Development of novel substrates and sampling techniques for the analysis of drugs and model environmental pollutants via surface enhanced Raman spectroscopy (SERS)

Surface Enhanced Raman Spectroscopy (SERS) has shown promise for the analysis environmental and pharmaceutically relevant compounds due to its tremendous enhancement of Raman signals, and the large amount of structural information provided by the technique. Despite these advantages, SERS has not been established as a routine analytical tool due to limitations in the analytical figures of merit such as reproducibility and linear dynamic range. This is due in part to gross decomposition of sample analytes upon irradiation with the incident laser beam which significantly broaden and diminish the intensities of observed spectral bands. The primary goals in this project are to develop new substrates and sampling techniques to overcome the above mentioned problems. This work presents the use of a Sample Translation Technique (STT) as a means to minimize the thermal and photolytic effects commonly seen in SERS. The rapid spinning of the sample minimizes the effective residence time of analytes and substrate within the irradiated zone without reducing the spectral acquisition time or the density of analyte within the zone. The technique was studied by acquiring SERS spectra of various environmental and pharmaceutically relevant compounds such as Naproxen USP, riboflavin, folic acid, Rhodamine 6G, and 4-aminothiophenol using silver islands on glass and silver-polydimethylsiloxane composites as SERS substrates. In all cases, the collected spectra show improvements upon spinning at laser powers as low as 4.2 (+/-0.1) mW. Further studies with a series naphthalene, phenol, and benzoic acid derivatives as model environmental pollutants showed a considerable improvement in the reproducibility and the sensitivity of SERS. A linear dynamic range of at least two orders of magnitude with detection limits as low as 2.9 x 10-8 M and precision of less than ten percent relative standard deviation was also observed. The experiments show that different variables like pH and matrix composition can affect the sorption of the analyte onto the SERS substrate. The effect of these variables in the analysis of real samples is presented. The potential use of liquid chromatography for isolating the model pollutants from detrimental matrix components in natural waters is also shown.