Respiratory Au nucleation and microelectrode techniques reveal key features of bacterial conductive matrix

Abstract

The previously reported relay network conductivity model has shed some light on the structure and mechanisms behind long-distance extracellular electron transfer (EET) in Geobacter biofilms. The structuration of c-type cytochromes (c-Cyt) in supramolecular complexes and their interactions with pili, as a requirement for achieving external intermolecular ET, were put forward. Such an arrangement supports a redox gradient-driven process limited by potential loss along the biofilm, which ultimately limits technological developments on bioanodes. Geobacter cells display wide respiratory versatility, including uranium, palladium, silver and gold salt reduction, which often yields nanoparticles (NPs). Here, we took advantage of the ability of G. sulfurreducens to produce monodisperse AuNPs (G.Au) to interpret and improve the EET mechanism. Both metabolic stratification and co-localization of c-Cyt and AuNPs were analyzed by TEM microscopy and Raman spectroscopy to evaluate the relation between these elements and reveal the spatial organization of redox proteins, giving support to the 2-fold increase in the current density production that was measured as a consequence of improving cell connectivity with gold nucleation. The final corroboration of specific interactions between AuNPs and c-Cyt came from the electrophoretic analysis of the nanostructure isolated fractions. We observed that electrons accumulated in the absence of polarization reduced Au(III) throughout the biofilm and can also be drained through a poised microelectrode located at 100 μm from the basal electrode used for biofilm growth, thus probing no predetermined directionality in the EET network, other than that dictated by the potential. While presenting gold nucleation as an alternative to overcome limitations in current production, these results corroborate main concepts of the relay network model, pushing towards more efficient applications for bio-hybrid nanostructured materials in the field of bioelectronics.