The oxidation states of encapsulated ferrocenium (Fc+ filled in calix [4] arene heterodimers) in the form of a self-assembled monolayer (SAM) immobilized on a gold surface were recognized by a 2.2 μm diameter ultramicroelectrode (tip) in combination with scanning electrochemical microscopy (SECM). By approaching the SECM tip biased at− 0.10 V to the vicinity of the encapsulated Fc+ moieties, an electrochemical current at the tip was ...