Immobilized biocatalyst: The oxygen-tolerant, membrane-bound hydrogenase of Ralstonia eutropha H16 is immobilized by a His-tag (see picture; green) onto a gold surface (yellow) modified with nickel–nitrilotriacetic acid (red/black). Catalytic activity towards hydrogen is investigated by surface-enhanced infrared absorption (SEIRA) spectroscopy. Switching redox states and related structural changes of the Ni–Fe active site are followed by the CO and CN− ligand stretching modes.