Conducting polymer nanostructures for implantable biosensing applications

Conductive polymers such as polyaniline, polypyrrole and PEDOT (poly(3,4-ethylenedioxythiophene)) have been used extensively to enhance the sensitivity, versatility and biocompatibility of electrochemical- and biosensors. Both their electrochemical stability and their mechanical properties make them highly suitable for applications that require sensors to be implanted in the body for longer periods of time. In addition, they provide great opportunities for nanostructuring, due to the inherently bottom-up nature of electropolymerization from monomer solution, by which the active surface area can be increased. In this project, various conducting polymer nanostructures were created on planar metal and polymer electrodes using templatefree electropolymerization methods. Several methods, all template-free and performed in aqeous environment, were used. The most succesful method, because of its versatility, simplicity, controllability and its potential to create high surface area structures, relies on reduction of the polymerization current in steps. This promotes heterogeneous growth of polymer in later steps, creating nanowires and hierarchical structures. After the formation of nanostructures on gold and PEDOT planar electrodes, the changes in electrochemical characteristics were investigated using cyclic voltammetry and amperometry, by measuring the changes in their interaction with a well-known redox couple, hexacyanoferrate(II) and -(III). The results show that modification of gold electrodes with conducting polymer nanostructures has strong effects on charge transport and may significantly increase the sensitivity of a sensor based on this platform. Electrochemical impedance spectroscopy confirms that all created nanostructures decrease charge transfer resistance and diffusion impedance to some degree, and some decrease them by orders of magnitude. Adding nanostructures to PEDOT electrodes is less effective, with only polyaniline nanowires demonstrating an increased sensitivity and decreased diffusion impedance. In addition to this method being a simple, cheap and controllable way to increase the active surface area and sensitivity of electrochemical sensors, there are several other advantages of using it to modifiy electrodes. The electropolymerization process lends itselfwell to effective immobilization of bioreceptors on the surface, and because of the large surface area of the nanostructures, the number of accessible binding sites can be increased, increasing the sensitivity of the biosensor. This stable immobilization, as well as the electrochemical stability of the polymer itself and mechanical properties that match those of soft tissue better than those of more conventional electrode materials do, make it possible for sensors to operate inside the body for longer periods of time.Mechanical, Maritime and Materials EngineeringPrecision and Microsystems EngineeringMicro & Nano EngineeringMNE 2016.03