Lucio Angnes

Screen-printed tyrosinase-containing electrodes for the biosensing of enzyme inhibitors

Disposable amperometric inhibition biosensors have been microfabricated by screen printing a tyrosinase-containing carbon ink. The decrease in the substrate (catechol) steady-state current, caused by the addition of various pesticides and herbicides, offers convenient quantitation of micromolar levels of these pollutants. Unlike esterasebased disposable strips, the tyrosinase thick-film devices can be fabricated by incorporating the enzyme within the carbon ink. and do not require a prolonged incubation step in the presence of the inhibitor. The effect of experimental variables, such as the enzyme loading or substrate concentration, is assessed. Applicability to an untreated river water sample is illustrated. Such use of single-use devices for monitoring toxins addresses the problem of irreversible enzyme inhibition, and holds great promise for on-site field analysis.

Scanning tunneling microscopic probing of surface fouling during the oxidation of nicotinamide coenzymes

Abstract Scanning tunneling microscopy is used to image adsorbed products resulting from the electro-oxidation of dihydronicotinamide adenine dinucleotide (NADH) on highly oriented pyrolytic graphite. Defined images of the passivated layer, with scattered protrusions of different sizes and shapes, are observed. The exact structure is strongly dependent on the anodization time, NADH concentration, or anodization potential, with higher surface coverages being observed for higher charges (passing during the anodization). The implications of these findings on the analytical/sensing utility of nicotinamine coenzymes are discussed.

Remarkably selective metallized-carbon amperometric biosensors

Abstract The use of metallized carbon transducers results in remarkably selective amperometric biosensors. Such focus on the transducer eliminates major electroactive interferences in the first place, and hence circumvents the need for anti-interference layers. The remarkable selectivity of metal-dispersed carbons is attributed to their strong, preferential, electrocatalytic action towards the detection of the enzymatically-liberated hydrogen peroxide and reduced nicotinamide cofactor (NADH). Such activity allows tuning of the operating potential to the region (+ 0.1 to −0.2 V) where unwanted reactions do not occur. This article reviews the development of metallized-carbon biosensors, with particular emphasis on metallized carbon-paste enzyme electrodes, miniaturized devices based on electrochemical deposition of the metal/enzyme layer and metal-dispersed sensor strips.

Electrocatalysis and amperometric detection of organic peroxides at modified carbon-paste electrodes

Cobalt-phthalocyanine modified carbon-paste electrodes are shown to catalyze the electro-oxidation of organic peroxides. Cyclic voltammetry offers useful insights into the catalytic behavior. Such behavior is exploited for developing an effective amperometric detection scheme for butanone peroxide, cumene hydroperoxide and tert-butyl hydroperoxide with optimum response at a potential of +0.70 V (vs. Ag/AgCl). Highly sensitive and stable flow injection measurements, with detection limits of 2.4-8.3 ng and relative standard deviations of 1.7-1.8% (n = 30), are reported. Applicability to measurements in drinking water is illustrated.

Computerized pipettes with programmable dispension

Abstract New computer controlled pipettes are used to establish several duspensuon patterns and thus to enhance considerably the power of batch injection analysis (BIA) The unique delivery functions offered by these pro- grammable pipettes permit advanced BIA operations with greater versatility and solution handling capability Particularly attractive are the abilities to induce the dispersion (via prolonged or reversed sample dispensions), to inject simultaneously two (air-segmented) sample! standard solutions, to inject accurately extremely small samples (down to 05 bol) at different rates, or to perform acidubase tutratuons Repetitive oscillations of the duspension greatly improve the contact of reactants, as illustrated for enzymatic reactions High precision accrues from the highly reproducible (operated-independent) piston movement Repetitive injections of 1 ~zl solutions are performed with a sample throughput of 300 per hour, with a relative standard deviation of 1 5% The effect of these novel dispensuon patterns on the BIA response are evaluated using potentiometric and amperometric detection schemes Prospects of using these pipettes in other automated analytical systems are discussed

Vegetable tissue from latania sp.: an extraordinary source of naturally immobilized enzymes for the detection of phenolic compounds

This paper describes the exciting characteristics of Latania sp. as a source of naturally immobilized polyphenol oxidase enzymes. Tissues from fruits of this palm tree present remarkably high enzymatic activity and this characteristic was explored for amperometric quantification of catechol and dopamine. These compounds were determined by the electrochemical reduction of their oxidation products on glassy carbon, after a biocatalytic conversion to quinone products. The quantification of these compounds was achieved in three different ways: by using an on-line tissue-based reactor associated with a flow injection analysis system; by incorporation of unripe tissue in carbon paste electrodes; and by exploring the use of dried tissue to construct reactors or modified carbon paste electrodes. This appears to be the first time that dry tissues have been utilized as an enzymatic source to construct enzymatic reactors, employed in the quantification of phenolic products. The last approach can address the unavailability of vegetable tissues (even out of the harvest season), and permits the dried material to be processed in a grinder, producing particles that can be selected accordingly to their size. Very high sensitivity, excellent reproducibility and long-term stability are some of the advantages obtained by using this tissue as a polyphenol oxidase source. Analogous measurements of phenol, based on the phenolase activity of Latania sp., are also reported.

Fast batch injection analysis of H2O2 using an array of Pt-modified gold microelectrodes obtained from split electronic chips

A fast and robust analytical method for amperometric determination of hydrogen peroxide (H(2)O(2)) based on batch injection analysis (BIA) on an array of gold microelectrodes modified with platinum is proposed. The gold microelectrode array (n=14) was obtained from electronic chips developed for surface mounted device technology (SMD), whose size offers advantages to adapt them in batch cells. The effect of the dispensing rate, volume injected, distance between the platinum microelectrodes and the pipette tip, as well as the volume of solution in the cell on the analytical response were evaluated. The method allows the H(2)O(2) amperometric determination in the concentration range from 0.8 μmolL(-1) to 100 μmolL(-1). The analytical frequency can attain 300 determinations per hour and the detection limit was estimated in 0.34 μmolL(-1) (3σ). The anodic current peaks obtained after a series of 23 successive injections of 50 μL of 25 μmolL(-1) H(2)O(2) showed an RSD<0.9%. To ensure the good selectivity to detect H(2)O(2), its determination was performed in a differential mode, with selective destruction of the H(2)O(2) with catalase in 10 mmolL(-1) phosphate buffer solution. Practical application of the analytical procedure involved H(2)O(2) determination in rainwater of São Paulo City. A comparison of the results obtained by the proposed amperometric method with another one which combines flow injection analysis (FIA) with spectrophotometric detection showed good agreement.

Electrodeposition of platinum and palladium particles into base-hydrolyzed cellulose acetate films. Electrocatalytic/permselective surface microstructures

Abstract The behavior and advantages of new surface coatings, prepared by electrodeposition of platinum and palladium microparticles into base-hydrolyzed cellulose acetate (CA) films on glassy carbon electrodes, are described. The multifunctional character of these microstructures arises from the coupling of the permselective properties of CA with the catalytic power of dispersed metal particles. Control of the film porosity, via base hydrolysis, affects the size and distribution of the metal particles, and hence offers tailoring of the catalytic response. In particular, access to the catalytic sites (based on solute size) is manipulated via change of the hydrolysis time. Various experimental variables affecting the catalytic and transport properties are explored using different techniques. Scanning electron microscopy offers useful insights into the distribution of the metal microparticles within the CA matrix. Amperometric detection for flow injection and liquid chromatography systems is greatly improved by the multifunctional character of the metal/CA coatings.

Unveiling the Structure of Polytetraruthenated Nickel Porphyrin by Raman Spectroelectrochemistry

The structure of polytetraruthenated nickel porphyrin was unveiled for the first time by electrochemistry, Raman spectroelectrochemistry, and a hydroxyl radical trapping assay. The electrocatalytic active material, precipitated on the electrode surface after successive cycling of [NiTPyP{Ru(bipy)2Cl}4](4+) species in strong aqueous alkaline solution (pH 13), was found to be a peroxo-bridged coordination polymer. The electropolymerization process involves hydroxyl radicals (as confirmed by the characteristic set of DMPO/(•)OH adduct EPR peaks) as reaction intermediates, electrocatalytically generated in the 0.80-1.10 V range, that induce the formation of Ni-O-O-Ni coordination polymers, as evidenced by Raman spectroelectrochemistry and molecular modeling studies. The film growth is halted above 1.10 V due to the formation of oxygen gas bubbles.

Batch Injection Spectroscopy

Abstract A novel non-flow approach for high-speed optical assays of microliter sample volumes is described. The method is based on the passage of well-defined sample zones, emerging from a computerized pipette, through a radiation path. Sharp absorbance transient peaks accrue from the reproducible travel pattern of the intact samples. Measurements are performed within a modified sample compartment of a conventional spectrophotometer. Repetitive optical measurements at very high rates of 360–1800 samples per hour yield relative standard deviations of 0.3–0.7% with no observable carry over. Such performance characteristics, coupled with the simplicity and low cost, hold a great promise for routine automated assays.