Lúcio Angnes

Performance of screen-printed carbon electrodes fabricated from different carbon inks

The screen-printing (thick-film) technology is widely used for the mass-production of disposable electrochemical sensors. The practical utility of screen-printed electrodes has been developed despite the fact that little is known about the nature of electrode reactions at these important microfabricated sensors. Given the complexity of carbon electrodes, in general, and differences in the composition of commercial carbon inks (used for fabricating screen-printed sensors), the question arises how such differences and complexity affect the electrochemical reactivity. The aim of the present work was to compare the electrochemical behavior and electroanalytical performance of thick-film carbon sensors fabricated with four different commercial carbon inks. The resulting sensors exhibit a wide range of electrochemical reactivities for benchmark redox systems and different background currents. Such differences have a profound effect upon the electroanalytical performance, as was indicated from pulse-voltammetric, amperometric and stripping operations. The choice of the carbon strip (ink) should depend upon the specific technique used.

Electrochemistry of a tetraruthenated cobalt porphyrin and its use in modified electrodes as sensors of reducing analytes

Abstract The properties and analytical applications of a new polymetallic species containing four [Ru(bipy) 2 Cl] + (bipy = 2,2′-bipyridine) groups coordinated to the pyridine residues of the meso -tetra(pyridyl)porphynate cobalt(III) complex are reported. The cyclic voltammograms of the tetraruthenated cobalt porphyrin, in DMF solution, exhibit an intense reversible wave at E 1 2 = 0.93 V associated with the Ru III II redox couple, and two irreversible cathodic waves at E pc = −1.37 and −1.63 V, involving the stepwise reduction of the bipyridine ligands in each peripheral ruthenium complex. Three successive reductions of the cobalt porphyrin center can be detected in the −0.5 to −0.7 V range, using spectroelectrochemical techniques. Stable, homogeneous films of the polymetallic porphyrin can be generated by dip-coating on to electrode surfaces, exhibiting pronounced electrochemical activity. The modified electrodes have been successfully employed for analytical purposes, using conventional and flow injection methods, showing excellent performance in the determination of reducing analytes, such as nitrite and sulphite ions, even at ppb levels.

Amperometric detection of nitrite and nitrate at tetraruthenated porphyrin-modified electrodes in a continuous-flow assembly

The modification of a glassy carbon surface by coating with an electrostatically assembled film of tetraruthenated cobalt porphyrin/(meso-tetra(4-sulphonatephenyl)porphyrinate zinc(II) yields an indicator electrode that allows the determination of nitrite to be performed with a limit of detection of 0.1 μM in a flow injection configuration. The dynamic range extends up to 1000 μM and the repeatability of the measurements was evaluated to be 1.5% with a throughput of 50 samples per hour. The efficiency of the bilayered film to mediate the electron transfer allows the determinations to be performed at a less positive potential (+0.75 V) with enhanced sensitivity. The coating also prevents the surface poisoning and its stability is maintained over several weeks. The same detector was used for determination of nitrate after reduction to nitrite in a reductor column containing copperised cadmium. This method was used for the determination of nitrate and nitrite in mineral water, saliva and cured meats, the results being in agreement with certified values and those obtained by using recommended procedures.

Modified microelectrodes and multivariate calibration for flow injection amperometric simultaneous determination of ascorbic acid, dopamine, epinephrine and dipyrone.

Flow injection amperometric quantification of ascorbic acid (AA), dopamine (DA), epinephrine (EP) and dipyrone (DI) in mixtures (in the microgram g-1 range) was successfully performed by using an array of microelectrodes with units modified by the electrodeposition of different noble metals, together with multivariate calibration analysis. The four groups of microelectrodes utilized included a pure gold electrode and electrodes modified by electrodeposition of platinum, palladium or a mixture of platinum + palladium. The array of microelectrodes was inserted in a flow cell and the amperometric data acquisition was performed with a four-channel potentiostat. The analysis of the resulting signals was carried out by a multivariate calibration method, using a group of 16 standard mixtures selected by a two-level factorial design. The analysis of synthetic samples and pharmaceutical compounds containing AA and DI led to very similar values to those obtained by the classical iodimetric analysis. The average absolute errors (in microgram g-1) calculated for each analyte were 0.3, 0.2, 0.4 and 0.4 for AA, DA, EP and DI, respectively.

Carbon Film Resistors as Electrodes: Voltammetric Properties and Application in Electroanalysis

The voltammetric properties of electrodes made from carbon film resistors of 2 Ω have been studied and compared with those of commercially available glassy carbon, before and after electrochemical preconditioning in perchloric acid. The potential window and background currents have been measured in different aqueous solutions. Cyclic voltammetry of hexacyanoferrate(II) oxidation has been carried out and compared with glassy carbon and diamond film electrodes. Promising results have been obtained for the anodic stripping voltammetry of metal ions, particularly zinc ion, without interference from hydrogen evolution, and augurs well for substituting mercury electrodes in stripping analysis.

Determination of sulfur dioxide in wines by gas-diffusion flow injection analysis utilizing modified electrodes with electrostatically assembled films of tetraruthenated porphyrin

Abstract Reported is a new method for the determination of both free and total SO 2 in wine, specially designed for FIA. The method involves separation of the analyte using a flow through gas diffusion unit, in line with an amperometric detector consisting of a glassy carbon electrode modified with electrostatically assembled films of a tetraruthenated porphyrin. The results demonstrate a good accuracy, comparable with those obtained using the conventional iodometric method. In addition, the method can be readily applied to red, white and rose wines; it is free from interference of the other compounds of wine, consumes small amounts of sample and is faster than the iodometric method.

Electrochemical detection of NADH and dopamine in flow analysis based on tetraruthenated porphyrin modified electrodes

Abstract Modified electrodes with molecular films of a tetraruthenated cobalt-porphyrin complex have been employed in the amperometric detection of NADH and dopamine. The electrochemical oxidation of these bioanalytes can be monitored at the onset of the Ru III II redox wave (E ° = 0.70 V vs Ag AgCl ) , exhibiting a very sharp peak consistent with a rapid and efficient electron transfer process. Because of the rectifying properties of the molecular films, a high selectivity for reduced species has been observed in scanning measurements. Very good signal to noise ratios have been obtained in flow analysis, ensuring a detection limit in the ppb range, for NADH and dopamine.

Combination of ultrasonic extraction and stripping analysis : An effective and reliable way for the determination of Cu and Pb in lubricating oils

The determination of metals in lubricating oil has been used as an important way to prevent components failures, to provide environmental information and in some cases, to identify adulteration. In this work, an effective and simple procedure is proposed for Cu and Pb determination in lubricating oils. An ultrasonic bath was employed for extraction of these elements from oil samples in a mixture 1:1 (v/v) of concentrated HCl and H(2)O(2). A very efficient extraction of Cu and Pb (approximately 100%) was attained after 30 min of ultrasound, allowing the simultaneous determination of both metals using square-wave anodic stripping voltammetry at thin-film gold electrodes. The extraction procedure was performed in 4 mL polypropylene closed vessels and dozens of samples could be treated simultaneously in the same ultrasonic bath. The regions of the ultrasonic bath, where the maximum efficiency of extraction was attained were evaluated. Over the optimized region, 30 samples can be treated simultaneously. Used lubricating oils from automotive engines were analyzed by using the optimized extraction procedure.

Electrostatically Assembled Films for Improving the Properties of Tetraruthenated Porphyrin Modified Electrodes

Modified electrodes, suitable for use in flow injection analysis, have been obtained by the electrostatic assembly of films constituted by two different porphyrins, namely, μ-meso(tetrapyridyl)porphyrinatecobalt(III)tetrakis[bis(bipyridine)(chloro)ruthenium(II)] and meso-tetra(4-sulphonatephenyl)porphyrinatezinc(II), forming alternating layers onto glassy carbon surfaces. The films are strongly adherent and highly stable, and can be used for at least a week without loss of sensitivity. In addition, they exhibit an enhanced amperometric response and good protection against fouling, e.g., by phenol oxidation products. Their usefulness in flow injection analysis of nitrite and sulfite ion have been demonstrated. Excellent signal to noise ratios have been obtained for sulfite down to 10−6 mol dm−3, and for nitrite down to 10−7 mol dm−3.

Eletrodos fabricados por "silk-screen"

A review dealing with the use of screen-printing technology to manufacture disposable electrodes is presented, covering in details virtually all the publications in the area up to early 1997 and including 206 references. The elements and different strategies on constructing modified electrodes are highlighted. Commercial and Home-made ink recipes are discussed. Microelectrode arrays, built by the combination of photostructuring and screen-printing technologies to the mass production of advanced disposable sensors, are also discussed. Future research trends are predicted.