Antibacterial silver nanoparticle dispersion

Abstract

To provide an antibacterial silver nanoparticle dispersion that exhibits superior antibacterial effect while maintaining superior dispersion stability.SOLUTION: The antibacterial silver nanoparticle dispersion of the present invention contains silver nanoparticles, a dispersant, and a polar solvent, wherein the silver nanoparticles have an average primary particle size of 20 nm or smaller and a secondary particle diameter (D50) of 200 nm or smaller, and the dispersant includes at least one selected from an alkylamine and an oxygen-containing amine, and also includes a nitrogen-containing organosilane compound.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an antibacterial silver nanoparticle dispersion. [Background technology]

[0002] Nano-sized silver particles (silver nanoparticles) are expected to be used in a wide range of fields due to their antibacterial, bactericidal, metallic, and optical properties. For example, Patent Document 1 discloses that a suspension of nano-sized silver nanoparticles can be obtained by heating and stirring a mixture of a silver compound, an alkylamine, and an alcohol compound. Patent Document 2 discloses the use of a mixture containing an aqueous silver nitrate solution, butanol, and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane as a biofilm-inhibiting coating material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-31542 [Patent Document 2] Special Publication No. 2005-523987 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0004] The mixture containing silver nanoparticles described in the above document may have a large particle size (secondary particle size) of aggregated particles in polar solvents such as alcohols. This may result in a decrease in the dispersibility of the silver nanoparticles, leading to precipitation or silver mirror formation. In particular, when used for antibacterial purposes, precipitation of silver nanoparticles may result in insufficient antibacterial effect or in the inability to use the composition as an antibacterial composition.

[0005] An object of the present invention is to provide an antibacterial silver nanoparticle dispersion that has excellent dispersion stability and exhibits excellent antibacterial effect. [Means for solving the problem]

[0006] As a result of extensive research into solving the above problems, the present inventors have found a solution comprising the following configuration, and have completed the present invention. (1) The antibacterial silver nanoparticle dispersion according to the present invention comprises silver nanoparticles, a dispersant, and a polar solvent. The silver nanoparticles have an average primary particle size of 20 nm or less and a secondary particle size (D50) of 200 nm or less. The dispersant comprises at least one of an alkylamine and an amine containing an oxygen atom, and a nitrogen-containing organosilane compound. (2) The antibacterial silver nanoparticle dispersion according to (1) above, wherein the alkyl amine has an alkyl group moiety having 5 to 16 carbon atoms. (3) The antibacterial silver nanoparticle dispersion according to (1) or (2) above, which has a zeta potential of zero or positive when the silver nanoparticle concentration is 50 ppm. (4) The antibacterial silver nanoparticle dispersion according to (3) above, having a zeta potential of 25 mV or more. [Effects of the Invention]

[0007] The antibacterial silver nanoparticle dispersion according to the present invention has the above-mentioned constitution, and therefore has excellent dispersion stability and exhibits excellent antibacterial effects. DETAILED DESCRIPTION OF THE INVENTION

[0008] As described above, an antibacterial silver nanoparticle dispersion according to one embodiment of the present invention contains silver nanoparticles, a dispersant, and a polar solvent.

[0009] The silver nanoparticles contained in the antibacterial silver nanoparticle dispersion according to one embodiment have an average primary particle diameter of 20 nm or less. When the silver nanoparticles have an average primary particle diameter of 20 nm or less, the antibacterial silver nanoparticle dispersion according to one embodiment exhibits an excellent antibacterial effect. When the silver nanoparticles have an average primary particle diameter exceeding 20 nm, a sufficient antibacterial effect is not exhibited.

[0010] The method for measuring the average primary particle diameter of silver nanoparticles is not limited. For example, the primary particle diameter of silver nanoparticles may be observed using a field emission-scanning electron microscope (FE-SEM) to determine the average primary particle diameter. The number of silver nanoparticles observed may be, for example, about 30.

[0011] Specifically, ethanol is added to the silver nanoparticle dispersion and diluted until it becomes observable, to obtain a diluted silver nanoparticle dispersion. The dilution concentration is not limited as long as the particle diameter of the nanoparticles can be observed. Next, an appropriate amount of the diluted silver nanoparticle dispersion is dropped onto a Si chip, dried at a temperature that does not cause sintering of the particles, preferably at room temperature, until the ethanol evaporates, and the image is observed using an FE-SEM. Next, 30 nanoparticles are randomly selected, and the narrowest diameter portion of each nanoparticle is measured. The arithmetic mean of these measurements is calculated as the average primary particle diameter of the silver nanoparticle dispersion.

[0012] Furthermore, the silver nanoparticles contained in the antibacterial silver nanoparticle dispersion according to one embodiment have a secondary particle diameter (D50) of 200 nm or less. By virtue of the silver nanoparticles having a secondary particle diameter (D50) of 200 nm or less, the antibacterial silver nanoparticle dispersion according to one embodiment has excellent dispersion stability. If the silver nanoparticles have a secondary particle diameter (D50) exceeding 200 nm, the dispersion stability decreases, and precipitation or silver mirroring becomes more likely to occur.

[0013] The method for measuring the secondary particle diameter (D50) of silver nanoparticles is not limited. For example, a dispersion containing 1% by mass of silver nanoparticles is prepared, and the secondary particle diameter in the dispersion is measured using a nanoparticle diameter measurement device. Specifically, ethanol is added to the prepared highly polar silver nanoparticle dispersion to prepare a dispersion containing 1% by mass of silver nanoparticles. Next, approximately 5 mL of the prepared silver nanoparticle dispersion is added to the cell of the nanoparticle diameter measurement device, and the particle diameter in the dispersion is measured under the set conditions of a measurement time of 30 seconds and two measurements, and the D50 value is taken as the secondary particle diameter.

[0014] The dispersant contained in the antibacterial silver nanoparticle dispersion according to one embodiment includes at least one of an alkylamine and an amine containing an oxygen atom, and a nitrogen-containing organosilane compound. Hereinafter, "at least one of an alkylamine and an amine containing an oxygen atom" may be simply referred to as "alkylamine, etc." Examples of alkylamines include monoalkylamines, dialkylamines, and trialkylamines. From the viewpoint of antibacterial properties, monoalkylamines and dialkylamines are preferred, with monoalkylamines being more preferred. The number of carbon atoms in the alkyl group is not limited; for example, the alkyl group may preferably have 5 to 16 carbon atoms, more preferably 6 to 12 carbon atoms.

[0015] The alkyl group moiety may have a linear or branched structure. From the viewpoint of antibacterial properties, a linear structure is preferred because it facilitates the production of silver nanoparticles with a small primary particle size. A linear structure has less bulky molecules than a branched structure, and the molecular structure has less steric hindrance. As described below, a molecular structure with less steric hindrance makes it easier to produce silver nanoparticles with a small primary particle size. In the case of dialkylamines and trialkylamines, the alkyl group moieties may be the same or different alkyl groups. In the case of dialkylamines and trialkylamines, the "number of carbon atoms in the alkyl group moiety" does not refer to the number of carbon atoms in each alkyl group, but refers to the sum of the number of carbon atoms in each alkyl group. For example, in the case of dibutylamine, the butyl group has four carbon atoms, so the number of carbon atoms in the alkyl group moiety of dibutylamine is 8.

[0016] Examples of alkylamines contained in the antibacterial silver nanoparticle dispersion according to one embodiment include butylamine (carbon number: 4), amylamine (carbon number: 5), hexylamine (carbon number: 6), octylamine (carbon number: 8), dodecylamine (carbon number: 12), hexadecylamine (carbon number: 16), 2-ethylhexylamine (carbon number: 8), 2-butyl-n-octan-1-amine (carbon number: 12), dibutylamine (carbon number: 8), triethylamine (carbon number: 6), tributylamine (carbon number: 12), 1,10-diaminodecane (carbon number: 10), and bis(hexamethylene)triamine (carbon number: 12). These alkylamines may be used alone or in combination of two or more.

[0017] When producing silver nanoparticles, multiple silver atoms generated as the reaction progresses gather together to form particle nuclei. As more silver atoms are subsequently incorporated, they grow into silver nanoparticles. If too many silver atoms are incorporated, the primary particle size tends to become coarse. The use of alkylamines makes it easier to obtain silver nanoparticles with small primary particle sizes. Specifically, alkylamines adsorb to the surface of silver nanoparticles, making it easier to prevent excessive silver atoms from being incorporated into the silver nanoparticles.

[0018] As described above, alkylamines include monoalkylamines, dialkylamines, and trialkylamines. Among these alkylamines, monoalkylamines are preferred. Monoalkylamines have a small steric hindrance in terms of molecular structure and are more likely to adsorb to the surface of silver nanoparticles. This makes it easier to inhibit excessive incorporation of silver atoms into silver nanoparticles, making it easier to obtain silver nanoparticles with a small primary particle size.

[0019] Examples of amines containing oxygen atoms include amines having at least one functional group selected from the group consisting of a hydroxy group, a ketone group, a carboxy group, an alkoxy group, and an ether group. Specific examples of amines containing oxygen atoms include 5-amino-1-hexanol (carbon number: 6), 3-amino-2-cyclohexen-1-one (carbon number: 6), 7-aminoheptanoic acid (carbon number: 7), methyl 6-aminohexanoate (carbon number: 7), 2-ethylhexyloxypropylamine (carbon number: 11), and 3-ethoxypropylamine (carbon number: 5). These amines containing oxygen atoms may be used alone or in combination of two or more. Furthermore, they may be used in combination with one or more of the above-mentioned alkylamines.

[0020] The content of alkylamine etc. is not limited, and is, for example, preferably 0.15 to 1000 parts by mass, more preferably 1.0 to 500 parts by mass, and even more preferably 1.2 to 500 parts by mass, relative to 100 parts by mass of silver nanoparticles. When the content of alkylamine is 0.15 to 1000 parts by mass, the average primary particle size of the silver nanoparticles can be controlled to be small, and excellent antibacterial properties can be obtained.

[0021] Nitrogen-containing organosilane compounds have the general formula R 1 n SiR 2 (4-n) It is a compound represented by R 1 R is the same or different and represents an alkoxy group or alkyl group having 1 to 3 carbon atoms. 2 represents groups, which may be the same or different, containing at least one amino group; n represents an integer of 1, 2, or 3;

[0022] Examples of the nitrogen-containing organic silane compound contained in the antibacterial silver nanoparticle dispersion liquid according to one embodiment include 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethylamino)propyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldiethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldiethoxysilane, Silane coupling agents such as aminopropylmethyltriethoxysilane, [3-(trimethoxysilyl)propyl]dimethyl(octadecyl)aminium chloride, N,N-dimethyl-N-[3-(triethoxysilyl)propyl]-1-octadecaneaminium chloride, and [3-(trimethoxysilyl)propyl]dimethyl(octadecyl)aminium chloride, as well as N,N'-bis(trimethylsilyl)urea, dimethylsilylamine, and aminomethyltrimethylsilane.

[0023] The nitrogen-containing organosilane compound is preferably one containing one or more alkoxy moieties bonded to a silane and one or more amino groups. From the viewpoint of dispersion stability, it is more preferable to use one or more alkoxy moieties bonded to a silane and two or more amino groups. These nitrogen-containing organosilane compounds may be used alone or in combination of two or more.

[0024] The content of the nitrogen-containing organosilane compound is not limited, and is, for example, preferably 0.1 to 2000 parts by mass, more preferably 1 to 2000 parts by mass, even more preferably 10 to 1000 parts by mass, and particularly preferably 30 to 500 parts by mass, relative to 100 parts by mass of silver nanoparticles. When the nitrogen-containing organosilane compound is 0.1 to 2000 parts by mass, the secondary particle diameter (D50) of the silver nanoparticles in the dispersion can be reduced, and dispersion stability is further improved.

[0025] The total content of the alkylamine or the like and the nitrogen-containing organic silane compound is not limited as long as it is the sum of the above-mentioned respective contents, and is preferably 0.25 parts by mass or more and 2500 parts by mass or less, more preferably 16 parts by mass or more and 2000 parts by mass or less, and even more preferably 180 parts by mass or more and 1000 parts by mass or less, relative to 100 parts by mass of silver nanoparticles.

[0026] The polar solvent contained in the antibacterial silver nanoparticle dispersion liquid according to one embodiment is not limited as long as it is a polar solvent capable of dispersing silver nanoparticles. Examples of such polar solvents include water, methanol, ethanol, 1-propanol, 2-propanol, acetone, 1-butanol, ethylene glycol, dimethyl sulfoxide, N,N-dimethylformamide, and acetonitrile. These polar solvents may be used alone or in combination. The polar solvent is used so that the concentration of silver nanoparticles contained in the dispersion liquid is preferably 0.00002% by mass or more and 95% by mass or less.

[0027] The method for producing the antibacterial silver nanoparticle dispersion according to one embodiment is not limited, and it can be obtained, for example, by the following method.

[0028] First, powdered silver(I) oxide is dispersed in a solvent to prepare a silver(I) oxide dispersion. The solvent is not limited as long as it can disperse silver(I) oxide and dissolve formic acid and alkylamines, which will be added in a later step. Examples of such solvents include alcohols, ethers, and hydrocarbons, with hydrocarbons being preferred. Examples of hydrocarbons include open-chain hydrocarbons such as hexane, heptane, octane, and nonane, as well as cyclic hydrocarbons such as cyclohexane and methylcyclohexane.

[0029] Next, formic acid is added to the obtained silver(I) oxide dispersion to obtain silver(I) formate. The reaction between silver(I) oxide and formic acid is an exothermic reaction. Therefore, from the viewpoint of safety, formic acid is preferably added dropwise over a period of, for example, 1 minute or more while stirring the silver(I) oxide dispersion at room temperature (40°C or less). Silver(I) oxide reacts with formic acid to obtain silver(I) formate. Formic acid is preferably used in a proportion of 39.7 parts by mass or more and 55.6 parts by mass or less per 100 parts by mass of silver(I) oxide. Most of the unreacted formic acid is dissolved in the solvent.

[0030] Next, an alkylamine or the like is added to the resulting solution containing silver(I) formate. The type of alkylamine or the like and the amount added are as described above, and a detailed description thereof will be omitted. The alkylamine or the like may be added after the temperature of the solution containing silver(I) formate is adjusted to 5°C or higher and 70°C or lower. During and after the addition of the alkylamine or the like, the solution temperature is preferably controlled to 5°C or higher and 70°C or lower.

[0031] If the liquid temperature is 5°C or higher and 70°C or lower, particle coarsening or sintering between particles is less likely to occur, making it easier to obtain more uniform particles. The alkylamine or the like may be added all at once in this step, or may be added in multiple installments. For example, some of the alkylamine or the like may be added in this step, and the remaining alkylamine or the like may be added when adding the nitrogen-containing organosilane compound described below.

[0032] When an alkylamine or the like reacts with silver(I) formate, a silver(I) formate amine complex is obtained. The resulting silver(I) formate amine complex dissolves in a solvent. The silver(I) formate amine complex is an unstable compound. Therefore, upon formation, a decomposing reduction reaction occurs, generating silver atoms in the system. As these silver atoms grow, silver nanoparticles are obtained. At least a portion of the surface of the resulting silver nanoparticles is coated with molecules of an alkylamine or the like (dispersant), and a dispersion is obtained in which the silver nanoparticles are dispersed in the solvent. Some of the alkylamine or the like reacts with the remaining formic acid to form an amine formate addition salt.

[0033] Next, the solvent is removed from the dispersion. The solvent may be distilled off, for example, under reduced pressure. The residue after solvent removal contains silver nanoparticles whose surfaces are coated with molecules such as alkylamine, addition salts of formic acid such as alkylamines, and alkylamines. The addition salts of formic acid such as alkylamines and alkylamines are removed from this residue. A method for removing the addition salts of formic acid such as alkylamines and alkylamines is, for example, to add aqueous ethanol to the residue and dissolve the addition salts of formic acid such as alkylamines and alkylamines in the aqueous ethanol. On the other hand, the silver nanoparticles whose surfaces are coated with molecules such as alkylamines are separated without being dispersed in the aqueous ethanol.

[0034] By removing the addition salt of alkylamine or the like and the aqueous ethanol in which the alkylamine or the like has been dissolved, silver nanoparticles whose surfaces are coated with molecules of alkylamine or the like remain. The addition salt of alkylamine or the aqueous ethanol in which the alkylamine or the like has been dissolved can be removed, for example, by decantation. For further purification, the addition and removal of aqueous ethanol may be repeated.

[0035] Thereafter, for example, the system is heated to 40°C and maintained at a reduced pressure of 20 hPa for 30 minutes or more (vacuum concentration) to remove ethanol and water from the system. If this removal process is insufficient, moisture will remain in the system, which may lead to coarsening of the secondary particle size when the dispersion is prepared. As a result, dispersion stability may be affected. The conditions for vacuum concentration are not particularly limited as long as the ethanol and water from the system can be sufficiently removed without affecting the silver nanoparticles whose surfaces are coated with molecules such as alkylamine. The above values ​​are merely examples, and the heating temperature may preferably be 20°C or higher and 300°C or lower. Vacuum concentration is preferably carried out at a pressure of 1 hPa or higher and 1000 hPa or lower for 1 minute or longer and 100 hours or shorter.

[0036] Next, a nitrogen-containing organosilane compound is added to a polar solvent and dissolved. The nitrogen-containing organosilane compound and the amount of nitrogen-containing organosilane compound added are as described above, and a detailed explanation will be omitted. Silver nanoparticles whose surfaces are coated with molecules such as alkylamine are added to the resulting solution and stirred thoroughly (for example, for 10 minutes or more), causing some of the molecules such as alkylamine that coat the silver nanoparticles to be detached, and the silver nanoparticles are coated with the nitrogen-containing organosilane compound (dispersant). As a result, the silver nanoparticles can be stably dispersed in the polar solvent.

[0037] If necessary, a polar solvent may be added to adjust the silver concentration. This method allows for the production of an antibacterial silver nanoparticle dispersion in which silver nanoparticles having an average primary particle diameter of 20 nm or less and a secondary particle diameter (D50) of 200 nm or less are dispersed. The antibacterial silver nanoparticle dispersion according to one embodiment obtained in this manner does not produce precipitates to the extent that they cannot be redispersed, nor does it produce silver mirror-like deposits. It has excellent dispersion stability and also exhibits excellent antibacterial effects.

[0038] An antibacterial silver nanoparticle dispersion according to one embodiment contains dispersed therein at least one type of silver nanoparticles: those whose surfaces are coated with molecules such as alkylamines; those whose surfaces are coated with molecules of a nitrogen-containing organosilane compound; and those whose surfaces are coated with molecules such as alkylamines and molecules of a nitrogen-containing organosilane compound. Furthermore, the antibacterial silver nanoparticle dispersion according to one embodiment may contain at least one of the alkylamines and the nitrogen-containing organosilane compound in a free state.

[0039] From the viewpoint of antibacterial properties, the zeta potential of the silver nanoparticle dispersion is preferably zero or positive. The zeta potential is the zeta potential of a silver nanoparticle dispersion with a silver nanoparticle concentration of 50 ppm. One of the antibacterial mechanisms of silver nanoparticles is that they may exhibit antibacterial activity when they come into contact with the cell surface of bacterial cells. In a normal aqueous environment with a near-neutral pH, bacterial cells are negatively charged due to dissociation of the carboxyl and phosphate groups exposed on the bacterial cell surface. In this case, if the zeta potential of the particles is zero or positive, electrostatic repulsion is less likely to occur when bacteria approach the particles, improving the contact rate. As a result, antibacterial properties are enhanced. To further enhance antibacterial properties, the zeta potential may be positive, particularly 25 mV or higher.

[0040] The antibacterial effect is exhibited in both dry and wet states. The antibacterial effect in the dry state refers to the antibacterial effect when the silver nanoparticle dispersion is applied to a solid object and then dried. The antibacterial effect in the wet state refers to the antibacterial effect when the silver nanoparticle dispersion is added to a liquid object.

[0041] The above-described manufacturing method is described based on an example using formic acid. However, the present invention is not limited to formic acid. Silver compounds such as silver oxalate, silver nitrate, and silver chloride obtained using other acids, such as oxalic acid, nitric acid, and hydrochloric acid, may also be used. Depending on the silver compound used, the temperature conditions during the reaction with alkylamines or the like may be appropriately set from the viewpoints of reaction efficiency, particle size, sintering, and the like. For example, in the case of silver oxalate, it is preferable to control the temperature control conditions during the reaction with alkylamines or the like to 50°C or higher and 120°C or lower. [Example]

[0042] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples. The raw materials used in the examples and comparative examples are as follows. <Raw materials> Silver oxide (I): Toyo Chemical Industry Co., Ltd. Silver oxalate: Manufactured by Oura Kikinzoku Kogyo Co., Ltd. Octylamine: manufactured by Tokyo Chemical Industry Co., Ltd. Amylamine: manufactured by Tokyo Chemical Industry Co., Ltd. Hexylamine: manufactured by Tokyo Chemical Industry Co., Ltd. Dodecylamine: manufactured by Tokyo Chemical Industry Co., Ltd. 3-Ethoxypropylamine: manufactured by Koei Chemical Co., Ltd. Dibutylamine: manufactured by Kishida Chemical Co., Ltd. Triethylamine: Kishida Chemical Co., Ltd. KBM-903: 3-aminopropyltrimethoxysilane, manufactured by Shin-Etsu Chemical Co., Ltd. KBM-602: N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, manufactured by Shin-Etsu Chemical Co., Ltd. KBE-603: 3-(2-aminoethylamino)propyltriethoxysilane, manufactured by Shin-Etsu Chemical Co., Ltd. N,N'-bis(trimethylsilyl)urea: manufactured by Kishida Chemical Co., Ltd. KBM-4803: 8-glycidoxyoctyltrimethoxysilane, manufactured by Shin-Etsu Chemical Co., Ltd. KBM-5803: 8-methacryloxyoctyltrimethoxysilane, manufactured by Shin-Etsu Chemical Co., Ltd. KBM-22: Dimethyldimethoxysilane, manufactured by Shin-Etsu Chemical Co., Ltd.

[0043] Example 1 107.4 parts by mass of powdered silver(I) oxide was dispersed in 2000 parts by mass of methylcyclohexane to obtain a dispersion of silver(I) oxide. The resulting dispersion was stirred at room temperature (25°C), and 42.7 parts by mass of formic acid was added dropwise to the dispersion over approximately 1 minute. The silver(I) oxide and formic acid reacted to obtain silver(I) formate. When all of the silver(I) oxide was converted into aggregates of silver(I) formate, the temperature of the reaction solution began to drop. Most of the unreacted formic acid was dissolved in the methylcyclohexane.

[0044] When the temperature of the reaction solution dropped to 30°C, 131.8 parts by mass of octylamine was added to the reaction solution. The octylamine reacted with silver(I) formate to produce a silver(I) formate amine complex. The resulting silver(I) formate amine complex was dissolved in methylcyclohexane.

[0045] The silver(I) formate amine complex is an unstable compound. Therefore, upon formation, a decomposing reduction reaction occurs, generating silver atoms in the system. Silver nanoparticles are obtained as these silver atoms grow. At least a portion of the surface of the resulting silver nanoparticles is coated with octylamine molecules, and a dispersion is obtained in methylcyclohexane. Some of the octylamine reacts with the remaining formic acid to form an amine formate addition salt.

[0046] The resulting silver nanoparticle dispersion was then placed in a recovery flask, and the methylcyclohexane was distilled off under reduced pressure. The residue contained silver nanoparticles whose surfaces were coated with octylamine molecules, the octylamine addition salt of formic acid, and octylamine. 1,330 parts by mass of ethanol and 270 parts by mass of water were added to this residue. Most of the octylamine addition salt of formic acid and octylamine dissolved in the mixed solvent of ethanol and water (aqueous ethanol). Meanwhile, the silver nanoparticles whose surfaces were coated with octylamine molecules did not disperse in the aqueous ethanol and separated.

[0047] The supernatant, containing the octylamine addition salt of formic acid and most of the octylamine dissolved therein, was removed by decantation, and silver nanoparticles coated with octylamine molecules were recovered. 1,330 parts by weight of ethanol and 270 parts by weight of water were added to the recovered material, and the same procedure was repeated three times. After three cycles, the recovered material was concentrated under reduced pressure at 40°C and 20 hPa for 30 minutes, and the ethanol and water in the system were removed.

[0048] This procedure yielded silver nanoparticles with a surface coating of 19.9 parts by mass of octylamine molecules per 100 parts by mass of silver nanoparticles. The amount of amine coating was determined by drying the washed silver nanoparticles in the air, then calcining them, and calculating the amount of amine coating from the remaining silver solids. The drying conditions were 110°C for 10 minutes, and the calcination conditions were a heating rate of 50°C / min and a 30-minute hold at 800°C.

[0049] Next, 150 parts by mass of KBM-903 was added to 1,000 parts by mass of ethanol and dissolved. 119.9 parts by mass (solids) of silver nanoparticles coated with octylamine molecules were added to this solution and stirred at 300 rpm for 10 minutes in an air atmosphere. Some of the octylamine molecules coating the silver nanoparticles were detached, leaving them coated with 3-aminopropyltrimethoxysilane. As a result, the silver nanoparticles were stably dispersed in the polar solvent.

[0050] Ethanol was then added to adjust the concentration, resulting in a silver nanoparticle dispersion with an average primary particle diameter of 5 nm and a secondary particle diameter (D50) of 7.1 nm. This dispersion contained silver at a concentration of 1% by mass. The average primary particle diameter of this dispersion was determined by observing the primary particle diameter of 30 randomly selected silver nanoparticles at 500,000x magnification using a field emission scanning electron microscope (FE-SEM) (JSM-7900F, manufactured by JEOL Ltd., accelerating voltage 10 kV). The secondary particle diameter (D50) of the resulting dispersion was measured using a nanoparticle size measurement device (Nano track wave II, manufactured by Microtrack Bell Co., Ltd.). The concentrations of other components are listed in Table 1.

[0051] Example 2 As shown in Table 1, silver nanoparticles whose surfaces were coated with 26.6 parts by mass of dodecylamine molecules per 100 parts by mass of silver nanoparticles were obtained using the same procedure as in Example 1, except that 189.0 parts by mass of dodecylamine was used instead of octylamine. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that KBM-602 was added to 1,000 parts by mass of ethanol instead of KBM-903. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 10 nm and a secondary particle diameter (D50) of 10.4 nm. The concentrations of each component are shown in Table 1.

[0052] Example 3 As shown in Table 1, silver nanoparticles whose surfaces were coated with 9.4 parts by mass of hexylamine molecules per 100 parts by mass of silver nanoparticles were obtained using the same procedure as in Example 1, except that hexylamine was used instead of octylamine. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that KBM-602 was added to 1,000 parts by mass of ethanol instead of KBM-903, and 10.5 parts by mass of hexylamine was added. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 7 nm and a secondary particle diameter (D50) of 19.9 nm. The concentrations of each component are shown in Table 1.

[0053] Example 4 As shown in Table 1, silver nanoparticles whose surfaces were coated with 5.4 parts by mass of amylamine molecules per 100 parts by mass of silver nanoparticles were obtained using the same procedure as in Example 1, except that amylamine was used instead of octylamine. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that KBM-602 was added to 1,000 parts by mass of ethanol instead of KBM-903, and 14.5 parts by mass of amylamine was added. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 11 nm and a secondary particle diameter (D50) of 48.3 nm. The concentrations of each component are shown in Table 1.

[0054] Example 5 As shown in Table 1, silver nanoparticles coated with 19.9 parts by mass of octylamine molecules per 100 parts by mass of silver nanoparticles were obtained using the same procedure as in Example 1. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that KBM-602 was added to 1,000 parts by mass of ethanol instead of KBM-903. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 5 nm and a secondary particle diameter (D50) of 10.0 nm. The concentrations of each component are shown in Table 1.

[0055] Example 6 107.4 parts by mass of powdered silver(I) oxide was dispersed in 2000 parts by mass of methylcyclohexane to obtain a dispersion of silver(I) oxide. The resulting dispersion was stirred at room temperature (25°C), and 42.7 parts by mass of formic acid was added dropwise to the dispersion over approximately 1 minute. The silver(I) oxide and formic acid reacted to obtain silver(I) formate. When all of the silver(I) oxide was converted into aggregates of silver(I) formate, the temperature of the reaction solution began to drop. Most of the unreacted formic acid was dissolved in the methylcyclohexane.

[0056] When the temperature of the reaction solution dropped to 30°C, the same procedure as in Example 1 was used, except that 83.9 parts by mass of octylamine was added to the reaction solution, to obtain silver nanoparticles whose surfaces were coated with 1.3 parts by mass of octylamine molecules per 100 parts by mass of silver nanoparticles.

[0057] Next, 18.6 parts by mass of octylamine and 150 parts by mass of KBM-602 were added to 1,000 parts by mass of ethanol and dissolved. To this solution, 101.3 parts by mass (solids) of silver nanoparticles coated with octylamine molecules were added and stirred at 300 rpm for 10 minutes in an air atmosphere. A portion of the octylamine attached to the silver nanoparticles was released, leaving them coated with N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane. As a result, the silver nanoparticles were stably dispersed in the polar solvent. Further, ethanol was added to adjust the concentration, resulting in a silver nanoparticle dispersion with an average primary particle diameter of 20 nm and a secondary particle diameter (D50) of 14.5 nm. This dispersion contained silver at a concentration of 1% by mass. The concentrations of other components are listed in Table 1.

[0058] Example 7 Silver nanoparticles coated with 1.3 parts by mass of octylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 6. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 6, except that 18.6 parts by mass of octylamine was not added to 1,000 parts by mass of ethanol. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 20 nm and a secondary particle diameter (D50) of 18.4 nm. The concentrations of each component are shown in Table 1.

[0059] Example 8 As shown in Table 2, silver nanoparticles coated with 19.9 parts by mass of octylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 5. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 5, except that 0.1 parts by mass of KBM-602 was added to 1,000 parts by mass of ethanol. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 5 nm and a secondary particle diameter (D50) of 192.7 nm. The concentrations of each component are shown in Table 2.

[0060] Example 9 As shown in Table 2, silver nanoparticles coated with 4.1 parts by mass of 3-ethoxypropylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 1, except that 3-ethoxypropylamine was used instead of octylamine. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that 15.8 parts by mass of 3-ethoxypropylamine per 100 parts by mass of silver was added to 1,000 parts by mass of ethanol, and KBM-602 was added instead of KBM-903. The silver nanoparticles contained in the resulting silver nanoparticle dispersion had an average primary particle diameter of 15 nm and a secondary particle diameter (D50) of 199.9 nm. The concentrations of each component are shown in Table 2.

[0061] Example 10 As shown in Table 2, silver nanoparticles coated with 0.3 parts by mass of dibutylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 1, except that dibutylamine was used instead of octylamine. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that 19.6 parts by mass of dibutylamine per 100 parts by mass of silver was added to 1,000 parts by mass of ethanol, and KBM-602 was added instead of KBM-903. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 20 nm and a secondary particle diameter (D50) of 35.1 nm. The concentrations of each component are shown in Table 2.

[0062] Example 11 As shown in Table 2, silver nanoparticles coated with 0.2 parts by mass of triethylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 1, except that triethylamine was used instead of octylamine. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that 19.7 parts by mass of triethylamine per 100 parts by mass of silver was added to 1,000 parts by mass of ethanol, and KBM-602 was added instead of KBM-903. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 15 nm and a secondary particle diameter (D50) of 36.4 nm. The concentrations of each component are shown in Table 2.

[0063] Example 12 As shown in Table 2, silver nanoparticles coated with 19.9 parts by mass of octylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 1. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that KBE-603 was added to 1,000 parts by mass of ethanol instead of KBM-903. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 5 nm and a secondary particle diameter (D50) of 20.1 nm. The concentrations of each component are shown in Table 2.

[0064] Example 13 As shown in Table 2, silver nanoparticles coated with 19.9 parts by mass of octylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 1. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that N,N'-bis(trimethylsilyl)urea was added to 1,000 parts by mass of ethanol instead of KBM-903. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 5 nm and a secondary particle diameter (D50) of 179.7 nm. The concentrations of each component are shown in Table 2.

[0065] Example 14 140.9 parts by mass of silver oxalate and 240.1 parts by mass of octylamine were mixed and heated in an oil bath at 110°C for 30 minutes while stirring. After cooling, 2000 parts by mass of ethanol was added to the resulting mixture and subjected to centrifugation. The resulting precipitate was then air-dried to obtain silver nanoparticles whose surfaces were coated with 15.5 parts by mass of octylamine molecules per 100 parts by mass of silver.

[0066] Next, a silver nanoparticle dispersion was obtained in the same manner as in Example 1, except that 4.4 parts by mass of octylamine per 100 parts by mass of silver was added to 1,000 parts by mass of ethanol and KBM-602 was added instead of KBM-903. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 7 nm and a secondary particle diameter (D50) of 17.4 nm. The concentrations of each component are shown in Table 2.

[0067] (Comparative Example 1) 107.4 parts by mass of powdered silver(I) oxide was dispersed in 2000 parts by mass of methylcyclohexane to obtain a dispersion of silver(I) oxide. The resulting dispersion was stirred at room temperature (25°C), and 42.7 parts by mass of formic acid was added dropwise to the dispersion over approximately 1 minute. The silver(I) oxide and formic acid reacted to obtain silver(I) formate. When all of the silver(I) oxide was converted into aggregates of silver(I) formate, the temperature of the reaction solution began to drop. Most of the unreacted formic acid was dissolved in the methylcyclohexane.

[0068] When the temperature of the reaction solution dropped to 30°C, the same procedure as in Example 1 was used, except that 10.9 parts by mass of octylamine was added to the reaction solution, to obtain silver nanoparticles whose surfaces were coated with 0.4 parts by mass of octylamine molecules per 100 parts by mass of silver.

[0069] Next, 19.5 parts by mass of octylamine and 150 parts by mass of KBM-602 were added to 1,000 parts by mass of ethanol and dissolved. To this solution, 100.4 parts by mass (solids) of silver nanoparticles coated with octylamine molecules were added and stirred at 300 rpm for 10 minutes in an air atmosphere. A portion of the octylamine attached to the silver nanoparticles was released, leaving them coated with N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane. As a result, the silver nanoparticles were stably dispersed in the polar solvent. Further, ethanol was added to adjust the concentration, resulting in a silver nanoparticle dispersion with an average primary particle diameter of 30 nm and a secondary particle diameter (D50) of 162.0 nm. This dispersion contained silver at a concentration of 1% by mass. The concentrations of other components are shown in Table 3.

[0070] (Comparative Example 2) As shown in Table 3, silver nanoparticles coated with 19.9 parts by mass of octylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 1. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that KBM-22 was used in place of KBM-903 in 1,000 parts by mass of ethanol. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 5 nm and a secondary particle diameter (D50) of 288.1 nm. The concentrations of each component are shown in Table 3.

[0071] (Comparative Example 3) As shown in Table 3, silver nanoparticles coated with 19.9 parts by mass of octylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 1. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that KBM-4803 was used in place of KBM-903 in 1,000 parts by mass of ethanol. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 5 nm and a secondary particle diameter (D50) of 5,266.2 nm. The concentrations of each component are shown in Table 3.

[0072] Comparative Example 4 As shown in Table 3, silver nanoparticles coated with 19.9 parts by mass of octylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 1. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that KBM-5803 was used in place of KBM-903 in 1000 parts by mass of ethanol. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 5 nm and a secondary particle diameter (D50) of 3334.1 nm. The concentrations of each component are shown in Table 3.

[0073] (Comparative Example 5) 107.4 parts by mass of powdered silver(I) oxide was dispersed in 2000 parts by mass of ethanol to obtain a dispersion of silver(I) oxide. The obtained dispersion was stirred at room temperature (25°C), and 42.7 parts by mass of formic acid was added dropwise to the dispersion. The silver(I) oxide and formic acid reacted to obtain silver(I) formate. When all of the silver(I) oxide was converted into aggregates of silver(I) formate, the temperature of the reaction solution began to drop. Most of the unreacted formic acid was dissolved in ethanol.

[0074] When the temperature of the reaction solution dropped to 30°C, 170.0 parts by mass of KBM-602 was added to the reaction solution. KBM-602 reacted with silver(I) formate to produce a silver(I) formate amine complex. The resulting silver(I) formate amine complex was dissolved in ethanol.

[0075] The silver(I) formate amine complex is an unstable compound. Therefore, upon formation, a decomposing reduction reaction occurs, generating silver atoms in the system. Silver nanoparticles are obtained as these silver atoms grow. At least a portion of the surface of the resulting silver nanoparticles is coated with N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, and a dispersion is obtained by dispersing the silver nanoparticles in ethanol. Some of the N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane reacts with the remaining formic acid to form the formate amine addition salt.

[0076] Ethanol was then added to adjust the concentration, yielding a silver nanoparticle dispersion with an average primary particle diameter of 30 nm and a secondary particle diameter (D50) of 5592.8 nm. This dispersion contained silver at a concentration of 1% by mass.

[0077] (Comparative Example 6) As shown in Table 3, silver nanoparticles coated with 19.9 parts by mass of octylamine molecules per 100 parts by mass of silver were obtained using the same procedure as in Example 1. Next, a silver nanoparticle dispersion was obtained using the same procedure as in Example 1, except that 150 parts by mass of octylamine was used instead of KBM-903 in 1,000 parts by mass of ethanol. The silver nanoparticles contained in the obtained silver nanoparticle dispersion had an average primary particle diameter of 5 nm and a secondary particle diameter (D50) of 382.3 nm. The concentrations of each component are shown in Table 3.

[0078] <Evaluation> The silver nanoparticle dispersions obtained in Examples 1 to 14 and Comparative Examples 1 to 6 were evaluated for (1) dispersion stability, (2) antibacterial activity in a dry state (bacterial mortality rate (R)), (3) zeta potential, and (4) antibacterial activity in a wet state (antibacterial activity linked to zeta potential). The results are shown in Tables 1 to 3.

[0079] (1) Dispersion stability 50 g of the silver nanoparticle dispersion was placed in a 70 mL glass bottle and allowed to stand for 30 days at 25° C. After 30 days, the silver nanoparticle dispersion was visually inspected for the presence or absence of precipitates and evaluated according to the following criteria. A: If no precipitate is observed. B: A precipitate was observed, but it was easily dispersed by shaking the glass bottle. C: When a precipitate was observed and a silver mirror had formed on the wall of the glass bottle.

[0080] (2) Antibacterial properties in a dry state (bacterial mortality rate (R)) The antibacterial properties were evaluated under the following conditions in accordance with JIS Z 2801:2010 (film adhesion method) according to the JIS standard. Test bacterial species: Staphylococcus aureus subsp. Aureus Rosenbach 1884 NBRC 12732 was used. Inoculum: A glycerol stock of Staphylococcus aureus was added to 10 mL of nutrient broth medium and incubated overnight at 37°C with shaking. The culture was then mixed and the bacterial concentration was measured using an absorption spectrophotometer (OD660). When the bacterial concentration was 2.5 x 10 5 ~1.0×10 6 The inoculum was prepared by diluting the mixture with nutrient bouillon medium at a concentration of 1 / 500 so that the concentration was 1 / 500 cells / mL. Test piece: 0.2 g of silver nanoparticle dispersion diluted to 2 ppm by mass with 60% by mass aqueous ethanol was applied to an acrylic plate measuring 5 cm in length and width and 2 mm in thickness, and then dried overnight at 25°C. Covering film: Polyethylene film 4 cm long and wide and 0.05 mm thick (High Clean Poly Bag No. 2, manufactured by Asoh Co., Ltd.). Inoculation of bacterial solution: After inoculating 0.4 mL of bacterial solution onto the surface of the test piece, the test piece was tightly covered with a covering film. Cultivation of test specimens: The test specimens inoculated with the bacterial solution were cultivated for 24 hours at 35°C in an atmosphere with a relative humidity of 90% or more. Measurement of viable bacterial count: The viable bacterial count was measured on two test pieces (n=2) immediately after inoculation and after 24 hours of incubation. Calculation of antibacterial activity value (R): It was calculated using the formula: antibacterial activity value (R) = UA. U indicates the average logarithm of the viable bacterial count of the untreated test piece 24 hours after inoculation. A indicates the average logarithm of the viable bacterial count of the antibacterial treated test piece 24 hours after inoculation. It is also called the bacterial cell death rate (R), and a bacterial cell death rate (R) of 100 indicates that almost all the bacteria have been killed.

[0081] (3) Zeta potential The silver nanoparticle dispersion was diluted with distilled water to a silver concentration of 50 ppm, and the zeta potential was measured using a zeta potential measuring device (ELSZ-2, manufactured by Otsuka Electronics Co., Ltd.). <Measurement conditions> Cell: Standard flow cell Measurement temperature: 25℃ Accumulation count: 5 times

[0082] (4) Antibacterial properties in wet conditions (antibacterial properties linked to zeta potential) Test bacterial species: Staphylococcus aureus subsp. Aureus Rosenbach 1884 NBRC 12732 was used. Inoculum: A glycerol stock of Staphylococcus aureus was added to 10 mL of nutrient broth medium and incubated overnight at 37°C with shaking. The culture was then mixed and the bacterial concentration was measured using an absorption spectrophotometer (OD660). When the bacterial concentration was 1.0 x 10 7 ~5.0×10 7 The inoculum was prepared by diluting with nutrient broth medium to give a concentration of bacteria / mL.

[0083] A 10 mL nutrient broth and a silver nanoparticle dispersion were added to a sterilized test tube to obtain a culture medium. The silver nanoparticle dispersion was added so that the silver concentration in the resulting culture medium was 50 ppm. The mixture was then inverted 10 times to disperse the silver nanoparticles in the culture medium. Next, 0.1 mL of the prepared inoculum was added to the resulting culture medium and cultured with shaking. Shaking culture was performed at 37°C for 24 hours at 20 rpm. 0.025 mL of the culture medium after shaking culture was inoculated onto a nutrient agar medium and the culture medium was spread over the entire medium with a cone rod. The culture medium with the spread culture medium was then cultured inverted at 37°C for 24 hours to prepare the test medium. The cloudy culture medium after shaking culture, which is presumed to contain Staphylococcus aureus, had a high bacterial cell concentration, so it was diluted approximately 30,000 times with nutrient broth medium and inoculated to facilitate observation.

[0084] On the other hand, a control was prepared by inverted culture using the same procedure as above, except that 10 mL of culture medium only (i.e., culture medium without silver nanoparticle dispersion) was used. For the control, because the bacterial cell concentration in the culture medium after shaking culture was high, the culture medium was diluted approximately 3,000,000 times with normal bouillon medium and inoculated to make observation easier.

[0085] Next, the mortality rate of bacterial cells in the liquid was calculated using the following formula (I) and evaluated according to the following criteria: The log reduction (logarithmic reduction value) of the medium in which the number of bacterial cells became 0 was calculated as log(1). log reduction = log (number of cells in the control (cells / mL)) - log (number of cells in the test medium (cells / mL)) (I) <Evaluation criteria> A: When log reduction is 6 or more. B: When log reduction is 2 or more but less than 6. C: When log reduction is less than 2.

[0086] [Table 1]

[0087] [Table 2]

[0088] [Table 3]

[0089] As shown in Tables 1 and 2, the silver nanoparticle dispersions of the present invention obtained in Examples 1 to 14 were all rated A or B for dispersion stability, demonstrating excellent dispersion stability. Furthermore, the silver nanoparticle dispersions of the present invention obtained in Examples 1 to 14 all exhibited a bacterial cell mortality rate of nearly 100, demonstrating excellent antibacterial effects even in a dry state. The results of antibacterial properties linked to the zeta potential indicate that excellent antibacterial effects are also observed in a wet state, with particularly significant effects being observed when the zeta potential is 25 mV or higher.

[0090] On the other hand, as shown in Table 3, the silver nanoparticle dispersion obtained in Comparative Example 1 was rated A for excellent dispersion stability, but did not exhibit satisfactory antibacterial effects. The silver nanoparticle dispersions obtained in Comparative Examples 2 to 6 showed precipitates and silver mirror formation on the wall of the glass bottle, indicating poor dispersion stability. Therefore, the silver nanoparticle dispersions obtained in Comparative Examples 2 to 6 were not evaluated for antibacterial properties.

Claims

1. silver nanoparticles, a dispersant, and a polar solvent; The silver nanoparticles have an average primary particle size of 20 nm or less and a secondary particle size (D50) of 200 nm or less, The dispersant contains at least one of an alkylamine and an amine containing an oxygen atom, and a nitrogen-containing organosilane compound. Antibacterial silver nanoparticle dispersion.

2. The antibacterial silver nanoparticle dispersion according to claim 1 , wherein the alkyl amine has an alkyl group moiety having 5 to 16 carbon atoms.

3. 3. The antibacterial silver nanoparticle dispersion according to claim 1, wherein the zeta potential is zero or positive when the concentration of the silver nanoparticles is 50 ppm.

4. The antibacterial silver nanoparticle dispersion according to claim 3 , wherein the zeta potential is 25 mV or more.

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