1 Department of Micro- and Nanotechnology, Technical University of Denmark
Electrochemical systems are well established tools used to determine the presence of target analytes in a broad range of fields such as clinical, environmental, food, or industrial applications. Inexpensive, simple, versatile, and highly reproducible, the thick-film technology of screen-printing (SP) features several advantages and thus appeared as the evident fabrication process for the development of disposable electrochemical sensors in this project. Due to the popularity of this technology, a very expansive variety of SP products is currently available on the market, especially in terms of inks for electronically conducting materials and dielectric materials. However, the precise composition of these materials is kept as proprietary information from the manufacturers. In this thesis, an assortment of some of the most commonly used products was investigated. It was demonstrated that significant differences in terms of electrochemical, mechanical and electrical properties exist between these products. The effect of electrochemical and thermal treatments of the screen-printed materials was also investigated. This study resulted in the selection of an optimal electrochemical system used for further electrochemical investigations in this thesis. Based on these electrochemical systems, the fabrication of potentiometric pH-sensors featuring a photo-curable polyurethane membrane as ion-selective-membrane (ISM) was then studied. The choice of the membrane was motivated by the fact that such material is very attractive from a technological point of view since compatible with standard photo-lithographic processes and thus easier to streamline than commonly used polyvinylchloride membranes. Prior to the membrane deposition on the screen-printed electrodes, a series of electrode treatments were used in order to increase the double layer capacitance of the graphite based sensors and thereby increase their potential stability. Electrochemical activation by cyclic voltammetry and optimized thermal treatment of the graphite sensors were used. The final potentiometric pH-sensor was composed of a coated-wire electrode (CWE) and a quasi-reference electrode (QRE) and displayed the excellent pH response of -60.8 ± 1.7 mV/pH over a six day period, which is very close to the theoretical Nernstian value. In order to further improve the stability of the CWEs, the conducting polymer PEDOT-PSS was deposited between the graphite electrode and the ISM to act as ion-to-electron transducing material. In this type of solid-state electrochemical system, the potential stability is defined by the redox capacitance instead of the double layer capacitance as in the case of CWEs. The dependence of the thickness of the PEDOT-PSS layer on the capacitance and the pH response of the final pHsensor were investigated. It was observed that the thicker the PEDOT-PSS layer, the higher the capacitance of the sensor but, unexpectedly, the lower the pH-response of the final sensor. In order to support these results and understand them in depth, additional experiments are needed. Moreover, the choice of the CP as ion-to-electron transducer for pH-selective electrodes needs to be investigated in more details. Alternatively, polymers that have proven to be suitable for such purpose such as polyaniline or polypyrrole could be used. Finally, the development of a screen-printed voltammetric system for pH monitoring was attempted. The main characteristic of this system was that it integrated both sensing and reference electroactive species in the graphite matrix of the working electrode. Sensors were fabricated by screen-printing a graphite paste loaded with phenanthraquinone (PAQ) as a pH-sensitive moiety (i.e., indicator species) and dimethylferrocene (Fc) as a pH-insensitive moiety. This represented a much simpler and faster technique compared to, for example, covalent chemical derivatization on carbon materials with electroactive species. Moreover, to my knowledge, the use of SP for the development of this type of voltammetric systems has surprisingly only been recently investigated by D. K. Kampouris et al.  despite its undeniable advantages. Electrochemical measurements highlighted the promising performances of such electrochemical system. It was shown that the oxidation peak of the Q/QH2 redox couple and the reduction peak of the Fc/Fc+ redox couple could be successfully used to monitor pH. A super Nernstian pH response was displayed by the sensors which still remains not very well understood. However, the sensors responded to pH changes in a very reproducible way despite their very simple fabrication processes. Moreover, the developed voltammetric system presented the major advantage of limiting the potential issues stemming from the reference half-cell. Indeed, since the measurement principle was based on evaluating the difference between the redox peaks of the two electrochemical species, potential drift of the RE was thus not as crucial as for other common electrochemical setups.