Single nanoparticle spectroscopy: Plasmonic properties and biosensing applications
Nehl, Colleen Lorraine
Hafner, Jason H.
Doctor of Philosophy
Single particle dark field spectroscopy has been combined with high-resolution scanning electron and atomic force microscopy to study the scattering spectra of individual gold nanoparticles. This technique has been applied to single gold/silica nanoshells, and single gold nanostars. For nanoshells, the plasmon resonant peak energies match those calculated by Mie theory based on the nanoshell geometry. The resonance line widths fit Mie theory without the inclusion of a size-dependent electron surface scattering term, which is often included to fit ensemble measurements. Single particle spectroscopy has also been applied to star-shaped gold nanoparticles which are ca. 100 nm in diameter. The gold nanostars were fabricated by a modified seed-mediated, surfactant-directed synthesis which is similar to a method known to produce gold nanorods in high yield. The yield, monodispersity, and initial investigations into the growth mechanism of the nanostar synthesis are described in detail. Through correlated structural characterization by electron microscopy, each scattering component can be assigned to different points on the nanostars. The plasmon resonances were also found to be extremely sensitive to the local dielectric environment, yielding sensitivities as high as 1.41 eV photon energy shift per refractive index unit. These properties suggest that gold nanostars may be highly valuable for certain biosensing and microscopic imaging paradigms. To test their properties as molecular sensors, single nanostar spectra were monitored upon exposure to alkane thiols (mercaptohexadecanoic acid) and proteins (bovine serum albumin) known to bind gold surfaces. The observed shifts are consistent with the effects of these molecular layers on the surface plasmon resonances in continuous gold films. The results suggest that localized surface plasmon resonance sensing with single nanoparticles is analogous to the well developed field surface plasmon resonance sensors, and will push the limits of sensitivity.
Physical chemistry; Optics