The surface diffusion of a cobalt bis-terpyridine, Co(tpy)2-containing tripodal compound (1·2PF6), designed to noncovalently adsorb to graphene through three pyrene moieties, was studied by scanning electrochemical microscopy on single-layer graphene. An initial boundary approach was designed in which pico-liter droplets (radii ∼15-50μm) of the tripodal compound were deposited on an single-layer graphene electrode, yielding microspots in which a monolayer of the tripodal molecules was initially confined. The time evolution of the electrochemical activity of these spots was detected at the aqueous phosphate buffer/single-layer graphene interface by scanning electrochemical microscopy, in both generation/collection and feedback modes. The tripodal compound microspots exhibited differential reactivity with respect to the underlying graphene substrate in two different electrochemical processes. For example, during the oxygen reduction reaction, adsorbed 1·2PF6 tripodal molecules generate more H2O2 than the bare graphene surface. This product was detected with spatial and temporal resolution using the scanning electrochemical microscopy tip. The tripodal compound also mediated the oxidation of a Fe(II) species, generated at the scanning electrochemical microscopy tip, under conditions in which single-layer graphene showed slow interfacial charge transfer. In each case, scanning electrochemical microscopy images, obtained at increasing times, showed a gradual decrease in the electrochemical response due to radial diffusion of the adsorbed molecules outward from the microspots onto the unfunctionalized areas of the single-layer graphene surface. This response was fit to a simple surface diffusion model, which yielded excellent agreement between the two experiments for the effective diffusion coefficients: Deff = 1.6 x 10-9cm2/s and Deff = 1.5 x 10-9cm2/s for G/C and feedback modes, respectively. Control experiments ruled out alternative explanations for the observed behavior, such as deactivation of the Co(II/III) species or of the single-layer graphene, and verified that the molecules do not diffuse when confined to obstructed areas. The non-covalent nature of the surface functionalization, together with the surface reactivity and mobility of these molecules, provides a means to couple the superior electronic properties of graphene to compounds with enhanced electrochemical performance, a key step toward developing dynamic electrode surfaces for sensing, electrocatalysis, and electronic applications.

Quantification of the Surface Diffusion of Tripodal Binding Motifs on Graphene Using Scanning Electrochemical Microscopy. Rodríguez-López, J., Ritzert, N.L., Mann, J.A., Tan, C., Dichtel, W.R., Abruña, H.D.: Journal of the American Chemical Society, 2012, 134[14], 6224-36