Photochemically-, chemically-, and pH-controlled electrochemistry at functionalized spiropyran monolayer electrodes

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 < lambda < 350 nm, yields at pH = 7.0 the protonated nitromerocyanine monolayer state, MRH(+) state. Further irradiation of the MRH(+) monolayer, lambda > 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 degrees 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, lambda > 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 application of a dinitrospiropyran monolayer (2) electrode in the presence of the dinitrophenyl antibody, DNP-Ab. (Mercaptobutyl)dinitrospiropyran 2 was assembled as a monolayer on a Au electrode. The dinitrospiropyran monolayer, SP state, exhibits antigen features for the DNP-Ab, where the protonated dinitromerocyanine monolayer, MRH(+) state, lacks antigen features for the DNP-Ab. Association of the DNP-Ab to the SP monolayer electrode blocks the electrooxidation of DHPAA or DOPA. Photochemical isomerization of the SP monolayer to the MRH(+) state, 320 nm < lambda < 350 nm, results in the release of DNP-Ab and the activation of the electrooxidation of DHPAA and DOPA. By the reversible photoisomerization of the monolayer between the SP and MRH(+) states in the presence of DNP-Ab, cyclic amperometric transduction of the optical signals recorded by the monolayer is accomplished.