Electrode photoisomerizable redox

M. Lion-Dagan, S. Marx-Tibbon, E. Katz, and I. Willner, Photoswitchable electrical communication of glucose oxidase and glutathione reductase with electrode surfaces through photoisomerizable redox mediators, Angew. Chem. Int. Ed. Engl. 34, 1604-1606 (1995). 

The signal-triggered functions of these molecular assemblies have to be first characterized in bulk solution. Then, extensive efforts have been directed to integrate these photoswitchable chemical assemblies with transducers in order to tailor switchable molecular devices. The redox properties of photoisomerizable mono-layers assembled on an electrode surface are employed for controlling interfadal electron transfer [16]. Specifically, electrical transduction of photonic information recorded by photosensitive monolayers on electrode supports can be used in developing monolayer optoelectronic systems [16-19]. Electrodes with receptor sites exhibiting controlled binding of photoisomerizable redox-active substrates from the solution [20] also allow the construction of molecular optoelectronic devices. 

Light-Switchable Activation of Redox Proteins by Means of Photoisomerizable Command Interfaces Associated with Electrodes... 

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

Photoswitchable electrical communication between enzymes and electrodes has also been achieved by the application of photoisomerizable electron-transfer mediators [195, 199]. DilTusional electron mediators (viologen or ferrocene derivatives) were functionalized with photoisomerizable spiropyran/merocyanine units. These mediators can be reversibly photoisomerized from the spiropyran state to the merocyanine state (360 < A < 380 nm) and back (A > 475 nm). An enzyme multilayer array composed of glutathione reductase or glucose oxidase was electrically contacted only when the photoactive group linked to the redox relay (viologen or ferrocene derivative, respectively) was in the spiropyran state. 

The immobilization of a photoisomerizable material that can be switched by light between redox-active and redox-inactive or conductive and insulating states offers an encouraging route toward integrated molecular memory devices. Figure 7.2 shows a photoisomer state A in which the molecular unit is redox-inactive and no electronic signal is transduced. Photoisomerization of the chemical component to state B generates a redox-active assembly, and the electron transfer between the electrode and the chemical modifier yields an amperometric (electrochemical) indicator of the state of the system. 

FIG. 7.8 Electronk transduction of photoswitchable bioeiectrocatal)rtic functions of redax enzymes by the tethering of photoisomerizable units to the protein. (R is a dtffusional electron mediator that electrically contacts the redox site of the protein with the electrode support.)... 

Fe(CN)6] , electrochemically contacted at a photoisomerizable command interface (lla/llb). Figure 7.15 shows the impedance features (as Nyquist plots) of the nitrospiropyran (11a) and protonated nitromerocyanine (lib) electrodes in the presence of [Fe(CN)6] as a redox probe. The impedance spectra show a larger resistance to interfacial electron transfer when the monolayer is in the neutral dinitrospiropyran state (Ret = 60 kll) than when it is in the positively charged protonated merocyanine state (Ret = 48 kQ) (Figure 7.15, curves b and a). The heterogeneous rate constants for electron transfer between the electrode and the redox probe were calculated to be 0.82 X 10" and 1.1 x 10" cm s" for the 11a and 1 lb-monolayer modified Au-electrodes, respectively. 

FIG. 7.19 Electronic transduction of phocoswitchable bioelectrocacalytic functions of enzymesf proteins by the application of a photoisomerizable command Interfece that controis the electrical contact between die redox enzyme/protain and the electrode. 

The combination of a photoisomerizable spiropyran and Ab-DNP was also applied in the development of reversible amperometric immunosensors.49 The amperometric immunosensor was based on a 6,8-dinitrospiropyran-assembled monolayer on an Au electrode (Figure 6b). When Ab-DNP associated with the spiropyran monolayer as the electrode, the electrode surface was insulated toward the solubilized redox probe, and its amperometric response decreased. After irradiation (360nmantigen monolayer that resulted in a dissociation of Ab-DNP from the monolayer. The rinsed electrode revealed a high amperometric response, indicating that the Ab-DNP was washed off. Thus, the two-step illumination of the monolayer enables its reversible cyclic performance, which would demonstrate a general means for cyclic operation of an amperometric immunosensor. 

The interactions between the photoisomerizable monolayer and charged substrates may also be controlled by the pH of the system [91] At pHs below 8.6, the merocyanine form of the monolayer is protonated (i.e. cationic), so negatively charged redox probes are attracted to the electrode and are electrochemically enhanced. At pH values above 8.6, however, the merocyanine is deprotonated (i.e. zwitterionic), so the electrochemistry of the... 

In another example, a mixed monolayer composed of a photoisomerizable component and an electrochemical catalyst was applied to switch the electrocatalytic properties of a modified electrode between ON - and OFF -states. A gold electrode surface functionalized with a spiropyran-monolayer and pyrroloquinoline quinone (PQQ) moieties incorporated into the mono-layer was applied to control the electrocatalytic oxidation of NADH by light [92]. The positively charged merocyanine-state interface resulted in the repulsion of Ca2+ cations (promoters for the NADH oxidation by the PQQ), thus resulting in the inhibition of the electrocatalytic process. In the nitrospiropyran-state the monolayer does not prevent association of the PQQ-catalyst and Ca2+-promoter, so provides efficient electrocatalytic oxidation of NADH. Similar results have been achieved by a combination of the photo- and thermal effects resulting in the isomerization of the spiropyran-monolayer with the incorporated PQQ-catalyst [93], Other photoisomerizable materials such as an azobenzenealkanethiol derivative mixed with a ferrocene-redox component have also been used to control the electrocatalyzed electron transfer process between a command interface and a dissolved redox probe [94]. 

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