Light-controlled electron transfer reactions at photoisomerizable monolayer electrodes by means of electrostatic interactions: active interfaces for the amperometric transduction of recorded optical signals

Abstract

Photoisomerizable nitrospiropyran (SP)/nitromerocyanine (MR) monolayer assembled on Au-electrodes provides active interfaces for controlling, by light electron transfer, reactions at the electrode surface. The functionalized electrodes act as `photo-command' interfaces for the amperometric transduction and amplification of recorded optical signals. The nitrospiropyran monolayer, SP state, undergoes light-induced isomerization to the protonated nitromerocyanine monolayer, MRH + state. The positively charged MRH +-monolayer interface, by means of electrostatic interactions, allows control of electrochemical transformation at the electrode interface. Electrooxidation of 3-hydroxytyramine (dopamine), ( 3), is retarded at the MRH +-monolayer electrode as compared to its electrooxidation by the SP-monolayer electrode. In contrast, electrochemical oxidation of 3,4-dihydroxyphenylacetic acid, DHPAA, ( 4), is enhanced at the MRH +-monolayer electrode as compared to the SP-electrode state. By cyclic photoisomerization of the monolayer between the MRH + and SP states, the amperometric responses of the electrode are tuned to high and low values in the presence of the two substrates. Another photo-command surface includes a mixed monolayer of pyrroloquinoline quinone, PQQ, and nitrospiropyran units. In the PQQ-SP-monolayer configuration, effective electrocatalyzed oxidation of NAD(P)H proceeds in the presence of Ca 2+ ions. Photoisomerization of the monolayer to the PQQ-MRH + state blocks the electrocatalytic oxidation of NADPH. The system is used for the cyclic amplified amperometric transduction of optical signals recorded by the monolayer. The SP/MRH +-monolayer electrode is also employed to control bioelectrocatalyzed transformations. Electrostatic attraction of ferrocene-modified glucose oxidase, Fc-GOx, by the MRH +-monolayer electrode, facilitates the electrocatalyzed oxidation of glucose, whereas in the presence of the SP-monolayer electrode the bioelectrocatalytic process is inhibited. The enzyme Fc-GOx, enables the cyclic, amplified amperometric transduction of optical signals recorded by the photoactive monolayer. A mixed monolayer consisting of nitrospiropyran and pyridine units assembled on a Au-electrode provides a functionalized interface that controls the binding of cytochrome c (Cyt. c) to the monolayer and the resulting electrical contact of Cyt. c with the electrode. With the pyridine-SP monolayer configuration, Cyt. c associates to the pyridine sites and reveals effective electrical communication with the electrode surface. In the pyridine-MRH +-monolayer state, Cyt. c is electrostatically repelled from the pyridine sites and its electrical contact with the electrode is blocked. The photostimulated association and dissociation of Cyt. c to and from the photoisomerizable monolayer is microgravimetrically analyzed by a quartz-crystal microbalance.