CN108753283B - Method for safely and simply preparing double-doped nitrogen and phosphorus carbon quantum dots - Google Patents

Method for safely and simply preparing double-doped nitrogen and phosphorus carbon quantum dots Download PDF

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CN108753283B
CN108753283B CN201810866243.1A CN201810866243A CN108753283B CN 108753283 B CN108753283 B CN 108753283B CN 201810866243 A CN201810866243 A CN 201810866243A CN 108753283 B CN108753283 B CN 108753283B
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瞿金清
蔡育铨
万芳
魏娇
陈晓翰
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Abstract

The invention discloses a safe and simple method for preparing double-doped nitrogen and phosphorus carbon quantum dots, which comprises the steps of dissolving amino acid and carbon precursors in deionized water, adding a phosphoric acid solution, and stirring to prepare a clear solution; then, treating the mixed solution in an ultrasonic machine for 1-2 hours; heating the solution subjected to ultrasonic treatment in an oil bath at 90-150 ℃ for 1-5 hours to prepare a nitrogen and phosphorus co-doped carbon dot solution, and cooling to room temperature; removing large particle impurities in the solution by centrifugation, and dialyzing in a dialysis bag to remove unreacted raw materials and other small particle impurities. The invention has the advantages of safe and nontoxic raw materials, mild conditions and simple process. The carbon quantum dots emitting blue, green, yellow or orange red light can be obtained by changing the types of amino acids and adjusting the raw material ratio, and the like, and can be applied to the fields of biological imaging, ion detection and the like.

Description

Method for safely and simply preparing double-doped nitrogen and phosphorus carbon quantum dots
Technical Field
The invention relates to carbon quantum dots, in particular to a safe and simple method for preparing double-doped nitrogen and phosphorus carbon quantum dots, belonging to the field of nano material science.
Background
Carbon quantum dots, also known as carbon dots, refer to quasi-spherical carbon nanoparticles having a particle diameter generally within 10nm, which were unexpectedly prepared in 2004 by Xu in arc discharge for single-walled carbon nanotubes (j.am.chem.soc.2004,126, 12736-12737), and were found by Sun in 2006 to have high fluorescence quantum efficiencies (j.am.chem.soc.2006,128, 7756-7757), thereby gaining attention gradually. Compared with the traditional organic dye or quantum dot, the carbon dot has low cytotoxicity, good biocompatibility, simple preparation method and good physical and optical properties, and has attracted great attention in the fields of biological imaging, photocatalysis, biomedicine and the like, for example, the carbon dot is used as a drug carrier, a gene transport carrier or a fluorescent probe for diagnosing human tumor tissues and the like.
Methods for preparing carbon dots include top-down methods (electrochemical oxidation, arc, laser ablation, etc.) and bottom-up methods (microwave-assisted, hydrothermal, strong acid oxidation, etc.). However, the above method or operation conditions are complicated and harsh, and require special equipment, or require strong acid, strong alkali, high temperature and high pressure, which is dangerous to some extent.
The defects of the carbon dots prepared at present are obvious, the fluorescence quantum efficiency of the carbon dots is low, emitted light is concentrated in blue-green light, and the application of the carbon dots is limited to a certain extent. There are a number of studies to find that it is possible to red-shift the emission peak and improve the fluorescence quantum efficiency by doping other atoms on the carbon dots, most of which are nitrogen-and phosphorus-doped carbon dots. In the past, most of the researches introduce-NH on the surface of a carbon point by mixing a carbon precursor with organic amine such as ethylenediamine, polyethyleneimine and the like2However, the ethylenediamine is flammable, has strong corrosivity and great harm to human bodies, and the polyethyleneimine has strong toxicity, so that the polyethyleneimine has certain risks when being applied to organisms, and needs to find another safer material for improving the performance.
Disclosure of Invention
In order to solve the problems of complicated preparation, poor fluorescence performance and high toxicity of the nitrogen-doped agent of the existing carbon quantum dots, the invention provides a method for preparing nitrogen-phosphorus co-doped carbon dots by using amino acid as a nitrogen-doped source and phosphoric acid as an oxidant, pretreating the raw materials by ultrasound, and then heating in an oil bath. The preparation method is safe and simple, green and environment-friendly, the fluorescence performance of the carbon dots is good, and the fluorescence of the carbon dots can be red-shifted from blue light to red light along with the change of the types of amino acids.
The amino acid is used as a biological micromolecule, is safe and non-toxic, has wide sources, more importantly, the amino acid structure has amino and carboxyl, is easy to generate polycondensation reaction, can be used for preparing carbon dots, is doped with nitrogen in the carbon dots, and is oxidized by adding phosphoric acid, so that nitrogen and phosphorus co-doped carbon dots are prepared.
The oil bath heating has the advantages of simple operation, easy control of reaction conditions and the like, however, the research on the preparation of the carbon dots by the oil bath method is few at present, and particularly, the carbon dots are prepared by the oil bath heating at a lower temperature, if the method can be successfully realized, the operation is safer, and the energy consumption and the cost for preparing the carbon dots can be reduced.
A method for safely and simply preparing double-doped nitrogen and phosphorus carbon quantum dots comprises the following steps:
1) dissolving amino acid and carbon precursor in deionized water, then adding phosphoric acid solution, and stirring to prepare clear solution; the carbon precursor is one or two of glucose, citric acid monohydrate or cyclodextrin; the molar ratio of the amino acid to the carbon precursor is 1: 2-2: 1;
2) transferring the solution to an ultrasonic machine for ultrasonic treatment, wherein the ultrasonic time is 1-2 h;
3) heating the solution subjected to ultrasonic treatment in an oil bath at the temperature of 80-150 ℃, refluxing for 1-5 h, gradually changing the solution from clear and transparent to brown and turbid, and naturally cooling to room temperature to prepare carbon dots simultaneously doped with nitrogen and phosphorus;
4) centrifuging the solution cooled in the step 3), removing large-particle impurities, dialyzing the clear solution in a dialysis bag, and removing unreacted raw materials and other small-particle impurities to obtain the high-purity fluorescent carbon dots.
To further achieve the object of the present invention, preferably, the amino acids refer to 20 amino acids required for protein synthesis in human body. The amino acid is preferably one of glutamic acid, aspartic acid, tryptophan, tyrosine, serine and histidine; the configuration is L type.
Preferably, the mass fraction of the phosphoric acid solution is 80-90%; the volume ratio of the phosphoric acid solution to the deionized water is 1: 1-2.0.
Preferably, the ultrasonic frequency is 40-80 kHz.
Preferably, the heating time of the oil bath is 1-5 h.
Preferably, the rotation speed of the centrifugation is in the range of 1-3 ten thousand revolutions per minute.
Preferably, the cut-off molecular weight of the dialysis bag is 500-1000 Da.
Compared with the prior art, the invention has the following advantages:
1) amino acid is used as a nitrogen-doped source instead of amines such as polyethyleneimine and ethylenediamine, the nitrogen-doped source is wide in material supply, safe and nontoxic, and nitrogen and phosphorus co-doped carbon dots are prepared by adding phosphoric acid for oxidation, so that the fluorescence property of the carbon dots is enhanced;
2) the ultrasonic treatment combined oil bath heating method is used for replacing a hydrothermal method and a microwave-assisted method, so that the reaction conditions of high temperature and high pressure are avoided, oil bath heating can be carried out at the temperature lower than 100 ℃, the reaction is safer, and carbon dots can be prepared quickly and on a large scale;
3) according to the difference of the amino acid types, the molar ratio of the amino acid to the carbon precursor, the reaction time and the like, carbon dots which emit different fluorescence can be obtained, and the requirements of multiple purposes can be met;
4) the carbon points show that the compound has rich hydroxyl groups, good solubility in water, amino groups, and can be connected with other substances through chemical reaction or hydrogen bond action, and the compound is applied to the fields of cell detection, human body imaging and the like.
Drawings
FIG. 1 is a UV absorption spectrum of tryptophan-carbon dots prepared in example 1.
FIG. 2 is a graph showing an infrared absorption spectrum of tryptophan-carbon points prepared in example 1.
FIG. 3 is a fluorescence spectrum of the tryptophan-carbon dot prepared in example 1 when excited at 510 nm.
FIG. 4 is the fluorescence spectra of tryptophan-carbon spots of different concentrations in example 1 when excited by 510nm excitation light.
FIG. 5 is the fluorescence spectra of tryptophan-carbon spots prepared in example 4 at different amino acid/glucose molar ratios under excitation light of 510 nm.
FIG. 6 is a fluorescence spectrum of tryptophan-carbon spots synthesized in example 5 under different heating times.
FIG. 7 is a fluorescence spectrum of tryptophan-carbon spots synthesized in example 6 under different sonication times.
FIG. 8 is a graph of the fluorescence spectra of tryptophan-carbon dot solutions at different pH's for example 7.
FIG. 9 is a graph showing fluorescence spectra of solutions of example 8 at different ratios of folate to tryptophan-carbon sites.
Detailed Description
For better understanding of the present invention, the following further describes the preparation method and effects of the present invention with reference to the accompanying drawings and examples, but the embodiments of the present invention are not limited thereto.
Example 1
Preparation of tryptophan-carbon dots:
weighing 0.005mol of tryptophan and 0.005mol of glucose, pouring into a beaker, adding 5ml of deionized water, adding 5ml of phosphoric acid solution with the mass fraction of 80%, and stirring until the solution is completely dissolved to form a clear and transparent solution;
and (3) placing the beaker in an ultrasonic machine for ultrasonic treatment for 2 hours, wherein the ultrasonic frequency is 40 Hz. The solution did not change in appearance after sonication.
Transferring the solution into a 25ml round-bottom flask, heating and refluxing in a constant-temperature heating magnetic stirrer, keeping the temperature at 90 ℃, and heating for 5 hours, wherein the color of the solution gradually changes from clear and transparent to red and finally changes to mauve. After heating, cooling to room temperature. The carbon dots are properly diluted, are red under sunlight, and emit orange red fluorescence under the irradiation of 365nm ultraviolet light.
Centrifuging for 10min under 10000rmp to remove large-particle impurities, dialyzing in a dialysis bag with cut-off molecular weight of 1000Da, changing water every 24h for two days, and removing unreacted raw materials and small-particle impurities.
FIG. 1 is a graph of the UV absorption spectrum of tryptophan-carbon dots, which characterizes the UV absorption characteristics of the carbon dots. The carbon dot has an absorption peak at 510nm, indicating that the maximum absorption wavelength is 510 nm.
FIG. 2 is a graph of an infrared absorption spectrum of tryptophan-carbon point, 3464cm-1Is the stretching vibration of O-H, the peak intensity at the position is very large, which indicates that the carbon point has more hydroxyl groups, and the condition is consistent with the good solubility of the carbon point in water. 2375cm-1The stretching vibration of P-H shows that P-H groups can be introduced into carbon points by doping phosphorus atoms into the raw materials, so that the phosphorus atoms are successfully doped into the carbon points, and the fluorescence property of the carbon points is improved. 1640cm-1Is an elongation of C ═ NVibration contraction 1156cm-1The stretching vibration of C-N shows that nitrogen atoms on amino acid are doped into carbon dots, and in conclusion, the nitrogen and phosphorus co-doped carbon dots are successfully prepared.
FIG. 3 is a fluorescence spectrum of tryptophan-carbon dots under 510nm excitation, and it can be seen that the emission peak of the carbon dots is around 610nm, and a distinct orange-red fluorescence is emitted.
FIG. 4 is a graph of fluorescence spectra of tryptophan-carbon dots at different concentrations under excitation at 510nm, and it can be seen that as the concentration of carbon dots increases, the fluorescence intensity decreases and the emission peak is red-shifted.
The appearance and the particle size of the carbon quantum dots are characterized by a transmission electron microscope, and the carbon quantum dots are quasi-spherical carbon nano particles with the particle size within 10 nm.
Example 2
Preparation of glutamic acid-carbon dot:
weighing 0.0005mol of glutamic acid and 0.005mol of glucose, pouring into a beaker, adding 5ml of deionized water, pouring 5ml of phosphoric acid solution with the mass fraction of 85%, and stirring until the glutamic acid and the glucose are completely dissolved to form a clear and transparent solution;
the beaker is placed in an ultrasonic machine for ultrasonic treatment for 2 hours, and the ultrasonic frequency is 50 kHz.
The solution was transferred to a 25ml round bottom flask and heated to reflux in a thermostatically heated magnetic stirrer, the temperature being maintained at 150 ℃ and heated for 1 hour, the color of the solution gradually changing from clear to transparent to brown during heating. After heating, cooling to room temperature. The obtained high-concentration carbon dots are light yellow under sunlight after being diluted, and emit yellow light under the irradiation of 365nm ultraviolet light.
Centrifuging for 10min under 20000rmp to remove large particle impurities, dialyzing in dialysis bag with molecular weight cutoff of 500Da, changing water every 24 hr for 4 days to remove unreacted raw materials and small particle impurities.
Example 3
Preparation of aspartic acid-carbon dots:
weighing 0.005mol of aspartic acid and 0.005mol of citric acid monohydrate, pouring into a beaker, adding 5ml of deionized water, adding 5ml of phosphoric acid solution with the mass fraction of 90%, and stirring until the solution is completely dissolved to form a clear and transparent solution;
the beaker is placed in an ultrasonic machine for ultrasonic treatment for 2 hours, and the ultrasonic frequency is 60 kHz. The solution did not change in appearance after sonication.
The solution was transferred to a 25ml round bottom flask and heated to reflux in a thermostatically heated magnetic stirrer, the temperature being maintained at 120 ℃ for 2 hours, the color of the solution gradually changing from clear to transparent to red and finally to dark brown during heating. After heating, cooling to room temperature. The carbon dots are properly diluted, brown under sunlight and emit blue light under 365nm ultraviolet irradiation.
Centrifuging for 10min under 30000rmp to remove large particle impurities, dialyzing in a dialysis bag with cut-off molecular weight of 800Da, changing water every 24h for two days, and removing unreacted raw materials and small particle impurities.
Example 4
Preparation of tryptophan-carbon dots at different amino acid/glucose molar ratios:
under the condition that the total concentration of glucose and tryptophan is 1mol/L, weighing raw materials with the molar ratio of glucose to amino acid of 2:1, 3:2, 1:1, 2:3 and 2:1 respectively into 5 beakers, adding 5ml of deionized water and 5ml of phosphoric acid solution with the mass fraction of 85%, and stirring until the deionized water and the phosphoric acid solution are completely dissolved to form a clear and transparent solution;
the beakers were respectively placed in an ultrasonic machine for ultrasonic treatment for 2 hours at an ultrasonic frequency of 40 kHz. The solution did not change in appearance after sonication.
Respectively transferring the solution into a 25ml round-bottom flask, heating and refluxing in a constant-temperature heating magnetic stirrer, keeping the temperature at 100 ℃, and heating for 2 hours, wherein the color of the solution gradually changes from clear and transparent to red and finally changes to mauve. After heating, cooling to room temperature. The carbon dots are properly diluted, are red under sunlight, and emit orange red fluorescence under the irradiation of 365nm ultraviolet light.
Centrifuging for 10min under 10000rmp to remove large-particle impurities, dialyzing in a dialysis bag with cut-off molecular weight of 1000Da, changing water every 24h for two days, and removing unreacted raw materials and small-particle impurities.
FIG. 5 is a graph of the fluorescence spectra of tryptophan-carbon spots prepared at different amino acid/glucose molar ratios under excitation at 510nm, and it can be seen that the emission peak is red-shifted with increasing glucose content, but the fluorescence intensity decreases with decreasing intensity.
Example 5
Preparation of Tryptophan-carbon dots at different heating times
Weighing 0.005mol of tryptophan and 0.005mol of glucose, pouring into a beaker, adding 5ml of deionized water, adding 5ml of phosphoric acid solution with the mass fraction of 85%, and stirring until the solution is completely dissolved to form a clear and transparent solution;
the beaker is placed in an ultrasonic machine for ultrasonic treatment for 2 hours, and the ultrasonic frequency is 40 kHz. The solution did not change in appearance after sonication.
Transferring the solution into a 25ml round-bottom flask, heating and refluxing in a constant-temperature heating magnetic stirrer, keeping the temperature at 90 ℃, and heating for 1-5 hours, wherein the color of the solution gradually changes from clear and transparent to red and finally changes to mauve. After heating, cooling to room temperature. The carbon dots are properly diluted to be brown under sunlight, and the carbon dots increase along with heating time under the irradiation of 365nm ultraviolet light, and the luminescence is red-shifted from green light to orange light and then blue-shifted to green light.
Centrifuging for 10min under 10000rmp to remove large-particle impurities, dialyzing in a dialysis bag with cut-off molecular weight of 1000Da, changing water every 24h for two days, and removing unreacted raw materials and small-particle impurities.
FIG. 6 is a fluorescence spectrum of synthesized carbon dots under different heating times. It can be seen that the intensity of the fluorescence emission peak increases and then decreases with the increase of the heating time, and the fluorescence intensity of the carbon spot reaches the maximum value with the heating time of 3 h.
Example 6
Preparation of tryptophan-carbon dots at different ultrasound times:
weighing 0.005mol of tryptophan and 0.005mol of glucose, pouring into a beaker, adding 5ml of deionized water, adding 5ml of phosphoric acid solution with the mass fraction of 85%, and stirring until the solution is completely dissolved to form a clear and transparent solution;
and (3) placing the beaker in an ultrasonic machine for ultrasonic treatment for 40-140 min, wherein the ultrasonic frequency is 40 kHz. The solution did not change in appearance after sonication.
Transferring the solution into a 25ml round-bottom flask, heating and refluxing in a constant-temperature heating magnetic stirrer, keeping the temperature at 90 ℃, and heating for 2 hours, wherein the color of the solution gradually changes from clear and transparent to red and finally changes to mauve. After heating, cooling to room temperature. The carbon dots are properly diluted to be brown under sunlight, and the carbon dots increase along with heating time under the irradiation of 365nm ultraviolet light, and the luminescence is red-shifted from green light to orange light and then blue-shifted to green light.
Centrifuging for 10min under 10000rmp to remove large-particle impurities, dialyzing in a dialysis bag with cut-off molecular weight of 1000Da, changing water every 24h for two days, and removing unreacted raw materials and small-particle impurities.
FIG. 7 is a fluorescence spectrum of synthesized carbon dots under different heating times. It can be seen that the excitation wavelength and fluorescence intensity of the carbon dot solution basically tend to increase and then decrease with the increase of the ultrasound time, and the fluorescence intensity is maximum when the ultrasound time is 120 min.
Example 7
Preparation of tryptophan-carbon dot solutions with different pH values:
the tryptophan-carbon dot prepared in example 1 was weighed into a beaker, and 4 parts of the tryptophan-carbon dot were weighed, designated as 1, 2, 3 and 4, and 30mL of a NaOH solution having a pH of 12, deionized water, an HCl solution having a pH of 4 and an HCl solution having a pH of 2 were added thereto, respectively, and dissolved by stirring.
FIG. 8 is a graph of the fluorescence spectra of tryptophan-carbon dot solutions at different pH. 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 greatest when the pH value is 4, which indicates that the carbon dot solution has the strongest fluorescence under weakly alkaline conditions.
Example 8
Preparation of folic acid-tryptophan-carbon dot:
0.3g of the tryptophan-carbon dot prepared in example 1 was weighed into a beaker, 3 parts, labeled 1, 2 and 3, were weighed, and 30ml of deionized water was added, respectively. 0.05g of folic acid is added into the solution No. 2 and stirred evenly, and 0.1g of folic acid is added into the solution No. 3 and stirred evenly.
FIG. 9 is a fluorescence spectrum of a solution at different ratios of folic acid to carbon dots. It can be seen that the fluorescence of the solution gradually decreases with increasing folate concentration, as the ratio of folate to tryptophan-carbon points increases to 1:1, fluorescence quenching occurs. The tryptophan-carbon dots prepared by the technology can be connected with the folic acid through chemical reaction or hydrogen bond action to cause fluorescence quenching, so that the tryptophan-carbon dots can be applied to the fields of cell detection, human body imaging and the like.
As can be seen from the above examples, the carbon dots of the invention have abundant hydroxyl groups, good solubility in water, and amino groups, can be used for connecting other substances such as folic acid through chemical reaction or hydrogen bond, and are applied to the fields of cell detection, human body imaging and the like. Compared with the common carbon dots, the carbon dots have the condition that the emission peak is red-shifted along with the increase of the concentration, so that the concentration of the carbon dots can be adjusted to obtain a solution which emits light from blue to red, and the carbon dots can be applied to the field of multicolor imaging.
In the prior art, the carbon dots are prepared under the conditions of strong acid, strong alkali, high temperature and high pressure, and have certain danger. The method can prepare the carbon dots by carrying out ultrasonic pretreatment on the raw materials and then carrying out oil bath heating at a low temperature, has simple and safe operation and low energy consumption, and can quickly prepare a large number of carbon dots. The method takes glucose as a precursor, amino acid as a nitrogen source, phosphoric acid as an oxidant and water as a solvent, has wide raw material sources, is safe and nontoxic, and can be used for preparing the carbon dots doped with nitrogen and phosphorus simultaneously and has good fluorescence performance.
According to the invention, different carbon dots emitting different fluorescence can be obtained according to the difference of the amino acid types, the molar ratio of the amino acid to the carbon precursor, the reaction time and the like, and the requirements of multiple purposes can be met.
The present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents and are included in the scope of the present invention.

Claims (5)

1. A method for safely and simply preparing double-doped nitrogen and phosphorus carbon quantum dots is characterized by comprising the following steps:
1) dissolving amino acid and carbon precursor in deionized water, then adding phosphoric acid solution, and stirring to prepare clear solution; the carbon precursor is one or two of glucose, citric acid monohydrate or cyclodextrin; the molar ratio of the amino acid to the carbon precursor is 1: 2-2: 1; the amino acid refers to 20 amino acids required by human body for synthesizing protein;
2) transferring the solution to an ultrasonic machine for ultrasonic treatment, wherein the ultrasonic time is 1-2 h; the ultrasonic frequency is 40-80 kHz;
3) heating the solution subjected to ultrasonic treatment in an oil bath at the temperature of 80-150 ℃, refluxing for 1-5 h, gradually changing the solution from clear and transparent to brown and turbid, and naturally cooling to room temperature to prepare carbon dots simultaneously doped with nitrogen and phosphorus;
4) centrifuging the solution cooled in the step 3), removing large-particle impurities, dialyzing the clear solution in a dialysis bag, and removing unreacted raw materials and other small-particle impurities to obtain the high-purity fluorescent carbon dots.
2. The safe and simple method for preparing the double-doped nitrogen and phosphorus carbon quantum dot according to claim 1, wherein the mass fraction of the phosphoric acid solution is 80-90%; the volume ratio of the phosphoric acid solution to the deionized water is 1: 1-2.0.
3. The method for safely and simply preparing the double-doped nitrogen and phosphorus carbon quantum dot according to claim 1, wherein the rotation speed of the centrifugation is in a range of 1-3 ten thousand revolutions per minute.
4. The method for safely and simply preparing the double-doped nitrogen and phosphorus carbon quantum dot as claimed in claim 1, wherein the molecular weight cut-off of the dialysis bag is 500-1000 Da.
5. The method for safely and simply preparing the double-doped nitrogen and phosphorus-carbon quantum dot according to claim 1, wherein the dialysis time is 2-4 days, and the water is changed every 24 hours.
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