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

Abstract

The invention provides a preparation method and application of nitrogen-doped carbon dots with excellent stability. The preparation method comprises the following steps: 1, adding citric acid and 2-amino-2-methyl-1-propanol in a molar ratio of 1: (0.8-3.5) into a solvent, and uniformly stirring to obtain a precursor solution; 2, carrying out one-step heating reaction on the precursor solution to form a carbon dot solution; and 3, separating in the carbon dot solution to obtain the nitrogen-doped fluorescent carbon dot solution. According to the invention, citric acid and 2-amino-2-methyl-1-propanol are used as raw materials, and a branched structure formed by dimethyl limits the resonance energy transfer effect between a carbon nucleus fluorescence center of a carbon dot and a nitrogen-related surface state fluorescence center, so that the stability of carbon dot fluorescence in various complex environments is ensured, and the fluorescence intensity of the carbon dot is improved. And the obtained carbon dots have excellent selectivity and sensitivity to iron ions, and show a huge application prospect of the carbon dots as an efficient iron ion detection probe.

Description

Technical Field

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

[0002] As an emerging carbon nanomaterial, carbon dots have been widely studied and applied due to their excellent fluorescence properties, low toxicity, photostability, environmental friendliness and good biocompatibility. Carbon dots have shown significant advantages in many fields such as medical imaging, drug delivery, chemical and biological sensing, especially in biological imaging and sensing, and have shown great potential for detecting metal ions. In recent years, in order to further regulate the fluorescence properties of carbon dots and improve their fluorescence quality, researchers usually use strategies such as doping and surface modification for processing.

[0003] When traditional nitrogen-doped carbon dots are used as metal ion probes, they are often easily disturbed by complex environments due to the rich functional groups on their surfaces, especially nitrogen-related functional groups. For example, fluorescence quenching is easily caused under light or other extreme conditions (such as high salt, strong acid and alkali, etc.), which limits their application in complex or harsh environments. In addition, the fluorescence characteristics of carbon dots with poor stability in repeated detections will also change with the detection time, resulting in insufficient accuracy and repeatability of the detection results. In addition, the detection of metal ions in real environments, such as ion detection in actual water samples or biological samples, is also easily affected by various environmental factors, resulting in 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, takes a short time, has low cost, is economical and environmentally friendly, and the nitrogen-doped carbon dots prepared by the present invention have excellent fluorescence properties, good stability, and strong application value.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a method for preparing nitrogen-doped fluorescent carbon dots with excellent stability, which comprises the following steps: 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.

[0006] Step 2: subjecting the precursor solution obtained in step 1 to a one-step heating reaction to form a carbon dot solution.

[0007] Step 3: Separate the nitrogen-doped fluorescent carbon dot solution from the carbon dot solution obtained in step 2.

[0008] The present invention uses citric acid and 2-amino-2-methyl-1-propanol as raw materials and N,N-dimethylformamide as solvent environment. The branched structure formed by the dimethyl group in 2-amino-2-methyl-1-propanol limits the resonance energy transfer effect between the carbon core fluorescence center of the carbon dots and the nitrogen-related surface state fluorescence center. In addition, the present invention can obtain nitrogen-doped fluorescent carbon dots with excellent stability by heating in a reaction kettle for a short time (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.

[0009] Preferably, the solvent is N,N-dimethylformamide.

[0010] Preferably, the amount of citric acid relative to N,N-dimethylformamide is 1 / 80 to 1 / 15 g / mL.

[0011] Preferably, the reaction temperature in step 2 is 160° C. to 200° C., and the reaction time is 6 h to 10 h.

[0012] 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.

[0013] 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.

[0014] In a second aspect, the present invention provides a nitrogen-doped fluorescent carbon dot, which is prepared by the aforementioned preparation method.

[0015] The carbon-doped fluorescent carbon dots showed good stability under long-term storage (0-50 days), different acid-base 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 varied within ±10%.

[0016] In a third aspect, the present invention provides an application of nitrogen-doped fluorescent carbon dots in the detection of iron ions.

[0017] 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 to be tested; and the change in fluorescence intensity at 330 nm is linearly related to the concentration of iron ions.

[0018] 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.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 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 its great application prospect as an efficient iron ion detection probe.

[0020] 2. The present invention uses N,N-dimethylformamide as the solvent environment, and can obtain nitrogen-doped fluorescent carbon dots with excellent stability by heating in a reactor for a short time (one-step solvent thermal method). The raw materials are easily available, the cost is low, the preparation method is simple, the time is short, the risk is low, and it is easy to achieve mass production.

[0021] 3. The nitrogen-doped carbon dots obtained by the present invention have excellent stability, and the fluorescence peak position and fluorescence intensity change very little in a complex environment, and can be used for fluorescence detection of iron ion concentration in deionized water and actual water samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is 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, and the inset is an enlarged view of the fluorescence spectra of Comparative Examples 1 to 3.

[0023] Figure 2 is the fluorescence spectra of the carbon dots in Example 4 of the present invention under different conditions; 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 365 nm UV light irradiation times in the range of 0-5 hours. Figure 2 The insets are the corresponding fluorescence intensity changes.

[0024] 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.

[0025] Figure 4 is a fluorescence spectrum of carbon dots in Example 5 of the present invention detecting different iron ion concentrations in deionized water; Figure 4 The inset is a linear relationship diagram between fluorescence intensity and iron ion concentration. DETAILED DESCRIPTION

[0026] 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 described embodiments 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 work are within the scope of protection of the present invention.

[0027] Example 1 A method for preparing nitrogen-doped fluorescent carbon dots with excellent stability comprises the following steps: 1) Take 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), dissolve them in N,N-dimethylformamide solvent, stir them thoroughly to obtain a mixed solution, pour the mixed solution into a 50 mL high-pressure reactor, seal it and place it in an oven, heat it at 200 ° C for 6 hours, and cool it naturally to obtain a carbon dot solution.

[0028] 2) The carbon dot solution was centrifuged at a speed of 8000 r / min for 30 min to obtain nitrogen-doped fluorescent carbon dots. The obtained carbon dot fluorescence emission spectrum is as follows: Figure 1 shown.

[0029] Example 2 A method for preparing nitrogen-doped fluorescent carbon dots with excellent stability. The difference between this embodiment and embodiment 1 is 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 embodiment 1. The obtained carbon dot fluorescence emission spectrum is as shown in FIG. Figure 1 shown.

[0030] Example 3 A method for preparing nitrogen-doped fluorescent carbon dots with excellent stability. The difference between this embodiment and embodiment 1 is 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 embodiment 1. The obtained carbon dot fluorescence emission spectrum is as follows: Figure 1 shown.

[0031] Comparative Example 1 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.

[0032] Comparative Example 2 The difference between this comparative example and Example 1 is that 2-amino-2-methyl-1-propanol is replaced by 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 to obtain nitrogen-doped fluorescent carbon dots, and its fluorescence emission spectrum is as shown in Figure 1 shown.

[0033] Comparative Example 3 The difference between this comparative example and Example 1 is that 2-amino-2-methyl-1-propanol is replaced by 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 shown in Figure 1 shown.

[0034] from Figure 1 It can be seen that the carbon dots prepared in Comparative Examples 1-3 have blue and green dual-emission fluorescence under 330nm excitation, that is, carbon core luminescence and nitrogen-related surface state luminescence. And as the number of methyl groups in the raw material 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, and the steric hindrance effect inhibits the resonance energy transfer between the two fluorescence centers, so that the carbon dots prepared in Examples 1-3 only emit blue fluorescence.

[0035] Example 4 Comprehensive stability test of nitrogen-doped fluorescent carbon dots (prepared in Example 2): For the storage stability test: the carbon dot solution prepared in Example 2 was exposed to air for different time periods (0-50 days), and the carbon dot solution was added to a BR buffer solution with a pH value of 6.8 at regular intervals. The obtained carbon dot fluorescence emission spectrum is 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%.

[0036] For the stability test of UV irradiation: irradiate the carbon dot solution with 365nm UV light for different time (0-300 minutes). At regular intervals, add the carbon dot solution into BR buffer solution with pH value of 6.8. The obtained carbon dot fluorescence emission spectrum is as follows: Figure 2 As shown in part (b), the fluorescence of the carbon dots gradually decreases, and after irradiation for 300 minutes, the fluorescence change range of the carbon dots is within 9%.

[0037] For the stability test of pH value change, the carbon dots were added into BR buffer solutions with different pH values ​​(3.0-13.5), mixed thoroughly and left 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%.

[0038] For the stability test of salt solution concentration changes: the carbon dots were added into NaCl solutions of different concentrations (0-5 mol / L), mixed thoroughly and left 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%.

[0039] Example 5 A method for detecting iron ions is as follows: Prepare various metal ion solutions (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+ ); add the carbon dot solution prepared in Example 2 to different metal ion solutions respectively, mix well and let stand for 5 minutes; test the fluorescence emission spectrum 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 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 Fe 3+ A clear quenching effect was observed.

[0040] In order to detect the sensitivity of the iron ion detection method provided in this embodiment, the detection of iron ion solutions with different concentrations is carried out as follows: Iron ions were dissolved in deionized water to prepare multiple groups of solutions with different iron ion concentrations; 3 mL of ion solution was taken from each group, and 50 uL of carbon dot solution prepared in Example 2 was added to each group, mixed evenly, and then allowed to stand for 5 minutes, and the fluorescence emission spectrum was tested under 330 nm excitation. The results are shown in FIG. Figure 4 As shown, 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, and 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.

[0041] In order to test the compatibility of the iron ion detection method provided in this embodiment with different water bodies, the following tests were performed: Iron ions were dissolved in mineral water, tap water, and lake water, and multiple groups of solutions with iron ion concentrations of 0-40 μM were prepared; each group of solutions was added with carbon dots, mixed evenly, and then allowed to stand for 5 minutes, and the fluorescence emission spectrum was tested under 330 nm excitation. The results are shown in Table 1. When testing actual water samples, it has a low RSD value (relative standard deviation) and a recovery rate between 93.0% and 103.0%.

[0042] Table 1 Comparison of the detection accuracy of nitrogen-doped fluorescent carbon dots for different concentrations of iron ion solutions in different water bodies sample Addition concentration (μM) Detection concentration (μM) Spiked recovery rate (%) 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 5 4.79±0.12 95.8 2.53 Lake 15 15.44±0.41 102.9 2.66 Lake 24 24.61±0.57 102.5 2.31 Lake 32 32.24±0.37 100.7 1.14 Lake 40 40.01±0.37 100.7 0.92 The above is only a preferred embodiment of the present invention, and is not any formal or substantial limitation of the present invention. 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, and these improvements and supplements should also be regarded as the protection scope of the present invention. Any technician familiar with this profession, without departing from the spirit and scope of the present invention, can make some changes, modifications and evolutions of the technical content disclosed above, which are equivalent embodiments of the present invention; at the same time, any changes, modifications and evolutions of any equivalent changes 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 in that: 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; 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, characterized in that: The solvent is N,N-dimethylformamide.

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

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

5. The method for preparing nitrogen-doped fluorescent carbon dots with excellent stability according to claim 1, characterized in that: 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.

6. The method for preparing nitrogen-doped fluorescent carbon dots with excellent stability according to claim 5, characterized in that: 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.

7. A nitrogen-doped fluorescent carbon dot, characterized in that: It is prepared by the preparation method as claimed in claim 1.

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

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

10. The use according to claim 8, characterized in that: The iron ion concentration in the tested iron ion solution is 0.2 μM to 40 μM.

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