Demonstra-se, pela primeira vez usando experimentos simples de voltametria cíclica, que a pré-polarização anódica de eletrodos de diamante dopado com boro (DDB) conduz a superfícies parcialmente bloqueadas. Eletrodos de DDB anodicamente pré-polarizados, com diferentes níveis de dopagem de boro, apresentaram respostas de voltametria cíclica sigmoidais para o par redox ferro/ferricianeto a baixas velocidades de varredura (0,5 mV s-1). Por outro lado, quando os mesmos eletrodos são pré-polarizados catodicamente, observaram-se respostas quase-reversíveis e reversíveis na mesma velocidade de varredura, correspondendo a superfícies eletroquimicamente heterogêneas. Assim, as superfícies de DDB podem atuar parcialmente bloqueadas ou com eletroatividade heterogênea, dependendo da polarização, anódica ou catódica, respectivamente.

It is demonstrated, for the first time using simple cyclic voltammetry experiments, that the anodic pre-polarization of boron-doped diamond electrodes (BDD) leads to partially blocked surfaces. Anodically pre-polarized BDD electrodes, with different boron-doping levels, presented sigmoidal cyclic voltammetry profiles to the ferro/ferri-cyanide redox couple under low scan rate (0.5 mV s-1). On the other hand, when the same electrodes are cathodically pre-polarized, quasi-reversible and reversible responses were observed under the same scan rate, corresponding to surfaces with heterogeneous electroactivity. Thus, BDD surfaces can act as partially blocked or with heterogeneous electroactivity, depending on the polarization, anodic or cathodic, respectively.

The potentiality of the use of ultrasound radiation in association with a boron-doped diamond electrode was evaluated on the voltammetric determination of the pesticide carbaryl. Improvements in the sensitivity, limit of detection and reproducibility of the measurements were observed due to both, the enhancement of mass transport and the cleaning of the electrode surface provided by ultrasound. Satisfactory recovery levels for carbaryl in pure water (96-98%) and pineapple juice (89-92%) for quiescent and sonovoltammetric methodologies were obtained. These methodologies can be alternative tools for the analyses of pesticides in fruit samples, mainly the insonated condition that improve the analytical performance and dispense intermediary cleanings of the electrode surface.

The beneficial effects of the ultrasound (US) like the cleaning of electrode surface and enhancement of mass transport were evaluated in association with potentiostatic electrolyses for the degradation of pentachlorophenol (PCP) at 3.0 V vs. Ag/AgCl, using a boron-doped diamond (BDD) electrode during 270 minutes. Different decay levels of the PCP spectrum bands in 220, 251 and 321 nm, respectively, were observed after application of ultrasound without electrochemical process (18.1, 17.7 and 19.8 %), silent electrolyses (29.3, 71.6 and 70.8 %), pulsed sonoelectrolysis (31.0, 75.1 and 76.3%) and sonoelectrolyses (39.2, 80.0 and 82.6 %). For silent and sonoelectrolyses processes, cleaning/reactivation of the BDD surface by acetonitrile and/or electrochemical treatment was necessary. The pulsed sonolectrolysis were carried out purposely without cleaning/reactivation of the surface. The results showed greater PCP degradation for insonated studies than those obtained for the silent electrolyses, due to the increase of mass transport, minimization of the electrode fouling and the combined generation of hydroxyl radicals by both ultrasound and the polarized BDD surface. These tools (US and BDD), especially the pulsed sonoelectrolysis, can improve the degradation of pesticides and their metabolites in the environment and enable the use of sonoelectrochemistry for wastewater remediation.

This work describes a comparative study of the electrocrystallization of Ni and Ni-P on Pt ultramicroelectrodes using chronoamperometric measurements. It was possible to confirm that in all cases a progressive nucleation was the predominant mechanism. Moreover, the application of the Atomistic Theory to the experimental rate of nuclei formation showed that the number of atoms in the critical nucleus was zero, except for Ni-P on Pt at low overpotentials were a value of one was observed. Furthermore, the physical characterisation of the different deposits on Pt by atomic force microscopy allowed observing the coalescence of the hemispherical nuclei of Ni and Ni-P at t max thus confirming the results obtained from the current-time analysis.

The aim of this review is to present and discuss the applications of ultrasound in electrochemical systems such as in sonoelectroanalysis and sonoelectrolysis for the electrochemical combustion of organic compounds. Initially, theoretical and experimental aspects are discussed, particularly those related to the enhancement of mass transport and the surface cleaning effects. Some results are included to illustrate alternative geometries for the experimental measurements and the working electrodes used in these systems. In the sequence, the available publications are presented and discussed to demonstrate that ultrasound combined with electrochemical techniques is a powerful set-up for the detection of analytes such as metals and/or organic compounds in hostile media and for the effective destruction of toxic organic substances. At the end, a table summarizes the results already published in the literature.

Fruits juices are natural sources of several compounds that present antioxidant action. Together with the fruits, they contribute with almost 40% of the antioxidant capacity in a healthy diet avoiding and preventing diseases deriving from oxidative stress. The present study determined the antioxidant capacity of seven samples of industrialized fruits juices applying CRAC (Ceric Reducing/Antioxidant Capacity) assay, a new electrochemistry assay that evaluates, by means of chronoamperometric measurements, the ability of a sample in reducing species Ce4+ in acid media. At the end of the assay was obtained the following classification: cashew > guava > grape > mango > apple > orange > passion fruit.

A determinação sonovoltamétrica do 4-nitrofenol foi estudada sobre um eletrodo de diamante dopado com boro. Foram observadas melhorias significativas na sensibilidade analítica devido à limpeza da superfície eletródica e ao aumento no transporte de espécies ao eletrodo proporcionados pelo ultra-som. Desta maneira, para os processos de oxidação e redução, os limites de detecção diminuíram de 11.7 para 3.87 e de 6.38 para 2.57 µg L-1, respectivamente.

The sonovoltammetric determination of 4-nitrophenol was studied on a boron-doped diamond electrode. Significant improvements in the analytical sensibility were observed due to electrode surface cleaning and the enhancement in the transport of species to the electrode surface provided by ultrasound. Thus, for the oxidation and reduction process, the limit of detection diminished from 11.7 to 3.87 and from 6.38 to 2.57 µg L-1, respectively.

This work describes the development of electroanalytical methodologies for the determination of atrazine, ametrine and simazine by square wave voltammetry on a mercury electrode. For pure atrazine and pure ametrine, the detection limits (DL) were 3.7 and 4.3 µg L-1, respectively, while they increased to 4.8 and 6.5 µg L-1 in the presence of 3.0 x 10-6 mol L-1 of the other component (a mathematical deconvolution program was used in the mixture cases). The voltammetric response for simazine could not be separated from that of atrazine and measurements were carried out only in pure simazine solutions (DL: 7.5 µg L-1).

RESUMO Com o objetivo de aprofundar investigações anteriores, foram realizadas modificações superficiais diretas em eletrodos de diamante dopados com boro (DDB), com óxidos metálicos (PtOx, RuO2, IrO2 e PbO2) e com alguns compósitos mistos, utilizando a técnica Sol-Gel. Estes materiais foram estudados por microscopia de força atômica (MFA) e os resultados indicaram a formação de sítios de deposição heterogênea. Técnicas eletroquímicas foram utilizadas para estabelecer a atividade catalítica para a reação de desprendimento de oxigênio (RDO) e para reação de oxidação de etanol em meio ácido utilizando os compósitos de PtOx e PtOx-RuO2. Testou-se ainda a estabilidade de um depósito de PtOx, recoberto por um filme de Nafion®, por meio de operação contínua de longa duração. Estudos de voltametria cíclica comparativa mostraram que a área superficial ativa dos novos materiais mudou consideravelmente com o método de modificação proposto. O eletrodo de IrO2/BDD mostrou o melhor desempenho para a RDO, iniciando o processo de oxidação em ~ 1.4 V vs. HESS, valor este 200 mV menor do que o observado sobre um eletrodo de PtOx/BDD. Foi observada uma pequena perda da atividade do eletrodo de PtOx/BDD (6%) depois de realizada eletrólise de 4 horas, enquanto que 1000 ciclos voltamétricos deixaram a superfície praticamente inalterada. Os estudos realizados mostram que as atividades satisfatórias dos eletrodos preparados pelo método Sol-Gel podem constituir ferramentas interessantes de pesquisa, visando principalmente a utilização destes materiais em aplicações práticas.

Continuing previous investigations, direct surface modifications of boron-doped diamond (BDD) electrodes with metal oxides (PtOx, RuO2, IrO2 and PbO2) and with some mixed composites were carried out by the Sol-Gel technique. The materials were studied by atomic force microscopy (AFM) to determine their surface topologies and by electrochemical techniques to establish the catalytic activity towards the oxygen evolution reaction (OER) and also, for the PtOx and PtOx-RuO2 composites, the ethanol oxidation reactions in acid media. The stability of PtOx coating covered by a Nafion® film was also tested by long-term operation. The AFM results indicated sites of heterogeneous deposition and the electrochemical studies demonstrated that the active surface area changed considerably with the proposed method of modification. The IrO2/BDD electrode showed the best performance to the OER with the onset of the oxidation current at ~1.4 V, a value 200 mV lower than for the PtOx/BDD electrode. The enhanced stability of PtOx/BDD electrodes achieved by the application of a Nafion® film and already reported in acid media was further proved using the ethanol oxidation reaction. Only a small loss of activity (6%) was observed after 4-hours electrolysis while one-thousand voltammetric cycles left the surface practically unchanged. In addition, preliminary studies for the same reaction on PtOx/BDD and PtOx-RuO2/BDD electrodes demonstrated the excellent activity of these mixed Sol-Gel coatings on the BDD surface and the possibility of further investigations for practical applications.

The aim of this work is to discuss selected applications of electroanalytical techniques for the detection of pesticides in foods and beverages, published in the last ten years. The applications involved different working electrodes for the electroanalytical determination of pesticides, namely amperometric biosensors, cholinesterase-based biosensors, polymer-modified electrodes, ultramicroelectrodes and hanging mercury drop electrodes. They were used for several voltammetric and amperometric techniques in different analytical procedures for the detection and quantification of different classes of pesticides in different food matrices.

The aim of this work is to discuss some selected applications of square wave voltammetry published in the last five years. The applications focused here cover several electroanalytical fields such as: determination of pesticides; molecules with biological activity; metals and other environmental pollutants. Special attention is given to the work developed in the Grupo de Materiais Eletroquímicos e Métodos Eletroanalíticos - IQSC - USP concerning the utilization of square wave voltammetry, with different kinds of electrodes, for the determination of pesticides in natural waters and active principles in pharmaceutical formulations. The new methodology is simple, fast and sensitive when compared with the traditional ones such as chromatography and spectrophotometry. The satisfactory results obtained provide alternative procedures for the quality control of drugs and the monitoring of pesticides in natural environments.

As reações eletródicas que ocorrem em ambos extremos da extensa janela eletroquímica do eletrodo de diamante dopado com boro foram analisadas por curvas de polarização visando determinar as respectivas energias de ativação aparentes para a etapa determinante da velocidade em cada caso. Encontrou-se que a reação de desprendimento de hidrogênio ocorre por um mecanismo Volmer-Heyrovsky, com a primeira etapa sendo a determinante da velocidade. A energia de ativação aparente foi calculada como sendo 150 kJ mol-1, indicando a formação de um intermediário do tipo M-H, típico da etapa de Volmer. Por outro lado, a energia de ativação aparente para a reação de desprendimento de oxigênio foi encontrada como sendo 106 kJ mol-1, o que também indica a etapa de adsorção do mecanismo como a determinante da velocidade. Desta forma, demonstrou-se que a interação entre as moléculas de água e a superfície do eletrodo de diamante é fortemente inibida, o que explica a extensa janela de potenciais observada para este eletrodo.

The electrochemical processes occurring at both edges of the wide electrochemical window of the boron doped diamond (BDD) electrode were studied by polarization curves experiments to evaluate the apparent energy of activation for the rate determining step in each reaction. It was found that the hydrogen evolution reaction occurs by a Volmer-Heyrovsky mechanism with the first step being the RDS. Moreover, the apparent energy of activation calculated from the Tafel plots presented a value as high as 150 kJ mol-1, indicating the formation of the M-H intermediate that is characteristic for the Volmer step. On the other hand, the apparent energy of activation for the oxygen evolution reaction was found to be 106 kJ mol-1 suggesting that the RDS in this mechanism is the initial adsorption step. In this way, it was demonstrated that the interaction between water molecules and the electrode surface is strongly inhibited on BDD thus justifying the extended potential window observed for this material.

The anodic voltammetric behavior of 4-chlorophenol (4-CF) in aqueous solution has been studied on a Boron-doped diamond electrode using square wave voltammetry (SWV). After optimization of the experimental conditions, 4-CF was analyzed in pure and natural waters using a Britton-Robinson buffer with pH = 6.0 as the supporting electrolyte. Oxidation occurs at 0.80 V vs Ag/AgCl in a two-electron process controlled by adsorption of the species. The detection limits obtained were 6.4 µg L-1 in pure water and 21.5 µg L-1 for polluted water taken from a local creek, respectively. The combination of square wave voltammetry and diamond electrodes is an interesting and desirable alternative for analytical determinations.

O comportamento voltamétrico do 4-nitrofenol foi estudado em meio aquoso sobre o eletrodo de diamante dopado com boro usando a voltametria de onda quadrada. Após a otimização dos parâmetros experimentais, o estudo foi realizado em tampão Britton-Robinson com pH 6,0. Nestas condições foi observado somente um pico de oxidação em torno de 1,0 V vs. Ag/AgCl com transferência de dois elétrons e um processo controlado pela adsorção das espécies na superfície do eletrodo. O limite de detecção (LD) obtido foi de 2,8 mig L-1. Este resultado foi comparado com o obtido nas mesmas condições experimentais mas utilizando o pico de redução do 4-nitrofenol em torno de -0,8 V vs Ag/AgCl (LD = 4,2 mig L-1). Os dois valores obtidos para o LD se encontram dentro do limite exigido pela legislação para água potável (30 mig L-1). A combinação da voltametria de onda quadrada com o eletrodo de diamante dopado com boro mostrou ser uma excelente e desejável alternativa para a determinação analítica de compostos orgânicos.

The anodic voltammetric behavior of 4-Nitrophenol on a Boron-doped diamond electrode in aqueous solution has been studied using square wave voltammetry. After optimization of the experimental conditions, that model molecule was analyzed in pure water solutions using a Britton-Robinson buffer with pH 6.0 as the supporting electrolyte. Oxidation occurs at 1.0 V vs. Ag/AgCl in an irreversible two-electron process controlled by adsorption of the species. The detection limit (DL) obtained was 2.8 mug L-1. This result was comparable to that obtained from reduction of the molecule at -0.8 V vs. Ag/AgCl under the same experimental conditions (DL = 4.2 mug L-1). Both DL values are within the limits required by the legislation for drinking water (30 mug L-1). The combination of square wave voltammetry and diamond electrodes has proved to be an interesting and desirable alternative for the analytical determination of organic molecules.

The theoretical aspects of square wave voltammetry were discussed. Reversible, irreversible and quase-reversible electrode reactions were analyzed and the correlations between parameters like frequency, period, square wave potential and amplitude were showed. In this way, diagnostic relationships allow to characterize the electrode process. The analytical applications were discussed in base of the increment in the analytical response (current) due to the characteristics of the developed equations and the unique mode of collecting the electrode response, i.e., the direct and reverse signals. Finally, recent advances in the basic theory, as the applications to the hydrodynamic electrode and the ultramicroelectrode were also analyzed, and the multiple pulses square wave voltammetry was also introduced.