Photoswitchable biomaterials as grounds for optobioelectronic devices

Abstract

Optobioelectronic systems provide a means for the electronic transduction of recorded optical signals. The methodologies to assemble optobioelectronic devices are reviewed. One method involves the chemical modification of redox enzymes with photoisomerizable units and their integration with electrode surfaces. In one photoisomer state the protein structure is perturbed and its bioelectrocatalytic activities are blocked. In the second photoisomer state, the tertiary structure of the protein is retained and the enzyme exhibits bioelectrocatalytic functions. This method is exemplified by the chemical modification of glucose oxidase (GOx) with photoisomerizable nitrospiropyran units and with the reconstitution of apo-GOx with the photoisomerizable nitrospiropyran-FAD (flavin adenine dinucleotide phosphate) cofactor. The two systems enable the cyclic amplified amperometric transduction of optical signals recorded by the photoactive proteins. The second approach to assemble optobioelectronic systems involves the organization of photoisomerizable monolayers on electrode surfaces acting as ‘optical command surfaces’ for the control of the electrical contact between redox proteins (or redox enzymes) and the electrode. This method is exemplified by the control of the electrical contact of cytochrome c (Cyt. c) with a functionalized electrode consisting of a mixed monolayer of pyridine/nitrospiropyran assembled onto an Au-electrode. The ON-OFF photostimulated electrical contact of Cyt. c with the electrode is coupled to mediated electron transfer cascades, i.e. bioelectrocatalyzed reduction of oxygen by cytochrome oxidase (COx). The systems allow the amplified amperometric transduction of optical signals recorded by the photoisomerizable monolayer-electrode. The photochemically-triggered activation or deactivation of the electrical communication between Cyt. c and the electrode, originate from the association or repulsion of the protein from the photoisomerizable monolayer-electrode interface. This enables the microgravimetric transduction of the optical signals recorded by the monolayer using a quartz crystal microbalance (QCM). The photostimulated electrical contact of Cyt. c and the electrode, and the subsequent activation of an electron transfer cascade via the electrobiocatalyzed reduction of oxygen by COx, allow the amplified amperometric transduction of optical signals recorded by the monolayer-functionalized-electrode. Reversible immunosensors based on photoisomerizable antigen monolayers assembled onto electrodes represent another configuration of an optobioelectronic device. Assembly of an antigen monolayer on electrodes yields an active interface for the amperometric transduction of the association of the complementary antibody (Ab) to the monolayer. Binding of the Ab to the monolayer blocks the electrical contact of a redox enzyme, i.e. ferrocene-modified glucose oxidase (Fc-GOx), with the electrode, and inhibits the electrobiocatalyzed oxidation of glucose. A photoisomerizable antigen assembled as a monolayer on an electrode allows the tailoring of reversible immunosensing interfaces. In one photoisomer state, the monolayer acts as an active interface for amperometric detection of the antibody. The complementary photoisomer state lacks affinity for the Ab and allows the washing-off of the antibody and the regeneration of the active antigen monolayer by a second illumination cycle. This approach is exemplified by the application of a dinitrospiropyran monolayer assembled onto an Au-electrode as a reversible sensing interface for the dinitrophenyl antibody (DNP-Ab). The dinitrospiropyran monolayer, SP-state, acts as an active interface for the association of the DNP-Ab. Photoisomerization of the monolayer to the dinitromerocyanine configuration, MRH +-state, allows the washing-off of the DNP-Ab and regeneration of the active SP-monolayer electrode by a secondary photoinduced isomerization. The reversible photostimulated binding and dissociation of DNP-Ab to and from the electrode is transduced by the application of Fc-GOx as redox probe and is further supported by microgravimetric QCM analyses.