1 Department of Micro- and Nanotechnology, Technical University of Denmark2 Department of Chemistry, Technical University of Denmark
This thesis presents fabrication strategies to produce different types of all-polymer electrochemical sensors based on electrodes made of the highly conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT). Three different systems are presented, fabricated either by using microdrilling or by hot embossing. Microdrilling was applied to fabricate arrays of circularas well as tubular electrodes, while hot embossing was used to produce planar electrodes. Arrays of circular electrodes were produced by using a new fast prototyping strategy for fabrication of microelectrodes. Electrical resistance-controlled microdrilling was applied to drill through an insulating polymer, covering a conductive layer of PEDOT. The sudden drop in electrical resistance between the metal drill and the PEDOT layer upon physical contact was employed as stop criterion for the drilling process. Arrays of 3x 3 microelectrodes of diameter 30 µm or 100 µm, respectively, with center-to-center electrode spacings of either 130 µm or 300 µm were fabricated. Their functionality was verified by amperometry on potassium ferro-/ferricyanide. Comparison of the experimentally obtained results to finite element modeling of the respective electrode configurations showed that the conducting polymer electrodes approach the steady state currents predicted from modeling, but at a much slower rate than expected. This wasshown to be caused by the use of electro active PEDOT electrodes. Subtraction of the latter contribution gave an approach to steady state currents within a few seconds, which was in very good agreement with the modeled response time. Arrays of tubular electrodes were fabricated by drilling through a cyclic olefin copolymer (COC)foil which was modified with spin coated layers of polystyrene, PEDOT, and a second layer of polystyrene. The drilling process resulted in a cylindrical drilling shaft, later used as microfluidic channel, and a tubular electrode integrated in the shaft sidewall. A modification of the backside of the COC foil with a polystyrene and a PEDOT layer before the drilling process allowed the simultaneous fabrication of an additional planar electrode at the end of the drilling shaft. Arrays of ten Ø 100 µm tubular electrodes (center-to-center spacing of 300 µm) were produced. The reproducibility of the fabrication method was confirmed by consistent amperometric responses of independent fabricated electrode arrays towards potassium ferrocyanide. A sensor application was demonstrated by amperometric detection of hydrogen peroxide concentrations in the range of 0.1 to5 mM. Planar electrodes were fabricated by hot embossing of a microfluidic channel with sloped sidewalls into a PEDOT covered COC bulk material. During embossing the PEDOT layers at the sloped sidewalls became part of the channel while retaining electrical contact to the surface layer. Consequently, the PEDOT layers at the sloped area could be used as electrodes. Electrodes placed on opposite channel sidewalls were spatially separated by the microfluidic channel in between. The electrical insulation of the PEDOT electrodes from the PEDOT at the channel bottom was achieved by embossing a vertical step. Functionality of the final thermally bonded system was shown by amperometric detection of physiologically relevant glucose concentrations (0–10 mM).