Photoswitchable antigen-antibody interactions studied by impedance spectroscopy

A thiolated dinitrospiropyran (1a) is assembled as a monolayer onto a Au electrode. The monolayer exhibits reversible photoisomerizable properties, and irradiation of the 1a monolayer state, 360 nm < lambda < 380 nm, yields the protonated merocyanine (1b) monolayer state, whereas illumination of the latter monolayer electrode, lambda > 495 nm, regenerates the 1a monolayer electrode. The la monolayer interface acts as antigen for the dinitrophenyl antibody, DNP-Ab, while the 1b state lacks an affinity for DNP-Ab. This stimulates the association of the antibody to the la monolayer interface, while photoisomerization of the monolayer to the 1b state allows one to wash off DNP-Ab and regenerate the la monolayer assembly by a secondary illumination process. The photoisomerization of the monolayer interface and the binding or dissociation of DNP-Ab to and from the photoisomerizable monolayer are studied by impedance spectroscopy, in the presence or absence of a solubilized redox probe. The derived equivalent circuits enable us to characterize the monolayer parameters associated with the photoisomerization and the interactions of DNP-Ab with the monolayer. The non-Faradaic impedance measurements reveal only capacitance changes of the double-layer interface as a result of photoisomerization and the association of DNP-Ab to the monolayer. Faradaic impedance measurements using [Fe(CN)(6)](3-/4-) as a molecular redox probe or ferrocene-tethered glucose oxidase, GOx-Fc, and glucose as a bioelectrocatalytic redox probe, allow for the determination of the electron-transfer resistance, the electron-transfer rate constants, and the characteristic time constants of the equivalent circuits corresponding to the different states of the photoisomerizable monolayer in the absence and presence of DNP-Ab. Faradaic impedance appears to be an effective transduction signal for the photoisomerization occurring upon optical activation of the monolayer and for the photostimulated binding and dissociation of DNP-Ab to and from the monolayer, respectively. The system represents an optoelectronic "OR" gate.