Multicolor fluorescent emission lignin-based carbon dots and preparation method and application thereof

Abstract

The application discloses a kind of multicolor fluorescent emission lignin-based carbon dots and preparation method and application thereof.The preparation method of the multicolor fluorescent emission lignin-based carbon dots includes the following steps: (1) lignin and organic acid are added to water, microwave treatment is carried out, and A carbon dots are obtained;(2) A carbon dots and oxidizing agent are added to water to carry out oxidation reaction, and B carbon dots are obtained;(3) B carbon dots are added to water, and esterification reaction is carried out by adding alcohol compound and acidic catalyst, and C carbon dots are obtained;(4) C carbon dots and reducing agent are added to water in proportion to carry out reduction reaction, and D carbon dots are obtained;(5) D carbon dots and alkaline catalyst are added to water, and alkaline hydrolysis reaction is carried out, and the multicolor fluorescent emission lignin-based carbon dots are obtained.The lignin-based carbon dots prepared by the application can realize blue, green, yellow and red fluorescent emission by microwave digestion treatment and surface chemical group regulation method.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of lignin-based nanomaterials, and particularly relates to a multi-color fluorescent emission lignin-based carbon dot and a preparation method and application thereof. BACKGROUND

[0002] As a new type of zero-dimensional carbon nanomaterial, carbon dots have the advantages of good water solubility, low toxicity, good biocompatibility and easy surface modification. With the deepening of the concept of carbon neutralization and the promotion of the goal of sustainable development, it is crucial for the clean production of carbon dots to use low-carbon and environmentally friendly precursors and simple preparation methods to prepare carbon dots with flexible fluorescence regulation characteristics.

[0003] Currently, the precursors of carbon dots are mostly organic synthetic polymers, which have high cost, high toxicity and environmental pollution problems. Lignin, as a natural aromatic heterocyclic macromolecule, has a rich aromatic structure and a rich endogenous heteroatom in its structure. At present, most lignin is used for combustion to produce heat as industrial waste, which causes a certain waste of resources. Therefore, it is necessary to develop a low-carbon and environmentally friendly, simple preparation method of multi-color fluorescent emission lignin-based carbon dots. SUMMARY

[0004] The primary purpose of the present application is to overcome the shortcomings and deficiencies of the prior art and provide a preparation method of multi-color fluorescent emission lignin-based carbon dots. The present application uses natural lignin as raw material and prepares multi-color fluorescent emission lignin-based carbon dots by flexibly regulating the interaction between the surface chemical groups.

[0005] Another purpose of the present application is to provide multi-color fluorescent emission lignin-based carbon dots prepared by the method.

[0006] Still another purpose of the present application is to provide the application of the multi-color fluorescent emission lignin-based carbon dots.

[0007] The purposes of the present application are achieved by the following technical solutions:

[0008] A preparation method of multi-color fluorescent emission lignin-based carbon dots, comprising the following steps:

[0009] (1) lignin and organic acid are added to water, stirred and mixed, then subjected to microwave treatment, filtered, and the filtrate is taken, then subjected to dialysis, freeze-dried to obtain lignin carbon dots (referred to as A carbon dots);

[0010] (2) the lignin carbon dots obtained in step (1) and an oxidizing agent are added to water, stirred and reacted at 40-70 DEG C, after the reaction is completed, dialysis, freeze-drying to obtain oxidized lignin-based carbon dots (referred to as B carbon dots);

[0011] (3) adding the oxidized lignin-based carbon dots obtained in step (2) into water, and then adding an alcohol compound and an acidic catalyst, and reacting at 60-80°C, and after the reaction is completed, dialysis, freeze-drying, to obtain esterified lignin-based carbon dots (referred to as C carbon dots);

[0012] (4) adding the esterified lignin-based carbon dots obtained in step (3) and a reducing agent into water at a mass ratio of 1:(0.2-0.8), and stirring and reacting at 25-50°C, and after the reaction is completed, dialysis, freeze-drying, to obtain reduced lignin-based carbon dots (referred to as D carbon dots);

[0013] (5) adding the reduced lignin-based carbon dots obtained in step (4) and an alkaline catalyst into water, and performing alkaline hydrolysis at 60-100°C, and after the reaction is completed, dialysis, freeze-drying, to obtain multi-color fluorescent emission lignin-based carbon dots (referred to as E carbon dots).

[0014] The lignin in step (1) includes at least one of alkali lignin, dealkali lignin, sodium lignosulfonate, and calcium lignosulfonate.

[0015] The organic acid in step (1) includes at least one of benzoic acid, formic acid, acetic acid, oxalic acid, lactic acid, citric acid, malic acid, salicylic acid, succinic acid, and tartaric acid; preferably at least one of benzoic acid, citric acid, acetic acid, and tartaric acid.

[0016] The mass ratio of the lignin and the organic acid in step (1) is 1:(1-3).

[0017] The amount of water in step (1) is 80-120 mL of water per gram of lignin; preferably 100 mL of water per gram of lignin.

[0018] The microwave treatment in step (1) is preferably microwave treatment in a microwave digestion instrument.

[0019] The microwave treatment in step (1) is preferably microwave treatment at a frequency of 2000 MHz and a power of 1000 W for 60-90 min.

[0020] The dialysis in step (1) is dialysis for 24-72 h using a dialysis bag with a molecular weight cut-off of 500-800 Da; preferably dialysis for 48 h using a dialysis bag with a molecular weight cut-off of 500 Da.

[0021] The dialysis liquid used in steps (1), (2), (3), (4), and (5) is water.

[0022] The freezing drying conditions in steps (1), (2), (3), (4) and (5) are: freezing drying at -50 to -60℃ for 12 to 36h; preferably: freezing drying at -55℃ for 24h.

[0023] The oxidizing agent in step (2) comprises at least one of sulfuric acid (solution), nitric acid (solution), hydrogen peroxide (or hydrogen peroxide solution), potassium periodate, sodium periodate, potassium permanganate and potassium dichromate.

[0024] The concentration of the sulfuric acid solution is preferably 2mol / L.

[0025] The concentration of the nitric acid solution is preferably 5mol / L.

[0026] The mass concentration of the hydrogen peroxide solution is preferably 27%.

[0027] The mass ratio of the lignin carbon dots to the oxidizing agent in step (2) is 1:(1-3); preferably 1:(1-2).

[0028] In steps (2), (3), (4) and (5), the amount of water can be added according to actual needs; in each step, preferably 80-120mL of water is added per gram of the lignin; more preferably 100mL of water is added per gram of the lignin.

[0029] The temperature of the stirring reaction in step (2) is preferably 40-60℃.

[0030] The time of the stirring reaction in step (2) is preferably 1-3h.

[0031] The dialysis in steps (2), (3), (4) and (5) is dialysis for 24-72h using a dialysis bag with a molecular weight cut-off of 800-1500Da; preferably dialysis for 48h using a dialysis bag with a molecular weight cut-off of 1000Da.

[0032] The alcohol compound in step (3) comprises at least one of tert-butyl alcohol, benzyl alcohol, 2-nitrobenzyl alcohol and 4-bromomethylbenzyl alcohol.

[0033] The mass ratio of the oxidized lignin-based carbon dots to the alcohol compound in step (3) is 1:(1-2) (preferably the alcohol compound is in excess).

[0034] The acidic catalyst in step (3) comprises at least one of sulfuric acid (solution), phosphoric acid (solution), benzene sulfonic acid, 2-aminobenzenesulfonic acid, m-aminobenzenesulfonic acid and acrylic acid cation exchange resin.

[0035] The concentration of the sulfuric acid solution is preferably 2mol / L.

[0036] The amount of the acid catalyst added in step (3) is 3% to 7% of the mass of the alcohol compound; preferably 3% to 4% of the mass of the alcohol compound.

[0037] The temperature of the reaction in step (3) is preferably 60 to 70°C.

[0038] The time of the reaction in step (3) is 3 to 6h; preferably 4 to 6h.

[0039] The reducing agent in step (4) includes at least one of sodium borohydride, potassium borohydride, vitamin C, sodium sulfide, sodium citrate, hydrazine hydrate and hydroxylamine hydrochloride; preferably at least one of sodium borohydride and vitamin C.

[0040] In step (4), when the mass ratio of the esterified lignin-based carbon dots and the reducing agent is controlled to be 1:0.8, 1:0.6, 1:0.4 and 1:0.2 respectively, single-fluorescent-emission lignin-based carbon dots can be obtained, which are blue-fluorescent lignin-based carbon dots, green-fluorescent lignin-based carbon dots, yellow-fluorescent lignin-based carbon dots and red-fluorescent lignin-based carbon dots in turn; therefore, by adjusting the mass ratio of the esterified lignin-based carbon dots and the reducing agent to be within the range of 1:(0.2 to 0.8), the product (in powder or solution state) obtained by the present application includes blue-fluorescent, green-fluorescent, yellow-fluorescent and red-fluorescent lignin-based carbon dots, i.e. multi-color fluorescent-emission carbon dots.

[0041] The temperature of the stirring reaction in step (4) is preferably 30 to 50°C.

[0042] The time of the stirring reaction in step (4) is preferably 12 to 24h.

[0043] The basic catalyst in step (5) includes at least one of sodium hydroxide and potassium hydroxide.

[0044] The reduced lignin-based carbon dots and the basic catalyst in step (5) are 1:(1 to 2).

[0045] The temperature of the basic hydrolysis reaction in step (5) is preferably 70 to 80°C.

[0046] The time of the basic hydrolysis reaction in step (5) is 3 to 8h; preferably 5 to 8h.

[0047] A multi-color fluorescent-emission lignin-based carbon dot is prepared by any one of the above-mentioned methods.

[0048] The multi-color fluorescent-emission lignin-based carbon dot is used in the preparation of carbon nanomaterials and / or biological drugs.

[0049] The application of the multi-color fluorescent emission lignin-based carbon dots in the fields of photoelectric conversion and sensing, ion detection, photoconversion film, information encryption or decryption, etc.

[0050] The application has the following advantages and effects relative to the prior art:

[0051] 1. The natural lignin raw material used in the application is low in price, green and environmentally friendly, and renewable.

[0052] 2. The lignin-based carbon dots prepared by the microwave digestion treatment and surface chemical group regulation method are capable of realizing blue, green, yellow and red fluorescent emission, and are simple, efficient and low in carbon emission.

[0053] 3. The carbon dot surface oxidation-reduction method used in the application precisely regulates the relative content of the electron-withdrawing group carboxyl and the electron-donating group hydroxyl, effectively activates the fluorescent emission migration of the lignin-based carbon dots by utilizing the antagonistic effect between the two groups, and realizes the multi-color fluorescent emission of the lignin-based carbon dots.

[0054] 4. The carbon dots prepared in the application can be used in many fields such as biological medicine, photoelectric conversion and sensing, ion detection, photoconversion film, information encryption and decryption, etc. BRIEF DESCRIPTION OF DRAWINGS

[0055] Figure 1 is the fluorescent emission diagram and CIE coordinate diagram of the blue fluorescent lignin-based carbon dots prepared in Example 1 in the application under different excitation wavelengths.

[0056] Figure 2 is the fluorescent emission diagram and CIE coordinate diagram of the green fluorescent lignin-based carbon dots prepared in Example 2 in the application under different excitation wavelengths.

[0057] Figure 3 is the fluorescent emission diagram and CIE coordinate diagram of the yellow fluorescent lignin-based carbon dots prepared in Example 3 in the application under different excitation wavelengths.

[0058] Figure 4 is the fluorescent emission diagram and CIE coordinate diagram of the red fluorescent lignin-based carbon dots prepared in Example 4 in the application under different excitation wavelengths.

[0059] Figure 5 is the fluorescent emission diagram and CIE coordinate diagram of the blue fluorescent lignin-based carbon dots prepared in Comparative Example 1 in the application under different excitation wavelengths.

[0060] Figure 6 is the fluorescent emission diagram and CIE coordinate diagram of the blue fluorescent lignin-based carbon dots prepared in Comparative Example 2 in the application under different excitation wavelengths. DETAILED DESCRIPTION

[0061] The present application will be further described in detail below with reference to examples, but the embodiments of the present application are not limited thereto. Each raw material and reagent used in the following examples can be obtained commercially, except for specifically indicated. The experimental method in the following examples, unless otherwise specified, is generally carried out according to the conventional conditions.

[0062] The lignin raw material described in the present application includes alkali lignin, calcium lignosulfonate, sodium lignosulfonate and dealkali lignin, etc. Among them, the lignin raw material used in the examples and comparative examples is purchased from National Pharmaceutical Reagent Co., Ltd.; the dealkali lignin is purchased from Macklin Reagent Co., Ltd.

[0063] The organic acid described in the present application includes formic acid, benzoic acid, acetic acid, oxalic acid, lactic acid, citric acid, malic acid, salicylic acid, succinic acid, tartaric acid, etc.; the oxidizing agent includes sulfuric acid, nitric acid, hydrogen peroxide, potassium periodate, sodium periodate, potassium permanganate, potassium dichromate, etc.; the acidic catalyst includes sulfuric acid, phosphoric acid, benzene sulfonic acid, 2-aminobenzenesulfonic acid, m-aminobenzenesulfonic acid, acrylic acid cation exchange resin, etc.; the reducing agent includes sodium borohydride, potassium borohydride, vitamin C, sodium sulfide, sodium citrate, hydrazine hydrate, hydroxylamine hydrochloride, etc.; the basic catalyst includes sodium hydroxide, potassium hydroxide, etc. Among them, the organic acid, oxidizing agent, acidic catalyst, reducing agent, basic catalyst used in the examples and comparative examples are of analytical grade purity, purchased from National Pharmaceutical Reagent Co., Ltd.

[0064] The alcohol compound described in the present application includes tert-butyl alcohol, benzyl alcohol, 2-nitrobenzyl alcohol (also known as 2-p-nitrobenzyl alcohol), 4-bromomethylbenzyl alcohol, etc. Among them, the alcohol compound used in the examples and comparative examples is of analytical grade purity, purchased from Macklin Reagent Co., Ltd.

[0065] Example 1

[0066] A preparation method of a lignin-based carbon dot, specifically comprising the following steps:

[0067] (1) 1 g of alkali lignin and 1 g of benzoic acid were added to 100 mL of water for stirring and dispersing treatment, and then transferred to a microwave digestion instrument for microwave treatment for 60 min (frequency 2000 MHz, power 1000 W), and then filtered and separated to obtain a filtrate and a residue; the filtrate was dialyzed in water for 48 h using a 500 Da dialysis bag, and then freeze-dried at -55℃ for 24 h to obtain a lignin carbon dot (A carbon dot);

[0068] (2) The carbon dots A obtained in step (1) were added to 100 mL of water, and then a 5 mol / L nitric acid solution (the mass ratio of carbon dots A to nitric acid was 1:1) was added, and the mixture was stirred at 50°C for 1 h. The reaction solution was then dialyzed in water using a 1000 Da dialysis bag for 48 h, and freeze-dried at -55°C for 24 h to obtain oxidized lignin-based carbon dots (carbon dots B).

[0069] (3) The B carbon dots obtained in step (2) and tert-butanol (mass ratio of 1:1) were added to 100 mL of water, and a 2 mol / L sulfuric acid solution (the amount of sulfuric acid added was 3% of the mass of tert-butanol) was added as a catalyst. After reacting at 60°C for 4 h, the reaction solution was dialyzed in water using a 1000 Da dialysis bag for 48 h, and freeze-dried at -55°C for 24 h to obtain esterified lignin-based carbon dots (C carbon dots);

[0070] (4) The C dots obtained in step (3) and sodium borohydride reducing agent (mass ratio of 1:0.8) were added to 100 mL of water and stirred at 30°C for 12 h. The reaction solution was dialyzed in water using a 1000 Da dialysis bag for 48 h and freeze-dried at -55°C for 24 h to obtain reduced lignin-based carbon dots (D dots).

[0071] (5) The D carbon dots obtained in step (4) and sodium hydroxide (mass ratio of 1:1.2) were added to 100 mL of water and reacted at 70 °C for 5 h. After the alkaline hydrolysis reaction was completed, the reaction solution was dialyzed in water using a 1000 Da dialysis bag for 48 h. After freeze-drying at -55 °C for 24 h, blue fluorescent lignin-based carbon dots (E carbon dots) were obtained.

[0072] Example 2

[0073] (1) 1 g of dealkalized lignin and 2 g of citric acid were added to 100 mL of water for stirring and dispersion treatment, and then transferred to a microwave digester for microwave treatment for 90 min (frequency 2000 MHz, power 1000 W), and then filtered to obtain a filtrate and a filter residue; the filtrate was dialyzed in water using a 500 Da dialysis bag for 48 h, and freeze-dried at -55°C for 24 h to obtain lignin carbon dots (A carbon dots);

[0074] (2) The carbon dots A obtained in step (1) were added to 100 mL of water, and then hydrogen peroxide was added (the mass ratio of carbon dots A to hydrogen peroxide was 1:2), and the reaction was stirred at 40°C for 3 h. The reaction solution was then dialyzed in water using a 1000 Da dialysis bag for 48 h, and freeze-dried at -55°C for 24 h to obtain oxidized lignin-based carbon dots (carbon dots B).

[0075] (3) The B carbon dots obtained in step (2) are added to 100 mL of water with 4-bromomethylbenzyl alcohol (mass ratio 1:1.5) under the catalysis of benzenesulfonic acid (4% of the mass of 4-bromomethylbenzyl alcohol is added) at 70°C for 4 h, and then the reaction solution is dialyzed in water for 48 h using a 1000 Da dialysis bag, and then freeze-dried at -55°C for 24 h to obtain esterified lignin-based carbon dots (C carbon dots);

[0076] (4) The C carbon dots obtained in step (3) are added to 100 mL of water with vitamin C reducing agent (mass ratio 1:0.6) and stirred at 50°C for 24 h, and then the reaction solution is dialyzed in water for 48 h using a 1000 Da dialysis bag, and then freeze-dried at -55°C for 24 h to obtain reduced lignin-based carbon dots (D carbon dots);

[0077] (5) The D carbon dots obtained in step (4) are added to 100 mL of water, and then a potassium hydroxide catalyst (mass ratio of D carbon dots to potassium hydroxide 1:2) is added, and then the mixture is reacted at 80°C for 8 h, and then the reaction solution is dialyzed in water for 48 h using a 1000 Da dialysis bag after the alkaline hydrolysis reaction is completed, and then freeze-dried at -55°C for 24 h to obtain green fluorescent lignin-based carbon dots (E carbon dots).

[0078] Example 3

[0079] (1) 1 g of alkali lignin and 1.5 g of acetic acid are added to 100 mL of water and stirred and dispersed, and then transferred to a microwave digestion instrument for microwave treatment for 80 min (frequency 2000 MHz, power 1000 W), and then filtered and separated to obtain a filtrate and a residue; the filtrate is dialyzed in water for 48 h using a 500 Da dialysis bag, and then freeze-dried at -55°C for 24 h to obtain lignin carbon dots (A carbon dots);

[0080] (2) The A carbon dots obtained in step (1) are added to 100 mL of water, and then potassium permanganate (mass ratio of potassium permanganate to A carbon dots 1:1) is added, and then the mixture is stirred at 60°C for 1 h; the reaction solution is dialyzed in water for 48 h using a 1000 Da dialysis bag, and then freeze-dried at -55°C for 24 h to obtain oxidized lignin-based carbon dots (B carbon dots);

[0081] (3) The B carbon dots obtained in step (2) are added to 100 mL of water with 2-p-nitrobenzyl alcohol (mass ratio 1:1.2) under the catalysis of phosphoric acid (4% of the mass of 2-p-nitrobenzyl alcohol is added) at 70°C for 4 h, and then the reaction solution is dialyzed in water for 48 h using a 1000 Da dialysis bag, and then freeze-dried at -55°C for 24 h to obtain esterified lignin-based carbon dots (C carbon dots);

[0082] (4) The C-carbon dots obtained in step (3) were added to 100 mL of water with sodium borohydride reducing agent (mass ratio 1:0.4), and stirred at 40°C for 18 h; the solution after reaction was dialyzed in water for 48 h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain reduced lignin-based carbon dots (D-carbon dots);

[0083] (5) The D-carbon dots obtained in step (4) were added to 100 mL of water, and then sodium hydroxide catalyst (mass ratio of D-carbon dots to sodium hydroxide 1:1.5) was added, and reacted at 70°C for 5 h; after the completion of the alkaline hydrolysis reaction, the solution after reaction was dialyzed in water for 48 h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain yellow fluorescent lignin-based carbon dots (E-carbon dots).

[0084] Example 4

[0085] (1) 1 g of sodium lignosulfonate and 3 g of tartaric acid were added to 100 mL of water and stirred and dispersed, and then transferred to a microwave digestion instrument for microwave treatment for 80 min (frequency 2000 MHz, power 1000 W), and then filtered and separated to obtain a filtrate and a residue; the filtrate was dialyzed in water for 48 h using a 500 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain lignin carbon dots (A-carbon dots);

[0086] (2) The A-carbon dots obtained in step (1) were added to 100 mL of water, and then a 2 mol / L sulfuric acid solution was added (mass ratio of A-carbon dots to sulfuric acid 1:1.5), and stirred at 50°C for 3 h; then the solution after reaction was dialyzed in water for 48 h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain oxidized lignin-based carbon dots (B-carbon dots);

[0087] (3) The B-carbon dots obtained in step (2) were added to 100 mL of water with tert-butyl alcohol (mass ratio 1:2), and reacted at 70°C for 6 h under the catalysis of 2-aminobenzenesulfonic acid (added amount 4% of the mass of tert-butyl alcohol); then the solution after reaction was dialyzed in water for 48 h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain esterified lignin-based carbon dots (C-carbon dots);

[0088] (4) The C-carbon dots obtained in step (3) were added to 100 mL of water with vitamin C reducing agent (mass ratio 1:0.2), and stirred at 50°C for 24 h; then the solution after reaction was dialyzed in water for 48 h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain reduced lignin-based carbon dots (D-carbon dots);

[0089] (5) The D carbon dots obtained in step (4) and potassium hydroxide (mass ratio 1:2) were added to 100 mL of water, and reacted at 70°C for 6 h. After the completion of the alkaline hydrolysis reaction, the reaction solution was dialyzed in water for 48 h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain red fluorescent lignin-based carbon dots (E carbon dots).

[0090] Comparative Example 1

[0091] In the present comparative example, the mass ratio of C carbon dots to reducing agent was 1:0.1, and the rest of the preparation process was the same as the above examples. The specific preparation method is as follows:

[0092] (1) 1 g of alkali lignin and 1 g of benzoic acid were added to 100 mL of water and stirred and dispersed, then transferred to a microwave digestion instrument for microwave treatment for 60 min (frequency 2000 MHz, power 1000 W), then filtered and separated to obtain filtrate and residue; the filtrate was dialyzed in water for 48 h using a 500 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain lignin carbon dots (A carbon dots);

[0093] (2) The A carbon dots obtained in step (1) were added to 100 mL of water, and 5 mol / L nitric acid solution was added (mass ratio of A carbon dots to nitric acid was 1:1), and stirred at 50°C for 1 h; the reaction solution was dialyzed in water for 48 h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain oxidized lignin-based carbon dots (B carbon dots);

[0094] (3) The B carbon dots obtained in step (2) and tert-butyl alcohol (mass ratio 1:1) were added to 100 mL of water, and 2 mol / L sulfuric acid solution was added (3% of the mass of tert-butyl alcohol) under the catalysis of concentrated sulfuric acid solution, and reacted at 60°C for 4 h; the reaction solution was dialyzed in water for 48 h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain esterified lignin-based carbon dots (C carbon dots);

[0095] (4) The C carbon dots obtained in step (3) and sodium borohydride reducing agent (mass ratio 1:0.1) were added to 100 mL of water, and stirred at 30°C for 12 h; the reaction solution was dialyzed in water for 48 h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24 h to obtain reduced lignin-based carbon dots (D carbon dots);

[0096] (5) The D carbon dots obtained in step (4) and sodium hydroxide (mass ratio of 1:1.2) were added to 100 mL of water, and reacted at 70°C for 5h. After the completion of the alkaline hydrolysis reaction, the reaction solution was dialyzed in water for 48h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24h to obtain blue fluorescent lignin-based carbon dots (E carbon dots).

[0097] Comparative Example 2

[0098] In the present comparative example, the mass ratio of C carbon dots to reducing agent was 1:1, and the rest of the preparation process was the same as the above examples. The specific preparation method is as follows:

[0099] (1) 1g of alkali lignin and 1g of benzoic acid were added to 100 mL of water and stirred and dispersed, then transferred to a microwave digestion instrument for microwave treatment for 60 min (frequency 2000 MHz, power 1000 W), then filtered and separated to obtain filtrate and residue; the filtrate was dialyzed in water for 48h using a 500 Da dialysis bag, and freeze-dried at -55°C for 24h to obtain lignin carbon dots (A carbon dots);

[0100] (2) The A carbon dots obtained in step (1) were added to 100 mL of water, and 5 mol / L nitric acid solution was added (mass ratio of A carbon dots to nitric acid was 1:1), and stirred at 50°C for 1h; the reaction solution was dialyzed in water for 48h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24h to obtain oxidized lignin-based carbon dots (B carbon dots);

[0101] (3) The B carbon dots obtained in step (2) and tert-butyl alcohol (mass ratio of 1:1) were added to 100 mL of water, and a 2 mol / L sulfuric acid solution was added (the amount of sulfuric acid added was 3% of the mass of tert-butyl alcohol) to catalyze the reaction at 60°C for 6h; the reaction solution was dialyzed in water for 48h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24h to obtain esterified lignin-based carbon dots (C carbon dots);

[0102] (4) The C carbon dots obtained in step (3) and sodium borohydride reducing agent (mass ratio of 1:1) were added to 100 mL of water, and stirred at 30°C for 12h; the reaction solution was dialyzed in water for 48h using a 1000 Da dialysis bag, and freeze-dried at -55°C for 24h to obtain reduced lignin-based carbon dots (D carbon dots);

[0103] (5) The D carbon dots obtained in step (4) and sodium hydroxide (mass ratio of 1:1.2) were added to 100 mL of water and reacted at 70 °C for 6 h. After the alkaline hydrolysis reaction was completed, the reaction solution was dialyzed in water using a 1000 Da dialysis bag for 48 h. After freeze-drying at -55 °C for 24 h, blue fluorescent lignin-based carbon dots (E carbon dots) were obtained.

[0104] Effect embodiment

[0105] The fluorescence emission spectra of the fluorescent lignin-based carbon dots prepared in Examples 1 to 4 and Comparative Examples 1 to 2 at different excitation wavelengths were measured using a fluorescence spectrum analyzer and converted into CIE coordinates. Figures 1-6 As shown: Figure 1 It shows that the blue fluorescent lignin-based carbon dots prepared in Example 1 of the present invention have an emission peak at 410 nm under the optimal excitation wavelength of 340 nm, and the CIE coordinates are (0.15, 0.14); Figure 2 It shows that the green fluorescent lignin-based carbon dots prepared in Example 2 of the present invention have an emission peak at 495 nm under the optimal excitation wavelength of 360 nm, and the CIE coordinates are (0.26, 0.41); Figure 3 It shows that the yellow fluorescent lignin-based carbon dots prepared in Example 3 of the present invention have an emission peak at 577 nm under the optimal excitation wavelength of 360 nm, and the CIE coordinates are (0.51, 0.49); Figure 4 The red fluorescent lignin-based carbon dots prepared in Example 4 of the present invention exhibited an emission peak at 577 nm at an optimal excitation wavelength of 390 nm, with CIE coordinates of (0.71, 0.29). The results indicate that, during the preparation process, when the mass ratio of C carbon dots to reducing agent was between 0.2 and 0.8, the lignin-based carbon dots achieved flexible control over their fluorescence emission from blue, green, yellow, to red. This is due to the antagonistic effect between the electron-withdrawing carboxyl groups and the electron-donating hydroxyl groups on the carbon dot surface, which promotes the red shift of the carbon dot fluorescence emission.

[0106] And in Figure 5 In the comparative example 1 (the mass ratio of C dots to reducing agent is 1:0.1), the fluorescent lignin-based carbon dots prepared still emit blue fluorescence at the optimal excitation wavelength of 370 nm, with an emission peak at 421 nm and a CIE coordinate of (0.15, 0.17); Figure 6 In the comparative example 2 (the mass ratio of C carbon dots to reducing agent is 1:1), the emission peak of the fluorescent lignin-based carbon dots prepared at the optimal excitation wavelength of 360 nm is at 432 nm, and the CIE coordinates are (0.14, 0.14), which belongs to the blue area.

[0107] The above results show that when the surface reduction degree of the carbon dots is too low (mass ratio is less than 1:0.2) or too high (mass ratio is higher than 1:0.8), the surface structure of the carbon dots is single, a single carboxyl or hydroxyl related emission state is generated, and it is difficult to realize long-wavelength fluorescent emission of the carbon dots, and the carbon dots still present a single common blue fluorescence. Therefore, within the protection scope of the present application, through a convenient surface chemical structure regulation strategy, the antagonism between the electron-withdrawing carboxyl group and the electron-donating hydroxyl group is utilized to realize multi-color fluorescent emission of a single lignin carbon dot, and technical means can be provided for clean production and development of the carbon dots.

[0108] The above examples are preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods, and are all included in the protection scope of the present application.

Claims

1. A method for preparing multicolor fluorescent emitting lignin-based carbon dots, characterized in that: The steps include: (1) adding lignin and an organic acid to water, stirring and mixing, and then subjecting the mixture to microwave treatment, filtering, collecting the filtrate, dialyzing, and freeze-drying to obtain lignin carbon dots; (2) adding the lignin carbon dots obtained in step (1) and an oxidant to water, stirring and reacting at 40-70° C., dialyzing, and freeze-drying after the reaction is completed to obtain oxidized lignin-based carbon dots; (3) adding the oxidized lignin-based carbon dots obtained in step (2) to water, then adding an alcohol compound and an acidic catalyst, and reacting at 60-80° C. After the reaction is completed, dialyzing and freeze-drying are performed to obtain esterified lignin-based carbon dots; (4) adding the esterified lignin-based carbon dots obtained in step (3) and the reducing agent to water in a mass ratio of 1:0.2-0.8, stirring and reacting at 25-50° C., dialyzing, and freeze-drying after the reaction to obtain reduced lignin-based carbon dots; (5) Adding the reduced lignin-based carbon dots obtained in step (4) and the alkaline catalyst into water, performing alkaline hydrolysis reaction at 60-100° C., dialyzing, and freeze-drying after the reaction is completed to obtain multi-color fluorescent emitting lignin-based carbon dots.

2. The method according to claim 1, wherein: The lignin in step (1) comprises at least one of alkali lignin, dealkalized lignin, sodium lignin sulfonate and calcium lignin sulfonate; The reducing agent described in step (4) includes at least one of sodium borohydride, potassium borohydride, vitamin C, sodium sulfide, sodium citrate, hydrazine hydrate and hydroxylamine hydrochloride.

3. The method according to claim 1, wherein: The organic acid in step (1) comprises at least one of benzoic acid, formic acid, acetic acid, oxalic acid, lactic acid, citric acid, malic acid, salicylic acid, succinic acid and tartaric acid; The oxidant in step (2) comprises at least one of sulfuric acid, nitric acid, hydrogen peroxide, potassium periodate, sodium periodate, potassium permanganate and potassium dichromate; The alcohol compound in step (3) includes at least one of tert-butyl alcohol, benzyl alcohol, 2-nitrobenzyl alcohol and 4-bromomethylbenzyl alcohol; The acidic catalyst in step (3) comprises at least one of sulfuric acid, phosphoric acid, benzenesulfonic acid, 2-aminobenzenesulfonic acid, m-aminobenzenesulfonic acid and acrylic acid cation exchange resin; The alkaline catalyst in step (5) includes at least one of sodium hydroxide and potassium hydroxide.

4. The method according to claim 1, wherein: The mass ratio of lignin to organic acid in step (1) is 1:1-3; The mass ratio of the lignin carbon dots to the oxidant in step (2) is 1:1-3; The mass ratio of the oxidized lignin-based carbon dots to the alcohol compound in step (3) is 1:1-2; The amount of the acidic catalyst added in step (3) is 3% to 7% of the mass of the alcohol compound; The reduced lignin-based carbon dots and the alkaline catalyst described in step (5) are in a ratio of 1:1 to 2.

5. The method according to claim 1, wherein: The dialysis in step (1) is performed using a dialysis bag with a molecular weight cut-off of 500 to 800 Da for 24 to 72 hours; The dialysis in steps (2), (3), (4) and (5) is performed using a dialysis bag with a molecular weight cut-off of 800 to 1500 Da for 24 to 72 hours; In steps (1), (2), (3), (4) and (5), the dialysate used for dialysis is water.

6. The method according to claim 1, wherein: The microwave treatment conditions in step (1) are: frequency 2000 MHz, power 1000 W, and microwave treatment time 60 to 90 min; The freeze-drying conditions described in steps (1), (2), (3), (4) and (5) are: freeze-drying at -50 to -60°C for 12 to 36 hours.

7. The method according to claim 1, wherein: The stirring reaction time in step (2) is 1 to 3 hours; The reaction time in step (3) is 3 to 6 hours; The stirring reaction time in step (4) is 12 to 24 hours; The alkaline hydrolysis reaction time in step (5) is 3 to 8 hours.

8. A multicolor fluorescent lignin-based carbon dot, characterized by: It is prepared by the method according to any one of claims 1 to 7.

9. Use of the multicolor fluorescent emitting lignin-based carbon dots according to claim 8 in the preparation of carbon nanomaterials and / or biopharmaceuticals.

10. Use of the multicolor fluorescent emitting lignin-based carbon dots according to claim 8 in photoelectric conversion and sensing, ion detection, photoconversion membrane, and information encryption or decryption.

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