Using nanoparticle optics for ultrasensitive chemical detection and surface-enhanced spectroscopy

This work describes a research effort focused on exploiting the unique optical properties of noble metal nanoparticles in order to develop ultrasensitive detection schemes for molecular analytes. The techniques are based on the localized surface plasmon resonance (LSPR), the collective oscillation of the conduction electrons of a noble metal nanoparticle that is excited by incident electromagnetic radiation. The first part of this work investigates surface-enhanced Raman scattering (SERS), a vibrational spectroscopy allows for study the molecular adsorbates on nanostructured metal surfaces. It is demonstrated that the sufficient enhancement is achieved to allow for the observation of Raman scattering from single molecules and molecules adsorbed to single nanoparticles. Additionally, wavelength-scanned SERS excitation spectroscopy is performed on well-characterized substrates in order to identify the experimental conditions that yield the largest SERS enhancement. It is shown that the size and shape distribution of the nanoscale roughness features on a SERS substrate play an important role in determining these conditions. It demonstrated that the novel fabrication technique angle-resolved nanosphere lithography provides increased control over nanoparticle size, shape, and interparticle spacing, all of which directly influence the optical properties of nanoparticle arrays. Electromagnetic coupling in nanoparticle arrays is also investigated both theoretically and experimental. It is shown that radiative dipolar coupling influences the spectral location of the LSPR for several nanoparticle array geometries. The final experimental work focuses on the development of a chemical sensing technology based on the spectral response of the LSPR to changes in the local refractive index of a nanoparticle. It is demonstrated that real-time sensing with zeptomole sensitivity can be performed by monitoring the LSPR of single nanoparticles. The remainder of the work outlines three activities that have been developed in order to educate high school students about concepts of nanoscience.