Electron Transfer in Binary Hemin-Modified Alkanethiol Self-Assembled Monolayers on Gold: Hemin's Lateral and Interfacial Interactions.

Orientated coupling of redox enzymes to electrodes by their reconstitution onto redox cofactors, such as hemin conjugated to self-assembled monolayers (SAMs) formed on the electrodes, poses the requirements for a SAM design enabling reconstitution. We show that the kinetics of electron transfer (ET) in binary SAMs of alkanethiols on gold composed of in situ hemin-conjugated 11-amino-1-undecanethiol (AUT) and diluting OH-terminated alkanethiols with 11, 6, and 2 methylene groups (MC11OH, MC6OH, and MC2OH) depends on both the SAM composition and surface density of hemin, Γheme. In AUT/MC11OH SAMs composed of equal linker/diluent lengths, the heterogeneous ET rate constant ks decreased with the Γheme and varied between 70 and 500 s-1. For shorter diluents, the ks of 245-330 s-1 (C6) and 300-340 s-1 (C2) showed a little (if any) Γheme dependence. In AUT/MC11OH SAMs, the increasing Γheme resulted in the steric crowding of hemin species and their neighboring lateral interactions in the plane of hemin localization, affecting the potential distribution at the SAM/electrode interface and inducing local electrostatic effects interfering with hemin oxidation. In AUT/MC6OH and AUT/MC2OH SAMs, hemin discharged at the plane of the closest approach to the gold surface, equal to the diluent length and permeable to electrolyte ions, which lessened those effects. All studied binary SAMs provided steric hindrance for protein reconstitution on the hemin cofactor conjugated to the extended AUT linker. Further use of SAM-modified electrodes with the covalently attached hemin as interfaces for heme proteins' reconstitution should consider SAMs with loosely dispersed redox centers terminating more rigid molecular wires. Such wires place hemin at fixed distances from the electrode surface and thus ensure the interfacial properties required for the effective on-surface reconstitution of proteins and enzymes.

Single-molecule DNA sequencing of widely varying GC-content using nucleotide release, capture and detection in microdroplets.

Despite remarkable progress in DNA sequencing technologies there remains a trade-off between short-read platforms, having limited ability to sequence homopolymers, repeated motifs or long-range structural variation, and long-read platforms, which tend to have lower accuracy and/or throughput. Moreover, current methods do not allow direct readout of epigenetic modifications from a single read. With the aim of addressing these limitations, we have developed an optical electrowetting sequencing platform that uses step-wise nucleotide triphosphate (dNTP) release, capture and detection in microdroplets from single DNA molecules. Each microdroplet serves as a reaction vessel that identifies an individual dNTP based on a robust fluorescence signal, with the detection chemistry extended to enable detection of 5-methylcytosine. Our platform uses small reagent volumes and inexpensive equipment, paving the way to cost-effective single-molecule DNA sequencing, capable of handling widely varying GC-bias, and demonstrating direct detection of epigenetic modifications.

Single-molecule detection of deoxyribonucleoside triphosphates in microdroplets.

A new approach to single-molecule DNA sequencing in which dNTPs, released by pyrophosphorolysis from the strand to be sequenced, are captured in microdroplets and read directly could have substantial advantages over current sequence-by-synthesis methods; however, there is no existing method sensitive enough to detect a single nucleotide in a microdroplet. We have developed a method for dNTP detection based on an enzymatic two-stage reaction which produces a robust fluorescent signal that is easy to detect and process. By taking advantage of the inherent specificity of DNA polymerases and ligases, coupled with volume restriction in microdroplets, this method allows us to simultaneously detect the presence of and distinguish between, the four natural dNTPs at the single-molecule level, with negligible cross-talk.

Design of maleimide-functionalised electrodes for covalent attachment of proteins through free surface cysteine groups.

Mixed two-component monolayers on glassy carbon are prepared by electrochemical oxidation of N-(2-aminoethyl)acetamide and mono-N-Boc-hexamethylenediamine in mixed solution. Subsequent N-deprotection, amide coupling and solid-phase synthetic steps lead to electrode-surface functionalisation with maleimide, with controlled partial coverage of this cysteine-binding group at appropriate dilution for covalent immobilisation of a model redox-active protein, cytochrome c, with high coverage (≈7.5 pmol cm(-2) ).

Scanning electrochemical microscopy: using the potentiometric mode of SECM to study the mixed potential arising from two independent redox processes.

This study demonstrates how the potentiometric mode of the scanning electrochemical microscope (SECM) can be used to sensitively probe and alter the mixed potential due to two independent redox processes provided that the transport of one of the species involved is controlled by diffusion. This is illustrated with the discharge of hydrogen from nanostructured Pd hydride films deposited on the SECM tip. In deareated buffered solutions the open circuit potential of the PdH in equilibrium between its ß and α phases (OCP(ß→α)) does not depend on the tip-substrate distance while in aerated conditions it is found to be controlled by hindered diffusion of oxygen. Chronopotentiometric and amperometric measurements at several tip-substrate distances reveal how the flux of oxygen toward the Pd hydride film determines its potential. Linear sweep voltammetry shows that the polarization resistance increases when the tip approaches an inert substrate. The SECM methodology also demonstrates how dissolved oxygen affects the rate of hydrogen extraction from the Pd lattice. Over a wide potential window, the highly reactive nanostructure promotes the reduction of oxygen which rapidly discharges hydrogen from the PdH. The flux of oxygen toward the tip can be adjusted via hindered diffusion. Approaching the substrate decreases the flux of oxygen, lengthens the hydrogen discharge, and shifts OCP(ß→α) negatively. The results are consistent with a mixed potential due to the rate of oxygen reduction balancing that of the hydride oxidation. The methodology is generic and applicable to other mixed potential processes in corrosion or catalysis.

A His-tagged Melanocarpus albomyces laccase and its electrochemistry upon immobilisation on NTA-modified electrodes and in conducting polymer films.

The article describes the construction, immobilisation and electrochemistry of histidine tagged laccase from Melanocarpus albomyces. A facile method of functionalisation of glassy carbon electrodes with nitrilotriacetic acid (NTA) using diazonium grafting and solid state chemistry is described. NTA-modified electrodes are shown to bind laccase which reduces oxygen at neutral pH in the presence of soluble redox mediator. Laccase-modified electrodes are also prepared by enzyme immobilisation within poly(aniline)/poly(vinylsulfonate) films. The polymer is found to efficiently retain the enzyme as well as provide direct electrical contact between the electrode and the enzyme active centre. Cyclic voltammetry reveals the direct electron transfer to the enzyme is dependent on the redox state of the polymer film.

Direct electrochemistry and Os-polymer-mediated bioelectrocatalysis of NADH oxidation by Escherichia coli flavohemoglobin at graphite electrodes.

Escherichia coli flavohemoglobin (HMP), which contains one heme and one FAD as prosthetic groups and is capable of reducing O2 by its heme at the expense of NADH oxidized at its FAD site, was electrochemically studied at graphite (Gr) electrodes. Two signals were observed in voltammograms of HMP adsorbed on Gr, at -477 and -171 mV vs. Ag|AgCl, at pH 7.4, correlating with electrochemical responses from the FAD and heme domains, respectively. The electron transfer rate constant for ET reaction between FAD of HMP and the electrode was estimated to be 83 s⁻¹. Direct bioelectrocatalytic oxidation of NADH by HMP was not observed, presumably due to impeded substrate access to HMP orientated on Gr through the FAD-domain and/or partial denaturation of HMP. Bioelectrocatalysis was achieved when HMP was wired to Gr by the Os redox polymers, with the onset of NADH oxidation at the formal potential of the particular Os complex (+140 mV or -195 mV). Apparent Michaelis constants K(M)(app) and j(max) were determined, showing bioelectrocatalytic efficiency of NADH oxidation by HMP exceeding the one earlier shown with diaphorase, which makes HMP very attractive as a component of bioanalytical and bioenergetic devices.

High-throughput synthesis and electrochemical screening of a library of modified electrodes for NADH oxidation.

We report the combinatorial preparation and high-throughput screening of a library of modified electrodes designed to catalyze the oxidation of NADH. Sixty glassy carbon electrodes were covalently modified with ruthenium(II) or zinc(II) complexes bearing the redox active 1,10-phenanthroline-5,6-dione (phendione) ligand by electrochemical functionalization using one of four different linkers, followed by attachment of one of five different phendione metal complexes using combinatorial solid-phase synthesis methodology. This gave a library with three replicates of each of 20 different electrode modifications. This library was electrochemically screened in high-throughput (HTP) mode using cyclic voltammetry. The members of the library were evaluated with regard to the surface coverage, midpeak potential, and voltammetric peak separation for the phendione ligand, and their catalytic activity toward NADH oxidation. The surface coverage was found to depend on the length and flexibility of the linker and the geometry of the metal complex. The choices of linker and metal complex were also found to have significant impact on the kinetics of the reaction between the 1,10-phenanthroline-5,6-dione ligand and NADH. The rate constants for the reaction were obtained by analyzing the catalytic currents as a function of NADH concentration and scan rate, and the influence of the surface molecular architecture on the kinetics was evaluated.

Mass transport controlled oxygen reduction at anthraquinone modified 3D-CNT electrodes with immobilized Trametes hirsuta laccase.

Carbon nanotubes covalently modified with anthraquinone were used as an electrode for the immobilization of Trametes hirsuta laccase. The adsorbed laccase is capable of oxygen reduction at a mass transport controlled rate (up to 3.5 mA cm(-2)) in the absence of a soluble mediator. The storage and operational stability of the electrode are excellent.

Monolayer anthracene and anthraquinone modified electrodes as platforms for Trametes hirsuta laccase immobilisation.

Surface modification techniques are essential to the construction of enzyme based elements of biofuel cells and biosensors. In this article we report on the preparation and characterisation of modified carbon electrodes which were used as supports for the immobilisation of laccase from Trametes hirsuta. The electrodes were electrochemically modified with diamine or diazonium linkers followed by attachment of either anthracene or anthraquinone head groups using solid phase chemical methodology. These well defined surfaces were found to effectively bind laccase and to provide direct electrical contact to the enzyme active site, as evidenced by XPS, EIS and voltammetry, respectively. The influence of the type of linker and head group on enzyme binding and bioelectrocatalytic activity are evaluated.