Fabrication and Optimization of a Gold Nanorod Vertical Array for Sensing Applications
The goal of this dissertation project was to explore the application of an ordered self-assembly of vertically standing gold nanorod array in advancing sensing performance. There were three specific aims: first, to establish a synthesis method that would improve gold nanorod quality in terms of monodispersity, shape, purity, and yield especially at a longer aspect ratio; second, to optimize the parameters to form and characterize a reproducible gold nanorod vertical array with high uniformity over a relatively large area; and third to explore possible applications of the orderly assembled nanorod substrate as a signal enhancer. The gold nanorods were synthesized using a seed-mediated approach using the slow reducing agent, hydroquinone. A range of gold nanorod aspect ratios were synthesized (LSPR 600-1200 nm), demonstrating tunability. Overall, the major improvements achieved by this method were the superior advancement in nanoparticle shape control, particularly at long wavelengths, and the establishment of a reliable purification protocol that maximized removal of spheres and other contaminants while circumventing aggregation. The formation of a pattern of ordered, vertically aligned gold nanorods, termed as a gold nanorod vertical array, was achieved by establishing the multi-dependent parameters for gold nanorod aspect ratio, concentration, CTAB concentration, drop deposition volume, NaCl concentration, and temperature and humidity in order to optimize the density of the gold nanorod packing and the coverage of the uniform array area. Two types of arrays were assembled and characterized, marking the optimal parameters at play to produce gold nanorods arrays of aspect ratios 2.05 (LSPR 606 nm) to 3.6 (LSPR 776 nm), which correspond to the resonance with the popular laser wavelengths 633 nm (visible region) and 785 nm (near-infrared region). The arrays were characterized by Scanning Electron Microscope and a cleaning/sample deposition protocol was established that was confirmed not to disturb the array quality. Lastly, the application of the vertical array in the enhancement of localized electric field was explored with COMSOL simulation and fluorescence and Raman applications. The closely packed nanoarray generated "hot spots" among the neighboring nanorods. These hot spots are characterized by intensified electric field as demonstrated in simulation results. Fluorescence of FAM dye was studied to calculate the fluorescence enhancement factor due to the placement of the fluorophore molecules in close proximity to the hot spots on the gold nanorod substrate. FAM was conjugated to the gold nanorod surface using double stranded DNA. The stability and consistent dimensions for double stranded DNA were exploited to distance the fluorophore from the array plasmonic surface with high precision. The calculation of enhancement factor showed a maximal enhancement when the fluorophore was at a distance of 45 bp (16.2 nm) from the gold nanorod surface. This trend was conserved across gold nanorod aspect ratios. Gold nanorods with LSPR 631 nm demonstrated the greatest enhancement factor for all three distances as compared to LSPR 740 and 838 nm, which was attributed to the proximity of the plasmon resonance wavelength to the absorbance and emission wavelengths of FAM dye. The potential for the vertical array as a SERS substrate was demonstrated by very high signal intensity at 1 nM concentrations for the detection of Maltol. This study shows promise for the enhancement of radiative signals across multiple sensing modes such as fluorescence, Raman spectroscopy, and infrared spectroscopy, with applications ranging from analytical and biochemical research, to forensics, detection, and catalysis applications.