Circuit and Contact Effects on Electron Transport in Protein Junctions

Practical limitations of ultra-thin protein film junctions, such as reliable 4-probe measurements, hinder quantitative estimates of protein-electrode contact resistances. The total junction resistance consists of multiple components, including circuit, contact, and protein resistance. DC measurements cannot isolate the pure protein resistance component. This complexity is clarified by impedance measurements on a series of protein junctions with systematically changed electrode pairs and protein film widths (~5-50 nm). In biomolecular junctions of proteins, more resistive contacts significantly influence transport across the presumably insulating protein films. Protein resistance estimated from impedance fitting and zero-length extrapolation provides the contact resistance. The influence of electrode contributions was examined by including (challenging) metal-protein-metal permanent contacts in micro-pore devices (MpD). Results indicate unexpectedly low protein resistance. Additionally, we introduce an impedance-derived empirical parameter that characterizes the electrical properties of specific protein junctions independent of electrode configurations.