Electrochemical water oxidation was also attempted using guest 3, which was shown to be active (though sluggish) for this reaction in solution (Figure ). Unfortunately, we could not observe an unambiguous catalytic response for water oxidation versus the background. To nevertheless confirm the activity of HGCbound guests, we were able to demonstrate catalytic oxidation of a water-soluble phosphine to the corresponding phosphine oxide with HGC-bound 3 on ITO as a model reaction (Figure ). This system also showed similar stability as the case for NH 3 oxidation in organic media. Regeneration of catalytic activity upon readsorption of 3 indicates the robustness of the host on the surface (Figure ).