A high-stability carbon dot and its preparation method and application

Abstract

The present invention discloses a highly stable carbon dot, its preparation method, and application. This material is prepared by a solvothermal method using biomass as a precursor and an ionic liquid as a solvent. This material exhibits excellent fluorescence stability, with fluorescence being highly stable in environments with a pH of 1.89-11.82, various ionic solutions, and high-salt solutions. It also exhibits excellent resistance to photobleaching, overcoming the poor stability of existing fluorescent materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical materials and functional materials, and relates to a fluorescent material and a preparation method and application thereof, and specifically to a high-stability carbon dot and a preparation method and application thereof, a fluorescent probe including the carbon dot and a preparation method and application thereof. Background Art

[0002] Fluorescent materials (including organic fluorescent molecules, carbon dots, and quantum dots) are widely used in biomedicine, optical sensing, information encryption, and advanced anti-counterfeiting due to their designable structures, tunable luminescence properties, high fluorescence quantum yields, high sensitivity, and low cost. However, due to their large number of organic functional groups, fluorescent materials often respond to environmental changes (such as light irradiation, ions, and acidity and alkalinity), resulting in limited stability. Therefore, constructing highly stable fluorescent materials whose fluorescence properties remain stable despite environmental changes is extremely challenging.

[0003] Due to their advantages such as low toxicity, good biocompatibility, adjustable luminescence properties, high fluorescence quantum yield, low cost and simple preparation process, carbon dots have been widely used in the fields of bioimaging, sensing, solid-state luminescence, anti-counterfeiting and so on. "Green" carbon dots refer to carbon dots synthesized using "green precursors" (naturally occurring substances, renewable natural products and their derivatives) as carbon sources. Currently, researchers have studied various "green precursors" in an attempt to achieve the preparation of carbon dots with excellent performance using simple, economical, efficient and environmentally friendly methods. However, due to the poor solubility of "green precursors" and the difficulty in designing and modifying the materials, the prepared carbon dots generally have low fluorescence quantum yields and poor stability. Summary of the Invention

[0004] To overcome the deficiencies of the prior art, the present invention proposes a method for preparing carbon dots. The carbon dots prepared by this method have high fluorescence stability, and the fluorescence properties are minimally affected by acids, bases, and salts.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for preparing carbon dots comprises: dissolving biomass in an ionic liquid to obtain a biomass solution, and performing a solvent thermal reaction to obtain carbon dots.

[0007] According to the present invention, the preparation method specifically comprises the following steps:

[0008] 1) dissolving biomass in an ionic liquid to obtain a biomass solution;

[0009] 2) adding a co-solvent to the biomass solution and transferring it to a reactor;

[0010] 3) performing a solvothermal reaction;

[0011] 4) Separating and purifying the product obtained by the reaction to obtain the carbon dots.

[0012] According to the present invention, the cationic group of the ionic liquid is selected from at least one of an alkyl quaternary ammonium salt cationic group, an alkyl quaternary phosphonium salt cationic group, a substituted or unsubstituted imidazolium cationic group, and a substituted or unsubstituted pyridinium cationic group.

[0013] The present invention uses biomass (such as natural high-molecular polysaccharides; specifically, cellulose and its derivatives, starch, chitosan, chitin and alginic acid) as a precursor and an ionic liquid (specifically, the cationic group of the ionic liquid is selected from at least one of an alkyl quaternary ammonium salt cationic group, an alkyl quaternary phosphonium salt cationic group, a substituted or unsubstituted imidazolium cationic group, and a substituted or unsubstituted pyridinium cationic group) as a solvent to prepare the carbon dots through a solvothermal method. The carbon dots prepared based on this method have high fluorescence stability, and the fluorescence properties are minimally affected by acids, bases and salts.

[0014] According to the present invention, the biomass is selected from natural high molecular weight polysaccharides; specifically, at least one selected from starch, cellulose and its derivatives, chitosan or chitin.

[0015] According to the present invention, the degree of polymerization (DP) of the biomass is 200-3000, for example, 220, 250, 300, 400, 500, 600, 700, 800, 900, 1000, 1200, 1500, 2000, 2500, 2800.

[0016] According to the present invention, the cation in the ionic liquid is selected from at least one of an alkyl quaternary ammonium salt cation group, an alkyl quaternary phosphonium salt cation group, a substituted or unsubstituted imidazolium cation group, and a substituted or unsubstituted pyridinium cation group. Specifically, the alkyl quaternary ammonium salt cation group, the alkyl quaternary phosphonium salt cation group, the substituted or unsubstituted imidazolium cation group, and the substituted or unsubstituted pyridinium cation group are as defined above.

[0017] According to the present invention, the anions in the ionic liquid are selected from halogen ions (such as F - 、Cl - Br - or I - ), carboxylate ions (such as formate ions, acetate ions, etc.), and at least one of phosphate ions.

[0018] Specifically, the ionic liquid is selected from at least one of the following ionic liquids: 1-ethyl-3-methylimidazolium chloride ionic liquid, 1-ethyl-3-methylimidazolium bromide ionic liquid, 1-allyl-3-methylimidazolium chloride ionic liquid, 1-allyl-3-methylimidazolium bromide ionic liquid, 1-butyl-3-methylimidazolium chloride ionic liquid, 1-butyl-3-methylimidazolium bromide ionic liquid, 1-ethyl-3-methylimidazolium acetate ionic liquid, 1-allyl-3-methylimidazolium acetate ionic liquid, 1-butyl-3-methylimidazolium acetate ionic liquid, N-ethylpyridinium chloride Ionic liquids, N-ethylpyridinium bromide ionic liquids, 1,3-dimethylimidazolium dimethyl phosphate ionic liquids, 1-ethyl-3-methylimidazolium diethyl phosphate ionic liquids, 3-methylimidazolium formate ionic liquids, N-methylpyridinium formate ionic liquids, 1-ethyl-3-methylimidazolium formate ionic liquids, 1-butyl-3-methylimidazolium formate ionic liquids, tetrabutyl quaternary ammonium chloride, tetraethyl quaternary ammonium chloride, triethyloctyl quaternary ammonium chloride, triethyloctadecyl quaternary ammonium chloride, tetrabutyl quaternary phosphonium chloride, tetraethyl quaternary phosphonium chloride, triethyloctyl quaternary phosphonium chloride, triethyloctadecyl quaternary phosphonium chloride.

[0019] According to the present invention, in the solvent thermal reaction, the concentration of the biomass solution is 0.01 to 10 wt %; the reaction temperature is 140 to 220° C.; and the reaction time is 6 to 48 hours.

[0020] According to the present invention, a co-solvent is selected for the solvothermal reaction; specifically, the co-solvent is selected from at least one of the following organic solvents: N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone, and pyridine.

[0021] According to the present invention, the mass ratio of the co-solvent to the ionic liquid is 0:1-1:1 (ie, the content of the co-solvent is 0-50%).

[0022] The present invention also provides carbon dots prepared by the above-mentioned carbon dot preparation method.

[0023] According to the present invention, the surface of the carbon dots contains nitrogen.

[0024] According to the present invention, the nitrogen element is bound to the surface in the form of at least one of an alkyl quaternary ammonium salt cationic group, an alkyl quaternary phosphonium salt cationic group, a substituted or unsubstituted imidazolium cationic group, and a substituted or unsubstituted pyridinium cationic group.

[0025] According to the present invention, the nitrogen element is bound to the surface in the form of substituted or unsubstituted imidazolium cationic groups.

[0026] According to the present invention, the alkyl quaternary ammonium salt cationic group (-N + The alkyl groups in (R)3) are the same or different and are independently selected from C 1-20 Specifically, the alkyl quaternary ammonium salt cationic group is selected from -N + (CH3)3, -N + (CH2CH3)3, -N + (CH2CH2CH3)3, -N + (CH2CH2CH2CH3)3, -N + (CH2CH2CH2CH2CH3)3 or -N + (C6H5)3.

[0027] According to the present invention, the alkyl quaternary phosphonium salt cationic group (-P + The alkyl groups in (R)3 are the same or different and are independently selected from C 1-20 Specifically, the alkyl quaternary phosphonium salt cationic group is selected from -P + (CH3)3, -P + (CH2CH3)3, -P + (CH2CH2CH3)3, -P + (CH2CH2CH2CH3)3, -P + (C6H5)3, -P + (C6H 11 )3.

[0028] According to the present invention, the substituent of the substituted or unsubstituted imidazolium cationic group may be selected from alkyl or alkenyl groups; specifically, C 1-6 Alkyl or C 2-6 alkenyl; illustratively, the substituent is selected from at least one of -CH3, -CH2CH3, -CH2CH2CH3, -CH(CH3)2, -CH2CH2CH2CH3, -CH(CH3)CH2CH3, -CH2CH(CH3)2, -C(CH3)3, n-pentyl and its isomers, n-hexyl and its isomers, -CH=CH2, -CH=CHCH3, -CH2CH=CH2, 1-butenyl and its isomers, 1-pentenyl and its isomers, 1-hexenyl and its isomers.

[0029] According to the present invention, the substituent of the substituted or unsubstituted pyridinium cationic group may be selected from alkyl or alkenyl groups; specifically, C 1-6 Alkyl or C 2-6alkenyl; illustratively, the substituent is selected from at least one of -CH3, -CH2CH3, -CH2CH2CH3, -CH(CH3)2, -CH2CH2CH2CH3, -CH(CH3)CH2CH3, -CH2CH(CH3)2, -C(CH3)3, n-pentyl and its isomers, n-hexyl and its isomers, -CH=CH2, -CH=CHCH3, -CH2CH=CH2, 1-butenyl and its isomers, 1-pentenyl and its isomers, 1-hexenyl and its isomers.

[0030] According to the present invention, the nitrogen element is connected to the surface of the carbon dots via chemical bonds.

[0031] According to the present invention, the carbon dots are nanomaterials. Specifically, the size of the carbon dots is 1 nm to 100 nm.

[0032] According to the present invention, the carbon dots are fluorescent materials.

[0033] According to the present invention, the carbon dots remain stable in solutions with different pH values ​​and high salt concentrations. Specifically, their fluorescence properties remain stable in solutions with different pH values ​​and high salt concentrations.

[0034] The present invention also provides an application of the carbon dots, which is used in fluorescent probes.

[0035] The present invention also provides a ratiometric fluorescent probe, wherein the fluorescent reference of the probe is the carbon dots.

[0036] The present invention also provides uses of the ratiometric fluorescent probe, which is used in fields such as visualization detection, solid-state luminescence, anti-counterfeiting and encryption.

[0037] Beneficial effects of the present invention

[0038] The present invention discloses a method for preparing carbon dots. Specifically, the present invention uses biomass as a precursor and an ionic liquid as a solvent to prepare the high-stability carbon dots through a solvothermal method.

[0039] The carbon dots provided by the present invention have excellent stability. Their fluorescence is very stable in an environment of pH = 1.89-11.82, various ionic solutions and high salt solutions. They also have excellent resistance to photobleaching, overcoming the problem of poor stability of existing fluorescent materials.

[0040] The carbon dots provided by the present invention can be used as fluorescence references to construct ratiometric fluorescent probes, and have important application prospects in the fields of visual detection, solid-state luminescence, anti-counterfeiting and encryption. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1Transmission electron microscope image and particle size distribution of carbon dots in Example 1. Testing instrument: Hitachi HT7700 transmission electron microscope, accelerating voltage 120 kV.

[0042] Figure 2 X-ray photoelectron spectrometer spectrum of carbon dots in Example 1. Testing instrument: ESCALab250Xi multifunctional photoelectron spectrometer from Thermo Scientific, with the excitation source being monochromated Al-K α X-ray.

[0043] Figure 3 Infrared spectrum and X-ray diffraction spectrum of carbon dots in Example 1. Testing instruments: Nicolet 6700 infrared spectrometer from Thermal Fisher and Empyrean polycrystalline X-ray diffractometer from PANalytical, with the excitation source being monochromatized Cu K α X-ray.

[0044] Figure 4 Visible light and fluorescence photos, and fluorescence emission spectra of carbon dots in buffer solutions of different pH values ​​in Example 1. Camera: Sony α7, excitation wavelength: 365 nm; Testing instrument: Hitachi F7000 fluorescence spectrometer.

[0045] Figure 5 Comparison of fluorescence emission intensities of carbon dots in different metal ions and high-concentration salt solutions in Example 1. Test instrument: Hitachi F7000 fluorescence spectrometer.

[0046] Figure 6 Comparison of fluorescence emission intensity of carbon dots under 10W UV lamp irradiation for 1-10 hours in Example 1. Test instrument: Hitachi F7000 fluorescence spectrometer.

[0047] Figure 7 In Example 6, carbon dots were used as a fluorescence reference to construct a ratiometric fluorescence probe to obtain fluorescence images, fluorescence spectra, and fluorescence intensity changes under different pH environments. Test instrument: Hitachi F7000 fluorescence spectrometer. DETAILED DESCRIPTION

[0048] The technical solutions of the present invention will be described in further detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are encompassed within the scope of protection that the present invention is intended to protect.

[0049] Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by known methods.

[0050] Example 1

[0051] 0.1296 g of cellulose (DP 220) was dissolved in 9.072 g of 1-allyl-3-methylimidazolium chloride ionic liquid (AmimCl), and 3.888 g of DMF was added. The mixture was transferred to a reactor and reacted at 180°C for 12 hours. After the reaction, the mixture was cooled to room temperature, and water was added to the reaction system. The mixture was then dialyzed against water for 48 hours and freeze-dried to obtain the final carbon dots.

[0052] The transmission electron microscope images and particle size distribution of the carbon dots prepared in Example 1 are shown in Figure 2. Figure 1 As shown, the average particle size is 5.57nm, and the X-ray photoelectron spectrometer spectrum is as follows Figure 2 As shown, the infrared spectrum and X-ray diffraction pattern are as shown Figure 3 As shown, it is proved that the surface of carbon dots is rich in N-heterocyclic imidazolium cations.

[0053] When the carbon dots are dispersed in buffer solutions with different pH values, there is no significant change in the fluorescence intensity and fluorescence images. Figure 4 When dispersed in different metal ions and high concentration salt solutions, the fluorescence intensity does not change significantly, as shown in Figure 5 As shown. After 10W UV lamp irradiation for 10 hours, there is no obvious change in fluorescence intensity. Figure 6 It is shown that the prepared carbon dots have good pH stability, salt resistance and photobleaching resistance.

[0054] Example 2

[0055] 1.0 g of starch (DP 400) was dissolved in 20.0 g of 1-butyl-3-methylimidazolium chloride ionic liquid (BmimCl), and 5.0 g of DMAc was added. The mixture was transferred to a reactor and reacted at 160°C for 24 hours. After the reaction, the mixture was cooled to room temperature, and water was added to the reaction system. The mixture was then dialyzed against water for 24 hours and freeze-dried to obtain the final carbon dots.

[0056] Example 3

[0057] 0.5 g of cellulose (DP 600) was dissolved in 24.5 g of 1-butyl-3-methylimidazolium chloride ionic liquid (BmimCl) in a reaction vessel and reacted at 140°C for 48 hours. After the reaction, the mixture was cooled to room temperature and water was added. The mixture was repeatedly extracted with ethyl acetate (at least three times) and rotary evaporated to obtain the final carbon dots.

[0058] Example 4

[0059] 1 g of chitosan (DP 800) was dissolved in 20 g of 1-ethyl-3-methylimidazolium acetate ionic liquid (EmimAc) and transferred to a reactor. The mixture was reacted at 220°C for 6 h. After cooling to room temperature, the reaction solution was precipitated in 400 mL of isopropanol. The precipitate was filtered, washed three times with ethanol, and dried under vacuum to obtain the final carbon dots.

[0060] Example 5

[0061] 0.2 g of cellulose (DP 800) was dissolved in 30 g of 1-allyl-3-methylimidazolium chloride ionic liquid (AmimCl), and 10.0 g of DMF was added. The mixture was transferred to a reactor and reacted at 220°C for 6 h. After the reaction, the mixture was cooled to room temperature, and water was added to the reaction system. The mixture was then dialyzed against water for 48 h and freeze-dried to obtain the final carbon dots.

[0062] After testing, the carbon dots of Examples 2-5 have similar properties to those of Example 1 (test results are consistent with Figure 1-6 The results shown are similar).

[0063] Example 6

[0064] Ratiometric fluorescent probes

[0065] The carbon dots of Examples 1-5 were used as fluorescent references to construct ratiometric fluorescent probes. Specifically, the CA-FITC / CDs ratiometric fluorescent probe was prepared as follows:

[0066] 1.5 mL of CA-FITC / DMF solution of different concentrations was measured, 0.2 mL of CDs / DMF solution (concentration: 10 mg / mL) was added dropwise, and after mixing evenly, 0.3 mL of BR buffer solution with a pH value of 1.89-11.82 was added dropwise. After mixing evenly, fluorescence spectrum testing and visible light and fluorescence photography were performed.

[0067] Upon testing, the probe showed that as the pH value increased, the color of the solution gradually deepened under visible light, and under ultraviolet light, the fluorescence of the solution gradually changed from blue to cyan, green, and bright yellow-green. The fluorescence spectrum showed that as the pH value increased, the yellow-green fluorescence first appeared from nothing, and the fluorescence intensity increased significantly after pH>7.01, and remained stable after pH>9.95. At the same time, as the pH value increased, the ratio of the yellow-green fluorescence intensity to the blue fluorescence intensity (I Green / I Blue ) increases slowly at first and then sharply at pH > 8.96. Adjusting the ratio of CDs to CA-FITC changes the visual color and response range.

[0068] Example 6 Carbon dots as fluorescence reference to construct ratiometric fluorescent probes under different pH environments: fluorescence photos, fluorescence spectra, and fluorescence intensity changes. Figure 7 As shown, the pH response of the ratiometric fluorescent probe was achieved.

[0069] The above describes the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for preparing carbon dots, characterized in that: The preparation method comprises the following steps: 1) dissolving the biomass in the ionic liquid to obtain a biomass solution; 2) Adding co-solvent to the biomass solution and transferring it to the reactor; 3) perform a solvothermal reaction; 4) separating and purifying the product obtained from the reaction to obtain the carbon dots; In step 1), the biomass is selected from at least one of starch and cellulose; the cationic group of the ionic liquid is selected from at least one of substituted or unsubstituted imidazolium cationic groups, and the substituent of the substituted or unsubstituted imidazolium cationic group is selected from C 1-6 Alkyl or C 2-6 Alkenyl; the anion in the ionic liquid is selected from at least one of halogen ions; In step 2), the co-solvent is selected from at least one of the following organic solvents: N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone, and pyridine; In step 3), the reaction temperature of the solvent thermal reaction is 140~220 o C.

2. The preparation method according to claim 1, characterized in that The substituents of the substituted or unsubstituted imidazolium cationic group are selected from at least one of -CH3, -CH2CH3, -CH2CH2CH3, -CH(CH3)2, -CH2CH2CH2CH3, -CH(CH3)CH2CH3, -CH2CH(CH3)2, -C(CH3)3, n-pentyl and its isomers, n-hexyl and its isomers, -CH=CH2, -CH=CHCH3, -CH2CH=CH2, 1-butenyl and its isomers, 1-pentenyl and its isomers, 1-hexenyl and its isomers.

3. The preparation method according to claim 1, characterized in that The ionic liquid is selected from at least one of the following ionic liquids: 1-ethyl-3-methylimidazolium chloride ionic liquid, 1-ethyl-3-methylimidazolium bromide ionic liquid, 1-allyl-3-methylimidazolium chloride ionic liquid, 1-allyl-3-methylimidazolium bromide ionic liquid, 1-butyl-3-methylimidazolium chloride ionic liquid, and 1-butyl-3-methylimidazolium bromide ionic liquid.

4. The preparation method according to claim 1, characterized in that The degree of polymerization (DP) of the biomass is 200-3000.

5. The preparation method according to any one of claims 1 to 4, characterized in that In the solvothermal reaction, the concentration of the biomass solution is 0.01-10 wt %; And / or, the reaction time of the solvent thermal reaction is 6 h to 48 h.

6. A carbon dot, characterized in that: The carbon dots are prepared by the preparation method according to any one of claims 1 to 5.

7. The carbon dots according to claim 6, characterized in that The surface of the carbon dots contains nitrogen.

8. The carbon dots according to claim 7, characterized in that The nitrogen element is bound to the surface in the form of substituted or unsubstituted imidazolium cationic groups.

9. Use of the carbon dots according to any one of claims 6 to 8 in fluorescent probes.

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