Fabrication and Characterization of Organic Polymer-based Memristive Biosensors

In recent years, research activities have seen a significant surge in the development of novel sensors, including those based on the memristive effect. Memristive biosensors, in particular, could pave the way towards the in-memory sensing and in-memory computing concepts, which nowadays are becoming increasingly intriguing and useful as alternatives to traditional Von Neumann architectures. These biosensors have demonstrated the ability to perform biosensing by establishing a direct correlation between the voltage gap (the distance between the voltages corresponding to the minima of the current in the semilogarithmic current-to-voltage plot) and the analyte of interest, enabling highly sensitive detection with a very low limit of detection. This experimental master’s thesis explores a straightforward approach to fabricating and characterizing organic polymer-based memristive devices. Facile and cost-effective techniques, such as spray coating, are used. The devices have a simple structure consisting of two electrodes, which are created by patterning a photoresist on polyimide in a single step using a negative photolithography process. A double layer of titanium and gold is then deposited using e-beam and thermal evaporation, respectively, followed by a lift-off process. The active material layer, made from a blend of poly(3,4 ethyldioxythiophene) poly(styrenesulfunate) (PEDOT:PSS) and poly(vinyl alcohol) (PVA), is subsequently deposited by spray coating to connect the two electrodes. The devices are then functionalized using two distinct methods. The first involves the application of an ion-selective membrane for ammonium detection. The second is a more intricate biofunctionalization process aimed at detecting prostate-specific antigens (PSA) using anti-prostate-specific antibodies, which is the core focus of this work. Both functionalization processes draw insights from the literature. Throughout this research, various morphological characterization techniques are employed, including optical microscopy, stylus surface profilometry, and atomic force microscopy. These techniques are used to assess the success of the fabrication steps and to investigate the morphological properties of the PEDOT:PSS-PVA layer. Electrical characterization is used to analyze the devices' response to different analyte concentrations. Each device is tested under low-voltage conditions (below 1 V to prevent water electrolysis) using a voltage loop to obtain the characteristic non-pinched hysteresis, as reported in the literature. The resulting data is then analyzed to examine the relationship between the voltage gap and the various analyte concentrations. As expected, the devices tested for ammonium ion detection did not yield promising results, likely due to the insufficient charge of the ions for effective memristive biosensing. In contrast, the results from PSA detection were promising and successful within the range of clinical interest. However, future work is needed to reduce device variability and refine the fabrication process. This could pave the way for further studies and potentially allow these devices to serve as building blocks in in-memory sensing and in-memory computing platforms, once they are fully developed. Finally, it should be noted that this thesis does not address the theoretical aspects and questions that arise from working with such a novel technology. Therefore, it would be valuable to dedicate additional efforts to exploring the underlying physics of these devices.