Desenvolvimento de um sensor para a especiação do enxofre em amostras ambientais

Abstract

A diversidade de águas sulfúreas, bem como a existência do enxofre em amostras ambientais, levou-nos a que se tentasse desenvolver um método capaz de fazer a especiação deste elemento. Havendo já vários métodos rotinizados para a especiação do enxofre inorgânico e apenas um para o orgânico, este trabalho visou desenvolver um sensor que determinasse o enxofre orgânico a fim de poder quantificar todas as formas de enxofre existentes numa determinada amostra. Este sensor, baseado nos cristais piezoeléctricos de quartzo, utiliza um microorganismo, o fungo Sporothrix, que se alimenta de enxofre orgânico e que vive nas fontes de águas sulfúreas. Assim, a primeira parte deste trabalho consistiu na recolha do fungo Sporothrix, no estudo das suas características e da sua cultura em laboratório de modo a que pudesse vir a integrar o sensor. Na segunda parte do trabalho ensaiaram-se as diversas formas de imobilização do fungo sobre o cristal a fim de construir o sensor adequado. Fez-se ainda o estudo do consumo da L-metionina pelo fungo, que resultou positivo. O fungo Sporothrix consumiu, ao fim de alguns dias, toda a metionina que se lhe forneceu. A L-metionina é um aminoácido, que, de acordo com a literatura, constituía uma das espécies com enxofre à qual o fungo respondia mais facilmente e que pôde ser a solução de ensaio neste trabalho. Seguiu-se o ensaio das metodologias que permitiriam a determinação do enxofre orgânico. A maior parte delas resultou infrutífera. O método da cultura líquida, que consistiu em colocar o sensor em contacto com o fungo, mostrou resultar, tendo-se obtido, com este método uma curva de calibração que virá a ser aperfeiçoada. Finalmente e com a cultura líquida pôde ainda ser reconhecida a metionina em todos os ensaios em que estava presente o fungo Sporithrix.

The presence of sulfur in environmental samples namely its occurrence of sulfur-rich waters prompted for the development of a new method, to perform the speciation of this element. Although there were several routinely used methods for the speciation of inorganic sulfur, for the organic there was only one. The aim of this work was to develop a sensor for the quantification of organic sulfur in water samples. This sensor, based on a combination of a microbalance based on piezoelectric quartz crystals and a microorganism, the Sporothrix fungus, would combined the very low detection capabilities of the microbalance with the very high specificity of the microorganism. The first part of this work deals with the confirmation of this specific fungus in sulfur-rich waters, the study of its characteristics and of its culture in order to use it as a sensor. For the second part several options were studied for the combination of microbalance/fungus to be used as a sensor. The consumption of organic sulfur by the fungus was confirmed using different liquid cultures with different concentrations of L-methionine. This was elected as the workhorse compound throughout this study because according to the literature, once the fungus is exposed to it, a vigorous growth occurs. Several methodologies using the immobilized fungus on the piezoelectric quartz crystal were used, but none of them gave positive results. Finally, the so called “method of the liquid culture”, in which the fungus was not on the crystal but rather in solution, seemed to give promising results based on the fact that a calibration line of the time of the fungus to consume L-methionine versus the concentration was obtained. In order to be sure that this combination of the fungus and the piezoelectric quartz crystal was functioning as a sensor for organic sulfur, experiments were made where L-methionine was added to water in which the fungus could be present or not. It was observed that in the solutions without the fungus the crystal responded to the variations of the characteristics of the solution, namely density and viscosity; in the solutions with the fungus, after a while the sensor got back to its original value, showing that the fungus had consumed the L-methionine. The conjunction of both facts, the above mentioned calibration line and the response to the change of the characteristics of an L-methionine solution, showed that the proposed device can be used as a sensor for organic sulfur.