Transparent nanostructured metal oxides for chemical biosensors: towards point-of-care environments

Abstract

There is an increasing need for developing innovative, versatile and low-cost point-of-care (POC) systems capable of screening for at early stages of development. POC systems usually consist of a biosensor part integrated in an electronic circuit and eventually a microfluidic system to manage the body fluid samples. The aim of this doctoral thesis is to investigate several ways of improving POC technology.

On the one hand, biosensors currently integrated into POC systems have limitations. A wide variety of important analytes cannot be properly detected and quantified, and methods supported by a powerful electronic systems that supply the necessary energy to trigger a measurable event that can be monitored are required. For this, adequate sensing substrates are required that allow the coupling of analytes and other biomolecules and enable the detection of chemical reactions occurring at their surfaces. Besides, the complex electronic circuitry capable of simultaneously exciting the sensor and monitoring its response must be redesigned into a low-cost and miniaturized format to be integrated into POC systems.

Electrochemical and optical biosensors have become relevant in point-of-care technology due to the versatility of POC systems based on such transducing principles, which provide the sensors with high sensitivities and specificities. In particular, sensitivity may become badly affected by the miniaturization of sensors and devices. Thus, the need for reducing the surface of sensing electrodes and yet maintaining the sensitivity has boosted the research and development of nanostructured surfaces. The high surface-to- volume ratio (SVR) presented by nanostructures makes them extremely interesting for the detection of biomolecules, since an increase of surface enables the interaction with a big amount of small-sized molecules and this implies an increase of sensitivity and the possibility to reduce the sensor size. In this thesis, nanostructured indium tin oxide (ITO) is proposed as working electrode (WE) for electrochemical biosensors.

The first part of this thesis consists in a study of ITO properties and its electrical, optical, electrochemical and structural characterization both as a thin film and as nanostructured electrodes prepared by electron beam evaporation onto silicon and glass substrates. Moreover, the interaction of nanostructured ITO with some molecules known as crosslinkers, which allow subsequent functionalization of surfaces with biomolecules, has also been studied in the frame of this thesis. Finally, several immunoassays were performed using nanostructured ITO as substrate, with special attention to the detection of several concentrations of tumour necrosis factor α (TNF-α).

On the other hand, several electrochemical sensor mechanisms were studied. These were based upon different ways of electrically attacking the sensor and processing its response, and included potentiometry, amperometry and electrochemical impedance spectroscopy. A low-cost and miniaturized device implementing electrochemical impedance spectroscopy measurements was designed and developed for the detection of several concentrations of TNF–α biomarker with an array of eight parallel gold-based microelectrodes. Besides, we also designed the electronics for performing two-electrode amperometry and potentiometry. The latter was tested on nanostructured ITO electrodes covered with a doped conducting polymer, which was sensitive to pH changes in aqueous media.

To synthesize, this thesis gathers several proposals for improving current POC systems, regarding both the biosensor and the electronic parts, employing an important biomarker in the biomedical area for the measurements and proofs of concept, and being such approaches extensible to the environmental field.

Existe una creciente necesidad de desarrollar sistemas de punto de cuidado (POC) innovadores, versátiles y de bajo coste, capaces de detectar enfermedades en estadios de desarrollo tempranos. Un sistema POC consiste en un biosensor integrado en un sistema electrónico y eventualmente un sistema microfluídico que gestione las muestras de fluidos biológicos. El objetivo general de esta tesis es investigar distintas posibilidades de mejorar la tecnología POC.

Los sistemas biosensores actualmente integrados en sistemas POC son limitados, y requieren de métodos que se apoyen en un potente sistema electrónico. Para esto, se requiere por un lado de sustratos adecuados que permitan tanto el acoplo de biomoléculas como la detección de reacciones químicas ocurridas en su superficie; por otro lado, es necesaria una potente circuitería electrónica que pueda ser miniaturizada y de bajo consumo.

Los biosensores electroquímicos y ópticos han tomado mucha relevancia en sistemas POC debido a su alta sensibilidad y especificidad, aunque éstas pueden verse gravemente afectadas por la miniaturización de los dispositivos. Por ello, la elevada relación superficie-volumen que presentan las nanoestructuras las hace especialmente interesantes para biodetección. En esta tesis, se propone el óxido de indio dopado con estaño (ITO) nanoestructurado como material de electrodo de trabajo en biosensores electroquímicos.

En la primera parte de esta tesis se han estudiado las propiedades del ITO, y se ha caracterizado eléctrica, óptica, electroquímica y estructuralmente, a partir de muestras preparadas sobre silicio y sobre vidrio mediante evaporación por haz de electrones. Se ha estudiado su interacción con moléculas crosslinkers, y se han realizado varios inmunoensayos usando el ITO nanoestructurado como sustrato.

Se han estudiado distintos mecanismos de medida de sensores electroquímicos; por espectroscopía de impedancias se han detectado distintas concentraciones del biomarcador TNF-α. Se ha diseñado la electrónica necesaria para realizar medidas de amperometría con dos electrodos, así como de potenciometría, ensayando este último sistema con ITO nanoestructurado y modificado sensible a cambios de pH.

En síntesis, esta tesis recoge algunas propuestas para la mejora de los sistemas POC actuales, tanto en la cuestión biosensora como en la parte electrónica, usando para las medidas y pruebas de concepto un biomarcador importante en el ámbito biomédico.