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.

Voltammetric behavior of benznidazole at a DNA-electrochemical biosensor.

Benznidazole is a drug used commonly as a therapeutic agent against Chagas' disease in Brazil. To clarify the cytotoxic action of benznidazole the electrochemical reduction of benznidazole has been investigated using a DNA-electrochemical biosensor, prepared by modification of a glassy carbon electrode with DNA, and the results compared with reduction at a bare glassy carbon electrode. The dependence of peak potential with pH follows slopes of 59 and 52 mV per pH unit in acid media, respectively, which corresponds to a mechanism involving the same number of electrons and protons. In neutral and alkaline solution no significant dependence of peak potential with pH was found. During the electrochemical reduction of benznidazole the formation of the hydroxylamine derivative occurs, involving a total of four electrons. The potentials for reduction were less negative when using the DNA-modified glassy carbon electrode than at the bare glassy carbon electrode although the mechanism was the same, and at pH 7.51 the peak current was four times higher than that obtained with the bare electrode. The DNA-biosensor enabled pre-concentration of the drug onto the electrode surface and the in situ damage caused to the DNA on the electrode surface by the product of benznidazole reduction could be detected electrochemically. The results are in agreement with the hypothesis that the hydroxylamine derivative is the reactive species responsible for the cytotoxic action of benznidazole.