Surface functionalization of a silicon nanowire field-effect transistor (SiNW-FET) for miRNA detection

Abstract

Micro RNAs (miRNAs) are promising biomarkers of various physiological and pathological processes. Since miRNAs are found in very low concentrations (e.g., attomolar), their detection requires highly sensitive devices. Recently, silicon nanowire field-effect transistors (SiNW-FETs) have drawn attention due to their high sensitivity and low detection limits. SiNW-FETs can be tailored to detect miRNA by employing surface functionalization strategies. The miRNA recogni-tion event that occurs at the sensor surface is transduced by the SiNW-FET in a detectable electronic signal. The main goals of the thesis are to propose and discuss different possibilities to functionalize a SiNW-FET for miRNA detection, and to experimentally evaluate the potential to detect miRNA with provided SiNW-FET devices. Additionally, different methods for regeneration of a SiNW-FET for miRNA detection are proposed and discussed since, in the future, the SiNW-FETs are meant to be integrated in in vitro cell culture platforms. Ideally, sensors for in vitro cell platforms would allow multiple sampling that can be enabled through surface regeneration. The thesis is divided into two major parts: a theoretical and an experimental approach. In the theoretical part, four functionalization methods are proposed. Two methods involve treating SiNW-FETs with (3-aminopropyl)triethoxysilane (APTES) and attaching DNA probes via glutar-aldehyde (GA) or L- ascorbic acid (AA) crosslinkers. AA-based functionalization is a novel method that has not been yet reported for design of miRNA biosensor. The other two methods allow DNA probes to be directly attached to (3-glycidoxypropyl)trimethoxysilane- (GPTMS) or 1,1´-carbonyldiimidazole- (CDI) modified silicon nanowires (SiNWs). To enable miRNA SiNW-FET reusability, the thesis proposes thermal-, chemical-, and en-zymes-based methods to detach the miRNA molecules from the DNA probes. The enzyme-based regeneration proposed in the thesis is a novel approach that has not been yet reported in miRNA SiNW-FET regeneration. Next, the provided SiNW-FET devices were electrically characterized in order to evaluate their performance. In addition, the main environmental disturbances were tested to understand how the external factors affect the sensors. For miRNA detection experiments, the functionalization method based on APTES and GA was employed to attach DNA probes on the surface of SiNW-FETs. Even though the final miR-NA tests revealed possible positive results, a clear statement about whether miRNA was de-tected cannot be made due to the challenges related to the SiNW-FETs (e.g., gate current leakages) and uncertainties of the functionalization scheme (e.g., hydrophobicity of the resulted surface). Nevertheless, the thesis’s proposed functionalization and regeneration approaches, the miRNA handling and experiments protocols, and the test observations offer valuable information for future research.