Abstract
Self-assembled molecular monolayers (SAMs) have long been recognized as crucial “bridges” between redox enzymes and solid electrode surfaces, on which the enzymes undergo direct electron transfer (DET)—for example, in enzymatic biofuel cells (EBFCs) and biosensors. SAMs possess a wide range of terminal groups that enable productive enzyme adsorption and fine-tuning in favorable orientations on the electrode. The tunneling distance and SAM chain length, and the contacting terminal SAM groups, are the most significant controlling factors in DET-type bioelectrocatalysis. In particular, SAM-modified nanostructured electrode materials have recently been extensively explored to improve the catalytic activity and stability of redox proteins immobilized on electrochemical surfaces. In this report, we present an overview of recent investigations of electrochemical enzyme DET processes on SAMs with a focus on single-crystal and nanoporous gold electrodes. Specifically, we consider the preparation and characterization methods of SAMs, as well as SAM applications in promoting interfacial electrochemical electron transfer of redox proteins and enzymes. The strategic selection of SAMs to accord with the properties of the core redox protein/enzymes is also highlighted.
Highlights
Self-assembled molecular monolayers (SAMs) are surface monolayers that spontaneously bind to metal surfaces on which, for example, the unique metal-S bonding between metal and thiols offers a versatile pathway to tailor interfacial properties for electrochemical and bioelectrochemical applications [1,2,3,4]
The cysteine mutant with a surface thiol group made it possible to form stable thioether bonds with maleimide groups via click-chemistry, thereby controlling the orientations of the MtCDH mutant on the electrode surface and revealing efficient electrochemical glucose oxidation. Another example reported by Meneghello and associates clearly indicated that the direct electron transfer (DET)-type bioelectrocatalysis of cellobiose dehydrogenase (CDH) is highly sensitive to the cysteine residues introduced at particular positions [115]
self-assembled molecular monolayers (SAMs)-modified electrodes with no significant difference in enzyme loading as disclosed by surface immobilization, which suggests that the secondary structure of the MvBOD is retained
Summary
Self-assembled molecular monolayers (SAMs) are surface monolayers that spontaneously bind to metal surfaces on which, for example, the unique metal-S bonding between metal and thiols offers a versatile pathway to tailor interfacial properties for electrochemical and bioelectrochemical applications [1,2,3,4]. To achieve efficient DET, it is important to consider the detailed surface characteristics of both enzyme and electrode for favorable enzyme orientation, leading to minimized electron tunneling distance. SAMs have been introduced into bioelectrocatalysis to serve as a bridge for gentle protein/enzyme immobilization on gold or other metal surfaces [36,38,39]. Metallic nanomaterials exhibit excellent electronic conductivity and large surface area, with promising potential in improving the catalytic response and stability of redox enzymes [25]. We review recent studies of SAMs in the DET-type bioelectrocatalysis of both atomically planar and nanostructured gold electrode surfaces. The use of structurally versatile SAMs and suitable electrode nanostructure supports achieving well-defined orientation for DET is highlighted next.
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