Surface enhanced Raman spectroscopy for single molecule detection and biosensing

The aim of this thesis is to design plasmonic nano-gaps capable of detecting materials down to sufficiently low concentrations such that single molecule characteristics are observed. We begin first, by discussing the theory of plasmonics. Then, we assess the recent literature on the subject to develop an understanding in the field of plasmonics and to describe the fundamental concepts behind how plasmonic nano-sensors operate. Also, this allows us to show where our research fits in.The aim of this thesis is to design plasmonic nano-gaps capable of detecting materials down to sufficiently low concentrations such that single molecule characteristics are observed. We begin first, by discussing the theory of plasmonics. Then, we assess the recent literature on the subject to develop an understanding in the field of plasmonics and to describe the fundamental concepts behind how plasmonic nano-sensors operate. Also, this allows us to show where our research fits in.The second area of research involves practical Surface Enhanced Raman Spectroscopy (SERS) experiments from our optimized nano-gaps. The nano-gaps were doped with the molecular dyes Rhodamine 6G and Crystal Violet at concentrations of 2x10⁻⁷ M. SERS measurements revealed differences in the relative intensities of their respective SERS peaks at low concentrations when compared to the SERS spectra measured from gaps doped the same dye at higher concentrations of 2x10⁻⁵ M. Time dependent SERS measurements showed that the SERS signal is stable over a long period of time, indicating the observed relative intensity changes are due to changes in molecular orientation from one gap to another, demonstrating that our optimized nano-gaps have single molecule sensitivity. When exciting at 532 nm, the 118 nm silver spheres used to form the nanogap with the silver film below were shown to enhance the Raman signal by 4:2x relative to the 200 nm silver nano-spheres, and up to 7:73x relative to the 60 nm silver nanospheres. When compared to our simulation results for the same structures excited with a Gaussian source with NA = 0:55, we showed the information collected from the Raman study correlated well with the theoretical data.Following our work investigating single molecule characterisation of fluorescent materials, we began looking at trace levels of a conjugated polymer (F8-PFB). The previous investigation had been from a purely electromagnetic enhancement perspective using a secondary polymer matrix buffer which was optically transparent in the region of interest for the Raman spectra of our target molecule. This polymer provided a barrier between the target material and the metallic nanostructure, thereby minimizing the potential of photo-induced chemical processes in the Raman signal. In this study, the material itself forms the basis of the cavity between the particle and the film below. This system classifies the single molecule regime via the observation of intensity blinking events, which are characteristic of Single Molecule SERS (SM-SERS).We also demonstrated the biosensing applications of our research, where nanoparticle clusters on a metallic film were used to produce spectra from bio-molecules undergoing conformation changes as a result of UV light exposure. The SERS spectra revealed decreased intensity from the Tryptophan (Trp) modes and appearance of disulphide bonds as time under UV light exposure progresses for lysozyme.Our final chapter shows that by using nanoparticles coupled to different substrates such as Distributed Bragg Reflectors (DBRs) and dielectric slabs, the hybrid modes improved the Quality-factor (Q-factor) of the scattering spectra. Therefore, these systems theoretically have great potential for refractive index sensing with high sensitivity to binding activity of molecular targets. The highest Q-factor of the systems we investigated was the 200 nm gold particle coupled to the 2 μm dielectric slab at 22:48, followed by the same particle deposited on a 700 nm stop-band DBR at 7:41.