Amperometric detection of benzoyl peroxide in pharmaceutical preparations using carbon paste electrodes with peroxidases naturally immobilized on coconut fibers.

This paper describes the applications of a new carbon paste electrode containing fibers of coconut (Cocus nucifera L.) fruit, which are very rich in peroxidase enzymes naturally immobilized on its structure. The new sensor was applied for the amperometric quantification of benzoyl peroxide in facial creams and dermatological shampoos. The amperometric measurements were performed in 0.1 mol L(-1) phosphate buffer (pH 5.2), at 0.0 V (versus Ag/AgCl). On these conditions, benzoyl peroxide was rapidly determined in the 5.0-55 micromol L(-1), with a detection limit of 2.5 micromol L(-1) (s/n=3), response time of 4.1 s (90% of the steady state) and sensitivity limit of 0.33 A mol L(-1) cm(-2). The amperometric results are in good agreement with those obtained by spectrophotometric technique, used as a standard method.

Biosensing hydrogen peroxide utilizing carbon paste electrodes containing peroxidases naturally immobilized on coconut (Cocus nucifera L.) fibers.

A novel unmediated hydrogen peroxide biosensor based on the incorporation of fibrous tissue of coconut fruit in carbon paste matrix is presented. Cyclic voltammetry and amperometry were utilized to characterize the main electrochemical parameters and the performance of this new biosensor under different preparation and operation conditions. The resulting H2O2-sensitive biosensors respond rapidly (7 s to attain 90% of the signal), was operated at -0.15 V, presented linear response between 2.0x10(-4) and 3.4x10(-3) mol L(-1), the detection limit was estimated as 4.0x10(-5) mol L(-1). Its operation potential was situated between -0.2 and 0.1 V and the best pH was determined as 5.2. Electrodes containing 5% (w/w) of coconut fiber presented the best signal and their lifetime was extended to 3 months. The apparent Michaelis-Menten constant KM(app) and Vmax were estimated to be 8.90 mmol L(-1) and 6.92 mmol L(-1) microA(-1), respectively. The results obtained for determination of hydrogen peroxide in four pharmaceutical products (antiseptic solution, contact lenses cleaning solution, hair coloring cream and antiseptic dental rinse solution) were in agreement with those obtained by the spectrophotometric method. An additional advantage of these biosensors is the capacity to measure hydrogen peroxide even in samples with relatively low pH. To demonstrate the enzymatic activity of the coconut tissue, a very simple way was created during this work. Coconut fibers were immersed in H2O2 solution between two glass slides. Sequential images were taken to show the rapid generation of O2, attesting the high activity of the enzymes.

Amperometric determination of dipyrone in pharmaceutical formulations with a flow cell containing gold electrodes from recordable compact discs.

A simple, rapid, and precise amperometric method for quantification of dipyrone in pharmaceutical formulations is presented. The proposed method permits determinations in the 10(-7) mol L(-1) of the analyte and enables 90 determinations h(-1), employing only 100 microL of sample per determination. This method is based on the direct quantification of dipyrone in many pharmaceutical products, avoiding cumbersome processes such as previous separations, solvent extraction, or sample filtration. This new procedure was applied to commercial pharmaceutical tablets, and the results obtained were in excellent agreement with the ones obtained by the classical iodometric method.

Gold electrodes from compact discs modified with platinum for amperometric determination of ascorbic acid in pharmaceutical formulations.

A simple, rapid and precise amperometric method has been developed for quantification of ascorbic acid (AA) in pharmaceutical formulations using flow-injection analysis (FIA). A slice of recordable compact disc (CD) modified by electrodeposition of platinum was employed as the working electrode. The proposed flow system allows determinations in the 1 mumol l(-1) of the analyte and enables 90 determinations per h, employing only 150-mul sample. The method permits the direct quantification of ascorbic acid in many pharmaceutical products, avoiding cumbersome processes as previous separations, solvent extraction or sample filtration. This new procedure was applied to commercial pharmaceutical tablets and the results obtained were identical than the ones obtained by the classical iodometric method. The calibration plots for freshly prepared ascorbic acid standards were highly linear in the concentration range of 1-10 mumol l(-1) with a relative standard deviation (R.S.D.) <1%. For all real samples studied, the deviations were situated between 0.5 and 8.7%.

Gold electrodes from recordable CDs

Gold electrodes are widely used in electrochemistry and electroanalytical chemistry. The notable performance when used in stripping analysis of many ionic species and the extraordinary affinity of thio compounds for its surface make these electrodes very suitable for many applications. This paper reports a simple and novel way to construct gold electrodes (CDtrodes) using recordable CDs as the gold source. The nanometer thickness of the gold layer of recordable disks (50-100 nm) favors the construction of band nanoelectrodes with areas as small as 10(-6) cm2. The plane surface can be easily used for the construction of conventional-sized gold electrodes for batch or flow injection analysis or even to obtain electrodes as large as 100 cm2. The low price of commercial recordable CDs allows a "one way use". The evaluation and applicability of these electrodes in the form of nanoelectrodes, in batch and associated with flow cells, are illustrated in this paper.

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.

Gold electrodes from recordable CDs for mercury quantification by flow injection analysis.

The development of a new methodology for the construction of very efficient flow cells for mercury detection by potentiometric stripping analysis, employing the thin gold layer of recordable CDs as working electrode is reported. This new source of electrodes (CDtrodes) show very attractive performance, similar to that obtained with commercial gold electrodes, with superior versatility. The low cost of this new source of "gold electrodes" allows a frequent replacement of the electrode, avoiding cumbersome clean-up treatments. Various experimental parameters have been optimized to yield low detection limits (0.25 ng/mL of mercury for 5 min deposition at 0.3 V) and good precision (standard deviation of 1.9% was obtained for 15 repetitive measurements using 10 ng/mL of mercury). Standard curves were found to be linear over the range of 0.5-100 microg L(-1) of mercury. The flow cells developed were used for the quantification of mercury in oceanic and tap water.

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.

Measurement of bradykinin, protein Na+, CL-, K+, and Zn+ concentrations in rat urine after intraperitoneal injection of Tityus serrulatus venom

Experiments were carried out in vivo by injecting Tityus serrulatus crude venom in rats followed by biological assays on the isolated guinea-pig ileum, to show the effects of scorpion venom on kallikrein-kinin system. Our results showed effects such as significant decrease of total kinin rate and a decrease of total kinin rate and a decrease of Zn++, Na+, Cl- and K+ ions in rat urine 24 and 48 hours after the injection of Tityus serrulatus crude venom.

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.

Analytical data: Potentiometric study of the association constant of isothiocyanic acid.

The association constant of isothiocyanic acid, HNCS, has been determined potentiometrically and found to be 0.135 +/- 0.007 and 0.168 +/- 0.007 at ionic strengths (NaClO(4)) of 2.0 and 4.0M respectively. The indicator electrode was Hg(SCN)(2-)(4)/Hg. For accuracy and precision the metal phase was used in the form of a dropping electrode. Because the association constant is so low, liquid-junction potentials would cause significant errors, since a strong acid was added to the working solutions in the procedure, and an estimate of this effect was taken into account.