Doctoral Dissertations

Date of Award

5-2004

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Michael J. Sepaniak

Committee Members

Robert J. Hinde, S. Douglass Gilman, Kevin Robinson

Abstract

Surface Enhanced Raman Spectroscopy (SERS) has shown promise for the analysis of 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 the fact that the continuous irradiation of the laser beam over the SERS substrate can promote the rapid decomposition of sample analytes which significantly broaden and diminish the intensities of observed spectral bands. Further irradiation can promote thermal or photolytic fragmentation of analytes, thereby altering the observable bands and possibly leading to a misinterpretation of analytical data. The primary goals in this project are to develop new substrates and sampling techniques to overcome the above mentioned problems.

The initial part of 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. By spinning the sample rapidly, the effective residence time of analytes and substrate within the irradiated zone is dramatically decreased without reduction of spectral acquisition time or the density of analyte in the zone. The technique is first 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. Specific differences in the appearance of the spectra and the potential use of STT for improved SERS qualitative and quantitative determinations are presented. Although the combined use of STT-SERS and silver-polydimethylsiloxane nanocomposites (Ag-PDMS) showed promise in the analysis of aromatic compounds, the results demonstrated that new methods and protocols were needed to effectively implement SERS as a routine analytical technique.

Consequently, further studies were performed to optimize the technique for the analysis of a series of naphthalene, phenol, and benzoic acid derivatives as model environmental pollutants. The presence of these chemicals in water constitutes a serious public health issue due to the toxicity, persistence and chemical activity of these chemicals in the environment. The STT technique showed a considerable improvement in the reproducibility and the sensitivity of SERS. The experimental results showed a linear dynamic range of at least two orders of magnitude with detection limits as low as 2.9x10-8 M and precision of less than ten percent relative standard deviation. These experiments allowed the identification and optimization of different experimental variables such as irradiation time, translation rate, and pH. The pH studies also revealed that the composition of the sample matrix can promote the selective sorption of the analyte to the SERS active substrate.

The above mentioned results indicate that the composition of the sample matrix is an important variable in SERS. Therefore, the final part of this work was devoted to studying how different variables such as pH and matrix composition can affect the sorption and SERS activity of model pollutants. The results show that the conjugate bases of weak acids can interact more efficiently with the substrate, leading to an increased signal at higher pH, while amino-aromatic compounds interact more efficiently at a lower pH. The sorption of these chemicals is an essential step in the process and has been attributed to the absorption of the analyte into the PDMS followed by its adsorption to the metallic surface. In addition, the presence of moderate concentrations (1x10-4 M) of a supporting electrolyte such as nitrate or fluoride can improve the sorption of 4-hydroxybenzoic acid to the Ag-PDMS nanoparticles. Other ions such as phosphate and chloride cause rapid oxidation of the substrates even at concentrations as low as 1x10-5 M. 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.

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