Maria Gatta

Study of the electrochemical deposition and properties of cobalt oxide species in citrate alkaline solutions

Abstract Glassy carbon electrode substrates coated with thin films of cobalt oxyhydroxide were investigated in moderately alkaline medium by cyclic voltammetry. The cobalt oxide formation process was accomplished in alkaline solution (30 mM NaOH) containing millimolar concentrations of CoCl 2 and ligand species such as sodium citrate. The cobalt oxide/hydroxide films were obtained either by voltage cycling between 0.4 and 1.1 V versus SCE or under potentiostatic conditions at moderate positive potentials (i.e. 0.5 V). The resulting electrodeposited film was characterized electrochemically in alkaline solutions by cyclic voltammetry and, independently of the electrodeposition procedure adopted, reveals three well-defined sets of redox waves related to the Co(OH) 2 /Co 2 O 3 , Co 2 O 3 /CoOOH and CoOOH/CoO 2 electrochemical transitions. The charge/discharge processes and the oxygen evolution reaction during continuous potential cycling were severely inhibited in the presence of increasing concentrations of electroinactive electrolytes (i.e. NaNO 3 , K 2 SO 4 , Na 3 PO 4 , etc.). The electrochemical behaviour of the electrodeposited cobalt oxide/hydroxide film in alkaline solution and in the presence of various organic molecules was also investigated.

Amperometric determination of underivatized amino acids at a nickel-modified gold electrode by anion-exchange chromatography

A nickel-based composite electrode obtained by anodic electrodeposition of nickel (III) oxyhydroxide film on the gold electrode substrate was characterized as an amperometric sensor and successfully applied to the determination of underivatised amino acids in flow-through systems. The electrodeposition of nickel oxyhydroxide films was obtained by cycling a gold electrode between 0.0 V and +1.0 V vs. a saturated calomel electrode in a 80 microM Ni2+ solution buffered at pH 10 with NaHCO3/Na2CO3. The resulting Au-Ni composite electrode exhibits good stability in alkaline medium and can be used as an amperometric sensor of underivatised amino acids at a fixed applied potential (+0.55 V vs. Ag/AgCl). The detection limits (S/N=3) for all investigated compounds ranged between 5 and 30 pmol injected, while the linear ranges spanned over two or three orders of magnitude. The contents of several free amino acids in two sample cheeses from different brands were evaluated by calibration graphs.

Determination of electroactive organic acids by anion-exchange chromatography using a copper modified electrode

An ion-chromatographic method combined with electrochemical detection at a copper-based chemically modified glassy carbon electrode (Cu-GC) has been shown to provide a simple analytical approach for the determination of some common organic acids in alkaline medium. Under the optimized isocratic chromatographic conditions (i.e. 0.1 M NaOH plus 80 mM CH3COONa), organic acids such as gallic, ascorbic, gluconic, lactobionic, galacturonic and glucuronic acid could be separated in less than 20 min. Under constant potential amperometric detection (i.e. 0.55 V vs. Ag-AgCl) the Cu-GC modified electrode allowed detection limits between 2 and 5 pmol for all investigated organic acids while the linear dynamic range spanned generally over three orders of magnitude. Examples of applications included the separation and quantitation of some common organic acids in vinegar, honey and tea samples, are given.

Electrodeposition and Characterization of Nickel-Copper Alloy Films as Electrode Material in Alkaline Media

A new and simple electrochemical procedure of anodic codeposition of nickel and copper oxide-hydroxide species on the gold electrode substrate from alkaline solutions is described. The electrodeposited alloy film, with an average composition Ni:Cu of 0.67 (i.e., Ni 40 Cu 60 ) was investigated by electrochemical, scanning electron microscopy, and X-ray photoelectron spectroscopy technique. The resulting deposited alloys appear uniform, smooth, adherent to the gold substrate, and homogeneous, independently of the electrochemical procedure adopted for the preparation (i.e., potentiostatic or potentiodynamic conditions). The presence of copper oxyhydroxide species in deposited film imparts an extensive stabilization of the β/β crystallographic forms of the nickel oxyhydroxide under prolonged repetitive potential cycling in alkaline medium. The electrocatalytic activity of the resulting composite Ni-Cu film was investigated toward the oxidation of carbohydrates using glucose as a model compound.

Determination of histamine by high-pH anion-exchange chromatography with electrochemical detection

Abstract A simple, sensitive and rapid method is described for determining histamine in alkaline media by anion-exchange chromatography with amperometric detection at a copper-based modified glassy carbon electrode. The influence of copper loading, applied potential and flow rate on the amperometric signal was evaluated. Under optimal chromatographic conditions, the detection limit for histamine (S/N=3) was 0.1 μM (corresponding to 5 pmol injected by a 50-μl sample loop). The linear range spanned from 5 to 500 μM (correlation coefficient=0.998). The proposed method allows a rapid and convenient quantitation of histamine in real matrices, such as canned tuna and red wine, without time-consuming clean-up or derivatisation procedures.

Applications of a copper-modified gold electrode for amperometric detection of polar aliphatic compounds by anion-exchange chromatography

Abstract An Au/Cu bimetallic electrode is investigated as a sensor in amperometric flow-through detection in alkaline solutions. Experimental conditions such as detection mode, pH, copper loading and flow-rate on sensitivity and stability are evaluated. Constant-potential detection at 0.550 V or 0.350 V and pulsed detection modes are tested after anion-exchange chromatography of alditols, carbohydrates and amino acids. The detection limits (S/N=3) of carbohydrates at 0.550 V and 0.350 V ranged between 4.6–9.0 pmol and 1.0–4.0 pmol, respectively. Those of amino acids were 9.0–39 pmol. In pulsed amperometric detection mode, molar sensitivities and detection limits were higher than those obtained in DC mode. Linear dynamic ranges spanned over four (alditols and carbohydrates) and three (amino acids) orders of magnitude. The Au/Cu CME retained 94–96% of its response after several hours operations in flowing streams. As an application, some alditols, carbohydrates and amino acids were determined in soluble coffee and red wine.

Electrocatalysis and Detection of Carbohydrates byAnion-Exchange Chromatography at a Gold Substrate Electrode Modified with Nickel Cyanide Ions

Chemically modified electrodes obtained by direct interaction between gold substrate and tetracyanonickelate(II) ions in alkaline medium were characterized by cyclic voltammetry toward the electrooxidation of carbohydrates. Satisfactory performance with respect to catalytic stability, background current stability and lowering of detection limit was obtained for the analysis of ten oligosaccharides contained in a synthetic sample by anion-exchange chromatography in alkaline solutions. Under constant applied potential of 0.55 V (vs. Ag/AgCl), the detection limits (S/N=3) for all investigated compounds ranged between 3 and 6 pmol injected for melezitose and turanose, respectively. The dynamic linear ranges spanned generally three orders of magnitude above the limit of detection. The effects of the concentration of tetracyanonickelate(II) ion on the catalytic activity, amperometric stability and chromatographic separations were examined.