Probing the interactions between dye molecules and metallic nanoparticles - Implications for surface enhanced spectroscopies
The work in this thesis focuses on improving the understanding of two key aspects of the interaction between dye molecules and metallic nanoparticles, with particular relevance to Surface Enhanced Raman Spectroscopy (SERS). This is manifested from two main branches of experimental work; the first is concerned with improving the reproducibility of SERS sample preparation using colloidal solutions while the second focuses on directly measuring the absorption spectra of commonly used dye molecules on the surface of colloidal silver nanoparticles. In the first body of work of the thesis, a major step towards improving SERS in colloidal solutions is achieved by highlighting a crucial, but unnoticed possible source of error for such samples; by comparing average enhancement factor measurements on colloidal solutions prepared using different analyte (dye) dilution methods, it is shown that large dye dilution factors can cause extreme variations in nanoparticle coverage across the entire sample. This not only causes analyte-dependent enhancement factors (which is highly undesirable) but can also lead to false identification of single-molecule SERS experiments using the well established bi-analyte method. The errors associated with large dilution factors are interpreted as a competition between dye diffusion and adsorption kinetics. Time dependent fluorescence quenching measurements and finite element modelling (FEM) in COMOSL show that in any system where adsorption competes with diffusion, large dilution factors should be avoided. A simple protocol of half-half dilutions of analytes is proposed as a standard method to be adopted when preparing colloidal solutions for SERS to ensure uniform distribution of analytes is achieved. The second body of work is an experimental investigation of the modification of the energy levels of commonly used dye molecules adsorbed to spherical silver nanoparticles at sub-monolayer concentrations. Through the use of a novel integrating sphere setup, the absorption spectra of Rhodamine 6G, Nile Blue, Rhodamine 700 and Crystal Violet are successfully measured on the surface of silver colloids at ultra-low concentrations where dye-dye interactions are negligible. These results indicate that for most dyes, absorption pectra on the colloid surface are shifted and/or broadened with respect to the free dye in solution. In the most extreme case, a blue shift of almost 90 nm for Crystal Violet suggests a strong chemical interaction with the silver surface. A Mie theory shell model of dye-coated silver spheres is found to accurately reproduce the measured evolution of absorption spectra as the dye concentration on the colloid surface is increased but overestimates the enhancement in absorption, which is interpreted as a result of the adsorption geometry of dyes on the surface, not captured by the shell model. Finally, through careful wavelength dependent SERS measurements, the SERS Raman excitation profile of Crystal Violet is measured and shown to be closely linked to the modified absorbance as obtained in the integrating sphere setup. A standard optical transform model for computing the Raman excitation profile from the modified absorbance is applied and gives good agreement with the measured SERS data. These results represent a direct indication of chemical modifications of resonant molecules used in SERS studies.