Electronic transduction of photostimulated binding interactions at photoisomerizable monolayer electrodes: Novel approaches for optobioelectronic systems and reversible immunosensor devices

Photoisomerizable monolayers assembled onto electrode supports act as 'command interfaces' for controlling the binding interactions of biomaterials with the functionalized surfaces. The light-induced binding and dissociation of the biomaterials to and from the electrodes, respectively, are electronically transduced. Two systems, including the photostimulated binding and dissociation of cytochrome c (Cyt c) and of anti-DNP antibody to and from functionalized surfaces, are discussed. The application of the systems as optobioelectronic devices and reversible immunosensors is addressed. A mixed monolayer consisting of pyridine and nitrospiropyran (1a) photoisomerizable units assembled on a Au-electrode acts as a command interface for the light-controlled association and dissociation of Cyt c to and from the monolayer. Cyt c binds to the pyridine/1a-monolayer electrode, resulting in electrical contact between the redox protein and the electrode. Photoisomerization of the mixed monolayer to the pyridine/protonated merocyanine state (1b) results in the electrostatic repulsion of Cyt c and its dissociation from the electrode support. This blocks the electrical contact between Cyt c and the electrode. By the cyclic photoisomerization of the mixed monolayer between the la and 1b states, reversible 'ON' 'OFF' amperometric transduction of the affinity interactions between the redox protein and the interface is accomplished. Coupling of the photostimulated electrical contact between Cyt c and the electrode surface to the Cyt c-mediated bioelectrocatalyzed reduction of O₂ by cytochrome oxidase provides a means to amplify the transduced electronic signal. A photoisomerizable thiolated dinitrospiropyran (2a) monolayer, assembled on solid supports, acts as a light-active antigen interface that enables the photocontrolled binding and dissociation of anti-dinitrophenyl antibody (DNP-Ab) to and from the interface. The dinitrospiropyran (2a) layer acts as an antigen for the DNP-Ab, whereas the protonated dinitromerocyanine (2b) lacks antigen features for the DNP-Ab. By reversible photoisomerization of the monolayer between the 2a and 2b states, cyclic binding and dissociation of DNP-Ab to and from the monolayer interface is accomplished. The association and dissociation of the DNP-Ab to and from the 2a- and 2b-monolayer states are electronically transduced, using amperometric, Faradaic impedance and microgravimetric, quartz crystal microbalance analyses. The photostimulated binding of an antibody to a photoisomerizable antigen monolayer provides a novel method to design reversible immunosensor devices.