The RE was located in one of the lateral channels of the glass cell (Figure The supporting electrolyte was injected into the cell with a mechanical syringe (APEMA, model PU11U) at a controlled flow which varied in the range 0.016 -0.032 cm 3 .min -1 , that is, the resident time of the solution in the FE-AFM cell was around one minute, which it was enough to maintain an appropriated oxygen concentration on the working electrode.