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 trans-quinone monolayer electrode is pH-dependent, consistent with a two-electron and two-proton redox process. Photoisomerization of the trans-quinone monolayer (305 nm < λ < 320 nm) generates the ana-quinone monolayer that lacks electrochemical activity. Upon photoisomerization of the ana-quinone monolayer to the trans-quinone state (λ > 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 redoxactive 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 transquinone 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.