Mixed Ionic and Electronic Charge Transport in Conductive Protein Fibers Revealed with DC Electrical Measurements

Naturally conductive protein nanowires have inspired efforts to engineer electrical conductivity into synthetic fibrous proteins for the development of bioelectronic materials and devices. A comprehensive analysis of charge transport in these systems requires a combination of various measurement methods, instruments and electrode designs. Measurements under direct current (DC) typically focus on charge transport without distinguishing between charged species, requiring alternating current (AC) and electrochemical methods to probe additional phenomena. Here, ionic and electronic charge transport mechanisms are separately studied within nanowires on interdigitated micro-electrodes under DC. This study also deconvolutes the effects of humidity, salts and polyethylene glycol (PEG) on protein conductivity. As a model system, the M13 bacteriophage, a filamentous protein assembly that is an ideal scaffold for engineering charge transport is used. The M13 phage is also compared with two previously studied conductive protein fibers, Geobacter-derived protein nanowires (e-PN) and engineered aromatic curli fibers. This study observes both transient ionic charge transport and steady-state electronic conductivity in the M13 phage and curli fibers, whereas e-PN materials predominantly exhibited electronic charge transport. The results show that transient and steady-state examinations of sensitive DC measurements in protein fibers help better understand mixed transport in these materials with particularly low conductivity.