Abstract
Direct electrochemical electron transfer (DET) of oxidoreductases has attracted increasing attention in pure and applied bioelectrochemistry (e.g. biosensors and biofuel cells) over the last decades. We report here a systematic study of DET-type bioelectrocatalysis of the membrane-bound redox enzyme fructose dehydrogenase (FDH, Gluconobacter sp.), on variable-length and variably terminated thiol self-assembled monolayers (SAMs) both on Au(111) and nanoporous gold (NPG) electrodes. FDH on Au(111) modified by short-chain moderately hydrophilic 2-mercaptoethanol (BME) SAMs exhibits the highest DET activities and largest DET-capable fraction. Fitting of theoretical polarization curves to the data and homology modeling/docking of FDH offer further mechanistic insight. The dependence of the DET efficiency of FDH on the length and differently terminated carbon chain is systematically presented. The decreased DET rate with increasing chain length is associated with increasingly unfavourable long-range electron tunnelling, and not with lowered enzyme loading. The porous structure of NPG is favorable for FDH bioelectrocatalysis by improving both efficient enzyme orientation and operational stability. Overall, our study maps systematically the controlled local environmental structural flexibility of the Au/SAM/enzyme/solution interface, a paradigm for thiol modified surfaces in biosensors and bioelectronics.
Highlights
Electron transfer (ET) between an electrode surface and the redox/catalytic center(s) of oxidoreductases is one of the most important processes in enzyme bioelectronics including biosensors and enzymatic biofuel cells (EBFCs) [1,2]
Au(111) electrodes with atomically planar surfaces are an ideal platform to investigate the effects of self-assembled monolayers (SAMs) with variable terminal groups on the direct ET (DET) behaviour of FDH, i.e. -NH2 (MEA), -OH (BME), -COOH (MPA) and -CH3 (PPT) (Fig. S1)
Our primary overarching objectives in the present study were to map the bioelectrocatalysis of FDH as a biochemically and biotechnologically broadly important enzyme, in new molecular mechanistic detail
Summary
Electron transfer (ET) between an electrode surface and the redox/catalytic center(s) of oxidoreductases is one of the most important processes in enzyme bioelectronics including biosensors and enzymatic biofuel cells (EBFCs) [1,2]. A similar study by Murata and associates reported that FDH anchored on gold nanoparticles (AuNPs) functionalized with 2-mercaptoethanol (BME) SAMs exhibits the highest current density in DET-type bioelectrocatalysis, exceeding that of both 3-mercaptopropionic acid (MPA, negative) and 2aminoethanethiol (AET, positive) [25]. The porous NPG structure gives notably larger amounts of active enzyme capable both of DET and of MET-type catalytic current densities, as well as significantly higher operational stability compared with Au(111)-electrodes. Such a comprehensive study, involving most of the interfacial structure-functional properties of a complex bioelectrochemical interface, has not been reported before. All chemicals were used as received and all solutions prepared with Millipore water (18.2 MΩ cm)
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