UCLA

This thesis asks a different question than most cortisol-sensing work. It asks not whether cortisol can be detected, but whether the field-effect transistors (FETs) used for detection remain electrically stable over anticipated monitoring periods. Previous work has demonstrated cortisol detection using aptamer-functionalized indium oxide field-effect transistors, but stability over prolonged intervals remains underexplored. Importantly, the surface-preparation steps required before aptamer attachment affect FET electrical stability, and it remains unclear whether this stability is preserved through these steps.This thesis addresses these questions in three stages. The first stage involved a pilot experiment in which bare, untreated In₂O₃ thin-film FETs were exposed to phosphate-buffered saline (PBS), a simple surrogate body fluid. The devices exhibited distinct electrical failure patterns, including increased gate leakage, loss of output saturation, and progressive threshold shifts. This motivated the second stage of the thesis, which examined the surface-preparation workflow used before aptamer functionalization. The workflow began with bare devices, then added dodecanethiol to passivate the gold source and drain electrodes, and progressively incorporated additional pre-aptamer surface chemistry used in the Andrews group’s sensing protocol.The third stage evaluated how electrical states of the FETs changed over time after different stages of surface chemistry. The surface-condition groups were compared under matched air controls and PBS exposure. The results identify how successive surface-preparation steps and environmental exposure contribute to the electrical stability of In₂O₃ FETs, providing a basis for distinguishing sources of instability as the devices progress toward fully assembled aptamer-based sensors. One separate supplemental document (Supplemental Data S1–S3) provides complete device-level and raw-detail electrical records for the Chapter 3 protocol-development experiments, Chapter 4 surface-processing characterization, and Chapter 5 longitudinal environmental-exposure study.