A series of STM images showing in situ observations of redox-gated electron-tunneling resonance arising from single azurin molecules. The images were obtained by using the azurin/octanethiol/Au(111) system in NH4Ac buffer (pH 4.6) with a fixed bias voltage (defined as Vbias = ET – ES) of –0.2 V but variable substrate overpotentials (vs. the redox potential of azurin, +100 mV vs. SCE): +200 (A), +100 (B), 0 (C), –100 (D), and –200 mV (E). Scan area is 35 × 35 nm.