Synthesis of copper and silver nanoparticles with extracts of berries: A green chemistry approach

Abstract

Nanoparticles are usually defined as particles in a size range of 1-100 nm, and these particles often have different properties than the non-nanoscale material of the same chemical composition. In recent years, as the interest and use of nanoparticles has increased and so has the interest in understanding their behaviour, impact and fate in the environment. In the last decades, there has been special focus on the search for more eco-friendly synthesis strategies, especially in the pharmaceutical industry. The three main areas where the principles of green chemistry could be applied in the synthesis of metal nanoparticles are: the capping agent, the reducing agent and the solvent. Along with the use of reagents and solvents with low toxicity, a low power consumption is desirable. The blueberry extracts (V. myrtillus and V. gaultheroides) are the known reducing agents as well as the capping agents. Since the extract is aqueous there was no need for toxic or dangerous solvents to be employed in the synthesis either. Finally, the synthesises were performed at relatively low temperatures (50°C and 70°C), which also positively affects the power consumption.

The main aim of this study was to apply the principles of green chemistry on the synthesis of metallic copper and silver nanoparticles, using extract of blueberries to reduce the metal precursors instead of more environmentally damaging and toxic materials. Employing valuable agricultural by-products also represent an added value for large scale production. Since the size and structure of metal nanoparticles has been shown to be important for the antimicrobial effect, the nanoparticles size and shape were characterised by transmission electron microscopy (TEM). The antimicrobial potential of different types of nanoparticles were compared, and correlated to the manufacturing procedure. To assure the applicability of the developed system in treatment of skin infections, the cytotoxicity of nanoparticles was assayed in keratinocytes.

We managed to synthesise the copper and silver nanoparticles by using extracts of two different species of blueberry as a reducing agent and the size and structure of the metal nanoparticles were characterised by TEM analysis. Although the copper nanoparticles did not exhibit expected antibacterial effect under the conditions applied in our study, the silver nanoparticles did exhibit antibacterial effect, although less than detected for the metal precursor salt solution alone. This might be correlated to the fact that the nanoparticle suspensions were not stable; the nanoparticles (and extract) clearly aggregated and precipitated upon storage. Most likely for the same reason, all nanoparticle suspensions showed toxicity toward keratinocytes at all concentrations tested in the cytotoxicity assay.

To conclude whether the synthesised nanoparticles are indeed promising antibacterials, as well as address apparent toxicity, further experiments should be performed. These can be done by varying the concentration of extract and metal precursor or by non-toxic stabilizers, such citrate from other plant sources, in order to obtain nanoparticle suspensions with longer shelf-life. The temperature of synthesis and stirring method could also be optimized.