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- Application of a lab-made electronic nose for extra virgin olive oils commercial classification according to the perceived fruitiness intensityPublication . Teixeira, Guilherme Gobbi; Dias, L.G.; Rodrigues, Nuno; Marx, Ítala; Veloso, Ana C.A.; Pereira, J.A.; Peres, António M.An electronic nose, comprising nine metal oxide sensors, has been built aiming to classify olive oils according to the fruity intensity commercial grade (ripely fruity or light, medium and intense greenly fruity), following the European regulated complementary terminology. The lab-made sensor device was capable to differentiate standard aqueous solutions (acetic acid, cis-3-hexenyl, cis-3-hexen-1-ol, hexanal, 1-hexenol and nonanal) that mimicked positive sensations (e.g., fatty, floral, fruit, grass, green and green leaves attributes) and negative attributes (e.g., sour and vinegary defects), as well as to semi-quantitatively classify them according to the concentration ranges (0.05–2.25 mg/kg). For that, unsupervised (principal component analysis) and supervised (linear discriminant analysis: sensitivity of 92% for leave-one-out cross validation) classification multivariate models were established based on nine or six gas sensors, respectively. It was also showed that the built E-nose allowed differentiating/discriminating (sensitivity of 81% for leave-one-out cross validation) extra virgin olive oils according to the perceived intensity of fruitiness as ripely fruity, light, medium or intense greenly fruity. In conclusion, the gas sensor device could be used as a practical preliminary non-destructive tool for guaranteeing the correctness of olive oil fruitiness intensity labelling.
- Enose lab made with vacuum sampling: quantitative applicationsPublication . Teixeira, Guilherme Gobbi; Peres, António M.; Estevinho, Leticia M.; Geraldes, Pedro; Garcia-Cabezon, Cristina; Martin-Pedrosa, Fernando; Rodriguez-Mendez, Maria Luz; Dias, L. G.A lab-made electronic nose (Enose) with vacuum sampling and a sensor array, comprising nine metal oxide semiconductor Figaro gas sensors, was tested for the quantitative analysis of vapor–liquid equilibrium, described by Henry’s law, of aqueous solutions of organic compounds: three alcohols (i.e., methanol, ethanol, and propanol) or three chemical compounds with different functional groups (i.e., acetaldehyde, ethanol, and ethyl acetate). These solutions followed a fractional factorial design to guarantee orthogonal concentrations. Acceptable predictive ridge regression models were obtained for training, with RSEs lower than 7.9, R2 values greater than 0.95, slopes varying between 0.84 and 1.00, and intercept values close to the theoretical value of zero. Similar results were obtained for the test data set: RSEs lower than 8.0, R2 values greater than 0.96, slopes varying between 0.72 and 1.10, and some intercepts equal to the theoretical value of zero. In addition, the total mass of the organic compounds of each aqueous solution could be predicted, pointing out that the sensors measured mainly the global contents of the vapor phases. The satisfactory quantitative results allowed to conclude that the Enose could be a useful tool for the analysis of volatiles from aqueous solutions containing organic compounds for which Henry’s law is applicable.
- Discrimination of three bacteria species using a potentiometric electronic tonguePublication . Teixeira, Guilherme Gobbi; Katel, Anna; Veloso, Ana C.A.; Dias, L.G.; Dias, Teresa; Peres, António M.The detection, monitoring and/or prevention of microorganism growing is of utmost relevance in several research fields, from food to environmental areas, being an important topic either from an academic or an industrial point of view. Conventional methods like plating techniques are the most widely used, being needed novel and faster screening methodologies like electronic noses, electronic tongue (E-tongue) and impedance based methods. In the present work, a potentiometric E-tongue (Fig.1), comprising 40 lipid polymeric sensor membranes with cross-sensitivity, was used to identify and discriminate three bacteria (Enterococcus faecalis ATCC29212; Staphylococcus aureus ATCC653 and Escherichia coli ATCC29998) at two concentrations levels (low and high). Brain Heart Infusion Broth medium was used for cultivating each of the three microorganisms, which were then individually inoculated into 1L Erlenmeyer flask (working volume of 300 mL) and incubated overnight (batch mode) at 35ºC, on a rotary shaker (90 rpm). After incubation, the biomass was spectrophotometrically determined, being measured the optical density at 550 nm. The cultures were split in volumes of 50 mL. The cells of each sample were harvested (centrifugation at 9000 rpm for 10 min) and stored at -20ºC. The obtained biomass was dried at 30ºC, and stored at -20ºC. Before E-tongue analysis, the cells were rehydrated with 20 mL of deionized water for 30 minutes at room temperature and aqueous sample solutions with different cell concentrations were obtained. Each E-tongue analysis took five minutes, enabling establishing a pseudo-equilibrium between the samples and the sensors´membranes, being the signals potentiometric profiles recorded. The classification performance of E-tongue was assessed by applying a linear discriminant analysis (LDA) coupled with the meta-heuristic simulated annealing (SA) variable selection algorithm. The preliminary results showed that E-tongue-LDA-SA predicting model could be established, based on the information gathered by a sub-set of 15 sensors, allowing to correctly classify (100%) (Fig.2) and 85% (original and leave-one-out cross-validation procedure, respectively) of the samples according to the microorganism and respectively concentration level.
- Analysis of phenolic content in grape seeds and skins by means of a bio-electronic tonguePublication . Garcia-Cabezon, Cristina; Teixeira, Guilherme Gobbi; Dias, L.G.; Salvo-Comino, Coral; García-Hernandez, Celia; Rodriguez-Mendez, Maria Luz; Martin-Pedrosa, FernandoA bio-electronic tongue has been developed to evaluate the phenolic content of grape residues (seeds and skins) in a fast and easy way with industrial use in mind. A voltammetric electronic tongue has been designed based on carbon resin electrodes modified with tyrosinase combined with electron mediators. The presence of the phenoloxydase promotes the selectivity and specificity towards phenols. The results of multivariate analysis allowed discriminating seeds and skins according to their polyphenolic content. Partial least squares (PLS) has been used to establish regression models with parameters related to phenolic content measured by spectroscopic methods i.e., total poliphenol content (TPC) and Folin–Ciocalteu (FC) indexes. It has been shown that electronic tongue can be successfully used to predict parameters of interest with high correlation coefficients (higher than 0.99 in both calibration and prediction) and low residual errors. These values can even be improved using genetic algorithms for multivalent analysis. In this way, a fast and simple tool is available for the evaluation of these values. This advantage may be due to the fact that the electrochemical signals are directly related to the phenolic content.
- Voltametria de onda quadrada na análise de compostos fenólicos com elétrodos compósitos de grafite: 4-clorofenol, fenol e 4-nitrofenolPublication . Teixeira, Guilherme Gobbi; Dias, L.G.; Medeiros, Flávia Vieira da SilvaOs compostos fenólicos estão entre os químicos a que os humanos mais estão expostos. Atividades domésticas, industriais, naturais e a agricultura facilitam a entrada dos compostos fenólicos no ambiente aquático. Métodos como a cromatrografia gasosa com detectores de captura eletrônica, ionização de chama e espectroscopia de massa são utilizados para identificação e quantificação de alguns compostos fenólicos em amostras ambientais e biológicas. Embora essas técnicas apresentem alta precisão e exatidão, requerem aparatos onerosos, exigem técnicos especializados, consomem muito tempo de preparação da amostra, além de exigirem constante manutenção. Neste âmbito, as técnicas de voltametria cíclica e de onda quadrada são adequadas para desenvolver metodologias rápidas, por aplicação de elétrodos que permitam a quantificação destas substâncias, tendo como base reações de oxidação-redução. Por isso, o objetivo geral deste trabalho foi avaliar o desempenho de elétrodos compósitos de carbono (com base em resina epoxílica e grafite) construídos com ou sem aditivos (Al2O3 ou Ag), na análise de 4-clorofenol, fenol e 4-nitrofenol e comparar as suas capacidades analíticas com um elétrodo comercial de carbono vítreo. Verificou-se por voltametria cíclica que todos os elétrodos compósitos e o de carbono vítreo funcionam corretamente, na análise de uma solução redox reversível, tendo-se obtido razão entre intensidades de pico catódica e anódica próximas da unidade para todos os elétrodos. De forma geral, os elétrodos permitiram medir concentrações inferiores a 1x10-5 mol/L para o 4-clorofenol e 4-nitrofenol e de 2x10-4 mol/L para o fenol, confirmadas através de calibrações individuais, cujos modelos de calibração apresentaram coeficientes de correlação superiores a 0,99 para todos os elétrodos testados. Para alguns dos elétrodos foi possível a medição de concentrações de 4-clorofenol ou 4-nitrofenol com níveis inferiores ou iguais a 5x10-8 mol/L. Nos ensaios efetuados com soluções de mistura dos três compostos fenólicos, verificou-se que os elétrodos testados permitiram a análise de concentração total dos compostos fenólicos e de 2 compostos fenólicos (4-clorofenol e fenol), obtendo-se relações lineares entre as concentrações previstas para o modelo e as experimentais com declives próximos da unidade (com valores de ordenada na origem não significativos) e coeficientes de determinação superiores a 0,991. Também se obtiveram modelos de previsão aceitáveis para a quantificação de cada composto fenólico isolado, predominando os dos compostos 4-clorofenol e 4-nitrofenol, mas não para todos elétrodos usados. Considerando o objetivo de aplicar estes elétrodos na análise de águas naturais, verificou-se que a matriz de água de furo e rio não alterava de forma acentuada o sinal voltamétrico quando comparado com o voltamograma obtido para a solução branco, com exceção da água do mar. Os resultados obtidos mostraram que o elétrodo com melhor comportamento analítico para a análise voltamétrica das diferentes águas naturais é o elétrodo compósito C, considerando as sensibilidades em relação aos compostos fenólicos testados.
- Discrimination of three bacteria species using a potentiometric electronic tonguePublication . Teixeira, Guilherme Gobbi; Katel, Anna; Veloso, Ana C.A.; Dias, L.G.; Dias, Teresa; Peres, António M.The microorganisms were inoculated into the broth medium and grown 90 rpm and 37º overnight in a rotatory incubator