New electrochemical sensors for decentralized analysis

Resumen

This thesis explores simple, robust and cost-effective analytical tools based on electrochemical techniques for relevant molecules determination. Mainly, potentiometry technique is used as a simple to operate, low-power consumption and versatile technique to generate and expand new affordable platforms in the wearable and point-of-care fields over a wide range of applications such as healthcare and fitness, environment and security missions among others. The increasing population and aging of the human being has risen huge challenges for the present society to maintain welfare. Hence, new opportunities are emerging in order to solve the issues presented to the society. Healthcare expenditure, pollution and security are some of the main issues that concern society. The development of low-cost manufacturing processes and cost-effective tools to solve this problems are crucial objectives for research and innovation today. For this reason, in analytical chemistry, we must generate new tools to determine, detect and monitor essential markers and provide an accurate feedback to society. In order to provide useful tools to society, the development of sensors and methods of detection for capturing data in decentralized settings and thus decrease the expenditure in lab-based analysis is the main goal of this thesis. Advances in potentiometry such as the solid-state technology provides the ability to develop robust interfaces able to interact with the chemical information of our environment (biological fluids, environmental samples, etc.) in a convenient way. This interaction yields a huge amount of data that is transferred and processed via Internet to possibly solve the present society issues. Therefore, the use of low-cost materials such as commercial carbon fibers, commodity textiles and paper or even commercial balloons to build potentiometric sensors has been addressed along the thesis. The technology already developed by URV Nanosensors group enable to fabricate a full potentiometric cell using different substrates for a completely decentralized setting. Ion-selective electrodes are used to build robust solid-state sensors for an outstanding contribution in the wearable chemical sensors area. Besides, a new approach to develop enzyme-based potentiometric electrodes on paper substrate for biomolecules monitoring is successfully presented. This thesis has principally focus in two directions: First, the research and development of new sensors/interfaces by the use of tailored inks and materials to fabricate affordable ion-selective sensors with wireless transmission data systems is provided. Moreover, new approaches in the development of highly sensitive potentiometric enzyme-based tools taking advantage of the Nafion polymer features are generated. Second, considerations in the design of novel platforms for the end-user application is reflected by the use of wearable approaches such as textile that mimics conventional cloth or even patches integrated directly on the skin. In this case, the main objective is the use of these electrochemical sensors to acquire reliable data without compromising the integrity of the wearer. Interestingly, both analytical performance of the sensor and the design of the sampling cell are of the same significance for a successful fabrication of the device. All in all, the development of the sensor and considerations on the design have been taking into account during all the work in order to satisfy the applicability of the research in real scenarios. The thesis has been divided into the following chapters: • Chapter 1 is mainly focus in the introduction to the information age and the needs of this area in the present society. It also provides an overview of the principal elements involved in this system: interfaces/sensors, internet of things, big data and information economy. It makes emphasis in the field of sensors, especially in the affordable and wearable chemical sensors as well as give some insights in the use of advanced materials. • Chapter 2 illustrates briefly the fundamentals of electrochemistry and explains more in detail potentiometry through a theoretical approach. It also describes the principles of the most used type of sensor in this thesis, solid-contact ion-selective electrodes as well as some analytical parameters. • Chapter 3 provides information about the experimental characterization techniques, fundamentals, procedures and instrumentation common over the chapters. For instance, electrochemical impedance spectroscopy, cyclic voltammetry, adsorptive stripping voltammetry and potentiometry are described. • Chapter 4 & 5 corresponds to the development of full wearable potentiometric sensors in commercial carbon fiber substrate and commodity textile by the use of ion-selective membrane and solid-state reference membranes. On-body measurements for sodium concentration in sweat matrix during cycling exercise is demonstrated. Besides, the versatility of sensor printing techniques over different textiles substrates is achieved. • Chapter 6 follows the wearable approach, but it has been focused in a chemiresistor sensor to determine liquid amount and designed for monitoring epidermal sweat loss. First, the mechanism is explained by a phenomenological approach. Second, a full characterization of the liquid sensor is performed. Finally, on-body test with wireless integration is presented in detail. • Chapter 7 & 8 approaches a novel technique to develop enzyme-based potentiometric electrodes. Accordingly, chapter 7 focus on the understanding of hydrogen peroxide detection mechanism using a Nafion coated electrode. Subsequently, chapter 8 contains the main information to develop a paper-based enzymatic electrode for glucose determination in biological samples. • Chapter 9 describes a new methodology to study the resiliency of conductive nanocomposite inks used to fabricate electrochemical sensors in a conventional rubber balloon. Moreover, a demonstration of the sensor performance to monitor explosive compounds is also presented. • Chapter 10 points out main conclusions derived from the experimental work and suggests future prospects. Basically, in this thesis, the development of fully electrochemical cells using different commodity substrates (filter paper, textiles, and rubber) has been accomplished. A wearable potentiometric sensor has been developed in different substrates (commercial carbon fibers and textiles) for electrolytes monitoring in sweat. Moreover, a proposal related to the calibration issues of potentiometric sensors has been addressed. Furthermore, sensors build in or with commodity materials allows to decrease the manufacturing costs thus enabling the development of disposable sensors. In this manner, sensors might undergo a previous calibration curve in the manufacturing facilities, and then, be ready to use by the final users. Of course further studies on the stability and storage must be dictated to drive the sensor as a final product. The versatility of the printing technique over different substrates have also been probed, potentiometrically characterized and integrated in a wireless potentiometer for multi-ion monitoring. Besides, tailored materials have been developed to endure daily mechanical stress conditions, such as stretching, bending or wrinkling without compromising the analytical performance of the system towards physiological parameters monitoring such as different electrolytes in sweat. In addition to the electrolyte concentration monitoring in biological fluids, the need of liquid loss monitoring is interesting to determine the physiological status of the human body. For this reason, a wearable patch for sweat loss quantification on human body has been developed. A paper-based sensor provides high accurate sensitivity to water content by changing its conductivity. The sensor has been widely characterized for a suitable user application as well as it has been integrated with a Bluetooth reader for on-site use. Hence, the chemiresistor is relevant for monitoring the dehydration status in both health and sport fields. This type of sensor provides the system the ability to avoid the use of a reference electrode as well as a calibration free approach. Interestingly, the combination of both technologies allows to measure the exact amount of electrolyte loss thus knowing the exact amount of salts that the human body must recover. This application is useful for healthcare, wellbeing and especially long-term fitness activities among others. Huge efforts have been devoted to develop potentiometric enzyme-based sensors for biomolecule determination. First, an outstanding potential sensitivity has been demonstrated by the use of Nafion ion exchange membrane in addition to redox sensitive ability of platinum substrate for hydrogen peroxide detection (byproduct of the enzymatic reaction involved by oxidases). This sensing mechanism has been conceptualized with the development of an enzymatic paper-based sensor where glucose oxidase is entrapped between Nafion membrane layers in order to determine glucose concentration in biological fluids. Eventually, advanced materials that are able to overcome common mechanical deformation as well as provide a suitable platform for the development of an electrochemical cell has been accomplished. For this reason, a new approach for the fabrication of screen-printed electrodes over a commercial rubber balloon has been realized. The study of stress-enduring inks over an expandable platform is crucial to overcome daily potential mechanical deformations without losing the electrochemical performance of the printed device. Last but not least, the applicability of the balloon-embedded sensor is demonstrated towards explosive compounds monitoring. All in all, this thesis has paved a new way to develop cost-effective sensors for present growing social demands. The demonstration of a successful development of low-cost electrochemical sensors has been presented along the thesis. Moreover, the fabrication of robust devices for the use in decentralized settings has been exemplified. These new devices are ready to be utilized as interfaces or gateways for gathering valuable chemical information. Later, this information will be economically exploited along the information based economy chain. Hence, electrochemical sensors have a crucial role for building highly valuable information networks thus leading to high social impact and new businesses creation.