Direct electron transfer (DET) reactions between redox enzymes and electrodes can be maximized by oriented immobilization of the enzyme molecules onto an electroactive surface modified with functionalized gold nanoparticles (AuNPs). Here, we present such strategy for obtaining a DET-based laccase (Lc) cathode for O(2) electroreduction at low overpotentials. The stable nanostructured enzymatic electrode is based on the step-by-step covalent attachment of AuNPs and Lc molecules to porous graphite electrodes using the diazonium salt reduction strategy. Oriented immobilization of the enzyme molecules on adequately functionalized AuNPs allows establishing very fast DET with the electrode via their Cu T1 site. The measured electrocatalytic waves of O(2) reduction can be deconvoluted into two contributions. The one at lower overpotentials corresponds to immobilized Lc molecules that are efficiently wired by the AuNPs with a heterogeneous electron transfer rate constant k(0) ≫ 400 s(-1).

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A new strategy for oriented covalent immobilization of Trametes hirsuta laccase on gold electrodes is presented. The strategy is based on the gold surface modification with a mixed monolayer of an aromatic diazonium salt derivative and 6-mercapto-1-hexanol for further use as scaffold for the enzyme's covalent linkage. This strategy offers a variety of advantages such as high stability and laccase-friendly support morphology, which turns it into a suitable metal-enzyme interface. Conditions aiming at optimum orientation for direct electron transfer (DET) via the T1 copper site were studied.Current density values up to 40 µA·cm -2 were measured for the electrocatalytic reduction of O 2 in absence of redox mediators. This strategy is a big step forward in the development of laccase-modified gold electrodes for bioelectrocatalytic reduction of O 2 .

Electrochemical or faradaic impedance spectroscopy (EIS) using the ferri/ferrocyanide couple as a redox probe at gold working electrodes was evaluated with respect to its ability to monitor consecutive surface modification steps. As a model reaction, the reversible hybridization and dehybridization of DNA was studied. Thiol-modified single stranded DNA (ssDNA, 20 bases, capture probe) was chemisorbed to a gold electrode and treated with a solution of short thiols to release nonspecifically adsorbed DNA before hybridization with complementary ssDNA (20 bases, target) was carried out. Reversible dehybridization was achieved by intense rinsing with pure water. The experimental procedures were optimized by kinetic surface plasmon resonance (SPR) and quartz crystal microbalance with dissipation (QCM-D) measurements to maximize the increase in reflectivity or decrease in frequency upon hybridization before hybridization/dehybridization was also monitored by EIS. In contrast to SPR and QCM-D, repeatable EIS measurements were not possible at first. Combined SPR/EIS and QCM-D/EIS measurements revealed that during EIS the gold surface is seriously damaged due to the presence of CN(-) ions, which are released from the ferri/ferrocyanide redox probe. Even at optimized experimental conditions, etching the gold electrodes could not be completely suppressed and the repeatability of the EIS measurements was limited. In three out of four experimental runs, only two hybridization/dehybridization steps could be monitored reversibly by EIS. Thereafter etching the gold electrode significantly contributed to the EIS spectra whereas the QCM-D response was still repeatable. Hence great care has to be taken when this technique is used to monitor surface modification at gold electrodes.

Immobilization of hydrogenases onto electrodes is of great interest for developing biofuel cells that use H 2 as a fuel. In this way, hydrogenases replace Pt as electrocatalyst for oxidizing H 2 in the anode. We have developed a method of covalent bonding of Desulfovibrio gigas Ni-Fe hydrogenase and Desulfovibrio vulgaris Hildenborough Ni-Fe-Se hydrogenase to gold electrodes modified with a self assembled monolayer (SAM) of 4-aminothiophenol for measuring high electrocatalytic currents of H 2 -oxidation in the absence of redox mediators. Electrochemical measurements and atomic force microscopy characterization show that direct electron transfer between enzyme and the Au support is due to formation of an organized monolayer of hydrogenase over the SAM-modified surface.

The ACS journal of surfaces and colloids 27: 6449-6457 (2011) 2 ABSTRACTThe interaction of redox enzymes with electrodes is of great interest for studying the catalytic mechanisms of redox enzymes and for bioelectronic applications. Efficient electron transport between the biocatalysts and the electrodes has achieved more success with soluble than with membrane enzymes due to the higher structural complexity and instability of the latter proteins. In this work we report a strategy for immobilizing a membrane-bound enzyme onto gold electrodes with a controlled orientation in its fully active conformation. The immobilized redox enzyme is the Ni-Fe-Se hydrogenase from Desulfovibrio vulgaris Hildenborough, which catalyzes H 2 -oxidation reversibly and is associated to the cytoplasmic membrane by a lipidic tail. Gold surfaces modified with this enzyme and phospholipids have been studied by atomic force microscopy (AFM) and electrochemical methods. The combined study indicates that by a two-step immobilization procedure the hydrogenase can be inserted via its lipidic tail onto a phospholipidic bilayer formed over the gold surface, only allowing mediated electron transfer between enzyme and electrode. On the other hand, a one-step immobilization procedure favours formation of a hydrogenase monolayer over the gold surface with its lipidic tail inserted in a phospholipid bilayer formed on top of the hydrogenase molecules. This latter method has allowed for the first time efficient electron transfer between a membrane-bound enzyme in its native conformation and an electrode.

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For the first time a complete characterization by infrared spectroscopy of a Ni-Fe-Se hydrogenase in its different redox states is reported. The Ni-Fe-Se hydrogenase was isolated from Desulfovibrio vulgaris Hildenborough. Two different electron paramagnetic resonance silent and air-stable redox states that are not in equilibrium were detected. Upon reduction of these states the catalytically active states Ni-R and Ni-C appear immediately. These states are in redox equilibrium and their formal redox potential has been measured. Putative structural differences between the redox states of the active site of the Ni-Fe-Se hydrogenase are discussed.