A joint experimental and computational search for authentic nano-electrocatalytic effects: electrooxidation of nitrite and L-ascorbate on gold nanoparticle-modified glassy carbon electrodes

Voltammetry of Carbon Nanotubes and the Limitations of Particle-Modified Electrodes: Are Carbon Nanotubes Electrocatalytic?

The use of carbon nanotubes (CNTs) as electrocatalysts is summarized; the limitations of using voltammetry based on CNT-modified electrodes is explained; and the role of mass transport, as well as electrode kinetics, with respect to dictating the voltammetric responses is discussed. The use of single-entity electrochemistry to at least complement, if not replace, ensemble voltammetry is advocated along with other caveats, notably purity, with respect to CNT voltammetry.

Electrochemical Sensor Based on Iron(II) Phthalocyanine and Gold Nanoparticles for Nitrite Detection in Meat Products

Nitrites are widely used in the food industry, particularly for the preservation of meat products. Controlling the nitrate content in food is an important task to ensure people's health is not at risk; therefore, the search for, and research of, new materials that will modify the electrodes in the electrochemical sensors that detect and control the nitrate content in food products is an urgent task. In this paper, we describe the electrochemical behavior of a glass carbon electrode (GCE), modified with a Fe(II) tetra-tert-butyl phthalocyanine film (FePc(tBu)₄/GCE), and decorated with gold nanoparticles (Au/FePc(tBu)₄/GCE); this electrode was deposited using gas-phase methods. The composition and morphology of such electrodes were examined using spectroscopy and electron microscopy methods, whereas the main electrochemical characteristics were determined using cyclic voltammetry (CV) and amperometry (CA) methods in the linear ranges of CV 0.25-2.5 mM, CA 2-120 μM in 0.1 M phosphate buffer (pH = 6.8). The results showed that the modification of bare GCEs, with a Au/FePc(tBu)₄ heterostructure, provided a high surface-to-volume ratio, thus ensuring its high sensitivity to nitrite ions of 0.46 μAμM^-1. The sensor based on the Au/FePc(tBu)₄/GCE has a low limit of nitrite detection at 0.35 μM, good repeatability, and stability. The interference study showed that the proposed Au/FePc(tBu)₄/GCE exhibited a selective response in the presence of interfering anions, and the analytical capability of the sensor was demonstrated by determining nitrite ions in real samples of meat products.

Electrochemistry under confinement

Although the term 'confinement' regularly appears in electrochemical literature, elevated by continuous progression in the research of nanomaterials and nanostructures, up until today the various aspects of confinement considered in electrochemistry are rather scattered individual contributions outside the established disciplines in this field. Thanks to a number of highly original publications and the growing appreciation of confinement as an overarching link between different exciting new research strategies, 'electrochemistry under confinement' is the process of forming a research discipline of its own. To aid the development a coherent terminology and joint basic concepts, as crucial factors for this transformation, this review provides an overview on the different effects on electrochemical processes known to date that can be caused by confinement. It also suggests where boundaries to other effects, such as nano-effects could be drawn. To conceptualize the vast amount of research activities revolving around the main concepts of confinement, we define six types of confinement and select two of them to discuss the state of the art and anticipated future developments in more detail. The first type concerns nanochannel environments and their applications for electrodeposition and for electrochemical sensing. The second type covers the rather newly emerging field of colloidal single entity confinement in electrochemistry. In these contexts, we will for instance address the influence of confinement on the mass transport and electric field distributions and will link the associated changes in local species concentration or in the local driving force to altered reaction kinetics and product selectivity. Highlighting pioneering works and exciting recent developments, this educational review does not only aim at surveying and categorizing the state-of-the-art, but seeks to specifically point out future perspectives in the field of confinement-controlled electrochemistry.

Toward point-of-care management of chronic respiratory conditions: Electrochemical sensing of nitrite content in exhaled breath condensate using reduced graphene oxide

We present a portable non-invasive approach for measuring indicators of inflammation and oxidative stress in the respiratory tract by quantifying a biomarker in exhaled breath condensate (EBC). We discuss the fabrication and characterization of a miniaturized electrochemical sensor for detecting nitrite content in EBC using reduced graphene oxide. The nitrite content in EBC has been demonstrated to be a promising biomarker of inflammation in the respiratory tract, particularly in asthma. We utilized the unique properties of reduced graphene oxide (rGO); specifically, the material is resilient to corrosion while exhibiting rapid electron transfer with electrolytes, thus allowing for highly sensitive electrochemical detection with minimal fouling. Our rGO sensor was housed in an electrochemical cell fabricated from polydimethyl siloxane (PDMS), which was necessary to analyze small EBC sample volumes. The sensor is capable of detecting nitrite at a low over-potential of 0.7 V with respect to an Ag/AgCl reference electrode. We characterized the performance of the sensors using standard nitrite/buffer solutions, nitrite spiked into EBC, and clinical EBC samples. The sensor demonstrated a sensitivity of 0.21 μA μM^-1 cm^-2 in the range of 20-100 μM and of 0.1 μA μM^-1 cm^-2 in the range of 100-1000 μM nitrite concentration and exhibited a low detection limit of 830 nM in the EBC matrix. To benchmark our platform, we tested our sensors using seven pre-characterized clinical EBC samples with concentrations ranging between 0.14 and 6.5 μM. This enzyme-free and label-free method of detecting biomarkers in EBC can pave the way for the development of portable breath analyzers for diagnosing and managing changes in respiratory inflammation and disease.

Recent Advances in Voltammetry

Recent progress in the theory and practice of voltammetry is surveyed and evaluated. The transformation over the last decade of the level of modelling and simulation of experiments has realised major advances such that electrochemical techniques can be fully developed and applied to real chemical problems of distinct complexity. This review focuses on the topic areas of: multistep electrochemical processes, voltammetry in ionic liquids, the development and interpretation of theories of electron transfer (Butler-Volmer and Marcus-Hush), advances in voltammetric pulse techniques, stochastic random walk models of diffusion, the influence of migration under conditions of low support, voltammetry at rough and porous electrodes, and nanoparticle electrochemistry. The review of the latter field encompasses both the study of nanoparticle-modified electrodes, including stripping voltammetry and the new technique of 'nano-impacts'.

Strong negative nanocatalysis: oxygen reduction and hydrogen evolution at very small (2 nm) gold nanoparticles

The electron transfer kinetics associated with both the reduction of oxygen and of protons to form hydrogen at gold nanoparticles are shown to display strong retardation when studied at citrate capped ultra small (2 nm) gold nanoparticles. Negative nanocatalysis in the hydrogen evolution reaction (HER) is reported for the first time.

Prolonged exposure of graphite oxide to soft X-ray irradiation during XPS measurements leads to alterations of the chemical composition

X-ray photoelectron spectroscopy is an indispensable technique to analyse the structural and chemical features of graphene-based materials. Such detailed characterization is often imperative prior to the usage of these materials for various applications. In this work, the effects of the continuous irradiation with soft X-rays during the analysis of graphite oxide with X-ray photoelectron spectroscopy were investigated. Prolonged exposure to the X-ray source was observed to cause an overall decrease in the amount of oxygen-containing groups, which could consequently lead to the wrong interpretation of the XPS analysis result. Proper and careful consideration on the time span of XPS analysis of graphite oxide is, thus, highly advocated.

A kinetic study of oxygen reduction reaction and characterization on electrodeposited gold nanoparticles of diameter between 17 nm and 40 nm in 0.5 M sulfuric acid

Kinetic and mechanistic studies of the oxygen reduction reaction (ORR) in oxygen saturated 0.5 M sulfuric acid at 298 K at a gold macroelectrode and at an electrodeposited gold nanoparticle-modified glassy carbon electrode are reported. The conditions of electrodeposition are optimized to obtain small nanoparticles of diameter from 17 nm to 40 nm. The mechanism and kinetics of ORR on the gold macroelectrode are investigated and compared with those obtained for nanoparticle-modified electrodes. The mechanism for this system includes two electron and two proton transfers and hydrogen peroxide as the final product. The first electron transfer step corresponding to the reduction of O2 to O2(-)˙ is defined as the rate determining step. No significant changes are found for the nanoparticles here employed: electron transfer rate constant (k0) is k0,bulk = 0.30 cm s(-1) on the bulk material and k0,nano = 0.21 cm s(-1) on nanoparticles; transfer coefficient (α) changes from αbulk = 0.45 on macro-scale to αnano = 0.37 at the nano-scale.

Nitrite electrochemical biosensing based on coupled graphene and gold nanoparticles

Biofunctionalized graphene-gold nanoparticle (AuNP) hybrids were prepared using a facile approach of in situ growth, with homogeneous distribution of AuNPs on the graphene nanosheets. Hemoglobin (Hb) was immobilized on the graphene-AuNP composites to fabricate biosensors for determination of nitrite (NO2(-)). A pair of well-defined redox peaks was observed for Hb immobilized on the graphene-AuNP hybrids with a formal potential (E(0')) of -0.314 V in 0.1 M phosphate buffered saline (0.15 M NaCl, pH 7.0). The novel biosensors exhibited many advantages, such as wide linear response range (from 0.05 to 1000 µM, R(2)=0.997), low detection limit (0.01 µM, a signal to noise ratio of 3), high sensitivity (0.15 μA μM(-1) cm(-2)), and excellent selectivity. These constructed biosensors were further used for determination of nitrite in pickled radish. The results obtained were in good agreement with those using spectrophotometry based on the National Food Safety Standard (GB 5009.33-2010), which indicates that these novel and sensitive biosensors have promising application for determination of nitrite in food.