CN108753283A - A kind of safe and simple method for preparing codope nitrogen and phosphorus carbon quantum dot - Google Patents

Abstract

The invention discloses a safe and simple method for preparing double-doped nitrogen and phosphorus-carbon quantum dots. The method first dissolves amino acid and carbon precursors in deionized water, adds phosphoric acid solution, and stirs to form a clear solution; then the mixed solution Put it in an ultrasonic machine for processing, the ultrasonic time is 1-2h; heat the ultrasonicated solution in an oil bath at 90-150°C for 1-5 hours to prepare a nitrogen and phosphorus co-doped carbon dot solution, and cool to room temperature; The large particle impurities in the solution are removed by centrifugation, and then dialyzed in a dialysis bag to remove unreacted raw materials and other small particle impurities. The raw materials of the invention are safe and non-toxic, the conditions are mild and the process is simple. Carbon quantum dots emitting blue, green, yellow or orange-red light can be obtained by changing the type of amino acid and adjusting the ratio of raw materials, which can be applied in the fields of biological imaging and ion detection.

Description

A Safe and Simple Method for Double-doped Nitrogen and Phosphorus Carbon Quantum Dots

technical field

The invention relates to carbon quantum dots, in particular to a method for safely and simply preparing double-doped nitrogen and phosphorus-carbon quantum dots, and belongs to the field of nanometer material science.

Background technique

Carbon quantum dots, also known as carbon dots, refer to quasi-spherical carbon nanoparticles with a particle size generally within 10nm. They were accidentally prepared by Xu in 2004 when preparing single-walled carbon nanotubes by arc discharge (J.Am.Chem.Soc .2004,126,12736–12737), and was found by Sun in 2006 to have high fluorescence quantum efficiency (J.Am.Chem.Soc.2006,128,7756–7757), thus gradually gaining people’s attention. Compared with traditional organic dyes or quantum dots, carbon dots have low cytotoxicity, good biocompatibility, simple preparation method, good physical and optical properties, and have achieved great results in bioimaging, photocatalysis, biomedicine and other fields. Concern, such as drug carriers, gene transport carriers, or fluorescent probes for the diagnosis of human tumor tissues, etc. have a lot of research.

Methods for preparing carbon dots include top-down methods (electrochemical oxidation method, arc method, laser ablation method, etc.) and bottom-up methods (microwave-assisted method, hydrothermal method, strong acid oxidation, etc.). However, the above methods or operating conditions are complex and harsh, requiring the use of specific equipment, or requiring strong acid and alkali, high temperature and high pressure conditions, and have certain risks.

The currently prepared carbon dots have obvious defects, the fluorescence quantum efficiency of carbon dots is low, and the emitted light is also concentrated in blue-green light, which limits the application of carbon dots to a certain extent. A large number of studies have found that the emission peak can be red-shifted and the fluorescence quantum efficiency can be improved by doping other atoms on the carbon dots, among which the carbon dots doped with nitrogen and phosphorus are the most researched and prepared. In previous studies, most of the fluorescent properties were improved by mixing carbon precursors and organic amines such as ethylenediamine, polyethyleneimine, etc., and introducing -NH ₂ on the surface of carbon dots. However, ethylenediamine is flammable, highly corrosive, and harmful to the human body. Large, polyethyleneimine is highly toxic, and there is a certain risk when used in vivo. It is necessary to find another safer material to improve its performance.

Contents of the invention

In order to solve the problems of cumbersome preparation of existing carbon quantum dots, poor fluorescence performance, and high toxicity of nitrogen doping agents, the present invention provides a method that uses amino acid as a nitrogen doping source, phosphoric acid as an oxidant, pretreats the raw materials by ultrasonic, and then heats them in an oil bath. A method for preparing nitrogen and phosphorus co-doped carbon dots. The preparation method is safe and simple, green and environmentally friendly, and the fluorescence performance of the carbon dots is good, and the fluorescence of the carbon dots can redshift from blue light to red light with the change of amino acid species.

The amino acid of the present invention is a kind of biological small molecule, which is safe, non-toxic, and has a wide range of sources. More importantly, the amino acid structure has amino groups and carboxyl groups, which are prone to polycondensation reactions, and can be used to prepare carbon dots. Phosphoric acid oxidation to prepare nitrogen and phosphorus co-doped carbon dots.

The oil bath heating of the present invention has the advantages of simple operation and easy control of reaction conditions. However, there are few researches on the preparation of carbon dots by the oil bath method, especially the preparation of carbon dots by oil bath heating at a lower temperature. If it can be successfully realized , not only the operation is safer, but also the energy consumption and cost of preparing carbon dots can be reduced.

A safe and simple method for preparing double-doped nitrogen and phosphorus-carbon quantum dots, comprising the following steps:

1) Dissolving the amino acid and the carbon precursor in deionized water, then adding it to the phosphoric acid solution, and stirring to form a clear solution; the carbon precursor is one or both of glucose, citric acid monohydrate or cyclodextrin species; the molar ratio of amino acid and carbon precursor is 1:2～2:1;

2) Transfer the solution to an ultrasonic machine for ultrasonic treatment, and the ultrasonic time is 1 to 2 hours;

3) Heat the ultrasonicated solution in an oil bath at 80-150°C and reflux for 1-5 hours. The solution gradually turns from clear and transparent to brownish-yellow and turbid, and is naturally cooled to room temperature. carbon dots;

4) Centrifuge the cooled solution in step 3) to remove large particles of impurities, and dialyze the clear liquid in a dialysis bag to remove unreacted raw materials and other small particles of impurities to obtain high-purity fluorescent carbon dots.

To further realize the purpose of the present invention, preferably, the amino acids refer to 20 kinds of amino acids required by the human body for protein synthesis. The amino acid is preferably one of glutamic acid, aspartic acid, tryptophan, tyrosine, serine and histidine; the configuration is L-form.

Preferably, the mass fraction of the phosphoric acid solution is 80-90%; the volume ratio of the phosphoric acid solution to deionized water is 1:1-2.0.

Preferably, the ultrasonic frequency is 40-80kHz.

Preferably, the heating time in the oil bath is 1-5 hours.

Preferably, the rotational speed range of the centrifuge is 10,000-30,000 rpm.

Preferably, the molecular weight cut-off of the dialysis bag is 500-1000Da.

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

1) Amino acids are used to replace polyethyleneimine, ethylenediamine and other amines as nitrogen doping sources, and the raw materials are widely available, safe and non-toxic, and nitrogen and phosphorus co-doped carbon dots are prepared by adding phosphoric acid oxidation to enhance the fluorescence of carbon dots performance;

2) Ultrasonic treatment combined with oil bath heating method is used to replace hydrothermal method and microwave-assisted method, which avoids the reaction conditions of high temperature and high pressure, and oil bath heating can be carried out under the condition of lower than 100°C, the reaction is safer, and it can be fast and large-scale Preparation of carbon dots;

3) Depending on the type of amino acid, the molar ratio of amino acid/carbon precursor, reaction time, etc., carbon dots with different emission of different fluorescence can be obtained, which can meet the needs of various purposes;

4) Carbon dots are rich in hydroxyl groups, have good solubility in water, and contain amino groups, which can connect other substances through chemical reactions or hydrogen bonds, and are used in cell detection, human imaging and other fields.

Description of drawings

Fig. 1 is the ultraviolet absorption spectrogram of the tryptophan-carbon dot prepared in Example 1.

FIG. 2 is an infrared absorption spectrum diagram of tryptophan-carbon dots prepared in Example 1.

FIG. 3 is a fluorescence spectrum diagram of tryptophan-carbon dots prepared in Example 1 when excited at 510 nm.

Fig. 4 is the fluorescence spectrum of different concentrations of tryptophan-carbon dots when excited by 510nm excitation light in Example 1.

Fig. 5 is the fluorescence spectrum of tryptophan-carbon dots prepared with different amino acid/glucose molar ratios when excited by 510nm excitation light in Example 4.

Fig. 6 is a fluorescence spectrum diagram of tryptophan-carbon dots synthesized in Example 5 under different heating times.

Fig. 7 is a fluorescence spectrum diagram of tryptophan-carbon dots synthesized in Example 6 at different ultrasonic times.

Fig. 8 is a fluorescence spectrum diagram of the tryptophan-carbon dot solution in Example 7 at different pHs.

Fig. 9 is a fluorescence spectrum diagram of the solution of Example 8 at different ratios of folic acid and tryptophan-carbon dots.

Detailed ways

In order to better understand the present invention, the preparation method and its effects of the present invention will be further described below in conjunction with the accompanying drawings and examples, but the embodiments of the present invention are not limited thereto.

Example 1

Preparation of tryptophan-carbon dots:

Weigh 0.005mol of tryptophan and 0.005mol of glucose into a beaker, add 5ml of deionized water, add 5ml of phosphoric acid solution with a mass fraction of 80%, stir until completely dissolved, and form a clear and transparent solution;

The beaker was placed in an ultrasonic machine for ultrasonic treatment for 2 h, and the ultrasonic frequency was 40 Hz. There was no change in the appearance of the solution after sonication.

The solution was transferred to a 25ml round bottom flask, heated to reflux in a constant temperature heating magnetic stirrer, the temperature was maintained at 90°C, and heated for 5 hours. During the heating process, the color of the solution gradually changed from clear and transparent to red, and finally became purple. After heating, cool to room temperature. After appropriate dilution, the carbon dots are red under sunlight and orange-red fluorescence under 365nm ultraviolet light.

Centrifuge at 10,000rmp for 10min to remove large particle impurities, and then dialyze in a dialysis bag with a molecular weight cut-off of 1000Da, change the water every 24h, and dialyze for two days to remove unreacted raw materials and small particle impurities.

Figure 1 is the ultraviolet absorption spectrum of tryptophan-carbon dots, which characterizes the ultraviolet absorption characteristics of carbon dots. Carbon dots have an absorption peak at 510nm, indicating that the maximum absorption wavelength is 510nm.

Figure 2 is the infrared absorption spectrum of tryptophan-carbon dots, 3464cm ^-1 is the stretching vibration of OH, the peak intensity at this place is very large, indicating that there are more hydroxyl groups on the carbon dots, which is related to the dissolution of carbon dots in water Good match. 2375cm ^-1 is the stretching vibration of PH, which shows that adding phosphorus atoms into the raw materials can introduce PH groups into the carbon dots, and successfully incorporate phosphorus atoms into the carbon dots, which improves the fluorescence performance of the carbon dots. 1640cm ^-1 is the stretching vibration of C=N, and 1156cm ^-1 is the stretching vibration of CN, indicating that the nitrogen atoms on the amino acid are incorporated into the carbon dots. In summary, nitrogen and phosphorus co-doped carbon dots were successfully prepared.

Figure 3 is the fluorescence spectrum of tryptophan-carbon dots when excited at 510nm. It can be seen that the emission peak of the carbon dots is around 610nm, and emits obvious orange-red fluorescence.

Fig. 4 is the fluorescence spectrum diagram of different concentrations of tryptophan-carbon dots when excited at 510nm. It can be seen that with the increase of the concentration of carbon dots, the fluorescence intensity decreases and the emission peak red shifts.

Characterizing its morphology and particle size with a transmission electron microscope can prove that the carbon quantum dots are quasi-spherical carbon nanoparticles with a particle size within 10nm.

Example 2

Preparation of glutamate-carbon dots:

Weigh 0.0005mol glutamic acid and 0.005mol glucose and pour them into a beaker, add 5ml deionized water, then pour 5ml phosphoric acid solution with a mass fraction of 85%, and stir until completely dissolved to form a clear and transparent solution;

The beaker was placed in an ultrasonic machine for ultrasonic treatment for 2 h, and the ultrasonic frequency was 50 kHz.

The solution was transferred to a 25ml round bottom flask, heated to reflux in a constant temperature heating magnetic stirrer, the temperature was maintained at 150°C, and heated for 1 hour. During the heating process, the color of the solution gradually changed from clear and transparent to brown. After heating, cool to room temperature. After dilution, the obtained high-concentration carbon dots are light yellow in sunlight, and emit yellow light under 365nm ultraviolet light.

Centrifuge at 20,000rmp for 10min to remove large particle impurities, and then dialyze in a dialysis bag with a molecular weight cut-off of 500Da, change the water every 24h, and dialyze for 4 days in total to remove unreacted raw materials and small particle impurities.

Example 3

Preparation of aspartic acid-carbon dots:

Weigh 0.005mol of aspartic acid and 0.005mol of citric acid monohydrate into a beaker, add 5ml of deionized water, add 5ml of phosphoric acid solution with a mass fraction of 90%, stir until completely dissolved, and form a clear and transparent solution;

The beaker was placed in an ultrasonic machine for ultrasonic treatment for 2 h, and the ultrasonic frequency was 60 kHz. There was no change in the appearance of the solution after sonication.

The solution was transferred to a 25ml round bottom flask, heated to reflux in a constant temperature heating magnetic stirrer, the temperature was maintained at 120°C, and heated for 2 hours. During the heating process, the color of the solution gradually changed from clear and transparent to red, and finally turned into dark brown. After heating, cool to room temperature. After appropriate dilution, the carbon dots are brown in sunlight and blue in 365nm ultraviolet light.

Centrifuge at 30,000rmp for 10min to remove large particle impurities, and then dialyze in a dialysis bag with a molecular weight cut-off of 800Da, change the water every 24h, and dialyze for two days to remove unreacted raw materials and small particle impurities.

Example 4

Preparation of tryptophan-carbon dots with different amino acid/glucose molar ratios:

Under the condition that the total concentration of glucose and tryptophan is 1mol/L, weigh the raw materials with molar ratios of glucose and amino acid of 2:1, 3:2, 1:1, 2:3, and 2:1 and place them in 5 beakers respectively , add 5ml of deionized water and 5ml of phosphoric acid solution with a mass fraction of 85%, and stir until completely dissolved to form a clear and transparent solution;

The beakers were respectively placed in an ultrasonic machine for ultrasonic treatment for 2 h, and the ultrasonic frequency was 40 kHz. There was no change in the appearance of the solution after sonication.

Transfer the solutions to 25ml round-bottomed flasks, heat to reflux in a constant temperature heating magnetic stirrer, keep the temperature at 100°C, and heat for 2 hours. During the heating process, the color of the solution gradually changes from clear and transparent to red, and finally turns purple. . After heating, cool to room temperature. After appropriate dilution, the carbon dots are red under sunlight and orange-red fluorescence under 365nm ultraviolet light.

Centrifuge at 10,000rmp for 10min to remove large particle impurities, and then dialyze in a dialysis bag with a molecular weight cut-off of 1000Da, change the water every 24h, and dialyze for two days to remove unreacted raw materials and small particle impurities.

Figure 5 is the fluorescence spectrum of tryptophan-carbon dots prepared with different amino acid/glucose molar ratios when excited at 510nm. It can be seen that with the increase of glucose content, the emission peak red shifts, but the fluorescence intensity decreases accordingly.

Example 5

Preparation of tryptophan-carbon dots with different heating times

Weigh 0.005mol of tryptophan and 0.005mol of glucose into a beaker, add 5ml of deionized water, add 5ml of phosphoric acid solution with a mass fraction of 85%, stir until completely dissolved, and form a clear and transparent solution;

The beaker was placed in an ultrasonic machine for ultrasonic treatment for 2 h, and the ultrasonic frequency was 40 kHz. There was no change in the appearance of the solution after sonication.

Transfer the solution to a 25ml round bottom flask, heat to reflux in a constant temperature heating magnetic stirrer, keep the temperature at 90°C, and heat for 1-5 hours. During the heating process, the color of the solution gradually changes from clear and transparent to red, and finally turns purple red. After heating, cool to room temperature. After appropriate dilution, the carbon dots are brown under sunlight, and the carbon dots grow with the heating time under the irradiation of 365nm ultraviolet light, and the luminescence changes from green red to orange and then blue to green.

Centrifuge at 10,000rmp for 10min to remove large particle impurities, and then dialyze in a dialysis bag with a molecular weight cut-off of 1000Da, change the water every 24h, and dialyze for two days to remove unreacted raw materials and small particle impurities.

Fig. 6 is a fluorescence spectrum diagram of carbon dots synthesized under different heating times. It can be seen that with the increase of heating time, the fluorescence emission peak intensity first increases and then decreases, and the fluorescence intensity of carbon dots reaches the maximum value when the heating time is 3h.

Example 6

Preparation of tryptophan-carbon dots with different ultrasonic time:

Weigh 0.005mol of tryptophan and 0.005mol of glucose into a beaker, add 5ml of deionized water, add 5ml of phosphoric acid solution with a mass fraction of 85%, stir until completely dissolved, and form a clear and transparent solution;

Place the beaker in an ultrasonic machine for ultrasonic treatment for 40-140 minutes, and the ultrasonic frequency is 40 kHz. There was no change in the appearance of the solution after sonication.

The solution was transferred to a 25ml round bottom flask, heated to reflux in a constant temperature heating magnetic stirrer, the temperature was maintained at 90°C, and heated for 2 hours. During the heating process, the color of the solution gradually changed from clear and transparent to red, and finally became purple. After heating, cool to room temperature. After appropriate dilution, the carbon dots are brown under sunlight, and the carbon dots grow with the heating time under the irradiation of 365nm ultraviolet light, and the luminescence changes from green red to orange and then blue to green.

Centrifuge at 10,000rmp for 10min to remove large particle impurities, and then dialyze in a dialysis bag with a molecular weight cut-off of 1000Da, change the water every 24h, and dialyze for two days to remove unreacted raw materials and small particle impurities.

Fig. 7 is a fluorescence spectrum diagram of carbon dots synthesized under different heating times. It can be seen that with the increase of ultrasonic time, the excitation wavelength and fluorescence intensity of the carbon dot solution basically increase first and then decrease. When the ultrasonic time is 120min, the fluorescence intensity is the largest.

Example 7

Preparation of different pH tryptophan-carbon dot solutions:

Weigh 0.3 g of tryptophan-carbon dot prepared in Example 1 into a beaker, weigh 4 parts, labeled as 1, 2, 3, 4, add 30 mL of NaOH solution with pH=12, deionized water, pH= 4 HCl solution, pH = 2 HCl solution, stirred to dissolve.

Fig. 8 is a graph of fluorescence spectra of different pH tryptophan-carbon dot solutions. It can be seen that the fluorescence intensity of the carbon dot solution is the lowest under alkaline conditions, and the fluorescence intensity is the highest when pH=4, indicating that the fluorescence of the carbon dot solution is the strongest under weakly alkaline conditions.

Example 8

Preparation of folic acid-tryptophan-carbon dots:

Weigh 0.3 g of tryptophan-carbon dots prepared in Example 1 into a beaker, weigh 3 parts, labeled as 1, 2, and 3, and add 30 ml of deionized water respectively. Add 0.05g of folic acid to No. 2 solution and stir evenly; add 0.1g of folic acid to No. 3 solution and stir evenly.

Fig. 9 is a graph of fluorescence spectra of solutions with different ratios of folic acid and carbon dots. It can be seen that the fluorescence of the solution gradually weakens as the concentration of folic acid increases, and when the ratio of folic acid and tryptophan-carbon dots increases to 1:1, fluorescence quenching occurs. It shows that the tryptophan-carbon dots prepared by this technology can be connected to folic acid through chemical reaction or hydrogen bond, so that the fluorescence can be quenched, so it can be applied in the fields of cell detection, human body imaging and so on.

It can be seen from the above examples that the carbon dots of the present invention are rich in hydroxyl groups, have good solubility in water, and contain amino groups, and can be connected to other substances such as folic acid through chemical reactions or hydrogen bonds, and can be used in cell detection, human body imaging and other fields. Compared with the general carbon dots, the carbon dots of the present invention have the red shift of the emission peak with the increase of the concentration, so the concentration of the carbon dots can be adjusted to obtain a solution that emits light from blue to red, which can be applied in the field of multicolor imaging .

The preparation of carbon dots in the prior art mostly requires the conditions of strong acid and alkali, high temperature and high pressure, which has certain risks. In the present invention, carbon dots can be prepared by ultrasonically pretreating raw materials and then heating in an oil bath at a low temperature, the operation is simple, safe, low in energy consumption, and a large amount of carbon dots can be rapidly prepared. The invention uses glucose as a precursor, amino acid as a nitrogen source, phosphoric acid as an oxidant, and water as a solvent. The source of raw materials is wide and safe and non-toxic. The prepared carbon dots are simultaneously doped with nitrogen and phosphorus, and the carbon dots have good fluorescence performance.

The present invention can obtain carbon dots that emit different fluorescence according to the amino acid type, the molar ratio of amino acid/carbon precursor, reaction time, etc., and can meet the needs of various purposes.

The present invention is not restricted by the above-mentioned embodiments, and any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent alternatives and are included in the protection of the present invention. within range.

Claims (8)

1. A method for safely and simply preparing double-doped nitrogen and phosphorus-carbon quantum dots is characterized in that it comprises the steps: 1) Dissolving the amino acid and the carbon precursor in deionized water, then adding it to the phosphoric acid solution, and stirring to form a clear solution; the carbon precursor is one or both of glucose, citric acid monohydrate or cyclodextrin species; the molar ratio of amino acid and carbon precursor is 1:2～2:1; 2) Transfer the solution to an ultrasonic machine for ultrasonic treatment, and the ultrasonic time is 1 to 2 hours; 3) Heat the ultrasonicated solution in an oil bath at 80-150°C and reflux for 1-5 hours. The solution gradually turns from clear and transparent to brownish-yellow and turbid, and is naturally cooled to room temperature. carbon dots; 4) Centrifuge the cooled solution in step 3) to remove large particles of impurities, and dialyze the clear liquid in a dialysis bag to remove unreacted raw materials and other small particles of impurities to obtain high-purity fluorescent carbon dots. 2. The method for safely and simply preparing double-doped nitrogen and phosphorus-carbon quantum dots according to claim 1, wherein said amino acids refer to 20 kinds of amino acids required for human body to synthesize proteins. 3. according to the method for safe and simple preparation double-doped nitrogen and phosphorus-carbon quantum dots described in claim 1, it is characterized in that, the massfraction of described phosphoric acid solution 80-90%; Described phosphoric acid solution and deionized water The volume ratio is 1:1‐2.0. 4. The method for safely and simply preparing double-doped nitrogen and phosphorus-carbon quantum dots according to claim 1, wherein the ultrasonic frequency is 40-80kHz. 5. The method for safely and simply preparing double-doped nitrogen and phosphorus-carbon quantum dots according to claim 1, characterized in that the heating time in the oil bath is 1-5 hours. 6. The method for safely and simply preparing double-doped nitrogen and phosphorus-carbon quantum dots according to claim 1, characterized in that, the rotational speed range of the centrifuge is 1-30,000 rpm. 7. The method for safely and simply preparing double-doped nitrogen and phosphorus-carbon quantum dots according to claim 1, wherein the molecular weight cut-off of the dialysis bag is 500-1000Da. 8. The method for safely and simply preparing double-doped nitrogen and phosphorus-carbon quantum dots according to claim 1, wherein the time of the dialysis is 2-4 days, and the water is changed every 24 hours.