Silicon, Silicon Carbide, and Gallium Nitride Nanowire Biosensors




Williams, Elissa H.

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Semiconductor nanostructures, such as silicon (Si), silicon carbide (SiC), and gallium nitride (GaN) nanowires (NWs), arranged as the active sensing element in an electrical device, present many advantages over the conventional methods used for biological detection. While Si NWs have demonstrated compatibility towards functionalization protocols, as well as the sensitive and selective electrical recognition of a wide range of biomolecules, SiC and GaN NWs have not yet been studied extensively in their propensities towards biofunctionalization. As a result, a solution-based functionalization protocol was developed for the specific attachment of the streptavidin (SA) protein to Si, SiC, and GaN NWs for the development of portable, highly sensitive, and selective biosensors. Successful SA immobilization on the functionalized Si, SiC, and GaN NW surface was proven using fluorescence microscopy, field emission scanning electron microscopy, high resolution transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. After demonstrating Si, SiC, and GaN NW functionalization and biocompatibility, Si NW field-effect-transistor (FET) type devices were fabricated for sensing bacterial cell membrane interactions and mechanisms. Spontaneous formation of a lipid bilayer around Si NW devices was found to occur upon exposure to 50 nm liposomes consisting of phosphatidylcholine (PC) and phosphatidylglycerol (PG), the lipids present in an E. coli cell membrane. Lipid bilayer formation on the Si NWs was detected by real-time electrical resistivity changes and it was found that the negative charge associated with the PC/PG layer resulted in an induced negative gate on the NW surface and caused a 1-2 % decrease in NW conductance. A strong detergent, Tween20, was found to remove the lipid bilayer from the NW surface very quickly and result in the return of NW conductance to its baseline value in buffer. Formation and destruction of the lipid bilayer on the NW devices was also confirmed by confocal fluorescence microscopy. The E. coli cell membrane encapsulated Si NW FET based biosensors demonstrate a novel platform to electrically probe bacterial cell membrane reactions.