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Antigen monolayers assembled onto Au-electrodes or Au-electrodes associated with quartz crystals act as active interfaces for the amperometric or microgravimetric analysis of the complementary antibody and provide the grounds for the development of electrochemical and piezoelectric immunosensors. The antigen monolayer of Nε-2,4-dinitrophenyl-l-lysine is assembled on an electrode. The anti-dinitrophenyl antibody, anti-DNP-Ab, is sensed by the antigen monolayer, and the formation of the antigen−antibody complex at the monolayer interface is probed by the insulation of the electrode toward a redox probe in the electrolyte solution. The formation of the antibody−antigen complex is amplified by the application of the anti-antibody or the use of a ferrocene-functionalized redox-enzyme, glucose oxidase, as redox probe. A 3,5-dinitrosalicylic acid antigen monolayer bound to Au-electrodes associated with a quartz crystal is used as active interface for the microgravimetric, quartz-crystal-microbalance analysis of the anti-DNP-Ab. Photoisomerizable antigen monolayer electrodes provide the basis for tailoring reversible immunosensors. The dinitrospiropyran monolayer, SP-state, is assembled on Au-electrodes or quartz crystals. The monolayer exhibits reversible photoisomerizable features, and irradiation of the SP-monolayer, 360 < λ < 380 nm, yields the protonated merocyanine monolayer, MRH+-state. Further irradiation of the MRH+-monolayer electrode, λ > 495 nm, restores the SP-monolayer electrode. The SP-monolayer acts as antigen for anti-DNP-Ab, whereas the MRH+-monolayer lacks antigen properties for anti-DNP-Ab. This enables the amperometric or piezoelectric transduction of the formation of the antigen−anti-DNP-Ab complex at the SP-monolayer interface. By photoisomerization of the monolayer to the MRH+-state, the Ab is washed-off from the sensing interface. Subsequent light-induced isomerization of the monolayer to the SP-state regenerates the electrochemically or piezoelectrically active sensing interfaces.

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...

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