Electrochemical Detection of pH-Responsive Grafted Catechol and Immobilized Cytochrome c onto Lipid Deposit-Modified Glassy Carbon Surface

Characterization of the inner membrane cytochrome ImcH from Geobacter reveals its importance for extracellular electron transfer and energy conservation

Electroactive bacteria combine the oxidation of carbon substrates with an extracellular electron transfer (EET) process that discharges electrons to an electron acceptor outside the cell. This process involves electron transfer through consecutive redox proteins that efficiently connect the inner membrane to the cell exterior. In this study, we isolated and characterized the quinone-interacting membrane cytochrome c ImcH from Geobacter sulfurreducens, which is involved in the EET process to high redox potential acceptors. Spectroscopic and electrochemical studies show that ImcH hemes have low midpoint redox potentials, ranging from -150 to -358 mV, and connect the oxidation of the quinol-pool to EET, transferring electrons to the highly abundant periplasmic cytochrome PpcA with higher affinity than to its homologues. Despite the larger number of hemes and transmembrane helices, the ImcH structural model has similarities with the NapC/NirT/NrfH superfamily, namely the presence of a quinone-binding site on the P-side of the membrane. In addition, the first heme, likely involved on the quinol oxidation, has apparently an unusual His/Gln coordination. Our work suggests that ImcH is electroneutral and transfers electrons and protons to the same side of the membrane, contributing to the maintenance of a proton motive force and playing a central role in recycling the menaquinone pool.

Electrografting a Hybrid Bilayer Membrane via Diazonium Chemistry for Electrochemical Impedance Spectroscopy of Amyloid-β Aggregation

Herein, a novel hybrid bilayer membrane is introduced as a platform to study the aggregation of amyloid-β_1–42 (Aβ_1–42) peptide on surfaces. The first layer was covalently attached to a glassy carbon electrode (GCE) via diazonium electrodeposition, which provided a highly stable template for the hybrid bilayer formation. To prepare the long-chain hybrid bilayer membrane (lcHBLM)-modified electrodes, GCE surfaces were modified with 4-dodecylbenzenediazonium (DDAN) followed by the modification with dihexadecyl phosphate (DHP) as the second layer. For the preparation of short-chain hybrid bilayer membrane (scHBLM)-modified electrodes, GCE surfaces were modified with 4-ethyldiazonium (EDAN) as the first layer and bis(2-ethylhexyl) phosphate (BEHP) was utilized as the second layer. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to characterize the bilayer formation. Both positively charged [Ru(NH₃)₆]³⁺ and negatively charged ([Fe(CN)₆]^3-/4-) redox probes were used for electrochemical characterization of the modified surfaces using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). EIS results showed a decrease in charge transfer resistance (R_ct) upon incubation of Aβ_1–42 on the hybrid bilayer-modified surfaces. This framework provides a promising electrochemical platform for designing hybrid bilayers with various physicochemical properties to study the interaction of membrane-bound receptors and biomolecules on surfaces.

Electrografted anthraquinone to monitor pH at the biofilm-anode interface in a wastewater microbial fuel cell

Electrografted anthraquinone on graphite was used as a probe to monitor the pH change at the biofilm-electrode interface at the anode of a microbial fuel cell inoculated with wastewater. The grafting procedure was optimized so that the pH-dependent electrochemical response of the grafted quinone did not overlay with that of the electroactive biofilm. The variation of the formal potential of the grafted quinone as a function of pH was linear over the pH range 1-10 with a slope of - 64 mV. This allowed to monitor the interfacial pH change over three weeks of biofilm colonization of the electrode. During that time the interfacial pH decreased from neutrality to 5.3 while the anolyte only acidified down to pH 6.2. This finding is relevant as local pH change usually leads to alterations of the bioenergetics process of microbial communities and hence on the performance of bioelectrochemical devices.

Facile Electrochemical Demethylation of 2-Methoxyphenol to Surface-Confined Catechol on the MWCNT and Its Efficient Electrocatalytic Hydrazine Oxidation and Sensing Applications

Owing to its biological significance, preparation of stable surface-confined catechol (CA) is a long-standing interest in electrochemistry and surface chemistry. In this connection, various chemical approaches such as covalent immobilization (using amine- and carboxylate-functionalized CA, diazotization-based coupling, and Michael addition reaction), self-assembled monolayer on gold (thiol-functionalized CA is assembled on the gold surface), CA adsorption on the ad-layer of a defect-free single-crystal Pt surface, π-π bonding, CA pendant metal complexes, and CA-functionalized polymer-modified electrodes have been reported in the literature. In general, these conventional methods are involved with a series of time-consuming synthetic procedures. Indeed, the preparation of a surface-fouling-free surface-confined system is a challenging task. Herein, we introduce a new and facile approach based on electrochemical demethylation of 2-methoxyphenol as a precursor on the graphitic surface (MWCNT) at a bias potential, 0.5 V vs Ag/AgCl in neutral pH solution. Such an electrochemical performance resulted in the development of a stable and well-defined redox peak at E ^o' = 0.15 (A2/C2) V vs Ag/AgCl within 10 min of preparation time in pH 7 phosphate buffer solution. Calculated surface excess (16.65 × 10^-9 mol cm^-2) is about 10-1000 times higher than the values reported with other preparation methods. The product (catechol) formed on the modified electrode was confirmed by collective electrochemical and physicochemical characterizations such as potential segment analysis, TEM, Raman, IR, UV-vis, GC-MS, and NMR spectroscopic techniques, and thin-layer chromatographic studies. The electrocatalytic efficiency of the surface-confined CA system was demonstrated by studying hydrazine oxidation and sensing reactions in a neutral pH solution. This new system is found to be tolerant to various interfering biochemicals such as uric acid, xanthine, hypoxanthine, glucose, nitrate, hydrogen peroxide, ascorbic acid, Cu²⁺, and Fe²⁺. Since the approach is simple, rapid, and reproducible, a variety of surface-confined CA systems can be prepared.