Brazil has the greatest plant diversity on the planet, distributed in different types of biomes. These plants are important sources of biologically active natural products, which are derived from various drugs marketed worldwide. This paper presents an electrochemical study of three unusual dimeric flavonoids, pharmacologically active, isolated and identified for the first time by our research group, in a Brazilian plant (Fridericia platyphylla). The results showed that oxidation processes are favored at higher pH, and mass transport was controlled by diffusion. Brachydins derivatives, Bra-A was oxidized at the lowest potential value (0.48 V vs. Ag/AgCl, KCl(sat)) and Bra-B and Bra-C, presented the highest oxidation potentials (ca. 0.71 and ca. 0.57 V vs. Ag/AgCl, KCl(sat), respectively). This study shows that electrochemistry is one more tool that would help us focus on future bio-pharmacological investigations of these unusual compounds.

Abstract The release of industrial effluents into the environment causes widespread contamination of aquatic systems. Adsorption is seen as one of the most promising treatment processes, and lignocellulosic materials have gained prominence as adsorbents. This study investigates the potential of rice hulls, either in natura or treated with nitric acid, as adsorbents for removal of the dye. The adsorbents were characterized by infrared spectroscopy, solid state 13C-NMR, thermogravimetric analysis, and pH at point of zero charge. The dye adsorption experiments were carried out in batch mode, using different experimental conditions. The kinetic adsorption data could be fitted using the model of Elovich. The Freundlich model provided the best fit to the isothermal data. The thermodynamic parameters confirmed the spontaneity of the adsorption process. These adsorbents offer an alternative for dye removal, with advantages including biomass availability and low cost.

This paper describes a procedure based on multicommutation in flow analysis system for photometric determination of hydroquinone in pharmaceutical gel formulations. The procedure involves oxidation of hydroquinone by Fe (III) and subsequent reaction between the generated Fe (II) species with 1,10-phenanthroline, producing a complex detected at 510 nm. Under optimized conditions, the proposed system showed a linear response in the range between 5 to 120 µmol L-1 (r = 0.9995), a limit of detection (3σ) 0.52 µmol L-1 and a relative standard deviation (RSD) of 1.5% for a 80 µmol L-1 hydroquinone standard solution. Others profitable features such as an analytical frequency of 72 samples per hour; a reagent consumption of 0.432 and 0.117 mg 1-10-phenantroline and FeCl3.6H2O per determination, respectively; and a waste generation of 4.0 ml per determination were also achieved.

This paper presents an analytical procedure for the determination of folic acid employing commutation flow analysis process and detection by chemiluminescence. The procedure is based on the reaction of hexacyanoferrate(III) with folic acid, followed of luminol oxidizing reaction in alkaline medium, resulting in emission of radiation at 425 nm. After optimization the experimental variables, the proposed procedure afforded the following useful features. A linear response ranging from 0.1 to 1.00 mg mL-1 folic acid (R2= 0.993), a detection limit (3σ criterion) 0.046 mg mL-1, a sampling rate of 156 determination per hour, a relative standard deviation less than 0.5% (n = 6) for a 0.6 mg mL->1 folic acid standard solution and a waste generation of 0.86 mL per determination.

This paper describes the feasibility of using direct current electrical resistivity measurements to evaluate AISI 304 austenitic stainless steel sensitization. ASTM A262 – Practice A and double loop electrochemical potentiodynamic reactivation (DL-EPR) tests were performed to assess the degree of sensitization (DoS) qualitatively and quantitatively, and electrical resistivity (ER) was measured by the four-point direct-current potential drop method. The results indicate that the DoS increases rapidly while the ER decreases gradually in response to increasing sensitization. Thereafter, the two parameters tend to remain approximately constant. This behavior may be due to the rapid increase in the volume fraction of M23C6 precipitates over a sensitization time of 4 to 6 hours, thereafter remaining relatively constant. The ER results, which were corroborated by other techniques used in this study, confirm the promising potential of this property to monitor the sensitization phenomenon in AISI 304 steel.

The aim of this study was to evaluate the pH, ultimate tensile strength (UTS), and micro-shear bond strength (µSBS) of two self-adhesive resin cements to enamel and dentin. Sound bovine incisors (n = 10) and two self-adhesive resin cements (i.e., RelyX U-100 and seT PP) were used. The pH of the resin cements was measured using a pH-indicator paper (n = 3). Specimens for UTS were obtained from an hourglass-shaped mold. For µSBS, cylinders with internal diameter of 0.75 mm and height of 0.5 mm were bonded to the flat enamel and dentin surfaces. Bonded cylinders were tested in the shear mode using a loop wire. The fracture mode was also evaluated. The cement seT PP showed a low pH; U-100 showed significantly higher UTS (49.9 ± 2.0) than seT PP (40.0 ± 2.1) (p < 0.05) and high µSBS to enamel (10.7 ± 3.7). The lowest µSBS was found for seT PP to dentin (0.7 ± 0.6); seT PP to enamel (4.8 ± 1.7), and for U-100 to dentin (7.2 ± 1.9), showing an intermediate µSBS value (p < 0.05). Adhesive failure was the most frequently observed failure mode. The resin cement that presented the lowest pH and UTS also presented the lowest micro-shear bond strength to enamel and dentin.

A construção e a aplicação de um sensor biomimético para a determinação de 4-metilbenzilideno cânfora (4-MBC), um protetor de radiação ultravioleta (UV), são descritas. O sensor foi preparado pela modificação de um eletrodo de pasta de carbono com um complexo de cloreto de ferro(III) com ftalocianina, FePcCl. As medidas amperométricas conduzidas com o sensor, sob um potencial aplicado de 0,0 V vs. Ag|AgCl em uma mistura de tetraidrofurano e 0,1 mol L-1 H2SO4 (30:70 em volume), mostraram uma resposta linear no intervalo de 1,8×10-4 to 9,0×10-4 mol L-1. Uma investigação detalhada da seletividade da resposta frente a outros nove filtros UV também foi realizada. Um mecanismo de resposta do sensor foi proposto e os resultados para a quantificação de 4-MBC em protetores solares comerciais e em águas de piscinas e de rios são apresentados.

The construction and application of a biomimetic sensor for determination of 4-methylbenzylidene camphor (4-MBC), an ultraviolet (UV) radiation protector, are described. The sensor was prepared by modifying a carbon paste electrode with iron(III) phthalocyanine chloride (FePcCl). Amperometric measurements carried out with the sensor under an applied potential of 0.0 V vs. Ag|AgCl in a mixture of tetrahydrofuran and 0.1 mol L-1 H2SO4 solution (30:70 volume ratio) showed a linear response range from 1.8×10-4 to 9.0×10-4 mol L-1. A detailed selectivity investigation for other nine UV filters was also performed. A sensor response mechanism was proposed and the results for 4-MBC quantification in commercial sunscreens and in water from swimming pools and rivers are presented.

Direct methanol fuel cells (DMFCs) without external pumps or other ancillary devices for fuel and oxidant supply are known as passive DMFCs and are potential candidates to replace lithium-ion batteries in powering portable electronic devices. This paper presents the results obtained from a membrane electrode assembly (MEA) specifically designed for passive DMFCs. Appropriated electrocatalysts were prepared and the effect of their loadings was investigated. Two types of gas diffusion layers (GDL) were also tested. The influence of the methanol concentration was analyzed in each case. The best MEA performance presented a maximum power density of 11.94 mW cm-2.

A construção de um sensor amperométrico para compostos fenólicos usando um eletrodo de pasta de carbono modificado com ftalocianina de manganês e histidina é relatado. Sob condições otimizadas, em 0 mV vs Ag/AgCl numa solução tampão fosfato 0,1 mol dm-3 (pH 7,0) contendo H2O2 250 µmol dm-3, uma resposta linear para catecol entre 20 e 130 µmol dm-3, com sensibilidade de 0,56 (± 0,02) µA dm³ mol-1 cm-2, foi obtida. O limite de detecção foi 1,1 µmol dm-3 e o tempo de resposta foi de 2 s. O sensor apresentou resposta estável durante 40 determinações sucessivas. A repetibilidade avaliada em termos de desvio padrão relativo foi de 3,6% para n = 5 e [catecol] = 10 µmol dm-3. As respostas do sensor para outros compostos fenólicos também foram investigadas em detalhes. Finalmente, a aplicabilidade do sensor foi testada em medicamento contendo dopamina e os resultados comparados favoravelmente com os obtidos com o método oficial.

The construction of an amperometric sensor for phenolic compounds using a carbon paste electrode modified with manganese phthalocyanine and histidine is reported. Under optimized conditions, at 0 mV vs Ag/AgCl in 0.1 mol dm-3 phosphate buffer solutions (pH 7.0) containing 250 µmol dm-3 H2O2, a linear response for catechol from 20 up to 130 µmol dm-3 was obtained with a sensitivity of 0.56 (± 0.02) µA dm³ µmol-1 cm-2. The detection limit was 1.1 µmol dm-3 and the response time was 2 s. The sensor presented stable response during 40 successive determinations. The repeatability, evaluated in terms of relative standard deviation, was 3.6% for n = 5 and [catechol] = 10 µmol dm-3. The sensor responses for other phenolic compounds were also investigated in details. Finally, the applicability of the sensor was tested in a pharmaceutical containing dopamine and the results compared favorably with those obtained with the official method.

O composto 5-(4-pyridinyl)-1,3,4-oxadiazole-2-thiol (Hpyt) adsorve espontaneamente sobre ouro formando SAMs ("Self-Assembled Monolayers") que, de acordo com os resultados eletroquímicos e de STM ("Scanning Tunneling Microscopy"), contêm poros através dos quais as moléculas dos complexos [Fe(CN)6]4- e [Ru(NH3)6]3+ acessam a superfície livre de modificação. Para o complexo [Fe(CN)6]4-, a dependência da corrente faradáica com o pH da solução eletrolítica permitiu o cálculo do pKa da molécula de Hpyt adsorvida sobre ouro (4,2). Os parâmetros termodinâmicos, DHads and DGads, para o processo de adsorção desta molécula foram estimados em -20,01 e -39,39 kJ mol-1, respectivamente, utilizando-se o modelo de Langmuir. O processo redox da metaloproteína citocromo c foi estudado utilizando-se a SAM de Hpyt. A constante de velocidade heterogênea de transferência de elétrons foi calculada em 2,29 × 10-3 cm s-1.

5-(4-pyridinyl)-1,3,4-oxadiazole-2-thiol (Hpyt) spontaneously adsorbs on gold forming SAMs (self-assembled monolayers) that, based on STM (Scanning Tunneling Microscopy) and electrochemical data, contain pinholes through which [Fe(CN)6]4- and [Ru(NH3)6]3+ probe molecules access the underlying gold electrode. For the former molecule, the dependence of the faradaic current on the electrolyte solution pH value allowed the evaluation of the surface pKa as 4.2. The thermodynamic parameters DHads and DGads for the Hpyt adsorption process could be described by the Langmuir model and were calculated as -20.01 and -39.39 kJ mol-1, respectively. Electrodic redox reaction of cytochrome c metalloprotein was accessed by using the Hpyt SAM with a heterogeneous electron transfer rate constant of 2.29 × 10-3 cm s-1.

A oxidação eletrocatalítica de hidrazina foi estudada sobre um eletrodo de grafite pirolítico ordinário modificado com tetrapiridinoporfirazina de ferro (FeTPyPz) com as técnicas de voltametria cíclica e de eletrodo de disco rotatório. Análise dos voltamogramas registrados a diferentes velocidades de varredura do potencial e das curvas de polarização para diferentes velocidades de rotação do eletrodo mostraram que a reação de eletroxidação de hidrazina sobre FeTPyPz processa-se de acordo com um mecanismo envolvendo 4 elétrons e com a formação de N2 como principal produto. Os parâmetros cinéticos sugerem que a segunda etapa de transferência de carga é a etapa determinante da velocidade da reação. A atividade eletrocatalítica do complexo FeTPyPz depende do potencial formal do processo redox Fe(II)/Fe(I), que apresentou bom ajuste num gráfico do tipo vulcano formado por diferentes ftalocianinas de ferro, indicando que este potencial formal é um bom indicador da reatividade destes complexos.

The electrocatalytic oxidation of hydrazine was studied using an ordinary pyrolytic graphite electrode modified with iron tetrapyridinoporphyrazine complex (FeTPyPz), employing cyclic voltammetry and rotating disk electrode techniques. Analyses of the voltammograms recorded at different potential scan rates and the polarization curves at different electrode rotation rates showed that the reaction of electrooxidation of hydrazine on FeTPyPz occurs via 4-electrons with the formation of N2 as main product. The kinetic parameters suggest that the second electron transfer step is rate controlling. The activity of FeTPyPz depends on its Fe(II)/Fe(I) formal potential and fits well in a volcano plot that includes several iron phthalocyanines, indicating that such formal potential is a good reactivity index for these complexes.

Descreve-se a construção e aplicação de um sensor biomimético para determinação de paracetamol em diversos tipos de amostras. O sensor foi construído modificando a superfície de um eletrodo de carbono vítreo com membrana de Nafion® dopada com tetrapiridinoporfirazina de ferro (FeTPyPz). Esse sensor apresentou melhor desempenho em tampão acetato 0,1 mol L-1 e pH 3,6. Nessas condições o potencial de oxidação do paracetamol foi de 445 mV vs. Ag|AgCl. O sensor apresentou uma faixa de resposta linear entre 4,0 e 420 µmol L-1, sensibilidade de 46,015 mA L mol-1 cm-2, limite de quantificação de 4,0 µmol L-1 e limite de detecção de 1,2 µmol L-1. Estudos eletroquímicos e de seletividade demonstraram a propriedade catalítica da FeTPyPz como sendo similares a da enzima P450 na oxidação do paracetamol. O sensor foi usado na determinação de paracetamol em formulações comerciais e no acompanhamento de sua degradação eletroquímica em efluentes provenientes de um reator em escala piloto.

This work describes the construction and application of a biomimetic sensor for paracetamol determination in different samples . The sensor was prepared by modifying a glassy carbon electrode surface with a Nafion® membrane doped with FeTPyPz. The best performance of the sensor in 0.1 mol L-1 acetate buffer was at pH 3.6. Under these conditions, an oxidation potential of paracetamol was observed at 445 mV vs. Ag|AgCl. The sensor presented a linear response range between 4.0 and 420 µmol L-1, a sensitivity of 46.015 mA L mol-1 cm-2, quantification and detection limits of 4.0 µmol L-1 and 1.2 µmol L-1, respectively. A detailed investigation about its electrochemical behavior and selectivity was carried out. The results suggested that FeTPyPz presents catalytic properties similar to P450 enzyme for paracetamol oxidation. Finally, the sensor was applied for paracetamol determination in commercial drugs and for the monitoring of its degradation in an electrochemical batch reactor effluent.