Nanowire-based chemical/biological sensor fuses

A novel type of nanowire-based sensor in which the signal transduction involves breaking an electrical connection between two electrodes is demonstrated in this thesis. Functionalized nanowires are manipulated using dielectrophoresis to form single-nanowire bridges across microelectrode junctions and anchored onto electrode surfaces through the specific interaction of biomolecular recognition elements. Once in place, chemical or enzymatic cleavage of a recognition site along the bonds linking the nanowire to the electrodes allows the nanowire to be easily removed from electrodes; this removal of nanowires can be detected in real time via changes in the AC electrical response. This form of sensing is inherently digital in nature as the removal of a single nanowire produces a sudden change in the current between electrodes and is essentially a chemically/biologically selective fuse. This sensing approach can be a general method for digital chemical and/or biological sensing using individual nanowires.;The material issues surrounding the use of nanowire bridges for sensing are discussed. Electrode materials including carbon and oxidized titanium are evaluated. In this thesis, photochemical grafting of organic alkenes to the surface of TiO2 thin film is demonstrated, and the resulting layers provide a starting point for covalently linking DNA oligonucleotides to the TiO2 thin films. The resulting DNA-modified surfaces exhibit excellent selectivity toward complementary target sequences in solution and the surfaces can withstand 25 cycles of hybridization and denaturation with little or no degradation. Furthermore, the use of simple masking methods provides a way to directly control the spatial location of the grafted layers, thereby providing a way to photopattern the spatial distribution of biologically active molecules to the TiO2 surfaces.