“Process for preparing stable suspensions of metal nanoparticles and the stable colloidal suspensions obtained thereby”

Process for preparing stable suspensions of metal nanoparticles and the stable colloidal suspensions obtained thereby Field of the invention The present invention relates to the field of suspensions of nanometer-sized metal particles and to their preparation methods. State of the art Because of their versatility and the numerous areas where they find application, metal nanoparticle suspensions are of great interest for industry. In particular, by virtue of their physicochemical characteristics, metal nanoparticles have important applications in various fields: biomedical, optical and electronic devices, and catalysts. In the medico-biological sector, said nanoparticles are widely studied for their antibacterial and antifungal properties, the antibacterial effect reported to increase with increasing concentration of metal in the suspension and for sizes less than 50 nm. The antimicrobial effect of silver can be utilized on various types of materials: prostheses (e.g. silver-loaded hydroxyapatite), plastic sanitary materials, epidermis, materials for dental use, steel, ceramic tiles, textiles and also for water treatment. From the literature it transpires that there is a need for suspensions that are time stable, obtained from reagents with low environmental impact. Recently, nanoparticulate silver has been investigated for applications in a more specific biomedical field; for example, interactions between nanoparticulate silver and viruses such as HIV have been observed, demonstrating its ability to inhibit them. Furthermore, other studies report the ability of nano-metal to destroy tumour cells. Other applications that exploit the optical properties typical of nanometric silver and other noble metals, characterized by the phenomenon of surface plasmon resonance, are surface Raman spectroscopy, optical devices and sensors, diagnostic medicine and biological imaging. Silver nanoparticles are also investigated for their catalytic properties, which are particularly important if they are synthesized together with other metals or oxides (supported catalysts). The optical and biological properties of silver nanoparticles, and the possibility of transferring the synthesis technique to industry, mainly depend on characteristics such as: high concentration, stability of the suspensions in time and dimension control. The methods proposed in the literature cannot provide for all these requirements together. Indeed, faced with the innumerable applications for nanoparticulate silver, many synthesis methods are reported in the state of the art which are often able to control both particle shape and size. In the many published studies, however, it is noted that for colloidal systems the considered concentrations are in most cases very low, and usually between 0.001 and 0.005 M even if the concentration is defined as high. Cases are reported with 0.05-0.06 M concentrations and a maximum of 0.2 M, but they involve syntheses consisting of precipitation of the solid or the presence of a stabilizing polymer in such excess as to form a metallopolymer composite. Also, the stability over time of the synthesized systems is rarely referred to, with, in one case, a maximum stability of 24 hours being reported for a 0.2 M concentration. The use of low concentrations enables smaller and more stable particles to be obtained, but for the purpose of industrial use and scale-up it is important to be able to work with medium-high concentrations in order to make the production cycles economically advantageous, by synthesizing a concentrated system which can be diluted in subsequent stages if necessary. Furthermore, a higher concentration enables the chromic characteristics of silver to be exploited and its antibacterial and antifungal properties to be enhanced, given that it allows more concentrated suspensions to be applied to the surfaces to be treated. The optimization of a synthesis carried out at low concentration is difficult to r...