Interface Electrostatics Dictates the Electron Transport via Bioelectronic Junctions

Different batches of Si wafers with nominally the same specifications were found to respond differently to identical chemical surface treatments aimed at regrowing Si oxide on them. We found that the oxides produced on different batches of wafer differ electrically, thereby affecting solid-state electron transport (ETp) via protein films assembled on them. These results led to the another set of experiments, where we studied this phenomenon using two distinct chemical methods to regrow oxides on the same batch of Si wafers. We have characterized the surfaces of the regrown oxides and of monolayers of linker molecules that connect proteins with the oxides and examined ETp via ultrathin layers of the protein bacteriorhodopsin, assembled on them. Our results illustrate the crucial role of (near) surface charges on the substrate in defining the ETp characteristics across the proteins. This is expressed most strikingly in the observed current's temperature dependences, and we propose that these are governed by the electrostatic landscape at the electrode-protein interface rather than by intrinsic protein properties. This study's major finding, relevant to protein bioelectronics, is that protein-electrode coupling in junctions is a decisive factor in ETp across them. Hence,surface electrostatics can create a barrier that dominates charge transport and controls the transport mode across the junction. Our findings' wider importance lies in their relevance to hybrid junctions of Si with (polyelectrolyte) biomolecules, a likely direction for future bioelectronics. A remarkable corollary of presented results is that once an electron is injected into the protein, transport within the proteins is so efficient that it does not encounter a measurable barrier down to 160 K.