Committee Members:
Prof. Eric Vogel, Advisor, Advisor, MSE
Prof. Natalie Stingelin, MSE/ChBE
Prof. Rosario Gerhardt, MSE
Prof. Oliver Brand,  ECE
Prof. Peter Hesketh, ME

The Impact of Mass Transport and Surface Charging on The Performance of Field Effect Transistor Biosensors

Abstract: 

Biosensors are critical components of the healthcare system and are used for a wide variety of applications including screening of serological diseases, early detection of infectious viruses, and chronic disease treatment.  This work aims to improve the performance of FET biosensors through elucidation of fundamental material concepts that control sensor performance and reliability. Previous work has suggested that FET sensors made from semiconducting nanowires have improved limits of detection compared to traditional planar FETs. In this work, a general model was develop including the effects of reaction, diffusion, and convection applicable to both planar and nanowire sensors. The model shows that, given enough flow, the accelerated diffusion observed by a cylindrical nanowire can be ignored. Rather, the sensor size has a greater effect under flow with nanometer sized sensors capable of reaching the reaction-limited regime. After a molecule successfully attaches, it must generate a measurable signal. One aspect often ignored is that surfaces themselves are charged before any biomolecule attachment. This surface charge can also be variable, depending on solution pH, as is the case for amphoteric surface sites of metal oxides. This pH dependent surface charge arises as a pH sensitivity of the sensor. A theoretical model was previously developed demonstrating that a pH sensitive surface such as a metal oxide can pin the surface potential, reducing the surface potential shift caused by biomolecule attachment. This work uses gFETs to investigate the relationship between pH sensitivity and biosensitivity. The results show that the existence of pH sensitivity does not simply cause a decrease in biosensitivity. Rather, this decrease does not occur unless sufficient surface charging is present, which is dependent on multiple factors such as ion concentration and the acid dissociation constant of the surface groups. Furthermore, the results suggest that the effect of ionic screening may be overestimated as has been previously demonstrated in literature. Another challenge encountered with FET sensors is integration into complete sensor systems. Two sensor prototypes were developed here. First, a multiplexed sensor chip was developed for use in a 3D printed, wireless, biosensor capsule designed for continuous monitoring of cell properties in bioreactors. The multiplexed sensor chip can detect pH, glucose, lactic acid, and protein biomarker detection is under development. Additionally, a Au EGFET E. coli sensor was developed for use in a drone-operated biowarfare defense system. E. coli sensing was demonstrated with a LOD of 107. The performance of FET sensors depends upon fundamental material concepts such as mass transport and surface charging.