A preparation method and application of nitrogen-doped carbon dots with excellent stability

Abstract

The present invention provides a preparation method and application of nitrogen-doped carbon dots with excellent stability; the preparation process is as follows: First, citric acid and 2-amino-2-methyl-1-propanol in a molar ratio of 1: (0.8 to 3.5) are added to a solvent, and stirred to obtain a precursor solution. Second, the precursor solution is subjected to a one-step heating reaction to form a carbon dot solution. Third, a nitrogen-doped fluorescent carbon dot solution is separated in the carbon dot solution. The present invention uses citric acid and 2-amino-2-methyl-1-propanol as raw materials, and a branched structure composed of dimethyl groups to limit the resonance energy transfer effect between the carbon nuclear fluorescence center of the carbon dot and the nitrogen-related surface state fluorescence center, thereby ensuring the stability of the carbon dot fluorescence in a variety of complex environments, and the obtained carbon dots have excellent selectivity and sensitivity to iron ions, demonstrating its huge application prospects as an efficient iron ion detection probe.

Description

Technical Field

[0001] The present invention relates to the field of fluorescent nanomaterials, and in particular to a method for preparing nitrogen-doped fluorescent carbon dots with excellent stability and an application thereof in iron ion detection. Background Art

[0002] Carbon dots, an emerging carbon nanomaterial, have been widely studied and applied due to their excellent fluorescence properties, low toxicity, photostability, environmental friendliness, and good biocompatibility. They demonstrate significant advantages in a variety of fields, including medical imaging, drug delivery, chemical and biosensing. In particular, they hold great promise for detecting metal ions in bioimaging and sensing. In recent years, researchers have employed strategies such as doping and surface modification to further manipulate the fluorescence properties and enhance their quality.

[0003] When used as metal ion probes, traditional nitrogen-doped carbon dots are often susceptible to interference from complex environments due to the abundance of surface functional groups, particularly nitrogen-related ones. For example, fluorescence quenching occurs under illumination or other extreme conditions (such as high salt concentrations, strong acids and bases), limiting their application in complex or harsh environments. Furthermore, the fluorescence characteristics of carbon dots exhibited by these probes, which are characterized by poor stability, can vary over time during repeated detection, resulting in inaccurate and reproducible detection results. Furthermore, real-world metal ion detection applications, such as ion detection in actual water or biological samples, are also susceptible to various environmental factors, leading to poor detection results. Summary of the Invention

[0004] The purpose of the present invention is to solve the problem of poor stability of traditional carbon dots under complex environmental conditions such as high salt, strong acid and alkali, and light, and to provide a method for preparing nitrogen-doped fluorescent carbon dots with excellent stability and its application in the field of metal ion detection. The preparation method of the present invention is simple, requires very few raw materials, is time-saving, low-cost, and is economical and environmentally friendly. The nitrogen-doped carbon dots prepared by the present invention have excellent fluorescent properties, good stability, and strong application value.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a method for preparing nitrogen-doped fluorescent carbon dots with excellent stability, comprising the following steps:

[0007] Step 1: Add citric acid and 2-amino-2-methyl-1-propanol in a molar ratio of 1: (0.8-3.5) to a solvent and stir evenly to obtain a precursor solution.

[0008] Step 2: The precursor solution obtained in step 1 is subjected to a one-step heating reaction to form a carbon dot solution.

[0009] Step 3: Separate the nitrogen-doped fluorescent carbon dot solution from the carbon dot solution obtained in step 2.

[0010] The present invention uses citric acid and 2-amino-2-methyl-1-propanol as raw materials and N,N-dimethylformamide as the solvent environment. The branched structure formed by the dimethyl group in 2-amino-2-methyl-1-propanol limits the resonance energy transfer between the carbon core fluorescence center and the nitrogen-related surface state fluorescence center of the carbon dots. In addition, the present invention can obtain nitrogen-doped fluorescent carbon dots with excellent stability by heating in a reactor for a short time (a one-step solvent thermal method). The raw materials are easily available and the cost is low. The preparation method is simple, the time is short, the risk is low, and mass production is easy to achieve.

[0011] Preferably, the solvent is N,N-dimethylformamide.

[0012] Preferably, the amount of citric acid relative to N,N-dimethylformamide is 1 / 80 to 1 / 15 g / mL.

[0013] Preferably, the reaction temperature in step 2 is 160° C. to 200° C., and the reaction time is 6 h to 10 h.

[0014] Preferably, the specific process of step three is: cooling the carbon dot solution obtained in step two to room temperature, and then centrifuging to obtain a nitrogen-doped fluorescent carbon dot solution.

[0015] Preferably, the centrifugal conditions are a rotation speed of 6000 r / min to 10000 r / min and a centrifugal time of 20 min to 40 min.

[0016] In a second aspect, the present invention provides nitrogen-doped fluorescent carbon dots, which are prepared by the aforementioned preparation method.

[0017] The carbon-doped fluorescent carbon dots showed good stability under long-term storage (0-50 days), different acidic and alkaline environments (pH: 3-13.5), different NaCl concentrations (0-5 mol / L), and long-term ultraviolet light irradiation (0-5 hours). The fluorescence peak position remained basically unchanged, and the fluorescence intensity changed within ±10%.

[0018] In a third aspect, the present invention provides an application of nitrogen-doped fluorescent carbon dots in the detection of iron ions.

[0019] Preferably, the concentration of iron ions is determined by the change in fluorescence intensity at 330 nm before and after the nitrogen-doped fluorescent carbon dots are added to the solution being tested; and the change in fluorescence intensity at 330 nm is linearly related to the concentration of iron ions.

[0020] Preferably, the iron ion concentration in the iron ion solution to be measured is 0.2 μM to 40 μM, and the fluorescence intensity has good linearity with the iron ion concentration.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The present invention uses citric acid and 2-amino-2-methyl-1-propanol as raw materials, and uses a branched structure composed of dimethyl groups to limit the resonance energy transfer between the carbon core fluorescence center of the carbon dots and the nitrogen-related surface state fluorescence center, thereby ensuring the stability of the carbon dot fluorescence in a variety of complex environments. The obtained carbon dots have excellent selectivity and sensitivity to iron ions, demonstrating their great application prospects as efficient iron ion detection probes.

[0023] 2. The present invention uses N,N-dimethylformamide as the solvent environment and heats the reaction vessel for a short time (one-step solvent thermal method) to obtain nitrogen-doped fluorescent carbon dots with excellent stability. The raw materials are readily available and the cost is low. The preparation method is simple, time-consuming, low-risk, and easy to achieve mass production.

[0024] 3. The nitrogen-doped carbon dots obtained in the present invention have excellent stability, with little change in fluorescence peak position and fluorescence intensity in complex environments, and can be used for fluorescence detection of iron ion concentration in deionized water and actual water samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 3 is a graph of the fluorescence emission spectra of the carbon dots obtained in Examples 1 to 3 of the present invention and Comparative Examples 1 to 3, with an excitation wavelength of 330 nm. The inset is an enlarged view of the fluorescence spectra of Comparative Examples 1 to 3.

[0026] Figure 2 1 is a fluorescence spectrum of carbon dots under different conditions in Example 4 of the present invention; Figure 2 Part (a) corresponds to different storage times in the range of 0-50 days, part (b) corresponds to different pH environments in the range of 3.0-13.5, part (c) corresponds to different NaCl concentrations in the range of 0-5 mol / L, and part (d) corresponds to different 365nm UV light irradiation times in the range of 0-5 hours. Figure 2 The insets are the corresponding fluorescence intensity changes.

[0027] Figure 3 This is a diagram showing the selectivity effect of the change in fluorescence intensity of the carbon dots in Example 5 of the present invention when detecting different metal ions in deionized water.

[0028] Figure 4 1 is a fluorescence spectrum of the carbon dots in Example 5 of the present invention detecting different iron ion concentrations in deionized water; Figure 4 The inset shows the linear relationship between fluorescence intensity and iron ion concentration. DETAILED DESCRIPTION

[0029] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] Example 1

[0031] A method for preparing nitrogen-doped fluorescent carbon dots with excellent stability comprises the following steps:

[0032] 1.89 g of citric acid and 751 μL of 2-amino-2-methyl-1-propanol (the molar ratio of citric acid to 2-amino-2-methyl-1-propanol is 1:0.8) were dissolved in N,N-dimethylformamide and stirred thoroughly to obtain a mixed solution. The mixed solution was poured into a sealed 50 mL autoclave and placed in an oven. The reaction was heated at 200°C for 6 hours and then naturally cooled to obtain a carbon dot solution.

[0033] 2) The carbon dot solution was centrifuged at 8000 r / min for 30 min to obtain nitrogen-doped fluorescent carbon dots. The fluorescence emission spectrum of the obtained carbon dots was as follows: Figure 1 shown.

[0034] Example 2

[0035] A method for preparing nitrogen-doped fluorescent carbon dots with excellent stability. Compared with Example 1, this embodiment differs in that: 0.44 g of citric acid and 539 μL of 2-amino-2-methyl-1-propanol (the molar ratio of citric acid to 2-amino-2-methyl-1-propanol is 1:2.4) are taken, and heated at 180° C. for 8 hours. The rest is exactly the same as in Example 1. The obtained carbon dot fluorescence emission spectrum is as follows: Figure 1 shown.

[0036] Example 3

[0037] A method for preparing nitrogen-doped fluorescent carbon dots with excellent stability. Compared with Example 1, this embodiment differs in that: 0.378 g of citric acid and 657 μL of 2-amino-2-methyl-1-propanol (the molar ratio of citric acid to 2-amino-2-methyl-1-propanol is 1:3.5) are taken, and the reaction is heated at 160° C. for 10 hours. The rest is exactly the same as in Example 1. The obtained carbon dot fluorescence emission spectrum is shown as follows: Figure 1 shown.

[0038] Comparative Example 1

[0039] The difference between this comparative example and Example 1 is that 2-amino-2-methyl-1-propanol is replaced by ethanolamine, and the molar ratio of ethanolamine to citric acid remains unchanged. The rest is exactly the same as Example 1. The obtained carbon dot fluorescence emission spectrum is as follows: Figure 1 shown.

[0040] Comparative Example 2

[0041] This comparative example is different from Example 1 in that 2-amino-2-methyl-1-propanol is replaced with 2-methyl-2-propanol, and the molar ratio of 2-amino-2-methyl-1-propanol to citric acid remains unchanged. The rest is exactly the same as Example 1, and nitrogen-doped fluorescent carbon dots are obtained. The fluorescence emission spectrum is shown in FIG. Figure 1 shown.

[0042] Comparative Example 3

[0043] This comparative example is different from Example 1 in that 2-amino-2-methyl-1-propanol is replaced with 1-amino-2-methyl-2-propanol, and the molar ratio of 2-amino-2-methyl-1-propanol to citric acid remains unchanged. The rest is exactly the same as Example 1. The obtained carbon dot fluorescence emission spectrum is as follows: Figure 1 shown.

[0044] from Figure 1 As can be seen from the graph, the carbon dots prepared in Comparative Examples 1-3 all exhibit blue and green dual fluorescence emission under 330nm excitation, namely carbon core luminescence and nitrogen-related surface state luminescence. Furthermore, as the number of methyl groups in the raw materials increases, the ratio of green fluorescence to blue fluorescence gradually decreases, and the resonance energy transfer efficiency between the carbon core fluorescence center and the nitrogen-related surface state fluorescence center gradually decreases. In contrast, the amino groups in the carbon dots prepared in Examples 1-3 are protected by two methyl branches. The steric hindrance effect inhibits the resonance energy transfer between the two fluorescence centers, resulting in the carbon dots prepared in Examples 1-3 emitting only blue fluorescence.

[0045] Example 4

[0046] Comprehensive stability test of nitrogen-doped fluorescent carbon dots (prepared in Example 2):

[0047] For the storage stability test, the carbon dot solution prepared in Example 2 was exposed to air for different time periods (0-50 days). The carbon dot solution was added to a BR buffer solution with a pH value of 6.8 at regular intervals. The fluorescence emission spectrum of the carbon dots was as follows: Figure 2 As shown in part (a), the fluorescence of the carbon dots gradually increases, and during the 50-day storage period, the fluorescence change range of the carbon dots is within 8%.

[0048] For the stability test of UV irradiation: irradiate the carbon dot solution with 365nm UV light for different time periods (0-300 minutes). At regular intervals, add the carbon dot solution into BR buffer solution with a pH value of 6.8. The fluorescence emission spectrum of the carbon dots is as follows: Figure 2 As shown in part (b), the fluorescence of the carbon dots gradually decreases. After 300 minutes of irradiation, the fluorescence change range of the carbon dots is within 9%.

[0049] For the stability test of pH value change, the carbon dots were added into BR buffer solution with different pH values ​​(3.0-13.5), mixed thoroughly and allowed to stand for 5 minutes. The fluorescence emission spectra of the carbon dots were as follows: Figure 2 As shown in part (c), the fluorescence variation range of carbon dots is within ±8%.

[0050] For the stability test of salt solution concentration changes: carbon dots were added into NaCl solutions of different concentrations (0-5 mol / L), mixed thoroughly and allowed to stand for 5 minutes. The fluorescence emission spectra of the carbon dots were as follows: Figure 2 As shown in part (d), the fluorescence of the carbon dots gradually decreases. After adding 5 mol / L salt solution, the fluorescence change range of the carbon dots is within 9%.

[0051] Example 5

[0052] A method for detecting iron ions is as follows:

[0053] Prepare various metal ion solutions with a concentration of 40 μM (Na + , K + , Ca 2+ , Ba 2+ , Zn 2+ , Mn 2 + , Cu 2+ , Ni 2+ , Cr 3+ , Cd 2+ , Ag + , Co 2+ , Mg 2+ , Fe 2+ , Fe 3+ ); The carbon dot solution prepared in Example 2 was added to different metal ion solutions, mixed evenly, and then allowed to stand for 5 minutes; the fluorescence emission spectrum was tested under 330nm excitation. The results are as follows Figure 3 As shown, it can be seen that Fe 3+ The fluorescence intensity of carbon dots in the ion solution decreased significantly, while the fluorescence intensity of carbon dots in the other metal ion solutions remained almost unchanged, indicating that nitrogen-doped fluorescent carbon dots have good selectivity as a probe for detecting iron ion concentration. After adding various metal ions, only Fe3+ A clear quenching effect was observed.

[0054] To test the sensitivity of the iron ion detection method provided in this embodiment, the detection of iron ion solutions with different concentrations was performed as follows:

[0055] Iron ions were dissolved in deionized water to prepare multiple solutions with different iron ion concentrations. 3 mL of ion solution was added to each group, and 50 μL of the carbon dot solution prepared in Example 2 was added. After mixing evenly, the mixture was allowed to stand for 5 minutes, and the fluorescence emission spectrum was measured under 330 nm excitation. The results are shown in Figure 2. Figure 4 As shown in Figure 2, the nitrogen-doped fluorescent carbon dots with excellent stability prepared in Example 2 have good iron ion concentration detection performance. As the iron ion concentration increases, the fluorescence intensity of the solution gradually decreases. When the iron ion concentration is in the range of 0-40 μM, there is a linear relationship between the initial fluorescence intensity F0 at 415 nm and the fluorescence intensity F after the iron ion concentration increases. The linear equation is (F0-F) / F0 = 0.015[Fe 3+ ]–0.002 (R 2 =0.998)), with a detection limit of 0.2 μM.

[0056] To test the compatibility of the iron ion detection method provided in this embodiment with different water bodies, the following tests were performed:

[0057] Iron ions were dissolved in mineral water, tap water, and lake water to prepare multiple solutions with iron ion concentrations ranging from 0 to 40 μM. Carbon dots were added to each solution, mixed thoroughly, and allowed to stand for 5 minutes. Fluorescence emission spectra were then measured under excitation at 330 nm. The results are shown in Table 1. When tested in real water samples, the fluorescence emission spectra exhibited low RSDs (relative standard deviations) and recoveries ranging from 93.0% to 103.0%.

[0058] Table 1 Comparison of the detection accuracy of nitrogen-doped fluorescent carbon dots for different concentrations of iron ion solutions in different water bodies

[0059] sample Addition concentration (μM) Detection concentration (μM) Spike recovery (%) Relative standard deviation (%) mineral water 5 4.68±0.16 93.6 3.39 mineral water 15 14.89±0.25 99.2 1.67 mineral water 24 24.59±0.26 102.5 1.05 mineral water 32 32.54±0.20 101.7 0.62 mineral water 40 40.15±0.91 100.3 2.26 Tap water 5 4.93±0.24 98.6 4.83 Tap water 15 15.34±0.39 102.8 2.51 Tap water 24 24.57±0.37 102.3 1.49 Tap water 32 32.66±0.39 102.0 1.18 Tap water 40 40.98±0.48 102.4 1.21 lake water 5 4.79±0.12 95.8 2.53 lake water 15 15.44±0.41 102.9 2.66 lake water 24 24.61±0.57 102.5 2.31 lake water 32 32.24±0.37 100.7 1.14 lake water 40 40.01±0.37 100.7 0.92

[0060] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form or substance. It should be pointed out that ordinary technicians in this technical field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention. Any equivalent changes, modifications and evolutions made by technicians familiar with this profession without departing from the spirit and scope of the present invention by using the technical content disclosed above are all equivalent embodiments of the present invention; at the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for preparing nitrogen-doped fluorescent carbon dots with excellent stability, characterized by: The following steps are involved: Step 1: adding citric acid and 2-amino-2-methyl-1-propanol in a molar ratio of 1: (0.8-3.5) to a solvent to obtain a precursor solution; the solvent is N,N-dimethylformamide; Step 2: heating the precursor solution obtained in step 1 to react and form a carbon dot solution; Step 3: Separate the nitrogen-doped fluorescent carbon dot solution from the carbon dot solution obtained in step 2.

2. The method for preparing nitrogen-doped fluorescent carbon dots with excellent stability according to claim 1, wherein: The amount of citric acid used relative to N,N-dimethylformamide is 1 / 80 to 1 / 15 g / mL.

3. The method for preparing nitrogen-doped fluorescent carbon dots with excellent stability according to claim 1, wherein: The reaction temperature in step 2 is 160° C. to 200° C., and the reaction time is 6 h to 10 h.

4. The method for preparing nitrogen-doped fluorescent carbon dots with excellent stability according to claim 1, wherein: The specific process of step three is: cooling the carbon dot solution obtained in step two to room temperature, and then centrifuging to obtain a nitrogen-doped fluorescent carbon dot solution.

5. The method for preparing nitrogen-doped fluorescent carbon dots with excellent stability according to claim 4, wherein: The centrifugal conditions are as follows: a rotation speed of 6000 r / min to 10000 r / min and a centrifugal time of 20 min to 40 min.

6. A nitrogen-doped fluorescent carbon dot, characterized in that: It is prepared by the preparation method according to claim 1.

7. Use of the nitrogen-doped fluorescent carbon dots as claimed in claim 1 in the detection of iron ions.

8. The use according to claim 7, characterized in that: The iron ion concentration was determined by the change in fluorescence intensity at 330 nm before and after the nitrogen-doped fluorescent carbon dots were added to the test solution. The change in fluorescence intensity at 330 nm was linearly related to the iron ion concentration.

9. The use according to claim 7, characterized in that: The iron ion concentration in the tested iron ion solution is 0.2 μM to 40 μM.

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