Synthesis of Responsive Polymer Brushes for Sensing Applications

During the past decades the field of sensors has been subject to much attention due to an increased demand for (bio)sensors and environmental monitoring. The efforts have been concentrated to create sensors which are reliable, highly sensitive and selective, small and fast responding. Surface modification by polymer coating has been proven an excellent method to introduce selectivity on actuators. Among the various techniques that allow the formation of polymer thin film, polymer brushes have gained attention along the past decades due to their unique structure and the possibility offered by controlled/"living" surface-initiated radical polymerization technique to generate polymer thin film with precisely controlled thickness, composition and architecture. Polymer brushes have found numerous applications including nonbiofouling surfaces and cell adhesive surfaces, protein binding and immobilization, chromatography supports, membrane functionalization, responsive surface, antibacterial coatings or low friction surfaces. Despite their interesting properties and the numerous reports describing the potential of polymer brush as responsive surface, their use for "real" sensing applications has receive little or no attention so far. This Thesis describes how polymer brushes can be employed as selective surface modification for sensing application. We aimed at synthesizing polymer thin film able to detect analytes of interest, with a particular focus on low detection limit and high selectivity. After a short introduction to the field of polymer brushes (Chapter 1), Chapter 2 presents a review of the work accomplished in the field of responsive polymer brushes with an emphasis on solvent responsive, thermoresponsive, pH- and ion-sensitive polymer brushes. The pH-induced swelling and collapse of surface-tethered, weak polyelectrolyte brushes is of interest not only for the development of responsive surface coatings but also for the pH controlled transport or adsorption. Chapter 3 discusses results of an extensive series of quartz crystal microbalance (QCM) experiments that aimed at further understanding the influence of brush thickness and density on the pH-responsiveness of poly(methacrylic acid) (PMAA) brushes and developing strategies that allow to engineer the pH responsiveness and dynamic response range of PMAA based brushes. It was observed that due to their high grafting density, the apparent pKa of surface-tethered PMAA differs from that of the corresponding free polymer in solution and also covers a broader pH range. The pKa of the PMAA brushes was found to depend both on brush thickness and density; thicker brushes showed a higher pKa value and brushes of higher density started to swell at higher pH. The second part of this section demonstrates the feasibility of the N-hydroxysuccinimide-mediated post-polymerization modification to engineer the pH responsiveness of the PMAA brushes. By using appropriate amine functionalized acids, it was possibl