MONOMOLECULAR LANGMUIR-BLODGETT-FILMS AT ELECTRODES - ELECTROCHEMISTRY AT SINGLE-MOLECULE GATE SITES

The Langmuir-Blodgett (L-B) technique is used to coat electrode surfaces with bifunctional monolayer films that are designed to carry out two functions: passivation and gating. We demonstrate that incorporation of ubiquinone (Q(50)) in otherwise passivating L-B monolayer films allows us to control access to the electrode surface via a controlled number of single molecule gate sites. The electrode passivation is accomplished with mixed octadecanethiol/octadecanol (C18SH/C18OH) monolayers, as described previously (Bilewicz, R.; Majda, M. Langmuir 1991, 7, 2794). A detailed characterization of the passivating properties of these L-B monolayers is provided in view of the Amatore-Saveant-Tessier (A-S-T) theory (Amatore, C.; Saveant, J.-M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39). The incorporation of low levels (10(-12)-10(-17) mol/cm(2)) of ubiquinone allows electroreduction of probe species such as Ru(NH3)(6)(3+). At Q(50) concentrations below 10(-15) mol/cm(2), these monolayers behave as a random array of individual, 5-Angstrom-radius disk microelectrodes each created by a single ubiquinone molecule. We postulate that a loose conformation of the ubiquinone's isoprenoid chain creates a channel that allows for a direct approach of Ru(NH3)(6)(3+) ions to the electrode surface. Electrochemical results are evaluated on the basis of A-S-T theory using direct comparison of the experimental and calculated cyclic voltammetric responses. Negative deviations in the observed voltammetric response compared with the theory at the Q(50) concentrations higher than 10(-15) mol/cm(2) are explained by postulating aggregation and loss of Q(50) during L-B transfer.