The well-known electrochemical probe Fe(CN)63-/Fe(CN)64- is widely used for estimating the electrochemically active area of electrodes modified with carbon nanotubes, conductive polymers, enzymes, etc. In this study, we used the platinum electrode, smooth or platinized with different roughness factors, to demonstrate that such a redox couple fails to respond to a surface roughness variation. We determined the roughness factors of the Pt surfaces by atomic force microscopy (AFM) images, which yielded values between 2.72 and 25.91. Almost the same values were found by using the charge of the hydrogen monolayer desorption obtained from steady-state cyclic voltammetry experiments performed in an acid medium. They were then compared with those provided by peak current in voltammetry or chronoamperometry with Fe(CN)63-/Fe(CN)64- which all yielded values nearly to one. Such comparison demonstrates that the electrochemical behavior of the redox probe is an outer sphere reaction with a quite small interaction with the electrode surface, thus not being suitable to be related with active areas.

Um novo sensor baseado em nanotubos de carbono de paredes múltiplas modificado com bastões de SnO2 foi desenvolvido para a determinação eletroquímica de levofloxacino. A morfologia, a estrutura, e o comportamento eletroquímico do eletrodo compósito foram caracterizados por microscopia eletrônica de varredura, energia dispersiva de raios X e voltametria cíclica, respectivamente. Voltametria de pulso diferencial em solução tampão fosfato pH 6,0 permitiu a aplicação de um método para determinar níveis de levofloxacino em um intervalo de 1,0-9,9 µmol L-1, com limite de detecção calculado em 0,2 µmol L-1 (72,0 mg L-1).

A new sensor based on multi-walled carbon nanotubes modified with SnO2 rods for the electrochemical determination of levofloxacin has been investigated. The morphology, the structure, and the electrochemical performance of the composite electrode were characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, and cyclic voltammetry, respectively. Differential pulse voltammetry in phosphate buffer solution at pH 6.0, allowed the application of a method to determine levofloxacin levels in a range of 1.0-9.9 µmol L-1, with a limit of detection calculated at 0.2 µmol L-1 (72.0 mg L-1).

A afinidade de dois pesticidas muito utilizados com diversas especificidades, isto é, o fungicida Thiram e o herbicida Picloram, por íons chumbo foi estudado por medidas eletroquímicas. O perfil voltamétrico de redissolução anódica para o íon metálico estudado foi significativamente afetado pela natureza química dos pesticidas (ligantes) e uma nova interpretação foi feita sobre a natureza, força e possível estequiometria dos complexos formados. Thiram causou um pronunciado efeito no sinal de redissolução do Pb. Algoritmos de linearização dos dados de titulação permitiram calcular a constante de estabilidade condicional para o Thiram (log K'= 6,8) e Picloram (log K' = 6,0). Foi encontrado que ambos os pesticidas contribuíram para a formação dos respectivos complexos 1:1 com Pb em uma alta porcentagem (aproximadamente 83%).

The affinity of two widely used pesticides of diverse specificities, i.e., the fungicide Thiram and the herbicide Picloram, for lead ions has been studied by an electrochemical approach. The anodic stripping voltammetric profile of the studied metallic ion was significantly affected by the chemical nature of pesticide (ligand) used and new light has been shed on the nature, strength and possible stoichiometry of formed complexes. Thiram caused a pronounced effect on the stripping signals of Pb. Linearization algorithms of titration data with the metal allowed the conditional stability constant to be calculated for Thiram (log K' = 6.8) and Picloram (log K' = 6.0). It was found that both pesticides contribute to the formation of the respective 1:1 complexes with Pb in a high percentage (ca. 83%).

This paper reports the development of multiple square wave voltammetry and the possibilities of its use for electroanalytical determinations of organic and inorganic compounds with the improvement of the signal-to-noise ratios and detection limits 2-3 orders of magnitude lower than those obtained with conventional square wave voltammetry. The theoretical aspects and analytical applications were demonstrated as an increased analytical response (current) and application of different pulse modes for different redox processes. Preliminary results obtained for several redox systems using different electrode surfaces are shown, demonstrating also that MSWV represents an excellent alternative for the determination of ultra-traces of organic and inorganic compounds.

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.

This work reports the use of square wave voltammetry (SWV) to analyse the electrochemical reduction of dichlorvos (2, 2-dichlorovinyl-dimethylphosphate) in spiked pure and natural waters. SWV measurements were carried out in 0.5 mol L-1 Na2SO4 aqueous solutions at pH 5, prepared with water originated from three different sources, namely, one sample of purified water and others from two urban creeks in São Carlos County. In all cases, two reduction peaks were observed, at potentials of -0.15 and -1.05 V vs Ag/AgCl, with both current and potential being dependent on pesticide concentration. This allowed the calculation of the following detection limits: 1.0, 2.5 and 3.0x10-8 mol L-1 for purified, Gregorio creek and Monjolinho creek waters, respectively, in a working range between 2.0x10-7 and 1.4x10-6 mol L-1. Recovery measurements found values higher than 80% in all cases, for an added concentration of 4.0 x 10-7 mol L-1 of dichlorvos in each solution. All analytical experiments were performed in triplicate and showed a standard deviation always less than 3%.

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.

The electrochemical behavior of paraquat on Pt, Au and carbon fiber ultramicroelectrodes were studied in laboratory samples by square wave voltammetry at high frequencies. The results showed two reversible peaks for paraquat reduction, in agreement to the literature data. The first peak was associated to the reduction of paraquat molecule in solution, with the further adsorption of the intermediate on the electrode surface. This adsorbed species undergoes to electroreduction in a reaction associated to the second voltammetric peak. The variation in pH and square wave parameters showed the best conditions to reduce paraquat as pH 5.0, frequency as high as 1000 s-1, scan increment of 2 mV and square wave amplitude of 50 mV. At such conditions, a variation of paraquat concentrations from 4.3 x 10-6 to 1.66 x 10-4 mol L-1 presented values for the detection limit equal to 3.9, 6.2 and 20.3 ppb on Pt, Au and carbon, respectively, at 1000 s-1. These values are quite below17 the allowed limit of paraquat in drinking water.

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.

O uso da voltametria de onda quadrada para a determinação eletroanalítica do picloram em meio aquoso é discutido neste trabalho. Encontrou-se que a reação ocorre por um mecanismo irreversível, envolvendo a transferência de quatro elétrons, com o reagente adsorvido na superfície do eletrodo. Uma aplicação prática foi levada a efeito, com amostras de águas contaminadas artificialmente com o herbicida. Estas amostras consistiam de água pura e de três diferentes amostras, coletadas em três diferentes riachos da região, dois dos quais eram altamente poluídos com efluentes domésticos e industriais. Os limites de detecção calculados variaram entre 11 (para a água pura) e 39 ppb para a amostra mais contaminada. Estes valores são suficientemente baixos para indicar a metodologia em aplicações de análise de traços.

This paper describes the use of square wave voltammetry for the electroanalytical determination of picloram in aqueous media. The reaction was found to occur via an irreversible mechanism involving the transference of four electrons, with the reagent adsorbed in the electrode surface. The practical application was carried out with spiked water samples originated from purified water and from three different local creeks, two of which are highly polluted with industrial and domestic effluents. The calculated detection limits ranged from 11 (for pure and unpolluted waters) to 39 ppb for the most contaminated sample. These low values indicate that such methodology can be conveniently applied in trace analysis.

This work reviews recent studies of underpotential deposition (UPD) of several metals on Pt and Au substrates performed in the Grupo de Materiais Eletroquímicos e Métodos Eletroanalíticos (IQSC -- USP, São Carlos). The UPD Cu, Cd and Pb on Pt were analysed in terms of their influence in the oxygen evolution reaction. Partial blockage of surface active sites, promoted by Pb ad-atoms, resulted in a change from water to hydrogen peroxide as the final product. The Ag UPD on Pt and Au substrates was also discussed in this work. A detailed model of charge calculation for Ag monolayer was developed and confirmed by the rotating ring-disk data. The partial charge transfer in UPD studies was analysed in the Cd/Pt and Cd/Au systems and a value of 0.5 was found for the adsorption electrovalence of Cd ad-ions. The Sn/Pt UPD systems were studied from the point of view of the valences of metallic ions in solution. The deposition from Sn(IV) generates a full monolayer with a maximum occupation of approximately 40% of the surface active sites (340 µC cm-2) plus 105 µC cm-2 of Hads (half monolayer). Changing the metallic ion for Sn(II), it was possible to deposit a full monolayer (210 µC cm-2) without any detectable Hads. Finally, the effect of anions was discussed in the Zn/Pt and Zn/Au systems. Here, the hydrogen evolution reaction (her) and the hydrogen adsorption/desorption were used in order to investigate the maximum coverage of the surface with Zn ad-atoms. The full monolayer, characterised by the complete absence of Hads, was achieved only in 0.5 M HF solutions.

This paper describes the development and characterisation of Ni-Co coatings to be used as anodes in water electrolysis. Chemical oxidation of the surface was performed through thermal treatment at 400ºC for 10 h. The resulting surfaces were analysed by X-ray diffraction, EDX, SEM, cyclic voltammetry and constant current electrolysis. The electrochemical oxidation occurring on bare surfaces during electrolysis promotes the formation of thick oxide layers resulting in loss of activity. In oxidised surfaces the chemical Ni-Co oxide grown during the thermal treatment prevents further oxidation thus retaining their activity towards oxygen evolution. An optimum condition for the growth of mixed oxide with high activity was found for the bath containing 50 g L-1 CoSO4.