Synthesis of Palladium Nanoparticles by Electrode-Respiring <i>Geobacter sulfurreducens</i> Biofilms

Electroactive microorganisms such as <i>Geobacter sulfurreducens</i> can couple organic electron donor oxidation to the respiration of electrode surfaces, colonizing them in the process. These microbes can also reduce soluble metal ions, such as soluble Pd, resulting in metallic nanoparticle (NP) synthesis. Such NPs are valuable catalysts for industrially relevant chemical production; however, their chemical and solid-state syntheses are often energy-intensive and result in hazardous byproducts. Utilizing electroactive microbes for precious metal NP synthesis has the advantage of operating under more sustainable conditions. By combining <i>G. sulfurreducens</i>'s ability to colonize electrodes and synthesize NPs, we performed electrode cultivation ahead of biogenic Pd NP synthesis for the self-assembled fabrication of a cell-Pd biomaterial. <i>G. sulfurreducens</i> biofilms were grown in electrochemical reactors with added soluble Pd, and electrochemistry, spectroscopy, and electron microscopy were used to confirm (1) metabolic current production before and after Pd addition, (2) simultaneous electrode respiration and soluble Pd reduction over time, and (3) biofilm-localized Pd NP synthesis. Utilizing electroactive microbes for the controlled synthesis of NPs can enable the self-assembly of novel cell-nanoparticle biomaterials with unique electron transport and catalytic properties.