Towards a Quantitative Understanding of Surface Enhanced Raman Phenomena by Using Internal References
Ameer, Fathima Suraiya
CommitteeWipf, David O.
Mlsna, Todd E.
Foster, Stephen C.
Accurate determination of the surface enhanced Raman scattering (SERS) enhancement factor (EF) is critically important for a fundamental understanding of the SERS phenomenon. Experimental quantification of SERS EFs is challenging. A series of instrument-, analyte-, and SERS-substrate related issues can affect the SERS intensity and thus compromise the reliability of the measured SERS EFs. This dissertation presents a series of computational and experimental studies that enhance the quantitative understanding of the SERS signal variation and identify ways to enhance the reliability of the SERS EF determination. Chapter I presents an overview of works described in this dissertation. The gold nanoparticle (AuNP) inner filter effect on SERS measurements is demonstrated in Chapter II. Using dithiopurine and ethanol as model analytes, we demonstrate that the nanoparticle will modify the analytes’ Raman signal through two competitive mechanisms: enhancing the Raman signal of the analyte on the nanoparticle surface through electromagnetic enhancement, and attenuating the analyte Raman signal through photon extinction. The significance of the AuNP inner filter effect is quantitatively evaluated using ethanol as the internal reference. A solvent internal reference method is presented in Chapter III for quantifying the SERS EFs of analytes adsorbed onto AuNPs and AgNPs. One of the key findings is that while an analyte’s SERS EF varies significantly as a function of nanoparticle aggregation, its peak SERS EF depends only on the types and sizes of nanoparticles, but not on experimental conditions including concentrations of analyte, nanoparticle, and aggregation reagent. Chapter IV presents a SERS internal reference method for the determination of the resonance Raman EFs in the SERS study of rhodamine 6G (R6G) adsorbed onto AuNPs and AgNPs. The most striking finding is that the AgNP binding reduces, instead of enhancing, the R6G resonance enhancement. Finally, the wavelength-dependent correlation between UV-vis intensities and SERS EFs of aggregated AuNPs and AgNPs were investigated under three fixed excitation wavelengths (532, 632, and 785 nm). The nanoparticle UV-vis intensity is an excellent indicator for identifying the optimal aggregation state for AgNP-based SERS acquisitions under each of the three excitation wavelengths and for the AuNP-based SERS under a 632 nm excitation.