Method for producing water-soluble quantum dot, and water-soluble quantum dot

Abstract

Provided are: a method for producing a water-soluble quantum dot which has an excellent quantum yield in an aqueous solvent such as water and has a narrow FWHM emission spectrum; and a water-soluble quantum dot obtained by the production method. The method for producing a water-soluble quantum dot comprises: a ligand exchange step for adding a hydrophilic ligand to a hydrophobic quantum dot dispersion and exchanging a ligand bonded to the surface of the quantum dot with a hydrophilic ligand to obtain a hydrophilic quantum dot; a dispersion step for dispersing the hydrophilic quantum dot in an aqueous solvent to obtain an aqueous solvent dispersion of the hydrophilic quantum dot; a surface treatment step for adding a surface treatment agent to the aqueous solvent dispersion to perform surface treatment of the hydrophilic quantum dot in an aqueous solvent, thereby obtaining an aqueous solvent dispersion of the surface-treated hydrophilic quantum dot; and a shell formation step for adding a shell-forming agent to the aqueous solvent dispersion of the surface-treated hydrophilic quantum dot to form a shell on the surface of the surface-treated hydrophilic quantum dot, thereby obtaining an aqueous solvent dispersion of the water-soluble quantum dot.

Description

Method for producing water-soluble quantum dots and water-soluble quantum dots

[0001] The present invention relates to a method for producing water-soluble quantum dots and water-soluble quantum dots obtained by the method.

[0002] In recent years, the development of quantum dots as light-emitting materials has progressed. Cadmium-based quantum dots such as CdSe, CdTe, and CdS are being developed as representative quantum dots due to their excellent light-emitting properties. In addition, cadmium is highly toxic and has a high environmental impact, so InP and CuInS ２ The development of cadmium-free quantum dots such as ZnTeSe is expected.

[0003] Taking advantage of these excellent luminescence properties, a method is being used to measure biological substances such as proteins, viruses, and nucleic acids using molecular recognition bodies in which molecular recognition substances are bound to marker substances such as fluorescent substances. In order to use quantum dots as marker substances in vivo, the quantum dot surfaces must be coated with water-soluble ligands to make the quantum dots water-soluble.

[0004] As a method for coating the quantum dot surface with a water-soluble ligand to make the quantum dot water-soluble, for example, an example in Patent Document 1 describes contacting InP quantum dots with zinc acetate and mercaptoacetic acid in an aqueous solvent to obtain water-soluble quantum dots having an InP / ZnS core-shell structure modified with mercaptoacetic acid as a ligand. Patent Document 2 also discloses an example of water-soluble InP / ZnS core-shell quantum dots, and describes a method for producing them by reacting indium chloride, zinc chloride, tris(diethylamino)phosphine, and sodium propanesulfonate in an amine, followed by adding mercaptopropionic acid to obtain InP / ZnS core-shell quantum dots coordinated with mercaptopropionic acid, which are then dispersed in water.

[0005] It is generally known that water-solubilizing quantum dots significantly reduces quantum yield and worsens luminescence characteristics, such as a broadening of the full width at half maximum (FWHM) of the emission spectrum. This is thought to be due to defects occurring on the quantum dot surface due to ligand exchange during water-solubilization. The above patent document raises concerns that defects will occur on the quantum dot surface when the ligand exchange is performed with a compound having a mercapto group as a water-solubilizing ligand, resulting in a deterioration in luminescence characteristics.

[0006] To address this issue, Patent Document 3 describes that 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, thereby growing a ZnS shell layer while protecting defects on the quantum dot surface with zinc, thereby achieving excellent luminescence properties.

[0007] Chinese Patent Application Publication No. 102031111 Chinese Patent Application Publication No. 114136934 Chinese Patent Application Publication No. 114891508

[0008] As described in Patent Document 3, by forming a shell while protecting defects on the quantum dot surface that occur during ligand exchange, it is possible to impart excellent luminescence properties even in water. However, it is difficult to achieve luminescence properties that are equal to or better than those in organic solvents, and further investigation is required.

[0009] Therefore, an object of the present invention is to provide a method for producing water-soluble quantum dots that have an excellent quantum yield in an aqueous solvent such as water and can provide an emission spectrum with a narrow FWHM, and to provide water-soluble quantum dots obtained by the method.

[0010] As a result of extensive research to solve the above problems, the present inventors discovered that if 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, even water-soluble quantum dots have an FWHM equivalent to that of quantum dots obtained in an organic solvent, and the decrease in quantum yield can be suppressed, which led to the completion of the present invention.

[0011] That is, the present invention provides a method for producing water-soluble quantum dots, comprising: a ligand exchange step of adding hydrophilic ligands to a hydrophobic quantum dot dispersion and exchanging the ligands bound to the surfaces of the quantum dots for 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 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-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.

[0012] The present invention also provides water-soluble quantum dots, in which the quantum yield of water-soluble quantum dots obtained by exchanging the ligands bound to the surfaces of the hydrophobic quantum dots with hydrophilic ligands is greater than 0.8 relative to the quantum yield of the hydrophobic quantum dots.

[0013] According to the present invention, it is possible to produce water-soluble quantum dots that have excellent quantum yield in aqueous solvents and provide emission spectra with narrow FWHM, by an industrially advantageous method.

[0014] 1 is an emission spectrum of an aqueous dispersion of water-soluble quantum dots obtained in Example 1. FIG. 2 is an emission spectrum of an aqueous dispersion of water-soluble quantum dots obtained in Example 2. FIG. 3 is an emission spectrum of an aqueous dispersion of water-soluble quantum dots obtained in Example 3.

[0015] <Hydrophobic quantum dot dispersion> The hydrophobic quantum dot dispersion used in the present invention is a dispersion of hydrophobic quantum dots in a solvent, and the hydrophobic quantum dots are quantum dots having hydrophobic ligands on their surfaces. The hydrophobic quantum dot dispersion may be obtained by reacting raw material compounds consisting of elements that constitute the hydrophobic quantum dots in a solvent, or may be obtained by dispersing commercially available hydrophobic quantum dots in a solvent.

[0016] The solvent for the dispersion is preferably a non-polar organic solvent, 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 solvent may be one type or a mixture of two or more types.

[0017] Examples of 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 preferred, and quantum dots having a core-shell structure in which the core has at least InP-based quantum dots obtained by reacting a phosphorus source and an indium source, and the shell has a coating compound other than InP.

[0018] Examples of the hydrophobic ligands that constitute the hydrophobic quantum dots include carboxylic acid derivatives, phosphine derivatives, amine derivatives, and phosphonic acid.

[0019] Preferred examples of the carboxylic acid derivatives include those in which the alkyl group in the molecule is linear and has from 2 to 24 carbon atoms. Specific examples of carboxylic acids in which the alkyl group in the molecule is linear and has from 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 from 12 to 20 carbon atoms are particularly preferred, and oleic acid is most preferred.

[0020] The phosphine derivative is preferably a primary to tertiary alkylphosphine, and preferred examples include those in which the alkyl group in the molecule is linear and has from 2 to 18 carbon atoms. The alkyl groups in the molecule may be the same or different. Specific examples of alkylphosphines in which the alkyl group is linear and has from 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, trialkylphosphines are preferred, and trioctylphosphine is most preferred.

[0021] 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 from 2 to 18 carbon atoms, and aromatic alkylamines in which the alkyl group in the molecule has from 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 from 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 from 6 to 12 carbon atoms include aniline, diphenylamine, triphenylamine, monobenzylamine, dibenzylamine, tribenzylamine, naphthylamine, dinaphthylamine, and trinaphthylamine. Furthermore, the phosphonic acid is preferably a monoalkylphosphonic acid in which the alkyl group in the molecule is a linear alkyl group having from 2 to 18 carbon atoms.

[0022] When the hydrophobic quantum dots used in the present invention have a core-shell structure, suitable coating compounds for the shell include AlN, AlP, and Al ２ O ３ , GaN, GaP, Ga ２ O ３ , SiO ２ , CdS, CdSe, CdSeS, CdTe, CdSeTe, CdTeS, ZnS, ZnSe, ZnSeS, ZnTe, ZnSeTe, ZnTeS, ZnO, ZnOS, ZnSeO, and ZnTeO, etc. In the present invention, the coating compound is preferably obtained by reaction with at least a zinc source.

[0023] 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.

[0024] <Ligand Exchange Step> The ligand exchange reaction in the present invention is a step of adding hydrophilic ligands to a hydrophobic quantum dot dispersion liquid to exchange the ligands bound to the surface of the quantum dots with hydrophilic ligands, thereby obtaining hydrophilic quantum dots.

[0025] 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, from the viewpoint of the stability of the hydrophilic quantum dots and improving the quantum yield, thiols, amine-containing thiols, amines, alcohols, thiol-containing alcohols, phosphonic acids, carboxylic acids, and thiol-containing carboxylic acids are preferred, and thiol-containing carboxylic acids are particularly preferred.

[0026] Examples of the thiol-containing carboxylic acid include thioglycolic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiomalic acid, cysteine, acetylcysteine, homocysteine, penicillamine, etc. 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.

[0027] The temperature when adding the hydrophilic ligand 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 the 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 the quantum dots.

[0028] The atmosphere in which the hydrophilic ligands are added to the quantum dot dispersion 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. By the above steps, a dispersion containing hydrophilic quantum dots is obtained.

[0029] Next, the hydrophilic quantum dots are separated from the dispersion containing the hydrophilic quantum dots. Examples of separation methods include common methods such as centrifugation, decantation, and suction filtration. In this process, in order to remove unnecessary hydrophobic ligands, it is preferable to add a solvent capable of dissolving the ligands and repeat the procedure of separating the hydrophilic quantum dots by centrifugation or the like multiple times. Through the above steps, hydrophilic quantum dots are obtained.

[0030] <Dispersion Step> 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.

[0031] 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.

[0032] The hydrophilic quantum dot aqueous solvent dispersion can be 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 an ammonium salt form, and then disperse them in the aqueous solvent, from the viewpoint of obtaining a uniform dispersion.

[0033] Furthermore, 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 them into a solution containing impurities and the hydrophilic quantum dots, and dispersing the separated hydrophilic quantum dots in an aqueous solvent. The separation of the solution containing impurities from the hydrophilic quantum dots can be performed by operations such as centrifugation, decantation, and suction filtration. Through the above process, an aqueous solvent dispersion of hydrophilic quantum dots is obtained.

[0034] <Surface Treatment Step> 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 the aqueous solvent, thereby obtaining an aqueous solvent dispersion of surface-treated hydrophilic quantum dots.

[0035] In the surface treatment step, defects generated on the quantum dot surface by the ligand exchange step are repaired in an aqueous solvent using a surface treatment agent, thereby stabilizing the quantum dots and making the surface uniform, thereby enabling stable shell formation in the shell formation step described below. Therefore, the water-soluble quantum dots finally obtained according to the present invention have an FWHM equivalent to that of quantum dots obtained in an organic solvent, and the decrease in quantum yield is suppressed.

[0036] 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; NF ３ Nitrogen halides such as NR ４ F and PR ４Examples of suitable surface treatment agents include halides of quaternary ammonium compounds and quaternary phosphonium compounds such as F (R represents H, or a saturated, unsaturated, or aromatic hydrocarbon having 1 to 8 carbon atoms, which may be linear or branched, and which 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, the surface treatment agent is preferably a metal halide or a metal carboxylate, from the viewpoint of further improving the quantum yield. Examples of the metal halides include zinc fluoride, zinc chloride, zinc bromide, and zinc iodide, with zinc chloride being particularly preferred. Examples of the metal carboxylates include zinc acetate, zinc trifluoroacetate, zinc myristate, zinc oleate, and zinc benzoate, with zinc acetate being particularly preferred.

[0037] 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 will vary depending on the type of 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.

[0038] 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.

[0039] Methods for adding the surface treatment agent in the surface treatment step include adding the surface treatment agent directly to the reaction solution and adding the surface treatment agent dissolved or dispersed in a solvent to the reaction solution. When adding the surface treatment agent dissolved or dispersed in a solvent to the reaction solution, the solvent is not particularly limited as long as it is a water-soluble solvent, and examples include water, methanol, ethanol, 2-propanol, ethylene glycol, propylene glycol, acetone, acetonitrile, N-methylpyrrolidone, dimethylformamide, and tetrahydrofuran, with water being particularly preferred. By the above steps, an aqueous solvent dispersion of surface-treated hydrophilic quantum dots is obtained.

[0040] <Shell Formation Step> The shell formation step in the present invention is a step of adding a shell-forming agent to an 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.

[0041] The shell-forming agent is preferably 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. From the viewpoint of improving quantum yield in particular, a mixture of a thiol-containing carboxylic acid as the sulfur source and a zinc carboxylate, zinc perchlorate, or zinc halide as the zinc source is preferred, and a mixture of a thiol-containing carboxylic acid and a zinc halide is particularly preferred.

[0042] Examples of the thiol-containing carboxylic acids include thioglycolic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiomalic acid, cysteine, acetylcysteine, homocysteine, penicillamine, etc. Among these, from the viewpoints 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.

[0043] As the zinc halide, zinc fluoride, zinc chloride, zinc bromide and zinc iodide are preferred, with zinc chloride being particularly preferred.

[0044] The shell can be formed by adding a shell-forming agent to the aqueous solvent dispersion of the surface-treated hydrophilic quantum dots. After adding the shell-forming agent, the surface-treated hydrophilic quantum dots are treated at preferably 30° C. to 200° C., more preferably 50° C. to 150° C., and particularly 70° C. to 120° C. for preferably 1 minute to 600 minutes, more preferably 5 minutes to 240 minutes, and particularly 10 minutes to 120 minutes, thereby forming a shell on the surface of the surface-treated hydrophilic quantum dots.

[0045] The atmosphere in the shell formation 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.

[0046] Methods for adding the shell-forming agent in the shell formation step include a method in which the shell-forming agent is added directly to the reaction solution, and a method in which the shell-forming agent is added to the reaction solution in a dissolved or dispersed state in a solvent. When adding the shell-forming agent to the reaction solution in a dissolved or dispersed state in a solvent, the solvent is not particularly limited as long as it is a water-soluble solvent, and examples include water, methanol, ethanol, 2-propanol, ethylene glycol, propylene glycol, acetone, acetonitrile, N-methylpyrrolidone, dimethylformamide, and tetrahydrofuran, with water being particularly preferred. By the above steps, an aqueous solvent dispersion of water-soluble quantum dots is obtained.

[0047] 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, suction filtration, etc. 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.

[0048] The water-soluble quantum dots obtained by the production method of the present invention exhibit excellent luminescence properties even in aqueous solvents because the decrease in quantum yield compared to the hydrophobic quantum dots before solubilization is suppressed. That is, when 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 solubilization is taken as the quantum yield maintenance value, this maintenance value is greater than 0.8, preferably 0.82 or greater. Therefore, the water-soluble quantum dots can be suitably used in fields where hydrophobic quantum dots could not be used, such as in the field of bioimaging as fluorescent probes.

[0049] 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 aqueous dispersion of the obtained quantum dots was measured using an absolute PL quantum yield measurement device (Quantaurus-QY, manufactured by Hamamatsu Photonics K.K.) 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 The maintenance value of quantum yield was calculated using the following formula: (Maintenance value of quantum yield) = (Quantum yield of water-soluble quantum dots) / (Quantum yield of quantum dots obtained in Synthesis Example)

[0050] 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, yielding a 1-octadecene solution of indium zinc myristate. The resulting 1-octadecene solution of indium zinc myristate was heated to 40°C under a nitrogen atmosphere, and 2.51 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added. The mixture was then maintained at 40°C and allowed to cool naturally to 20°C. This yielded a yellow solution containing an InP quantum dot precursor. Subsequently, the InP quantum dot precursor solution was heated to 250°C to yield 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, heated to 120°C under reduced pressure with 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. 18.17 g of the InP quantum dot reaction solution obtained above was added, and the temperature was raised to 230°C and maintained for 30 minutes. Then, the temperature was further raised to 300°C and maintained for 60 minutes, thereby obtaining an oleylamine / dioctylamine dispersion of InP / ZnSe quantum dots having an InP core and a ZnSe shell. After cooling the resulting dispersion to 240 ° C, 7.54 g of zinc oleate, 7.54 g of trioctylphosphine, and 4.35 g of trioctylphosphine sulfide were added and held for 90 minutes to obtain an oleylamine / dioctylamine dispersion of multi-shell quantum dots with InP cores and ZnSe and ZnS shells. After cooling the resulting 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 added to this dispersion, and the mixture was stirred and centrifuged to recover the InP / ZnSe / ZnS quantum dots as a precipitate. The recovered InP / ZnSe / ZnS quantum dots were suspended in 7.03 g of octane to obtain a dispersion of purified InP / ZnSe / ZnS quantum dots in octane. The luminescence properties of the resulting dispersion were measured, and the results are shown in Table 1.

[0051] 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, yielding a 1-octadecene solution of indium myristate. The resulting 1-octadecene solution of indium myristate was heated to 40°C under a nitrogen atmosphere, and 2.51 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added. The mixture was then maintained at 40°C and allowed to cool to 20°C. This yielded a yellow solution containing InP quantum dot precursors. Subsequently, the InP quantum dot precursor solution was heated to 280°C to yield 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, heated to 120°C under reduced pressure with 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. 18.19 g of the InP quantum dot reaction solution obtained above was added, and the temperature was raised to 230°C and maintained for 30 minutes. Then, the temperature was further raised to 300°C and maintained for 60 minutes, thereby obtaining an oleylamine / dioctylamine dispersion of InP / ZnSe quantum dots having an InP core and a ZnSe shell. The resulting 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 added. The mixture was then held for 90 minutes to obtain an oleylamine / dioctylamine dispersion of multi-shell quantum dots with InP cores and ZnSe and ZnS shells. The resulting dispersion was then cooled to room temperature, and 50 g of acetone was added, followed by stirring and centrifugation to recover InP / ZnSe / ZnS quantum dots as a precipitate. The recovered InP / ZnSe / ZnS quantum dots were suspended in 8.67 g of toluene to obtain a toluene dispersion of InP / ZnSe / ZnS quantum dots. Another 50 g of acetone was added to this dispersion, followed by stirring and centrifugation to recover InP / ZnSe / ZnS quantum dots as a precipitate.The recovered InP / ZnSe / ZnS quantum dots were suspended in 7.03 g of octane to obtain a dispersion of purified InP / ZnSe / ZnS quantum dots in octane. The luminescence properties of the resulting dispersion were measured, and the results are shown in Table 1.

[0052] 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, yielding a 1-octadecene solution of indium myristate. The resulting 1-octadecene solution of indium myristate was heated to 40°C under a nitrogen atmosphere, and 2.51 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added. The mixture was then maintained at 40°C and allowed to cool to 20°C. This yielded a yellow solution containing InP quantum dot precursors. Subsequently, the InP quantum dot precursor solution was heated to 280°C to obtain a base InP quantum dot reaction solution. 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., yielding a 1-octadecene solution of indium myristate. The resulting 1-octadecene solution of indium myristate was heated to 40 ° C. under a nitrogen atmosphere, and 3.76 g of a trioctylphosphine solution containing 10% by mass of tris(trimethylsilyl)phosphine was added. The mixture was then maintained at 40 ° C. for 10 minutes and allowed to cool naturally to 20 ° C. This yielded a yellow solution containing InP quantum dot precursors. The previously prepared base InP quantum dot reaction solution was reheated to 260 ° C., and a 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, heated to 120° C. under reduced pressure with 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. 15.29 g of the InP quantum dot reaction solution obtained above was added, the temperature was raised to 230 ° C and held for 30 minutes, and then further raised to 300 ° C and held for 60 minutes to obtain an oleylamine / dioctylamine dispersion of InP / ZnSe quantum dots having an InP core and a ZnSe shell. The resulting dispersion was cooled to 240 ° C, and 7.54 g of zinc oleate, 7.54 g of trioctylphosphine, and 4.35 g of trioctylphosphine sulfide were added and held for 90 minutes to obtain an oleylamine / dioctylamine dispersion of multi-shell quantum dots having an InP core and ZnSe and ZnS stacked shells. The resulting dispersion was cooled to room temperature, and 50 g of acetone was added and stirred. The InP / ZnSe / ZnS quantum dots were collected as a precipitate by centrifugation. The recovered 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 added to this dispersion, followed by stirring and centrifugation to recover the InP / ZnSe / ZnS quantum dots as a precipitate. The recovered 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 luminescence characteristics of the resulting dispersion were measured, and the results are shown in Table 1.

[0053] 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 maintained for 10 minutes, yielding InP / ZnSe / ZnS quantum dots in which the ligands had been 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. The supernatant was removed again. This process was repeated three times to obtain a solid. The solid was suspended in 5 ml of ethanol and centrifuged to recover InP / ZnSe / ZnS quantum dots as a precipitate. (Dispersion Step) The recovered InP / ZnSe / ZnS quantum dots were dispersed by adding 3.6 ml of a 40% aqueous solution of tetraethylammonium hydroxide, followed by adding 6 ml of tetrahydrofuran and 60 ml of 2-propanol and stirring, followed by centrifuging to recover InP / ZnSe / ZnS quantum dots as a precipitate. The recovered InP / ZnSe / ZnS quantum dots were dispersed by adding 1.6 ml of pure water, followed by adding 60 ml of 2-propanol and stirring, followed by centrifuging to recover InP / ZnSe / ZnS quantum dots as a precipitate. This process 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 Step) 3 ml of an InP / ZnSe / ZnS quantum dot aqueous dispersion (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 using nitrogen gas to create a nitrogen atmosphere. The internal temperature was heated to between 80°C and 100°C while stirring, and the flask was maintained for 30 minutes. After that, the flask was cooled to room temperature to obtain a surface-treated InP / ZnSe / ZnS quantum dot aqueous dispersion. (Shell Formation Step) 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 to the obtained surface-treated InP / ZnSe / ZnS quantum dot aqueous dispersion.After depressurization, the pressure was returned to atmospheric pressure using nitrogen gas to create a nitrogen atmosphere. The internal temperature was heated to between 80°C and 100°C while stirring, held for 30 minutes, and then cooled to room temperature to obtain an aqueous dispersion of water-soluble InP / ZnSe / ZnS quantum dots. The resulting aqueous dispersion of water-soluble InP / ZnSe / ZnS quantum dots was centrifuged to recover water-soluble InP / ZnSe / ZnS quantum dots as a precipitate. The recovered precipitate was dispersed in 2.25 ml of pure water and 0.10 ml of a 35% aqueous solution of tetraethylammonium hydroxide, and then centrifuged to remove insoluble impurities, yielding a purified aqueous dispersion of water-soluble InP / ZnSe / ZnS quantum dots. The luminescence characteristics of the resulting aqueous dispersion were measured, and the results are shown in Table 1. The emission spectrum of the resulting aqueous dispersion was also measured, and the results are shown in Figure 1.

[0054] Example 2 An aqueous dispersion of purified water-soluble InP / ZnSe / ZnS quantum dots was obtained by the same procedure as in Example 1, except that the octane dispersion of orange InP / ZnSe / ZnS quantum dots obtained in Synthesis Example 2 was used in the ligand exchange step. The luminescence 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 Figure 2.

[0055] Example 3 An aqueous dispersion of purified water-soluble InP / ZnSe / ZnS quantum dots was obtained by the same procedure as in Example 1, except that the octane dispersion of red InP / ZnSe / ZnS quantum dots obtained in Synthesis Example 3 was used in the ligand exchange step. The luminescence 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 Figure 3.

[0056] Example 4: An InP / ZnSe / ZnS quantum dot aqueous dispersion was obtained by performing the same operations as in Example 1 up to the dispersion step, except that the octane dispersion of red InP / ZnSe / ZnS quantum dots obtained in Synthesis Example 3 was used in the ligand exchange step. (Surface Treatment Step) 0.12 g of anhydrous zinc chloride was dissolved in 0.60 ml of pure water to obtain an aqueous solution of anhydrous zinc chloride. 0.05 ml of this aqueous solution of anhydrous zinc chloride, 2.0 ml of γ-butyrolactone, and 0.03 ml of the 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 flask was returned to atmospheric pressure with nitrogen gas to create a nitrogen atmosphere. The flask was heated to an internal temperature of 80°C to 100°C while stirring, maintained at this temperature for 30 minutes, and then cooled to room temperature to obtain an aqueous dispersion of surface-treated InP / ZnSe / ZnS quantum dots. (Shell Formation Step) 0.05 ml of anhydrous zinc chloride aqueous solution and 0.02 ml of 3-mercaptopropionic acid were added to the resulting surface-treated InP / ZnSe / ZnS quantum dot aqueous dispersion. After reducing the pressure, the pressure was returned to atmospheric pressure using nitrogen gas to create a nitrogen atmosphere. The internal temperature was heated to between 190°C and 205°C while stirring, and the mixture was maintained for 30 minutes. After that, the mixture was cooled to room temperature to obtain a water-soluble InP / ZnSe / ZnS quantum dot aqueous dispersion. The resulting water-soluble InP / ZnSe / ZnS quantum dot aqueous dispersion was centrifuged to recover water-soluble InP / ZnSe / ZnS quantum dots as a precipitate. The recovered precipitate was dispersed in 1.0 ml of pure water and 0.1 ml of a 35% aqueous solution of tetraethylammonium hydroxide. Insoluble impurities were then removed by centrifugation to obtain a purified water-soluble InP / ZnSe / ZnS quantum dot aqueous dispersion. The luminescence characteristics of the resulting aqueous dispersion were measured, and the results are shown in Table 1.

[0057] Example 5: An InP / ZnSe / ZnS quantum dot aqueous dispersion was obtained by performing the same operations as in Example 1 up to the dispersion step, except that the octane dispersion of orange InP / ZnSe / ZnS quantum dots obtained in Synthesis Example 2 was used in the ligand exchange step. (Surface Treatment Step) 0.044 g of zinc acetate dihydrate was dissolved in 2.0 ml of pure water to obtain a zinc acetate aqueous solution. 1.0 ml of this zinc acetate aqueous solution, 5.0 ml of pure water, and 0.5 ml of InP / ZnSe / ZnS quantum dot aqueous dispersion (quantum dot concentration: 0.020 mmol / ml) were weighed into a glass flask with a lid. After reducing the pressure, the flask was returned to atmospheric pressure with nitrogen gas to create a nitrogen atmosphere. The internal temperature was heated to between 80°C and 100°C while stirring, maintained at this temperature for 30 minutes, and then cooled to room temperature to obtain a surface-treated InP / ZnSe / ZnS quantum dot aqueous dispersion. (Shell Formation Step) 1.0 ml of aqueous zinc acetate solution and 0.16 ml of 3-mercaptopropionic acid were added to the resulting surface-treated InP / ZnSe / ZnS quantum dot aqueous dispersion. After reducing the pressure, the pressure was returned to atmospheric pressure using nitrogen gas to create a nitrogen atmosphere. The internal temperature was heated to between 80°C and 100°C while stirring, and the mixture was maintained for 30 minutes. After that, the mixture was cooled to room temperature to obtain a water-soluble InP / ZnSe / ZnS quantum dot aqueous dispersion. The resulting water-soluble InP / ZnSe / ZnS quantum dot aqueous dispersion was centrifuged to recover water-soluble InP / ZnSe / ZnS quantum dots as a precipitate. The recovered precipitate was dispersed in 0.30 ml of pure water and 0.03 ml of a 35% aqueous solution of tetraethylammonium hydroxide. Insoluble impurities were then removed by centrifugation to obtain a purified water-soluble InP / ZnSe / ZnS quantum dot aqueous dispersion. The luminescence characteristics of the resulting aqueous dispersion were measured, and the results are shown in Table 1.

[0058] Comparative Example 1 The aqueous dispersion of 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.

[0059] Comparative Example 2 The aqueous dispersion of purified InP / ZnSe / ZnS quantum dots obtained in the dispersing step in Example 2 was used as water-soluble quantum dots, and the luminescence characteristics were measured. The results are shown in Table 1.

[0060] Comparative Example 3 The aqueous dispersion of 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.

[0061] 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, and InP / ZnSe / ZnS quantum dots were recovered 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 resulting dispersion was placed in a single-neck flask in a nitrogen atmosphere, and 1 ml of 28% aqueous ammonia, 0.5 ml of 1-butanol, and 0.25 ml of mercaptopropionic acid were added. The mixture was heated with stirring to an internal temperature between 55°C and 65°C. After cooling, 7.5 ml of acetonitrile was added and centrifuged. The resulting solid was dispersed in 1.5 ml of pure water to obtain a mixed dispersion of InP / ZnSe / ZnS quantum dots. To the resulting InP / ZnSe / ZnS quantum dot dispersion, 1 ml of aqueous Zn-mercaptopropionic acid solution, prepared by mixing 0.14 g of anhydrous zinc chloride, 1 ml of mercaptopropionic acid, and 4 ml of pure water, was added. The mixture was heated with stirring to an internal temperature of 70 to 80°C, and then irradiated with ultraviolet light (365 nm wavelength) for 40 minutes while stirring. After 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 mixture was centrifuged twice. The resulting solid was dispersed in 1 ml of pure water to obtain a water-soluble InP / ZnSe / ZnS quantum dot aqueous dispersion. The luminescence characteristics of the resulting aqueous dispersion were measured, and the results are shown in Table 1.

[0062]

[0063] As shown in Table 1, the quantum dots water-soluble by the manufacturing method of the present invention have an FWHM equivalent to that of the hydrophobic quantum dots used as raw materials, and it can be seen that the decrease in quantum yield is also suppressed.

Claims

1. A method for producing water-soluble quantum dots, comprising: a ligand exchange step of adding hydrophilic ligands to a hydrophobic quantum dot dispersion liquid to exchange the ligands bonded to the surfaces of the quantum dots for 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.

2. 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 at least a phosphorus source and an indium source, and the shell is a coating compound other than an InP-based compound.

3. The shell is AlN, AlP, Al ２ O ３ , GaN, GaP, Ga ２ O ３ , SiO ２ 3. The method for producing water-soluble quantum dots according to claim 1 or 2, wherein the water-soluble quantum dot is at least one selected from the group consisting of CdS, CdSe, CdSeS, CdTe, CdSeTe, CdTeS, ZnS, ZnSe, ZnSeS, ZnTe, ZnSeTe, ZnTeS, ZnO, ZnOS, ZnSeO and ZnTeO.

4. 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.

5. The method for producing water-soluble quantum dots according to claim 1 or 2, wherein the aqueous solvent is water.

6. 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.

7. 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.

8. A water-soluble quantum dot, in which the quantum yield of the water-soluble quantum dot obtained by exchanging the ligands bound to the surface of the hydrophobic quantum dot with hydrophilic ligands is greater than 0.8 relative to the quantum yield of the hydrophobic quantum dot.

9. The water-soluble quantum dot according to claim 8, which is a quantum dot of a core-shell structure 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.

10. The shell is AlN, AlP, Al ２ O ３ , GaN, GaP, Ga ２ O ３ , SiO ２ 10. The water-soluble quantum dot according to claim 9, which is at least one selected from the group consisting of CdS, CdSe, CdSeS, CdTe, CdSeTe, CdTeS, ZnS, ZnSe, ZnSeS, ZnTe, ZnSeTe, ZnTeS, ZnO, ZnOS, ZnSeO, or ZnTeO.

11. The water-soluble quantum dot according to claim 8 or 9, wherein 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.

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