Reversible Associative and Dissociative Interactions of Glucose Oxidase with Nitrospiropyran Monolayers Assembled onto Gold Electrodes:  Amperometric Transduction of Recorded Optical Signals

Photoisomerizable nitrospiropyran monolayers assembled onto Au surfaces provide active electrodes for controlling electrical communication of glucose oxidase, GO, and ferrocene-modified glucose oxidase, Fc-GO, with the electrode interface. A thiolated nitrospiropyran monolayer was assembled onto Au electrodes by treatment with 1-(4-mercaptobutyl)-3,3-dimethyl-6'-nitrospiro[indolin-2,2'-[1-2H]benzopyran] (3). The monolayer undergoes reversible photoisomerization to nitrospiropyran (SP (3)) monolayer state and nitromerocyanine (MRH+ (4)) monolayer. With ferrocenecarboxylic acid as a diffusional electron mediator, the electrobiocatalyzed oxidation of glucose by GO is inhibited in the presence of the MRH+-monolayer electrode as compared to the system that includes the SP-monolayer electrode. The inhibition phenomenon originates from electrostatic binding of GO to the MRH+-monolayer electrode that perturbs the interfacial redox process with the diffusional electron mediator, ferrocenecarboxylic acid. The interfacial electron-transfer rate between ferrocenecarboxylic acid and the electrode is 10-fold slower in the presence of the MRH+-monolayer electrode and GO as compared to the SP-monolayer electrode and GO. Quartz crystal microbalance analyses and determination of the respective capacity currents reveal electrostatic attraction of GO to the MRH+-monolayer electrode. With ferrocene-modified glucose oxidase, Fc-GO, the electrobiocatalyzed oxidation of glucose is enhanced in the presence of the MRH+-monolayer electrode as compared to the SP-monolayer electrode. The modified enzyme, Fc-GO, exhibits direct electrical communication with the electrode surface. Electrostatic attraction of Fc-GO to the MRH+-monolayer electrode increases the biocatalyst concentration at the electrode surface and facilitates electrochemical oxidation of glucose. Similar results are observed upon organization of the nitrospiropyran monolayer on Au electrodes by a stepwise method that includes covalent linkage of 1-(β-carboxyethyl)-3,3-dimethyl-6'-nitrospiro[indolin-2,2'-[1-2H]benzopyran] (1) to a cystamine monolayer-modified Au electrode. The reversible photoisomerizable properties of the nitrospiropyran monolayer electrodes allow the cyclic modulation of the electrocatalytic anodic currents in the systems that include GO and Fc-GO as biocatalysts. The assemblies represent systems for the amplification and amperometric transduction of optical signals recorded by monolayer electrodes. They can also serve as a model for mimicking basic functions of the vision process.