Valberes B. Nascimento

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.

Coconut-based plant tissue reactor for biosensing of catechol in flow injection analysis

Abstract A novel plant tissue biosensor is proposed in this paper. Catechol is determined by the reduction of its oxidation product on glassy carbon electrode after a biocatalytic conversion to quinone in an on-line coconut-based reactor by flow injection analysis. The remarkably high polyphenol oxidase activity of the tissue is successfully associated to its high stability and physical feasibility to manufacture the bioreactor. The sensor retains its enzyme activity for at least one week. Catechol can be determined at micromolar range in a rate of up to 60–90 samples per hour with good precision (RSD 2%). Optimization of the experimental parameters including bioreactor and cell design is addressed. An interference study is also included showing the limitations and other possibilities of the use for this plant material. Applicability to river water analysis and waste water is illustrated. Such use of biocatalytic material apart from the electrochemical detector still holds great advantages for easy screening of new material sources for biosensing applications.

Electroanalytical determination of carbendazim by square wave adsorptive stripping voltammetry with a multiwalled carbon nanotubes modified electrode

A preconcentrating/voltammetric multiwalled carbon nanotube modified glassy carbon electrode (MWCNT–GCE) has been developed for stripping analysis of carbendazim (Methyl Benzimidazol-2-yl Carbamate—MBC), based on dispersing MWCNT in water. The effect of experimental variables, such as the dispersion and loading of MWCNT, was assessed. A quasi-reversible behavior for MBC in acetic acid/acetate buffer 0.1 mol L−1 (pH 4.7) was verified and its high effective pre-concentration was attributed to the high adsorption capability and enormous surface area of the MWCNT. No evidence of carry-over effect, combined with the easiness of electrode preparation, led to the development of a highly sensitive and reliable method with an experimental work range from 0.256 to 3.11 µmol L−1 with a detection limit of 10.5 ppb for a short (60 s) accumulation period. Measurement of MBC in a river water sample was demonstrated. The accuracy of the method for real sample analysis was assessed by estimating the apparent recovery (93 ± 2.9% and 86 ± 4.1% for 4.3 × 10−7 mol L−1) for a MBC spiked river water sample.

Automatic determination of chlorine without standard solutions using a biamperometric flow-batch analysis system

This study presents an automatic analysis system that does not require the use of standard solutions. The system uses an electrochemical flow cell for in line generation of the standards, and operates under the standard addition technique. The versatility of this system was demonstrated by the development of a one key touch fully automatic method for the determination of total available chlorine in real samples. The extremely simple, accurate and inexpensive method was based simply on the biamperometric monitoring of the well known redox reaction of chlorine with iodide ions in a flow-batch system, where the produced iodine (triiodide ions) generates an electrical current proportional to the chlorine concentration in the sample. The flow-batch parameters were optimized to maximize the sensitivity without losses on the precision of the analysis. An excellent linear dependence between the biamperometric signal and the chlorine concentration for the standard additions and a good agreement between the proposed approach and a reference method were obtained. The method was successfully applied to determine chlorine in several different bleach and chlorinated water samples (r=0.9995, LOD=8.261 x 10(-7) mol L(-1)) and could be easily extended to other oxidants and samples. Comparison to a reference method and recoveries close to 100% demonstrated the reliability of the proposed method. In addition, low residue disposal and reagent consumption, allied with high accuracy and precision, make it very promising for routine applications.

Flow-injection electrochemical determination of citric acid using a cobalt(II)–phthalocyanine modified carbon paste electrode

The electrocatalytic activity of cobalt(II)-phthalocyanine immobilized in a carbon paste electrode for citric acid oxidation under a diffusion-controlled electrochemical process was employed to develop a selective flow injection amperomettric analysis method. Under optimized conditions, rapid and reliable determinations of citric acid (65 samples per hour) were achieved for analyses of commercial fruit juices with minor pretreatment of the samples. A linear response was accomplished ranging from 0.25 to 15 mmol L(-1) (r(2)=0.9979) with an estimated detection limit of 0.117 mmol L(-1). Alternatively, a very sensitive and selective assay was developed for the micromolar range, obtaining a linear response ranging from 3.81 to 91.2 μmol L(-1) (r(2)=0.9993) with an estimated detection limit of 2.54 μmol L(-1), by using square wave voltammetric methodology. The speed and simplicity of the proposed approaches compares advantageously to the all other citric acid determination methods.

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.

Flow-through Cell Based on an Array of Gold Microelectrodes Obtained From Modified Integrated Circuit Chips

The construction of a flow-through cell incorporating an array of gold microelectrodes is described and its application to flow injection analysis with amperometric detection is presented. Simple modification of almost any conventional integrated circuit chip, used as an inexpensive source of pre-assembled gold micro-wires, leads to the rapid and successful preparation of arrays of 8–48 elements. The polymeric encapsulation material from the top face of the chip is removed by abrasion until the gold micro-wires (used to interconnect the silicon circuit to the external contact pins of the chip) are disrupted and their transversal (elliptical) sections become exposed. Once polished, the flat and smooth top surface of the gold microelectrode-array chip (MEAC) is provided with a spacer and fitted under pressure against an acrylic block with the reference and auxiliary electrodes, to form the electrochemical (thin-layer) flow cell, while the contact pins are plugged into a standard IC socket. This design ensures autonomous electric contact with each electrode and allows fast dismantling for polishing or substitution. The performance of flow cells with MEACs was investigated utilizing the technique of reverse pulse amperometry without oxygen removal. A method was established for the determination of the copper concentration in sugar cane spirit, regulated by law for beverages. Samples from industrial producers and small-scale (alembic) brewers were compared. With a 24 MEAC, a detection limit of 30 µg l -1 of copper (4.7 × 10 -7 mol l -1 of Cu II for 100 µl injections) was calculated. Routine operation was established at a frequency of 60–90 determinations per hour. Intercomparison with atomic absorption spectrometric determinations resulted in excellent agreement.

Biamperometric Determination of Tetracycline in Pharmaceuticals

Abstract A fast, reliable, and low cost biamperometric flow‐injection method, with an error of 1.3% and an analytical throughput of 55 samples h−1, for determination of tetracycline hydrochloride in pharmaceuticals capsules is proposed. The analytical curve was linear (r=0.998) in the range 10 to 50 mg l−1 using Fe(CN)6 3− and NaOH solutions as reagent and carrier stream/supporting electrolyte, respectively. A relative standard deviation of 1.6% (10 sequential injections of 30.0 mg l−1) was verified with detection and quantification limits of 0.6 and 3.4 mg l−1, respectively.

In-line electrochemical reagent generation coupled to a flow injection biamperometric system for the determination of sulfite in beverage samples

This work reports an in-line electrochemical reagent generation coupled to a flow injection biamperometric procedure for the determination of SO3(2-). The method was based on a redox reaction between the I3(-) and SO3(2-) ions, after the diffusion of SO2 through a gas diffusion chamber. Under optimum experimental conditions, a linear response ranging from 1.0 to 12.0 mg L(-1) (R=0.9999 and n=7), a detection and quantification limit estimated at 0.26 and 0.86 mg L(-1), respectively, a standard deviation relative of 0.4% (n=10) for a reference solution of 4.0 mg L(-1) SO3(2-) and sampling throughput for 40 determinations per hour were achieved. Addition and recovery tests with juice and wine samples were performed resulting in a range between 92% and 110%. There were no significant differences at a 95% confidence level in the analysis of eight samples when comparing the new method with a reference procedure.

Flow Injection Determination of Metronidazole through Spectrophotometric Measurement of the Nitrite Ion Produced upon Alkaline Hydrolysis

A new method for metronidazole determination, based on spectrometric monitoring of a diazonium salt produced in-line by alkaline hydrolysis released nitrite ions, was developed and successfully applied to pharmaceutical tablets (r = 0.9993, 2.0-20.0 mg L-1, DL = 0.7 mg L-1) with no interference from common ingredients accompanying the drug.