Chronoamperometry in phosphate buffer saturated with O₂. 

Detail of a single, iron-containing porphyrin upon switching between high potential (0.7 VRHE, ORR inactive) and low potential (0.4 VRHE, ORR active). Only the contrast of the Fe-OEP molecule changes, concomitantly to the potential switch. In the lower right corner, a diffusing molecule gets trapped in the second layer.
The active molecule (low potential) appears indiscernible from the metal-free OEP.
I_t=0.75 nA; tip kept at a constant potential of 0.3 VRHE, resulting in a bias between 0.4 and 0.1 V (imaging of unoccupied states). The movie is accelerated by a factor of 2.

Movie at constant potential of 0.7 V_RHE (ORR off) in phosphate buffer under Ar with traces of O₂. 

The iron is present as Fe(III) and no redox process occurs. Traces of O₂ react with some Fe-OEP molecules that are embedded among metal-free OEPs. The resulting brightness makes the individual assignment possible and allows to study the diffusion of adsorbates within the layer that appears to be much more dynamic than expected.

I_t=0.9 nA; tip is kept at a constant potential of 0.1 VRHE, resulting in a tip-sample bias of 0.60 V (imaging of unoccupied states). The movie is accelerated by a factor of 5.