Identificador: TDX:276
Autores: Pérez Hernández, Javier
Resumen:
The organic form of mercury (methylmercury) is highly toxic, affecting the nervous system and even causing death. In the last years, human activities on coal combustion, waste incineration, gold mining and other industrial processes have raised the level of mercury in the atmosphere, rivers and other sources. Several public bodies have demonstrated that the direct detection of inorganic mercury (the precursor of mehtylmercury) will be beneficial in order to prevent mercury contamination. The detection of inorganic mercury through simple and low cost systems is possible by using colorimetric chemical sensors.<br/>Thus, several research groups worldwide have shown that the use of molecular probes, which change their optical properties upon the binding of inorganic mercury, is a promising topic for the development of detector devices for pollutant species.<br/>This Thesis describes the most remarkable aspects in the development of an optical fiber probe designed for mercury determination in aqueous samples. The research arises from the discovery of a novel molecule (IUPAC name bis(2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium(II) bistetrabutylammonium bis-thiocyanate) that upon mercury binding induces a color change from dark red-purple to orange in solution. The selectivity towards mercury of this ruthenium complex is high when compared to other known chemical reagents. Yet, in this work, we have been able to increase the selectivity through a fully multivariate calibration of the absorbance measurements. We have analyzed the mercury-containing solutions under the co-existence of higher concentrations (19.5 mg L-1) of other potential competitors such as Cd2+, Pb2+, Cu2+ and Zn2+ ions. Our experimental model is based on partial least squares (PLS) linear regression and other general techniques as wavelet, orthogonal signal correction, genetic algorithm and statistical feature selection that have been used to refine, a priori, the analytical data. In summary, we have demonstrated that the root mean square error of mercury prediction with statistical feature selection, as compared to the absence of pre-treatment, can be reduced from 10.5 to 5.2 percent, which improves the prediction ability of the calibration model by a factor of 2.<br/>On the other hand, the possibility of working in solid-liquid phase increases the integration ability of the molecule in a device, making easier the measurement process. Nevertheless, the immobilization of the molecule onto a surface constitutes one of the challenges of this Thesis.<br/>Some preliminary works with the Ru(II) complex focussed on the immobilization of the molecule onto TiO2 mesoporous thin films.<br/>However, some leaching problems were apparent when aqueous samples were analysed. Accordingly, we have improved the water stability of the molecule by anchoring the dye onto Al2O3 nanoparticles thin films treated with sulphuric acid. Moreover, the good optical properties of the alumina support allow a better transparency of the films, which translates in a higher amount of available spectral absorbance information.<br/>A compact mercury read-out system has been achieved by coating an unclad optical fiber piece with Al2O3 paste. The proof-of-principle is based on the optical changes of the reagent upon Hg2+ ions exposure, which modulates the light intensity transmitted through the optical core.<br/>There are many theoretical studies that explore a particular research case of the evanescent optical fibers. As the alumina cladding has higher refractive index than the core, both evanescent field and radiative mode may appear in the modified cladding. This Thesis exposes a brief explanation of this behavior in order to understand the mechanisms of the response of our mercury optical fiber probe. Moreover, several experiments have been carried out in mercury aqueous samples so as to find the proper working conditions, such as the optimum dye concentration adsorbed onto the alumina cladding, which has a great effect on the device performance. Finally, mercury quantification has been possible through multivariate calibration, direct partial least squares being the most robust procedure if we take into account the fact that the response of this kind of optical probes varies significantly from one to another. A root mean square error for mercury predictions of 11.5 percent has been achieved within a range from 0 to 6 mg L-1 of Hg2+ ions.<br/>Overall, this thesis work has illustrated all the steps that come into play in the design of an optical fiber chemical-based probe, providing a simplified measurement process and a lower cost if it is compared to traditional analysis equipment. As far as the author is concerned, an optical fiber probe for mercury determination is presented for the first time.
Repositori URV