Permeability and Dynamics of a Monolayer are Mediated by ITO Support Surface-Modification.

Abstract

Indium tin oxide (ITO) has been extensively used as a transparent conductor. The surface chemistry of ITO is amenable to reactions similar to those used to modify silica, but a long-standing issue has been understanding the density and robustness of the ITO surface-modification. We report on the formation of chemically bound Cd2+-complexed octadecylphosphonic acid (ODPA) monolayer formed on a Langmuir trough and deposited using Langmuir-Blodgett (LB) methodology onto an ITO surface, either in its native form or functionalized with phosphonate (RPO32-). The organization of the Langmuir monolayer depends on the pH and presence of Cd2+ in the aqueous subphase on which it is formed and on the functionalization of the ITO surface. We probe the permeability of the resulting LB-support interface electrochemically and the motional freedom characteristic of chromophores contained within the monolayer using fluorescence recovery after photobleaching (FRAP). Our data demonstrate that without modification of the ITO surface the monolayer is significantly permeable by the electrophores used (ferrocene and Ru3+), and surface modification to produce covalently bound phosphonate functionality results in a monolayer that is impermeable to the electrophores. FRAP studies reveal a relatively rigid monolayer aliphatic chain region for deposition on either native or modified ITO, suggesting direct Cd2+-ITO interactions.