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 (Q50) 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 octadecanethioVoctadecano1 (ClsSWC180H) monolayers, as described previously (Bilewicz, R.; Majda, M. Langmuir 1991,7,2794). Adetailed characterization ofthe 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-l2mol/cm2) of ubiquinone allows electroreduction of probe species such as Ru(NH&~+. At Q50 concentrations below moVcm2, these monolayers behave as a random array of individual, 5-A-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)s3+ 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 Q50 concentrations higher than mol/cm2 are explained by postulating aggregation and loss of Q50 during L-B transfer.