Amperometric transduction and amplification of optical signals recorded by a phenoxynaphthacenequinone monolayer electrode: Photochemical and pH-gated electron transfer

A phenoxynaphthacenequinone photoisomerizable monolayer was assembled onto an Au electrode. The resulting ''trans''-quinone monolayer exhibits poor electrochemical reversibility due to a nondensely-packed configuration. Treatment of the trans-quinone monolayer with 1-tetradecanethiol yields a densely-packed monolayer that exhibits electrochemical reversibility. The electrochemical response of the irans-quinone monolayer electrode is pH-dependent, consistent with a two-electron and two-proton redox process. Photoisomerization of the trans-quinone monolayer (305 nm < lambda < 320 nm) generates the ana-quinone monolayer that lacks electrochemical activity. Upon photoisomerization of the ana-quinone monolayer to the trans-quinone state (lambda > 430 nm), the electroactivity of the monolayer is restored. By cyclic photoisomerization of the electrode between the ana- and trans-quinone states, reversible amperometric transduction of the recorded optical signals was accomplished. Coupling of redox-active materials, such as Fe(CN)(6)(3-) or N,N'-dibenzyl-4,4'-bipyridinium (BV2+) to the photoisomerizable electroactive monolayer electrode allows vectorial electron transfer and amplification of the electrical response of the trans-quinone monolayer by the electrocatalyzed reduction of Fe(CN)(6)(3-) or BV2+. The vectorial electron transfer from the trans-quinone monolayer to BV2+ is gated by the pH of the medium. The trans-quinone monolayer electrode was coupled to the redox mediator BV2+ and the enzyme nitrate reductase. In the presence of NO3-, the multicomponent system in the trans-quinone state leads to the bioelectrocatalyzed reduction of nitrate and the transduction of an amplified cathodic current. In this system, the vectorial reduction of BV2+ to BV+. yields an electron mediator that activates the biocatalyzed process. By cyclic photoisomerization of the monolayer between the ana- and trans-quinone states, reversible light-induced activation and deactivation of the vectorial electron transfer in the system is accomplished. The functionalized electrode assemblies provide a means for the amplified amperometric transduction of recorded optical signals.