Chemical sensors for environmental applications

Resumen

New analytical systems for evaluation, control and monitoring of parameters of environmental interest are being extensively implemented in order to fulfil the international regulation, with which a sustainable environment may be guaranteed. In this Thesis, chemical sensors fabricated with semiconductor technology are presented as simple, fast, reliable and low-cost attractive alternatives for the development of analytical instrumentation required in the environmental field. The work presented in this Thesis is included in one of the main research lines developed by the Grupo de Transductores Químicos at the Instituto de Microelectrónica de Barcelona. This is the application of miniaturized sensors for environmental monitoring. In this context, a variety of sensors have been developed. On one hand, ISFETs and conductivity electrodes are firstly presented, these being assembled in a probe for environmental monitoring purposes. On the other hand, the fabrication and thorough characterization of a new set of voltammetric microelectrodes (amperometric cells and ultramicroelectrode arrays-UMEAs) is presented together with interesting applications of these devices in the environmental field. This work was performed under the frame of a project funded by the Ministerio de Educación y Ciencia (MICROLAMB, DPI2003-08229-C03-01). The main body of the Thesis is constituted by the papers published by the author arranged in the different chapters, but with slight variations in order to either include some extra-data or facilitate the Thesis reading. Its presentation is meant to show how the technology and applications were getting more and more complex but, at the same time, being of higher performance and more versatile. Throughout the Chapters, the technology and characterization of the different devices is described in detail. As an introduction (Chapter 1), a comprehensive review focused on the technologies used for the fabrication of miniaturized sensors and the state-of-the art in the development of sensors for environmental monitoring purposes is presented. The objectives addressed, following a coherent path way, are presented in Chapter 2. The results are presented in two main parts. The first part (Chapter 3) refers to the development of an analytical probe that integrates several microsensors for monitoring pH, redox potential (ORP), conductivity and temperature and the fabrication of a versatile, portable, multiparametric system. Sensors used were already fabricated and characterized at the IMB. ISFETs for pH and electric cells based on a 4-electrode configuration for conductivity and ORP measurements were applied. In this part, the main effort was directed towards the appropriate encapsulation of sensors in a probe that withstood the harsh conditions of bentonite barriers, such as high pressure and temperature for a suitable working period of time. This system was applied to measure pore water coming from bentonite barriers, using a model fabricated at the Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT). Also, the whole system was validated with wastewater samples. The second part presents the design, fabrication and characterization of amperometric and voltammetric microelectrode and ultramicroelectrode array (UMEAs) devices (Chapters 4 and 6, respectively), and their analytical application (Chapters 4, 5, 7 and 8). They were fabricated by means of the Si/SiO2/metal photolithographic techniques at the Clean Room facilities of the Instituto de Microelectrónica de Barcelona. This is the first time that such fabrication process was performed in our group. Thus, a thorough evaluation in terms of fabrication efficiency and analytical performance of the resulting transducers was carried out by combining voltammetric techniques and optical tools (AFM, SEM, transmission light and fluorescence optical imaging). Once the response characteristics were examined and taking into account the excellent performance of the fabricated transducers and the feasibility of the applied technology, different applications were developed. The first application described corresponds to the monitoring of free chlorine in swimming-pool waters (Chapter 4). A flow injection analysis (FIA) system integrating an amperometric microsensor was developed. In this chapter, the excellent sensor performance in the flow system and its working and storage stability is demonstrated. Chapter 5 reports on the modification of the surface of amperometric microsensors with a graphite polymer composite. The resulting modified sensor device was successfully applied to estimate the Electrochemical Oxygen Demand (EOD) of several types of wastewater samples coming from parenteral food and winemaking processes. UMEAs are presented in Chapter 6. Apart from their characterization using the techniques described above, a new approach for the modification of UMEAs was carried out, which consisted of the simple electrodeposition of gold nanoparticles (GNP) over their surface. It is demonstrated that reproducible and rougher surfaces can be obtained which, in turn, greatly improve the analytical performance of these electrochemical transducers while keeping their microelectrode inherent properties. GNP-modified UMEAs were applied for the detection of copper by using under potential deposition anodic stripping voltammetry technique (Chapter 7). The performance of the developed device is demonstrated by the analysis of this target analyte in soil samples. Chapter 8 reports on the improved performance of amperometric biosensors when GNP- modified UMEAs were used as transducers. This is demonstrated by developing a peroxidase-based biosensor using these platforms. A self-assembled monolayer of a certain thiolated molecule was used for the stable anchorage of the biorecognition element on the GNP surface. This biosensor was successfully applied to the quantification of catechol as a target analyte of environmental interest. Overall, this Thesis is meant to show feasible miniaturized analytical approaches based on the use of microfabricated transducers for the rapid and reliable monitoring of different parameters in matrices of environmental interest, which could be considered as a good alternative to those expensive and cumbersome systems currently applied for such purposes.