Surface Enhanced Raman Spectroscopy (SERS) for Characterization of Particles in Solution

This dissertation focuses on the development of a 2D planar Surface Enhanced Raman Spectroscopy (SERS) substrate and demonstrates its utility through in situ detection and the monitoring of the dynamics of macromolecular particles via correlation spectroscopy.

A highly-enhancing SERS substrate was fabricated via thermal evaporation of noble metals into a nano-porous aluminum oxide (AAO) template. Characterization of the substrate was carried out by imaging the surface using scanning electron microscopy (SEM), measuring the SERS signal from a self-assembled monolayer of thiophenol, mapping the thiophenol SERS signal, and mapping the dark-field scattering from the substrate. The data indicate that the substrate shows an extraordinary enhancement factor (108) and uniform SERS response (RSD ~10%) due to its high density of enhancing sites.

The substrate was incorporated into a commercial flow cell to facilitate the label- free, in situ detection of single macromolecular particles including lipid vesicles (DPPC) and polystyrene (PS) beads. The substrate facilitates detection by providing enhancement of signal from particles in solution approaching the surface. The duration of the observed signal demonstrates the capability of monitoring the diffusion of macromolecular species in solution on the millisecond time scale.

Finally, the SERS substrate was utilized to demonstrate Surface Enhanced Raman Correlation Spectroscopy (SERCS) for assemblies interacting with the substrate. Auto- and cross-correlation of the time dependent SERS signal allows for the monitoring of diffusion dynamics for particles ranging in radius from 50 to 500nm while simultaneously rejecting extraneous signal fluctuations. When the extension of the SERS field away from the surface is approximated to be 5nm, the diffusion constants measured are on the order of 10-10-10-11 cm2/s, a factor of 40 slower than Stokes- Einstein predictions. The slow diffusion constants suggest that particles are interacting with the substrate and experiencing hindered diffusion near the surface.