WO2021072939A1

WO2021072939A1 - Oil-soluble carbon quantum dots and preparation method therefor

Info

Publication number: WO2021072939A1
Application number: PCT/CN2019/123184
Authority: WO
Inventors: 张和凤; 方来平
Original assignee: 汕头大学
Priority date: 2019-10-14
Filing date: 2019-12-05
Publication date: 2021-04-22
Also published as: CN110734051B; CN110734051A

Abstract

A type of oil-soluble carbon quantum dots and a preparation method therefor. The preparation comprises using a surfactant as a sole carbon source to perform one-step carbonization to obtain oil-soluble carbon quantum dots; the oleophilic end of the surfactant contains 12-30C, and a hydrophilic group thereof has poor structure thermal stability and decomposes easily by heating. The surfactant comprises one among an alkyl anionic series, alkyl cationic series, Tween series, Span series, glycosyl series, betaine series, lecithin series and amino acid series. The one-step carbonization method comprises a solvothermal method and a solvent-free solid phase reaction method. The present method has various types of carbon sources, a broad range of sources, is low-priced and has easy availability; the obtained oil-soluble carbon quantum dots are similar in appearance, uniform in size, and contain nanocrystalline structures. The present method uses surfactants containing different elements as carbon sources to prepare heteroatom-doped oil-soluble carbon quantum dots.

Description

A class of oil-soluble carbon quantum dots and preparation method thereof

Technical field

The invention belongs to the field of new materials, and particularly relates to a type of oil-soluble carbon quantum dots and a preparation method thereof.

Background technique

Carbon quantum dot (CQD) is also called carbon dot (C-dot) because of its wide source of raw materials, low price, easy preparation, low toxicity, good biocompatibility, and light Stability and resistance to photobleaching are receiving more and more attention, and they are considered to be the best materials to replace traditional inorganic quantum dots and semiconductor quantum dots in terms of drug loading, biological imaging, and fluorescent probes. However, more than 95% of the currently reported carbon quantum dots are water-soluble, and there is still a lack of better solutions in terms of large-scale preparation and particle size control, which is an important reason that restricts its further development and application.

So far, although "top-down" methods such as arc discharge, laser ablation and electrochemical oxidation of macrographitic carbon sources have been used to prepare carbon quantum dots, the purity of the carbon quantum dots produced by these methods is very low. , And has a very wide size distribution. Before use, the product needs to be purified and size classified by multiple long-term dialysis and other methods. Later, "bottom-up" synthesis methods such as incomplete combustion, thermal cracking, microwave-assisted thermal cracking of small organic molecules, and hydrothermal methods of biomass were developed. The carbon quantum dots synthesized by these "bottom-up" methods have a significant improvement in size distribution compared to the "top-down" method, but the resulting products still have a wider size distribution. Rhee et al. prepared carbon quantum dots with a size distribution of 1.0-5.0 nanometers through the carbonization reaction of glucose in micelles under optimal conditions. In this scale range, due to the strong size effect, the wider size distribution will cause great interference in establishing an accurate structure-activity relationship of carbon quantum dots. At present, whether it is "top-down" or "bottom-up", both methods have random characteristics, which can neither control yield nor accurately control product size; and the yield of carbon quantum dots and fluorescence quantum efficiency are not high.

In addition, since most carbon quantum dots are realized by carbonizing small organic molecules such as citric acid and ethylenediamine, or natural products such as orange juice, watermelon, cola, and rice, the resulting carbon quantum dots are almost all water-soluble . On the one hand, the water-soluble nature of this kind of quantum dots restricts its application in more aspects, on the other hand, the post-processing process of water-soluble quantum dots is relatively troublesome. The water-soluble carbon quantum dots must be replaced with long-term water and dialysis tape in the later stage The method of separation and purification is not only time-consuming and low-yield, but also objective conditions can not achieve the purpose of large-scale preparation.

Summary of the invention

The present invention uses inexpensive commercial surfactants as the sole carbon source, and does not require redundant synthesis steps. After carbonizing the hydrophilic ends, oil-soluble carbon quantum dots can be obtained; by utilizing the steric hindrance of long-chain alkanes carried by the surfactants It can obtain oil-soluble carbon quantum dots with uniform particle size; at the same time, using microwave as a heating source can realize rapid carbonization in a short time and greatly shorten the preparation time; thus, it solves the current single water solubility of carbon quantum dots. The problems of large-scale preparation and difficulty in particle size control provide the possibility for the rapid and large-scale preparation of oil-soluble quantum dots with uniform particle size.

A method for preparing oil-soluble carbon quantum dots mainly includes using a surfactant as the sole carbon source to perform one-step carbonization to obtain oil-soluble carbon quantum dots; the lipophilic end of the surfactant contains 12-30C and has a hydrophilic group structure Poor thermal stability, easy to heat and decompose.

The only carbon source used in the present invention is a surfactant. One end containing a hydrophilic group is easily carbonized under heating, while the long-chain alkane at the other end is not easily carbonized at the same temperature. Most of the prior art uses at least two substances as carbon sources, which has a big difference; a small amount uses only one substance as a carbon source. In the work of others, organic substances (such as amines, aldehydes) are used as carbon sources (non-ionic). , But its biocompatibility is still unclear. In the present invention, commercial anionic, nonionic and zwitterionic surfactants are used. There are many types of industrial surfactants. By selecting different surfactants, the fluorescent core structure and peripheral carbon chains of carbon dots can be easily adjusted to prepare carbon quantum dots with different heteroatom doping and different carbon chain lengths. The luminescence behavior and solubility can be adjusted. For example, carbon quantum dots prepared from sucrose esters contain only three elements: carbon, hydrogen, and oxygen. The outer periphery of carbon quantum dots is surrounded by eighteen-carbon long-chain alkanes; carbon quantum dots prepared from betaine contain carbon, hydrogen, and oxygen in the core. The four elements, nitrogen and nitrogen, are surrounded by twelve-carbon long-chain alkanes. Through experimentation, different surfactants can be used to obtain carbon quantum dots with different luminescence properties. For example, the strongest emission light of carbon quantum dots prepared using betaine is 425nm, and the strongest emission light of carbon quantum dots prepared using sucrose ester is 446nm. Under the same preparation method, the properties of carbon quantum dots depend on the surfactant raw materials used.

In view of the fact that most of the current carbon quantum dots are water-soluble, the present invention creatively proposes a method of directly carbonizing a hydrophilic group with a long-chain alkane surfactant, which realizes the rapid synthesis of oil-soluble carbon quantum dots, that is, under high temperature, The hydrophilic end of the surfactant can be carbonized to form a carbon dot core, but the lipophilic end is not carbonized to provide excellent oil solubility, thereby preparing oil-soluble carbon quantum dots. The present invention addresses the problem of wide particle size distribution in traditional methods. It utilizes the steric hindrance between alkane chains and the degree of carbonization is approximately the same within the same time. Therefore, the size of the final carbon quantum dots is uniform and exhibits uniformity in a good solvent. Monodisperse.

Preferably, the surfactants include alkyl anionic series (sodium alkylbenzene sulfonate, sodium alkyl sulfonate, sodium alkyl sulfate, sodium alkyl carboxylate), alkyl cationic series (octadecyl Trimethyl ammonium chloride, cetyl trimethyl ammonium chloride, dodecyl trimethyl ammonium chloride, octadecyl trimethyl ammonium bromide, dodecyl dimethyl ammonium oxide, Octadecyl methyl amine bromide,) Tween series (Tween 20, Tween 40, Tween 60, Tween 80), Span (Span 20, Span 40, Span 60, Span 80 ) Series, sugar-based series (sucrose fatty acid ester, coco glucoside), betaine series (cocamidopropyl betaine, lauryl betaine, thiobetaine), lecithin series (soy egg) Phospholipids, hydrogenated lecithin, egg yolk lecithin, krill lecithin, collagen peptide taurine lecithin, collagen peptide donkey-hide gelatin lecithin) and amino acid series (cocoyl glycine, cocoyl glutamate, lauroyl glycine) , Lauroyl glutamate, sodium cocoyl glycinate, sodium cocoyl glutamate, potassium cocoyl glycinate, potassium cocoyl glutamate). More preferably, it is one of undoped sucrose fatty acid ester, phosphorus-doped soybean lecithin, undoped Span 40, and nitrogen-doped dodecyl betaine.

Preferably, the method of carbonization includes a solvothermal method and a solvent-free solid phase reaction method.

Preferably, the solvothermal method for synthesizing oil-soluble carbon quantum dots mainly includes the following steps:

(1) Disperse the surfactant in a solvent with a high boiling point and low polarity and use an electric heating plate to heat it to 120-250°C and cool it to room temperature;

(2) Using diatomaceous earth as a medium, petroleum ether and ethyl acetate as eluents, the solvent is separated from the reaction solution, and insoluble impurities are removed, and oil-soluble carbon quantum dots are obtained after concentration.

Preferably, in step (1), 2-20 g of surfactant is added to every 50-400 mL of high boiling point and low polarity solvent.

Preferably, the high-boiling low-polarity solvent includes n-alkane series (n-decane to n-pentadecane), bromo-alkane series (bromooctane to bromotriacontane), decalin, toluene and diphenyl One or more of methyl ether.

The method is to simply and efficiently prepare oil-soluble carbon quantum dots with uniform size and excellent solubility in organic solvents by carbonizing the hydrophilic part of surfactant micelles dissolved in non-polar solvents. First, let the surfactant dissolve in a non-polar organic solvent at a medium temperature to form nanomicelles. As the temperature increases, the surfactant gradually changes from a single molecule to a hydrophilic end in the presence of the non-polar solvent. Reverse nanomicelles with the lipophilic end inward and outward. At this time, due to the steric hindrance of the lipophilic end long-chain alkane, the number of molecules of the surfactant forming the nanomicelles will not increase indefinitely, and the actual size of each nanomicelle will be uniform. As the temperature continues to rise, the hydrophilic end at the core of the nanomicelle can be carbonized, while the lipophilic end is retained. The micelles have the same degree of carbonization within the same time, so the uniform size and diameter approximately equal to 3 nanometers can be prepared. Carbon quantum dots.

For the solvothermal method, it needs to be prepared under a certain feeding concentration. If the concentration is too high, the surfactant will not dissolve completely and the dispersion will be uneven, and it will be easier to agglomerate during the heating process, and more black carbonized precipitate will be produced. The yield is low, the particle size is uneven and the fluorescence is not strong; the concentration is too small, the yield is low , The fluorescence performance of the prepared carbon quantum dots is not strong.

Preferably, the solvent-free solid-phase reaction method for synthesizing oil-soluble carbon quantum dots mainly includes the following steps:

(1) Stir the surfactant and silica gel uniformly, and use microwave heating for 5-120 minutes;

(2) After adding ethyl acetate, ultrasonication, suction filtration, separation of silica gel;

(3) Using diatomaceous earth as a medium, petroleum ether and ethyl acetate as eluents to remove insoluble impurities, and after concentration, oil-soluble carbon quantum dots are obtained.

At this time, the silica gel has the function of dispersing the surfactant to ensure that the carbon source is heated more uniformly, otherwise the surfactant will agglomerate in a large amount and be unevenly heated.

Preferably, the mass ratio of the surfactant to the silica gel is 1:1-10. Preferably, the method further includes performing column chromatography separation on the synthesized oil-soluble carbon quantum dots, separating insoluble impurities and spin-drying the solution to obtain a carbon quantum dot product with uniform particle size.

This method uses a microwave-assisted method to prepare oil-soluble carbon quantum dots on a large scale in a short time without adding any chemical reagents. After mixing the surfactant and silica gel uniformly, the hydrophilic end of the surfactant is directly carbonized by the microwave-assisted "one-step method", while the lipophilic end with long-chain alkanes cannot be carbonized. Without the addition of any chemical reagents, oil-soluble carbon quantum dots can be efficiently prepared on a large scale within a few minutes. The experiment process is simple, fast, and easy to operate. It not only overcomes the time-consuming problem of general traditional methods (hydrothermal method, solvothermal method), but also the entire preparation process does not add any chemical reagents, which is environmentally friendly.

The oil-soluble carbon quantum dots obtained by the above-mentioned preparation method of oil-soluble carbon quantum dots are prepared.

Compared with the prior art, the present invention uses commercial surfactants as carbon sources, which have a wide range of types, wide sources, low prices and easy availability, which can not only prepare carbon quantum dots in large quantities conveniently and quickly, but also reduce costs and reduce energy consumption; The present invention carbonizes the hydrophilic end of the surfactant and retains the lipophilic end with long chain alkanes, thereby rapidly preparing oil-soluble carbon quantum dots in one step, and the obtained oil-soluble carbon quantum dots are similar in shape and uniform in size. The invention adopts microwave as the heating source, which can quickly raise the temperature within a few minutes, which is beneficial to the low-cost and large-scale preparation of oil-soluble carbon quantum dots. The present invention can also use surfactants containing different elements as a carbon source, without adding other reactants, the next step to prepare heteroatom doped (such as N (nitrogen), S (sulfur), P (phosphorus)) Oil-soluble carbon quantum dots.

Description of the drawings

Figure 1 is a schematic diagram of the structure of a representative surfactant preferred in the present invention.

Figure 2 is a schematic diagram of the surfactant being carbonized with the participation of a solvent when the solvothermal method is adopted in the present invention.

Figure 3 shows the solubility of carbon quantum dots prepared by the present invention in different solvents.

Figure 4 A is the fluorescence emission spectrum of the oil-soluble carbon quantum dot solution prepared by the solvothermal method and sucrose fatty acid ester as the carbon source; B is the oil-soluble carbon quantum dot solution prepared by the solid phase method and sucrose fatty acid ester as the carbon source The fluorescence emission spectrum of the dot solution; C is the TEM image of the oil-soluble carbon quantum dot solution prepared with sucrose fatty acid ester as the carbon source; D is the carbon quantum dot prepared by the solvothermal method (left) and the solid phase method (right) Fluorescence of the solution under a fluorescent lamp and a 365nm ultraviolet lamp; the concentration of the solution is 5mg/mL.

Figure 5 A is the fluorescence emission spectrum of the oil-soluble carbon quantum dot solution prepared by the solvothermal method and Span 40 as the carbon source; B is the oil-soluble carbon quantum dot solution prepared by the solid phase method and Span 40 as the carbon source Fluorescence emission spectrum; C is the TEM image of the oil-soluble carbon quantum dot solution prepared by Span 40 as the carbon source; D is the carbon quantum dot solution prepared by the solvothermal method (left) and the solid phase method (right) in a fluorescent lamp Fluorescence chart under UV lamp and 365nm UV lamp; the concentration of the solution is 5mg/mL.

Figure 6 A is the fluorescence emission spectrum of the oil-soluble carbon quantum dot solution prepared by the solvothermal method and soybean lecithin as the carbon source; B is the oil-soluble carbon quantum dot solution prepared by the solid phase method and soybean lecithin as the carbon source Fluorescence emission spectra; C is the TEM image of the oil-soluble carbon quantum dot solution prepared by soybean lecithin as the carbon source; D is the carbon quantum dot solution prepared by the solvothermal method (left) and the solid phase method (right) in a fluorescent lamp Fluorescence chart under UV lamp and 365nm UV lamp; the concentration of the solution is 5mg/mL.

Figure 7 A is the fluorescence emission spectrum of the oil-soluble carbon quantum dot solution prepared by the solvothermal method and lauryl betaine as the carbon source; B is prepared by the solid phase method and lauryl betaine as the carbon source The fluorescence emission spectrum of the oil-soluble carbon quantum dot solution; C is the TEM image of the oil-soluble carbon quantum dot solution prepared with lauryl betaine as the carbon source; D is the solvothermal method (left) and the solid phase method (right ) Fluorescence chart of the prepared carbon quantum dot solution under fluorescent lamp and 365nm ultraviolet lamp; the concentration of the solution is 5mg/mL.

Figure 8 A is the fluorescence emission spectrum of the oil-soluble carbon quantum dot solution prepared by the solid-phase method and Tween 80 as the carbon source; B is the oil-soluble prepared by the solid-phase method and sodium cocoyl glutamate as the carbon source The fluorescence emission spectrum of the carbon quantum dot solution; the concentration of the solution is 10 mg/mL. C is the fluorescence emission spectrum of the oil-soluble carbon quantum dot solution prepared by the solid phase method and sodium dodecyl benzene sulfonate as the carbon source; D is the solid phase method and octadecyl trimethyl ammonium chloride as the carbon Fluorescence emission spectrum of the oil-soluble carbon quantum dot solution prepared by the source;

Fig. 9 A is a bar graph comparing the fluorescence intensity of the oil-soluble carbon quantum dot solution prepared by the solvothermal method and dodecyl betaine as the carbon source after one day and six months; B is the solvothermal method and ten Comparison of the fluorescence intensity of oil-soluble carbon quantum dot solutions prepared with dialkyl betaine as a carbon source under continuous UV wavelengths of 365 nm for different durations; the concentration of the solutions are both 2.5 mg/mL.

Fig. 10 shows a large amount of oil-soluble carbon quantum dots prepared by using a solvent-free solid-phase reaction method and using Span 40 as a carbon source.

Figure 11 is to explore the fluorescence quenching effect of different concentrations of iron ion solution on carbon quantum dot solution (4mg/mL).

Figure 12 is a solvothermal method, with sucrose ester as a carbon source, decalin as a solvent, and fluorescence emission spectra of carbon quantum dots prepared with different reaction concentrations.

Figure 13 is the fluorescence emission spectrum of the carbon quantum dot solution prepared by the solid phase method with ammonium lauryl sulfate as the carbon source; the concentration of the solution is 10 mg/mL.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings.

Example 1

One-step synthesis of oil-soluble carbon quantum dots using sucrose fatty acid ester as carbon source

(1) Solvent-free solid-phase reaction method: Put a 250 mL beaker containing 2 g of sucrose fatty acid ester (without doping) and 2 g of silica gel (200-300 mesh) into a microwave oven for 10 minutes, and then cool to room temperature. Add 200 mL of ethyl acetate, sonicate and filter with suction to remove silica gel. Using diatomaceous earth as a medium, petroleum ether and ethyl acetate as eluents, the product was separated by column chromatography, and the insoluble impurities were removed to obtain a carbon quantum dot solution, which was then spin-dried to obtain oil-soluble carbon quantum dots.

(2) Solvothermal method: Put a flask containing 2g of sucrose fatty acid ester (without doping) and 50mL of high-boiling low-polarity solvent n-hexadecane into an oil bath, and use an electric heating plate (jacket) to heat to 200°C , Cool to room temperature. After concentrating the reaction solution, using diatomaceous earth as the medium and petroleum ether as the eluent, the product is separated by column chromatography, and the low-polarity solvent is removed first. Petroleum ether and ethyl acetate are used as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

The flow chart of the solvothermal method of this embodiment is shown in Figure 2. As the temperature continues to increase, the surfactant in the presence of a non-polar solvent gradually changes from a single molecule to a hydrophilic end to an inward lipophilic end. Outward nanomicelles. At this time, due to the steric hindrance of the lipophilic end long-chain alkanes, the number of molecules of the surfactant forming the nanomicelles will not increase indefinitely, which results in the actual size of each nanomicelle being almost the same. When the temperature continues to rise, the hydrophilic end of the nanomicelles can be carbonized at high temperature, while the lipophilic end is retained. The carbonization degree of the micelles is similar in the same time, so oil-soluble carbon quantum dots of uniform size can be prepared.

Example 2

One-step synthesis of oil-soluble carbon quantum dots using soybean lecithin as carbon source

(1) Solvent-free solid-phase reaction method: A 250 mL beaker containing 2 g of soybean lecithin (phosphorus doped) and 2 g of silica gel (200-300 mesh) is placed in a microwave oven and heated for 15 minutes, and then cooled to room temperature. Add 200 mL of ethyl acetate, sonicate and filter with suction to remove silica gel. After the filtrate is concentrated, the product is separated by column chromatography using petroleum ether and ethyl acetate as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

(2) Solvothermal method: Put 2g soybean lecithin (phosphorus doped) and 50mL high-boiling low-polarity solvent n-hexadecane flask into an oil bath, use an electric heating plate (set) to heat to 200℃, Cool to room temperature. After the reaction solution is concentrated, the product is separated by column chromatography with petroleum ether as the eluent, and the low-polarity solvent is removed first. Petroleum ether and ethyl acetate are used as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

Example 3

One-step synthesis of oil-soluble carbon quantum dots using Span 40 as a carbon source

(1) Solvent-free solid-phase reaction method: A 250 mL beaker containing 2 g of Span 40 (no doping) and 2 g of silica gel (200-300 mesh) is placed in a microwave oven and heated for 20 minutes, and then cooled to room temperature. Add 200 mL of ethyl acetate, sonicate and filter with suction to remove silica gel. After the filtrate is concentrated, the product is separated by column chromatography using petroleum ether and ethyl acetate as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

(2) Solvothermal method: Put 2g Span 40 (undoped) and 50mL high-boiling low-polarity solvent n-hexadecane flask into an oil bath, and use an electric heating plate (set) to heat to 200°C, Cool to room temperature. After the reaction solution is concentrated, the product is separated by column chromatography with petroleum ether as the eluent, and the low-polarity solvent is removed first. Petroleum ether and ethyl acetate are used as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

Example 4

One-step synthesis of oil-soluble carbon quantum dots using lauryl betaine as carbon source

(1) Solvent-free solid-phase reaction method: A 250 mL beaker containing 2 g of dodecyl betaine (nitrogen doped) and 2 g of silica gel (200-300 mesh) is placed in a microwave oven and heated for 20 minutes, and then cooled to room temperature. Add 200 mL of ethyl acetate, sonicate and filter with suction to remove silica gel. After the filtrate is concentrated, the product is separated by column chromatography using petroleum ether and ethyl acetate as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

(2) Solvothermal method: Put 2g of dodecyl betaine (nitrogen doped) and 50mL of high boiling point low polarity solvent n-hexadecane flask into an oil bath, and use an electric heating plate (set) to heat to 200°C, cool to room temperature. After the reaction solution is concentrated, the product is separated by column chromatography with petroleum ether as the eluent, and the low-polarity solvent is removed first. Petroleum ether and ethyl acetate are used as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

Example 5

One-step synthesis of oil-soluble carbon quantum dots using Tween 80 as carbon source

Solvent-free solid-phase reaction method: Put a 250 mL beaker containing 2g Tween 80 (no doping) and 2g silica gel (200-300 mesh) into a microwave oven for 20 minutes, and then cool to room temperature. Add 200 mL of ethyl acetate, sonicate and filter with suction to remove silica gel. After the filtrate is concentrated, the product is separated by column chromatography using petroleum ether and ethyl acetate as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

Example 6

One-step synthesis of oil-soluble carbon quantum dots using sodium cocoyl glutamate as carbon source

Solvent-free solid-phase reaction method: Put a 250 mL beaker containing 2 g of sodium cocoyl glutamate (sodium doped) and 2 g of silica gel (200-300 mesh) into a microwave oven for 5 minutes, and then cool to room temperature. Add 200 mL of ethyl acetate, sonicate and filter with suction to remove silica gel. After the filtrate is concentrated, the product is separated by column chromatography using petroleum ether and ethyl acetate as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

Example 7

One-step synthesis of oil-soluble carbon quantum dots using sodium dodecylbenzene sulfonate as carbon source

Solvent-free solid-phase reaction method: Put a 250 mL beaker containing 2 g of sodium cocoyl glutamate (sodium doped) and 2 g of silica gel (200-300 mesh) into a microwave oven to heat for 20 minutes, and then cool to room temperature. Add 200 mL of ethyl acetate, sonicate and filter with suction to remove silica gel. After the filtrate is concentrated, the product is separated by column chromatography using petroleum ether and ethyl acetate as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

Example 8

One-step synthesis of oil-soluble carbon quantum dots using octadecyltrimethylammonium chloride as carbon source

Solvent-free solid-phase reaction method: A 250 mL beaker containing 2 g of sodium cocoyl glutamate (sodium doped) and 2 g of silica gel (200-300 mesh) is placed in a microwave oven and heated for 18 minutes, and then cooled to room temperature. Add 200 mL of ethyl acetate, sonicate and filter with suction to remove silica gel. After the filtrate is concentrated, the product is separated by column chromatography using petroleum ether and ethyl acetate as eluents to remove insoluble impurities to obtain a carbon quantum dot solution, and then the solution is concentrated to obtain oil-soluble carbon quantum dots.

Performance characterization and comparison

As shown in Figure 3, the carbon quantum dots prepared in Examples 1-6 of the present invention have good solubility in low-polar solvents (such as petroleum ether, ethyl acetate), and in high-polar solvents (such as methanol, water) The solubility is poor, which indicates that the carbon quantum dots prepared by the present invention are oil-soluble carbon quantum dots. However, carbon quantum dots prepared by traditional methods are mostly water-soluble products, and their application in oily environments is greatly restricted. For example, Wang et al. used L-cysteine and galactose as raw materials and refluxed in NaOH solution for different time periods to obtain water-soluble carbon quantum dots with different fluorescent colors; Ping Wang et al. used sodium alginate and ethylenediamine as raw materials to obtain Water-soluble carbon quantum dots. Yang et al. _{prepared N-doped strong fluorescent carbon quantum dots with C 3} N ₄ (carbon nitride) as raw materials; Yaling Wang et al. prepared N, S co-doped carbon quantum dots with ammonium persulfate and ascorbic acid as raw materials ; Monoj Kumar Barman et al. prepared N and P co-doped carbon quantum dots using diethylene triamine, citric acid, and phosphoric acid as raw materials. All the above examples are summarized in Table 1.

Table 1 Carbon source for preparing carbon quantum dots and the properties of the prepared carbon quantum dots

As shown in FIG. 4A, the emission wavelength of the carbon quantum dot solution prepared by the solvothermal method and sucrose fatty acid ester as the carbon source is dependent on the excitation wavelength. With the gradual increase of the excitation wavelength, the emission wavelength gradually red shifts and the fluorescence intensity first increases and then decreases. Under the excitation wavelength of 385nm, it has the largest fluorescence emission intensity, and the best emission wavelength is 447nm. As shown in FIG. 4B, the emission wavelength of the carbon quantum dot solution prepared by the solid phase method and sucrose fatty acid ester as the carbon source is dependent on the excitation wavelength. With the gradual increase of the excitation wavelength, the emission wavelength gradually red shifts and the fluorescence intensity first increases and then decreases. Under the excitation wavelength of 385nm, it has the largest fluorescence emission intensity, and the best emission wavelength is 452nm. As shown in Figure 4C, the results of the transmission electron microscope show that the carbon quantum dots prepared with sucrose fatty acid esters as the carbon source did not agglomerate and were uniformly dispersed on the copper mesh. According to statistics, the particle size of the carbon quantum dots prepared with sucrose fatty acid esters as the carbon source is about 2.66nm. From the high-resolution transmission electron microscope images, a lattice with a pitch of 0.2185nm can be seen, revealing that its internal structure contains graphite Olefin structure. As shown in Figure 4D, under a 365nm ultraviolet lamp, the carbon quantum dot solutions of the same concentration prepared by the solvothermal method (left) and the solid phase method (right) respectively emit blue fluorescence.

As shown in FIG. 5A, the emission wavelength of the carbon quantum dot solution prepared by the solvothermal method and the Span 40 as the carbon source is dependent on the excitation wavelength. With the gradual increase of the excitation wavelength, the emission wavelength gradually red shifts and the fluorescence intensity first increases and then decreases. Under the excitation wavelength of 345nm, it has the largest fluorescence emission intensity, and the best emission wavelength is 388nm. As shown in FIG. 5B, the emission wavelength of the carbon quantum dot solution prepared by the solid phase method and the Span 40 as the carbon source has excitation wavelength dependence. With the gradual increase of the excitation wavelength, the emission wavelength gradually red shifts and the fluorescence intensity first increases and then decreases. Under the excitation wavelength of 400nm, it has the largest fluorescence emission intensity, and the best emission wavelength is 478nm. As shown in FIG. 5C, it can be seen from the results of the transmission electron microscope that the carbon quantum dots prepared with the Span 40 as the carbon source did not agglomerate and were evenly dispersed on the copper mesh. According to statistics, the particle size of the carbon quantum dots prepared with Span 40 as the carbon source is about 2.85nm. From the high-resolution transmission electron microscope images, you can see the lattice with a pitch of 0.2132nm, revealing that its internal structure contains graphene. structure. As shown in Figure 5D, under a 365nm ultraviolet lamp, the carbon quantum dot solutions of the same concentration prepared by the solvothermal method (left) and the solid phase method (right) respectively emit blue fluorescence.

As shown in FIG. 6A, the emission wavelength of the carbon quantum dot solution prepared by the solvothermal method and soybean lecithin as the carbon source is dependent on the excitation wavelength. With the gradual increase of the excitation wavelength, the emission wavelength gradually red shifts and the fluorescence intensity first increases and then decreases. Under the excitation wavelength of 385nm, it has the largest fluorescence emission intensity, and the best emission wavelength is 452nm. As shown in FIG. 6B, the emission wavelength of the carbon quantum dot solution prepared by the solid phase method and soybean lecithin as the carbon source is dependent on the excitation wavelength. With the gradual increase of the excitation wavelength, the emission wavelength gradually red shifts and the fluorescence intensity first increases and then decreases. Under the excitation wavelength of 385nm, it has the largest fluorescence emission intensity, and the best emission wavelength is 463nm. As shown in Figure 6C, the results of the transmission electron microscope show that the carbon quantum dots prepared with soybean lecithin as the carbon source did not agglomerate and were evenly dispersed on the copper mesh. According to statistics, the particle size of the carbon quantum dots prepared with soybean lecithin as the carbon source is about 2.75nm. From the high-resolution transmission electron microscope images, you can see the lattice with a pitch of 0.2057nm, revealing that its internal structure contains graphene. structure. As shown in Figure 6D, under a 365nm ultraviolet lamp, the carbon quantum dot solutions of the same concentration prepared by the solvothermal method (left) and the solid phase method (right) respectively emit blue fluorescence.

As shown in FIG. 7A, the emission wavelength of the carbon quantum dot solution prepared by the solvothermal method and dodecyl betaine as the carbon source is dependent on the excitation wavelength. With the gradual increase of the excitation wavelength, the emission wavelength gradually red shifts and the fluorescence intensity first increases and then decreases. Under the excitation wavelength of 365nm, it has the largest fluorescence emission intensity, and the best emission wavelength is 425nm. TEM image of a carbon quantum dot solution prepared by solvothermal method and dodecyl betaine as a carbon source. As shown in FIG. 7B, the emission wavelength of the carbon quantum dot solution prepared by the solid phase method and dodecyl betaine as the carbon source is dependent on the excitation wavelength. With the gradual increase of the excitation wavelength, the emission wavelength gradually red shifts and the fluorescence intensity first increases and then decreases. Under the excitation wavelength of 405nm, it has the largest fluorescence emission intensity, and the best emission wavelength is 474nm. As shown in Figure 7C, from the results of the transmission electron microscope, it can be seen that the carbon quantum dots prepared with lauryl betaine as the carbon source did not agglomerate and were uniformly dispersed on the copper mesh. According to statistics, the particle size of the carbon quantum dots prepared with lauryl betaine as the carbon source is about 3.22nm. From the high-resolution transmission electron microscope images, you can see the lattice with a pitch of 0.2540nm, revealing its internal structure. Contains graphene structure. As shown in Figure 7D, under a 365nm ultraviolet lamp, the carbon quantum dot solutions of the same concentration prepared by the solvothermal method (left) and the solid phase method (right) respectively emit blue fluorescence.

Traditional methods for preparing carbon quantum dots "from top to bottom", such as laser ablation, oxidation-passivation, and electrochemical methods, all have the problem of wide particle size distribution and easy agglomeration. Sun et al. used the method of laser ablation of graphite powder and cement mixture, which not only consumes large energy and high cost, but also the prepared carbon quantum dots are agglomerated together with a large difference in particle size; and then further use amino-terminated polyethylene glycol to pair Surface passivation is performed to prepare carbon quantum dots with strong fluorescence but still uneven particle size distribution. Lu et al. used an electrochemical method to peel off the graphite electrode placed in the ionic liquid. Due to the randomness and uncertainty of the carbonization process, it was impossible to efficiently control the carbonization process, and the degree of carbonization varied greatly. Carbon quantum dots, carbon nanoribbons and graphene are prepared in one pot. In the case of carbon quantum dots, almost all of the carbon quantum dots are agglomerated together under a transmission electron microscope, and the dispersion is extremely poor. At the same time, "bottom-up" preparation methods such as hydrothermal, solvothermal, and microwave methods also have similar problems. Shin et al. dissolved α, β, γ-cyclodextrin in water and reacted at 160°C for 16 hours. The reaction process takes a long time, and the prepared carbon quantum dots have a particle size distribution of 70-150nm. The obtained carbon quantum dots not only have too wide particle size distribution, but also have a serious agglomeration effect. Wang et al. used a hydrothermal method to dissolve hydrochloric acid-glucosamine in water and react at 140°C for 12 hours to prepare carbon quantum dots of 15-70 nm. Zhang et al. directly carbonized glycerol by microwave, and obtained water-soluble carbon quantum dots with a particle size of 4-25 nm in one step. All the above examples are summarized in Table 2.

Table 2. Methods and characteristics for preparing carbon quantum dots

As shown in Figure 8, the oil-soluble carbon quantum dots prepared by solid-phase reaction method and Tween 80 as raw materials have the strongest excitation and emission wavelengths at 400nm and 475nm respectively; solid-phase reaction method and sodium cocoyl glutamate The oil-soluble carbon quantum dots prepared as raw materials have the strongest excitation and emission wavelengths at 420nm and 483nm, respectively. The strongest excitation and emission wavelengths of oil-soluble carbon quantum dots prepared by solid-phase method and sodium dodecylbenzene sulfonate are at 460nm and 534nm respectively; solid-phase reaction method and octadecyltrimethylammonium chloride The oil-soluble carbon quantum dots prepared as raw materials have the strongest excitation and emission wavelengths at 400nm and 474nm, respectively; at the optimal excitation wavelength, only carbon quantum dots prepared with sodium dodecylbenzene sulfonate as the carbon source are the solution It is yellow fluorescence, and the fluorescence color of the remaining solution is blue.

As shown in Figure 9, the carbon quantum dots prepared by the present invention have significant photostability and resistance to photobleaching. The carbon quantum dot solution prepared by the solvothermal method and dodecyl betaine as the carbon source, the fluorescence intensity comparison chart of the same concentration after one day and six months after the preparation, found that when the excitation wavelength is less than 360nm, the fluorescence intensity is even There is an increase. When the excitation wavelength is greater than 360nm, the fluorescence intensity only drops by about 14%-36%, which has good light stability. In terms of anti-photobleaching, the prepared carbon quantum dot solution was continuously irradiated under an ultraviolet lamp for 10.5 hours, and its fluorescence intensity changes were recorded in sections. It was found that there was almost no change in fluorescence intensity, indicating that the anti-photobleaching performance was very good.

As shown in Figure 10, referring to Example 3, the solvent-free solid-phase reaction method and the oil-soluble carbon quantum dots prepared in large quantities using Span 40 as a carbon source have the same performance as the solvent-free solid-phase reaction method of Example 3. The obtained oil-soluble carbon quantum dots are similar.

As shown in Figure 11, with Span 40 as the carbon source, the carbon quantum dots obtained by the solid-phase method can be used for iron ion detection. When 0.5 mL of Fe ³⁺ solution with a concentration of 0.019 mg/mL was added to 2 mL of carbon quantum dot solution with a concentration of 4 mg/mL, the fluorescence intensity decreased significantly, and the quenching rate reached more than 60%; when Fe ³⁺ solution When the concentration reached 0.31mg/mL, the carbon quantum dot solution was completely quenched, and the quenching rate reached 99%.

Comparative example 1

In the early exploration, the solvothermal method was used to prepare oil-soluble carbon quantum dots. The reaction solution used was large, the heating time was long (more than 2 hours), the required temperature was high (above 180℃), and the reaction feed volume was small (per 50mL non-polar 1g surfactant is added to the solvent). At the same time, if you want to obtain carbon quantum dots with higher fluorescence efficiency, it takes longer to carbonize at a lower temperature. If the time is not enough, the carbonization is not enough; conversely, it takes a shorter time at a higher temperature, and if the time is too long, the carbon source is complete. Carbonization fails.

For example, in an experiment, 5g of sucrose ester was added to 50mL of diphenyl ether, the surfactant was completely dissolved at 100°C, and heated at 180°C for 1 hour. As time passed, black carbonized solid matter gradually adhered to the inner wall of the reaction vessel. After the reaction was completed, the solid matter was filtered out, and the reaction filtrate had extremely weak luminescence. In another experiment, heating at 260°C for 1 hour under the same conditions, there were significantly more ineffective carbonized substances attached to the wall of the reaction vessel than in the same experiment at 180°C. After the reaction was filtered, the filtrate had an enhanced luminescence compared to 180°C.

The heating time at the same temperature is not as long as possible. If the heating time is short, the degree of carbonization will be insufficient, and carbon quantum dots will not be produced; if the heating time is too long, the carbonization will be excessive, and the carbon quantum dots will be transformed into carbon black and deactivated. In a control experiment, the reaction was carried out at 190°C for 1 hour, 2 hours, 3 hours, 5 hours, and 7 hours. The results showed that the carbon quantum dots prepared under 2 to 3 hours had stronger fluorescence intensity, and the fluorescence intensity gradually increased after 3 hours. Weaken. In short, after many experiments, heating time at 190℃-220℃ for 2-3 hours is the best fluorescent property of the prepared carbon quantum dots.

Fig. 12 is a solvothermal method, with sucrose ester as a carbon source, decalin as a solvent, and fluorescence emission spectra of carbon quantum dot solutions obtained with different feed concentrations. It can be concluded that under the same conditions, the feed concentration affects the fluorescence intensity of the obtained carbon quantum dots. If the concentration is too high, the surfactant will not dissolve completely and the dispersion will be uneven, and it will be easier to agglomerate during the heating process, and more black carbonized precipitate will be produced. The yield is low, the particle size is uneven and the fluorescence is not strong; the concentration is too small, the yield is low , The fluorescence performance of the prepared carbon quantum dots is not strong.

Comparative example 2

In the early exploration, a solvent-free solid-phase reaction method was used to prepare oil-soluble carbon quantum dots. For liquid surfactants, they are directly carbonized without adding silica gel, causing all the surfactants to be ejected from the beaker during the carbonization process. For solid surfactants, without the participation of silica gel, after carbonization is completed, the product will stick to the bottom of the beaker and cannot be taken out.

Comparative example 3

In the early exploration, the hydrophilic end of the surfactant is not easy to carbonize dodecyl ammonium sulfate, which is carbonized, the prepared carbon quantum dots have low yield and weak luminescence, low fluorescence intensity, and poor performance. Figure 13 is the fluorescence emission spectrum of carbon quantum dots prepared by solid-phase method and ammonium lauryl sulfate as the carbon source. It can be seen that the emission spectrum is not continuous, and only has weak fluorescence intensity at 340nm and 360nm. The overall luminous performance is relatively poor.

In summary, the invention points and key points of the present invention are: 1. Using a commercial surfactant as a carbon source, one-step synthesis of heteroatom-doped oil-soluble carbon quantum dots; 2. By using the surfactant itself With the steric hindrance of long-chain alkanes, oil-soluble carbon quantum dots with uniform size and not easy to agglomerate are prepared.

Claims

A method for preparing oil-soluble carbon quantum dots, which is characterized in that it mainly includes using a surfactant as the sole carbon source to perform one-step carbonization to obtain oil-soluble carbon quantum dots; the lipophilic end of the surfactant contains 12-30C, and The water-based structure has poor thermal stability and is easily decomposed by heating.
The method for preparing oil-soluble carbon quantum dots according to claim 1, wherein the surfactants include alkyl anionic series, alkyl cationic series, Tween series, Span series, sugar-based series, sugar beet One of alkali series, lecithin series and amino acid series.
The method for preparing oil-soluble carbon quantum dots according to claim 1, wherein the one-step carbonization method includes a solvothermal method and a solvent-free solid phase reaction method.
The method for preparing oil-soluble carbon quantum dots according to claim 3, wherein the solvothermal method mainly comprises the following steps:

(1) Disperse the surfactant in a solvent with a high boiling point and low polarity and use an electric heating plate to heat it to 120-250°C and cool it to room temperature;

(2) Using diatomaceous earth as a medium, petroleum ether and ethyl acetate as eluents, the solvent is separated from the reaction solution, and insoluble impurities are removed, and oil-soluble carbon quantum dots are obtained after concentration.
The method for preparing oil-soluble carbon quantum dots according to claim 3, wherein the solvent-free solid-phase reaction method mainly comprises the following steps:

(1) Stir the surfactant and silica gel uniformly, and use microwave heating for 5-120 minutes;

(2) After adding ethyl acetate, ultrasonication, suction filtration, separation of silica gel;

(3) Using diatomaceous earth as a medium, petroleum ether and ethyl acetate as eluents to remove insoluble impurities, and after concentration, oil-soluble carbon quantum dots are obtained.
The method for preparing oil-soluble carbon quantum dots according to claim 4, characterized in that, in step (1), 2-20 g of surfactant is added to every 50-400 mL of high boiling point and low polarity solvent.
The method for preparing oil-soluble carbon quantum dots according to claim 5, wherein the mass ratio of the surfactant to the silica gel is 1:1-10.
The method for preparing oil-soluble carbon quantum dots according to claim 4, wherein the high-boiling low-polarity solvent includes one of n-alkane series, brominated alkane series, decalin, toluene, and diphenyl methyl ether. Kind or multiple.
The oil-soluble carbon quantum dots obtained are prepared according to the method for preparing oil-soluble carbon quantum dots according to claim 1.