High Sensitivity Surface Enhanced Raman Scattering Detection of Tryptophan

Raman spectroscopy has the capability of providing detailed information about molecular structure, but the extremely small cross section of Raman scattering prevents this technique from applications requiring high sensitivity. Surface enhanced Raman scattering (SERS) on the other hand provides strongly increased Raman signal from molecules attached to metallic nanostructures. SERS is thus a promising technique for high sensitivity analytical applications. One particular area of interest is the application of such techniques for the analysis of the composition of biological cells. However, there are issues which have to be addressed in order to make SERS a reliable technique such as the optimization of conditions for any given analyte, understanding the kinetic processes of binding of the target molecules to the nanostructures and understanding the evolution and coagulation of the nanostructures, in the case of colloidal solutions. The latter processes introduce a delay time for the observation of maximum enhancement factors which must be taken into account for any given implementation of SERS.;In the present thesis the goal was to develop very sensitive SERS techniques for the measurement of biomolecules of interest for analysis of the contents of cells. The techniques explored could be eventually be applicable to microfluidic systems with the ultimate goal of analyzing the molecular constituents of single cells. SERS study of different amino acids and organic dyes were performed during the course of this thesis. A high sensitivity detection system based on SERS has been developed and spectrum from tryptophan (Trp) amino acid at very low concentration (10−8 M) has been detected. The concentration at which good quality SERS spectra could be detected from Trp is 4 orders of magnitude smaller than that previously reported in literature. It has shown that at such low concentrations the SERS spectra of Trp are qualitatively distinct from the spectra commonly reported in literature. These distinctions are attributed to the unique binding geometry of Trp molecules to the small silver nanoclusters at these low concentrations.;Background electrolytes in the solution can have a significant role in SERS experiment as it helps in the binding of molecules to the metallic structures and stabilizes the colloid in some cases We performed a study of effects of different electrolytes and an optimization of electrolytes has been carried out, which leads to the high enhancement reported in this thesis. The SERS detection has also been performed in microfluidic and flow cell geometries which enable a combination of high sensitivity of the SERS with the low volume requirements of microfluidic devices. A Teflon AF capillary was used for performing liquid core waveguide (LCW) SERS measurements. With this geometry the enhancement obtained was about a factor of 10 compared to that from cuvette so the detection limit could be further decreased by a factor of 10 in LCW reaching 10 −9 − 10−10M for Trp amino acid. The enhanced sensitivity and better understanding of the optimum conditions for SERS developed in the thesis are important since they now could allow the possibility of assays of the chemical constituents of single cells in future microfluidic systems.