Ionogel‐Electrode for the Study of Protein Tunnel Junctions under Physiologically Relevant Conditions

The study of charge transport through proteins is essential for understanding complicated electrochemical processes in biological activities while the reasons for the coexistence of tunneling and hopping phenomena in protein junctions still remain unclear. In this work, a flexible and conductive ionogel electrode is synthesized and is used as a top contact to form highly reproducible protein junctions. The junctions of proteins, including human serum albumin, cytochrome C and hemoglobin, show temperature‐independent electron tunneling characteristics when the junctions are in solid states while with a different mechanism of temperature‐dependent electron hopping when junctions are hydrated under physiologically relevant conditions. It is demonstrated that the solvent reorganization energy plays an important role in the electron‐hopping process and experimentally shown that it requires ≈100 meV for electron hopping through one heme group inside a hydrated protein molecule connected between two electrodes. A novel electrode material made of ionogel is used to conduct measurement of charge transport through protein molecules adsorbed on a surface of a self‐assembled monolayer. It is shown that tunneling through a protein junction can operate via temperature‐independent tunneling at solid states, and switch into the temperature‐dependent hopping with hydrated proteins under physiologically relevant conditions.