EP2030706B1 - Method of preparing nanoparticles of silver - Google Patents

Abstract

Preparation of silver nanoparticles having diameter of less than 100 nm, dispersed in polymer matrix at a concentration of greater than 1 M, comprises reacting an organic silver salt and polymeric agent for nucleation and stabilization of silver nanoparticles; mixing the mixture obtained at reduction potential reducer and having an affinity of coordination with silver ions; and concentrating and separating the polymer matrix containing silver nanoparticles.

Description

Technical area

The present invention relates to the field of nanotechnology. It relates, more particularly, to a process for preparing silver nanoparticles.

State of the art

Metal nanoparticles are widely studied for their optical, electrical, catalytic or biological properties. The size and shape of these particles greatly influence their characteristics. Numerous studies have been carried out in order to define processes that make it possible precisely to control the shape and the size of these different metallic nanoparticles. Various preparation routes have been tested for this purpose, such as chemical reduction, gas condensation, laser irradiation ...

More specifically, silver particles are of great interest. Firstly, their antimicrobial properties resulting from their interaction with thiol, amine, imidazole, carboxyl or phosphate functional groups of living organisms proteins make them suitable for a large number of applications in the medical field.

On the other hand, when the silver particles are dispersed in polymeric organic matrices, they can serve as conductors in electronic and electrotechnical applications. This use is doubly interesting, on the one hand because the resulting conductive formulations can be partially transparent and, on the other hand, because it is possible to induce sintering between the particles to create a crosslinked metal assembly of which the conductive properties are greatly improved.

In addition, it is also important to stabilize the formed particles so that they do not agglomerate and retain their properties.

However, this research has for the moment been undertaken only experimentally and the reaction conditions can not be transposed to be industrialized.

For example, a synthetic route has been proposed by Li and Al (J. Am.Chem., Vol 127, No. 10,2005 ), from silver acetate and alkylamine, in toluene and phenylhydrazine. However, such a reaction can not be used industrially for two major disadvantages. Firstly, the use of a nitrogen reducer is troublesome for possible electronic applications of the nanoparticles obtained, because there are still traces of nitrogen that are detrimental to the quality of the electronic device obtained. Then, although the publication mentions that the product of the reaction has a high silver concentration, it is only 0.5M. However, such a concentration is not high enough for such a synthesis is economically interesting. Indeed, it is necessary to use large volumes of reagents to obtain a sufficient amount of nanoparticles.

S. Mukherjee proposes the production of an Ag-PAM nanocomposite. A silver salt is mixed with a polymeric agent ( S. Mukherjee: "Nitrogen-mediated interaction in polyacrylamide-silver nanocomposites", Journal of Physics, Condensed Matter 18 2006, 11233-11242 ).

In addition, other conventional ways of preparing silver by reduction of Ag + ions generally involve reagents or toxic solvents (silver nitrate, DMF, etc.) and energetic reaction conditions (temperature, pressure). which also does not make them ideal solutions for industrialization, because they are delicate in terms of safety and ecology. Finally, conventional methods of nucleation / growth lead to particles that are too large and unusable for the intended applications.

The object of the present invention is therefore to propose an easily industrializable silver nanoparticle synthesis route which makes it possible to obtain these particles with good control of their size and shape.

Disclosure of the invention

• mise en réaction d'un sel organique d'argent et d'un agent polymérique de nucléation et de stabilisation des nanoparticules d'argent,
• mélange du milieu réactionnel obtenu précédemment à un réducteur à potentiel de réduction limité, de manière à ne pas agglomérer l'argent réduit, et présentant une affinité de coordination avec des ions Ag⁺,
• concentration et séparation de la matrice polymère contenant les nanoparticules d'argent.

More specifically, the invention relates to a method for preparing silver nanoparticles with a diameter of less than 100 nm, dispersed in a polymer matrix at a concentration greater than 1M, comprising the following steps:

• reacting an organic silver salt and a polymeric nucleating and stabilizing agent for silver nanoparticles,
• mixture of the reaction medium obtained above with a reducing agent with limited reduction potential, so as not to agglomerate the reduced silver, and having a coordination affinity with Ag ⁺ ions,
• concentration and separation of the polymer matrix containing the silver nanoparticles.

More particularly, the above process is particularly advantageous when the organic silver salt used is selected from silver acetate, silver acetylacetonate, silver citrate, silver lactate or silver pentafluoropropionate.

Very interesting results have been obtained by mixing the organic silver salt with a polymer based on polyvinylpyrrolidone (PVP), polyethylene glycol (PEG) or based on polypropylene glycol.

Thus, the method according to the invention does not involve toxic product or dangerous for the environment. In addition, the reaction conditions are mild and minimize the risks inherent in the reaction.

Brief description of the drawings

Other characteristics of the method will appear more clearly on reading the following description accompanied by the attached drawing showing images obtained by transmission electron microscopy (TEM) of silver particles obtained according to the method.

Mode (s) of realization of the invention

The process for the preparation of silver nanoparticles, according to the invention, comprises a first step of mixing 5 g of silver acetate with a solution of 5 g of polyvinylpyrrolidone (PVP) of molecular mass 10000 in 200 ml of water at a temperature of temperature between 40 and 60 ° C, typically at 50 ° C. PVP serves as a nucleating agent and stabilizer, to allow the formation of silver nanoparticles, while avoiding that they agglomerate.

A rise in temperature is carried out in 5 minutes to reach a temperature between 60 and 90 ° C, typically 75 ° C. The solution, white at the beginning of the reaction, then evolves towards a burne color. The reaction mixture is then left stirring for 45 minutes at 95 ° C. The solution then evolves slowly from a brown color to a green color. The heating is then stopped and the solution is left stirring to reach 35 ° C.

The reaction medium is then mixed with a solution of ascorbic acid at 20 mM. Ascorbic acid serves as a reducing agent. It has a coordination affinity with the Ag ⁺ ions, while having a limited reduction potential, so as not to agglomerate the reduced silver. Thus, the ascorbic acid can initially bind with the Ag ⁺ ions stably, allowing the electron transfer to be done in a second time, without agglomeration of silver particles. As an indication, the reduction potential of ascorbic acid is -0.41V. Other reducing agents with a reduction potential typically less than + 0.2V, preferably less than -0.2V, but greater than -1.5V, preferably greater than -1.2V, preferably greater than -1V may be envisaged. It should be noted, for example, that glucose (-1.87V reduction potential) is a too strong reducing agent and reduces Ag ⁺ ions but forming agglomerates. The potentials above are given according to the usual norm in Europe and extracted from: CRC Handbook Series in Organic Electrochemistry, Vol 1, 1976 .

It would also be possible to continuously add the reaction medium and the reducing agent, in a stoichiometric proportion.

When the reduction reaction is complete, i.e., typically after 30 minutes, the solution is centrifuged to concentrate the polymer matrix containing the silver nanoparticles. It will be noted that the evolution of the reduction reaction can be followed by UV / visible spectroscopy.

The analyzes carried out on the final product make it possible to determine that 80% of the silver introduced in the form of silver acetate is converted into metallic silver (Ag ⁰ ). The figures 1 and 2 are images obtained by transmission electron microscopy (TEM) that measure the size of nanoparticles and their distribution. The size of the nanoparticles obtained is between 3 and 50 nm.

Other experiments have been carried out with various organic silver salts, such as silver acetylacetonate, silver citrate, silver lactate or silver pentafluoropropionate. Similarly, polyethylene Glycol (PEG) and polypropylene glycol have also been used as replacements for PVP and these polymers can be used with different molecular weights. For the purposes of the claims, the term PVP, PEG or polypropylene glycol polymer includes copolymers having one of these monomers as a unit. Depending on the reagents used, the silver nanoparticles obtained have a diameter of less than 100 nm, more particularly less than 80 nm, more particularly less than 50 nm. Particles with a diameter of around 2 nm could be detected. These particles are dispersed in the polymer matrix at a concentration greater than 1 M, particularly greater than 2M, more particularly greater than 3M.

The conversion rate obtained, on the one hand, and the quality of the particles obtained (small size and uniformity of the dimensions), on the other hand, are remarkable compared to the other methods tested.

By way of comparison, mention may be made of another experimental protocol tested, comprising a first step of mixing 10 g of silver acetate and 1 g of 1500 molecular weight polyethylene glycol (PEG 1500) in 80 ml of 50% tert-butanol. ° C. PEG also serves as a reducer. Silver acetate forms a suspension in the alcohol and PEG solution. The mixture is stirred and its temperature is raised to about 75 ° C over a period of five minutes. The solution is stirred for 45 minutes at 80 ° C. The best conversion rate obtained with this protocol is about 50%.

Thus is proposed a process for preparing silver nanoparticles which makes it possible to obtain these particles with good control of their size and shape. At the level of industrialization, the various reagents mentioned above can be used and combined. However, the choice of silver acetate and PVP appears to offer the best combination in terms of yield, quality of the particles obtained, cost of reagents, safety of the reaction and ecology.

Claims (9)

1. A method for preparing silver nanoparticles which a diameter of less than 100nm, dispersed in a polymeric matrix at a concentration above 1M, including the following steps:

   i. reacting an organic silver salt and a polymeric agent for nucleation and stabilization of silver nanoparticles,

   ii. mixing the reaction medium obtained previously with a reducing agent having a defined reduction potential and having coordination affinity with Ag+ ions,

   iii. concentrating and separating the polymeric matrix containing the silver nanoparticles.
2. The method according to claim 1, characterized in that said organic silver salt is selected from silver acetate, silver acetylacetonate, silver citrate, silver lactate or silver pentafluoropropionate.
3. The method according to any of claims 1 or 2, characterized in that the polymer is based on polyvinylpyrrolidone (PVP) or polyethyleneglycol (PEG) or polypropyleneglycol.
4. The method according to claim 3, characterized in that the reacting takes place in an aqueous medium.
5. The method according to claim 4, characterized in that step i includes addition of water at a temperature comprised between 40 and 60°C, a phase for heating to a temperature comprised between 65 and 95°C and a cooling phase.
6. The method according to any of the preceding claims, characterized in that the reducing agent used is ascorbic acid.
7. The method according to any of the preceding claims, characterized in that the concentration and separation operation is carried out by centrifugation.
8. The method according to any of the preceding claims, characterized in that the diameter of the obtained silver nanoparticles is less than 50nm.
9. The method according to any of the preceding claims, characterized in that the obtained silver nanoparticles are dispersed in a polymeric matrix at a concentration above 2M, preferably above 3M.

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