MSE PhD Prospectus Defense of Ang Liu

3:00 pm on Friday, September 25, 2020

ABSTRACT: Detection of chemical molecules and biomarkers are important in basic and applied science as well as in medical diagnostics and medicine. In this work, n-channel enhancement-mode silicon nanowires (NWs) field-effect transistors (FET) - based nanobiosensors are introduced as potential point-of-care (POC) medical devices for health monitoring and disease diagnosis. The devices work on the semiconducting silicon NWs being used as the conductive channel between the source and drain terminals, and the selective bindings of charged entities (for example, H+ or proteins) to the pre-anchored covalently-crosslinked counterparts on the ion/biologically-sensitive surface of NWs modulate NWs’ conductances like applying a gate voltage changes the channel conductance. Because of the high surface-to-volume ratio and other outstanding characteristics of NWs, these devices can be used for label-free, real-time and highly sensitive detection of small molecules, DNA and proteins.

Here presents a novel top-down wafer fabrication process for mass production of Si NWs FET-based nanobiosensors, all the way from CAD design, e-beam/photo/soft lithography, thin film deposition, to dicing, surface functionalization and packaging. This method will enable scalable manufacturing and testing of complex nanosensor structures with high efficiency and precision. The systematic study of the device characteristics starts with the optimization of the drain-source/backgate voltages and the transconductance. pH detection results show that APTES-modified Si NWs surface will activate the pH-dependent linear response of conductances or total surface charge density. We also investigated the electric double layer (EDL) which forms around the charged proteins, and studied the barriers of Debye screening that exist in high-salt physiologically relevant electrolytic solutions, such as blood serum, which puts a limitation on biomarker detection. By applying an external static/dynamic electric field (DC/AC) by the on-chip patterned electrodes we discovered a modulation of local charge density in affinity sensors at a characteristic frequency which could be explained by the breakdown or change in polarization/capacitance/dielectric properties of EDL exposing the charges of the protein core. Future work will involve measuring zeta potential/streaming potential and metabolism sensing in which a new scheme has been proposed using an immobilized enzyme to catalyze a redox reaction.

COMMITTEE: ADVISOR Raj Mohanty, MSE, Physics; Shyam Erramilli, MSE, Physics, BME; Anna Swan, MSE, ECE; David Bishop, MSE, ECE, ME, BME, Physics; Final Defense Chair: Uday Pal, MSE, ME