Date of Award
Doctor of Philosophy (PhD)
Localized surface plasmon resonance (LSPR) involves the collective and coherent oscillation of dielectrically confined conduction electrons. The LSPR wavelength of noble metal nanoparticles (such as gold, silver and copper), which falls into the visible and near infrared range of the electromagnetic spectrum, is sensitive to the composition, size, shape, dielectric properties of the surrounding medium, and proximity to other nanostructures (plasmon coupling). Based on the sensitivity of the surface plasmon resonance to the changes in the dielectric properties of the surrounding medium and the enhancement of the electromagnetic (EM) field in proximity of metal nanostructures, two important classes of plasmonic sensors have evolved: refractometric LSPR and surface enhanced Raman scattering (SERS) sensors. SERS involves the large enhancement of the Raman scattering from analytes adsorbed on or in close proximity to a nanostructured metal surface.
Most of the SERS substrates based on individual nanostructures offer modest SERS enhancement. On the other hand, interstitial sites between assembled or lightly aggregated nanostructures, often termed as electromagnetic hotspots, offer large SERS signal enhancements, enabling the single molecule detection under ideal conditions. Although the assemblies of nanostructures are highly SERS-active, the SERS response is very sensitive to the assembly state, thus making it challenging to realize uniform and reproducible SERS substrates with high density of EM hotspots based on such traditional assemblies. Furthermore, the fabrication of SERS substrates based on the controlled assemblies of nanostructures involves either complex chemical methods or expensive lithographic techniques. Therefore, it is desirable to engineer nanostructures with inherent EM hotspots, which can significantly enhance the EM field and enable the sensitive detection of analytes using SERS.
Hollow and porous metal nanostructures are a novel class of plasmonic nanostructures that exhibit extraordinary optical and catalytic properties compared to their solid counterparts, due to a higher surface to volume ratio and the facile tunability of the LSPR wavelength over a broad range from visible to parts of near infrared. In this work, we design, synthesize, and comprehensively characterize the optical properties of hollow nanostructures including plasmonic nanocages and nanorattles comprised of gold nanostructures as cores and porous gold cube as shells. We demonstrate that hollow and porous plasmonic nanostructures exhibit a significantly higher refractive index sensitivity compared to other solid nanostructures of similar size, leading to LSPR sensors with higher sensitivity and lower limit-of-detection compared to biosensors based on solid counterparts. Furthermore, we demonstrate that plasmonic nanorattles host electromagnetic hotspots between the core and the shell, offering significantly higher SERS enhancement as compared to other solid nanostructures of similar size. Through a systematic study, we unveil the influence of size, shape and orientation of the plasmonic nanorattles on the optical properties and SERS enhancement. The work described here provides guidelines for the design of hollow plasmonic nanostructures for various sensing applications.
Parag Banerjee, Guy Genin, Jeremiah Morrissey, Bryce Sadtler,