Technical Field

The present invention relates to nanosized silica- silver(nano-silicasilver) . Particularly, the present invention relates to nanosized silica-silver in which nano- silver is combined with a silica molecule and a water soluble polymer, and to a method of preparing the same.

Background Art

Silicon (Si) , which is the second most abundant material on the earth, is known to be absorbed into plants to increase disease resistance and stress resistance (Role of Root hairs and Lateral Roots in Silicon Uptake by Rice

J.F.Ma et al. Ichii Plant Physiology (2001) 127: 1773-

1780) . In particular, an aqueous silicate solution, used to treat plants, is reported to exhibit excellent preventive effects on pathogenic microorganisms chiefly responsible for causing powdery mildew or downy mildew in plants, and as well, is known to promote the physiological activity of plants, accelerating the growth of plants and inducting disease resistance and stress resistance in plants

(Suppressive effect of potassium silicate on powdery mildew of strawberry in hydroponics T. Kanto et al. J GenPlant

Pathol (2004) 70: 207-211). However, silica has no direct

disinfecting effects on pathogenic microorganisms in plants.

Silver (Ag) is known as a powerful disinfecting agent for killing unicellular microorganisms by inactivating enzymes having metabolic functions in the microorganisms by oligodynamic action (T. N. Kim, Q. L. Feng, et al. J. Mater. Sci. Mater. Med., 9, 129 (1998)) . In addition to silver, although heavy metals, such as copper or zinc, may exert the same action, silver has the strongest antimicrobial effects. Also, silver is known to exhibit superb effects on algae. Research into silver as a substitute for chlorine or other toxic microbicides has been continuously progressing. Moreover, various inorganic antimicrobial agents that use silver have been developed to date.

Presently available silver-based inorganic antimicrobial agents are produced in the forms of silver- supported inorganic powders, silver colloids, metal silver powders, etc., of which silver-supported inorganic powders are the most used and thus are representative of a typical inorganic antimicrobial agent.

Silver in an ionic state is advantageous because it exhibits high antimicrobial activity. However, ionic silver is disadvantageous because it is unstable due to its high reactivity and thus may be easily oxidized or reduced into a metal depending on the surrounding atmosphere. Hence, silver causes discoloration by itself or allows other

materials to cause undesirable coloration, and it does not continuously exert antimicrobial activity. Meanwhile, silver in the form of a metal or oxide is advantageous because it is stable in the environment, however it disadvantageous because it should be undesirably used in a relatively increased amount due to its low antimicrobial activity.

Silver, having the above advantages and disadvantages, is presently receiving attention in the form of nano-particles. Various methods of preparing the nano- particles include mechanical grinding, coprecipitation, spraying, sol-gel manufacture, electrolysis, inverse microemulsion, etc. However, these methods are disadvantageous because the size of the particles formed is difficult to control, or high cost is required to prepare fine metal particles. For example, through the coprecipitation method, since the particles are prepared using an aqueous solution phase, the sizes, shapes and size distribution of the particles are impossible to control. Through the electrolysis and sol-gel methods, high preparation costs are required, and also, mass production is difficult. Although the inverse microemulsion method allows sizes, shapes and size distribution of the particles to be easily controlled, it may not be used in practice due to its complicated preparation processes.

On the other hand, methods of preparing nanometer sized particles using exposure to radioactive rays are

provided, which are advantageous because sizes, shapes and size distribution of the particles are easily controlled, and the particles may be prepared at room temperature. Also, preparation processes are simple, and therefore, mass production is possible at low costs.

Korean Patent No. 0425976 discloses a method of preparing a nanometer sized silver colloid using exposure to radioactive rays, and a nanometer sized silver colloid thus prepared. According to the above patent, a silver salt is dissolved in tertiary distilled water, added with sodium dodecylsulfate (SDS) , polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP) as a colloid stabilizer, purged with nitrogen, and then exposed to radioactive rays, to prepare a silver colloid. However, the silver colloid thus prepared has a particle size of 100 nm or larger, and must be used in a high concentration to exhibit antimicrobial actions on microorganisms, in particular, fungi.

In addition, Korean Patent Laid-open Publication No. 2003-0082065 (Application No. 10-2002-0020594) discloses a method of preparing a stable silver colloid, using a PVP used in Korean Patent No. 0425976, (1-vinylpyrrolidone) - acrylic acid copolymer, (1-vinylpyrrolidone)-vinylacetic acid copolymer, etc., as a polymer stabilizer.

In addition to the above methods, thorough attempts to provide a nano-silver having various functions such as antimicrobial action, purification and deodorization have been made. Still, less expensive methods of preparing

stable nano-silver through a simpler process are required.

Under these circumferences, ^' the present inventors have found that a silver salt, silicate and a water soluble polymer are mixed, and then exposed to radioactive rays, to prepare nanosized silica-silver -particles comprising nano- silver that is combined with silica molecule and water soluble polymer, which have a uniform size, are stable and exhibit excellent antimicrobial effects at a very low concentration, thereby completing the present invention.

Disclosure of the Invention

Accordingly, an object of the present invention is to provide nanosized silica-silver(nano-silicasilver) in which nano-silver is combined with silica molecule and water soluble polymer. Another object of the present invention is to provide a method of preparing nanosized silica-silver(nano- silicasilver) , comprising preparing a solution including a silver salt, silicate and water soluble polymer, and exposing the solution to radioactive rays.

Brief Description of the Drawings

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in

conjunction with the accompanying drawings, in which:

FIG. 1 shows a schematic flowchart for the preparation of nanosized silica-silver(nano-silicasilver) ;

FIG. 2 shows the stability of colloidal nanosized silica-silver in an aqueous environment;

FIG. 3 shows the absorption spectrum (403 run) of the nanosized silica-silver, water and silver ions;

FIG. 4 shows the absorbance (403 run) of the nanosized silica-silver varying with the concentration of sodium silicate (Na ₂ SiO ₃ ) ;

FIG. 5 shows the absorption spectrum (403 nm) of the nanosized silica-silver varying with the concentration of a water soluble polymer;

FIG. 6 shows the absorption spectrum (403 nm) of the nanosized silica-silver varying with the kind of water soluble polymer;

FIG. 7 shows the absorption spectrum (403 nm) of the nanosized silica-silver varying with the dose of radioactive rays; FIG. 8a shows the antifungal effect of nanosized silica-silver on a pathogenic fungus in plants, Bhizoctonia solani, and FIG. 8b shows the antifungal effect of nanosized silica-silver on Botrytis cinerea;

FIG. 9 shows the antifungal effect of nanosized silica-silver on young squash suffering from powdery mildew;

FIG. 10 shows the antifungal effect of nanosized silica-silver on Phytophthora infestans; and

FIG. 11 shows a transmission electron microscope of nanosized silica-silver prepared according to the present invention.

Best Mode for Carrying Out the Invention

According to an aspect, the present invention pertains to nanosized silica-silver(nano-silicasilver) in which nano-silver is combined with silica molecule and water soluble polymer.

In the present invention, the term "nanosized silica- silver(nano-silicasilver) " means a composite comprising nanosized silver particle and silica molecule that are combined with water soluble polymer. According to a specific aspect, nanosized silica-silver may be prepared by exposing a solution comprising silver salt, silicate and water soluble polymer to radioactive rays. One example of the composite is a structure in which nanosized silver particles formed from silver ion and silica molecule formed from silicate are, individually or together, surrounded by water soluble polymer via exposure to radioactive rays. The nanosized silica-silver, in a colloidal state, may be present as nano-particles separated from each other or be formed into loose spherical clusters (FIG. 11) . As such, the clusters may be simply separated when the temperature increases. As confirmed in the absorption spectrum of FIG. 3,

the nanosized silica-silver absorbs light of 403 nm which is the unique wavelength of nano-silver, and has a uniform nanoparticle size as shown in FIG. 11.

In the present invention, the particle size of the nanosized silica-silver preferably ranges from 0.5 to 30 nm, more preferably from 1 to 20 nm, and, most preferably, from 1 to 5 nm.

The nanosized silica-silver of the present invention exhibits high antimicrobial activity, and thus, may be used to prevent and disinfect pathogenic bacteria and fungi. In particular, since the nanosized silica-silver may exert excellent controlling effect on pathogenic fungi in plants even at a very low concentration, it is used at a low concentration to selectively control only pathogenic fungi in plants. In addition, when the nanosized silica-silver is applied once, prevention effects may be exhibited for 3 weeks or longer. Further, the nanosized silica-silver can control both spores and hyphae, and causes no chemical injury even if it is applied at a high concentration, and also, is harmless to the human body and to plants. Furthermore, the nanosized silica-silver of the present invention has high preservability, and may be used in the state of being diluted in tap water or agricultural water, compared to silver ions that are precipitated in the form of silver chloride along with chlorine ions in tap water.

According to another aspect, the present invention relates to a method of preparing nanosized silica-silver,

comprising (A) preparing a solution including silver salt, silicate and water soluble polymer, and (B) exposing the solution to radioactive rays.

The above method further includes bubbling (or purging) the solution with an inert gas, before, after, or before and after exposing the solution to radioactive rays. The inert gas includes nitrogen, argon, etc., of which nitrogen gas may be preferably used. The bubbling is preferably performed for 10 to 30 min. According to the above method, when the solution comprising silver salt, silicate and water soluble polymer is prepared, a radical scavenger is further included to scavenge radicals generated by exposure to radioactive rays. The radical scavenger includes, for example, alcohol, glutathione, vitamin-E, flavonoid, ascrobic acid, etc. Usable alcohols are exemplified by methanol, ethanol, n- propanol, iso-propanol (IPA) , butanol, etc. Of these alcohols, iso-propanol may be preferably used. The alcohol may be used in an amount of 0.1 to 20%, and preferably 3 to 10%, based on the total amount of the solution comprising silver salt, silicate and water soluble polymer.

Examples of the silver salt usable in the preparation of the nanosized silica-silver include silver nitrate (AgNO ₃ ) , silver perchlorate (AgClO ₄ ) , silver chlorate (AgClO ₃ ), silver chloride (AgCl), silver iodide (AgI), silver fluoride (AgF), silver acetate (CH ₃ COOAg), etc., of which a

silver salt (e.g.: silver nitrate), easily soluble in water, may be preferably used.

Examples of the water soluble polymer used in the preparation of the nanosized silica-silver include polyvinylpyrrolidone (PVP) , polyvinylalcohol (PVA) , polyacrylic acid and derivatives thereof, levan, flurane, gellane, water soluble cellulose, glucan, xanthane, water soluble starch, levan, corn starch, etc. Of these polymers, PVP may be preferably used. Examples of silicate used in the preparation of the nanosized silica-silver include sodium silicate, potassium silicate, calcium silicate, magnesium silicate, etc. Of these silicates, sodium silicate may be preferably used. Before disclosure in the present invention, the use of silicate in the preparation of nano-silver has not been found in any patent. The present inventors first used silicate, not silica, . to react with the silver salt, thereby providing nanosized silica-silver having high antimicrobial effects in which silica molecule and water soluble polymer are combined with nano-silver.

Upon the preparation of the nanosized silica-silver of the present invention, the silver salt and silicate react in a weight ratio range of silver salt:silicate of 1:0.05 to 1.3. Preferably, the above two materials react at a weight ratio of 1:1. The particle size of nanosized silica-silver may be adjusted depending on the amount of silicate. If silicate is used in a small amount, the particles become

large. Meanwhile, if silicate is excessively used relative to the silver salt, the particles are not formed.

Upon the preparation of the nanosized silica-silver of the present invention, the silver salt and the water soluble polymer react in a weight ratio range of silver salt:water soluble polymer of 1:0.5 to 2.5. Preferably, the above two materials react at a weight ratio of 1:1.

To prepare the nanosized silica-silver, radioactive rays, such as beta rays, gamma rays, X-rays, ultraviolet light, electron rays, etc., may be used. Preferably, gamma rays may be used at a dose of 10 to 30 kGy.

A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as the limit of the present invention.

Example 1: Preparation Of Nanosized silica-silver Comprising Silica Molecule And Water Soluble Polymer

1 g of sodium silicate (Na ₂ SiOs) , 1 g of silver nitrate (AgNO ₃ ) , 1 g of polyvinylpyrrolidone (PVP) , and 12 ml of isopropylalcohol (IPA) were added to 188 ml of distilled water, and then stirred. The resultant solution was bubbled for 20 min using nitrogen gas, and then exposed to gamma rays of 25 kGy, thus obtaining nanosized silica- silver.

FIG. 1 is a schematic flowchart showing the preparation of nanosized silica-silver comprising silica

molecule and water soluble polymer, according to the present invention. The solution, formed after being exposed to gamma rays, was yellow due to nano-silver particles, which means that the silica molecules and the water soluble polymer combined with silver particles through the above reaction, yielding stable nanosized silica-silver particles.

To confirm whether the particles thus prepared were nano-silver particles, test groups as shown in Table 1, below, were prepared and allowed to stand at room temperature for 24 hr, after which color change was observed.

TABLE 1

* : Dissolved solution prepared in Example 1

The test groups A and B are solutions prepared by exposing the dissolved solution to radioactive rays, and the test groups C and D are solutions in which Ag ⁺ ions were present without exposure to radioactive rays. The test groups SW and DW were control groups having no silver ions or silver particles.

Silver in the ionic state is easily oxidized, and are precipitated in the form of AgCl while browning in the presence of Cl ^" ions. Thus, the state of silver may be confirmed using tap water having Cl ^" ions. In the case where silver is present in the state of Ag ⁺ ions, it precipitates. Meanwhile, when silver is present in stable nano-silver particles, it shows yellow. The results are given in Table 2, below.

TABLE 2

As is apparent from Table 2, the test groups SW, D and DW were colorless without color change even after allowing to stand for 24 hr, which means that silver ions, chlorine ions, or silver ions and chlorine ions were all absent. However, the colorless test group C turned to reddish brown, which means that silver ions were formed into AgCl along with chlorine ions in tap water. In addition, the test groups A and B showed yellow without color change, which means that stable nanosized silica- silver particles comprising silica molecule and water soluble polymer were formed via exposure to radioactive

rays and AgCl precipitates were not formed even in the presence of chlorine ions. The color change is shown in FIG. 2.

The absorption spectrum of the nanosized silica- silver of the present invention is shown in FIG. 3. FIG. 3 shows the absorption spectra of the solutions of the test groups DW, B and D in Table 2, in which only the test group B absorbed light of 403 ran, which is the unique wavelength of nano-silver, and the test groups DW and D did not absorb light at the same wavelength.

From the result of the absorption spectrum, measured after the solution was allowed to stand, it was confirmed that stable nanosized silica-silver particles comprising silica molecule and water soluble polymer were formed by exposing the solution comprising sodium silicate, silver nitrate and PVP to radioactive rays.

FIG. 11 shows a photograph of the prepared nanosized silica-silver, observed using a transmission electron microscope (TEM) . As shown in FIG. 11, the nanosized silica-silver particles have uniform particle size distribution having a particle size of 1 to 5 nm smaller than 20 nm. The nanosized silica-silver particles may be independently separated or be formed into loose spherical clusters due to intermolecular attraction. As such, the clusters may be easily separated by heat.

Example 2: Preparation Of Nanosized silica-silver

Comprising Silica Molecule And Water Soluble Polymer

Nanosized silica-silver was prepared in the same manner as in Example 1, with the exception of varying the concentration of sodium silicate (Na ₂ SiC> ₃ ) from 0.5 to 2 g. The test groups having different concentrations are shown in Table 3, below.

TABLE 3

The absorbance and color of nanosized silica-silver varying with the concentration of sodium silicate as shown in Table 3 are shown in FIG. 4.

As shown in FIG. 4, when sodium silicate and silver nitrate are mixed at a ratio of 1:1, the highest absorbance is obtained. when sodium silicate and silver nitrate are mixed at a ratio of at least 1.5:1, the absorbance is somewhat decreased. Meanwhile, when sodium silicate and silver nitrate are mixed at a ratio of 0.5 or less:l, the size of the silver particles is increased as evidenced by an orange gold color.

As is apparent from the above result, since the amount of sodium silicate functions as an important factor

to prepare nanosized silica-silver, it is adjusted to regulate the particle size of the nanosized silica-silver.

Example 3: Preparation Of Nanosized silica-silver Comprising Silica Molecule And Water Soluble Polymer

Nanosized silica-silver was prepared in the same manner as in Example 1, with the exception of varying the concentration of polyvinylpyrrolidone (PVP) from 0.5 to 2 g-

The absorbance and color of nanosized silica-silver varying with the concentration of PVP are shown in Table 4, below, and FIG. 5.

TABLE 4

As shown in Table 4 and FIG. 5, when sodium silicate and silver nitrate are mixed at the same ratio, polyvinylpyrrolidone (PVP) may be used at a concentration 0.5 to 2 times that of sodium silicate (or silver nitrate).

Example 4: Preparation Of Nanosized silica-silver Comprising Silica Molecule And Water Soluble Polymer

Nanosized silica-silver was prepared in the same

manner as in Example 1, with the exception of using high levan or corn starch, instead of polyvinylpyrrolidone (PVP) .

The absorbance and absorption spectrum of the prepared nanosized silica-silver are shown in Table 5, below, and FIG. 6.

TABLE 5

As shown in Table 5 and FIG. 6, the nanosized silica- silver may be prepared using polysaccharides such as levan or corn starch, although it has lower absorbance than nanosized silica-silver prepared using polyvinylpyrrolidone (PVP) .

Example 5: Preparation Of Nanosized silica-silver Comprising Silica Molecule And Water Soluble Polymer

Nanosized silica-silver was prepared in the same manner as in Example 1, with the exception of varying the dose of radioactive rays.

The absorbance and absorption spectrum of the prepared nanosized silica-silver are shown in Table 6, below, and FIG. 7.

TABLE 6

As seen in Table 6 and FIG. 7, absorption occurred even at 10 kGy, and increased as the dose of radioactive rays was increased. Thus, the nanosized silica-silver may be prepared using radioactive rays of 10 kGy or more.

Experimental Example: Antifungal Effects Of Nanosized silica-silver Comprising Silica Molecule And Water Soluble Polymer

To confirm antifungal effects of nanosized silica- silver comprising silica molecule and water soluble polymer, antimicrobial effects on various pathogenic fungi were measured.

Experimental Example 1: Antifungal Effect On Rhizoctonia

A microorganism culture medium (PDA medium, Difco Co.

Ltd. ) was autoclaved, and then 25 ml were aliquotted into each of a plurality of petri dishes. Before the aliquotted medium was cured (at about 40°C) , it was mixed with a silica molecule from the test group A, mixed with the nanosized silica-silver prepared in Example 1 from the test group B, mixed with 20 nm sized silver particles from the test group C, and mixed with 100 nm sized silver particles from the

test group D, and then cooled, to prepare each medium. Into the prepared medium, a solid medium having sufficiently cultured Rhizoctonia solani as one of the pathogenic fungi in plants, which was removed on a circular piece having a diameter of 5 mm, was inoculated. Through culture at room temperature for 2 days, whether the growth of the microorganism had been inhibited was confirmed. The concentration of the mixed material of each test group was set to 6 ppm and 0.3 ppm. As shown in FIG. 8a, the test group A including only a silica molecule had the same result as a control group, regardless of the concentration. The test groups C and D, including 20 nm sized silver and 100 nm sized silver, respectively, had the same results as a control group at a concentration of 0.3 ppm. However, the test group B including the nanosized silica-silver of the present invention exhibited an excellent inhibitory effect on growth of Rhizoctonia solani, even at a low concentration of 0.3 ppm.

Experimental Example 2: Antifungal Effect On Botrytis

A microorganism culture medium (PDA medium, Difco Co.

Ltd. ) was autoclaved, and then 25 ml were aliquotted into each of a plurality of petri dishes. Before the aliquotted medium was cured (at about 40°C) , it was mixed with a silica molecule from the test group A, mixed with the nanosized silica-silver prepared in Example 1 from the test group C,

mixed with 20 nm sized silver particles from the test group B, and mixed with 100 nm sized silver particles from the test group D, and then cooled, to prepare each medium. Into the prepared medium, a solid medium having sufficiently cultured Botrytis cinerea as one of the pathogenic fungi in plants, which was removed on a circular piece having a diameter of 5 mm, was inoculated. Through culture at room temperature for 2 days, whether the growth of the microorganism had been inhibited was confirmed. The concentration of the mixed material of each test group was set to 6 ppm, 3 ppm, and 0.3 ppm.

As shown in FIG. 8b, the test group A including only a silica molecule had the same result as a control group, regardless of the concentration. The test groups B and D, including 20 nm sized silver and 100 nm sized silver, respectively, had the same results as a control group at a concentration of 0.3 ppm. However, the test group C including the nanosized silica-silver of the present invention had a higher inhibitory effect on growth of Botrytis cinerea than those of test groups including 20 nm sized silver and 100 nm sized silver, even at a low concentration of 3 ppm.

Experimental Example 3: Antifungal Effect On Powdery Mildew

To assay controlling effects of the nanosized silica- silver prepared in Example 1 on pathogenic fungi in plants,

the present experiment was carried out in a plastic film greenhouse of young squash plants infected with powdery mildew. 0.3 ppm nanosized silica-silver was uniformly applied onto young squash plants infected with powdery mildew. After the nanosized silica-silver was applied, the state of powdery mildew was observed for 3 weeks.

FIG. 9 is photographs showing the controlling effects on powdery mildew, 0, 3 and 7 days after the nanosized silica-silver was applied. Although powdery mildew was widespread on the leaves of young squash at day 0, controlling effects close to 100% were exhibited after the nanosized silica-silver was applied. After about 3 weeks, powdery mildew was not observed.

Experimental Example 4: Antifungal Effect On Phytophthora ±nfestans

The controlling effect of the nanosized silica-silver prepared in Example 1 on pathogenic fungi in plants was assayed using potatoes (cv. Desiree) infected with Phytophthora spp.

The upper surface of the potato leaf was coated with lens-paper, and then inoculated with zoospores of Phytophthora infestans at a concentration of 3 x 10 ⁴ zoospores/ml. Thereafter, humidity of 95% or more was maintained. 3 days after Phytophthora infestans was inoculated, the nanosized silica-silver was diluted 1000 times and then sprayed onto the potato leaf.

FIG. 10 is photographs showing the results of spray treatment 3 days after inoculation and the controlling effects 6 and 9 days after the spray treatment. From the photographs of FIG. 10, the nanosized silica-silver was confirmed to exhibit excellent controlling effects on Phytophthora lnfestans.

Industrial Applicability

As described above, the present invention provides nanosized silica-silver and a method of preparing the same. According to the present invention, stable nanosized silica-silver may be provided in a high concentration at room temperature via a simple process. The colloidal solution including the nanosized silica-silver particles thus prepared exhibits excellent controlling effects on pathogenic microorganisms in plants, at a low concentration.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.