Fabrication of a high aspect ratio biosensor for bacteria detection

Even if most of the micro-organisms we carry are harmless and even beneficial, some are dangerous and may be at the origin of different infectious diseases. For the health sector as well as for food industries, there is an urgent need for reliable, rapid and affordable solutions to detect, in the most accurate way possible, the concentration of those pathogens. Nowadays, traditional methods which exist can achieve a reasonable limit of detection but are not user friendly. Biosensors are an alternative that could, in the coming years, combine a compact design with a high sensitivity and selectivity. Among the numerous transducer means available, this master thesis will focus on an impedance-based interdigitated electrodes (IDE) sensor previously developed by Dr. Numa Couniot at the UCL. Our aim is to study the ability of thicker electrodes to capture and detect bacteria and to compare these results with standard electrodes. Indeed, increasing the electrodes thickness leads to a greater capacitance value but also to a larger cavity of concentrated electric field in which bacteria should settle. However, changing one parameter requires to re-think the whole process of manufacture. Indeed, aluminium, which was selected for an eventual integration with CMOS technology, can no longer be used as main component of electrodes. Furthermore, special resists must be used to reach thicknesses greater than 3 μm. This master thesis presents a new process based on electroplated nickel forming the sensor's electrodes. Once fabricated, sensors were tested under dry environment to compare their raw electric properties with theoretical values. For the impedance modulus, we obtained an average discrepancy less than 1%. Afterwards, they were tested under liquid environments with an inflow of Staphylococcus Epidermidis in PBS through a microfluidic system. To increase the adhesion of bacteria to the sensors' wall, we use polydopamine also known as a biological glue. The results show that the normalised admittance increased all along the incubation period during which bacteria can be deposited on electrode walls. We also calculated the sensitivity of each sensors as the weighted ratio of the difference between the admitance with and without bacteria. For the purpose of the comparison we defined a metric S_M which was the average of sensitivities calculated at 100 kHz and 1 MHz. An optimal sensitivity was observed with electrodes 'thickness of 5 µm. However, we can not certify that this result was reproductible. We provided nevertheless a list of improvements that could increase the certitudes.Master [120] : ingénieur civil biomédical, Université catholique de Louvain, 201