Synthesis of nitrogen-sulfur-carbon dots and preparation of pH-regulated Hg (II) / Cr (VI) / Mn (VI) sensing array

Abstract

The invention discloses synthesis of nitrogen-sulfur carbon dots and preparation of a pH-regulated Hg (II) / Cr (VI) / Mn (VII) sensing array, and belongs to the technical field of carbon nanomaterials. The preparation method comprises the following steps: dissolving phenolphthalein, m-phenylenediamine and L-cysteine in a sodium hydroxide aqueous solution, carrying out ultrasonic treatment, then carrying out a heating reaction, after the reaction is completed, naturally cooling to room temperature, filtering, dialyzing, and freeze-drying to obtain the nitrogen-sulfur-doped carbon quantum dots. The nitrogen and sulfur doped carbon quantum dots are torispherical nanoparticles, the particle size is distributed between 1.5 nm and 4 nm, and the average particle size is 2.52 + / -0.3 nm; the fluorescence is stable in a pH range of 5-7, and the optimal excitation / emission wavelength is 395nm / 504nm. According to a fluorescence sensor array based on nitrogen and sulfur doped carbon quantum dots, N and S-CDs buffer solutions with different pH values (5.0, 6.0 and 7.0) form a sensing element, and qualitative and quantitative detection of target ions in a complex environment is realized through fluorescence quenching difference caused by the action of metal ions (Hg < 2 + >, Cr < 6 + > and Mn < 7 + >) and the sensing element in combination with principal component analysis (PCA).

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon nanomaterials, and particularly relates to a preparation method of nitrogen and sulfur doped carbon quantum dots and their application in constructing a sensor array for detecting heavy metal ions. Background Art

[0002] Heavy metal ions, especially those with high biological toxicity such as mercury, chromium, and manganese, are difficult to be degraded by microorganisms, easily accumulate in nature, and are enriched through the food chain. The harm to environmental pollution and human health has become a major global issue. Therefore, it is very urgent to design sensitive and reliable methods for detecting metal ions such as mercury, chromium, and manganese. So far, various methods such as colorimetric detection technology, inductively coupled plasma mass spectrometry (ICP-MS), and electrochemical detection have been used for the quantitative and qualitative detection of metal ions. It is worth noting that patents such as CN114235725A, CN219224560U, and CN105352944A adopt atomic absorption spectrometry, atomic emission spectrometry, and inductively coupled plasma spectrometry. Although the detection accuracy is relatively high, there are still inherent problems such as long time consumption, complex operation, and expensive instrument equipment, which are difficult to meet the requirements of on-site rapid detection. Although these methods show excellent accuracy and sensitivity, most of their recognition strategies are based on the "lock and key" sensing mode, that is, one probe can only specifically recognize one target, which is not conducive to the recognition and detection of multiple target substances or mixed samples. For example, patents such as CN119555653A, CN117757469A, CN115287063A, and CN105044071A all adopt the technical route of using a single probe to detect a single heavy metal ion and cannot cope with the complex system of coexistence of multiple metal ions in the actual environment. In fact, multiple heavy metal ions often coexist in natural water or soil samples. Therefore, developing a simple, rapid method that can simultaneously detect, accurately identify, and quantify heavy metal ions under complex conditions is crucial for protecting the ecological environment and improving public health.

[0003] Fluorescent sensor array detection relies on the selective interaction with the analyte. Different targets produce different response patterns, and the analyte is identified through high-throughput methods such as linear discriminant analysis (LDA), principal component analysis (PCA), and hierarchical clustering analysis (HCA). It not only has high sensitivity, high selectivity, low cost, and simple operation, but more importantly, it can meet the need to simultaneously identify various metal ions and mixtures, showing excellent performance in the detection of metal ions. In recent years, progress has been made in fluorescent sensor arrays for detecting metal ions.

[0004] Although fluorescence sensor arrays have considerable advantages in terms of high sensitivity and strong discrimination ability, almost all reported fluorescence sensor arrays involve several fluorescent materials as sensor elements. Such a multi-probe system not only results in time-consuming and complex fabrication of the sensor array (as shown in patents such as CN105352944A), but also makes the fabrication of such a sensor array quite time-consuming and complex, leading to low detection efficiency. In addition, most of the fluorescent probes used have disadvantages such as difficult synthesis, poor photostability, and high biological toxicity. Therefore, it is crucial to develop a single-probe-based sensor array for detecting and differentiating multiple heavy metal ions. Summary of the Invention

[0005] Aiming at the problems existing in the existing metal ion detection technologies (such as atomic absorption spectrometry), including expensive equipment, complex pretreatment, and difficulty in differentiating mixed ions with a single probe, the present invention provides a nitrogen and sulfur co-doped carbon quantum dot, a preparation method thereof, and an application thereof.

[0006] To solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A preparation method of a nitrogen and sulfur co-doped carbon quantum dot, comprising the following steps: dissolving phenolphthalein, m-phenylenediamine, and L-cysteine in an aqueous sodium hydroxide solution, performing ultrasonic treatment, and then carrying out a heating reaction. After the reaction is completed, it is naturally cooled to room temperature and then filtered, dialyzed, and freeze-dried to obtain the nitrogen and sulfur co-doped carbon quantum dot.

[0008] Further, the molar ratio of phenolphthalein, m-phenylenediamine, and L-cysteine is 1:5-15:5-15.

[0009] Further, the concentration of sodium hydroxide in the aqueous sodium hydroxide solution is 0.005-0.01 mol / L.

[0010] Further, the time of the ultrasonic treatment is 10-20 min; the time of the freeze-drying is 48 h.

[0011] Further, the heating temperature is 120-200 °C, and the heating time is 5-12 h.

[0012] Further, the filtration operation is carried out using a 0.22 μm filter membrane; the dialysis operation is carried out using a dialysis membrane with a molecular weight cut-off of 500-1000 Da for 48 h.

[0013] A nitrogen and sulfur co-doped carbon quantum dot, which is a quasi-spherical nanoparticle with a particle size distribution between 1.5-4 nm and an average particle size of 2.52±0.3 nm; the surface functional groups include C-C, C-N, C-S, and oxygen-containing groups, and it is fluorescence-stable in the pH range of 5-7, and the optimal excitation / emission wavelength is 395 nm / 504 nm.

[0014] A fluorescence sensor array based on nitrogen and sulfur co-doped carbon quantum dots consists of sensing elements composed of N,S-CDs buffer solutions with different pH values (5.0, 6.0, 7.0). Qualitative and quantitative detection of target ions in complex environments is achieved by combining the fluorescence quenching differences caused by the interaction of metal ions (Hg 2+ , Cr 6+ and Mn 7+ ) with the sensing elements and principal component analysis (PCA).

[0015] The application of nitrogen and sulfur co-doped carbon quantum dots in constructing a sensor array for detecting heavy metal ions can detect the concentrations of Hg 2+ , Cr 6+ , and Mn 7+ in water in the range of 10 - 300 μM, with a detection limit lower than 5 μM, and can distinguish binary and ternary mixed ion systems.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] 1. The operation steps of the present invention are simple. Carbon dots with green fluorescence (N,S-CDs) are synthesized by a simple alkali-assisted hydrothermal method.

[0018] 2. The prepared N,S-CDs of the present invention have different fluorescence responses to metal ions under different pH conditions. Based on this, a novel fluorescence sensor array with a single probe is developed. By using the data matrix composed of different quenching behaviors shown when metal ions are mixed with different sensing elements, qualitative and quantitative analysis of three metal ions, Hg 2 + , Cr 6+ and Mn 7+ , is achieved in complex environments through principal component analysis (PCA).

[0019] 3. The prepared N,S-CDs of the present invention have good fluorescence properties and stability. The constructed sensing array only involves one fluorescent material, with simple and rapid synthesis, high detection efficiency, and high sensitivity, realizing the rapid identification of metal ions in mixed samples based on a single-probe sensor array. Description of the Drawings

[0020] To more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0021] Figure 1a is the TEM and HR-TEM images of N,S-CDs, and b is the particle size distribution diagram;

[0022] Figure 2 a is the XRD pattern of N,S-CDs, and b is the Raman spectrum;

[0023] Figure 3 is the FTIR spectrum of N,S-CDs;

[0024] Figure 4 a is the XPS survey spectrum of N,S-CDs, and b, c, d, e are the high-resolution elemental spectra of C 1s, N 1s, O 1s, and S 2p;

[0025] Figure 5 a is the UV-visible absorption spectrum, excitation spectrum, and emission spectrum of N,S-CDs, and b is the emission spectrum of N,S-CDs under different excitation lights;

[0026] Figure 6 a, b, c, d are the diagrams of the stability of N,S-CDs under xenon lamp, salt concentration, storage time, and pH, respectively;

[0027] Figure 7 a is the fluorescence spectrum of N,S-CDs after adding metal ions, b is the selectivity of N,S-CDs for metal ions, and c is the absorption spectrum of N,S-CDs after adding different metal ions;

[0028] Figure 8 a is the fluorescence quenching degree of metal ions on N,S-CDs under different pH conditions, and b is the heat map of the response of the sensor array to Hg 2+ 、Cr 6+ and Mn 7+ three metal ion response heat maps;

[0029] Figure 9 is the principal component analysis diagram of N,S-CDs@pH sensor array for distinguishing Hg 2+ 、Cr 6+ 、Mn 7+ at different concentrations;

[0030] Figure 10 a-c are the fluorescence response bar charts of N,S-CDs@pH sensor array to different concentrations of Hg 2+ 、Mn 7+ 、Cr 6+ ,and d-f are the heat maps obtained from the response bar charts of the sensor array to metal ions;

[0031] Figure 11 a-c are the N,S-CDs@pH sensor array for different concentrations of Hg 2+ 、Mn7+ , Cr 6+ PCA analysis diagram of; d - f are the fitting curves of PC1 with different concentrations of Hg2+, Mn7+ and Cr6+;

[0032] Figure 12 a and c are for Hg 2+ , Mn 7+ , Cr 6+ binary mixture PCA analysis of; b and d are for Hg 2+ , Mn 7+ , Cr 6+ ternary mixture PCA analysis of;

[0033] Figure 13 Using the N,S - CDs@pH sensor array to distinguish Hg with spiked concentrations of 50 μM and 100 μM in tap water samples 2+ , Mn 7+ , Cr 6+ . Detailed implementation methods

[0034] To deeply understand the present invention, we will describe it comprehensively and meticulously. However, the present invention has various implementation manners and is not limited to the specific examples listed herein. The presentation of these examples aims to deepen the comprehensive understanding of the disclosed content of the present invention.

[0035] Example 1

[0036] A preparation method of nitrogen and sulfur co - doped carbon quantum dots, comprising the following steps:

[0037] By one - step hydrothermal method, weigh 0.0318 g of phenolphthalein, 0.108 g of m - phenylenediamine and 0.12 g of L - cysteine according to a molar ratio of 1:10:10, dissolve them in 20 mL of NaOH (0.01 M) aqueous solution, ultrasonicate for 10 min to fully dissolve, transfer to a 50 mL hydrothermal reaction kettle and react in an oven at 160 °C for 8 h. After the reaction ends, wait for the obtained solution to cool naturally to room temperature, filter with a 0.22 μm filter membrane, then dialyze with a 500 - 1000 Da dialysis membrane for 48 h, and then freeze - dry for 48 h to obtain N,S - CDs powder. Using quinine sulfate (Φ R = 0.54) dissolved in 0.1 mol / L dilute sulfuric acid solution as the reference solution, the relative quantum yield of N,S - CDs is 9.59%.

[0038] Characterize the prepared N,S - CDs:

[0039] Characterization test 1

[0040] TEM, HR-TEM and particle size distribution diagrams of the N,S-CDs prepared in Example 1.

[0041] As Figure 1 shown, the N,S-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 N,S-CDs. The particle size of N,S-CDs is distributed between 1.5 and 4 nm, and the average particle size is 2.52 ± 0.3 nm.

[0042] Characterization Test 2

[0043] XRD and Raman spectra of the N,S-CDs prepared in Example 1.

[0044] As Figure 2 shown. Figure 2 Figure a is the XRD pattern. The results show that there is a typical broad peak at about 2θ = 23°, indicating that the N,S-CDs have good crystallization performance, which corresponds to the results obtained from the HRTEM image. The Raman spectrum is as Figure 2 shown in Figure b. Two characteristic peaks, the D peak and the G peak of N,S-CDs, are observed at 1371 cm -1 and 1575 cm -1 respectively. The D peak is related to sp3 hybridized carbon atoms, and the G peak comes from the planar vibration of sp2 carbon atoms. And ID / IG = 0.94, reflecting that the carbon atoms in N,S-CDs tend to ordered sp2 carbon, indicating the formation of carbon nuclei.

[0045] Characterization Test 3

[0046] Fourier transform infrared spectroscopy (FTIR) characterization of the N,S-CDs prepared in Example 1.

[0047] As Figure 3 shown. The results show that the absorption band in the range of 3216 - 3362 cm -1 of N,S-CDs corresponds to the stretching vibrations of -OH and N-H. The strong absorption peak at 1561 cm -1 is attributed to the stretching vibration of C=O; 1395 cm -1 belongs to the stretching vibration of C=C, 1241 cm -1 belongs to the stretching vibration of C-N, and the peak at 827.8 cm -1 belongs to the C-S bending vibration. It can be seen that N and S elements have been successfully doped into the framework of N,S-CDs.

[0048] Characterization Test 4

[0049] X-ray photoelectron spectroscopy (XPS) characterization of the N,S-CDs prepared in Example 1.

[0050] As Figure 4 shown. Figure 4 In a, the XPS full spectrum shows four characteristic peaks, corresponding to carbon, nitrogen, oxygen, and sulfur respectively. Figure 4 In b, c, d, and e, the high-resolution XPS spectra of C 1s, N 1s, O 1s, and S 2p are shown respectively. Figure 4 In b, the C 1s spectrum shows peaks at 284.8 eV, 285.68 eV, 286.58 eV, and 289.38 eV, corresponding to C-C, C-C / C-H, C-O, and O-C=O functional groups respectively. The N 1s spectrum ( Figure 4 c) can be decomposed into two peaks at 399.98 eV and 401.76 eV, corresponding to pyridine nitrogen and graphitic nitrogen respectively. In Figure 4 the O 1s spectrum of d, the peaks at 531.99, 532.89, and 533.74 eV are attributed to C=O, C-O, and C–OH functional groups. Figure 4 In e, the characteristic peaks of S 2p are located at 163.99 eV, 165.0 eV, 165.90 eV, and 169.53 eV, representing C-S-C, C-SO3-H, -SO, and C-S respectively. These results indicate that the surface of N,S-CDs has rich functional groups.

[0051] Characterization Test 5

[0052] UV-visible absorption spectrum, excitation spectrum, and emission spectrum of the N,S-CDs prepared in Example 1.

[0053] As Figure 5 shown in a, the emission spectra of N,S-CDs under different excitation lights are shown in Figure 5 b. The results show that: the aqueous solution of N,S-CDs appears as a nearly colorless transparent solution under sunlight, and emits bright green fluorescence under 365 nm ultraviolet lamp irradiation. The broad absorption band at 289 nm in the UV-Vis spectrum mainly corresponds to the π-π* transition of C-C in the sp2 hybrid domain of the carbon core structure, while the peak near 400 nm is mainly attributed to the n-π* transition of C-O and C-N surface functional groups. In addition, the optimal excitation and emission wavelengths of N,S-CDs are 395 nm and 504 nm. Figure 5 As shown in b, when the excitation wavelength is adjusted from 355 nm to 425 nm, the emission wavelength of N,S-CDs only redshifts by nearly 15 nm, which may be due to the regular and uniform structure of N,S-CDs.

[0054] Characterization Test 6

[0055] Xenon lamp stability, salt concentration stability, storage time stability, and pH stability diagrams of the N,S-CDs prepared in Example 1.

[0056] As Figure 6 shown in a, b, c, and d of Figure 6 a - c, under different xenon lamp irradiation times, different salt concentrations (KCl), and different storage times, the N,S-CDs maintained a stable fluorescence intensity, indicating their good photostability, excellent anti-photobleaching property, and salt tolerance, and having good application prospects in complex systems. In addition, the stability and fluorescence properties of N,S-CDs at different pH values were studied using BR buffer solution. As Figure 6 shown in d, in an acidic environment, the fluorescence intensity of N,S-CDs showed a slight decreasing trend with the increase of pH value, but the fluorescence intensity was generally stable in the pH range of 5 - 7. When the pH became alkaline, the fluorescence intensity of N,S-CDs decreased sharply, and as the pH value increased from 8 to 12, the fluorescence intensity of N,S-CDs also gradually decreased and was almost completely quenched. The reason may be due to the protonation and deprotonation of different functional groups in different pH environments.

[0057] Application Example 1

[0058] Detection of metal ions by the N,S-CDs prepared in Example 1.

[0059] As Figure 7 shown, as Figure 7 shown in a, after adding metal ions with the same concentration to the aqueous solution of N,S-CDs, Hg 2+ , Fe 3+ , Cr 6+ , Mn 7+ all decreased the fluorescence of the N,S-CDs aqueous solution, and the quenching degree increased in turn, while the influence of other metal ions on the fluorescence of the N,S-CDs aqueous solution was negligible. Figure 7 b is a bar chart of F / F0 after adding different metal ions, where F0 and F represent the fluorescence intensities of N,S-CDs at 504 nm before and after adding metal ions, respectively. It can be more intuitively shown in the figure that the fluorescence sensor of N,S-CDs has higher selectivity for Hg 2+ , Fe 3+ , Cr 6+ , Mn 7+ than other metal ions. In addition, Figure 7 c shows the absorption spectra of the N,S-CDs aqueous solution in the presence of different metal ions. The results confirmed that in the presence of Hg 2+ , Fe 3+ , Cr 6+ , Mn7+ In the presence of [specific metal ion], the absorption peak of N, S-CDs showed a shift, 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.

[0060] Application Example 2

[0061] The sensing array constructed by the N, S-CDs prepared in Example 1 in an acidic environment.

[0062] As Figure 8 shown, first, 60 μL of the N, S-CDs solution (3 mg / mL) was added to 2 mL of BR buffer solution with different pH values (5, 6, 7) as three sensing elements. The sensor array composed of these three sensing elements was used for qualitative and quantitative analysis of metal ions. As Figure 8 shown in Fig. a, when the detection condition was changed to BR solutions with different pH values, the fluorescence quenching of N, S-CDs caused by Fe 3+ could be well masked. This was because under acidic conditions, H + was prone to combine with Fe 3+ , increasing its solubility, resulting in the fluorescence of N, S-CDs being unaffected. When the pH condition was changed, N, S-CDs still had high selectivity and sensitivity to Hg 2+ , Cr 6+ and Mn 7+ . Moreover, the fluorescence responses of the N, S-CDs probe to Hg 2+ , Cr 6+ and Mn 7+ were different under different pH conditions. As can be seen from Fig. b, the fluorescence intensity of N, S-CDs was generally stable in the pH range of 5 - 7. Therefore, we proposed that the N, S-CDs solutions at three different pH values of 5.0, 6.0, and 7.0 could be selected as three sensing elements, and a fluorescence sensor array for metal ion pattern recognition based on pH regulation was developed. It can be seen from the fluorescence response bar chart ( Figure 8 Fig. a) and the heat map ( Figure 8 Fig. b) that the three metal ions had different responses to the three sensing elements, verifying the feasibility of using this sensor array for the analysis of three metal ions, namely Hg 2+ , Cr 6+ and Mn 7+ .

[0063] Application Example 3

[0064] The N, S-CDs@pH sensor array constructed by the N, S-CDs prepared in Example 1 for Hg 2+ , Cr 6+ and Mn 7+ and7+ Qualitative analysis

[0065] As Figure 9 shown, three N,S-CDs buffer solutions with pH values of (5.0, 6.0, 7.0) were prepared as the N,S-CDs@pH sensor array, and three metal ions with final concentrations of 20, 30, 50, 150, 200, and 300 μmol / L were added to the buffer solutions as targets. The fluorescence intensities F / F0 of N,S-CDs@pH before and after the addition of metal ions were calculated, where F0 and F represent the fluorescence intensities of N,S-CDs@pH at 504 nm before and after the addition of metal ions, respectively. Principal component analysis was performed on the F / F0 of the fluorescence data matrix "3 N,S-CDs@pH sensing elements × 3 metal ions × 5 replicates", and a principal component analysis graph was obtained. As 2+ Hg 6 + Cr 7+ and Mn Figure 9 shown, the results indicate that the three metal ions are three independent clusters, which proves that these three ions are independent of each other. As the detection concentration increases, changes in the relative positions of the three metal ions can be observed, confirming the feasibility, accuracy, and sensitivity of the developed pH-regulated N,S-CDs@pH sensor array for differentiating Hg 2+ Cr 6+ and Mn 7+ , laying a foundation for the detection of more complex samples.

[0066] Application Example 4

[0067] Quantitative analysis of Hg 2+ Cr 6+ and Mn 7+ using the N,S-CDs@pH sensor array constructed with N,S-CDs prepared in Example 1

[0068] As Figure 10 and 11 shown, after successfully differentiating the three metal ions, the N,S-CDs@pH sensor array was used for the quantitative analysis of Hg 2+ Cr 6+ and Mn 7+ to further determine the analytical performance of the constructed sensor array. A series of different concentrations of Hg 2+ Cr 6+ and Mn 7+ , such as 10 μM, 20 μM, 30 μM, 40 μM, and 50 μM, were respectively added to the pH-regulated N,S-CDs@pH sensor array to obtain fluorescence intensity data. Hg 2+ Cr 6+and Mn 7+ The bar chart and heat map of the fluorescence response to Figure 10 are shown as follows. It can be seen from the figure that Hg with different concentrations 2+ , Cr 6+ and Mn 7+ have different responses to the three sensing elements, and there are also obvious differences in the response capabilities of the three metal ions under the same concentration gradient. Compared with Cr 6+ and Mn 7+ , Hg 2+ obviously shows a weaker response ability. At the same time, the response ability of Cr 6+ is also stronger than that of Mn 7+ . This reflects from the side that the abilities of the three metal ions to act on the N,S-CDs@pH sensor array are different, providing practical feasibility for the high-precision identification of the array sensor.

[0069] After processing the data matrix with the PCA method, as Figure 11 shown, it can be seen that the clusters corresponding to Hg with different concentrations 2+ , Cr 6+ and Mn 7+ are arranged regularly according to the concentration gradient, and are relatively close when the concentration is low. As the ion concentration increases, the arrangement distance of the cluster groups represented by different concentrations gradually increases. This shows that the higher the ion concentration, the stronger the effect on the sensor array. Since PC2 in the figure is less than 3%, PC1 can be used to correlate with the concentrations of Hg 2+ , Cr 6+ and Mn 7+ . As shown in Figure 11 d-f, Hg 2+ is linearly correlated with PC1 at a concentration of 0 - 500 μmol / L ( Figure 11 d). At the same time, when it is 0 - 200 μmol / L, Mn 7+ is linearly correlated with PC1 ( Figure 11 e), and when it is 0 - 50 μmol / L, Cr 6+ is linearly correlated with the PC1 concentration ( Figure 11 f). These results indicate that the N,S-CDs@pH sensor array can distinguish metal ions with different concentrations, and this method can be applied to the discrimination of Hg with unknown concentration according to the above linear relationship 2+ , Cr 6+ and Mn7+.

[0070] Application Example 5

[0071] The N,S-CDs@pH sensor array constructed with the N,S-CDs prepared in Example 1 for Hg 2+ , Cr 6+, Mn 7+ Multivariate mixed analysis

[0072] As Figure 12 shown, binary mixtures of three metal ions with a total amount of 500 μM were selected (50% Hg 2+ + 50% Cr 6+ ; 50% Hg 2+ + 50% Mn 7+ ; 50% Cr 6+ + 50% Mn 7+ ), as well as ternary mixtures (25% Hg 2+ + 25% Mn 7+ + 50% Cr 6+ ; 50% Hg 2+ + 25% Mn 7+ + 25% Cr 6+ ; 25% Hg 2+ + 50% Mn 7+ + 25% Cr 6+ ) as target analytes for identification. The results are as Figure 12 shown. Whether it is a binary mixture or a ternary mixture, it can be clustered according to its composition and can be clearly distinguished. In addition, pure samples of single components were added to these mixtures for rearrangement, and the PCA graph could also distinguish these components, proving that the constructed N,S-CDs@pH sensor array can realize the analysis of mixed samples, showing excellent performance in complex environments and laying a good foundation for the detection of actual samples.

[0073] Application Example 6

[0074] Application of the N,S-CDs@pH sensor array constructed with N,S-CDs prepared in Example 1 in actual water samples.

[0075] As Figure 13 shown, in order to prove the applicability of the developed sensor array in monitoring actual water samples, the pH of tap water samples was adjusted to 5, 6, and 7 with BR buffer, and N,S-CDs solution was added to them to construct the sensing elements of tap water samples. Then, Hg with concentrations of 50 μM and 100 μM, Cr 2+ , and Mn 6+ were added to the tap water sample sensing elements respectively, and their fluorescence responses were measured. As 7+ shown, Figure 13As shown, the six samples are clearly distinguishable from each other, and their relative positions also change according to their different response capabilities. The two typical factors are 97.6% and 1.7% respectively, and accurate recognition of metal ions is obtained with an identification accuracy close to 100%. The practicality and stability of the N,S-CDs@pH sensor array in identifying different metal ions in actual samples are confirmed.

[0076] The content not detailed in the description of the present invention belongs to the prior art well-known to those skilled in the art. Although the illustrative specific embodiments of the present invention are described above for the understanding of those skilled in the art of the present technology, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art of the present technology, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions made using the concept of the present invention are within the scope of protection.

Claims

1. A preparation method of nitrogen and sulfur co-doped carbon quantum dots, characterized in that: It includes the following steps: Phenolphthalein, m-phenylenediamine, and L-cysteine are dissolved in an aqueous sodium hydroxide solution, ultrasonicated, and then subjected to a heating reaction. After the reaction is completed, it is naturally cooled to room temperature and then filtered, dialyzed, and freeze-dried to obtain nitrogen and sulfur co-doped carbon quantum dots.

2. The preparation method of a nitrogen and sulfur co-doped carbon quantum dot according to claim 1, wherein: The molar ratio of phenolphthalein, m-phenylenediamine, and L-cysteine is 1:5-15:5-15.

3. The preparation method of a nitrogen and sulfur co-doped carbon quantum dot according to claim 1, characterized in that: The concentration of sodium hydroxide in the aqueous sodium hydroxide solution is 0.005-0.01 mol / L.

4. The preparation method of a nitrogen and sulfur co-doped carbon quantum dot according to claim 1, wherein: The time for ultrasonic treatment is 10-20 min; the time for freeze-drying is 48 h.

5. The preparation method of a nitrogen and sulfur co-doped carbon quantum dot according to claim 1, characterized in that: The heating temperature is 120-200 °C, and the heating time is 5-12 h.

6. The preparation method of a nitrogen and sulfur co-doped carbon quantum dot according to claim 1, characterized in that: The filtration operation is carried out using a 0.22 μm filter membrane; the dialysis operation is carried out using a dialysis membrane with a molecular weight cut-off of 500-1000 Da for 48 h.

7. A nitrogen and sulfur co-doped carbon quantum dot prepared by the preparation method according to any one of claims 1 to 6, characterized in that: The nitrogen and sulfur co-doped carbon quantum dots are quasi-spherical nanoparticles with a particle size distribution between 1.5-4 nm and an average particle size of 2.52±0.3 nm; the surface functional groups include C-C, C-N, C-S, and oxygen-containing groups, and the fluorescence is stable in the pH range of 5-7, and the optimal excitation / emission wavelength is 395 nm / 504 nm.

8. A fluorescence sensor array of nitrogen and sulfur co-doped carbon quantum dots prepared by the preparation method according to claims 1 to 6, characterized in that: A sensing element is composed of N,S-CDs buffer solutions with different pH values. Through the fluorescence quenching difference caused by the interaction between metal ions and the sensing element, qualitative and quantitative detection of target ions in a complex environment is achieved by combining principal component analysis.

9. The fluorescence sensor array of nitrogen and sulfur co-doped carbon quantum dots according to claim 8, characterized in that: The different pH values are 5.0, 6.0, and 7.0 respectively; the metal ions are Hg 2+ , Cr 6+ , and Mn 7+ .

10. Use of the nitrogen and sulfur co-doped carbon quantum dots prepared by the preparation method according to any one of claims 1 to 6 in constructing a sensor array for detecting heavy metal ions, characterized in that: Capable of detecting Hg in water 2+ , Cr 6+ , Mn 7+ The concentration range is 10 - 300 μM, the detection limit is below 5 μM, and it can distinguish binary and ternary mixed ion systems.

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