Sensores optoquímicos integrados basados en guías de onda

Resumen

Con el fin de solucionar los problemas derivados de la falta de sensibilidad o la alta complejidad de los sensores ópticos integrados basados en medidas de absorbancia, a lo largo de la tesis se diseñó y construyó una plataforma versátil, viable para el desarrollo de dispositivos sensores basados en óptica integrada, para la determinación selectiva de diferentes analitos. Éste se basa en un dispositivo construido mediante guías de onda planas, integradas en una matriz de silicio. Además, para simplificar su diseño instrumental, el acoplamiento del dispositivo a la fuente de luz y al detector se realiza mediante fibras ópticas. El diseño planteado para cumplir los requisitos citados consiste en un circuito microfabricado de guías de onda ARROW y en una membrana químicamente activa, de un determinado grosor, depositada en una zona definida del circuito de guías, que actúa simultáneamente como parte de la estructura plana de guiado y como elemento de reconocimiento. El mecanismo de transducción se basa en el fenómeno de absorción, que es modulado por la membrana selectiva al interaccionar con el analito presente en la disolución. Se formularon diversas membranas poliméricas que incorporaban cromoionóforos NIR sensibles a protón de la familia de las cetocianinas y se formuló y caracterizó una membrana modelo selectiva de ión cadmio compuesta por la combinación de una de las cetocianinas (5ee) evaluadas previamente en un sensor integrado, con un ionóforo comercial (ETH 5435) selectivo de cadmio. Su caracterización espectroscópica (estabilidad química y fotoquímica, posición de bandas, respuesta a pH, pKa, tiempo de respuesta) demostró su excelente comportamiento analítico. Se obtuvieron membranas completamente reversibles, de alta sensibilidad y respuesta rápida y reproducible. Para aprovechar las características de respuesta óptimas de las membranas poliméricas desarrolladas, que incorporaban cetocianinas como cromoionóforos de protón, éstas se incorporaron como elemento de reconocimiento de sensores ópticos integrados. Se diseñó un sistema de análisis por inyección en flujo como metodología simple, automática y reproducible para la caracterización de las membranas desarrolladas, y se integró el sensor como detector. Los resultados obtenidos probaron que una de las ventajas que ofrece el uso de la nueva configuración es el incremento de la sensibilidad sin necesidad de aumentar la concentración de cromoionóforo de la membrana, posibilidad que vendría limitada por su solubilidad, y sin aumentar el grosor de ésta, lo que provocaría un incremento del tiempo de respuesta. Para ambas configuraciones y en idénticas condiciones experimentales, se determinó el tiempo de respuesta de las membranas. Los resultados mostraron como la variación total en unidades de absorbancia conseguido para cada salto de concentración se multiplicaba al utilizar el sensor, mientras que el tiempo de respuesta permanecía equivalente en ambas configuraciones, señal inequívoca de que el grosor de las membranas utilizadas era similar. The present work pretends to address some of the weak points of the absorbance techniques. Besides increasing the dye concentration in the membrane, which is limited by its solubility in the plasticizer, there are two effective alternatives to improve sensitivity: on the one hand, increasing the optical path length and on the other hand, changing the optically active reagent to another one with higher molar absortivity. With that in mind, we first designed and built a new integrated waveguide absorbance optode (IWAO) for chemical sensing based on radiation transmission through the core of a waveguide. The device consisted of a microfabricated planar waveguide circuit, based on antiresonant reflecting optical waveguide structures (ARROW), and on a chemically active membrane, of a suitable thickness, deposited in a defined region of the former and yielding one part of the light guiding planar structure. Concerning the transduction mechanism, it is established by absorbance/transmittance phenomena of the recognition optode membrane as it interacts with a given concentration of the analyte in the sample. We proceed evaluating the optical and analytical characteristics of a series of novel neutral pH indicators, a class of ketocyanine dyes in PVC membranes. Optical membranes were characterized initially with a conventional absorbance/transmittance flow cell in a continuous-flow system to perform a comparative study of the analytical response. All the parameters determining the membrane response were optimized in order to guarantee the same operational conditions, using the conventional technique as well as the IWAO. The absorbance change achieved with the conventional configuration was multiplied using the sensor almost for every step change, while the response times for both devices were comparable. As a result of the excellent elicited results, the developed IWAO became a very promising device, with large applicability that adapted existing absorbance schemes to the design and construction of novel specific optochemical sensors. Ketocyanine dyes appear to be completely soluble in the organic solvent used as the plasticizer, and no aggregation was observed during any of the experiments. The absorbance maxima of ketocyanine dyes in ethanol were slightly red-shifted (longer wavelength) in this less polar solvent, approaching the working wavelength of the IWAO optical system. Membranes showed good performance during calibrations and basicities were calculated from the equilibrium constant, KaMEM, of each ketocyanine in the membrane. Concerning their stability, photostability was studied in dry and humid conditions under the exposure of different light sources. None of the ketocyanine dyes suffered decomposition in the acid form, but all of them slowly decomposed at their basic form and it was possible to conclude that the tested dyes did not leach to the solution. Finally, analytical response characteristics as sensitivity, response time and reproducibility were checked. In conclusion, all of the prepared and tested membranes can be considered very sensitive, fast, and reversible optical transducers, suitable for pH determinations. The composition of a cadmium-selective membrane was optimized using one of the ketocyanine dyes characterized as a pH indicator in a PVC-polymeric membrane, in combination with a commercial cadmium ionophore. We performed a screen-printing like deposition technique to activate the planar sensor platform with this bulk optode. The sensor integration as a detector in a flow injection system (FIA) was proposed to obtain an automated, simple, and sufficiently reproducible analytical methodology with a high sample throughput. In conclusion, the proposed technologically improved IWAO made evident its appropriateness as a versatile platform for the development of ion-selective optical sensors, depending on the membrane composition, which also permitted to obtain disposable transducers. Actually, investigations are focused on improving ion-selective membranes as well as giving ruggedness and portability to the optical system for the future transference of technology and even though, taking profit of the employed technology to develop optical sensors based on other optical phenomena as fluorescence.