Localized Surface Plasmon Spectroscopy on Individual Self-Assembled Pd-Au@Silica Heterodimers

Single particle measurements characterize the heterogeneities present within batches of nanocrystals, revealing properties obscured by ensemble measurements. By understanding the properties of individual nanocrystals, specific subpopulations can be targeted as materials for catalysis, separations, and energy storage. This project uses localized surface plasmon spectroscopy to detect chemical changes in single palladium nanocubes (PdNCs); single particle dark-field spectroscopy is used to detect shifts in the localized surface plasmon resonance (LSPR) of plasmonic nanocrystals located near the PdNCs, revealing information about transformations in the PdNC.

LSPRs arise from the resonant optical properties of plasmonic nanoparticles under illumination. Incident light generates a collective oscillation of electrons along the nanoparticle surface, which is resonant at a specific wavelength (the LSPR wavelength). The LSPR is highly sensitive to the composition, shape, and size of the plasmonic nanoparticle, as well as its local electromagnetic environment. As a result, spectral shifts and intensity changes in the LSPR can detect real-time chemical changes in a variety of sample environments, such as in-vitro measurement of biomolecule binding kinetics. In this project, the LSPR shifts of a plasmonic nanocrystal are applied to probe chemical changes in a nearby catalytic nanoparticle (PdNC).

The plasmonic nanocrystals compared are gold nanorods (length: 74 ± 6 nm, width: 21 ± 1 nm) and gold nanospheres (50 – 100 nm). Both were synthesized in aqueous solutions of cationic surfactant (CTAB) with a seeded growth method. Similar reaction chemistry was used to synthesize PdNCs with edge lengths of 13 ± 4 nm, 41 ± 5 nm, and 66 ± 7 nm. As synthesized, both the plasmonic nanocrystals and PdNCs were cationically surface charged. Electrostatic self-assembly was enabled by applying a 6 ± 1 nm silica layer to the plasmonic nanocrystal surface. This inverted the surface charge and prevented alloying between the plasmonic nanocrystals and PdNCs.

For each combination of PdNC size and plasmonic nanocrystal tested, PdNC:plasmonic nanocrystal ratios were varied between 2:5 and 3:1, and CTAB concentrations were varied between 10 uM and 1,000 uM. The synthetic yield of these heterodimers were characterized using ensemble solution-phase UV/Vis measurements, and SEM and TEM images of drop cast samples. Dark field scattering spectra were taken on single heterodimers and correlated to SEM images, revealing different sensitivity to the chemical changes in the PdNCs for the Au nanorods and Au nanospheres.