Recent advances in graphite powder-based electrodes.

Graphite powder-based electrodes have the electrochemical performance of quasi-noble metal electrodes with intrinsic advantages related to the possibility of modification to enhance selectivity and their easily renewable surface, with no need for hazardous acids or bases for their cleaning. In contrast with commercial electrodes, for example screen-printed or sputtered-chip electrodes, graphite powder-based electrodes can also be fabricated in any laboratory with the form and characteristics desired. They are also readily modified with advanced materials, with relatively high reproducibility. All these characteristics make them a very interesting option for obtaining a large variety of electrodes to resolve different kinds of analytical problems. This review summarizes the state-of-the-art, advantages, and disadvantages of graphite powder-based electrodes in electrochemical analysis in the 21st century. It includes recent trends in carbon paste electrodes, devoting special attention to the use of emergent materials as new binders and to the development of other composite electrodes. The most recent advances in the use of graphite powder-modified sol-gel electrodes are also described. The development of sonogel-carbon electrodes and their use in electrochemical sensors and biosensors is included. These materials extend the possibilities of applications, especially for industrial technology-transfer purposes, and their development could affect not only electroanalytical green chemistry but other interesting areas also, for example catalysis and energy conversion and storage.

1-Furoylthiourea-Sonogel-Carbon electrodes: structural and electrochemical characterization.

A new type of sensor based on Sonogel-Carbon materials modified with 1-(2-furoyl)-3-(1-naphthyl)thiourea is presented to be used as an amperometric sensor for metals. Different percentages of modifier were tested in order to optimise the structural and mechanical properties, as well as the electrochemical behaviour. Scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), Fourier transform-infrared spectroscopy (FTIR), single crystal X-ray diffraction (XRD) and powder XRD were used for the structural characterization of these electrodes. The 1-(2-furoyl)-3-(1-naphthyl)thiourea did not modify significantly the structural and mechanical properties of the Sonogel-Carbon electrodes, being similar to other modifications carried out previously in these materials. For the study of the electrochemical response, anodic stripping differential pulse voltammetry (ASDPV) was employed, optimising other parameters of measurement such as pH of the buffer, potential and accumulation time and pulse amplitude. The electrochemical response of the modified electrodes improved significantly with respect to the non-modified electrode, giving good signals and acceptable detection limit.

Sonogel-carbon electrode based on hemin for detection of superoxide.

The Sonogel electrode was immersed in hemin solution to obtain the hemin-Sonogel-modified electrode by cyclic voltammetry (CV). It was applied to detect superoxide radicals produced by xanthine oxidase catalyzed hypoxanthine oxidation. The characterization of hemin-Sonogel was modified by impedance and CV.

Alumina sol-gel/sonogel-carbon electrode based on acetylcholinesterase for detection of organophosphorus pesticides.

Two new amperometric biosensors based on immobilization of acetylcholinesterase on a sonogel-carbon electrode for detection of organophosphorous compounds are proposed. The electrodes were prepared applying high-energy ultrasounds directly to the precursors. The first biosensor was obtained by simple entrapping acetylcholinesterase in Al(2)O(3) sol-gel matrix on the sonogel-carbon. The second biosensor was produced in a sandwich configuration. Its preparation involved adsorption of the enzyme and modification via a polymeric membrane such as polyethylene glycol and the ion-exchanger Nafion. The optimal enzyme loading was found to be 0.7 mIU. Both biosensors showed optimal activity in 0.2 M phosphate buffer, pH 7.0, at an operating potential of 210 mV. The detection limit achieved for chlorpyriphos-ethyl-oxon was 2.5x10(-10)M at a 10-min incubation time.

Investigation of biosensor signal bioamplification: comparison of direct electrochemistry phenomena of individual Laccase, and dual Laccase-Tyrosinase copper enzymes, at a Sonogel-Carbon electrode.

Direct electrochemistry of Trametes versicolor Laccase (LAC) was found at a Sonogel-Carbon electrode. The bioamplification, performed by dual immobilization of this enzyme and Mushroom Tyrosinase (TYR), of the bio-electrocatalytic reduction of O(2) was investigated. The calculated alpha transfer coefficients were 0.64 and 0.67, and the heterogeneous electron-transfer rate constants were 6.19 and 8.52 s(-1), respectively, for the individual LAC and dual LAC-TYR-based Nafion/Sonogel-Carbon bio-electrodes. The responses of the dual enzymes electrode to polyphenols were stronger than those of the individual LAC or TYR biosensors. Hypotheses are offered about the mechanism of bioamplification. The surfaces of the biosensors were also characterized by AFM.

The Sonogel-Carbon materials as basis for development of enzyme biosensors for phenols and polyphenols monitoring: a detailed comparative study of three immobilization matrixes.

Three amperometric biosensors based on immobilization of tyrosinase on a new Sonogel-Carbon electrode for detection of phenols and polyphenols are described. The electrode was prepared using high energy ultrasounds (HEU) directly applied to the precursors. The first biosensor was obtained by simple adsorption of the enzyme on the Sonogel-Carbon electrode. The second and the third ones, presenting sandwich configurations, were initially prepared by adsorption of the enzyme and then modification by mean of polymeric membrane such as polyethylene glycol for the second one, and the ion-exchanger Nafion in the case of the third biosensor. The optimal enzyme loading and polymer concentration, in the second layer, were found to be 285 U and 0.5%, respectively. All biosensors showed optimal activity at the following conditions: pH 7, -200 mV, and 0.02 mol l(-1) phosphate buffer. The response of the biosensors toward five simple phenols derivatives and two polyphenols were investigated. It was found that the three developed tyrosinase Sonogel-Carbon based biosensors are in satisfactory competitiveness for phenolic compounds determination with other tyrosinase based biosensors reported in the literature. The detection limit, sensitivity, and the apparent Michaelis-Menten constant K(m)(app) for the Nafion modified biosensor were, respectively, 0.064, 0.096, and 0.03 micromol, 82.5, 63.4, and 194 nA micromol(-1)l(-1), and 67.1, 54.6, and 12.1 micromol l(-1) for catechol, phenol, and 4-chloro-3-methylphenol. Hill coefficient values (around 1 for all cases), demonstrated that the immobilization method does not affect the nature of the enzyme and confirms the biocompatibility of the Sonogel-Carbon with the bioprobe. An exploratory application to real samples such as beers, river waters and tannery wastewaters showed the ability of the developed Nafion/tyrosinase/Sonogel-Carbon biosensor to retain its stable and reproducible response.