The present invention relates to a method for coating a substrate with silver nanoparticles.

Silver coatings on a substrate, for example a glass substrate, are well known in the art. Besides the well known Tollens ^' reaction, i.e. the test for aldehydes resulting in the formation of a silver mirror due to reduction of silver ions, respective further coatings are also known from US 5, 186,984, 3,978,271, 5,346,651, 4,894,290 and 4,652,465.

Moreover, US2007/0018140 Al discloses a method of producing metal nanoparticles, said method comprising (a) producing a metal alkanoate by reacting a metal precursor with an alkanoate of an alkali metal, alkali earth metal, or ammonium in an aqueous solution; (b) filtrating and drying the metal alkanoate; and (c) heat-treating the alkanoate of (b).

The methods of coating a substrate known in the art are somewhat difficult and non-straight forward to be carried out, especially at small and larger scales. Typically, prior art methods use either vacuum at lower temperature, or temperatures of at least 26CPC. Further, the production of conducting ink requires addition of non aqueous solvents like toluene and tetradecane, and the use of a ultra-sonicator.

It is therefore an object of the present invention to provide a method for coating a substrate with silver nanoparticles which overcomes the drawbacks of the prior art. Especially a method shall be provided which is simple, facile, straight forward and easy to execute, both at small and larger scales, with a possible application on industrial level.

This object is achieved by a method for coating a substrate with silver nanoparticles comprising the steps: contacting the substrate with a solution of silver Cg-Cig alkanoate, preferably oleate, in nitrobenzene, and refluxing the solution of silver Cg-Cis alkanoate in nitrobenzene at 215-25CPC, preferably 22CPC, for 1-5 hours, preferably 3 hours.

Preferably, the method additionally comprises the steps: removing the solution after refluxing from the substrate, washing the substrate, and drying the substrate.

In one embodiment washing is done at least once with acetone and/or hexane.

Even preferred the silver nanoparticles have a size of about 10-50 nm, and/or wherein the coating has a thickness of about 5-40 nm.

In one preferred embodiment the substrate is glass.

More preferred silver alkanoate is added to nitrobenzene if the nitrobenzene is already refluxing.

The invention also provides a substrate coated with silver nanoparticles obtained by a method according to the invention.

Surprisingly, it was found that the inventive method allows easy coating of a substrate, preferably a glass substrate, with a silver coating. The inventive method is simple, facile, straight forward and easy to execute, both at small and larger scales. It can be also applied on an industrial level. The method employed according to the present invention utilizes a silver alkanoate, especially a silver oleate as a source of silver. The silver oleate can be also considered as a capping agent. The inventive method involves the solvent to be heated, preferably followed by the addition of the silver salt while stirring. A substrate, for example glass plates (slides) can be placed into the solution prior to the addition of a silver salt. The reaction is carried out under reflux of the solvent for a period of time, preferably about 3 hours, and is then allowed to cool. The substrate can be removed from the reaction, can be optionally washed and dried, and the substrate is found to be coated with silver.

The coated substrate can be characterized using various analytical techniques such as AFM (Atomic Force Microscopy), SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy). The silver particles are preferably in the range of 10 bis 50 nm. It has been also found that the coating on the substrate has preferably a thickness of about 5-40 nm.

The coated substrate obtained by the method of the present invention can be, for example, used as silver mirror without any backing support necessary, as anti reflective coating, for example for buildings, cars and glasses, as biofilms, as casing of electronic equipments, in electronic circuit boards, or in biology and medicine for antibacterial, antiviral and anticancer activity.

The inventive method is based on a solvothermal process utilizing a single step process that can be used for coating substrates, such as glass surfaces, for use of reflective purposes and as mirror without the presence of any backing material.

Additional advantages and features of the subject matter of the present invention can be taken from the following detailed description by means of examples and the accompanying drawing wherein figure 1 shows a ^! H HMR spectrum of silver oleate prepared according to example 1 ; figures 2 and 3 show SEM images of silver nanoparticles as prepared according to example 2; and figures 4 and 5 show AFM images of silver nanoparticles coated on glass according to example 3.

Example 1

Preparation of silver oleate 10 grams of silver nitrate is introduced in a beaker and dissolved in a minimum quantity of water. 5 grams of sodium oleate is introduced in another beaker and dissolved in a minimum quantity of water, with heating and stirring, using a hot plate stirrer. After complete dissolution of sodium oleate, the solution is allowed to cool down to room temperature. Then, the silver nitrate solution is added to the oleate solution, slowly while being stirred, and the silver oleate is formed as a solid. The precipitate obtained is filtered and dried in an oven to give a white powder. This powder has been characterized and confirms the formation of silver oleate.

Example 2

Preparing a silver lining on an inner wall of a round bottom flask

50 ml of nitrobenzene is introduced into a round bottom flask, equipped with a water cooled condensor and heated to a reflux temperature of 22CPC, while being stirred using a magnetic hot plate stirrer. To the solvent, silver oleate (1 gram) is added, and the refluxing is then continued at the same temperature for about 3 hours. The contents of the round bottom flask is then, each twice, washed with acetone and hexane. A silver lining can be observed on the inner wall of the round bottom flask. The bottom of the flask clearly shown the uniform coating of silver.

Example 3

Preparing a silver lining on both walls of a glass plate.

Two plates of glass which needed to be coated are placed facing each other in a round bottom flask which contained 50 ml of nitrobenzene. The solvent is heated to reflux temperature of 22CPC. To this, 1 gram of silver oleate is added, and the reaction is then continued at the same temperature for about 3 hours. The content of the round bottom flask is subsequently washed, each twice, with acetone and hexane. The silver lining can be observed on the walls of the glass plates. Two glass plates can thus be uniformly coated on a single side of the glass. The inner wall of the round bottom flask is also coated. The material thus obtained has been characterized by electron microscopic techniques, like SEM, TEM and AFM. These results show that silver nanoparticles are formed and the coating is uniform. Both examples 2 and 3 clearly demonstrate that a silver coating on a substrate can be easily achieved by a simple chemical synthetic route. The silver coated glass plate when subjected to scanning electron microscopy gave results as illustrated in figures 2 and 3. These results suggest that the glass is well coated and the coating is covering the entire glass. The particles coating the glass have been found to be spherical in shape, and the atomic force microscopy results show that the thickness of a coating varies from 5-50 nm.

The features disclosed in the foregoing description and the claims may, both separately and in any combination thereof, be material for realizing the invention in diverse forms thereof.