence of the hydrogel, which showed that it had excellent selectivity. In summary, NS-CDs fluorescent hydrogel has the potential to be used as a solid sensor in the realization of Cr 6+ and Mn 7+ It has great application prospects in on-site detection and visual semi-quantitative analysis.

[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 ratios of phenolphthalein, m-phenylenediamine and glutathione are 1:5:1, 1:10:1, 1:15:1, 1:25:1, 1:30:1, 1:20:5, and 1:20:10, respectively), different reaction temperatures (i.e., the only difference from Example 1 is that the reaction temperatures in the oven are 140°C and 180°C, respectively), and different reaction times (i.e., the only difference from Example 1 is that the reaction times in the oven are 6 h and 10 h, respectively) were also verified. The results are as follows: Figure 1 As shown, Figure 1 It shows that the quantum yield of NS-CDs is the highest under the conditions of Example 1.

[0090] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing nitrogen-sulfur co-doped carbon quantum dots, characterized in that: The following steps are involved: Phenolphthalein, m-phenylenediamine and glutathione are dissolved in an alkaline solution and subjected to a hydrothermal reaction; After the hydrothermal reaction is completed, the nitrogen-sulfur co-doped carbon quantum dots are obtained by filtration, dialysis and freeze-drying in sequence.

2. The method for preparing nitrogen-sulfur co-doped carbon quantum dots according to claim 1, characterized in that: The molar ratio of the 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 0.01M NaOH solution or KOH solution.

4. The method for preparing nitrogen-sulfur co-doped carbon quantum dots according to claim 1, characterized in that: The temperature of the hydrothermal reaction is 140° C. to 180° C., and the time is 6 to 10 hours. 5 . Nitrogen-sulfur co-doped carbon quantum dots prepared by the preparation method according to claim 1 .

6. A fluorescent filter paper strip, characterized in that: The filter paper is placed in a 1-2 mg / mL NS-CD solution and soaked for 4-6 minutes, and then dried at 50°C to 60°C to obtain the fluorescent filter paper strip; the NS-CD solution is prepared by dissolving the nitrogen-sulfur co-doped carbon quantum dots according to claim 5 in water.

7. A fluorescent hydrogel kit, characterized in that: Dissolving agarose in ultrapure water, then adding NS-CD solution, mixing and solidifying to obtain the fluorescent hydrogel; 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; the NS-CD solution is prepared by dissolving the nitrogen-sulfur co-doped carbon quantum dots according to claim 5 in water.

8. The nitrogen-sulfur co-doped carbon quantum dots according to claim 5, the fluorescent filter paper strips according to claim 6, or the fluorescent hydrogel kit according to claim 7 in detecting Cr in water environment 6+ and Mn 7+ application.

9. Cr in a water environment 6+ and Mn 7+ The visual detection method is characterized in that The following steps are involved: The fluorescent filter paper strip according to claim 6 or the fluorescent hydrogel according to claim 7 is immersed in a water sample for 20 minutes, and then the blue fluorescence is observed to be dimmed or quenched under 365nm ultraviolet light; if the blue fluorescence is dimmed or quenched, it indicates that Cr is present in the water environment. 6+ or Mn 7+ .

10. Cr in water environment according to claim 9 6+ and Mn 7+ The visual detection method is characterized in that The pH of the water sample is 4-6.

Images