Silver nanoparticles (AgNPs) exhibit stronger antimicrobial activity than their silver salts. Current methods for AgNP synthesis use either potentially hazardous compounds or extreme synthesis conditions, which increase the environmental impact and cost of AgNPs. Bioreduction of Ag (I) to Ag (0)s in viable microorganisms or their cell-free extract does not require exogenous reducing or capping agents. However, the particle size distribution of biosynthetic AgNPs is too broad for practical applications. We hypothesize that the application of a mild electrochemical potential might narrow down the particle size distribution. We grow Shewanella sp. biofilms, a model electrochemically active microorganism, on carbon cloth electrodes in a bioelectrochemical reactor. Following removal of the microbially-produced flavins, the biofilms are exposed to various electrochemical potentials and the morphologies of the AgNPs produced are characterized via Transmission Electron Microscopy. We found that a low electrode potential (-0.4 V vs. Ag/AgCl sat. KCl) improves the particle size distribution of biosynthetic AgNPs, as the size distribution decreased from 80-700nm to 80-160nm upon the application of a mild electrical potential. This decrease in size distribution is possibly due to the increased reduction rate though the Omc A/mtr C cytochromes.
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We tested a variety of electrode surfaces, to determine suitable electrodes for bioelectrochemical synthesis. Our results show carbon felt and carbon cloth electrodes are suitable for this application. However graphite electrodes were found to be unsuitable, as AgNPs failed to be synthesized. This is probably due to the lower interaction between the outer membrane cytochromes and the electrode.