Synthesis and Characterization of Nanomaterials for Biosensing Applications
Open Access
- Author:
- Sioss, James
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 15, 2007
- Committee Members:
- Christine Dolan Keating, Committee Chair/Co-Chair
Thomas E Mallouk, Committee Member
Mary Beth Williams, Committee Member
Theresa Stellwag Mayer, Committee Member - Keywords:
- nanowires
silica coating
optical properties
biosensors - Abstract:
- The aim of this thesis is the development of nanoparticles with tailored structures and surface properties for biosensing applications. This work focuses on nanowires with diameters between 30 and 300 nm, made of metal or silicon. For many of the applications investigated here, Au and Ag striped nanowires (NWs) were used. One problem with Ag is that it can degrade in aqueous environment. Chapter 2 describes the use of citrate, a mild reducing agent, as an additive to prevent Ag degradation. We studied NWs stored in high salt hybridization buffer for undisturbed and continuously agitated samples. We found that NWs stored in high salt buffer were significantly degraded after two weeks, and much faster when agitated. The NWs were unidentifiable in less than one week when continuously agitated. We found that adding 40 mM citrate increased the stability of Ag NWs by 17 weeks over those stored in hybridization buffer. When the NWs were continuously agitated in citrate buffer, they remained stable for over two weeks. Derivatization of the NWs with biomolecules adds some protection. NWs coated with rhodamine tagged DNA attached via neutravidin-biotin chemistry are stable for 12 days in hybridization buffer. When 40 mM citrate is added to the buffer, they are stable for at least 63 days. Ag deterioration was coupled to loss of fluorescence of the labeled DNA and NW breakage. In chapter three, a simple, wet-chemical method for fabricating linear chains of Au and Ag nanoparticles by selectively etching alternating segments from striped metal nanowires is presented. Nanowires composed of sacrificial Ni segments as well as Au and/or Ag segments were prepared by templated electrodeposition in the pores of alumina membranes. After removal from the membrane, wires were coated with silica, and selective etching revealed the nanoparticle chains. Extinction spectra are presented as a function of interparticle spacing for nanoparticle chains of diameter ~100 and ~33 nm. Chapter 4 builds on the results of chapter 3. Silica coated Au nanoparticle chains were studied for use as surface enhanced Raman spectroscopy (SERS) substrates. NP chains were vortexed with bis pyridyl ethylene (BPE), a Raman active molecule. Raman intensity of BPE was measured for Au NP dimers and chains with diameter of ~100 nm. The chemistry of the silica coating was adjusted by mixing tetra ethoxy silane (TEOS) with a small amount of Bis (trimethoxysilyl ethyl) benzene (BTEB). This made the silica more hydrophobic, which allowed the hydrophobic molecule BPE to penetrate the coating. NP chains with diameters of ~33 nm were coated with TEOS/BTEB silica and BPE was adsorbed. Large differences in the Raman signal were recorded for the bare NWs, silica coated, and etched NP chains. There was also a large dfference in the Raman intensity for NP chains with large spacing (420 nm) and small spacing (30 nm). In chapter 5, striped metallic nanowires (NW) have been coated with a silica shell of controllable thickness (6 – 150 nm), and the performance of coated vs. uncoated has been compared. We find that the SiO2 coating does not interfere with identification of the metal striping pattern, and protects Ag segments from oxidation, extending the range of assay conditions under which barcoded NW can be used. Much higher and more uniform fluorescence intensities were observed for dye-labeled ssDNA bound to SiO2-coated as compared to uncoated NW. Simultaneous, multiplexed DNA hybridization assays for three pathogen-specific target sequences on SiO2-coated NW showed good discrimination of complementary from noncomplementary targets. Application of SiO2-coated NW in discrimination of single base mismatches corresponding to a mutation of the p53 gene was also demonstrated. Finally, we have shown that thiolated probe DNA resists desorption under thermocycling conditions if attached via siloxane chemistry to SiO2-coated NW, but not if it is attached via direct adsorption to bare Au/Ag NW. Finally, in chapter 6, the surface chemistry of Si and Rh NWs was investigated for the attachment of DNA probes for incorporation of cantilever arrays for mass based biosensing. Experiments were performed on bare and silica coated AuAg striped NWs to determine the optimum conditions of DNA probe attachment, hybridization of DNA and RNA target, and tagging the target with either fluorescently labeled DNA, or Au:DNA conjugates. It was found that RNA could be detected on bare and silica coated NWs with both fluorescent and Au:DNA tags even with its larger size and secondary structure,. The Au:DNA conjugates ranged in size from 12 to 50 nm, with the more massive NPs desired for their mass amplification abilities. The surface coverage of 50 nm Au:DNA conjugates was measuread as a function of NaCl concentration in the final buffer rinse. Resonant frequency measurements were made of SiNW before and after addition of 50 nm AuNP with 11-mercapto undecylamine. Finally, mass changes due to the binding of target DNA and Au:DNA conjugates to RhNW cantilevers were measured by resonant frequency changes.