Optimization of Surface-Protein Interactions for Next Generation Biosensors

Currently, diagnosis for serological diseases such as Ebola, HIV, and Lyme disease relies on enzyme-linked immunosorbent assays (ELISAs), which require centralized laboratories and several-day timescales to complete. However, emerging technologies such as potentiometric and electrochemical impedance biosensing can be developed into portable, label-free, point-of-care devices that require only hour timescales. Specifically, potentiometric sensing platforms can be miniaturized through cost-effective microfabrication, lend themselves to multiplexed and parallel sensing, and are easily integrated with other electronics. Despite the promise of these new label free technologies, device reliability inhibits commercialization and adoption. This work focuses on improving potentiometric sensing, primarily through understanding erroneous behavior at the sensor-solution interface. During biomolecular sensing, biomolecules in solution interact with the sensor surface. Ideally, protein recognition mechanisms are leveraged to allow only target proteins to attach to the surface, imparting signal selectivity. However, unwanted protein interactions with sensor surfaces cause signal instability and increase false-positive rates. Although commonly used to functionalize the sensing surface, carboxyl-terminated thiol self-assembled monolayers (COOH-SAMs) can have large defect densities, which in turn leads to large non-selective adsorption of proteins to hydrophobic surfaces exposed by these defects. A procedure is developed where the surface of COOH-SAMs is treated before functionalization to improve the reliability and quality of receptor attachment to the sensor surface. In this method, a preblocking protein orthogonal to the immunological system of interest is used to cover hydrophobic, non-selective sites on the sensor surface while still leaving carboxylic acid headgroups available for covalent functionalization. This methodology is advantageous when compared to standard blocking, where the receptor protein must be attached to the sensor prior to the blocking step. With traditional postblocking, non-selective adsorption and degradation of the receptor protein itself can occur, and the storage stability of the receptor must be considered since the sensor cannot be functionalized after blocking. Additionally, COOH-SAMs oxidize when exposed to ambient conditions. The impact of this degradation of sensing, as well as methods to prevent degradation, are explored. Beyond SAM-based sensors, there has been significant interest in biomedical applications of 2D materials, including potentiometric sensing. The inert basal plane of certain 2D materials, such as graphene, could lead to larger biosensing signals due to a decrease in surface pH response. However, there is conflicting literature on to what extent interaction from the substrate are transmitted through a 2D monolayer, and the subsequent effect on biomolecule interactions are unknown. Therefore, the degree to which the substrate influences graphene-protein interactions is explored. Finally, a section is dedicated to non-surface-layer sources of signal unreliability, including presentation of a model for sensor response time. The work presented in this thesis demonstrates initial steps towards reliable control over sensor-solution interfaces. Despite the current challenges facing label-free, portable biosensors, the work presented here provides a step towards reliable biosensing.Ph.D

Design Rules for Discovering 2D Materials from 3D Crystals

Two-dimensional (2D) materials are championed as potential components for novel technologies due to the extreme change in properties that often accompanies a transition from the bulk to a quantum-confined state. While the incredible properties of existing 2D materials have been investigated for numerous applications, the current library of stable 2D materials is limited to a relatively small number of material systems, and attempts to identify novel 2D materials have found only a small subset of potential 2D material precursors. Here I present a rigorous, yet simple, set of criteria to identify 3D crystals that may be exfoliated into stable 2D sheets and apply these criteria to a database of naturally occurring layered minerals. These design rules harness two fundamental properties of crystals—Mohs hardness and melting point—to enable a rapid and effective approach to identify candidates for exfoliation. It is shown that, in layered systems, Mohs hardness is a predictor of inter-layer (out-of-plane) bond strength while melting point is a measure of intra-layer (in-plane) bond strength. This concept is demonstrated by using liquid exfoliation to produce novel 2D materials from layered minerals that have a Mohs hardness less than 3, with relative success of exfoliation (such as yield and flake size) dependent on melting point.Bachelor of Scienc

Direct correlation between potentiometric and impedance biosensing of antibody-antigen interactions using an integrated system

A fully integrated system that combines extended gate field-effect transistor (EGFET)-based potentiometric biosensors and electrochemical impedance spectroscopy (EIS)-based biosensors has been demonstrated. This integrated configuration enables the sequential measurement of the same immunological binding event on the same sensing surface and consequently sheds light on the fundamental origins of sensing signals produced by FET and EIS biosensors, as well as the correlation between the two. Detection of both the bovine serum albumin (BSA)/anti-BSA model system in buffer solution and bovine parainfluenza antibodies in complex blood plasma samples was demonstrated using the integrated biosensors. Comparison of the EGFET and EIS sensor responses reveals similar dynamic ranges, while equivalent circuit modeling of the EIS response shows that the commonly reported total impedance change (DZtotal) is dominated by the change in charge transfer resistance (Rct) rather than surface capacitance (Csurface). Using electrochemical kinetics and the Butler-Volmer equation, we unveil that the surface potential and charge transfer resistance, measured by potentiometric and impedance biosensors, respectively, are, in fact, intrinsically linked. This observation suggests that there is no significant gain in using the FET/EIS integrated system and leads to the demonstration that low-cost EGFET biosensors are sufficient as a detection tool to resolve the charge information of biomolecules for practical sensing applications