Nitrogen-doped carbon dots with high quantum yield, preparation method thereof and method for detecting content of divalent mercury ion in water

Abstract

This invention relates to nitrogen-doped carbon dots with high quantum yield, their preparation method, and a method for detecting divalent mercury ions in water, belonging to the field of analytical detection technology. The nitrogen-doped carbon dots are obtained in one step using a solvothermal method with salicylic acid and nitric acid as raw materials and N,N-dimethylformamide as solvent. Experimental results show that the nitrogen-doped carbon dots have high quantum yield, and their quenching fluorescence intensity is comparable to that of Hg. 2+ The concentration is linearly correlated and can be used for Hg 2+ Quantitative detection of Hg, and the determination of Hg 2+ It exhibits excellent selectivity, anti-interference properties, and sensitivity. This nitrogen-doped carbon dot was combined with a microfluidic platform for the treatment of Hg in water. 2+ The detection limit for Hg content can be as low as 0.016 μM. The microfluidic platform avoids the use of large instruments and reduces reagent consumption, enabling on-site detection of Hg in water. 2+ Fast and convenient testing.

Description

Technical Field

[0001] This invention belongs to the field of analytical detection technology, and relates to nitrogen-doped carbon dots with high quantum yield, their preparation methods, and methods for detecting the content of divalent mercury ions in water. Background Technology

[0002] Hg 2+ Mercury is one of the most common and stable forms of mercury pollution in water bodies. It is easily absorbed and accumulated by organisms and can be further transferred to humans through the food chain. Long-term exposure to environments with excessive mercury ions can lead to damage to the central nervous system and kidneys, and in severe cases, endanger life. Therefore, detecting Hg in water is crucial. 2+ The content of [something] is of great significance.

[0003] Currently used for detecting Hg in water quality. 2+ The main methods for determining the content include instrumental analysis, biosensing, and fluorescence sensing. Among them, instrumental analysis requires expensive and large-scale instruments and professional testing personnel, and the entire testing process is time-consuming and laborious, with obvious data lag, making it difficult to meet the needs of on-site water quality testing and emergency warning of sudden accidents. Biosensing is difficult to adapt to the needs of long-term monitoring due to the instability and consistency of the activity of biological functional materials. In contrast, fluorescence sensing is based on the changes in the fluorescence intensity, lifetime, and anisotropy of the fluorophores in the fluorescent nanomaterials induced by the target analyte to detect heavy metal ions, thus meeting the needs of rapid on-site water quality testing and long-term monitoring. Although fluorescence sensing has certain advantages over the other two detection methods, it still has some problems: (1) The fluorescent nanomaterials used in fluorescence sensing can be carbon dots with excellent optical performance, high specific surface area and good biocompatibility. However, the carbon dots synthesized by current methods generally have disadvantages such as weak fluorescence intensity and low quantum yield, which are not conducive to the detection of Hg. 2+ (1) Content detection; (2) The reagent consumption in the fluorescence sensing method is large, which greatly increases the detection cost.

[0004] To address the problems existing in fluorescence sensing methods, it is necessary to develop a novel method for synthesizing carbon dots with superior performance and a method for detecting Hg in water based on these carbon dots. 2+ A new method. Summary of the Invention

[0005] In view of this, one objective of the present invention is to provide a method for preparing nitrogen-doped carbon dots with high quantum yield; a second objective of the present invention is to provide a nitrogen-doped carbon dot with high quantum yield; and a third objective of the present invention is to provide a method for detecting the content of divalent mercury ions in water.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] 1. A method for preparing nitrogen-doped carbon dots with high quantum yield, the method comprising: dissolving salicylic acid and nitric acid in N,N-dimethylformamide, and then reacting at 140-220°C for 2-10 h, and after the reaction is completed, naturally cooling to room temperature, and then centrifuging, filtering and purifying to obtain nitrogen-doped carbon dots with high quantum yield.

[0008] Preferably, the mass ratio of salicylic acid to nitric acid is 1:3 to 3:1; the ratio between the total mass of salicylic acid and nitric acid and the volume of N,N-dimethylformamide is 2:25 to 6:25, g:mL.

[0009] 2. The nitrogen-doped carbon dots with high quantum yield obtained by the preparation method described above.

[0010] 3. A method for detecting the content of divalent mercury ions in water, the method being as follows:

[0011] (1) The nitrogen-doped carbon dots with high quantum yield and deionized water are respectively introduced into the two liquid inlets of the microfluidic chip. After the nitrogen-doped carbon dots and the deionized water reach the detection port of the microfluidic chip, the fluorescent light source is aligned with the detection port, the excitation wavelength of the fluorescent light source is adjusted to 360nm, and the fluorescence intensity F0 at 406nm is measured.

[0012] (2) The nitrogen-doped carbon dots with high quantum yield and different concentrations of Hg were mixed. 2+ Standard solutions were introduced into the two inlets of the microfluidic chip, respectively, to react with the nitrogen-doped carbon dots and Hg at different concentrations. 2+ After the standard solution reaches the detection port of the microfluidic chip, the fluorescent light source is aligned with the detection port, and the excitation wavelength of the fluorescent light source is adjusted to 360 nm. The concentrations of Hg at 406 nm are then measured. 2+ The fluorescence intensity F of the nitrogen-doped carbon dots after quenching;

[0013] (3) Establish the fluorescence intensity quenching values ​​F0-F and Hg of nitrogen-doped carbon dots. 2+ The linear relationship between the concentrations of standard solutions was used to plot a standard curve of concentration versus fluorescence intensity quenching value.

[0014] (4) The nitrogen-doped carbon dots with high quantum yield and the water sample to be tested are respectively introduced into the two inlets of the microfluidic chip. After the nitrogen-doped carbon dots and the water sample to be tested reach the detection port of the microfluidic chip, the fluorescent light source is aligned with the detection port, the excitation wavelength of the fluorescent light source is adjusted to 360 nm, and the Hg in the water sample at 406 nm is measured. 2+The fluorescence intensity F1 of the nitrogen-doped carbon dots after quenching is calculated. Then, the fluorescence intensity quenching value F0-F1 is substituted into the standard curve described in step (3) to calculate the Hg in the water sample. 2+ The content of.

[0015] Preferably, the channel structure of the microfluidic chip includes two liquid inlets with a radius of 1 mm, at least one bent mixing channel, one detection port with a radius of 1 mm, and one waste liquid port with a radius of 1 mm; the bent mixing channel is located in the middle of the two liquid inlets; the bent mixing channel has a length of 2 cm, a width of 0.2 mm, and a spacing of 1.8 mm; the detection port is located at the lower end of the bent mixing channel; the waste liquid port is located at the lower end of the detection port.

[0016] The beneficial effects of this invention are as follows: 1. This invention provides nitrogen-doped carbon dots (N-CDs) with high quantum yield. These nitrogen-doped carbon dots are obtained in one step using a solvothermal method with salicylic acid and nitric acid as raw materials and N,N-dimethylformamide as solvent. Experimental results show that the quantum yield of these nitrogen-doped carbon dots can reach 53.33%, and the quantum yield of Hg... 2+ It exhibits excellent selectivity, anti-interference properties, and sensitivity. Furthermore, its fluorescence quenching degree is similar to that of Hg. 2+ There is a high correlation between concentrations, the linear detection range is 0–120 μM, and the detection limit is 7.091 nM, making it extremely suitable for Hg. 2+ The quantitative detection results met the WHO standards for drinking water. (The text also mentions Hg synthesized from other sources and methods, but this appears unrelated to the preceding sentence about Hg.) 2+ Compared to carbon dots used for detection, the nitrogen-doped carbon dots prepared in this invention exhibit a wider detection range and a lower detection limit. The fluorescence properties of the nitrogen-doped carbon dots and their ability to detect Hg were tested by examining the effects of environmental factors (such as acidity, alkalinity, salinity, and temperature). 2+ The results showed that Hg detection under normal water conditions was affected. 2+ The performance of the nitrogen-doped carbon dots is not affected by other environmental factors. Furthermore, spiked recovery experiments demonstrated that the recoveries of nitrogen-doped carbon dots in tap water samples ranged from 99.4% to 105.5%, while the recoveries in Jialing River samples ranged from 92.7% to 105.2%. The relative standard deviation (RSD) for the entire spiked recovery experiment was 1.1% to 2.4%. These results indicate that the nitrogen-doped carbon dots in this invention are a potential candidate for detecting Hg in complex real-world water samples. 2+ Fluorescent probes.

[0017] 2. This invention also provides a method for detecting the content of divalent mercury ions in water. This method combines the aforementioned nitrogen-doped carbon dots and a microfluidic chip for detecting mercury ions in water. 2+ The content is detected. The principle is that in the microfluidic chip, without Hg...2+ The fluorescence intensity of nitrogen-doped carbon dots quenched and Hg-treated 2+ The difference between the fluorescence intensities of quenched nitrogen-doped carbon dots (fluorescence intensity quenching value) and Hg 2+ The concentrations of the standard solutions showed a linear correlation, which was observed by detecting Hg in the test aqueous solution. 2+ Substituting the fluorescence intensity quenching values ​​of the nitrogen-doped carbon dots before and after quenching into the linear correlation curve yields the Hg concentration in the aqueous solution. 2+ The content of [specific element]. Because a microfluidic chip is incorporated into the detection process, the use of large instruments is avoided, reagent consumption is reduced, and rapid, convenient, and highly sensitive on-site water quality testing can be achieved.

[0018] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0020] Figure 1 This is a schematic diagram of the channel structure in the microfluidic chip prepared in Example 2;

[0021] Figure 2 The image shows the liquid phase mass spectrum of the nitrogen-doped carbon dots prepared in Example 1 (with the inset showing the main fluorescent chemical structure of the nitrogen-doped carbon dots).

[0022] Figure 3 The nitrogen-doped carbon dots (N-CDs) prepared in Example 1, and the nitrogen-doped carbon dots with Hg at a concentration of 10 μM. 2+ The mixed solution (N-CDs+Hg) 2+ (10 μM)), this nitrogen-doped carbon point and Hg at a concentration of 50 μM 2+ The mixed solution (N-CDs+Hg) 2+ Stability test results (50 μM) at different pH values;

[0023] Figure 4 The nitrogen-doped carbon dots (N-CDs) prepared in Example 1, and the nitrogen-doped carbon dots with Hg at a concentration of 10 μM. 2+ The mixed solution (N-CDs+Hg) 2+ (10 μM)), this nitrogen-doped carbon point and Hg at a concentration of 50 μM 2+ The mixed solution (N-CDs+Hg)2+ (50 μM) Stability test graphs in NaCl solutions of different concentrations;

[0024] Figure 5 This is a graph showing the change in fluorescence intensity of nitrogen-doped carbon dots prepared in Example 1 with UV lamp irradiation time.

[0025] Figure 6 This is a graph showing the change in fluorescence intensity of the nitrogen-doped carbon dots prepared in Example 1 over storage time.

[0026] Figure 7 The nitrogen-doped carbon dots prepared in Example 1 are Hg 2+ The specific detection results are shown in the figure, where (a) is Hg. 2+ (a) shows the specific detection results at a concentration of 50 μM, and (b) shows the Hg concentration. 2+ The specificity detection results at a concentration of 100 μM are shown in the figure.

[0027] Figure 8 For Hg 2+ The linear relationship between the concentration of Hg and the fluorescence quenching value of the nitrogen-doped carbon dots prepared in Example 1 is shown in the graph, where (a) represents the concentration of Hg in the range of 0–120 μM. 2+ (a) Linear relationship curve of fluorescence quenching value with nitrogen-doped carbon dots prepared in Example 1, and (b) Hg concentration in the range of 0.2–0.5 μM. 2+ Linear relationship curve between the fluorescence quenching values ​​of the nitrogen-doped carbon dots prepared in Example 1 and the fluorescence quenching values ​​of the nitrogen-doped carbon dots prepared in Example 1;

[0028] Figure 9 The nitrogen-doped carbon dots prepared in Example 1 were used to detect Hg in a microfluidic chip. 2+ The effect diagram shows that (a) represents the nitrogen-doped carbon dots as a function of Hg. 2+ (a) shows the trend of concentration change, and (b) shows the concentration of Hg at 0–22 μM on the microfluidic chip platform. 2+ Linear curves of fluorescence quenching values ​​of nitrogen-doped carbon dots (with insets showing Hg concentrations of 0.1–1 μM on a microfluidic chip platform). 2+ (Linear curve of fluorescence quenching value of nitrogen-doped carbon dots);

[0029] Figure 10 To dynamically monitor different concentrations of Hg in water samples using a microfluidic fluorescence sensing platform 2+ The graph shows the change in fluorescence intensity over monitoring time. Detailed Implementation

[0030] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0031] Example 1

[0032] The specific preparation method for nitrogen-doped carbon dots with high quantum yield is as follows:

[0033] Salicylic acid and nitric acid were weighed in a 1:1 mass ratio (total weight 4g) and placed together in 25mL of N,N-dimethylformamide. The mixture was sonicated for 1 hour to fully dissolve the N,N-dimethylformamide solution. The solution was then transferred to a 50mL polytetrafluoroethylene-lined reactor and placed in a 180℃ electric heating oven for 6 hours. After the reaction, the reactor was removed and allowed to cool naturally to room temperature. The product obtained in the reactor was then centrifuged at 8000rpm / min for 10 minutes. The resulting liquid was filtered through a 0.22μm microporous membrane. Finally, the filtrate was subjected to vacuum distillation at 80℃ in a rotary evaporator to remove residual N,N-dimethylformamide solvent, yielding a liquid nitrogen-doped carbon dot with high quantum yield. This nitrogen-doped carbon dot was stored at 4℃.

[0034] Example 2

[0035] The specific fabrication process for the microfluidic chip is as follows:

[0036] (1) Design the channel pattern of the microfluidic chip using CAD software, such as Figure 1 The design includes two inlets with a radius of 1 mm (corresponding to the inlets for nitrogen-doped carbon dots and the water sample to be tested, respectively). Eight bends in the mixing channels are designed between the two inlets, each 2 cm long and 0.2 mm wide, with a spacing of 1.8 mm between each channel. A detection port with a radius of 1 mm is designed at the lower end of each bend in the mixing channels, and a waste outlet with a radius of 1 mm is designed at the lower end of the detection port. (The nitrogen-doped carbon dots and the water sample to be tested are thoroughly mixed in the bend inlets before entering the detection port; after testing, the waste outlet is used for centralized collection.) After the channel pattern design is completed, a laser direct-write machine is used to create a mask template for the channel pattern.

[0037] (2) Sulfuric acid and hydrogen peroxide are mixed in a volume ratio of 2:1. After mixing, the mixture is boiled at 120°C for 30 minutes. Then, it is rinsed with a large amount of deionized water, dried with a nitrogen gun, and dried at 210°C for 30 minutes to obtain the piranha etching solution. The etching solution is then used to clean a 3-inch silicon wafer. Finally, a silicon dioxide oxide layer with a thickness of 300 nm is deposited on the surface of the cleaned silicon wafer to obtain the substrate of the microfluidic chip.

[0038] (3) A layer of SU8-2100 negative photoresist with a thickness of 1 mm is spin-coated on the substrate surface obtained in step (2) according to the gradient spin coating (step 1: spin coating at 500 rpm for 10 s with an acceleration of 100 rpm / s; step 2: spin coating at 1000 rpm for 30 s with an acceleration of 300 rpm / s). The substrate is pre-baked according to the temperature rise program of drying at 65℃ for 7 min and drying at 95℃ for 60 min. After the pre-baking is completed, the mask prepared in step (1) is covered on the substrate. Then, the substrate is exposed for 120 s using a binary exposure machine. After the post-baking is completed, the substrate is dried at 65℃ for 5 min and drying at 95℃ for 20 min. After the post-baking is completed, the substrate is immersed in SU-8 developer and developed in a shaker at 100 rpm for 20 min. After the development is completed, the substrate is washed with isopropanol and dried with a nitrogen gun. Finally, the substrate is hard-baked at 150℃ for 30 min to obtain the mold of the channel pattern.

[0039] (4) Add 1 mL of fluorinated polyether polymer (hydrophobic release agent) to the mold obtained in step (3), then use a spin coater to disperse it at a speed of 5000 rpm for 30 s, and then dry it at 80 ℃ for 30 min. After drying, cast polydimethylsiloxane PDMS (specifically, mix sylgard184 elastomer and its crosslinking agent at a mass ratio of 10:1, stir thoroughly, degas, and then cast). After casting, dry it again at 80 ℃ for 6 h. After demolding and cutting, the designed microfluidic chip channel is obtained.

[0040] (5) The quartz plate and the microfluidic chip channel obtained in step (4) were surface treated with a plasma cleaner (10% power, 30 Pa). After treatment for 30 seconds, the microfluidic chip channel was immediately attached to the surface of the quartz plate. Then, it was treated at 80°C for 30 minutes. Next, 0.5% perfluoroalkyltriethoxysilane (modifying reagent) was injected into the microfluidic chip channel with a syringe. Then, it was annealed at 110°C for 30 minutes to prepare the channel-designed microfluidic chip.

[0041] Example 3

[0042] A method for detecting Hg in water 2+ The method for determining the content is as follows:

[0043] (1) Using a flow injection pump (the flow injection pump has an automatic injection flow rate of 20 μM and an injection needle diameter of 16 mm), the nitrogen-doped carbon dots and deionized water prepared in Example 1 were introduced into the two inlets of the microfluidic chip prepared in Example 2 at a volume ratio of 1:1. After the nitrogen-doped carbon dots and deionized water were fully mixed through the bent mixing channel and reached the detection port of the microfluidic chip, the fluorescence light source was aligned with the detection port, the excitation wavelength of the fluorescence light source was adjusted to 360 nm, and the fluorescence intensity F0 at 406 nm was measured.

[0044] (2) A flow injection pump (automatic injection flow rate of 20 μM, injection needle diameter of 16 mm) was used to mix the nitrogen-doped carbon dots prepared in Example 1 with Hg at a concentration not exceeding 22 μM. 2+ The standard solutions were introduced into the two inlets of the microfluidic chip prepared in Example 2 at a 1:1 volume ratio, respectively, to nitrogen-doped carbon dots and Hg of different concentrations. 2+ After the standard solution is thoroughly mixed through the zigzag mixing channel and reaches the detection port of the microfluidic chip, the fluorescence light source is aligned with the detection port, and the excitation wavelength of the fluorescence light source is adjusted to 360 nm. The concentrations of Hg at 406 nm are then measured. 2+ The fluorescence intensity F of the nitrogen-doped carbon dots after quenching (the fluorescence intensity of the nitrogen-doped carbon dots relative to the Hg concentration at each concentration gradient) 2+ The standard solution was continuously and automatically injected over 1 hour, and the data was automatically recorded by the instrument every 5 minutes.

[0045] (3) Establish the fluorescence intensity quenching values ​​F0-F and Hg of nitrogen-doped carbon dots. 2+ The linear relationship between the concentrations of standard solutions was used to plot a standard curve of concentration versus fluorescence intensity quenching value.

[0046] (4) Using a flow injection pump (automatic injection flow rate of 20 μmol / L, injection needle diameter of 16 mm), the nitrogen-doped carbon dots prepared in Example 1 and the water sample to be tested were introduced into the two inlets of the microfluidic chip prepared in Example 2 at a volume ratio of 1:1. After the nitrogen-doped carbon dots and the water sample to be tested were fully mixed through the bent mixing channel and reached the detection port of the microfluidic chip, the fluorescence light source was aligned with the detection port, the excitation wavelength of the fluorescence light source was adjusted to 360 nm, and the Hg in the water sample at 406 nm was measured. 2+ The fluorescence intensity F1 of the nitrogen-doped carbon dots after quenching is calculated. Then, the fluorescence intensity quenching value F0-F1 is substituted into the standard curve in step (3) to calculate the content of divalent mercury ions in the water sample to be tested.

[0047] Performance testing

[0048] 1. Investigate the main fluorescent chemical structure of the nitrogen-doped carbon dots prepared in Example 1.

[0049] The liquid nitrogen-doped carbon dots prepared in Example 1 were subjected to liquid phase mass spectrometry (LPMS) analysis. The experimental results are as follows: Figure 2 As shown. From Figure 2 Analysis revealed that 2-hydroxy-N,N-dimethylbenzamide is the main fluorescent structure of this nitrogen-doped carbon site (as shown in the inset).

[0050] 2. The nitrogen-doped carbon dots prepared in Example 1 and the nitrogen-doped carbon dots were mixed with different concentrations of Hg. 2+ The mixed solution was then subjected to stability tests at different pH values.

[0051] Figure 3 The nitrogen-doped carbon dots (N-CDs) prepared in Example 1, and the nitrogen-doped carbon dots with Hg at a concentration of 10 μM. 2+ The mixed solution (N-CDs+Hg) 2+ (10 μM)), this nitrogen-doped carbon point and Hg at a concentration of 50 μM 2+ The mixed solution (N-CDs+Hg) 2+ (50 μM) Stability test results at different pH values. During the experiment, the acid-base environment of each sample solution was adjusted to a pH range of 1–13 using 0.1 M hydrochloric acid and 5% sodium hydroxide aqueous solution. Then, 1 mL of the corresponding sample solution was taken at each pH point for fluorescence intensity testing (3 measurements were taken at each pH point). Figure 3 The experimental results all indicate that the nitrogen-doped carbon dots remain stable in the pH range of 4 to 10, making them suitable for a wide range of water body detection applications.

[0052] 3. The nitrogen-doped carbon dots prepared in Example 1 and the nitrogen-doped carbon dots were mixed with different concentrations of Hg. 2+ The mixed solution was placed in NaCl solutions of different concentrations for stability testing.

[0053] Sodium chloride was dissolved in pure water to prepare a 2M sodium chloride stock solution. Seven 2.5mL centrifuge tubes were used. First, 1mL of the nitrogen-doped carbon dots prepared in Example 1 was added to each tube. Then, 0μL, 100μL, 200μL, 300μL, 400μL, 500μL, and 600μL of the sodium chloride stock solution were added to each tube, respectively. Finally, pure water was added to each centrifuge tube to a final volume of 2mL. After mixing the centrifuge tubes, they were incubated at room temperature for 1 minute before fluorescence detection. The same detection method was used for the nitrogen-doped carbon dots, except that the nitrogen-doped carbon dots (N-CDs) were replaced with nitrogen-doped carbon dots and 10μM Hg. 2+ The mixed solution (N-CDs+Hg) 2+ (10 μM)), nitrogen-doped carbon dots and Hg at a concentration of 50 μM 2+ The mixed solution (N-CDs+Hg) 2+The fluorescence properties were detected at (50 μM) respectively, and the experimental results are as follows: Figure 4 As shown. Figure 4 The experimental results all show that the nitrogen-doped carbon dots are stable in water bodies with a salt concentration of 0.5M (approximate to seawater salinity), and can be used for a wide range of water body detection.

[0054] 4. The photobleaching resistance of the nitrogen-doped carbon dots prepared in Example 1 was tested.

[0055] 1 mL of the nitrogen-doped carbon dots prepared in Example 1 was dissolved in 1 mL of pure water, and then irradiated under a 365 nm UV lamp for 360 min. Fluorescence detection was performed, and the experimental results are as follows. Figure 5 As shown. From Figure 5 It can be seen that the fluorescence intensity of the nitrogen-doped carbon dot did not change under continuous ultraviolet light irradiation, meaning that no bleaching phenomenon occurred after irradiation under ultraviolet light, indicating that it has good optical stability.

[0056] 5. The nitrogen-doped carbon dots prepared in Example 1 were subjected to storage environment stability tests.

[0057] Nitrogen-doped carbon samples were stored at room temperature under light-free conditions for 60 days. Fluorescence intensity was then recorded on days 1, 3, 5, 7, 10, 20, 30, 40, 50, and 60. Each sample was recorded three times, and the average fluorescence intensity was taken as the test result. The experimental results are as follows: Figure 6 As shown. From Figure 6 As can be seen, the fluorescence intensity of the nitrogen-doped carbon spot did not change significantly after 3 months of storage, which also indicates that it can be used for long-term field testing and maintains good stability.

[0058] 6. Perform selective testing on the nitrogen-doped carbon dots prepared in Example 1.

[0059] 1 mL of Hg at different concentrations was added to 1 mL of the nitrogen-doped carbon dots prepared in Example 1. 2+ (50μM, 100μM) and the same concentration of interfering cations and anions (including NO2) - F - NO3 - Cl - HCO3 - ,Br - CO3 2- I - PO4 2- SO4 2- Zn 2+ Al 3+ Pb 2+ Ag + Cr 6+ Mn2+ Cr 3+ Na + Fe 3+ Ca 2+ Fe 2+ Mg 2+ and Cd 2+ Then, fluorescence spectroscopy was used to detect the fluorescence intensity of the nitrogen-doped carbon dot. The experimental results are as follows: Figure 7 (a) and Figure 7 As shown in (b). Experimental results show that, compared to other interfering ions, regardless of Hg... 2+ Whether the concentration is high or low, the light intensity decreases significantly after nitrogen-doped carbon dots bind to it, indicating that the nitrogen-doped carbon dots can effectively identify Hg in the water. 2+ It can be applied to water body detection in a variety of scenarios.

[0060] 7. The quantum yield of the nitrogen-doped carbon dots prepared in Example 1 was determined.

[0061] Using quinine sulfate with a maximum excitation wavelength of 350 nm as a standard reference (the quantum yield of quinine sulfate dispersed in 0.1 M sulfuric acid is known to be 57.7%), the ultraviolet absorption value and fluorescence emission peak of the nitrogen-doped carbon dots prepared in Example 1 and quinine sulfate at 350 nm were measured respectively. The fluorescence emission was integrated, and the quantum yield of the nitrogen-doped carbon dots was calculated according to Equation (1) to be 53.33%.

[0062]

[0063] In equation (1), QY St For the quantum yield of quinine sulfate, I CDs and I St The integrated fluorescence emission intensities of nitrogen-doped carbon dots and quinine sulfate, respectively, are A. CDs and A St These represent the absorbance values ​​of nitrogen-doped carbon dots and quinine sulfate at an excitation wavelength of 350 nm, respectively. CDs and η St λ represents the refractive index of the nitrogen-doped carbon dots and the corresponding solvents for quinine sulfate, respectively. To reduce the reabsorption effect, the nitrogen-doped carbon dots and the standard reference are set at λ... ex The ultraviolet absorption at 350 nm remained below 0.05 au.

[0064] 8. Determination of the detection limit for the nitrogen-doped carbon dots prepared in Example 1.

[0065] (1) The fluorescence intensity F0 at 406 nm was measured using a Hitachi F-7000 fluorescence spectrophotometer. A four-way quartz cuvette containing 1 mL of nitrogen-doped carbon dots prepared in Example 1 was placed under an excitation wavelength of 360 nm.

[0066] (2) The test was performed using a Hitachi F-7000 fluorometer from Japan, with 1 mL of Hg at a concentration not exceeding 120 μM. 2+ Standard solutions were added to four-way quartz cuvettes containing 1 mL of nitrogen-doped carbon dots prepared in Example 1, and then the concentrations of Hg at 406 nm were measured at an excitation wavelength of 360 nm. 2+ The fluorescence intensity F of the nitrogen-doped carbon dots after quenching;

[0067] (3) Establish the fluorescence intensity quenching values ​​F0-F and Hg of nitrogen-doped carbon dots. 2+ The linear relationship between the concentrations of standard solutions was used to plot a standard curve of concentration versus fluorescence intensity quenching value (e.g.) Figure 8 (as shown in (a));

[0068] (4) Select Hg from the standard curve of concentration-fluorescence intensity quenching value in step (3). 2+ The concentration is in the linear range of 0.2–0.5 μM (e.g. Figure 8 As shown in (b), the slope of the linear interval is obtained, and then the nitrogen-doped carbon dot to Hg is calculated according to equations (2) and (3). 2+ The detection limit was 7.091 nM.

[0069]

[0070]

[0071] In equation (2), N represents the number of times the fluorescence intensity of a single nitrogen-doped carbon dot is measured, and the value is 11. For a certain concentration of Hg 2+ The average fluorescence intensity after mixing with nitrogen-doped carbon dots, measured N times; X i For a certain concentration of Hg 2+ The fluorescence intensity after mixing with nitrogen-doped carbon dots is the actual fluorescence intensity measurement value in the i-th time.

[0072] In equation (3), m represents the slope in the linear interval of 0.2 to 0.5 μM.

[0073] 9. The applicability and feasibility of the nitrogen-doped carbon dots prepared in Example 1 in actual water samples were evaluated using the spiked recovery method.

[0074] Because the nitrogen-doped carbon dots prepared in Example 1 exhibit Hg... 2+ Due to its strong selectivity and high sensitivity, this fluorescent probe was applied to detect Hg in real water samples. 2+ Because the actual water body contains Hg 2+The content was low, and the applicability and feasibility of this nitrogen-doped carbon point in actual water samples were evaluated using the spiked recovery method (as shown in Table 1). The spiked recovery experiment results showed that the spiked recovery rate of this nitrogen-doped carbon point ranged from 99.4% to 105.5% in tap water samples and from 92.7% to 105.2% in Jialing River samples. The relative standard deviation (RSD) of the entire spiked recovery experiment ranged from 1.1% to 2.4%.

[0075] Table 1. Hg detection of nitrogen-doped carbon dots prepared in Example 1 2+ Spiked recovery results (n=3)

[0076]

[0077] 10. To investigate the detection of Hg in water using the method described in Example 3. 2+ Effect

[0078] Figure 9 (a) shows the nitrogen-doped carbon dots in the microfluidic chip platform as a function of Hg. 2+ A trend graph of concentration changes. From Figure 9 As can be seen in (a), with Hg 2+ As the concentration increased from 0 μM to 22 μM, the fluorescence spectrum showed that the fluorescence intensity of the nitrogen-doped carbon dots at 406 nm gradually decreased, and the two showed a strong correlation. Figure 9 (b) shows Hg concentrations of 0–22 μM on a microfluidic chip platform. 2+ Linear curves of fluorescence quenching values ​​of nitrogen-doped carbon dots (with insets showing Hg concentrations of 0.1–1 μM on a microfluidic chip platform). 2+ Linear curve of fluorescence quenching value of nitrogen-doped carbon dots. (For Hg...) 2+ Linear fitting was performed between concentration and the fluorescence quenching degree of nitrogen-doped carbon dots. The results showed that although the linear correlation consisted of two parts, both parts exhibited high linear correlation within their respective concentration ranges. Within the first linear interval, the sensing platform was able to detect trace concentrations of Hg. 2+ (0.1 μM), at which point the decrease in fluorescence signal is related to Hg 2+ The concentration relationship is y = 3495.8x + 1461.2. Calculations show that the microfluidic detection platform can detect Hg... 2+ The detection limit is 0.016 μM, which meets the WHO standards for drinking water quality. The second linear interval is Hg. 2+ When the concentration is in the range of 0–22 μM, Hg 2+ The relationship between concentration and fluorescence quenching degree of nitrogen-doped carbon dots is y = 224.816x + 4313.675, with a correlation coefficient exceeding 0.98.

[0079] To illustrate the effect of the created microfluidic chip fluorescence sensing platform on Hg 2+ The stability of the detection was recorded at Hg 2+ The fluorescence intensity peaks were automatically detected every 5 minutes during dynamic monitoring at concentrations of 0.8 μM, 4 μM, 14 μM, and 22 μM. The results are as follows: Figure 10 As shown, from 0 min to 60 min, the microfluidic platform demonstrated its effectiveness against four concentrations of Hg. 2+ The small fluctuations in fluorescence intensity during detection indicate that the established microfluidic fluorescence sensing platform is effective for Hg. 2+ The detection is stable.

[0080] In summary, this invention provides a nitrogen-doped carbon dot with high quantum yield, its preparation method, and a method for detecting the content of divalent mercury ions in water. The nitrogen-doped carbon dot is obtained in one step using a solvothermal method with salicylic acid and nitric acid as raw materials and N,N-dimethylformamide as solvent. The prepared nitrogen-doped carbon dot has a high quantum yield and is effective against Hg. 2+ It exhibits excellent selectivity, anti-interference properties, and sensitivity; its quenching fluorescence intensity is comparable to that of Hg. 2+ The concentration showed a linear correlation. This nitrogen-doped carbon dot was combined with a microfluidic platform for the treatment of Hg in water. 2+ The detection limit for Hg content is low. Furthermore, this method avoids the use of large instruments and reduces reagent consumption, enabling on-site detection of Hg in water. 2+ It offers fast, convenient, and highly sensitive detection.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing nitrogen-doped carbon dots with high quantum yield, characterized in that: The preparation method is as follows: salicylic acid and nitric acid are dissolved in N,N-dimethylformamide, and then reacted at 140~220°C for 2~10h. After the reaction is completed, the mixture is naturally cooled to room temperature, and after centrifugation, filtration and purification, nitrogen-doped carbon dots with high quantum yield can be obtained. The mass ratio of salicylic acid to nitric acid is 1:3 to 3:1; the ratio between the total mass of salicylic acid and nitric acid and the volume of N,N-dimethylformamide is 2:25 to 6:25, g:mL.

2. Nitrogen-doped carbon dots with high quantum yield prepared by the preparation method according to claim 1.

3. A method for detecting the content of divalent mercury ions in water, characterized in that: The method is as follows: (1) The nitrogen-doped carbon dots with high quantum yield as described in claim 2 and deionized water are respectively introduced into the two liquid inlets of the microfluidic chip. After the nitrogen-doped carbon dots and the deionized water reach the detection port of the microfluidic chip, the fluorescent light source is aligned with the detection port, the excitation wavelength of the fluorescent light source is adjusted to 360nm, and the fluorescence intensity F0 at 406nm is measured. (2) The nitrogen-doped carbon dots with high quantum yield as described in claim 2 and different concentrations of Hg 2+ Standard solutions were introduced into the two inlets of the microfluidic chip, respectively, to react with the nitrogen-doped carbon dots and Hg at different concentrations. 2+ After the standard solution reaches the detection port of the microfluidic chip, the fluorescent light source is aligned with the detection port, and the excitation wavelength of the fluorescent light source is adjusted to 360 nm. The concentrations of Hg at 406 nm are then measured. 2+ The fluorescence intensity F of the nitrogen-doped carbon dots after quenching; (3) Establish the fluorescence intensity quenching values ​​F0-F and Hg of nitrogen-doped carbon dots. 2+ The linear relationship between the concentrations of standard solutions was used to plot a standard curve of concentration versus fluorescence intensity quenching value. (4) The nitrogen-doped carbon dots with high quantum yield as described in claim 2 and the water sample to be tested are respectively introduced into the two inlets of the microfluidic chip. After the nitrogen-doped carbon dots and the water sample to be tested reach the detection port of the microfluidic chip, the fluorescent light source is aligned with the detection port, the excitation wavelength of the fluorescent light source is adjusted to 360 nm, and the Hg in the water sample at 406 nm is measured. 2+ The fluorescence intensity F1 of the nitrogen-doped carbon dots after quenching is calculated. Then, the fluorescence intensity quenching value F0-F1 is substituted into the standard curve described in step (3) to calculate the Hg in the water sample. 2+ The content of.

4. The method according to claim 3, characterized in that: The microfluidic chip has a channel structure comprising two liquid inlets with a radius of 1 mm, one or more bent mixing channels, one detection port with a radius of 1 mm, and one waste liquid port with a radius of 1 mm. The bent mixing channel is located between the two liquid inlets. The bent mixing channel has a length of 2 cm, a width of 0.2 mm, and a spacing of 1.8 mm. The detection port is located at the lower end of the bent mixing channel. The waste liquid port is located at the lower end of the detection port.

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