JP2021088477A - Carbon quantum dot, and method of manufacturing the same - Google Patents

Abstract

To provide a carbon quantum dot capable of more widely adjusting luminous color; and further, to provide a method of manufacturing such a carbon quantum dot simply and efficiently by using safe and inexpensive raw material.SOLUTION: There is provided a method of manufacturing a carbon quantum dot exhibiting visible light-emitting ability, the method comprising a step (1) of preparing a solution containing (A) lignin or its carbonized product, (B) an aromatic compound comprising a hydroxy group and an amino group, and (C) aliphatic polyamine; and a step (2) of heating and carbonizing the solution prepared in the step (1).SELECTED DRAWING: Figure 1

Description

The present invention relates to carbon quantum dots (CQD) and a method for producing the same, and more particularly to carbon quantum dots using lignin as a raw material and a method for producing the same.

In recent years, quantum dots have been attracting attention as a light emitting material having extremely high brightness and being less likely to fade with light as compared with general organic dyes and fluorescent proteins. Quantum dots are ultrafine nanoparticles usually having a diameter of about 0.5 to about 100 nm, particularly about 2 to 10 nm, and have a structure that three-dimensionally confine electrons. By controlling the particle size of the quantum dots, the band gap can be controlled and the emission wavelength (color) can be adjusted.

As typical quantum dots, ultrafine nanoparticles composed of CdSe and CdTe of the genus II-VI, CuCl of the genus I-VII, InAs of the genus III-V, Si and C of the genus IV, etc. can be exemplified. Quantum dots containing transition metals such as CdSe cannot be applied to the biotechnology field due to concerns about toxicity and the like. On the other hand, from the viewpoints of biocompatibility, low toxicity, low environmental load, stability of raw material supply and cost reduction, particle dots composed of carbon, that is, carbon quantum dots are attracting attention.

As a method for producing such carbon quantum dots, for example, a production method in which a carbon target is laser ablated and then chemically treated, a method for producing from candle soot, a production method for chemically treating graphite oxide, and a fullerene are used. A method for producing from the conversion reaction of graphite and a method for chemically treating cheaper carbon raw materials such as carbon fiber and activated carbon have been reported. In these methods, the carbon raw material is chemically treated with sulfuric acid, nitric acid or a mixed acid thereof, so that the production conditions are very strict. Furthermore, after the reaction, it is necessary to neutralize the strong acid with a large amount of alkali. Therefore, there is a demand for a method for producing carbon quantum dots more easily and efficiently using safe and inexpensive raw materials.

Patent Document 1 discloses a method for producing luminescent nanocarbon, in which a positively charged citric acid solution and a negatively charged ethylenediamine solution are electrosprayed to heat and carbonize the obtained droplets. (See, for example, claim 1, paragraphs 0026 to 0027, paragraph 0030, etc.).

Patent Document 2 discloses a method for producing carbon quantum dots, which comprises mixing a carbon material such as activated carbon with oxygen peroxide and decomposing carbon in the carbon material by oxygen peroxide (for example, claim). Item 1, paragraph 0029, etc.).

Patent Document 3 discloses a method for producing carbon quantum dots, which comprises producing a solution containing (1) (A) a polyphenol and (B) an amine compound; and (2) heating and carbonizing the solution. (See, for example, claim 1, paragraphs 0014 to 0018, etc.).
Etc.).

Non-Patent Document 1 discloses a method for producing blue fluorescent carbon dots by a hydrothermal reaction using a lignin derivative.

Non-Patent Document 2 discloses a method for producing yellow fluorescent carbon dots by a hydrothermal reaction using a lignin derivative.

JP 2015-174945 Japanese Unexamined Patent Publication No. 2014-133685 JP-A-2018-35035

Sudheer Rai et. Al., “Lignin derived reduced fluorescence carbon dots with theranostic approaches: Nano-drug-carrier and bioimaging”, Journal of Luminescence, 190, 492-503 (2017).

Aye AyeMyint et. Al., “Water-soluble, lignin-derived carbon dots with high fluorescent emissions and their applications in bioimaging”, Journal of Industrial and Engineering Chemistry, 66, 387-395 (2018)

As described above, in the conventional method for producing carbon quantum dots, it is necessary to chemically treat the carbon raw material with sulfuric acid, nitric acid or a mixed acid thereof in order to decompose carbon. Therefore, there is a need for a further method for producing carbon quantum dots easily and efficiently using safe and inexpensive raw materials.

Further, from the viewpoint of effective utilization of carbon materials, there is a demand for a simple and efficient method for producing carbon quantum dots using biomass as a carbon material.

In the carbon quantum dots disclosed in Patent Documents 1 to 3 and Non-Patent Documents 1 and 2, emission colors up to blue, blue-green, and yellow have been reported. However, carbon quantum dots having orange or red emission colors have not yet been developed. Therefore, carbon quantum dots of various emission colors are required.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide carbon quantum dots capable of broadly adjusting the emission color. Another object of the present invention is to provide a simple and efficient method for producing such carbon quantum dots using safe and inexpensive raw materials.

As a result of diligent studies, the present inventors have obtained (A) a lignin or a carbonized product thereof, (B) an aromatic compound having a hydroxy group and an amino group, and (C) a carbide of a mixture containing an aliphatic polyamine. It has been found that the generated carbon quantum dots can be a multicolor light emitting material of blue, green, or orange. Further, the carbon quantum dots are safe in that a solution containing (A) lignin or a carbonized product thereof, (B) an aromatic compound having a hydroxy group and an amino group, and (C) an aliphatic polyamine is heated and carbonized. Moreover, it has been found that it can be produced by a simple and efficient method at low cost.

That is, the present invention provides the following carbon quantum dots and a method for producing the same as various embodiments thereof.
[1] A carbon quantum dot which is a carbide of a mixture containing (A) a lignin or a carbonized product thereof, (B) an aromatic compound having a hydroxy group and an amino group, and (C) an aliphatic polyamine.
[2] The carbon quantum dot according to the above [1], wherein the (A) lignin contains one or more selected from the group consisting of lignin sulfonic acid, kraft lignin, soda lignin, and organosolvrignin. ..
[3] The above [1] or [2], wherein the aromatic compound having a hydroxy group and an amino group (B) contains one or more selected from the group consisting of diaminophenol, dopamine, and tyrosine. The carbon quantum dots described in.
[4] The above [1] to the above [1], wherein the (C) aliphatic polyamine contains one or more selected from the group consisting of ethylenediamine, diaminobutane, diaminohexane, polyethyleneimine, polyallylamine, and polyamideamine. The carbon quantum dot according to any one of [3].
[5] Prepare a solution containing (1) (A) lignin or a carbide thereof, (B) an aromatic compound having a hydroxy group and an amino group, and (C) an aliphatic polyamine; and (2) the above (1). ) To heat and carbonize the solution prepared in
A method for producing carbon quantum dots, including.
[6] The production method according to the above [5], wherein the (A) lignin contains one or more selected from the group consisting of lignin sulfonic acid, kraft lignin, soda lignin, and organosolvrignin.
[7] The above [5] or [6], wherein the aromatic compound having a hydroxy group and an amino group (B) contains one or more selected from the group consisting of diaminophenol, dopamine, and tyrosine. The manufacturing method described in.
[8] The above [5] to the above [5], wherein the (C) aliphatic polyamine contains one or more selected from the group consisting of ethylenediamine, diaminobutane, diaminohexane, polyethyleneimine, polyallylamine, and polyamideamine. The method for producing carbon quantum dots according to any one of [7].
[9] The production method according to any one of the above [5] to [8], wherein the treatment of (2) is performed by irradiating with microwaves and / or by sealing and heating.
[10] The production method according to any one of [5] to [9] above, wherein the treatment of (2) is carried out by a solvothermal method using an organic solvent.

According to the present invention, it is possible to provide carbon quantum dots having a blue, green, or orange visible light emitting ability.
Further, according to the present invention, carbon quantum dots can be produced safely, inexpensively, easily and efficiently.

FIG. 1 is a diagram showing the results of ultraviolet-visible (UV-vis) absorption spectrum measurement and fluorescence spectrum measurement of Examples 2 to 4. FIG. 2 is a diagram showing the results of FT-IR spectrum measurement of Examples 2 to 4. FIG. 3-1 is a diagram showing the results of investigating the pH dependence of the zeta potential of the CQD aqueous solution of Example 2 (Blue). FIG. 3-2 is a diagram showing the results of investigating the pH dependence of the zeta potential of the CQD aqueous solution of Example 3 (Green). FIG. 3-3 is a diagram showing the results of investigating the pH dependence of the zeta potential of the CQD aqueous solution of Example 4 (Orange). FIG. 4 is a diagram showing images of CQDs of Examples 2 to 4 observed with an atomic force microscope (AFM). FIG. 5 is a diagram showing an X-ray diffraction pattern of the CQDs of Examples 2 to 4. FIG. 6 is a diagram showing the measurement results of the UV-Vis spectrum and the fluorescence spectrum of the aqueous solution obtained in Example 5. FIG. 7 is a diagram showing the assumed results of the UV-Vis spectrum and the fluorescence spectrum of the aqueous solution obtained in Example 6. FIG. 8 is a diagram showing the measurement results of the UV-Vis spectrum and the fluorescence spectrum of the aqueous solution obtained in Example 7. FIG. 9 is a diagram showing the measurement results of the UV-Vis spectrum and the fluorescence spectrum of the aqueous solution obtained in Example 8. FIG. 10 is a diagram showing the measurement results of the UV-Vis spectrum and the fluorescence spectrum of the aqueous solution obtained in Example 9. FIG. 11 is a diagram showing the measurement results of the UV-Vis spectrum and the fluorescence spectrum of the aqueous solution obtained in Comparative Example 1.

<1. Carbon quantum dots ＞
The present invention provides carbon quantum dots. The carbon quantum dots of the present invention have a blue, green, or orange visible light emitting ability. That is, it can be a carbon quantum dot having a light emitting ability in the wavelength range of about 450 nm to 620 nm.

In one embodiment of the invention, the carbon quantum dots are
(A) Lignin or its carbonized product,
It is a carbide of a mixture containing (B) an aromatic compound having a hydroxy group and an amino group, and (C) an aliphatic polyamine.

In the present invention, the component (A) "lignin" refers to a compound having a polyphenol structure, which exists as a main component of wood, bamboo, stems and other woody plants together with cellulose and hemicellulose, and is a target carbon. As long as quantum dots can be obtained, there are no particular restrictions. Examples of lignin include lignin sulfonic acid obtained from sulfite cooking effluent, kraft lignin obtained from kraft cooking effluent, soda lignin obtained from soda cooking effluent, organosolvrignin obtained by the organosolv method, and NWL. (Fractured lignin), blasted lignin, lignin extracted from wood with an organic solvent, and the like can be mentioned.

In the present invention, the term "lignin" may include a modified product (lignin derivative) having a basic skeleton of lignin. Examples of modified products (lignin derivatives) having a basic lignin skeleton include lignin sulfonic acid (including partially desulfonated lignin sulfonic acid, lignin sulfonic acid purified by UF (ultra-extrafiltration) treatment, etc.), and craft Examples include lignin, soda lignin, and organosorb lignin.

Examples of the lignin that can be used in the present invention include preferably lignin sulfonic acid, kraft lignin, soda lignin, organosolvrignin and the like, more preferably lignin sulfonic acid, kraft lignin and the like, and further preferably lignin. Examples include sulfonic acid. The lignin may be one kind or a mixture of two or more kinds.

The lignin of the component (A) may be carbonized in advance or preliminarily. The carbonized product of lignin may be completely carbonized or partially carbonized. The carbonized product of lignin used as the component (A) may be referred to as "carbonized lignin".

As the component (A), at least one of lignin and its carbonized product may be contained, and both lignin and its carbonized product may be contained.

In the present invention, the "aromatic compound having a hydroxy group and an amino group" of the component (B) means a compound having at least one amine group and one hydroxyl group, which is liquid or solid at normal temperature and pressure. It has the property of reacting with lignin or a carbonized product thereof and ethylenediamine, and is not particularly limited as long as the desired carbon quantum dots can be obtained. The aromatic compound of the component (B) is preferably safe and inexpensive. As the aromatic compound of the component (B), for example, diaminophenol, dopamine, tyrosine and the like are preferable, and diaminophenol and the like are more preferable. The aromatic compound of the component (B) may be one kind or a mixture of two or more kinds.

In the present invention, the "aliphatic polyamine" of the component (C) refers to an aliphatic compound having two or more amino groups or imino groups. Aliphatic polyamines may be acyclic or cyclic. Also, the aliphatic polyamine may be saturated or unsaturated. Further, the aliphatic polyamine may be branched. Preferred examples of the aliphatic polyamine include diamines such as ethylenediamine, diaminobutane and diaminohexane, polyvalent amines such as polyethyleneimine, polyallylamine and polyamideamine, and more preferably ethylenediamine and polyethyleneimine.

The number average molecular weight of the aliphatic amine that can be used as the component (C) can be preferably 300 to 100,000, more preferably 300 to 10000, and even more preferably 300 to 2000.

As an embodiment thereof, the carbon quantum dots of the present invention may have a visible light emitting ability from blue to green or orange. Among these, carbon quantum dots that emit blue light may have a high fluorescence quantum yield.

The fluorescence quantum yield of the carbon quantum dots of the present invention can be preferably 1% or more, more preferably 5% or more, still more preferably 10, 15, 18, or 20% or more.

The carbon quantum dots of the present invention can be a brown solution in one embodiment. It is preferable that the solution containing carbon quantum dots has a wide range of absorption from the near infrared region of 300 to 800 nm to the ultraviolet region.

_{In the IR spectrum, it is preferable that a peak indicating the presence of an amino group (-NH 2} ), a hydroxy group (-OH) and a carboxy group (-COOH) is observed on the surface of the carbon quantum dot. It is preferable to observe blue, green, orange (or red) visible light emission with excitation light of 300 to 480 nm.

When carbon quantum dots are observed using an atomic force microscope (AFM) and the height of the particles observed in the cross-sectional view of the AFM image is taken as the size of the carbon quantum dots, the size of the carbon quantum dots is 1 to 1. It is preferably 500 nm, more preferably 1 to 100 nm, preferably 1 to 50 nm, and particularly preferably 1 to 10 nm.

In various embodiments of the carbon quantum dots of the present invention, they can be used in the technical fields in which the quantum dots are conventionally used, for example, solar cells, displays, security inks, anti-counterfeiting, quantum dot lasers, bioimaging, etc. It can be used in technical fields such as biomarkers, medical imaging devices (imaging of cancer cells, protein analysis, cell tracking, etc.), lighting including LEDs, photocatalysts, and the like.

Since the carbon quantum dots of the present invention are mainly made of lignin and are not manufactured from materials of concern for toxicity such as quantum dots containing transition metals such as CdSe, the quantum dots containing such transition metals are not produced. Higher safety than dots. Therefore, the carbon quantum dots of the present invention can be applied to fields where high safety is required. Applications in fields requiring high safety include, for example, bioimaging, biomarkers, medical imaging devices (cancer cell imaging, protein analysis, cell tracking, etc.).

<2. Manufacturing method of carbon quantum dots>
The present invention provides, as a further embodiment, a method for producing carbon quantum dots. The carbon quantum dot manufacturing method of the present invention can manufacture carbon quantum dots safely, inexpensively, easily and efficiently. Further, according to the production method of the present invention, carbon quantum dots having blue, green, or orange visible light emitting ability can be produced.

In one embodiment of the present invention, the method for producing carbon quantum dots is
Prepare a solution containing (1) (A) lignin or a carbonized product thereof, (B) an aromatic compound having a hydroxy group and an amino group, and (C) an aliphatic polyamine; and (2) the above (1). To heat and carbonize the solution prepared in
including.

The step (1) includes preparing a solution containing (A) lignin or a carbonized product thereof, (B) an aromatic compound having a hydroxy group and an amino group, and (C) an aliphatic polyamine. In the method of preparing such a solution, each component (A), (B), and (C) may be mixed, and in the step (2), the solution is heated to obtain a target carbon quantum dot. As long as it can be obtained, there are no particular restrictions. The solution may be produced, for example, by separately producing a solution containing the component (A), a solution containing the component (B), and a solution containing the component (C), and then mixing them with each other. In addition, the pH of the solution may be adjusted as appropriate during production.

As the component (A) in the step (1), a lignin carbonized in advance or in advance can be used. The carbonized product of lignin that can be used as the component (A) can be obtained by a general method for obtaining a carbonized product. The carbonized product of lignin that can be used as the component (A) can be obtained by heat-treating lignin in an inert gas atmosphere or in a vacuum in a temperature range of 300 to 450 ° C., for example, using an electric furnace or the like. it can. When the lignin carbonized product as the component (A) is obtained, lignin and an aliphatic polyamine may be mixed and heat-treated in an electric furnace.

Of "lignin or its carbonized product" used as a component (A), "aromatic compound having a hydroxy group and an amino group" used as a component (B), and "aliphatic polyamine" used as a component (C). Each component is as described in the section "1. Carbon quantum dots" above.

The weight ratio ((A): (B): (C)) to the solution containing (A) lignin or a carbonized product thereof, (B) an aromatic compound, and (C) an aliphatic polyamine is the target carbon. As long as quantum dots can be obtained, there are no particular restrictions. It can be appropriately selected in relation to the reaction conditions and the like described later.

The weight ratio of (A) lignin or its carbonized product, (B) aromatic compound, and (C) aliphatic polyamine / (10 mL solvent) is: (A) lignin is 0.05 parts by weight or more, 0.3. It is preferable that the amount of (B) aromatic compound is 0.2 parts by weight or more and 1.2 parts by weight or less, and (C) aliphatic polyamine is 1 part by weight or more and 10 parts by weight or less. A) lignin is 0.08 parts by weight or more and 0.15 parts by weight or less, (B) aromatic compound is 0.3 parts by weight or more and 0.6 parts by weight or less, and (C) aliphatic polyamine is 1 part by weight. It is even more preferably 7 parts by weight or less, (A) lignin is 0.1 to 0.2 parts by weight, (B) aromatic compound is 0.3 to 0.4 parts by weight, and (C) aliphatic. The polyamine is particularly preferably 2 to 5 parts by weight.

The solvent that can be used when preparing the solution containing the components (A), (B), and (C) does not inhibit the carbonization progress of lignin, and the components (A), (B), and ( C) As long as the component is dissolved and the desired carbon quantum dot is obtained, there is no particular limitation. Examples of such a solvent include water and organic solvents such as alcohol, formamide, ethylene glycol and dimethylacetamide. Among these, water, formamide, etc. are preferably used from the viewpoint of adjusting the type of visible light and obtaining carbon quantum dots having a high fluorescence quantum yield.

In the description of the present invention, water refers to general water, and is not particularly limited as long as the desired carbon quantum dots can be obtained. In the description of the present invention, the water may be, for example, ion-exchanged water, pure water, distilled water, tap water, or the like. Water can contain an organic solvent as long as it does not adversely affect the carbonization of lignin and the desired carbon quantum dots can be obtained. Such organic solvents can include, for example, alcohols, formamides, ethylene glycols, dimethylacetamides and the like. A solvent containing water is also referred to as an aqueous medium.

As the step (2), the solution obtained in the process (1) is heated and carbonized to generate carbon quantum dots.

A method for producing carbon quantum dots can be carried out by heating and carbonizing a solution containing (A) lignin or a carbonized product thereof, (B) an aromatic compound, and (C) an aliphatic polyamine. As long as the carbon quantum dots to be obtained can be obtained, there is no particular limitation. Thiourea may be further added to the solution containing the components (A), (B), and (C) in order to increase the emission intensity of the carbon quantum dodd.

Examples of the heating method for generating carbon quantum dots include a method of sealing and heating (also referred to as "solvothermal method"; in the case of an aqueous medium, particularly referred to as "hydrothermal method"), and microwaves. A method of irradiating and heating (also referred to as "microwave method") can be used.

In the microwave method, a solution containing (A) lignin or a carbonized product thereof, (B) an aromatic compound, and (C) an aliphatic polyamine can be heated and carbonized to obtain a desired carbon quantum dot. Wherever possible, the microwave wattage and heating time can be selected as appropriate. The wattage of the microwave is, for example, preferably 100 W to 1500 W, more preferably 300 W to 1000 W, further preferably 300 W to 800 W, and particularly preferably 500 W to 800 W. The heating time by microwave is preferably 1 to 30 minutes, more preferably 1 to 15 minutes, further preferably 5 to 15 minutes, and particularly preferably 5 to 10 minutes.

In the sorbothermal method, a solution containing (A) lignin or a carbonized product thereof, (B) an aromatic compound, and (C) an aliphatic polyamine can be heated and carbonized to obtain a desired carbon quantum dot. As long as it is possible, the heating temperature and heating time under the hermetically sealed state can be appropriately selected. The heating temperature under sealing is, for example, preferably 100 ° C. to 350 ° C., more preferably 120 ° C. to 300 ° C., further preferably 150 ° C. to 280 ° C., and 200 ° C. to 250 ° C. Is particularly preferable. The heating time under sealing is preferably 1 to 48 hours, more preferably 2 to 24 hours, further preferably 3 to 12 hours, and particularly preferably 4 to 8 hours. ..

At the end of the reaction, the solution containing (A) lignin or a carbonized product thereof, (B) an aromatic compound, and (C) an aliphatic polyamine turns brown, and (A) lignin or a carbonized product thereof, (B). It can be judged by the absence of aromatic compounds or (C) aliphatic polyamines.

After the reaction, the mixture may be allowed to cool and the product may be purified as appropriate. Purification can be appropriately combined with commonly used methods. For example, insoluble matter and dispersion can be removed by centrifugal sedimentation or filtration by adding a solvent as appropriate. The supernatant from centrifugal sedimentation or the filtrate by filtration can be further purified by dialysis. The solution obtained through the process (2) can be dried under reduced pressure to obtain carbon quantum dots.

The desired carbon is used in the product obtained by heating and carbonizing a solution containing (A) lignin or a carbonized product thereof, (B) an aromatic compound having a hydroxy group and an amino group, and (C) an aliphatic polyamine. Quantum dots may be included, and other by-products may be mixed in the suspension in the form of insoluble matter or dispersion.

The carbon quantum dots that can be produced by the production method of the embodiment of the present invention can form a brown solution, and it is preferable that a wide range of absorption is observed in the blue to orange region of 450 to 620 nm.

Hereinafter, the present invention will be described in detail with reference to Examples, but these Examples are merely one aspect of the present invention, and the present invention is not limited to these Examples.
In the description of the examples, unless otherwise specified, the portion not considering the solvent is used as the standard of parts by weight and% by weight.

The components used in this example are shown below.

(A) Lignin (a1) Sodium lignin sulfonate (manufactured by Nippon Paper Industries, Ltd., trade name: Vanillex N, lignin sulfonate 91% as chemical composition (% solid content))
(A2) Craft lignin (derived from coniferous trees) (manufactured by Nippon Paper Industries, Ltd.)
(A3) Hydrolyzate before dissolution of dissolved kraft pulp (manufactured by Nippon Paper Industries, Ltd.)
(A4) Sodium lignin sulfonate (low molecular weight fraction) (manufactured by Nippon Paper Industries, Ltd., trade name: Sunpearl CP)
(A5) Sodium lignin sulfonate (manufactured by Nippon Paper Industries, Ltd., trade name: lignin sulfonate 97% as vanillex RN chemical composition (% solid content))

(B) Amine compound (b1) Amidol (manufactured by Wako Pure Chemical Industries, Ltd.)
(B2) m-phenylenediamine (aromatic amine containing no hydroxyl group)

(C) Ethylenediamine (manufactured by Wako Pure Chemical Industries, Ltd.)

<Example 1: Production of blue emitting carbon quantum dots: CQD (Blue)>
Mix 0.1 g / 5 mL (a1) lignin powder (without charcoal treatment), 2.5 mL ethylenediamine (EDA), 0.1 g / 5 mL thiourea and 0.4 g / 5 mL amidol well with a vortex mixer. Then, it became a black-blue solution. The pH meter F-53 was prepared using 5 mol / L NaOH so that the pH value of the mixed solution was about 12.0. This mixed solution is transferred to a pressure-resistant metal container for hydrothermal synthesis, which is a double-sealed double-sealed PTFE inner cylinder container and pressure-resistant stainless steel outer cylinder, and is subjected to solvothermal synthesis at 230 ° C. for 5 hours in an autoclave. (Solvent: formamide) was performed. After heating, the container was cooled overnight until it reached room temperature. The obtained solution was equally divided into two centrifuge tubes and centrifuged at 6000 rpm for 10 minutes using a centrifuge. The obtained supernatant was dialyzed (24 hours) using a dialysis membrane having a molecular weight cut off of 3500 to obtain a blue luminescent CQD (Blue) aqueous solution.

<Example 2: Production of blue emitting carbon quantum dots: CQD (Blue)>
0.8 g of (a1) lignin powder and 5 mL of EDA are mixed, transferred to a 25 mL quartz crucible, and then heated in a vacuum electric furnace under a nitrogen atmosphere (4.0 L / min) at 360 ° C. for 2.5 hours. By doing so, carbonized lignin was obtained.

A mixture of 0.1 g / 5 mL lignin powder, EDA 2.5 mL, 0.1 g / 5 mL thiourea and 0.4 g / 5 mL amidol was mixed well with a vortex mixer to give a black-blue solution. It was prepared by a pH meter F-53 using 5 mol / L NaOH so that the pH value of the mixed solution was about 12.0. This mixed solution is transferred to a pressure-resistant metal container for hydrothermal synthesis, which is a double-sealed double-sealed PTFE inner cylinder container and pressure-resistant stainless steel outer cylinder, and is subjected to solvothermal synthesis at 230 ° C. for 5 hours in an autoclave. (Solvent: formamide) was performed. After heating, the container was cooled overnight until it reached room temperature. The obtained solution was equally divided into two centrifuge tubes and centrifuged at 6000 rpm for 10 minutes using a centrifuge. The obtained supernatant was dialyzed (24 hours) using a dialysis membrane having a molecular weight cut off of 3500 to obtain a blue luminescent CQD (Blue) aqueous solution.

<Example 3: Production of green emitting carbon quantum dots: CQD (Green)>
A mixture of 0.5 g / 5 mL (a1) lignin powder (without carbonization), 5.0 mL EDA, 0.5 g / 5 mL thiourea and 2.0 g / 5 mL amidol is mixed well with a vortex mixer to darken. It became a brown solution. It was prepared by a pH meter using 5 mol / L NaOH so that the pH value of the mixed solution was about 12.0. This mixed solution is transferred to a pressure-resistant metal container for hydrothermal synthesis, which is a double-sealed double-sealed PTFE inner cylinder container and pressure-resistant stainless steel outer cylinder, and hydrothermal synthesis is performed in an autoclave at 230 ° C. for 5 hours. (Solvent: water) was performed. The obtained supernatant was dialyzed (24 hours) using a dialysis membrane having a molecular weight cut off of 3500 to obtain a green luminescent CQD (Green) aqueous solution.

<Example 4: Manufacture of orange emitting carbon quantum dots: CQD (Orange)>
After mixing 0.8 g of (a1) lignin powder and 5 mL of ethylenediamine (EDA) and transferring to a 25 mL quartz crucible, in a vacuum electric furnace under a nitrogen atmosphere (4.0 L / min), 360 ° C., 2. By heating for 5 hours, carbonized lignin was obtained.

A mixture of 0.5 g / 5 mL lignin powder, 5.0 mL EDA, 0.5 g / 5 mL thiourea and 2.0 g / 5 mL amidol was well stirred with a vortex mixer to give a black-blue solution. It was prepared by a pH meter using 5 mol / L NaOH so that the pH value of the mixed solution was about 12.0. This mixed solution is transferred to a pressure-resistant metal container for hydrothermal synthesis, which is a double-sealed double-sealed PTFE inner cylinder container and pressure-resistant stainless steel outer cylinder, and hydrothermal synthesis is performed in an autoclave at 230 ° C. for 5 hours. (Solvent: water) was performed. The obtained supernatant was dialyzed (24 hours) using a dialysis membrane having a molecular weight cut off of 3500 to obtain an orange luminescent CQD (Orange) aqueous solution.

<Example 5: Production of green emitting carbon quantum dots: CQD (Green)>
CQD was synthesized according to the production method of Example 3 except that (a2) craft lignin (derived from coniferous tree) was used instead of (a1) sodium lignin sulfonate (lignin powder, trade name: Vanillex N).
The obtained supernatant was dialyzed (24 hours) using a dialysis membrane having a molecular weight cut off of 3500 to obtain a green luminescent CQD (Green) aqueous solution.

<Example 6: Production of green emitting carbon quantum dots: CQD (Green)>
CQD was synthesized according to the production method of Example 3 except that (a3) a hydrolyzate before dissolution of dissolved kraft pulp was used instead of the lignin powder (trade name: Vanillex N) of (a1).
The obtained supernatant was dialyzed (24 hours) using a dialysis membrane having a molecular weight cut off of 3500 to obtain a green luminescent CQD (Green) aqueous solution.

<Example 7: Production of blue emitting carbon quantum dots: CQD (Blue)>
The production method of Example 3 except that (a4) sodium lignin sulfonate (low molecular weight fraction, trade name: Sunpearl CP) was used instead of the lignin powder (trade name: Vanillex N) of (a1). CQD was synthesized according to the above.
The obtained supernatant was dialyzed (24 hours) using a dialysis membrane having a molecular weight cut off of 3500 to obtain a green luminescent CQD (Blue) aqueous solution.

<Example 8: Production of green emitting carbon quantum dots: CQD (Green)>
CQD was synthesized according to the production method of Example 3 except that (a5) sodium lignin sulfonate (trade name: Vanillex RN) was used instead of the lignin powder (trade name: Vanillex N) of (a1). ..
The obtained supernatant was dialyzed (24 hours) using a dialysis membrane having a molecular weight cut off of 3500 to obtain a green luminescent CQD (Green) aqueous solution.

<Example 9: Production of green emitting carbon quantum dots: CQD (Green)>
0.5 g / 5 mL (a1) lignin powder (without carbonation), 5.0 mL branched polyethyleneimine (Brunched-PEI, Sigma Aldrich, model number 468533, number average molecular weight 423), 0.5 g / 5 mL thiourea And 2.0 g / 5 mL of amidol mixture was stirred well with a vortex mixer to give a dark brown solution. It was prepared by a pH meter using 5 mol / L NaOH so that the pH value of the mixed solution was about 12.0. This mixed solution is transferred to a pressure-resistant metal container for hydrothermal synthesis, which is a double-sealed double-sealed PTFE inner cylinder container and pressure-resistant stainless steel outer cylinder, and hydrothermal synthesis is performed in an autoclave at 230 ° C. for 5 hours. (Solvent: water) was carried out. The obtained supernatant was dialyzed (24 hours) using a dialysis membrane having a molecular weight cut off of 3500 to obtain a luminescent CQD aqueous solution.

<Comparative Example 1: Production of carbon quantum dots: using m-phenylenediamine>
CQD was synthesized according to the production method of Example 3, except that m-phenylenediamine, which is an aromatic amine containing no hydroxyl group of amidol, was used.
The UV-Vis spectrum and the fluorescence spectrum of the aqueous solution obtained in Comparative Example 1 are shown in FIG. Unlike the case where Amidol was used, green fluorescence was not observed, and the fluorescence was light blue emission showing a fluorescence peak wavelength at 490 nm. The fluorescence quantum yield was very low, 1% or less.

<Evaluation of carbon quantum dots>
The carbon quantum dots manufactured as described above were evaluated by performing the following measurements.

<Evaluation method of morphology of carbon quantum dot solution>
The condition of the produced carbon quantum dot solution was visually observed.
If the solution is a uniform solution or dispersion: good (G)
When a uniform solution cannot be obtained and a large amount of agglomerates are obtained: Impossible (NG)

<Measurement method of UV-vis absorption spectrum>
The solution of carbon quantum dots was diluted at a wavelength of 460 nm so that the absorbance was about 0.1. 2 mL of carbon quantum dot solution was placed in a two-sided quartz cell. The ultraviolet-visible absorption spectrum (UV-vis absorption spectrum) was measured using an ultraviolet-visible near-infrared spectrophotometer.
A wide range of absorption was observed from the near infrared to the ultraviolet region of 300 to 800 nm: good (G).
Absorption was not observed from near infrared to ultraviolet region of 300 to 800 nm: Impossible (NG)

<Measurement method of fluorescence spectrum>
A solution of about 1 mL of carbon quantum dots was placed in a four-sided quartz cell. The fluorescence spectrum was measured using a fluorometer.
Luminescence was observed and the fluorescence spectrum was measured: good (G)
Luminescence is observed and fluorescence spectrum cannot be measured: Impossible (NG)

<Measurement method of IR spectrum>
The water content of the carbon quantum dot solution was removed by vacuum drying to obtain a solid carbon quantum dot. The infrared absorption spectrum (IR spectrum) of the obtained solid carbon quantum dots was measured using a Fourier transform infrared spectrophotometer. _{It was investigated whether or not hydrophilic groups such as amine group (-NH 2} ), hydroxy group (-OH) and carboxy group (-COOH) were present on the surface of carbon quantum dots.

<Observation method with atomic force microscope (AFM)>
Carbon quantum dots were observed and their size evaluated using an atomic force microscope (AFM).
From the cross-sectional view of the AFM image, the height of the particles was taken as the size of the carbon quantum dots.

<Measurement method of XRD spectrum>
The XRD spectrum (X-ray diffraction pattern) of the solid carbon quantum dots was measured using an X-ray diffractometer. The lattice spacing was estimated from the diffraction peak.

<Measurement method of zeta potential>
Zeta potential measurements were performed to investigate the surface charge state of carbon quantum dots.
Zeta potential measurements of various carbon quantum dots were carried out to evaluate the particle surface charge. A carbon quantum dot solution having an absorption wavelength of 360 nm and an absorbance of about 0.1 was prepared. 600 μL of this solution and 100 μL of a 10 mM NaCl aqueous solution prepared in advance were placed in a cell for measuring the zeta potential, and the surface potential (zeta potential) was measured using a Zetasizer Nano ZSP manufactured by Malvern.

The results of Examples 1 to 4 and 9 are summarized in Table 1.

<Examples 2 to 4: Results of ultraviolet-visible (UV-vis) absorption spectrum measurement>
The aqueous solutions obtained in Examples 2 to 4 were all brownish brown aqueous solutions peculiar to CQD. Their UV-Vis spectra are shown in FIG. In FIG. 1, the horizontal axis represents wavelength and the vertical axis represents absorbance or photoluminescence intensity. The broken line shows the UV-Vis (ultraviolet-visible) absorption spectrum, and the solid line shows the fluorescence spectrum.

An absorption edge from around 700 nm, which is characteristic of CQD, was observed. Since the carbon material having a graphene structure in which the π-conjugated system was extended had an absorption end on the long wavelength side, it was suggested that the synthesized CQD had a graphene structure inside. The CQD (Blue: blue) solution of Example 2 shows a peak (shoulder) near about 350 nm. This peak coincides with the peak derived from the ^{n → π *} transition due to the benzene ring of the product and the chromogenic group having n electrons. Further, in the CQD (Green: green) of Example 3 and the CQD (Orange: orange) of Example 4, a broad peak was confirmed in the range of 400 to 600 nm, which is the wide conjugated double bond possessed by the CQD. It is considered that abundant functional groups containing elements such as N and O are present on the particle surface and are generated by electronic transitions to the surface levels formed by these functional groups.

<Examples 2 to 4: Results of fluorescence spectrum measurement>
The fluorescence spectra of the aqueous solutions obtained in Examples 2 to 4 are shown in FIG. In FIG. 1, the horizontal axis represents wavelength and the vertical axis represents absorbance or photoluminescence intensity. The solid line shows the fluorescence spectrum, and the broken line shows the UV-Vis (ultraviolet-visible) absorption spectrum.

From the fluorescence spectrum, the fluorescence wavelengths of various CQDs were 425 nm (Ex = 337 nm) in CQD (Blue: blue) of Example 2, 518 nm (Ex = 389 nm) in CQD (Green: green) of Example 3, and that of Example 4. The CQD (Orange: orange) was 594 nm (Ex = 480 nm). The difference between the excitation spectrum and the fluorescence spectrum of various CQD solutions (Stokes Shift) was about 120 nm. This large Stokes shift is not seen in organic fluorescent molecules and is characteristic of CQD.

<Examples 2 to 4: Results of FT-IR spectrum measurement>
The IR spectra of the CQDs of Examples 2 to 4 are shown in FIG. In FIG. 2, the horizontal axis represents wavenumber and the vertical axis represents transmittance.

It is clear that the CQD (Green) of Example 3 and the CQD (Orange) of Example 4 show similar peaks, while the CQD (Blue) of Example 2 shows significantly different peaks from the other two. .. In CQD (Blue), ^{the wide peak of 3300 cm -1} is the expansion and contraction vibration of the OH bond, ^{the peak of 2830 to 2900 cm -1} is the expansion and contraction vibration of CH (alkane), and ^{the peak of 1465 cm -1} is the C-H (alkane). ), 1650 cm ^-1 , 1100 cm ^-1 peak is attributed to C = C expansion and contraction vibration, and 1180 cm ^-1 peak is attributed to COC (annular) expansion and contraction vibration. It was considered that a hydroxyl group, which is a non-dissociative hydrophilic group, was present on the surface of the CQD (Blue) of Example 2.

In the CQD (Green) of Example 3, ^{the wide peak of 3300 cm -1} is the expansion and contraction vibration of the N—H bond, the 3050 cm ^-1 peak is the expansion and contraction vibration of CH (pyridine), and the peak of ^{2770 cm -1 is the CH (} stretching vibration of alkanes), stretching vibration of 1650 cm ^-1 peak is C = C, N-H bending of the peak of 1630 cm ^-1 of aromatic amines, the peak of 1330 cm ^-1 is the stretching vibration of C-N of an aromatic amine , 1520 cm ^-1 peak stretching vibration of C = C or C = N,, 1230 cm ^-1, a peak of 1034cm ^-1, the stretching vibration of the -SO ₃ H, 1150 cm ^-1 peak is C-O-C (ring) It was attributed to the expansion and contraction vibration of. The CQD (Orange) of Example 4 was the same as above. From these results, it can be seen that the surface of the CQD (Green) of Example 3 contains amino groups (-NH ₂ ) and sulfone groups (-SO _{3 H), which are dissociative hydrophilic groups, as surface functional groups.} it was thought.

<Examples 2 to 4: Results of sweater potential measurement>
The results of investigating the pH dependence of the zeta potential of the CQD aqueous solution of Examples 2 to 4 are shown in FIGS. 3-1, 3-2, and 3-3. The zeta potential of CQD (Blue) in Example 2 was almost zero. This indicates that the CQD (Blue) surface of Example 2 is free of dissociative functional groups. This is consistent with the fact that the main surface functional group of CQD (Blue) of Example 2 revealed by IR spectrum measurement is a pH-independent hydroxyl group.

The CQD (Green) of Example 3 was +1.44 mV (water), +13.4 mV (pH3.0), and -22.4 mV (pH 11.0), and the CQD (Orange) of Example 4 was +1.44 mV ( Water), +5.48 mV (pH 3.0), and -14.8 mV (pH 11.0). Unlike the CQD (Blue) of Example 2, the CQD (Green) of Example 3 and the CQD (Orange) of Example 4 were found to depend on the pH for the value and sign of the zeta potential. This indicates that dissociative functional groups are present on the surface of the CQD, and protonation or deprotonation occurs depending on the pH. Both the CQD (Green) of Example 3 and the CQD (Orange) of Example 4 were positively charged on the particle surface under acidic conditions (pH 3) and negatively charged under basic conditions (pH 11). it is conceivable that. This is consistent with the fact that the main surface functional groups of CQD (Green / Orange) revealed by IR spectrum measurement are amino groups and sulfone groups.

<Examples 2 to 4: Results of observation with an atomic force microscope (AFM)>
The images of CQD observed with the atomic force microscope (AFM) of Examples 2 to 4 are shown in FIG. The cross-sectional view in the figure is a cross-sectional profile (lower side of FIG. 4) from the straight line A point to the B point in the image. From the cross-sectional profile of the AFM, it was found that the size of the CQD was about 2-3 nm.

<Examples 2 to 4: XRD spectrum measurement results>
The X-ray diffraction pattern of CQD of Examples 2 to 4 is shown in FIG. In the CQD (Blue) of Example 2, diffraction peaks appeared in the vicinity of 2θ = 27 ° and 2θ = 40 °. The sharp peak of 2θ = 27 ° coincides with the analysis peak of the (002) plane of graphite (2θ = 25 °). In addition, the lattice spacing calculated from Black's equation was that CQD (Blue) was d = 3.4 Å, and the graphite plane spacing was 3.34 Å. Therefore, it was considered that the particle portion of CQD (Blue) had a highly crystalline graphite structure.

In both the CQD (Green) of Example 3 and the CQD (Orange) of Example 4, a broad diffraction peak appeared in the vicinity of 2θ = 25 °. No peak derived from the crystalline graphite structure observed in CQD (Blue) of Example 2 was observed. It was considered that the inside of the particles of CQD (Green) of Example 3 and CQD (Orange) of Example 4 mainly had an amorphous structure. Due to the influence of this amorphous structure, the lattice spacing is 3.80 Å for CQD (Green) in Example 3 and 3.75 Å for CQD (Orange) in Example 4, which is wider than graphite (d = 3.4 Å). The interval is shown.

<Example 5: Measurement results of UV-Vis spectrum and fluorescence spectrum>
The UV-Vis spectrum and fluorescence spectrum of the aqueous solution obtained in Example 5 (using (a2) as lignin) are shown in FIG. When (a1) and (a2) were compared as lignin sources, no significant difference was found in the fluorescence spectra between the two.

<Example 6: Measurement results of UV-Vis spectrum and fluorescence spectrum>
The UV-Vis spectrum (left graph) and fluorescence spectrum (right graph) of the aqueous solution obtained in Example 6 (using (a3) as lignin) are shown in FIG. When (a1) and (a3) were compared as the lignin source, it was clarified that the intensity of the fluorescence peak increased when (a1) was used.

<Example 7: Measurement results of UV-Vis spectrum and fluorescence spectrum>
The UV-Vis spectrum (left graph) and fluorescence spectrum (right graph) of the aqueous solution obtained in Example 7 (using (a4) as lignin) are shown in FIG. When (a1) and (a4) were compared as the lignin source, the intensity of the fluorescence peak increased when (a1) was used.

<Example 8: Measurement results of UV-Vis spectrum and fluorescence spectrum>
The UV-Vis spectrum (left graph) and fluorescence spectrum (right graph) of the aqueous solution obtained in Example 8 (using (a5) as lignin) are shown in FIG. When (a1) and (a5) were compared as the lignin source, the intensity of the fluorescence peak increased when (a1) was used.

<Example 9: Measurement results of UV-Vis spectrum and fluorescence spectrum>
The UV-Vis spectrum (left graph) and fluorescence spectrum (right graph) of the aqueous solution obtained in Example 9 are shown in FIG. A green CQD could be obtained using polyethyleneamine as the aliphatic polyamine. The CQD of Example 9 had a fluorescence peak wavelength at 526 nm, and the fluorescence quantum yield was 1.5%.

<Comparative Example 1: Measurement results of UV-Vis spectrum and fluorescence spectrum>
The UV-Vis spectrum (left graph) and fluorescence spectrum (right graph) of the aqueous solution obtained in Comparative Example 1 (using m-phenylenediamine instead of amidol) are shown in FIG. Unlike the case where Amidol was used, green fluorescence was not observed, and the fluorescence was light blue emission showing a fluorescence peak wavelength at 490 nm. The fluorescence quantum yield was very low, 1% or less.

Claims (10)

A carbon quantum dot which is a carbide of a mixture containing (A) a lignin or a carbonized product thereof, (B) an aromatic compound having a hydroxy group and an amino group, and (C) an aliphatic polyamine. The carbon quantum dot according to claim 1, wherein the lignin (A) comprises one or more selected from the group consisting of lignin sulfonic acid, kraft lignin, soda lignin, and organosolvrignin. The carbon quantum dot according to claim 1 or 2, wherein the aromatic compound having a hydroxy group and an amino group (B) contains one or more selected from the group consisting of diaminophenol, dopamine, and tyrosine. .. Any of claims 1 to 3, wherein the aliphatic polyamine (C) comprises one or more selected from the group consisting of ethylenediamine, diaminobutane, diaminohexane, polyethyleneimine, polyallylamine, and polyamideamine. The carbon quantum dot described in item 1. (1) Prepare a solution containing (A) lignin or a carbide thereof, (B) an aromatic compound having a hydroxy group and an amino group, and (C) an aliphatic polyamine; and (2) prepare in (1) above. Heating and carbonizing the solution,
A method for producing carbon quantum dots, including. The production method according to claim 5, wherein the lignin (A) comprises one or more selected from the group consisting of lignin sulfonic acid, kraft lignin, soda lignin, and organosolvrignin. The production method according to claim 5 or 6, wherein the aromatic compound having a hydroxy group and an amino group (B) comprises one or more selected from the group consisting of diaminophenol, dopamine, and tyrosine. Any of claims 5 to 7, wherein the aliphatic polyamine (C) comprises one or more selected from the group consisting of ethylenediamine, diaminobutane, diaminohexane, polyethyleneimine, polyallylamine, and polyamideamine. The method for producing carbon quantum dots according to item 1. The production method according to any one of claims 5 to 8, wherein the treatment of (2) is carried out by irradiating with microwaves and / or by sealing and heating. The production method according to any one of claims 5 to 9, wherein the treatment of (2) is carried out by a solvothermal method using an organic solvent.

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