Metal-doped carbon quantum dots and preparation method thereof

Abstract

The application belongs to the technical field of carbon quantum dots, and discloses a metal-doped carbon quantum dot and a preparation method thereof, which comprises the following steps: 1) reacting o-phenylenediamine with 2,5-dihydroxybenzaldehyde to obtain a Schiff base compound; 2) coordinating the Schiff base compound with a metal to obtain a metal-coordinated Schiff base compound; 3) dissolving the metal-coordinated Schiff base compound in a solvent, then adding a carboxyl-containing compound to completely dissolve the compound, and then performing a warming reaction; after the reaction is completed, centrifugal separation is performed to obtain supernatant, dialysis is performed, and freeze-drying is performed to obtain the metal-doped carbon quantum dot. The application uses a Schiff base metal complex as a precursor, nitrogen atoms in the Schiff base can form a coordination bond with metal ions, the formation of the chemical bond increases the stability of the complex, enables the doped metal to have a stable structure, and ensures that the metal is doped into the center of the carbon quantum dot rather than the surface, so that a higher doping rate and better product performance are obtained.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of carbon quantum dots, and particularly relates to a metal-doped carbon quantum dot and a preparation method thereof. BACKGROUND

[0002] Carbon quantum dots (CDs) can generally be defined as spherical carbon nanoparticles with excellent fluorescence performance and a size of less than 10 nm. With good dispersibility, biocompatibility, low toxicity, easy functional modification, abundant raw materials and low cost, carbon quantum dots have been widely applied in the fields of optoelectronic devices, photocatalysis, biological imaging, biomedical treatment, chemical sensing and biological treatment. Doping is an effective method for adjusting the physical and chemical properties of carbon dots (CDs), and has attracted more and more attention in recent years. After doping with heteroatoms, the electronic structure, nanostructure and chemical composition of CDs will change due to the atomic orbital overlap of heteroatoms and carbon atoms, and the push-pull electron effect of heteroatoms. Compared with non-metal atoms, metal ions have more electrons and unoccupied orbitals, and larger atomic radius. Doping CDs with metal ions can cause significant changes in optical, electronic and magnetic properties by changing the electron density distribution and energy gap of CDs.

[0003] Currently, there are some difficulties in the preparation process of metal-doped carbon quantum dots: 1) low metal doping rate, which is due to the generation of nano-metal oxides during the solvothermal process of metal ions, thereby bringing many negative effects; the generation of nano-metal oxides may interfere with the synthesis reaction of carbon quantum dots, thereby hindering the progress of the reaction, reducing the efficiency and yield of the reaction; the generation of nano-metal oxides may also affect the purity of the product, further reducing the doping rate; 2) the oxidation or reduction properties of metal ions have adverse effects on the performance of the product; metal ions with oxidation properties may undergo oxidation reactions with other substances in the reaction system, thereby changing the reaction path and product distribution, causing changes in the properties and structure of the generated carbon quantum dots, affecting their performance and application; metal ions with reducing properties may participate in reduction reactions, consuming oxidizing agents or reducing agents in the reaction system, thereby affecting the progress of the reaction; in addition, metal ions with reducing properties may also cause the generated carbon quantum dots to have too many negative charges on the surface, affecting their stability and solubility. SUMMARY

[0004] The present application solves the technical problems existing in the prior art, and provides a metal-doped carbon quantum dot and a preparation method thereof, which uses a Schiff base metal complex as a precursor, can stably dope metal into the center of carbon quantum dots, and obtain a higher doping rate and better product performance.

[0005] To solve the technical problems proposed in the present application, the present application provides a preparation method of metal-doped carbon quantum dots, comprising the following steps:

[0006] 1) 2,5-dihydroxybenzaldehyde is dissolved in ethanol to obtain a first solution, o-phenylenediamine is dissolved in ethanol to obtain a second solution, the second solution is added dropwise into the first solution for reaction, after the reaction is completed, the solid is retained by filtration, washed, and dried to obtain a Schiff base compound;

[0007] 2) the Schiff base compound is added into ethanol and heated to completely dissolve to obtain a third solution, a metal salt is dissolved in ethanol to obtain a fourth solution, the fourth solution is added dropwise into the third solution for coordination reaction, after the reaction is completed, the solid is retained by filtration, washed, and dried to obtain a metal-coordinated Schiff base compound;

[0008] 3) the metal-coordinated Schiff base compound is dissolved in a solvent, then a carboxyl-containing compound is added and ultrasonicated until completely dissolved, and then heated for reaction, after the reaction is completed, the supernatant is retained by centrifugal separation, dialyzed, and freeze-dried to obtain metal-doped carbon quantum dots.

[0009] In the above scheme, in the first solution, the molar amount of 2,5-dihydroxybenzaldehyde to the volume of ethanol is 1 mmol:(2-20) mL.

[0010] In the above scheme, in the second solution, the molar amount of o-phenylenediamine to the volume of ethanol is 1 mmol:(2-25) mL.

[0011] In the above scheme, the molar ratio of o-phenylenediamine to 2,5-dihydroxybenzaldehyde is 1:(2-3).

[0012] In the above scheme, the reaction temperature in step 1) is 60-80°C, and the reaction time is 3-8 h.

[0013] In the above scheme, in step 1), the obtained solid is washed with ethanol and dried at 40-70°C.

[0014] In the above scheme, the structural formula of the Schiff base compound is:

[0015]

[0016] In the above scheme, in the third solution, the molar amount of the Schiff base compound to the volume of ethanol is 1 mmol:(1-40) mL.

[0017] In the above scheme, in the fourth solution, the molar amount of the metal salt to the volume of ethanol is 1 mmol:(1-30) mL.

[0018] In the above scheme, the molar ratio of the Schiff base compound to the metal salt is 1:(1-2).

[0019] In the above scheme, the metal in the metal salt is one of manganese, iron, cobalt, nickel, copper, zinc, ruthenium and platinum.

[0020] In the above scheme, in step 2), the reaction temperature of the coordination reaction is 60-80℃, and the reaction time is 4-10h.

[0021] In the above scheme, in step 2), the obtained solid is washed with ethanol and dried at 40-70℃.

[0022] In the above scheme, the structural formula of the metal-coordinated Schiff base compound is:

[0023]

[0024] In the formula, M is one of Mn, Fe, Co, Ni, Cu, Zn, Ru and Pt.

[0025] In the above scheme, the solvent is one of water, ethanol and N-N dimethylformamide.

[0026] In the above scheme, the mass of the metal-coordinated Schiff base compound to the volume of the solvent is 1mg:(1-5)mL.

[0027] In the above scheme, the carboxyl-containing compound is one of citric acid, tartaric acid, malic acid, pyromellitic acid and amino acid.

[0028] In the above scheme, the mass ratio of the metal-coordinated Schiff base compound to the carboxyl-containing compound is 1:(5-20).

[0029] In the above scheme, the ultrasonic time is 5-30min.

[0030] In the above scheme, in step 3), the reaction temperature of the heating reaction is 120-200℃, and the reaction time is 4-8h.

[0031] In the above scheme, the molecular weight cut-off of the dialysis is 500-1200Da, and the dialysis time is 36-72h.

[0032] In the above scheme, the temperature of the freeze-drying is -40--80℃, and the time is 6-48h.

[0033] The application also provides a metal-doped carbon quantum dot prepared by the above method.

[0034] Compared with the prior art, the application has the following beneficial effects:

[0035] 1) The present invention uses a Schiff base metal complex as a precursor. The nitrogen atoms in the Schiff base can form coordination bonds with metal ions. The formation of this chemical bond increases the stability of the complex, enabling it to be structurally stably doped with metals and ensuring that the metal is doped into the center of the carbon quantum dots rather than the surface, thereby obtaining a higher doping rate and better product performance.

[0036] 2) The iron-doped carbon quantum dots prepared by the present invention have peroxidase mimicking activity, which can catalyze hydrogen peroxide to produce reactive oxygen species. The manganese-doped carbon quantum dots prepared by the present invention have superoxide dismutase activity, which can scavenge superoxide anions and have good antioxidant capacity.

[0037] 3) The preparation method of the metal-doped carbon quantum dots of the present invention has easy-to-obtain reaction raw materials, a simple synthesis method, and is easy to industrialize and produce. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a fluorescence image of the iron-doped carbon quantum dots prepared in Example 1 after being dissolved in N,N-dimethylformamide under 365nm laser irradiation.

[0039] Figure 2 This is a transmission electron microscopy image of the iron-doped carbon quantum dots prepared in Example 1.

[0040] Figure 3 This is the ultraviolet absorption spectrum of the iron-doped carbon quantum dots prepared in Example 1 in deionized water.

[0041] Figure 4 This is the fluorescence emission spectrum of the iron-doped carbon quantum dots prepared in Example 1 when excited at different excitation wavelengths in N,N-dimethylformamide.

[0042] Figure 5 This is the infrared spectrum of the iron-doped carbon quantum dots prepared in Example 1.

[0043] Figure 6 This is the ultraviolet absorption spectrum of the iron-doped carbon quantum dots prepared in Example 1 reacting with TMB+H2O2.

[0044] Figure 7 The color change of the solution of the iron-doped carbon quantum dots prepared in Example 1 in the reaction with TMB+H2O2.

[0045] Figure 8 This is the effect of the iron-doped carbon quantum dots prepared in Example 1 on the catalytic activity of the simulated enzyme under different coexisting substances.

[0046] Figure 9 This is the effect of the iron-doped carbon quantum dots prepared in Example 1 on the catalytic activity of the simulated enzyme at different storage times.

[0047] Figure 10XPS spectra of iron-doped carbon quantum dots prepared for Example 1 and Comparative Example 1.

[0048] Figure 11 Fluorescence emission spectra of iron-doped carbon quantum dots prepared for Comparative Example 1 and Comparative Example 2 excited in N,N-dimethylformamide at different excitation wavelengths.

[0049] Figure 12 UV absorption spectra of iron-doped carbon quantum dots prepared for Comparative Example 1 and Comparative Example 2 for TMB+H2O2 reaction.

[0050] Figure 13 Michaelis-Menten plots and Lineweaver-Burk plots of iron-doped carbon quantum dots prepared for Example 1, Comparative Example 1 and Comparative Example 2 analyzed with H2O2 as substrate.

[0051] Figure 14 Fluorescence spectra of manganese-doped carbon quantum dots prepared for Example 2 under 365 nm laser irradiation after dissolved in N,N-dimethylformamide.

[0052] Figure 15 Maximum excitation wavelength and maximum emission wavelength of manganese-doped carbon quantum dots prepared for Example 2 in N,N-dimethylformamide.

[0053] Figure 16 Superoxide anion scavenging effect of manganese-doped carbon quantum dots prepared for Example 2 at different concentrations.

[0054] Figure 17 DPPH scavenging effect of manganese-doped carbon quantum dots prepared for Example 2 at different concentrations.

[0055] Figure 18 ABTS scavenging effect of manganese-doped carbon quantum dots prepared for Example 2 at different concentrations. DETAILED DESCRIPTION

[0056] In order to better understand the present application, the content of the present application is further illustrated below in combination with examples, but the content of the present application is not limited only to the following examples.

[0057] Example 1

[0058] This example prepares an iron-doped carbon quantum dot, which comprises the following steps:

[0059] 1) 1.21 mmol of 2,5-dihydroxybenzaldehyde was dissolved in 3 mL of ethanol to obtain a first solution, 0.55 mmol of o-phenylenediamine was dissolved in 2 mL of ethanol to obtain a second solution, the second solution was added dropwise to the first solution, and heated to 65°C to reflux for 5 h, after the reaction was completed, the solid was filtered, washed with ethanol, and dried at 50°C to obtain a Schiff base compound;

[0060]

[0061] 2) 0.19 mmol of the Schiff base compound was added to 5 mL of ethanol and heated to 65°C to completely dissolve to obtain a third solution, 0.19 mmol of FeCl3·6H2O was dissolved in 2 mL of ethanol to obtain a fourth solution, the fourth solution was added dropwise to the third solution, and refluxed at 65°C for 5 h, after the reaction was completed, the solid was filtered, washed with ethanol, and dried at 50°C to obtain an iron-coordinated Schiff base compound;

[0062]

[0063] 3) 50 mg of the iron-coordinated Schiff base compound was dissolved in 50 mL of N,N-dimethylformamide, then 500 mg of citric acid was added and ultrasonically treated for 20 min until completely dissolved, then transferred to a reaction kettle, placed in a programmed temperature oven, the reaction temperature was set to 160°C, and reacted for 8 h, after the reaction was completed, the supernatant was separated by centrifugation, the clear liquid was dialyzed for 36 h using a dialysis bag with a molecular weight cutoff of 1000 Da, and then freeze-dried at -50°C under vacuum for 24 h to obtain iron-doped carbon quantum dots, denoted as salen-Fe-CDs.

[0064] Figure 1 The fluorescence diagram of the iron-doped carbon quantum dots prepared in this example under 365 nm laser irradiation after being dissolved in N,N-dimethylformamide can be clearly seen to be green fluorescence.

[0065] Figure 2 The transmission electron microscope diagram of the iron-doped carbon quantum dots prepared in this example can be seen by observation that the prepared carbon quantum dots are spherical and have good dispersibility, and analysis shows that the average particle size of the prepared carbon quantum dots is about 2.31 nm.

[0066] Figure 3 The ultraviolet absorption spectrum of the iron-doped carbon quantum dots prepared in this example in deionized water shows an absorption peak at 270 nm, which can be attributed to the π→π transition of the C=C double bond. *

[0067] Figure 4 ​The fluorescence emission spectrum of the iron-doped carbon quantum dots prepared in this example was obtained by exciting the sample in N,N-dimethylformamide at different excitation wavelengths, and the maximum excitation wavelength was 360 nm.

[0068] Figure 5 The infrared spectrum of the iron-doped carbon quantum dots prepared in Example 1 showed peaks at 3426 cm -1 , 2928 cm -1 , 1624 cm -1 , and 1455 cm -1 , which were attributed to C-H / C=O and C-N stretching vibrations; peaks at 1125 cm -1 and 652 cm -1 were signals of Fe-N and Fe-O stretching vibrations, respectively; these results demonstrated that the iron-doped carbon quantum dots had hydroxyl groups and nitrogen-containing compounds on the surface, and that iron had been successfully doped into the carbon quantum dots and formed chemical bonds with nitrogen and oxygen atoms.

[0069] The simulated enzyme activity of the iron-doped carbon quantum dots prepared in this example was tested by adding the iron-doped carbon quantum dots (0.3 mg / mL), H2O2 (0.33 mmol / L), and 3,3',5,5'-tetramethylbenzidine TMB (0.16 mmol / L) in an acetic acid buffer solution (pH 3.8, 0.2 mol / L) and reacting at room temperature for 3 min; at the same time, a control group without the addition of H2O2 was also tested. Figure 7 It was shown that the color of the TMB+salen-Fe-CDs system did not change, while the color of the TMB+salen-Fe-CDs+H2O2 system changed to blue, which was visible to the naked eye, and Figure 6 It was also shown that the TMB+salen-Fe-CDs+H2O2 system had an absorption signal at 652 nm, indicating that the iron-doped carbon quantum dots had peroxidase mimetic activity and could catalyze the production of reactive oxygen species from hydrogen peroxide, oxidizing colorless TMB to stable blue ox-TMB.

[0070] The enzyme stability of the iron-doped carbon quantum dots prepared in this example was tested. First, the effect of coexisting substances on the catalytic activity was tested by adding 0.1 mL (10 mmol / L) of coexisting substances to the TMB+salen-Fe-CDs+H2O2 system. It was found that Figure 8 the catalytic performance of the iron-doped carbon quantum dots was almost unaffected by these coexisting substances; secondly, the effect of storage time on the catalytic activity was tested by storing the prepared carbon quantum dots at room temperature for 15 days. It was found that Figure 9 the iron-doped carbon quantum dots still maintained high peroxidase mimetic activity after long-term storage.

[0071] Comparative Example 1

[0072] The comparative example prepared a kind of iron-doped carbon quantum dots (FeCl3-CDs), including the following steps:

[0073] 50mg FeCl3·6H2O is dissolved in 50mL N,N-dimethylformamide, then 500mg citric acid is ultrasonically dissolved for 20min, then transferred to the reaction kettle, placed in the program temperature oven, the reaction temperature is set to 160℃ reaction 8h, after the reaction is finished, centrifugal separation is retained supernatant, the clear solution is dialyzed in dialysis bag with molecular weight cut-off of 1000Da for 36h, then freeze-dried under vacuum condition at-50℃ for 24h, iron-doped carbon quantum dots are obtained, marked as FeCl3-CDs.

[0074] Comparative Example 2

[0075] The comparative example prepared a kind of iron-doped carbon quantum dots (EDTA-Fe-CDs), including the following steps:

[0076] 50mg FeCl3·6H2O is dissolved in 50mL N,N-dimethylformamide, then 500mg citric acid is ultrasonically dissolved for 20min, then transferred to the reaction kettle, placed in the program temperature oven, the reaction temperature is set to 160℃ reaction 8h, after the reaction is finished, centrifugal separation is retained supernatant, the clear solution is dialyzed in dialysis bag with molecular weight cut-off of 1000Da for 36h, then freeze-dried under vacuum condition at-50℃ for 24h, iron-doped carbon quantum dots are obtained, marked as FeCl3-CDs.

[0077] Figure 10 XPS diagram of iron-doped carbon quantum dots prepared for example 1 and comparative example 1, comparative example 1 is difficult to detect due to the small amount of iron doping in the product when the amount of 50mg FeCl3·6H2O is used, a group of FeCl3-CDs is prepared again by adjusting the amount of FeCl3·6H2O to 250mg for detection, according to the element content analysis results, the iron content doped in comparative example 1 is 0.82%, the iron content doped in example 1 is 0.65%, however, the amount of FeCl3·6H2O used in the preparation of carbon dots in comparative example 1 is 5 times the amount of iron complex schiff base compound used in the preparation of carbon dots in example 1, so actually the iron doping rate of example 1 is relatively high.

[0078] Figure 11 The fluorescence emission spectrum of iron-doped carbon quantum dots prepared for comparative example 1 and comparative example 2 is excited at different excitation wavelengths in N,N-dimethylformamide, it can be seen from the figure that the maximum emission wavelength of iron-doped carbon quantum dots prepared for comparative example 1 and comparative example 2 is 440nm, while Figure 4The maximum emission wavelength of the iron-doped carbon quantum dots prepared in Example 1 is at 520 nm. From the perspective of fluorescence performance, when a Schiff base iron complex is used as a precursor to prepare carbon dots, a larger effective conjugated structure can be formed during the carbonization process, which helps to promote the delocalization and transition of electrons, thereby affecting the luminescent properties of the carbon dots, including red shift of the emission wavelength.

[0079] The simulated enzyme activity of the iron-doped carbon quantum dots prepared in Comparative Example 1 and Comparative Example 2 was tested. Iron-doped carbon quantum dots (0.3 mg / mL), H2O2 (0.1 mmol / L), and TMB (0.25 mmol / L) were added to an acetic acid buffer solution (pH 4, 0.2 mol / L), and the reaction was carried out at room temperature for 3 min. The absorbance value at 652 nm was measured by a UV-Vis spectrophotometer, and the results are shown in Table 1. Figure 12 As can be seen, under the same conditions, the iron-doped carbon quantum dots prepared in Example 1 have the highest absorption value at 652 nm and better peroxidase-like activity.

[0080] In order to further compare the catalytic ability, H2O2 was used as a substrate to calculate the Michaelis constant (Km) of the three kinds of iron-doped carbon quantum dots. m The value of K m indicates the affinity of the substrate to the carbon dot mimetic enzyme, and the smaller the K m , the stronger the affinity. Figure 13 As shown, the K m of Comparative Example 1 is 0.11, the K m of Comparative Example 2 is 0.085, and the K m of Example 1 is 0.041. According to the value of K m , it can be seen that the iron-doped carbon quantum dots prepared in Example 1 have the strongest affinity to H2O2 and better peroxidase-like activity than the comparative examples.

[0081] Example 2

[0082] In this example, a manganese-doped carbon quantum dot is prepared, including the following steps:

[0083] 1) 7 mmol of 2,5-dihydroxybenzaldehyde was dissolved in 20 mL of ethanol to obtain a first solution, 3.5 mmol of o-phenylenediamine was dissolved in 10 mL of ethanol to obtain a second solution, the second solution was added dropwise to the first solution, and heated to 70°C to reflux for 4.5 h. After the reaction was completed, the solid was filtered, washed with ethanol, and dried at 60°C to obtain a Schiff base compound;

[0084] 2) 0.078 mmol of the Schiff base compound was added to 3 mL of ethanol and warmed to 65°C to completely dissolve to obtain a third solution, 0.078 mmol of Mn(OAc)2·4H2O was dissolved in 2 mL of ethanol to obtain a fourth solution, the fourth solution was added dropwise to the third solution, and the reaction was refluxed at 70°C for 5 h. After the reaction was completed, the solid was filtered, washed with ethanol, and dried at 60°C to obtain the manganese-coordinated Schiff base compound;

[0085] 3) 50 mg of the manganese-coordinated Schiff base compound was dissolved in 50 mL of N,N-dimethylformamide, then 500 mg of citric acid was added and ultrasonicated for 20 min until completely dissolved, then transferred to a reaction kettle and placed in a programmed temperature oven, the reaction temperature was set to 180°C and reacted for 8 h. After the reaction was completed, the supernatant was separated by centrifugation, the clear solution was dialyzed in a dialysis bag with a molecular weight cut-off of 1000 Da for 36 h, and then freeze-dried under vacuum at -50°C for 28 h to obtain the manganese-doped carbon quantum dots.

[0086] Figure 14 The manganese-doped carbon quantum dots prepared in this example were dissolved in N,N-dimethylformamide and irradiated with a 365 nm laser. As can be clearly seen from the fluorescence image, it emitted yellow-green fluorescence.

[0087] The optical properties of the manganese-doped carbon quantum dots prepared in this example were tested, and the absorption spectrum of the DMF solution was measured, as shown in Figure 15 , the maximum excitation wavelength was 350 nm, and the maximum emission wavelength was 520 nm.

[0088] The simulated superoxide dismutase activity of the manganese-doped carbon quantum dots prepared in this example was studied. Nitro blue tetrazolium chloride (NBT) was used as a probe to determine its superoxide anion scavenging effect. The test principle is that NBT can react with superoxide anion to generate formazan in the solution, which is blue and has a peak at 560 nm. When there is a superoxide anion scavenger, the peak value will decrease. As shown in Figure 16 , with the increase of the concentration of manganese-doped carbon quantum dots, the absorbance value of the solution at 560 nm decreased sharply, and the superoxide anion scavenging rate reached 80% at a concentration of 40 μg / mL, indicating that the manganese-doped carbon quantum dots prepared in this example had excellent superoxide anion scavenging rate.

[0089] The antioxidant properties of the manganese-doped carbon quantum dots prepared in this example were explored, and their activity in scavenging reactive oxygen free radicals was detected. DPPH (2,2-diphenyl-1-picrylhydrazyl) was chosen as a model free radical. DPPH exhibits a characteristic absorption peak at 515 nm. When DPPH comes into contact with a free radical scavenger, the peak value decreases due to the formation of DPPH complexes. The elimination of DPPH represents the antioxidant capacity. As shown in Figure 17As shown, with the concentration of manganese-doped carbon quantum dots increasing from 0 to 150 μg / mL, the peak intensity at 517 nm shows a continuous downward trend, and the DPPH clearance rate also rises to 89.84%, indicating that the manganese-doped carbon quantum dots prepared in this embodiment have high DPPH clearance activity and strong antioxidant activity.

[0090] In addition, ABTS (2,2-azino-bis (3-ethyl-benzothiazoline-6-sulfonic acid) diammonium salt) is also selected as a model group, and ABTS + Radicals, which need to obtain electrons from antioxidant molecules to form stable neutral molecules, are evaluated for radical scavenging activity of manganese-doped carbon quantum dots by reduction of their absorbance at 734 nm. As shown in FIG. 6, with the concentration of manganese-doped carbon quantum dots increasing from 0 to 150 μg / mL, the peak intensity at 517 nm shows a continuous downward trend, and the DPPH clearance rate also rises to 89.84%, indicating that the manganese-doped carbon quantum dots prepared in this embodiment have high DPPH clearance activity and strong antioxidant activity. Figure 18 As shown in FIG. 7, with the concentration of manganese-doped carbon quantum dots increasing from 5 to 150 μg / mL, the peak intensity at 734 nm shows a continuous downward trend, and the ABTS clearance rate also increases from 6.42% to 91.27%. The results show that the manganese-doped carbon quantum dots prepared in this embodiment have excellent antioxidant capacity and can effectively scavenge ABTS radicals. + As shown in FIG. 7, with the concentration of manganese-doped carbon quantum dots increasing from 5 to 150 μg / mL, the peak intensity at 734 nm shows a continuous downward trend, and the ABTS clearance rate also increases from 6.42% to 91.27%. The results show that the manganese-doped carbon quantum dots prepared in this embodiment have excellent antioxidant capacity and can effectively scavenge ABTS radicals.

[0091] Example 3

[0092] In this embodiment, a platinum-doped carbon quantum dot is prepared, including the following steps:

[0093] 1) 2 mmol of 2,5-dihydroxybenzaldehyde is dissolved in 10 mL of ethanol to obtain a first solution, 1 mmol of o-phenylenediamine is dissolved in 5 mL of ethanol to obtain a second solution, the second solution is added dropwise to the first solution, and heated to 68°C to reflux for 6 h. After the reaction is completed, the solid is filtered, washed with ethanol, and dried at 46°C to obtain a Schiff base compound;

[0094] 2) 1 mmol of the Schiff base compound is added to 5 mL of ethanol and heated to 65°C to completely dissolve to obtain a third solution, 1 mmol of PtCl4 is dissolved in 3 mL of ethanol to obtain a fourth solution, the fourth solution is added dropwise to the third solution, and refluxed at 68°C for 6 h. After the reaction is completed, the solid is filtered, washed with ethanol, and dried at 46°C to obtain a platinum-coordinated Schiff base compound;

[0095] 3) 25 mg of the platinum-coordinated Schiff base compound is dissolved in 25 mL of ethanol, then 400 mg of tartaric acid is added and ultrasonically dissolved for 25 min, then transferred to a reaction kettle, placed in a programmed temperature oven, the reaction temperature is set to 185°C, and reacted for 4 h. After the reaction is completed, the supernatant is separated by centrifugation, the clear solution is dialyzed in a dialysis bag with a molecular weight cutoff of 1000 Da for 48 h, and then freeze-dried at -60°C under vacuum conditions for 30 h to obtain platinum-doped carbon quantum dots.

[0096] Example 4

[0097] The embodiment prepares a copper-doped carbon quantum dot, including the following steps:

[0098] 1) 2.5 mmol of 2,5-dihydroxybenzaldehyde was dissolved in 15 mL of ethanol to obtain a first solution, 1 mmol of o-phenylenediamine was dissolved in 6 mL of ethanol to obtain a second solution, the second solution was added dropwise to the first solution, and heated to 76°C to reflux for 4 h, after the reaction was completed, the solid was filtered, washed with ethanol, and dried at 48°C to obtain a Schiff base compound;

[0099] 2) 1.5 mmol of the Schiff base compound was added to 8 mL of ethanol and heated to 70°C to completely dissolve to obtain a third solution, 1.6 mmol of CuCl2 was dissolved in 7 mL of ethanol to obtain a fourth solution, the fourth solution was added dropwise to the third solution, and refluxed at 76°C for 7 h, after the reaction was completed, the solid was filtered, washed with ethanol, and dried at 48°C to obtain a copper-coordinated Schiff base compound;

[0100] 3) 50 mg of the copper-coordinated Schiff base compound was dissolved in 60 mL of deionized water, then 700 mg of glycine was added and ultrasonically dissolved for 25 min, then transferred to a reaction kettle, placed in a program-controlled oven, the reaction temperature was set to 170°C, and reacted for 5 h, after the reaction was completed, the supernatant was separated by centrifugation, the clear liquid was dialyzed in a dialysis bag with a molecular weight cut-off of 1200 Da for 24 h, and then freeze-dried at -45°C under vacuum for 30 h to obtain a copper-doped carbon quantum dot.

[0101] The above embodiments are only examples for the purpose of clarity, and are not intended to limit the embodiments. Based on the above description, other different forms of changes or variations can be made by those of ordinary skill in the art, and it is not necessary or possible to exhaust all embodiments, and therefore the changes or variations thus extended are still within the protection scope of the present application.

Claims

1. A method for preparing metal-doped carbon quantum dots, characterized in that: The following steps are involved: 1) dissolving 2,5-dihydroxybenzaldehyde in ethanol to obtain a first solution, dissolving o-phenylenediamine in ethanol to obtain a second solution, and adding the second solution dropwise to the first solution to react. After the reaction is complete, filtering and retaining a solid, washing, and drying the solid to obtain a Schiff base compound; 2) adding a Schiff base compound to ethanol and heating it to completely dissolve it to obtain a third solution; dissolving a metal salt in the ethanol to obtain a fourth solution, wherein the metal in the metal salt is one of manganese, iron, cobalt, nickel, copper, zinc, ruthenium, and platinum; and dropwise adding the fourth solution to the third solution to carry out a coordination reaction. After the reaction is complete, filtering the solid, washing it, and drying it to obtain a metal-coordinated Schiff base compound; 3) dissolving the metal-coordinated Schiff base compound in a solvent selected from the group consisting of water, ethanol, and N-N-dimethylformamide; then adding a carboxyl-containing compound and sonicating until completely dissolved; the carboxyl-containing compound selected from the group consisting of citric acid, tartaric acid, and malic acid; heating the mixture to 120-200° C. for 4-8 hours, centrifuging the supernatant after the reaction, dialyzing the supernatant, and freeze-drying the supernatant to obtain metal-doped carbon quantum dots.

2. The method for preparing metal-doped carbon quantum dots according to claim 1, wherein: The molar ratio of o-phenylenediamine to 2,5-dihydroxybenzaldehyde is 1:2-3; the reaction temperature in step 1) is 60-80° C., and the reaction time is 3-8 hours.

3. The method for preparing metal-doped carbon quantum dots according to claim 1, wherein: The molar ratio of the Schiff base compound to the metal salt is 1:1-2; the reaction temperature of the coordination reaction in step 2) is 60-80° C., and the reaction time is 4-10 h.

4. The method for preparing metal-doped carbon quantum dots according to claim 1, wherein: The structural formula of the metal-coordinated Schiff base compound is: ， In the formula, M is selected from one of Mn, Fe, Co, Ni, Cu, Zn, Ru, and Pt.

5. The method for preparing metal-doped carbon quantum dots according to claim 1, wherein: The mass ratio of the metal-coordinated Schiff base compound to the carboxyl-containing compound is 1:5-20.

6. The method for preparing metal-doped carbon quantum dots according to claim 1, wherein: The mass ratio of the metal-coordinated Schiff base compound to the volume of the solvent is 1 mg:1-5 mL.

7. The method for preparing metal-doped carbon quantum dots according to claim 1, wherein: In the first solution, the volume ratio of the amount of 2,5-dihydroxybenzaldehyde to ethanol is 1 mmol:2-20 mL; in the second solution, the volume ratio of the amount of o-phenylenediamine to ethanol is 1 mmol:2-25 mL.

8. The method for preparing metal-doped carbon quantum dots according to claim 1, wherein: In the third solution, the volume ratio of the amount of the Schiff base compound to the ethanol is 1 mmol:1-40 mL; in the fourth solution, the volume ratio of the amount of the metal salt to the ethanol is 1 mmol:1-30 mL.

9. The method for preparing metal-doped carbon quantum dots according to claim 1, wherein: The molecular weight cut-off of the dialysis is 500-1200 Da, and the dialysis time is 36-72 h; the temperature of the freeze-drying is -40--80° C., and the time is 6-48 h.

10. Metal-doped carbon quantum dots prepared by the method according to any one of claims 1 to 9.

Images