Fully Integrated Electrochemical Sensor Based on Surface Activated Copper/Polymer Bonding for Lead Detection

Abstract

Lead (Pb) levels in tap water below the established water safety guideline are now considered harmful, thus detecting sub-parts-per-billion level Pb is important. This thesis reports on a miniaturized Copper (Cu)−based electrochemical sensor fabricated from thick film electrodes for their superior sensing performance. These thick film electrodes are based on highly conductive rolled-annealed Cu foil that has a compact bulk structure, but these advantages are often offset by the fact that RA Cu foil is difficult to bond to a substrate due to poor film-adhesion property and lack of mechanical interlocks. For this reason, we develop a direct bonding process for Cu/polymer. An integrated three-electrode planar configuration is then fabricated on the bonded specimen to achieve a fully-functional sensor that can detect 0.2 μg/L (0.2 ppb) Pb2+ ions from a 100 μL sample in only 30 s. This is the most rapid detection of Pb featured to date by an all Cu-based sensor. This thesis first focuses on improving substrate adhesion of RA Cu foil to liquid crystal polymer (LCP). This is achieved by a surface activated bonding process where Cu and LCP surfaces are treated with low-power reactive ion etching oxygen plasma followed by low-pressure contact at 230 °C. This treatment produces hydroxyl (OH−) groups on Cu and LCP surfaces making them highly hydrophilic. When Cu and LCP are contacted and heated, the OH− chains condense by dehydration and form an intermediate oxide layer. This layer mainly develops as Cu2O nanoparticles from the plasma-treated Cu side due to thermal oxidation in air. These nanoparticles diffuse into the polymer substrate when heated under mechanical pressure, resulting in a strongly bonded flexible specimen for the sensor. A simple, inexpensive, and production-friendly fabrication process is then developed for these sensors. Following direct bonding, flexible Cu/LCP is fed into a LaserJet printer for a one-step transfer of polyester resin−based electrode mask on Cu. This is followed by etching, packaging, and a chlorinating process to achieve a fully-functional integrated sensor. The sensing performance of directly bonded Cu/LCP is comparable to that of commercially available Cu/polyimide (PI) laminate. Our approach holds promise towards realizing low-cost integrated water quality monitoring systems.