A review on the recent progress of square-wave voltammetry is presented, covering the period of the last five years. The review addresses the new theoretical development of the technique as well as its application for mechanistic purposes, electrode kinetic measurements, biochemical and analytical applications. Besides, a few novel methodological modifications are proposed that might expand the scope and application of the technique.

Polyelectrolyte/gold nanoparticle multilayers composed of poly(L-lysine) (pLys) and mercaptosuccinic acid (MSA) stabilized gold nanoparticles (Au NPs) were built up using the electrostatic layer-by-layer self-assembly technique upon a gold electrode modified with a first layer of MSA. The assemblies were characterized using UV-vis absorption spectroscopy, cyclic and square-wave voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy. Charge transport through the multilayer was studied experimentally as well as theoretically by using two different redox pairs [Fe(CN) 6 ] 3-/4-and [Ru(NH 3 ) 6 ] 3+/2+ . This paper reports a large sensitivity to the charge of the outermost layer for the permeability of these assemblies to the probe ions. With the former redox pair, dramatic changes in the impedance response were obtained for thin multilayers each time a new layer was deposited. In the latter case, the multilayer behaves as a conductor exhibiting a strikingly lower impedance response, the electric current being enhanced as more layers are added for Au NP terminated multilayers. These results are interpreted quite satisfactorily by means of a capillary membrane model that encompasses the wide variety of behaviors observed. It is concluded that nonlinear slow diffusion through defects (pinholes) in the multilayer is the governing mechanism for the [Fe(CN) 6 ] 3-/4-species, whereas electron transfer through the Au NPs is the dominant mechanism in the case of the [Ru(NH 3 ) 6 ] 3+/2+ pair.

This review is focused on the basic principles, the main applications, and the theoretical models developed for various redox mechanisms in protein film voltammetry, with a special emphasis to square-wave voltammetry as a working technique. Special attention is paid to the thermodynamic and kinetic parameters of relevant enzymes studied in the last decade at various modified electrodes, and their use as a platform for the detection of reactive oxygen species is also discussed. A set of recurrent formulas for simulations of different redox mechanisms of lipophilic enzymes is supplied together with representative simulated voltammograms that illustrate the most relevant voltammetric features of proteins studied under conditions of square-wave voltammetry.

Hardcover Electrochemistry of Immobilized Particles and Droplets▶ There are no competing books on the market because the subject is new ▶ The necessary theoretical background as well as detailed information on the experiments is providedImmobilizing particles or droplets on electrodes is a novel and most powerful technique for studying the electrochemical reactions of three-phase systems. It gives access to a wealth of information, ranging from quantitative and phase analysis to thermodynamic and kinetic data of electrode processes. Three-phase electrodes with immobilized droplets provide information on the electrochemistry of redox liquids and of compounds dissolved in inert organic liquids. Such measurements allow the determination of the Gibbs energies of the transfer of cations and anions between immiscible solvents, and thus make it possible to assess the hydrophobicity of ions -a property that is of great importance for pharmaceutical applications, biological studies, and for many fields of chemistry. The monograph gives, for the first time, a comprehensive overview of the results published in more than 300 papers over the last 15 years. The experiments are explained in detail, applications from many different fields are presented, and the theoretical basis of the systems is outlined.

Ions can be transferred between immiscible liquid phases across a common interface, with the help of a three‐electrode potentiostat, when one phase is an organic droplet attached to a solid electrode and containing a redox probe. This novel approach has been used in studies to determine the Gibbs energy of anion and cation transfer, ranging from simple inorganic and organic ions to the ionic forms of drugs and small peptides. This method of studying ion transfer has the following advantages: 1) no base electrolytes are necessary in the organic phase; 2) the aqueous phase contains only the salt to be studied; 3) a three‐electrode potentiostat is used; 4) organic solvents such as n‐octanol and chiral liquids such as D‐ and l‐2‐octanol can be used; 5) the range of accessible Gibbs energies of transfer is wider than in the classic 4‐electrode experiments; 6) the volume of the organic phase can be very small, for example, 1 μL or less; 7) the experiments can be performed routinely and fast. Herein, the basic principle is outlined, as well as a summary of the results obtained to date, and a discussion on the theoretical treatments concerning the kinetic regime of the three‐phase electrodes with immobilized droplets.

The lipophilicity of the anionic forms of drugs and model compounds was assessed by their transfer across (i) the water-2-nitrophenyloctyl ether (NPOE), (ii) the water-nitrobenzene (NB), and (iii) the water-noctanol interfaces by using the three-phase electrode technique. The lipophilicities, expressed in terms of logarithm of partition coefficients, range for the studied anions from -3.46 to 0.68 (log P A -,aq QNPOE ) for NPOE, from -3.81 to 2.62 (log P A -,aq QNB ) for NB, and from -6.20 to -3.20 (log P A -,aq Qn-oct ) for n-octanol. Although NPOE shares with nitrobenzene the aromatic part and with n-octanol the hydrophobic carbon chain, only very weak correlation was observed between the NPOE-based data with the n-octanol-based data, and the same is true for the correlation of the NB-based and n-octanol-based data. However, there is a strong and even linear correlation between the NPOE-based and the NB-based data.

a b s t r a c tReactive oxygen species (ROS) are increasingly recognized as second messengers in many cellular processes. While high concentrations of oxidants damage proteins, lipids and DNA, ultimately resulting in cell death, selective and reversible oxidation of key residues in proteins is a physiological mechanism that can transiently alter their activity and function. Defects in ROS producing enzymes cause disturbed immune response and disease.Changes in the intracellular free Ca 2+ concentration are key triggers for diverse cellular functions. Ca 2+ homeostasis thus needs to be precisely tuned by channels, pumps, transporters and cellular buffering systems. Alterations of these key regulatory proteins by reversible or irreversible oxidation alter the physiological outcome following cell stimulation. It is therefore necessary to understand which proteins are regulated and if this regulation is relevant in a physiological-and/or pathophysiological context. Because ROS are inherently difficult to identify and to measure, we first review basic oxygen redox chemistry and methods of ROS detection with special emphasis on electron paramagnetic resonance (EPR) spectroscopy. We then focus on the present knowledge of redox regulation of Ca 2+ permeable ion channels such as voltage-gated (CaV) Ca 2+ channels, transient receptor potential (TRP) channels and Orai channels.© 2011 Elsevier Ltd. All rights reserved. Basic redox chemistry of oxygenMany chemical processes are linked to the exchange of electrons between two or more molecular entities. The transfer of one (or more) electron(s) is associated with oxidation (loss of electron) and reduction (gain of electron) of the components. Such reactions are also known as redox reactions. The processes of reactive oxygen species (ROS) formation and elimination are exclusively of redox nature.Molecular oxygen is the precursor of all ROS. It contains two unpaired electrons in separate (antibonding) orbitals in its outer electron sphere and is thus, by definition, a radical. Radicals have at least one unpaired electron in their orbital system and usually show high reactivity; transition metal ions also may have unpaired electrons, but are not called radicals. Although having radical character, molecular oxygen is chemically rather inert (luckily!), because high * Corresponding authors at: Institut für Biophysik, Gebäude 58, Universität des Saarlandes, D-66421 Homburg/Saar, Germany. Tel.: +49 6841 1626453; fax: +49 6841 1626060.E-mail addresses: ivan.bogeski@uks.eu (I. Bogeski), barbara.niemeyer@uks.eu (B.A. Niemeyer).

Coenzyme Q10 (CoQ10) is one of the essential components of the mitochondrial electron-transport chain (ETC) with the primary function to transfer electrons along and protons across the inner mitochondrial membrane (IMM). The concomitant proton gradient across the IMM is essential for the process of oxidative phosphorylation and consequently ATP production. Cytochrome P450 (CYP450) monoxygenase enzymes are known to induce structural changes in a variety of compounds and are expressed in the IMM. However, it is unknown if CYP450 interacts with CoQ10 and how such an interaction would affect mitochondrial function. Using voltammetry, UV–vis spectrometry, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), fluorescence microscopy and high performance liquid chromatography–mass spectrometry (HPLC–MS), we show that both CoQ10 and its analogue CoQ1, when exposed to CYP450 or alkaline media, undergo structural changes through a complex reaction pathway and form quinone structures with distinct properties. Hereby, one or both methoxy groups at positions 2 and 3 on the quinone ring are replaced by hydroxyl groups in a time-dependent manner. In comparison with the native forms, the electrochemically reduced forms of the new hydroxylated CoQs have higher antioxidative potential and are also now able to bind and transport Ca2+ across artificial biomimetic membranes. Our results open new perspectives on the physiological importance of CoQ10 and its analogues, not only as electron and proton transporters, but also as potential regulators of mitochondrial Ca2+ and redox homeostasis.