WO2012017446A2

WO2012017446A2 - Improved process for the preparation of stable suspension of nano silver particles having antibacterial activity

Info

Publication number: WO2012017446A2
Application number: PCT/IN2011/000474
Authority: WO
Inventors: Revathi Janardhanan; Hebalkar Neha; Narsinga Rao Tata
Original assignee: International Advanced Research Centre For Powder Metallurgy And New Materials (Arci)
Priority date: 2010-08-04
Filing date: 2011-07-19
Publication date: 2012-02-09
Also published as: GB201303768D0; WO2012017446A3; GB2496089A; GB2496089B

Abstract

This invention relates to an improved process for the preparation of stable suspension of nano silver particles having antibacterial activity, comprising the steps of: (i). Preparing a glucose solution and a silver salt solution in water separately wherein silver salt is selected from Silver Sulphate, Silver Accetate and Silver Nitrate; (ii) Mixing the two solutions, as prepared in step (i), and stirring the mixture for 5 to 10 minutes; (iii) Preparing a solution of surfactant in water and adding the solution, to the solution obtained in step (ii) and stirred slowly for 1 - 5 minutes; (iv) Adding the basic reagent diethyl amine to the solution obtained in step (iii) if silver sulphate or silver acetate is used, or adding the basic reagent Sodium carbonate to the solution obtained in step (iii) if silver salt, as mentioned in step(i), is used and (v) Adding anti foaming agent to the suspension obtained in step (iii).

Description

"IMPROVED PROCESS FOR THE PREPARATION OF STABLE SUSPENSION OF NANO SILVER PARTICLES HAVING ANTIBACTERIAL ACTIVITY"

FIELD OF THE INVENTION

The present invention relates to improved process for the preparation of stable suspension of nano silver particles having antibacterial activity even at lower concentrations of nano silver, containing nano silver particles of size 10-50 nm with silver content in the range 0.0135 to 0.135 wt % which is found to be stable for several months at lower concentrations and few months for the maximum concentration of 0.135 wt %.

BACKGROUND OF INVENTION

Silver has long been known to exhibit a strong toxicity to a wide range of micro-organisms; for this reason silver-based compounds have been used extensively in many bactericidal applications. Silver compounds have also been used in the medical field to treat burns and a variety of infections. Silver powders having a very fine and uniformly distributed sizes are desirable in many fields of industrial applications. A smaller particle size would in theory reduce the necessary quantity for providing desired electrical conductivity and antibacterial activity. The subject of preparing nanosized metal colloids has attracted a great deal of attention in recent years. Both physical and chemical methods had been reported in the literature. For chemical methods, the choice of the reducing agent is of course the major factor. Examples include g-radiation, hydrazine, sodium boro hydride among others. The reducing ability will determine the formation kinetics and hence reaction temperature. The reaction can be carried out in either aqueous solution or in organic solvent such as the polyol process. All these methods involve the reduction of relevant metal salts in the presence of a suitable protecting agent, which is necessary in controlling the growth of metal colloids through agglomeration. Common methods for size and shape control employ the capping agents, such as surfactants, ligands, polymers, or dendrimers to confine the growth in the nanometer regime. The surfactant molecules have the intrinsic property to adsorb into the interface, which are formed between two different phases. Thus, the surfactant molecules would adsorb into the surface of nuclei in solution. The adsorbed surfactant molecules from the solution prevent the coalescence of particles and control the rate of particle growth. By choosing the proper kind and/or concentration of surfactants, the size of particles formed in solution can be controlled in nm scale. The choice of the surfactant depends on the surface charge of the particle to be protected.

Bonet et al., "SYNTHESIS OF MONODISPERSE AU, PT, PD, RU AND IR NANOP ARTICLES IN ETHYLENE GLYCOL", (NanoStructured Materials, Vol. 11, No. 8, pp. 1277-1284, 1999) have reported the synthesis of Au, Pt, Pd, Ru and Ir nanoparticles with a narrow size distribution by chemical reduction of their corresponding metal species in ethylene glycol. Particle size was mainly controlled by varying the initial total metal concentration, the reaction temperature, and the concentration of Poly Vinyl Pyrollidone (PVP). Metal nano particles of size 5 - 20nm were obtained in the reaction. Metal particle agglomeration and sintering was prevented by the addition of PVP, a well known protective agent that also aids particle dispersion.

Nersisyan et al., "A NEW AND EFFECTIVE CHEMICAL REDUCTION METHOD FOR PREPARATION OF NANOSIZED SILVER POWDER AND COLLOID DISPERSION", (Materials Research Bulletin, 38 (2003) 949-956) have reported the preparation of Nanosized uniform silver powders and colloidal dispersions of silver from AgN03 by a chemical reduction method involving the intermediate preparation of Ag20 colloidal dispersion in the presence of Sodium Dodecyl Sulfate CH₃(CH₂)nOS0₃Na as a surfactant (Materials Research Bulletin 38 (2003) 949-956). Several reducing agents such as hydrazine hydrate (N2H4 H2O), formaldehyde (HCOH) and glucose

have been found to be preferable in this study from a practical point of view. The silver powder with the 60-120 nm particle size and colloidal dispersion with the particles size 10-20 nm and 0.5-2.0 wt. % concentration were synthesized by this method.

Monodisperse colloidal dispersions of bimetallic Ag-Pd particles have been prepared from ethylene glycol solutions of polyvinyl pyrrolidone(PVP) and silver and palladium nitrates by Silvert et al., (NanoStructured Materials. Vol. 7. No. 6. pp. 611618.1996). The average particle size of these colloids was found to depend on the amount of precursor and PVP added. A PVP / precursor weight ratio of 10.3 yields particles with 14 nm diameter, while a ratio of 47 yields 7 run particles.

Nirmalya K. Chaki et al., "SINGLE PHASE PREPARATION OF MONODISPERSED SILVER NANOCLUSTERS USING A UNIQUE ELECTRON TRANSFER AND CLUSTER STABILISING AGENT, TRIETHYL AMINE", (Chemical Communication, 2002, 76-77) have reported a simple and reproducible single phase preparation of 2.5 nm silver nanoclusters using silver benzoate along with triethylamine (TEA) and dodecanethiol (DDT); these spontaneously self- assemble to a two-dimensional array whereas in the absence of thiol only polydispersed nanoclusters are obtained.

US 20060045916 on "METHODS FOR PRODUCING SILVER NANOPARTICLES" discloses a method for making silver nanoparticles, and includes steps of reacting a silver salt with a phosphene amino acid to make silver nanoparticles. Exemplary phosphene amino acids include trimers, with a particular example being a trimeric amino acid conjugate containing one phosphene group. In an exemplary method of the invention, the silver nanoparticles may be produced in timer periods of less than about 30 minutes, and at temperatures of less than about 40 DEG C. Other methods of the invention are directed to methods for stabilizing silver nanoparticles.

It reveals a method for making silver nanoparticles, by reacting a silver salt with a phosphene amino acid to make silver nanoparticles of size 10 -20nm.

US6660058 on "PREPARATION OF SILVER AND SILVER ALLOYED NANOPARTICLES IN SURFACTANT SOLUTIONS" relates to the preparation of nanoparticles of silver (Ag) and silver alloyed with other elements such as platinum (Pt), palladium (Pd), gold (Au), aluminum (Al), cadmium (Cd) and sulfur (S) in surfactant solutions. The surfactant molecules have the intrinsic property to adsorb into the interface, which are formed between two different phases. Thus, the surfactant molecules would adsorb into the surface of nuclei in solution. The adsorbed surfactant molecules from the solution prevent the coalescence of particles and control the rate of particle growth. By choosing the proper kind and/or concentration of surfactants, the size of particles formed in solution can be controlled in nm scale. It focuses on the process for the preparation of nanoparticles of silver (Ag) and silver alloyed with other elements such as platinum (Pt), palladium (Pd), gold (Au), aluminum (Al), cadmium (Cd) and sulfur (S) in surfactant solutions. Hydrazine Hydrate was used as reducing agent and CTAB, SLS and Tween 20 were used as surfactants to obtain metal nano particles of size less than 20 nm.

Steve Lien-Chung Hsu et al., "SYNTHESIS OF CONTAMINATION-FREE SILVER NANOP ARTICLE SUSPENSIONS FOR MICRO-INTERCONNECTS" (Materials Letters 61 (2007) 3719-3722), synthesized suspensions of Silver nanoparticle of size 10 -20nm by chemical reduction from silver nitrate in a formaldehyde reductant and Poly Vinyl PyroUidone stabilizer using organic bases as the reaction promoter Two different organic bases of different basicity, triethylamine and pyridine, were used in the reaction. Effect of using a surfactant to control the particle size was not reported in this paper.

Siddhartha Shrivastava et al. "CHARACTERIZATION OF ENHANCED ANTIBACTERIAL EFFECTS OF NOVEL SILVER NANOPARTICLES", (Nanotechnology 18 (2007) 225103 (9pp)) reported the process for the preparation of silver nanoparticles in the range of 10-15 nm with increased stability and enhanced anti-bacterial potency. Silver nitrate, D-glucose and Hydrazine were used. Citric acid was used to maintain the pH at 7.4 which might help in stability of the suspension.

Modified tollens reaction based method for synthesis of nano silver suspension is reported already as mentioned in the prior art. The methods reported so far involves use of PVP as particle size controller and where in surfactant is used, hydrazine hydrate was used as reducing agent.

Nano silver particles are characterized by high surface area to volume ratio. Monodispersity and low particle size of Nanosilver particles would reduce their concentration required for efficient antibacterial action The formation of silver particles involves three distinct stages: (1) reduction of positive silver ions into silver atoms by modified tollens reaction as reported in the publication of Steve Lien-Chung Hsu et al mentioned earlier. The source of silver ions used in this process being silver nitrate. Glucose as a reducing agent and triethyl amine or pyridine as the catalyst have been used in the prior art (2) nucleation (the formation of centers of crystallization) and (3) crystal growth by coalescence of nuclei or diffusion of silver atoms into the surface of the nuclei.

Tollens reaction is a test for aldehydes. When an ammoniacal solution of silver nitrate was added to an aldehyde silver mirror is formed due to redox reaction. I.e. aldehyde gets oxidised to carboxylic acid and silver ions Ag⁺ gets reduced to silver Ag°. In modified tollens reaction diethyl amine is added to silver nitrate and glucose solution to obtain nano silver particles.

In the prior art process , when the use of modified Tollens reaction is employed as reported in the in the publication of Steve Lien-Chung Hsu et al mentioned earlier , silver nitrate is used as a source for silver ions. The nitrate ions react with the amine to form carcinogenic nitrous amine. Such a formation of carcinogenic amine is not good as it causes heath risk during large scale production of nano silver suspension.

In the present day environment nano silver suspensions are being increasingly used for various anti bacterial applications. To mention some examples in the textile industry the nano silver suspensions are used to produce antibacterial textiles for making baby care products, for making bed spreads etc for use in hospitals. In addition the textiles have negative electrostatic charge due to which the use of nano silver suspensions hitherto known , the anti bacterial activity reduces after many repeated washes of the textiles It would be beneficial if a nano silver suspension is made available which can be used for treating the textiles in which the anti bacterial activity is retained even after many washes In plastic industry, nano silver suspensions are used for making anti bacterial plastics for preparing health care products.

Further more the hitherto known nano silver suspensions are stable only for few months. It is beneficial if the stability of the nano silver suspension is increased substantially to met the present day requirements.

Hence there is need to develop an improved process for preparing nano silver suspensions which will not have carcinogenic nitrous amine in the suspension, the textiles & plastic products treated with the suspension retains the anti bacterial activity for a longer period of time and the suspension is stable for a longer period of time. OBJECTS OF INVENTION

The main objective of the present invention is to provide an improved process for the preparation of stable suspension of nano silver particles having antibacterial activity.

Another objective of the present invention is to provide an improved process for the preparation of stable suspension of nano silver particles having 0.01 to 0.135 wt % of nano silver , the size of nano silver in the suspension being in range of 10 to 50 nm.

Yet another objective of the invention is to prepare a nano silver suspension using a further modified Tollens reaction using glucose as a reducing agent and diethyl amine as catalyst.

A further objection of the invention is not to use silver nitrate as a source of silver ion in the process but to use either silver sulphate or silver acetate.

An additional objective of the invention is to prepare a stable suspension which should be stable for several month at lower concentration.

Yet another objective of the invention to carry out the reaction of the process at room temperature thereby making the process simple, cost effective and environmentally friendly.

Still another objective of the invention is to synthesize the suspension taking into consideration particle size of nano silver particles, antibacterial activity, stability of the suspension, cost effectiveness, easy up scaling and environmental friendliness.

The foregoing has outlined some of the pertinent objectives of the invention. These objectives should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of disclosure. Accordingly, other objectives and a full understanding of the invention and the detailed description of the preferred embodiment in addition to the scope of invention are to be defined by the claims.

STATEMENT OF INVENTION

Accordingly the present invention provides An improved process for the preparation of stable suspension of nano silver particles having antibacterial activity, comprising the steps of:

(i) Preparing a glucose solution and a silver salt solution in water separately wherein silver salt is selected from Silver Sulphate, Silver Accetate and Silver Nitrate;

(ii) Mixing the two solutions, as prepared in step (i), and stirring the mixture for 5 to 10 minutes;

(iii) Preparing a solution of surfactant in water and adding the solution, to the solution obtained in step (ii) and stirred slowly for 1 - 5 minutes;

(iv) Adding the basic reagent diethyl amine to the solution obtained in step (iii) if silver sulphate or silver acetate is used, or adding the basic reagent Sodium carbonate to the solution obtained in step (iii) if silver salt, as mentioned in step(i), is used and

(v) Adding anti foaming agent to the suspension obtained in step (iii). BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figl shows the XRD of the nano silver particles. Particles were collected by centrifuging the suspension at 8000 rpm.

Fig2 shows the SEM image of 0.0135wt % nano silver suspension. A drop of the suspension was placed on the carbon tape for SEM analysis. 10 - 50 nm mono dispersed nano silver particles were observed. Fig3 show the UV -Visible spectrum for 0.0135 wt % nano silver suspension before centrifuge and after centrifuge at 6000 rpm. The fwhm and X_max are 73 and 412 nm respectively.

Fig4 Schematic diagram of the reactor used for large scale synthesis of nano silver suspension.

While the invention is described in conjunction with the illustrated embodiment, it is not intended to limit the invention to such embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the inventions disclosure as defined by the claims.

DETAIL DESCRIPTION OF THE INVENTION

For the purpose of promoting an understanding of the principles of the invention, reference is now to be made to the embodiment illustres in the drawings and specific language is used to describe the same. It is nevertheless to be understood that no limitations of the scope of invention is hereby intended, such alterations and further modifications in the illustrated bag and such further applications of the principals of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

The present invention has been developed on the basis of many the important factors such as particle size, antibacterial activity, stability of the suspension, cost effectiveness, easy to upscale and environmentally friendly and useful for commercial production. An improved process to prepare nano silver suspension using a further modified tollens reaction has been developed.

Further modified Tollens reaction has been employed in the present invention. The modification, which has been effected to the Tollens reaction, comprises using glucose as a reducing agent and diethyl amine as the organic base as the catalyst and the source of silver ion being silver sulphate or silver acetate instead of silver nitrate. Silver nitrate is not used as the source in the process of the present invention wherein Diethyl amine is used and therefore there is no formation of carcinogenic nitrous amine in the further modified Tollens reaction of the present invention. When silver nitrate is used as a source for silver ions in the process sodium carbonate is used as a catalyst. In addition , the reaction takes place at room temperature which makes the process quiet simple, cost effective and environmentally friendly when compared with the other prior art processes.

According to the above process it is possible to prepare nano silver suspension having 0.01 to 0.135 wt % of nano silver of the particle size ranging from 10 - 50 nm mono dispersed particles. The concentration of silver sulphate or silver acetate or silver nitrate used may range from 0.001 to 0.05 M. The molar ratio of Ag ions: and glucose may vary from 1:1 to 1:10. The lower concentration suspensions were stable for 6 months whereas the maximum concentration of 0.135 wt % nano silver suspension was stable for a month.

When the surfactant molecules are present in the solution, surfactant molecules would be adsorbed onto the surface of silver nuclei. This surfactant film will hinder the coalescence of nuclei and controls the diffusion of silver atoms into the surface of nuclei. Thus, the particle size is controlled and it also alters the surface charge of the nano silver particles to maintain them suspended in water.

The Molar ratio of Surfactant and silver ions may vary from 1:0.25 to 1:1. The surfactant which can be used may be anionic, cationic or amphoteric. The cationic surfactant being preferred one. By way of examples of the cationic surfactant such as Cetyl trimethyl ammonium bromide (CTAB), Cetylpyridinium chloride (CPC) Polyethoxylated tallow amine (POEA) Benzethonium chloride. Examples of anionic surfactant are sodium dodecyl sulphate. Examples of amphoteric surfactant are Polysorbate (Tween 20). The amount of the base used can vary from 1:1 to 1:10. The anti foaming agent which can be used can be selected from oil, water, powder or silicone based defoamer and their amount ranges from 0.1 to 1 wt %.

For preparing the suspension on a large scale, a reactor as shown in fig 4 is used The reactor has Four Round flasks (1) for containing the four chemical solutions namely, silver sulphate, glucose, surfactant and the organic base , Diethyl amine. Each vessel is provided with angular valves (2) for controlling the flow rate of the solutions The reactor has a cylindrical vessel (4) with top cover (3) which is the reaction vessel. The angular valve connects the round flasks with the cylindrical vessel. The reaction vessel contains in situ baffles (5) to facilitate quick homogeneous mixing of the solutions added to reaction vessel. The cylindrical vessel is provided with a stirrer (6) for stirring.

The following Examples are given to illustrate the invention only and therefore should not be construed to limit the scope of the present invention.

Example 1:

100 ml of 0.05 moles of glucose solution was added to 100ml of 0.0025 moles of silver sulphate solution in a vessel and stirred for 5 minutes. Then 100ml of 0.0025 moles of CTAB was added to the silver sulphate and glucose solution and stirred for another 5 minutes to get a homogeneous solution. 100ml of 0.05 moles Diethyl amine in water was added to the above solution and stirred again for 5 minutes to mix homogeneously. The solution turned from yellow to bright red colour. The colour change to bright red colour indicates the completion of reaction thereby producing the suspension.

The suspension obtained as described above was characterised by SEM, UV -visible spectroscopy and XRD. The suspension was dried on a glass slide and the dried precipitate was analysed by XRD. The phase formed was confirmed by XRD as shown in Figure 1 of the drawing accompanying this specification. The SEM image shown in figure 2 confirms that the particle size of silver is the range of 10 - 50 nm.

Example 2:

100 ml of 0.15 moles of glucose solution was added to 100ml of 0.015 moles of silver nitrate solution in a vessel and stirred for 5 minutes. Then 100ml of 0.012 moles of CTAB was added to the silver nitrate and glucose solution and stirred for another 1 minute to get a homogeneous solution. 100ml of 0.15 moles Sodium carbonate in water was added to the above solution and stirred again for 5 minutes to mix homogeneously. The solution turned from yellow to bright red colour. The colour change to bright red colour indicates the completion of reaction thereby producing the suspension.

Example 3:

4 litres of 0.0025 moles of silver sulphate solution, 4 litres of 0.00125M CTAB solution and 4 litres of 0.05M Diethyl amine solution and 4 litres of 0.05 M glucose solution were taken in the four round bottomed flasks of the reactor shown in fig 4. The stirrer attached to the cylindrical vessel was switched on. Then the angular valve of the reactor containing the silver sulphate solution was opened so that the 4 litres of silver sulphate flows in to the cylindrical reaction vessel. While stirring condition the angular valve of the reactor containing the glucose solution was opened to add 4 litres of 0.05M glucose solution to 0.005M silver sulphate solution cylindrical reaction vessel and stirring continued for 2 minutes more to achieve homogeneous mixing. Then the angular valve of the round bottomed flask containing CTAB solution was opened to add 4 litres of 0.00125 Molar CTAB solution to 0.0025M silver sulphate and 0.05 Molar glucose solution in the cylindrical reaction vessel. Immediately after opening the valve of the reactor containing the CTAB solution the angular valve of the reactor containing the aqueous solution of Diethyl amine was opened to add 0.05Molar Diethyl amine to 0.0025 Molar silver sulphate, 0.05Molar glucose and 0.00125Molar CTAB solution in the cylindrical reaction vessel and stirred for another 2 minutes to get a homogeneous solution. The solution turned from yellow to bright red colour when the diethyl amine is added to yield 16 litres of 0.0135wt % nano silver suspension. The colour change to bright red colour indicates the completion of the reaction thereby producing the suspension.

Example 4;

4 litres of 0.05 moles of glucose solution, 4 litres of 0.0025 moles of silver sulphate solution, 4 litres of 0.00125M CTAB solution were prepared. 4 litres of 0.05M Diethyl amine in 0.00125 Molar CTAB solution was also prepared. . These solutions were taken in the four round flasks in the reactor shown in Fig 4. The stirrer was switched on. Then the angular valve of the flask containing silver sulphate solution was opened so that the 4 litres of silver sulphate solution flows in to the cylindrical reaction vessel. While stirring the angular valve of the flask containing glucose solution was opened to add 4 litres of 0.05M glucose solution to 0.005M silver sulphate solution in the reaction vessel and stirred for 2 minutes to achieve homogeneous mixing. Then the angular valve of the flask containing CTAB solution was opened to add 4 litres of 0.00125 Molar CTAB solution to 0.0025M silver sulphate and 0.05 Molar glucose solution in the reaction vessel. Immediately after opening the valve of the flask containing CTAB solution was opened completely, the angular valve of the flask containing Diethyl amine solution was opened to add 0.05 Molar Diethyl amine prepared in 0.00125 Molar CTAB to 0.0025 Molar silver sulphate, 0.05Molar glucose and 0.00125Molar CTAB solution in the reaction vessel and stirred for another 2 minutes to get a homogeneous solution. The solution turned from yellow to bright red colour to yield 16 litres of 0.0135wt % nano silver suspension. The colour change to bright red colour indicates the completion of reaction thereby producing the suspension. Surface Plasmon resonance peak in the visible region around 420nm observed in the UV visible spectrum as shown in figure 3 confirms the presence of nano sized silver particles in the suspension. The suspension prepared as above was tested for its stability using centrifuge at 6000 rpm. The absorbance value obtained in UV Visible spectrum is directly proportional to the concentration. Any drastic precipitation of nano silver particles due to instability in the suspension can be detected by measuring the absorbance of the suspension before and after centrifuge at 6000rpm. There was a negligible drop in absorbance for the suspension after centrifuge at 6000 rpm when compared with before centrifuge absorbance as shown in Figure 3.

Example 5:

100 ml of 0.15 moles of glucose solution was added to 100ml of 0.015 moles of silver nitrate or silver acetate solution in a vessel and stirred for 5 minutes. Then 100ml of 0.012 moles of CTAB or sodium lauryl sulphate or Polysorbate (Tween 20) solution was added to the silver nitrate or silver acetate and glucose solution and stirred for another 1 minute to get a homogeneous solution. 100ml of 0.15 moles Sodium carbonate in water was added to the above solution and stirred again for 5 minutes to mix homogeneously, thereby producing the suspension. ADVANTAGES OF THE INVENTION

1 The suspension prepared is a stable suspension of nano silver particles haying antibacterial activity , the nano silver particles having the size in range of 10 to 50 am.

2 The process is simple, cost effective, easy to upscale and environmentally friendly and capable of preparing the suspension commercially.

All documents cited in the description are incorporated herein by reference. The present invention is not intended to be limited in scope by the specific embodiments and examples which are intended as illustration of a number of aspects of the scope of this invention. Those skilled in art will know or to be able to ascertain using no more than routine experimentations many equivalents to the specific embodiments of the invention described herein.

It is to be further noted that present invention is susceptible to inodifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention which is further set forth under the following claims:

Claims

We Claim

1. An improved process for the preparation of stable suspension of nano silver particles

having antibacterial activity, comprising the steps of:

(v) Adding anti foaming agent to the suspension obtained in step (iii).

2. An improved process for the preparation of stable suspension of nano silver particles, as claimed in Claim 1, wherein the nano silver suspension consists of 0.01 to 0.135 wt % of nano silver and the particle size of the silver particles ranges from 10 - 50 nm

3. An improved process as claimed in claims 1 & 2 wherein the concentration of silver sulphate or silver acetate used ranges from 0.001 to 0.05 M, the molar ratio of Ag ions: and glucose ranges from 1:1 to 1:10.

4. An improved process as claimed in claims 1 to 3 wherein the surfactant used is selected from Cetyl trimethyl ammonium bromide (CTAB), cetylpyridinium chloride (CPC), Poy ethoxylated tallow amine (POEA), Benzethonium chloride, sodium dodecyl sulphate Sodium lauryl sulphate (SLS) or polysorbate (Tween 20).

5. An improved process as claimed in claims 1 to 4 wherein the amount of the basic reagent diethyl amine or Sodium carbonate used vary from 1:1 to 1:10

6. An improved process as claimed in claims 1 to 5 wherein the antifoaming agent is selected from oil, water, powder or silicone based defoamer and used in the range from 0.1 to 1 wt %

7. A improved process for the preparation of stable nano silver suspension having antibacterial activity substantially as herein described with reference to the Examples