Electrolyte-Gated Transistors for Neuromorphic Signal Processing and Biosensing

This thesis describes the design, fabrication and testing of neuromorphic electronic devices designed to detect target biomolecules in solution. Within these devices, the critical sensing layer was formed from a polymer treated using a plasma processing method known as plasma immersion ion implantation (PIII). Polymer layers with sub-micron thicknesses were effectively modified using a 2.5 kV Ar PIII process, introducing free radical covalent binding sites to their surfaces. When applied to the treated polymer layer, protein enzymes became covalently bound to the activated surface. This binding persisted for up to 24 hours after the PIII process. The PIII modified polymer sensing layers were integrated as gate dielectric layers in lateral three-terminal electrolyte-gated devices. These devices featured source, drain and gate electrodes with a liquid channel connecting the source and drain. This liquid channel was modulated using an applied gate potential. As well as demonstrating transistor-like output and transfer characteristics, the devices were capable of producing outputs resembling postsynaptic signals given pulsed gate voltage (presynaptic) inputs. The postsynaptic current through the electrolyte solution and between the source and drain electrodes was sensitive to electric double layers formed at these electrodes. The dynamic response of the double layers to the presynaptic input produced a current output that resembled a spiking postsynaptic signal. Paired-pulse depression, postsynaptic saturation and spike rate-dependent plasticity were all observed in the postsynaptic output characteristics. Covalently immobilizing horseradish peroxidase (HRP) on the plasma-modified dielectric gate layer of the devices significantly influenced the dynamics of the double layers (formed around the bound HRP), the channel conductance and the device characteristics. Hence, the ability to sense the presence of bound biomolecules via the modulation of the neuromorphic device output was demonstrated using the inexpensive HRP biomolecule. Tests were then performed with transactive response DNA binding protein (TDP-43), a biomarker for neurodegenerative diseases. Detection occurred when the target antigen (present in solution at physiologically realistic concentration) bound to TDP-43 antibodies which were covalently immobilised on the PIII modified gate-insulating layer within each device. This postsynaptic signal was sensitive to the presence of bound and unbound biomolecules. Crucially, the paired-pulse index, postsynaptic current decay, and the cumulative charge passed in tailored pulse sequences enabled the true detection of TDP-43 antigen-antibody binding within the device electrolyte, even in the presence of interfering biomolecules. The devices reported were designed for scalable production and are suitable for disposable use in population screening applications. In summary, the findings reported in this thesis demonstrate the potential for the electrolyte gated biosensing devices as low-cost, sensitive and rapid biosensing devices.<p></p>