and S18 in SI). Furthermore, impedance spectroscopy revealed the increased charge-transfer resistance after binding of host 5 to an ITO electrode, which is in accordance with previous observations on gold electrodes (Figure ). Submersion of the host-functionalized electrodes into guest solutions led to the formation of surface-bound HGCs accompanied by physisorbed guest species. The presence of the guests on the surface could be clearly determined by various spectroscopic methods and by electrochemical measurements (Figures S20, S21, S22 and S23). Removal of physisorbed species was achieved by treatment of the samples with 100 mM acetic acid in methanol, as was determined by the loss of guest fluorescence signal from a physisorbed sample on ZrO 2 (Figure )). Formation of the host-guest complexes could also be seen during cyclic voltammetry experiments: whereas physisorbed guests rapidly desorb after a few cycles, redoxfeatures of the HGCs could be observed even after extended cycling (Figure ). A binding constant for HGC formation of guest 3 with host 5 on ZrO 2 of K 11 surf = 3.3•10 5 M -1 was calculated based on a Langmuir-type model (Figure ).