JP2024177865A - Method for producing water-soluble quantum dots and water-soluble quantum dots - Google Patents

Abstract

To provide: a method for producing a water-soluble quantum dot which is excellent in quantum yield in an aqueous solvent such as water, and can obtain a light emission spectrum having a narrow FWHM; and a water-soluble quantum dot which is obtained by the production method.SOLUTION: A method for producing a water-soluble quantum dot includes: a ligand exchange step of adding a hydrophilic ligand to a hydrophobic quantum dot dispersion liquid and exchanging a ligand bonded to the surface of a quantum dot with a hydrophilic ligand to obtain a hydrophilic quantum dot; a dispersion step of dispersing the hydrophilic quantum dot into an aqueous medium to obtain an aqueous solvent dispersion liquid of the hydrophilic quantum dot; a surface treatment step of adding a surface treatment agent to the aqueous solvent dispersion liquid, subjecting the hydrophilic quantum dot to surface treatment in the aqueous medium to obtain an aqueous solvent dispersion liquid of the surface-treated hydrophilic quantum dot; and a shell formation step of adding a shell formation agent to the aqueous solvent dispersion liquid of the surface-treated hydrophilic quantum dot and forming a shell on the surface of the surface-treated hydrophilic quantum dot to obtain an aqueous solvent dispersion liquid of the water-soluble quantum dot.SELECTED DRAWING: None

Description

The present invention relates to a method for producing water-soluble quantum dots and water-soluble quantum dots obtained by the method.

In recent years, the development of quantum dots as luminescent materials has progressed. Cadmium-based quantum dots such as CdSe, CdTe, and CdS have been developed as representative quantum dots due to their excellent luminescent properties. In addition, cadmium is highly toxic and has a high environmental impact, so the development of cadmium-free quantum dots such as InP, CuInS ₂ , and ZnTeSe is expected.

Taking advantage of these excellent luminescence properties, a method is used to measure biological substances such as proteins, viruses, and nucleic acids using a molecular recognition body in which a molecular recognition substance is bound to a marker substance such as a fluorescent substance. Here, in order to use quantum dots as marker substances in the body, it is necessary to coat the quantum dot surface with a water-soluble ligand to make the quantum dots water-soluble.

As a method for coating the quantum dot surface with a water-soluble ligand to make the quantum dot water-soluble, for example, in the examples of Patent Document 1, it is described that InP quantum dots are contacted with zinc acetate and mercaptoacetic acid in an aqueous solvent to obtain water-soluble quantum dots with a core-shell structure of InP/ZnS modified with mercaptoacetic acid as a ligand. In addition, in the examples of Patent Document 2, water-soluble InP/ZnS core-shell quantum dots are also disclosed, and the method for producing them is described as reacting indium chloride, zinc chloride, tris(diethylamino)phosphine, and sodium propanesulfonate in an amine, then adding mercaptopropionic acid to obtain InP/ZnS core-shell quantum dots coordinated with mercaptopropionic acid, which are then dispersed in water.

It is generally known that when quantum dots are made water-soluble, the quantum yield drops significantly and the emission characteristics, such as the full width at half maximum (FWHM) of the emission spectrum becoming wider, are deteriorated. This is thought to be because defects occur on the quantum dot surface due to the ligand exchange during water-solubilization. In the above patent document, there is concern that defects will occur on the quantum dot surface when the ligand for water-solubilization is exchanged with a compound having a mercapto group, resulting in a decrease in emission characteristics.

In response to this issue, Patent Document 3 describes how, in order to obtain water-soluble quantum dots with high brightness and stability, after ligand exchange with mercaptopropionic acid, a mixed aqueous solution of zinc chloride and mercaptopropionic acid is added, and the defects on the quantum dot surface are protected by zinc while a ZnS shell layer is grown, resulting in excellent luminescence characteristics.

Chinese Patent Publication No. 102031111 Chinese Patent Publication No. 114136934 Chinese Patent Publication No. 114891508

As described in Patent Document 3, by forming a shell while protecting the defects on the quantum dot surface that occur during ligand exchange, it is possible to impart excellent luminescence properties even in water, but it is difficult to achieve luminescence properties equal to or better than those in organic solvents, and further investigation is required.

The object of the present invention is therefore to provide a method for producing water-soluble quantum dots that have excellent quantum yield in aqueous solvents such as water and produce a narrow FWHM emission spectrum, and to provide water-soluble quantum dots obtained by said method.

As a result of intensive research conducted by the inventors to solve the above problems, they discovered that if the ligand-exchanged quantum dots are surface-treated in an aqueous solvent to repair defects on the quantum dot surface, and then a shell is formed, the water-soluble quantum dots have an FWHM equivalent to that of quantum dots obtained in an organic solvent, and the decrease in quantum yield is also suppressed, leading to the completion of the present invention.

That is, the present invention provides a method for producing water-soluble quantum dots, which includes a ligand exchange step of adding hydrophilic ligands to a hydrophobic quantum dot dispersion liquid and exchanging the ligands bonded to the surface of the quantum dots with hydrophilic ligands to obtain hydrophilic quantum dots; a dispersion step of dispersing the hydrophilic quantum dots in an aqueous solvent to obtain an aqueous solvent dispersion of hydrophilic quantum dots; a surface treatment step of adding a surface treatment agent to the aqueous solvent dispersion liquid to perform surface treatment of the hydrophilic quantum dots in the aqueous solvent to obtain an aqueous solvent dispersion of surface-treated hydrophilic quantum dots; and a shell formation step of adding a shell formation agent to the aqueous solvent dispersion liquid of the surface-treated hydrophilic quantum dots to form a shell on the surface of the surface-treated hydrophilic quantum dots to obtain an aqueous solvent dispersion of water-soluble quantum dots.

The present invention also provides water-soluble quantum dots in which the quantum yield of the water-soluble quantum dots obtained by exchanging the ligands bound to the surface of the hydrophobic quantum dots with hydrophilic ligands is greater than 0.8 relative to the quantum yield of the hydrophobic quantum dots.

According to the present invention, it is possible to produce water-soluble quantum dots that have excellent quantum yield in aqueous solvents and produce emission spectra with narrow FWHM in an industrially advantageous manner.

1 is an emission spectrum of the aqueous dispersion of water-soluble quantum dots obtained in Example 1. 1 is an emission spectrum of the aqueous dispersion of water-soluble quantum dots obtained in Example 2. 1 is an emission spectrum of the aqueous dispersion of water-soluble quantum dots obtained in Example 3.

<Hydrophobic quantum dot dispersion>
The hydrophobic quantum dot dispersion liquid used in the present invention is a liquid in which hydrophobic quantum dots are dispersed in a solvent, and the hydrophobic quantum dots are quantum dots having hydrophobic ligands on their surfaces. The hydrophobic quantum dot dispersion liquid may be one obtained by reacting raw material compounds consisting of elements constituting the hydrophobic quantum dots in a solvent, or may be one obtained by dispersing commercially available hydrophobic quantum dots in a solvent.

The solvent for the dispersion liquid is preferably a non-polar organic solvent, and more preferably a solvent made of an aliphatic hydrocarbon. Examples of aliphatic hydrocarbons include saturated hydrocarbons such as n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-dodecane, n-hexadecane, and n-octadecane; and unsaturated aliphatic hydrocarbons such as 1-undecene, 1-dodecene, 1-tetradecene, 1-hexadecene, and 1-octadecene. The aliphatic hydrocarbon used as the solvent may be one type, or two or more types may be mixed together.

Quantum dots constituting the hydrophobic quantum dots include cadmium-based quantum dots such as CdSe, CdTe, and CdS, and cadmium-free quantum dots such as InP, CuInS, and ZnTeSe. In the present invention, quantum dots having a core-shell structure in which a shell is formed on the surface of a core are preferable, and quantum dots having a core-shell structure in which the core has at least an InP-based quantum dot obtained by reacting a phosphorus source with an indium source, and the shell has a coating compound other than InP-based, are particularly preferable.

Examples of hydrophobic ligands constituting the hydrophobic quantum dots include carboxylic acid derivatives, phosphine derivatives, amine derivatives, and phosphonic acid.

Preferred examples of the carboxylic acid derivatives include those in which the alkyl group in the molecule is linear and has 2 to 24 carbon atoms. Specific examples of carboxylic acids in which the alkyl group in the molecule is linear and has 2 to 24 carbon atoms include capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, elaidic acid, oleic acid, linolenic acid, and erucic acid. Among these, from the viewpoint of improving the quality of the obtained hydrophobic quantum dots, those in which the alkyl group in the molecule has 12 to 20 carbon atoms are particularly preferred, and oleic acid is the most preferred.

The phosphine derivative is preferably a primary to tertiary alkylphosphine, and the alkyl group in the molecule is preferably a straight-chain alkyl group having 2 to 18 carbon atoms. The alkyl groups in the molecule may be the same or different. Specific examples of alkylphosphines having a straight-chain alkyl group having 2 to 18 carbon atoms include monoethylphosphine, monobutylphosphine, monodecylphosphine, monohexylphosphine, monooctylphosphine, monododecylphosphine, monohexadecylphosphine, diethylphosphine, dibutylphosphine, didecylphosphine, dihexylphosphine, dioctylphosphine, didodecylphosphine, dihexadecylphosphine, triethylphosphine, tributylphosphine, tridecylphosphine, trihexylphosphine, trioctylphosphine, tridodecylphosphine, and trihexadecylphosphine. Among these, from the viewpoint of improving the quality of the obtained hydrophobic quantum dots, those in which the number of carbon atoms in the alkyl group in the molecule is 4 to 12 are particularly preferred, those which are trialkylphosphines are preferred, and trioctylphosphine is the most preferred.

The amine derivative is preferably a primary to tertiary alkylamine, and preferred examples include linear alkylamines in which the alkyl group in the molecule has 2 to 18 carbon atoms, and aromatic alkylamines in which the alkyl group has 6 to 12 carbon atoms. The alkyl groups in the molecule may be the same or different. Specific examples of linear alkylamines in which the alkyl group has 2 to 18 carbon atoms include monoethylamine, monobutylamine, monodecylamine, monohexylamine, monooctylamine, monododecylamine, monohexadecylamine, diethylamine, dibutylamine, didecylamine, dihexylamine, dioctylamine, didodecylamine, dihexadecylamine, triethylamine, tributylamine, tridecylamine, trihexylamine, trioctylamine, tridodecylamine, and trihexadecylamine. Specific examples of aromatic alkylamines in which the alkyl group has 6 to 12 carbon atoms include aniline, diphenylamine, triphenylamine, monobenzylamine, dibenzylamine, tribenzylamine, naphthylamine, dinaphthylamine, and trinaphthylamine.
The phosphonic acid is preferably a monoalkylphosphonic acid having a linear alkyl group having from 2 to 18 carbon atoms in the molecule.

When the hydrophobic quantum dots used in the present invention have a core-shell structure, examples of coating compounds suitable as the shell include AlN, AlP, _Al2O3 , GaN, GaP, _Ga2O3 , _SiO2 , _CdS , CdSe, CdSeS, CdTe, CdSeTe, CdTeS, ZnS, ZnSe, ZnSeS, ZnTe, ZnSeTe, _ZnTeS , ZnO, ZnOS, ZnSeO, and ZnTeO, etc. In the present invention, it is preferable that the coating compound is obtained by reaction with at least a zinc source.

The hydrophobic quantum dot dispersion liquid used in the present invention can be produced by a known method, for example, the method described in JP-A-2023-033157.

<Ligand exchange step>
The ligand exchange reaction in the present invention is a process in which hydrophilic ligands are added to a hydrophobic quantum dot dispersion liquid, and the ligands bonded to the surface of the quantum dots are exchanged for hydrophilic ligands to obtain hydrophilic quantum dots.

Examples of hydrophilic ligands include thiols, amine-containing thiols, amines, alcohols, thiol-containing alcohols, selenols, phosphites, phosphinic acids, phosphonic acids, phosphoric acids, phosphine oxides, phosphine sulfides, phosphine selenides, carboxylic acids, thiol-containing carboxylic acids, thiocarboxylic acids, thionocarboxylic acids, dithiocarboxylic acids, sulfenic acids, sulfinic acids, and sulfonic acids. Among these, thiols, amine-containing thiols, amines, alcohols, thiol-containing alcohols, phosphonic acids, carboxylic acids, and thiol-containing carboxylic acids are preferred from the viewpoint of the stability of the hydrophilic quantum dots and the improvement of the quantum yield, and thiol-containing carboxylic acids are particularly preferred.

Examples of the thiol-containing carboxylic acid include thioglycolic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiomalic acid, cysteine, acetylcysteine, homocysteine, and penicillamine. Among these, from the viewpoint of improving the water dispersibility and quantum yield of the quantum dots, thioglycolic acid, 3-mercaptopropionic acid, cysteine, and acetylcysteine are preferred, and 3-mercaptopropionic acid is more preferred.

The temperature when adding hydrophilic ligands to the quantum dot dispersion is preferably 0°C or higher and 350°C or lower, more preferably 20°C or higher and 300°C or lower, from the viewpoint of successfully promoting ligand exchange, and the treatment time is preferably 1 minute or higher and 600 minutes or lower, more preferably 5 minutes or higher and 240 minutes or lower. The amount of ligand added depends on the type of ligand, but is preferably 0.01 g/L or higher and 100,000 g/L or lower, more preferably 0.1 g/L or higher and 2,000 g/L or lower, relative to the reaction solution containing quantum dots.

The atmosphere when adding the hydrophilic ligand to the quantum dot dispersion liquid is preferably a non-oxidizing atmosphere such as an inert gas atmosphere, reduced pressure, or vacuum, from the viewpoint of preventing the denaturation of the quantum dots and the by-production of oxides. From the viewpoint of ease of operation, the non-oxidizing atmosphere is preferably an inert gas atmosphere such as nitrogen gas or argon gas.
Through the above steps, a dispersion liquid containing hydrophilic quantum dots is obtained.

Next, the hydrophilic quantum dots are separated from the dispersion liquid containing the hydrophilic quantum dots. Examples of the separation method include common methods such as centrifugation, decantation, and suction filtration. At this time, in order to remove unnecessary hydrophobic ligands, it is preferable to add a solvent capable of dissolving the ligands and repeat the operation of separating the hydrophilic quantum dots by centrifugation or the like several times.
Through the above steps, hydrophilic quantum dots are obtained.

<Dispersion process>
The dispersion step in the present invention is a step of dispersing the hydrophilic quantum dots in an aqueous solvent to obtain an aqueous solvent dispersion of the hydrophilic quantum dots.

The aqueous solvent is not particularly limited as long as it is a water-soluble solvent, and examples thereof include water, methanol, ethanol, 2-propanol, ethylene glycol, propylene glycol, acetone, acetonitrile, N-methylpyrrolidone, dimethylformamide, and tetrahydrofuran, with water being preferred.

The hydrophilic quantum dot aqueous solvent dispersion is obtained by dispersing the hydrophilic quantum dots in the aqueous solvent. The hydrophilic quantum dots can be dispersed in the aqueous solvent as they are, but for example, when the hydrophilic ligand is a carboxylic acid containing a thiol, it is preferable to react these with ammonia water or tetraethylammonium hydroxide, etc., to convert them into the form of an ammonium salt, and then disperse them in the aqueous solvent, from the viewpoint of obtaining a uniform dispersion.

In addition, since the hydrophilic quantum dots before the dispersion process may contain by-products or unreacted impurities, they may be subjected to a purification process. This purification process can be performed by adding an organic solvent such as methanol, ethanol, 2-propanol, acetone, acetonitrile, or tetrahydrofuran to a solution containing hydrophilic quantum dots to precipitate the hydrophilic quantum dots, separating the solution containing impurities from the hydrophilic quantum dots, and dispersing the separated hydrophilic quantum dots in an aqueous solvent. The solution containing impurities and the hydrophilic quantum dots can be separated by operations such as centrifugation, decantation, and suction filtration.
Through the above steps, an aqueous solvent dispersion of hydrophilic quantum dots is obtained.

<Surface treatment process>
The surface treatment step in the present invention is a step of adding a surface treatment agent to the aqueous solvent dispersion to perform surface treatment of the hydrophilic quantum dots in an aqueous solvent, thereby obtaining an aqueous solvent dispersion of the surface-treated hydrophilic quantum dots.

In the surface treatment step, defects that occur on the quantum dot surface in the ligand exchange step are repaired in an aqueous solvent using a surface treatment agent, stabilizing the quantum dots and making the surface uniform, enabling stable shell formation in the shell formation step described below. Therefore, the water-soluble quantum dots of the present invention finally obtained have a FWHM equivalent to that of quantum dots obtained in an organic solvent, and the decrease in quantum yield is also suppressed.

Examples of the surface treatment agent include metal halides such as halides of Li, Na, K, Be, Mg, Ca, Sr, Cu, Ag, Au, Hg, In, Sn, Pb, Zn, Cd, Al, and Ga; hydrogen halides such as HF, HCl, HBr, and HI; nitrogen halides such as _NF3 ; halides of quaternary ammonium compounds and quaternary phosphonium compounds such as NR ₄ F and PR ₄ F (R represents H, or a saturated, unsaturated, or aromatic hydrocarbon having 1 to 8 carbon atoms, which may be linear or branched, and may be substituted with a functional group); carboxylic acids, phosphonic acids, sulfonic acids, and acid halides thereof; and metal carboxylates, metal carbamates, and metal thiocarboxylates containing metals such as Li, Na, K, Be, Mg, Ca, Sr, Cu, Ag, Au, Hg, In, Sn, Pb, Zn, Cd, Al, and Ga. Among these, from the viewpoint of further improving the quantum yield, the surface treatment agent is preferably a metal halide or a metal carboxylate. The metal halide may be zinc fluoride, zinc chloride, zinc bromide, zinc iodide, etc., and zinc chloride is particularly preferred. The metal carboxylate may be zinc acetate, zinc trifluoroacetate, zinc myristate, zinc oleate, zinc benzoate, etc., and zinc acetate is particularly preferred.

The surface treatment of the hydrophilic quantum dots can be carried out by adding a surface treatment agent to the hydrophilic quantum dot aqueous solvent dispersion. The surface treatment is preferably carried out at a temperature of -20°C or higher and 200°C or lower, more preferably 0°C or higher and 150°C or lower, and particularly 20°C or higher and 120°C or lower, and preferably for 1 minute or longer and 600 minutes or shorter, more preferably 5 minutes or longer and 240 minutes or shorter, and particularly 10 minutes or longer and 120 minutes or shorter. The amount of the surface treatment agent added depends on the type of the surface treatment agent, but is preferably 0.01 g/L or higher and 10,000 g/L or lower, and more preferably 0.1 g/L or higher and 1,000 g/L or lower, relative to the hydrophilic quantum dot aqueous solvent dispersion.

The atmosphere in the surface treatment step is preferably a non-oxidizing atmosphere such as an inert gas atmosphere, reduced pressure, or vacuum, from the viewpoint of preventing denaturation of the quantum dots and by-production of oxides. From the viewpoint of ease of operation, the non-oxidizing atmosphere is preferably an inert gas atmosphere such as nitrogen gas or argon gas.

The method of adding the surface treatment agent in the surface treatment step includes a method of directly adding the surface treatment agent to the reaction liquid, and a method of adding the surface treatment agent dissolved or dispersed in a solvent to the reaction liquid. When the surface treatment agent is added by a method of adding the surface treatment agent dissolved or dispersed in a solvent to the reaction liquid, the solvent is not particularly limited as long as it is a water-soluble solvent, and examples of the solvent include water, methanol, ethanol, 2-propanol, ethylene glycol, propylene glycol, acetone, acetonitrile, N-methylpyrrolidone, dimethylformamide, and tetrahydrofuran, with water being particularly preferred.
Through the above steps, an aqueous solvent dispersion of surface-treated hydrophilic quantum dots is obtained.

<Shell Formation Process>
The shell formation step in the present invention is a step of adding a shell forming agent to the aqueous solvent dispersion of the surface-treated hydrophilic quantum dots to form a shell on the surface of the surface-treated hydrophilic quantum dots, thereby obtaining an aqueous solvent dispersion of water-soluble quantum dots.

As the shell-forming agent, a mixture of at least one sulfur source selected from thiols, thiol-containing carboxylic acids, thioureas, thiocarbamates, thiocarbonates, thiocarboxylic acids, thionocarboxylic acids, dithiocarboxylic acids, phosphine sulfides, and sulfides of Li, Na, K, Be, Mg, Ca, and Sr, and zinc carboxylate, zinc nitrate, zinc sulfate, zinc hypochlorite, zinc chlorite, zinc chlorate, zinc perchlorate, zinc bromate, zinc iodate, zinc carbonate, zinc borate, zinc hypophosphite, zinc phosphite, zinc phosphate, zinc trifluoromethanesulfonate, or zinc halide is preferable. In particular, from the viewpoint of improving the quantum yield, a mixture of thiol-containing carboxylic acids as the sulfur source and zinc carboxylate, zinc perchlorate, or zinc halide as the zinc source is preferable, and a mixture of thiol-containing carboxylic acids and zinc halide is particularly preferable.

Examples of the thiol-containing carboxylic acids include thioglycolic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiomalic acid, cysteine, acetylcysteine, homocysteine, and penicillamine. Among these, from the viewpoint of the water dispersibility and quantum yield of the quantum dots, thioglycolic acid, 3-mercaptopropionic acid, cysteine, and acetylcysteine are preferred, and 3-mercaptopropionic acid is more preferred.

Preferred examples of the zinc halide include zinc fluoride, zinc chloride, zinc bromide, and zinc iodide, with zinc chloride being particularly preferred.

The shell can be formed by adding a shell-forming agent to the aqueous solvent dispersion of the surface-treated hydrophilic quantum dots. After the addition of the shell-forming agent, a treatment is carried out preferably at 30°C or higher and 200°C or lower, more preferably at 50°C or higher and 150°C or lower, and particularly at 70°C or higher and 120°C or lower, for preferably 1 minute or longer and 600 minutes or shorter, more preferably at 5 minutes or longer and 240 minutes or shorter, and particularly at 10 minutes or longer and 120 minutes or shorter, to form a shell on the surface of the surface-treated hydrophilic quantum dots.

The atmosphere in the shell formation process is preferably a non-oxidizing atmosphere such as an inert gas atmosphere, reduced pressure, or vacuum, from the viewpoint of preventing denaturation of the quantum dots and by-production of oxides. From the viewpoint of ease of operation, the non-oxidizing atmosphere is preferably an inert gas atmosphere such as nitrogen gas or argon gas.

The method of adding the shell-forming agent in the shell formation step includes a method of directly adding the shell-forming agent to the reaction liquid, and a method of adding the shell-forming agent dissolved or dispersed in a solvent to the reaction liquid. When the shell-forming agent is added by a method of adding the shell-forming agent dissolved or dispersed in a solvent to the reaction liquid, the solvent is not particularly limited as long as it is a water-soluble solvent, and examples of the solvent include water, methanol, ethanol, 2-propanol, ethylene glycol, propylene glycol, acetone, acetonitrile, N-methylpyrrolidone, dimethylformamide, and tetrahydrofuran, with water being particularly preferred.
Through the above steps, an aqueous solvent dispersion of water-soluble quantum dots is obtained.

The water-soluble quantum dots can be separated from the obtained aqueous solvent dispersion of water-soluble quantum dots by common methods such as centrifugation, decantation, and suction filtration. The obtained water-soluble quantum dots are dispersed again in an aqueous solvent to remove insoluble impurities, thereby obtaining a purified aqueous solvent dispersion of water-soluble quantum dots.

The water-soluble quantum dots obtained by the manufacturing method of the present invention exhibit excellent luminescence properties even in aqueous solvents, since the decrease in quantum yield from the hydrophobic quantum dots before water-solubilization is suppressed. In other words, if the ratio (B%/A%) of the quantum yield (B%) of the water-soluble quantum dots to the quantum yield (A%) of the quantum dots before water-solubilization is taken as the quantum yield maintenance value, the maintenance value is greater than 0.8, preferably 0.82 or more, and therefore the water-soluble quantum dots can be suitably used in fields where hydrophobic quantum dots could not be used, for example, in the field of bioimaging as fluorescent probes.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited thereto. The properties in the examples were measured by the following methods.
(1) Emission spectrum, maximum fluorescence wavelength, full width at half maximum (FWHM) and quantum yield (PLQY)
The obtained aqueous dispersion of quantum dots was measured using an absolute PL quantum yield measurement device (Quantaurus-QY, manufactured by Hamamatsu Photonics KK) under measurement conditions of an excitation wavelength of 400 nm and a measurement wavelength of 200 to 950 nm.
(2) Maintenance value of quantum yield: Calculated according to the following formula.
(Quantum yield maintenance value)=
(Quantum yield of water-soluble quantum dots)/(Quantum yield of quantum dots obtained in the synthesis example)

Synthesis Example 1 Synthesis of green InP/ZnSe/ZnS quantum dots 1.04 g of indium myristate, 0.31 g of zinc myristate, and 0.06 g of indium acetate were added to 14.25 g of 1-octadecene, and the mixture was heated to 120° C. under reduced pressure with stirring and degassed for 1.5 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 40° C. to obtain a 1-octadecene solution of indium zinc myristate.
The obtained 1-octadecene solution of indium zinc myristate was kept at 40° C. under a nitrogen atmosphere, and 2.51 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added thereto, and the mixture was kept in that state for 10 minutes and then naturally cooled to 20° C. As a result, a yellow liquid containing an InP quantum dot precursor was obtained.
Next, the InP quantum dot precursor solution was heated to 250° C. to obtain a reaction solution of InP quantum dots.
7.54 g of zinc oleate, 4.09 g of zinc chloride, 5.40 g of trioctylphosphine selenide, 8.90 g of trioctylphosphine, 16.26 g of oleylamine, and 16.26 g of dioctylamine were mixed in a 200 mL reaction vessel, and heated to 120° C. under reduced pressure while stirring, and degassed for 30 minutes. After degassing, the pressure was returned to atmospheric pressure with nitrogen gas, and the temperature was raised to 180° C. under a nitrogen atmosphere, and 18.17 g of the reaction solution of InP quantum dots obtained above was added, and the temperature was raised to 230° C. and held for 30 minutes, and then further raised to 300° C. and held for 60 minutes, thereby obtaining an oleylamine/dioctylamine dispersion of InP/ZnSe quantum dots having InP in the core and ZnSe in the shell. The obtained dispersion was cooled to 240° C., and then 7.54 g of zinc oleate, 7.54 g of trioctylphosphine, and 4.35 g of trioctylphosphine sulfide were poured thereinto and held for 90 minutes to obtain an oleylamine/dioctylamine dispersion of multi-shell quantum dots having InP in the core and ZnSe and ZnS stacked in the shell.
After cooling the obtained dispersion to room temperature, 50 g of acetone was added, the mixture was stirred, and the InP/ZnSe/ZnS quantum dots were collected as a precipitate by centrifugation. The collected InP/ZnSe/ZnS quantum dots were suspended in 8.67 g of toluene to obtain a toluene dispersion of InP/ZnSe/ZnS quantum dots. 50 g of acetone was further added to this dispersion, the mixture was stirred, and the InP/ZnSe/ZnS quantum dots were collected as a precipitate by centrifugation. The collected InP/ZnSe/ZnS quantum dots were suspended in 7.03 g of octane to obtain an octane dispersion of purified InP/ZnSe/ZnS quantum dots. The results of measuring the luminescence characteristics of the obtained dispersion are shown in Table 1.

Synthesis Example 2 Synthesis of orange InP/ZnSe/ZnS quantum dots 1.43 g of indium myristate was added to 14.25 g of 1-octadecene, and the mixture was heated to 120° C. under reduced pressure with stirring and degassed for 1.5 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 40° C. to obtain a 1-octadecene solution of indium myristate.
The obtained 1-octadecene solution of indium myristate was kept at 40° C. under a nitrogen atmosphere, and 2.51 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added thereto, and the mixture was held for 10 minutes and then naturally cooled to 20° C. As a result, a yellow liquid containing an InP quantum dot precursor was obtained.
Next, the InP quantum dot precursor solution was heated to 280° C. to obtain a reaction solution of InP quantum dots.
7.54 g of zinc oleate, 4.09 g of zinc chloride, 5.40 g of trioctylphosphine selenide, 8.90 g of trioctylphosphine, 16.26 g of oleylamine, and 16.26 g of dioctylamine were mixed in a 200 mL reaction vessel, and heated to 120° C. under reduced pressure while stirring, and degassed for 30 minutes. After degassing, the pressure was returned to atmospheric pressure with nitrogen gas, and the temperature was raised to 180° C. under a nitrogen atmosphere, and 18.19 g of the reaction solution of InP quantum dots obtained above was added, and the temperature was raised to 230° C. and held for 30 minutes, and then further raised to 300° C. and held for 60 minutes, thereby obtaining an oleylamine/dioctylamine dispersion of InP/ZnSe quantum dots having InP in the core and ZnSe in the shell. The obtained dispersion was cooled to 240° C., and then 7.54 g of zinc oleate, 7.54 g of trioctylphosphine, and 4.35 g of trioctylphosphine sulfide were poured thereinto and held for 90 minutes to obtain an oleylamine/dioctylamine dispersion of multi-shell quantum dots having InP in the core and ZnSe and ZnS stacked in the shell.
After cooling the obtained dispersion to room temperature, 50 g of acetone was added, the mixture was stirred, and the InP/ZnSe/ZnS quantum dots were collected as a precipitate by centrifugation. The collected InP/ZnSe/ZnS quantum dots were suspended in 8.67 g of toluene to obtain a toluene dispersion of InP/ZnSe/ZnS quantum dots. 50 g of acetone was further added to this dispersion, the mixture was stirred, and the InP/ZnSe/ZnS quantum dots were collected as a precipitate by centrifugation. The collected InP/ZnSe/ZnS quantum dots were suspended in 7.03 g of octane to obtain an octane dispersion of purified InP/ZnSe/ZnS quantum dots. The results of measuring the luminescence characteristics of the obtained dispersion are shown in Table 1.

Synthesis Example 3 Synthesis of red InP/ZnSe/ZnS quantum dots 1.59 g of indium myristate was added to 14.25 g of 1-octadecene, and the mixture was heated to 120° C. under reduced pressure with stirring and degassed for 1.5 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 40° C. to obtain a 1-octadecene solution of indium myristate.
The obtained 1-octadecene solution of indium myristate was kept at 40° C. under a nitrogen atmosphere, and 2.51 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added thereto, and the mixture was held for 10 minutes and then naturally cooled to 20° C. As a result, a yellow liquid containing an InP quantum dot precursor was obtained.
Next, the InP quantum dot precursor solution was heated to 280° C. to obtain a reaction solution of InP quantum dots as a base.
In a separate reaction vessel, 2.39 g of indium myristate was added to 21.37 g of 1-octadecene, and the mixture was heated to 120° C. under reduced pressure with stirring and degassed for 1.5 hours. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and cooled to 40° C. to obtain a 1-octadecene solution of indium myristate.
The obtained 1-octadecene solution of indium myristate was kept at 40° C. under a nitrogen atmosphere, and 3.76 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added thereto, and the mixture was held for 10 minutes and then naturally cooled to 20° C. As a result, a yellow liquid containing an InP quantum dot precursor was obtained.
The previously prepared base InP quantum dot reaction solution was reheated to 260° C., and the newly prepared InP quantum dot precursor solution was added over 60 minutes to obtain an InP quantum dot reaction solution.
7.54 g of zinc oleate, 4.09 g of zinc chloride, 5.40 g of trioctylphosphine selenide, 8.90 g of trioctylphosphine, 16.26 g of oleylamine, and 16.26 g of dioctylamine were mixed in a 200 mL reaction vessel, and heated to 120° C. under reduced pressure while stirring, and degassed for 30 minutes. After degassing, the pressure was returned to atmospheric pressure with nitrogen gas, and the temperature was raised to 180° C. under a nitrogen atmosphere, and 15.29 g of the reaction solution of InP quantum dots obtained above was added, and the temperature was raised to 230° C. and held for 30 minutes, and then the temperature was further raised to 300° C. and held for 60 minutes, thereby obtaining an oleylamine/dioctylamine dispersion of InP/ZnSe quantum dots having InP in the core and ZnSe in the shell. The obtained dispersion was cooled to 240° C., and then 7.54 g of zinc oleate, 7.54 g of trioctylphosphine, and 4.35 g of trioctylphosphine sulfide were poured thereinto and held for 90 minutes to obtain an oleylamine/dioctylamine dispersion of multi-shell quantum dots having InP in the core and ZnSe and ZnS stacked in the shell.
After cooling the obtained dispersion to room temperature, 50 g of acetone was added, the mixture was stirred, and the InP/ZnSe/ZnS quantum dots were collected as a precipitate by centrifugation. The collected InP/ZnSe/ZnS quantum dots were suspended in 8.67 g of toluene to obtain a toluene dispersion of InP/ZnSe/ZnS quantum dots. 50 g of acetone was further added to this dispersion, the mixture was stirred, and the InP/ZnSe/ZnS quantum dots were collected as a precipitate by centrifugation. The collected InP/ZnSe/ZnS quantum dots were suspended in 7.03 g of octane to obtain an octane dispersion of purified InP/ZnSe/ZnS quantum dots. The results of measuring the luminescence characteristics of the obtained dispersion are shown in Table 1.

[Example 1]
(Ligand exchange step)
2 ml of the octane dispersion of green InP/ZnSe/ZnS quantum dots obtained in Synthesis Example 1 (quantum dot concentration 0.100 mmol/ml) and 10 ml of hexadecane were weighed into a 50 ml two-necked eggplant flask. The mixture was heated to 90°C under reduced pressure while stirring and degassed for 30 minutes. After degassing, the mixture was returned to atmospheric pressure with nitrogen gas and heated to 220°C under a nitrogen atmosphere, 4 ml of 3-mercaptopropionic acid was added, and the mixture was held for 10 minutes to obtain InP/ZnSe/ZnS quantum dots in which the ligands were substituted with 3-mercaptopropionic acid. After cooling to 160°C, 40 ml of octane was added and the mixture was cooled to room temperature. The supernatant was removed, 30 ml of hexane was added, and the mixture was stirred, and the supernatant was removed again. This was repeated three times to obtain a solid. The solid was suspended in 5 ml of ethanol, and the InP/ZnSe/ZnS quantum dots were collected as a precipitate by centrifugation.
(Dispersion process)
The collected InP/ZnSe/ZnS quantum dots were dispersed by adding 3.6 ml of 40% aqueous solution of tetraethylammonium hydroxide, and then 6 ml of tetrahydrofuran and 60 ml of 2-propanol were added and stirred, and the InP/ZnSe/ZnS quantum dots were collected as a precipitate by centrifugation. The collected InP/ZnSe/ZnS quantum dots were dispersed by adding 1.6 ml of pure water, and then 60 ml of 2-propanol was added and stirred, and the InP/ZnSe/ZnS quantum dots were collected as a precipitate by centrifugation. This was repeated twice, and the resulting precipitate was dispersed in 10 ml of pure water to obtain an aqueous dispersion of purified InP/ZnSe/ZnS quantum dots.
(Surface treatment process)
3 ml of an aqueous dispersion of InP/ZnSe/ZnS quantum dots (quantum dot concentration 0.020 mmol/ml), 0.082 g of anhydrous zinc chloride, and 5 ml of pure water were weighed into a glass flask with a lid. After reducing the pressure, the pressure was returned to atmospheric pressure with nitrogen gas to create a nitrogen atmosphere, and the internal temperature was heated to between 80°C and 100°C while stirring, and the mixture was held for 30 minutes, and then cooled to room temperature to obtain an aqueous dispersion of surface-treated InP/ZnSe/ZnS quantum dots.
(Shell Formation Process)
To the obtained aqueous dispersion of surface-treated InP/ZnSe/ZnS quantum dots, 0.090 g of anhydrous zinc chloride dissolved in 4.0 ml of pure water and 1.0 ml of 3-mercaptopropionic acid were added. After reducing the pressure, the pressure was returned to atmospheric pressure with nitrogen gas to create a nitrogen atmosphere, and the internal temperature was heated to between 80°C and 100°C while stirring, and the mixture was maintained for 30 minutes, and then cooled to room temperature to obtain an aqueous dispersion of water-soluble InP/ZnSe/ZnS quantum dots.
The water-soluble InP/ZnSe/ZnS quantum dots were collected as a precipitate from the obtained aqueous dispersion of water-soluble InP/ZnSe/ZnS quantum dots by centrifugation. The collected precipitate was dispersed by adding 2.25 ml of pure water and 0.10 ml of a 35% aqueous solution of tetraethylammonium hydroxide, and then insoluble impurities were removed by centrifugation to obtain an aqueous dispersion of purified water-soluble InP/ZnSe/ZnS quantum dots. The results of measuring the emission characteristics of the obtained aqueous dispersion are shown in Table 1. The results of measuring the emission spectrum of the obtained aqueous dispersion are shown in FIG. 1.

[Example 2]
In the ligand exchange step, the same procedure as in Example 1 was carried out, except that the octane dispersion of orange InP/ZnSe/ZnS quantum dots obtained in Synthesis Example 2 was used, to obtain a purified aqueous dispersion of water-soluble InP/ZnSe/ZnS quantum dots. The emission characteristics of the obtained aqueous dispersion were measured, and the results are shown in Table 1. The emission spectrum of the obtained aqueous dispersion was also measured, and the results are shown in FIG. 2.

[Example 3]
In the ligand exchange step, the same operation as in Example 1 was carried out, except that the octane dispersion of red InP/ZnSe/ZnS quantum dots obtained in Synthesis Example 3 was used, to obtain an aqueous dispersion of purified water-soluble InP/ZnSe/ZnS quantum dots. The emission characteristics of the obtained aqueous dispersion were measured, and the results are shown in Table 1. The emission spectrum of the obtained aqueous dispersion was also measured, and the results are shown in FIG.

[Example 4]
In the ligand exchange step, the same operation as in Example 1 was carried out up to the dispersion step, except that the octane dispersion of red InP/ZnSe/ZnS quantum dots obtained in Synthesis Example 3 was used, and an aqueous dispersion of InP/ZnSe/ZnS quantum dots was obtained.
(Surface treatment process)
0.12 g of anhydrous zinc chloride was dissolved in 0.60 ml of pure water to obtain an anhydrous zinc chloride aqueous solution. 0.05 ml of this anhydrous zinc chloride aqueous solution, 2.0 ml of γ-butyrolactone, and 0.03 ml of InP/ZnSe/ZnS quantum dot aqueous dispersion (quantum dot concentration 0.025 mmol/ml) were weighed into a glass flask with a lid. After reducing the pressure, the pressure was returned to atmospheric pressure with nitrogen gas to create a nitrogen atmosphere, and the internal temperature was heated to between 80°C and 100°C while stirring, and the mixture was held for 30 minutes, and then cooled to room temperature to obtain a surface-treated InP/ZnSe/ZnS quantum dot aqueous dispersion.
(Shell Formation Process)
To the obtained surface-treated InP/ZnSe/ZnS quantum dot aqueous dispersion, 0.05 ml of anhydrous zinc chloride solution and 0.02 ml of 3-mercaptopropionic acid were added. After reducing the pressure, the pressure was returned to atmospheric pressure with nitrogen gas to create a nitrogen atmosphere, and the internal temperature was heated to between 190°C and 205°C while stirring, and the mixture was maintained for 30 minutes, and then cooled to room temperature to obtain a water-soluble InP/ZnSe/ZnS quantum dot aqueous dispersion.
The obtained aqueous dispersion of water-soluble InP/ZnSe/ZnS quantum dots was centrifuged to recover the water-soluble InP/ZnSe/ZnS quantum dots as a precipitate. The recovered precipitate was dispersed by adding 1.0 ml of pure water and 0.1 ml of a 35% aqueous solution of tetraethylammonium hydroxide, and then centrifuged to remove insoluble impurities, thereby obtaining an aqueous dispersion of purified water-soluble InP/ZnSe/ZnS quantum dots. The results of measuring the luminescence characteristics of the obtained aqueous dispersion are shown in Table 1.

[Example 5]
In the ligand exchange step, the same operations as in Example 1 were carried out up to the dispersion step, except that the octane dispersion of orange InP/ZnSe/ZnS quantum dots obtained in Synthesis Example 2 was used, to obtain an aqueous dispersion of InP/ZnSe/ZnS quantum dots.
(Surface treatment process)
0.044g of zinc acetate dihydrate was dissolved in 2.0ml of pure water to obtain an aqueous zinc acetate solution. 1.0ml of this aqueous zinc acetate solution, 5.0ml of pure water, and 0.5ml of InP/ZnSe/ZnS quantum dot aqueous dispersion (quantum dot concentration 0.020mmol/ml) were weighed into a glass flask with a lid. After reducing the pressure, the pressure was returned to atmospheric pressure with nitrogen gas to create a nitrogen atmosphere, and the internal temperature was heated to between 80°C and 100°C while stirring, and then the mixture was held for 30 minutes and cooled to room temperature to obtain an aqueous dispersion of surface-treated InP/ZnSe/ZnS quantum dots.
(Shell Formation Process)
To the obtained aqueous dispersion of surface-treated InP/ZnSe/ZnS quantum dots, 1.0 ml of zinc acetate solution and 0.16 ml of 3-mercaptopropionic acid were added. After reducing the pressure, the pressure was returned to atmospheric pressure with nitrogen gas to create a nitrogen atmosphere, and the internal temperature was heated to between 80°C and 100°C while stirring, and the mixture was maintained for 30 minutes, and then cooled to room temperature to obtain an aqueous dispersion of water-soluble InP/ZnSe/ZnS quantum dots.
The obtained aqueous dispersion of water-soluble InP/ZnSe/ZnS quantum dots was centrifuged to recover the water-soluble InP/ZnSe/ZnS quantum dots as a precipitate. The recovered precipitate was dispersed by adding 0.30 ml of pure water and 0.03 ml of a 35% aqueous solution of tetraethylammonium hydroxide, and then centrifuged to remove insoluble impurities, thereby obtaining an aqueous dispersion of purified water-soluble InP/ZnSe/ZnS quantum dots. The results of measuring the luminescence characteristics of the obtained aqueous dispersion are shown in Table 1.

[Comparative Example 1]
The aqueous dispersion of the purified InP/ZnSe/ZnS quantum dots obtained in the dispersion step in Example 1 was used as water-soluble quantum dots, and the luminescence characteristics were measured. The results are shown in Table 1.

[Comparative Example 2]
The aqueous dispersion of the purified InP/ZnSe/ZnS quantum dots obtained in the dispersion step in Example 2 was used as water-soluble quantum dots, and the luminescence characteristics were measured. The results are shown in Table 1.

[Comparative Example 3]
The aqueous dispersion of the purified InP/ZnSe/ZnS quantum dots obtained in the dispersion step in Example 3 was used as water-soluble quantum dots, and the luminescence characteristics were measured. The results are shown in Table 1.

[Comparative Example 4]
0.5 ml of the octane dispersion of orange InP/ZnSe/ZnS quantum dots (quantum dot concentration 0.020 mmol/ml) obtained in Synthesis Example 3 was centrifuged to recover the InP/ZnSe/ZnS quantum dots as a precipitate. The recovered InP/ZnSe/ZnS quantum dots were dispersed in 1 ml of n-hexane to obtain an n-hexane dispersion of InP/ZnSe/ZnS quantum dots (quantum dot concentration 10 mg/ml).
1 ml of the obtained dispersion was placed in a one-neck flask in a nitrogen atmosphere, 1 ml of 28% ammonia water, 0.5 ml of 1-butanol, and 0.25 ml of mercaptopropionic acid were added, and the internal temperature was heated to between 55°C and 65°C with stirring. After cooling, 7.5 ml of acetonitrile was added and centrifuged, and the obtained solid was dispersed in 1.5 ml of pure water to obtain a mixed dispersion of InP/ZnSe/ZnS quantum dots.
To the obtained InP/ZnSe/ZnS quantum dot mixed dispersion, 1 ml of Zn-mercaptopropionic acid aqueous solution, which was a mixture of 0.14 g of anhydrous zinc chloride, 1 ml of mercaptopropionic acid, and 4 ml of pure water, was added, and the mixture was heated with stirring so that the internal temperature was 70°C to 80°C, and irradiated with ultraviolet light (wavelength 365 nm) for 40 minutes while stirring. After stopping the irradiation and 1 hour had elapsed since the start of stirring, the reaction solution was cooled to room temperature, 4.5 ml of acetonitrile was added, and the centrifugal separation was performed twice, and the obtained solid was dispersed in 1 ml of pure water to obtain a water-soluble InP/ZnSe/ZnS quantum dot aqueous dispersion. The results of measuring the luminescence characteristics of the obtained aqueous dispersion are shown in Table 1.

As shown in Table 1, the quantum dots water-soluble by the manufacturing method of the present invention have the same FWHM as the hydrophobic quantum dots used as the raw material, and it can be seen that the decrease in quantum yield is also suppressed.

Claims (11)

a ligand exchange step of adding hydrophilic ligands to the hydrophobic quantum dot dispersion to exchange the ligands bound to the surfaces of the quantum dots with hydrophilic ligands to obtain hydrophilic quantum dots;
A dispersing step of dispersing the hydrophilic quantum dots in an aqueous solvent to obtain an aqueous solvent dispersion of the hydrophilic quantum dots;
A surface treatment step of adding a surface treatment agent to the aqueous solvent dispersion to perform surface treatment of the hydrophilic quantum dots in the aqueous solvent to obtain an aqueous solvent dispersion of the surface-treated hydrophilic quantum dots; and
a shell-forming step of adding a shell-forming agent to the aqueous solvent dispersion of the surface-treated hydrophilic quantum dots to form a shell on the surface of the surface-treated hydrophilic quantum dots to obtain an aqueous solvent dispersion of water-soluble quantum dots;
A method for producing a water-soluble quantum dot having the formula: The method for producing water-soluble quantum dots according to claim 1, wherein the quantum dots in the hydrophobic quantum dot dispersion have a core-shell structure in which the core is an InP-based quantum dot obtained by reacting with at least a phosphorus source and an indium source, and the shell is a coating compound other than an InP-based compound. The _method for producing _water- soluble quantum dots according to claim 1 or 2, wherein the shell is at least one selected from the group consisting of AlN, AlP, _Al2O3 , GaN, GaP, _Ga2O3 , _SiO2 , CdS, CdSe, CdSeS, CdTe, CdSeTe, CdTeS, ZnS, ZnSe, ZnSeS, ZnTe, ZnSeTe, ZnTeS, ZnO, ZnOS, ZnSeO and ZnTeO. The method for producing water-soluble quantum dots according to claim 1 or 2, wherein the hydrophilic ligand used in the ligand exchange step is at least one selected from the group consisting of thiols, amine-containing thiols, amines, alcohols, thiol-containing alcohols, selenols, phosphites, phosphinic acids, phosphonic acids, phosphoric acids, phosphine oxides, phosphine sulfides, phosphine selenides, carboxylic acids, thiol-containing carboxylic acids, thiocarboxylic acids, thionocarboxylic acids, dithiocarboxylic acids, sulfenic acids, sulfinic acids, and sulfonic acids. The method for producing water-soluble quantum dots according to claim 1 or 2, wherein the aqueous solvent is water. The method for producing water-soluble quantum dots according to claim 1 or 2, wherein the surface treatment agent is a metal halide, a metal carboxylate, a hydrogen halide, a nitrogen halide, a halide of a quaternary ammonium compound or a quaternary phosphonium compound, or a carboxylic acid, a phosphonic acid, a sulfonic acid, or an acid halide thereof. The method for producing water-soluble quantum dots according to claim 1 or 2, wherein the shell-forming agent is a mixture of at least one sulfur source selected from thiols, thiol-containing carboxylic acids, thioureas, thiocarbamates, thiocarbonates, thiocarboxylic acids, thionocarboxylic acids, dithiocarboxylic acids, phosphine sulfides, and sulfides of Li, Na, K, Be, Mg, Ca, and Sr, and zinc carboxylate, zinc nitrate, zinc sulfate, zinc hypochlorite, zinc chlorite, zinc chlorate, zinc perchlorate, zinc bromate, zinc iodate, zinc carbonate, zinc borate, zinc hypophosphite, zinc phosphite, zinc phosphate, zinc trifluoromethanesulfonate, or a zinc halide. Water-soluble quantum dots in which the quantum yield of the water-soluble quantum dots obtained by exchanging the ligands bound to the surface of the hydrophobic quantum dots with hydrophilic ligands is greater than 0.8 relative to the quantum yield of the hydrophobic quantum dots. The water-soluble quantum dot according to claim 8, which is a core-shell quantum dot having an InP-based quantum dot composed of at least phosphorus and indium elements in the core and a coating compound other than InP in the shell. The water _- soluble quantum dot according to claim 9, wherein the shell is at least _{one selected from the group consisting of AlN, AlP, Al2O3, GaN, GaP, Ga2O3, SiO2 , CdS ,} CdSe, CdSeS, CdTe, CdSeTe, CdTeS, ZnS, ZnSe, ZnSeS, ZnTe, ZnSeTe, ZnTeS, ZnO, ZnOS, ZnSeO or ZnTeO. The ligand coordinated to the surface of the water-soluble quantum dot is at least one selected from the group consisting of thiols, amine-containing thiols, amines, alcohols, thiol-containing alcohols, selenols, phosphites, phosphinic acids, phosphonic acids, phosphoric acids, phosphine oxides, phosphine sulfides, phosphine selenides, carboxylic acids, thiol-containing carboxylic acids, thiocarboxylic acids, thionocarboxylic acids, dithiocarboxylic acids, sulfenic acids, sulfinic acids and sulfonic acids. The water-soluble quantum dot according to claim 8 or 9, wherein the ligand is at least one selected from the group consisting of thiols, amine-containing thiols, amines, alcohols, thiol-containing alcohols, selenols, phosphites, phosphinic acids, phosphonic acids, phosphoric acids, phosphine oxides, phosphine sulfides, phosphine selenides, carboxylic acids, thiol-containing carboxylic acids, thiocarboxylic acids, thionocarboxylic acids, dithiocarboxylic acids, sulfenic acids, sulfinic acids and sulfonic acids.