A photoisomerizable nitrospiropyran monolayer assembled on a Au electrode provides a functionalized interface for the photochemical, pH, and thermal control of electrochemical processes of charged electroactive redox probes. (Mercaptobutyl)nitrospiropyran 1 was assembled as a monolayer on a Au electrode. The monolayer exhibits reversible photoisomerizable features, and illumination of the nitrospiropyran monolayer, SP state, 320 nm < λ < 350 nm, yields at pH = 7.0 the protonated nitromerocyanine monolayer state, MRH+ state. Further irradiation of the MRH+ monolayer, λ > 495 nm, regenerates the SP state of the monolayer. The light-induced transformation of the monolayer between a neutral and a positively-charged interface allows the control of the electron transfer processes at the electrode interface. Electrooxidation of the negatively-charged (3,4-dihydroxyphenyl)acetic acid, DHPAA, is enhanced at the MRH+ monolayer electrode as compared to the SP-functionalized monolayer electrode. Electrooxidation of the positively-charged 3-hydroxytyramine (dopamine), DOPA, is inhibited at the MRH+ monolayer electrode as compared to its oxidation by the SP monolayer electrode. The control of the electrochemical oxidation of DHPAA and DOPA at the photoisomerizable monolayer electrode is attributed to the electrostatic interactions of the MRH+ monolayer electrode with the redox-active substrates. Electrostatic attraction of DHPAA and repulsion of DOPA by the MRH+ monolayer results in enhancement or inhibition of the electrochemical processes, respectively. By reversible isomerization of the monolayer between the SP and MRH+ states, cyclic amperometric transduction of the optical signals recorded by the monolayer is accomplished. In the presence of a mixture of oppositely-charged redox substrates, i.e. DHPAA and 2,5-bis[[2-(dimethylbutylammonio)ethyl]amino]-1,4-benzoquinone (3) or pyrroloquinoline quinone, PQQ, (4) and 3, photostimulated selective electrochemistry is accomplished in the presence of the photoisomerizable monolayer electrode. The transformation of the protonated nitromerocyanine monolayer, MRH+ state, generated at pH = 7.0, to the zwitterionic nitromerocyanine configuration, MR± state at higher pH, allows the pH-controlled electrooxidation of DHPAA and DOPA at the monolayer electrode. Similarly, thermal isomerization of the SP monolayer electrode, pH = 7.0, 60 °C, yields the MRH+ monolayer electrode. These thermochromic features of the monolayer are employed to respectively activate or deactivate the electrooxidation of DHPAA or DOPA at the functionalized electrode. By cyclic thermal isomerization of the SP monolayer to the MRH+ monolayer followed by photochemical isomerization of the MRH+ monolayer followed by photochemical isomerization of the MRH+ monolayer to the SP state, λ > 495 nm, the thermochromic and photochromic features of the monolayer are amperometrically transduced via the oxidation of DHPAA and DOPA, respectively. Electrochemical oxidation of DHPAA and DOPA is further accomplished by the applicatio...
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.
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 transquinone 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 (BV 2+ ) 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 BV 2+ . The vectorial electron transfer from the trans-quinone monolayer to BV 2+ is gated by the pH of the medium. The trans-quinone monolayer electrode was coupled to the redox mediator BV 2+ and the enzyme nitrate reductase. In the presence of NO 3 -, 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 BV 2+ 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.
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