Metallic nanoparticles are known to dramatically modify the spontaneous emission of nearby fluorescent molecules and materials. Here we examine the role of the nanoparticle plasmon resonance energy and nanoparticle scattering cross section on the fluorescence enhancement of adjacent indocyanine green (ICG) dye molecules. We find that enhancement of the molecular fluorescence by more than a factor of 50 can be achieved for ICG next to a nanoparticle with a large scattering cross section and a plasmon resonance frequency corresponding to the emission frequency of the molecule.

With clinical trials for photothermal tumor ablation using laser-excited tunable plasmonic nanoparticles already underway, increasing understanding of the efficacy of plasmonic nanoparticle-based photothermal heating takes on increased urgency. Here we report a comparative study of the photothermal transduction efficiency of SiO 2 /Au nanoshells, Au 2 S/Au nanoshells, and Au nanorods, directly relevant to applications that rely on the photothermal response of plasmonic nanoparticles. We compare the experimental photothermal transduction efficiencies with the theoretical absorption efficiencies for each nanoparticle type. Our analysis assumes a distribution of randomly oriented nanorods, as would occur naturally in the tumor vasculature. In our study, photothermal transduction efficiencies differed by a factor of 3 or less between the different types of nanoparticle studied. Both experiment and theory show that particle size plays a dominant role in determining transduction efficiency, with larger particles more efficient for both absorption and scattering, enabling simultaneous photothermal heating and bioimaging contrast enhancement.

Single particle dark field spectroscopy has been combined with high-resolution scanning electron and atomic force microscopy to study the scattering spectra of single gold/silica 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 surface scattering term, which is often included to fit ensemble measurements. These results suggest that plasmon spectral measurements of nanoparticle ensembles are broadened due to particle inhomogeneity. Noble metal nanoparticles exhibit particle plasmon resonances at optical frequencies, making them strong scatterers and absorbers of visible light with resonant peak wavelengths and line widths that are highly sensitive to the nanoparticle size, shape, and local environment. 1,2 These properties, coupled with recent advances in nanoparticle synthesis and assembly, 3 have stimulated interest in the use of plasmon resonant nanoparticles and nanostructures as biological and chemical sensors. 4 Plasmon resonances also allow for the manipulation and enhancement of local electromagnetic fields at nanoparticle surfaces, spurring applications in surface enhanced spectroscopies 5 and photonic devices. 6 The spectral extinction of noble metal nanoparticles has been studied experimentally on nanoparticle ensembles and compared to Mie theory calculations with the complex dielectric function of the metal 7 included as an empirical parameter. 2 Mie theory accurately predicts the dipole and higher order plasmon resonant energies as a function of particle size and environment. However, the resonant line widths, which physically correspond to the coherence lifetime of the plasmon excitation, are underestimated by Mie theory for small particles (<20 nm diameter). 1,[8][9][10][11] The observed line width broadening is typically explained by invoking a size-dependent modification of the bulk dielectric function to include contributions for surface scattering as the particle size becomes smaller than the electron mean free path. Inhomogeneous broadening due to varying particle size and shape, although difficult to assess, should also be considered 12 but is usually assumed to be unimportant. Recently, spectral scattering measurements have emerged which completely remove inhomogeneous broadening by studying a single nanoparticle. [13][14][15][16][17][18][19][20][21] In the case of spherical nanoparticles for which analytical Mie calculations are available, 14,16,21 these results suggest that the additional surface scattering term is not necessary. A recent report on the spectra of single Au/ Au 2 S nanoshells also suggests that electron surface scattering has reduced significance. 22 Here we report similar measurements on larger Au/silica nanoshells where phase retardation is significant.Gold nanoshells are versatile nanophotonic particles whose plasmon resonance can be tuned from the visible through the infrared by adjusting the ratio of their core a...