Influence of varying analyte concentration, environment, and composition on nanoshell-based SERS
Levin, Carly Sue
Halas, Naomi J.
Doctor of Philosophy
This thesis describes a series of experiments designed to examine the use of Au nanoshells as highly controllable surface enhanced Raman spectroscopy (SERS) nanosensor substrates. Individual Au nanoshells provide simple, scalable substrates for SERS with demonstrated strong electromagnetic field enhancements which exist near the molecule-substrate interface. The SERS spectral response is explored as a function of analyte concentration, environmental pH, and various analyte properties and composition. As a function of analyte concentration, the SERS response is exploited for the determination of packing density and molecular conformation of thiolated poly(ethylene glycol) (PEG) adsorbates on Au nanoshells. By varying the environmental pH when a pH-sensitive molecular adsorbate is attached to the Au nanoshells, the resultant SERS spectra allow for local pH monitoring with an average accuracy of +/-0.10 pH units across the operating range of the nanodevice. Changing analyte properties, such as carbon chain length for alkanethiol self-assembled monolayers (SAMs) on gold, produces a series of sharp resonances in the SERS spectra, suggesting coupling of the gold-sulfur bond stretch with the longitudinal acoustic, "accordion", vibrations of the molecular alkane chain. Further variation in chain termination or the addition of a phospholipid headgroup yields observable SERS spectral differences, providing unique fingerprints for each molecule. An associated phospholipid layer assembled onto an underlying alkanethiol SAM forms a hybrid bilayer on Au nanoshells, providing a way to spectrally monitor intercalation of the nonsteroidal anti-inflammatory drug (NSAID), ibuprofen. Low frequency SERS peaks for halogen, nitrogen, and oxygen containing molecules act as probes for metal-adsorbate binding, with spectral evidence for carbon monoxide adsorption occurring in the high frequency region. Finally, we demonstrate the synthesis and characterization of nanoparticles composed of magnetic cores with continuous Au shell layers that simultaneously possess both magnetic and plasmonic properties. The work presented in this thesis further demonstrates the emergence of Au nanoshells as versatile and valuable tools for sensing applications.
Analytical chemistry; Inorganic chemistry; Physical chemistry