Surface-enhanced Raman scattering (SERS) has long been the interest of researchers in chemistry, physics and engineering, especially since the discovery that SERS can probe into the system down to the single molecule (SM) level. Despite the large number of publications regarding the fabrication of SERS substrates, it has been a challenge in the field to quantify the SERS signal and universally compare substrates. Traditionally, enhancement factor (EF) is used as an indicator of substrate quality, but the EF calculation is hugely dependent on the estimation of the surface coverage and other factors that are determined largely subjectively. Therefore, this thesis aims at discussing other parameters that can also be used to evaluate different substrates. Six different SERS substrates of Ag or Au nanoparticles of different sizes were fabricated by nanosphere lithography (NSL) and characterized by electron microscopy and UV-vis spectroscopy. SERS substrates were mapped for different concentrations of a probe molecule. Through subsequent baseline correction and principle component analysis (PCA), the "intensity" of individual spectrum was obtained and the shapes of intensity histograms of each substrate were acquired. Instead of calculating EF, five criteria (six quantification methods in total) were employed to comprehensively evaluate the six substrates. These were density of hot spots (characterized by the number of zero-intensity events), enhancement (represented by mean intensity), spatial variation (calculated by RSD of intensity), repeatability (realized by cross correlation) and histogram shape (quantified by skewness and kurtosis). These new methods provide insights to the understanding of the properties of SERS substrates in terms of hot spots. Different substrates may exhibit better performance in terms of one criterion but worse in terms of others. Those variations in performance can be explained by their surface morphology. These more elaborated methods are believed to provide a more comprehensive approach to evaluate and compare substrates than the traditional EF values. The thesis also paves the way for future study on SM-SERS and fabricating better SERS substrates. / Graduate

Noble metal nanomaterials have numerous uses in plasmonic and surface enhanced Raman scattering (SERS) detection applications; however, upon the addition of analytes, nanomaterials often undergo uncontrolled aggregation which leads to inconsistent signal intensities. To overcome this limitation, the effect of gold nanosphere concentration, column purification, and surface chemistry functionalization using internally etched silica stabilization methods was investigated on SERS assays for small molecule detection. Nanostructure composition, size, shape, stability, surface chemistry, optical properties, and SERS-activity were monitored using localized surface plasmon resonance (LSPR or extinction) spectroscopy, transmission electron microscopy (TEM), and Raman spectroscopy. First, the behavior of citrate-stabilized gold nanospheres was monitored as a function of molecular surface coverage. Both extinction and SERS spectral intensities increased linearly below monolayer functionalization. Above this value, however, uncontrolled nanoparticle aggregation occurred and large but irreproducible SERS signal intensities were monitored. Next, gold nanoparticles were encapsulated with varying silica shell thicknesses and purified using traditional centrifugation steps and/or column chromatography. Relative to the traditionally purified (i.e. centrifuged) samples, the SERS responses from small molecules using the column purified nanoparticle samples followed a well-known SERS distance-dependence model. Thus, surface chemistry cannot form more than a 2 nm thick layer on gold nanospheres if SERS applications were targeted. To overcome these challenges, gold nanospheres encapsulated with a thick silica shell were made SERS-active by etching the internal silica layer near the metal surface. During the synthesis of these internally etched silica-coated gold nanospheres, the LSPR wavelength shift, a parameter related to the effective local refractive index near the gold core, was monitored instead of etching time, in order to produce nanostructures with more uniform internal silica etching from sample to sample. The SERS-activity of a target molecule using these nanostructures was measured as a function of LSPR wavelength shift. SERS signal intensity increased, which suggested that more analyte molecules were able to bind to the gold surface because of the larger pore size in the silica layer near the metal core. Further exploration of these findings should increase the integration of solution-phase nanoparticles in more predictable functions in future applications, resulting in more quantitative and reproducible molecular detection in complex sample matrices, including biological and environmental samples.

Surface enhanced Raman spectroscopy (SERS) is highly promising analytical technique for trace detection of analytes. It is particularly well suited for environmental analyses due to its high sensitivity, specificity, ease of operation and rapidity. The detection and characterization of environmental contaminants, using SERS is highly related to the uniformity, activity and reproducibility of the SERS substrate. In this thesis, SERS substrates were produced by gold nanoparticle formation on wax patterned chromatography paper. In situ reduction of hydrogen tetrachloroaurate (gold precursor) by trisodium citrate dihydrate (reducing agent) was used to produce gold nanoparticles within a paper matrix. These gold nanoparticle based SERS substrates were analyzed by FE-SEM, UV-Vis and Raman spectroscopy. This work discusses the SERS signal enhancements for Raman active MGITC dye for a series of substrates prepared by in situ reduction of gold salt and pre-produced gold nanoparticles. UV-Vis analysis was performed to understand the effect of different molar ratio (reducing agent to gold precursor) and reaction time on the size and shape of the localized surface plasmon resonance (LSPR) band that dictates the SERS enhancements. It was concluded that lower molar ratio (1:1 and 2:1) of citrate-to gold produced better SERS signal enhancements and broader LSPR band. Therefore, use of lower molar ratio (MR) was recommended for paper-based substrates using in situ-based reduction approach. / Master of Science