A Biotechnology Perspective on Silicon Nanowire FETs for Biosensor Applications

Abstract: The study of silicon nanowire-FET-based electronic biosensor applications is an emerging scientific field. These biosensors have the benefit of being theoretically extremely sensitive and reports of down to femtomolar (fM) levels of biomolecule detection have been reported. This thesis is written from a biotechnological perspective on the development of a silicon nanowire-FET biosensor. The thesis project was oriented towards developing a novel affinity-based silicon nanowire-FET biosensor based on small (2-3 nm) protein affinity-binders denoted Affibody molecules. The hypothesis was that a smaller biological detector element would reduce the effect of Debye screening of the charged biomarker. This hypothesis was neither proved nor disproved, and a substantial amount of time and effort was spent on improving the function of the different biosensor components. In paper I, a study on the effect of the redox state and pH at solvent-to-surface interfaces of the reference gate electrodes was done by using solutions with alternating pH and varying ratios of the Fe(CN)63-/ Fe(CN)64- redox pair. These experiments showed that the selection of reference gate electrode has major implications on the signal readout in terms of false response and current instability. While a current drop due to potential change on the surface of a platinum reference electrode was observed, no such thing was observed using a silver/silver chloride reference gate electrode. The conclusion is that it is critical for performance to use a reference gate electrode that has a stable electrode potential such as silver/silver chloride. In paper II, a discovery was made when intending to use nanowire joule heating to lyse HT-29 and MCF-7 cells. Using fringing electric fields irreversible electroporation of a cell on top of a nanowire was achieved at 600-1200 mVpeak-to-peak at 10 MHz for 2 ms. The process was monitored using a 3,3´-dihexyloxacarbocyanine iodide (DiOC6(3)) and Propidium Iodide (PI) live-dead dye kit. The nanowire-mediated electroporation method releases the cell content without the risk of heat denaturation and it is ultra-localized. To address the concern on how to control and monitor organosilane monolayer formation in the surface functionalization of silicon nanowires, a microwave-assisted method was evaluated in paper III. Using ellipsometry, AFM, ATR-FTIR and fluorescence scanning it was shown that less than 10 minutes of incubation in 1% (v/v) APTES in toluene at 75?C is needed for formation of a 0.7 nm monolayer. In paper IV, surface functionalization was further explored by using microdispensing of solutions of capture probes for localized functionalization of individual devices for a multiplexed silicon nanowire-FET biosensor application. Besides showing by fluorescent scanning that oligonucleotide or protein spots of ~ 100 ?m diameters could be deposited on individual silicon nanowires, the functionalization chemistry was validated by using the same protocol for immobilization of the Z domain from Staphylococcus aureus Protein A (SpA) on silicon dioxide-coated SPR sensor chips, followed by real-time detection of the binding of immunoglobulin G. The immunoglobulins as affinity-binders have a drawback due to large size and the importance of having the binding event near the device in silicon nanowire-FET biosensor due to the effect of Debye screening. In paper V, in an effort to further minimize the size of affinity-binders of potential value as capture probes in silicon nanowire-FET applications, a backbone-cyclized, minimized 2-helix affibody-molecule (ZHER2:342min) was designed and produced by Solid Phase Peptide Synthesis(SPPS). The 2-helix affibody-molecule was evaluated for in vivo molecular imaging of HER2-expressing tumours, which was demonstrated in mice carrying SKOV-3 xenografts.