Photoisomerization nitromerocyanine

The photoisomerizable enzyme monolayer electrode also revealed photoswitchable bioelectrocatalytic activity (Figure 7.10). In the presence of ferrocene carboxylic acid (5) as a diffusional electron transfer mediator, the nitrospiropyran-tethered GOx (4a) revealed a high bioelectrocatalytic activity, reflected by a high electrocatalytic anodic current. The protonated nitromerocyanine-GOx (4b) exhibited a two-fold lower activity, as reflected by the decreased bioelectrocatalytic current. By the reversible photoisomerization of the enzyme electrode between the 4a- and 4b-states, the current responses are cycled between high and low values (Figure 7.10, inset). 

Thus, the photoisomerization of the monolayer between the 46a-state and the protonated nitromerocyanine 46b-state provides a means to control the electrical features of the electrode surface, thereby regulating electron transfer at the electrode interface. The 46a-monolayer results in a neutral electrode surface while the 46b-monolayer gives a positively charged surface, causing the formation of an electrical double-layer at the electrode interface. Photoisomerization of the command interface resulting from the different electrochemical kinetics of the soluble redox probe can also be probed by Faradic impedance spectroscopy [90]. A small electron transfer resistance is found for the system when there is an attractive interaction between the charged redox probe and the command interface. Much larger electron transfer resistances are found upon photoisomerization to the state when repulsive interactions exist. 

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