Nanowire sensor for real-time chemical and biological detection

Silicon nanowire (SiNW) sensors have been developed by using CMOS-compatible top-down fabrication approach for conductivity-based detection of chemical (eg. pH level, K+, Na+ ions) and biological species (eg. 12-mer DNA oligomer and streptavidin) with fast-response and high-sensitivity. Top-down approach based on electron-beam lithography (EBL) and reactive ion etching (RIE) on silicon on insulator (SOI) substrate, as opposed to bottom-up synthesis approach employed by most SiNW sensors currently developed, enables compatibility with CMOS fabrication process, accurate alignment to other electrical components, flexible design of nanowire geometry and better control of electrical characteristics. The sensor transduction mechanism is simply a change in SiNW conductivity and this requires minimal instrumentation complexity. This sensor platform is capable of a real-time detection of chemical species with fast response speed and sensitive signal output. Also, the surface functionalization of SiNW with receptor molecules (eg. PNA or biotin) allows selective detection of target biomolecules (eg. DNA or streptavidin). However, nonselective and global surface functionalization has been problematic especially for the detection of biomolecules in extremely low concentration level in the past. Therefore, a novel scheme of selective surface functionalization has been developed based on nanoscale Joule heating of SiNWs in this research. This is method is expected to dramatically improve the sensitivity and detection limit of SiNW sensor by selectively binding target molecules on the SiNW sensors functionalized with probe molecules.; The top-down approach allowed highly reproducible fabrication of SiNWs down to 40 nm width in any desired geometries by taking advantage of the design flexibility of EBL. Furthermore, larger metal-silicon contact area connected seamlessly to SiNWs by EBL design facilitated minimized contact resistance between metal interconnection and SiNW, enabling good ohmic contact. SiNWs showed typical ohmic electrical resistor behaviors with an average electrical resistivity of 1.56x10-4 O·m. Integration of SiNW with PDMS microfluidic channel facilitated a real-time chemical and biological detection. First, pH level detection showed a good performance with large operation range of pH level (pH=4 to 10) with an average sensitivity of (DeltaR/R)/pH = 2.6%/pH and fast response speed (rise time = 7.9 seconds). Also, SiNW sensor was able to detect a small pH level difference (DeltapH=0.2) near neutral pH conditions (pH=7). A real time detection of pH=6.7-7.3 could be achieved with an average sensitivity of (DeltaR/R)/pH = 3.6%/pH and fast response speed (rise time = 8.5 seconds). Surface modification of the SiNW enabled the detection of biomolecules such as protein and DNA. SiNW sensor functionalized with NHS-sulfo-biotin was used for a detection of streptavidin and avidin molecules. The SiNW functionalized with peptide nucleic acid (PNA) molecules showed a sensitive detection of target DNA molecules with a concentration down to 5 pM. Also, +200 mV DC offset on the SiNW increased the sensitivity dramatically (5.5% conductance increase) compared to the case of no DC offset (1.3% conductance increase) for 5.5 muM of DNA via electrostatic attraction.; In addition, a novel method of selective surface functionalization of SiNWs based on polymer ablation via nanoscale Joule heating of SiNWs has been developed. Only the surfaces of selected SiNWs were open for surface functionalization while the surrounding areas were protected by inert polymer layers. It was demonstrated that a highly localized ablation of polytetrafluoroethylene (PTFE) thin film is possible via nanoscale Joule heating of SiNW. Subsequently, a selective surface functionalization of SiNW with 3-mercaptopropyl-trimethoxysilane (3-MPTMS) has been realized and visually verified by selective attachment of gold nanoparticles via gold-thiol affinity. By using this met...