Electrochemical determination of Saccharomyces cerevisiae sp using glassy carbon electrodes modified with oxidized multi-walled carbon nanotubes dispersed in water -Nafion®.

The electrochemical behavior of Saccharomyces cerevisiae sp was studied using a glassy carbon electrode (GCE) modified with Nafion-dispersed oxidized multi-walled carbon nanotubes (OMWCNT). The morphology was studied using scanning electron microscopy (SEM), showing that the yeast sticks to the carbon nanotube surface instead of the glassy carbon surface. The redox couple Fe(CN)6 4-/Fe(CN)6 3- was used to determine the electroactive area and the heterogeneous transfer constant, which increased 80.5% and 108% respectively by the presence of nanotubes. The studies of the pH effect showed that the anodic potential decreases at alkaline pH and that the highest current intensity occurs at a pH value of 7.00. Studies of the scan rate effect have shown that yeast oxidation is an irreversible mixed control process in which two electrons participate. The relationship between yeast concentration and the anodic current density was studied using different electrochemical techniques obtaining the best analytical parameters through chronoamperometry. The linear range was between 3.36 and 6.52 g L-1, the limit of detection (LOD) and the limit of quantification (LOQ) were 0.98 g L-1 and 3.36 g L-1 respectively, and the sensibility obtained was 0.086 µA L g-1 mm-2. These results show that the multi-walled carbon nanotubes in water and Nafion® allow obtaining an anodic signal corresponding to the yeast, which facilitates its quantification through electrochemical methodologies, favoring the reduction of analysis times and costs compared with other techniques.

Development and validation of voltammetric method for determination of amoxicillin in river water.

Amoxicillin is an antibiotic that can accumulate in aquatic environments and lead to the development of resistant bacteria; thus, its determination is of great importance. In this study, a glassy carbon electrode modified with reduced graphene oxide and Nafion was used as a sensor in a square-wave voltammetry method for determination of amoxicillin in river water samples from Guarapuava city, Brazil. The method was validated, using parameters and statistical tools recommended by the validation guidelines, in the range of 1.8-5.4 µmol L-1 (r = 0.922 and R2 = 85.1%). The analytical curve was constructed using external standard calibration in pure electrolyte, since the matrix effect was not significant. Results of linear regression analysis, lack of fit test and analysis of the residual plots pointed that the linear regression was significant, without lack of fit of linear model and that the variances had homoscedastic distribution. Both coefficients of regression curve were significant and, thus, they were included in the regression equation: Response = 7.0 + 3.5CAMX. The limits of detection and quantification were 0.36 and 1.2 µmol L-1, respectively. The method was selective towards interferents such as humic acids and benzylpenicillin. The relative standard deviations for repeatability and intermediate precision were adequate according to the limits established in literature. The mean recoveries were statistically equal to those obtained through a comparative chromatography method, so, the accuracy of the method was also adequate. Therefore, the method can be applied to the voltammetric determination of amoxicillin in river water, affording reliable and consistent measurements.

Modification of Graphene Oxide Membranes by the Incorporation of Nafion Macromolecules and Conductive Scaffolds.

Stable, reproducible and low-cost graphene oxide (GO)/Nafion (N) membranes were fabricated using electronically conductive carbon paper (CP) matts as a scaffold. The presence of polar groups in the Nafion molecule facilitates the strong interaction with functional groups in the GO, which increases GO dispersion and aids the retention of the composite into the CP scaffold. Distribution of GO/N was carefully characterized by X-ray diffraction work function measurements, Raman and scanning electron microscopy analyses. The performance of these membranes was tested with 1 M NaCl at standard conditions, finding 85% ion removal in the best membranes by a mixed ion rejection/retention mechanism. The Nafion provided mechanical stability and fixed negative charge to the membranes, and its micellar organization, segregation and confinement favored ion rejection in Nafion-rich areas. The good electronic conductivity of these membranes was also demonstrated, allowing for the application of a small potential bias to enhance membrane performance in future studies.

A Nafion Film Cover to Enhance the Analytical Performance of the CuO/Cu Electrochemical Sensor for Determination of Chemical Oxygen Demand.

We modified and evaluated the performance of a CuO/Cu electrochemical electrode for chemical oxygen demand (COD) determination by covering it with a Nafion (Nf) film. The resulting modified CuONf/Cu electrode sensor was used for the electrochemical determination of COD in river, slaughterhouse and estuarine water samples in order to evaluate its performance for this particular task. It was compared with the CuO/Cu sensor with no Nafion. The main electrochemical characteristics of interest, resistance, sensitivity, accuracy and reproducibility, were assessed by means of Linear Sweep Voltammetry using glucose as a standard. Results of these essays indicate that the procedure used produced smooth and firmly attached Nf films covering the whole copper surface. This sensor was shown to be resistant to interferences and effective in electro-oxidation of a wide range of organic compounds and therefore very useful for COD determination. Using the newly developed CuONf/Cu electrode an analytical linear range of 50 to 1000 mg·L-1 COD, with a detection limit of 2.11 mg·L-1 (n = 6) COD was achieved. The comparison shows that the CuONf/Cu sensor is more appropriate for COD determination than its counterpart with no Nafion.

Thermo-Optical Properties of Perfluorinated Sulfonic Acid Membranes: An Investigation of Hydration Based on Absorption Spectra.

The Nafion membranes are widely used in electrochemical applications such as fuel cells, chlor-alkali cells, and actuators-sensors. In this work, the thermal-optical characterization of Nafion in acid form was performed by photoacoustic spectroscopy, thermogravimetry, and differential scanning calorimetry. In the experimental procedure three distinct hydration levels were considered: (1) pristine membrane (λ ≅ H2O/-SO3H ≅ 5.6); (2) swelling process (λ ≅ 17.4); and (3) drying at controlled room temperature after swelling process (λ ≅ 6.5). The discovered behaviors showed significant irreversible structural changes induced by water retention in the membrane. These structural changes depend on the water population present in the clusters and also affect the directional thermal diffusivity of the membrane irreversibly.

A simple and sensitive method for determination of Norfloxacin in pharmaceutical preparations / Um método simples e sensível para determinação da norfloxacina em preparações farmacêuticas

In this approach, a new voltammetric method for determination of norfloxacin was proposed with high sensitivity and wider detection linear range. The used voltammetric sensor was fabricated simply by coating a layer of graphene oxide (GO) and Nafion composited film on glassy carbon electrode. The advantage of proposed method was sensitive electrochemical response for norfloxacin, which was attributed to the excellent electrical conductivity of GO and the accumulating function of Nafion under optimum experimental conditions, the present method revealed a good linear response for determination of norfloxacin in the range of 1×10^-8mol/L-7×10^-6 mol/L with a detection limit of 5×10^-9 mol/L. The proposed method was successfully applied in the determination of norfloxacin in capsules with satisfactory results...

Propos-se, por essa abordagem, novo método voltamétrico, com alta sensibilidade e faixa linear de detecção mais ampla, para a determinação de norfloxacino. O sensor voltamétrico utilizado foi fabricado simplismente por cobertura de camada de óxido de grafeno (GO) e filme de Nafion em eletrodo de cabrono vítreo. A vantagem do método proposto foi a resposta eletroquímica sensível para o norfloxacino, atribuída à condutividade elétrica excelente do GO e à função acumulada do Nafion. Sob condições experimentais ótimas, o presente método revelou boa resposta linear para a determinação do norfloxacino na faixa de limite de detecção de 1×10^-8mol/L-7×10^-6 mol/L. O método proposto foi aplicado com sucesso na determinação de norfloxacino em cápsulas, com resultados satisfatórios...