Stable, colourless and water-soluble electron-transfer mediators used in enzyme electrochemistry

Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent

The complex and indeterminant composition of NOM makes characterization of its redox properties challenging. Approaches that have been taken to address this challenge include chemical probe reactions, potentiometric titrations, chronocoulometry, and voltammetry. In this study, we revisit the use of direct voltammetric methods in aprotic solvents by applying an expanded and refined suite of methods to a large set of NOM samples and model compounds (54 NOM samples from 10 different sources, 7 NOM model compounds, and 2 fresh extracts of plant materials that are high in redox-active quinonoid model compounds dissolved in DMSO). Refinements in the methods of fitting the data obtained by staircase cyclic voltammetry (SCV) provided improved definition of peaks, and square wave voltammetry (SWV), performed under the same conditions as SCV, provided even more reliable identification and quantitation of peaks. Further evidence is provided that DMSO improves the electrode response by unfolding some of the tertiary structure of NOM polymers, thereby allowing greater contact between redox active functional groups and the electrode surface. We averaged experimental peak potentials for all NOM compounds and calculated potentials in water. Average values for Epa1, Epc1, and Ep1 in DMSO were -0.866 ± 0.069, -1.35 ± 0.071, and -0.831 ± 0.051 V vs. Ag/Ag+, and -0.128, -0.613, and -0.0930 V vs. SHE in water. In addition to peak potentials, the breadth of SCV peaks was quantified as a way to characterize the degree to which the redox activity of NOM is due to a continuum of contributing functional groups. The average breadth values were 1.63 ± 0.24, 1.28 ± 0.34, and 0.648 ± 0.15 V for Epa1, Epc1, and Ep1 respectively. Comparative analysis of the overall dataset-from SCV and SWV on all NOMs and model compounds-revealed that NOM redox properties vary over a narrower range than expected based on model compound properties. This lack of diversity in redox properties of NOM is similar to conclusions from other recent work on the molecular structure of NOM, all of which could be the result of selectivity in the common extraction methods used to obtain the materials.

Electrochemical analyses of redox-active iron minerals: a review of nonmediated and mediated approaches

Redox-active minerals are ubiquitous in the environment and are involved in numerous electron transfer reactions that significantly affect biogeochemical processes and cycles as well as pollutant dynamics. As a consequence, research in different scientific disciplines is devoted to elucidating the redox properties and reactivities of minerals. This review focuses on the characterization of mineral redox properties using electrochemical approaches from an applied (bio)geochemical and environmental analytical chemistry perspective. Establishing redox equilibria between the minerals and working electrodes is a major challenge in electrochemical measurements, which we discuss in an overview of traditional electrochemical techniques. These issues can be overcome with mediated electrochemical analyses in which dissolved redox mediators are used to increase the rate of electron transfer and to facilitate redox equilibration between working electrodes and minerals in both amperometric and potentiometric measurements. Using experimental data on an iron-bearing clay mineral, we illustrate how mediated electrochemical analyses can be employed to derive important thermodynamic and kinetic data on electron transfer to and from structural iron. We summarize anticipated methodological advancements that will further contribute to advance an improved understanding of electron transfer to and from minerals in environmentally relevant redox processes.

Redox properties of structural Fe in clay minerals. 2. Electrochemical and spectroscopic characterization of electron transfer irreversibility in ferruginous smectite, SWa-1

Structural Fe in clay minerals is an important, albeit poorly characterized, redox-active phase found in many natural and engineered environments. This work develops an experimental approach to directly assess the redox properties of a natural Fe-bearing smectite (ferruginous smectite, SWa-1, 12.6 wt % Fe) with mediated electrochemical reduction (MER) and oxidation (MEO). By utilizing a suite of one-electron-transfer mediating compounds to facilitate electron transfer between structural Fe in SWa-1 and a working electrode, we show that the Fe2+/Fe3+ couple in SWa-1 is redox-active over a large range of potentials (from E(H) = -0.63 V to +0.61 V vs SHE). Electrochemical and spectroscopic analyses of SWa-1 samples that were subject to reduction and re-oxidation cycling revealed both reversible and irreversible structural Fe rearrangements that altered the observed apparent standard reduction potential (E(H)(ø)) of structural Fe. E(H)(ø)-values vary by as much as 0.56 V between SWa-1 samples with different redox histories. The wide range of E(H)-values over which SWa-1 is redox-active and redox history-dependent E(H)(ø)-values underscore the importance of Fe-bearing clay minerals as redox-active phases in a wide range of redox regimes.