M. Scott Goodman, Ph.D., Professor of Chemistry
Date of Award
Forensic Science, M.S.
Jinseok Heo, Ph.D., Associate Professor of Chemistry
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William Durfee, Ph.D., Professor of Chemistry
Kimberly Bagley, Ph.D., Professor of Chemistry
Raman microscopy is a potentially useful method for detecting drug metabolites in fingerprints and imaging latent fingerprints without a physical or a chemical treatment. However, with normal Raman scattering signals, it is difficult to detect analytes in the µM concentration range, a typical order of the concentration of drug metabolites in sweat. Surface‒Enhanced Raman Scattering (SERS) can improve this intrinsically weak normal Raman scattering. SERS refers to an increased Raman scattering of molecules by 104‒1010 times when molecules are close to or adsorbed on a nano‒scale metal surface such as gold or silver. For the broader use of SERS in trace chemical analysis, it has long been an important issue to develop a simple and reproducible method to prepare SERS substrates.
My thesis research has been focused on assessing the SERS activity of citrate‒capped gold nanoparticles (AuNPs) having a sphere shape and an average diameter of 70 nm. The SERS activities of these AuNPs were tested in a deposit layer and a solution using Rhodamine 6G (R6G). Unlike other’s previous report that used smaller AuNPs, the SERS activity of the 70 nm AuNPs showed an inconsistency in the deposit layer. This suggests that a larger size of AuNPs may not have formed a well-ordered 2-D assembly of AuNPs, which resulted in inconsistent SERS activities depending on the locations of the AuNP deposit.
On the other hand, the SERS activities of AuNPs in a solution produced more reproducible and reliable data than those of the AuNP deposits. We found that the SERS signals of R6G were strongly dependent on the concentrations of AuNPs and R6G. To detect a μM concentration of R6G, at least ~109 particles/mL of AuNP solution was required. Although the characteristic Raman peak intensities of R6G exhibited an increasing trend with the increase of R6G concentration in the concentration range of 10−6 − 10−4 M, the relationship between the two was not linear. This must be associated with the saturation of adsorption sites of AuNPs in a higher concentration range of R6G. The R6G could be detected as low as 10−6 M concentration with the SERS effect of AuNPs in a solution.
Another interesting discovery of this research was that the aggregation of AuNP could increase the SERS signal of analyte, but the difficulty of controlling the extent of aggregation produced poor reproducibility in the SERS data. A future study will be focused on finding a method of producing controlled aggregation of AuNPs in a solution, which can be used for fabricating reproducible SERS substrates.
Johnson, Caitlin M., "Assessment of 70 nm Citrate‒Capped Gold Nanoparticles (AuNPs) for Surface‒Enhanced Raman Scattering (SERS) Substrate Using Rhodamine 6G" (2016). Forensic Science Theses. 12.
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