| Abstract
| - The selection of nanoparticles for achieving efficient contrast for biological and cell imaging applications, aswell as for photothermal therapeutic applications, is based on the optical properties of the nanoparticles. Weuse Mie theory and discrete dipole approximation method to calculate absorption and scattering efficienciesand optical resonance wavelengths for three commonly used classes of nanoparticles: gold nanospheres,silica−gold nanoshells, and gold nanorods. The calculated spectra clearly reflect the well-known dependenceof nanoparticle optical properties viz. the resonance wavelength, the extinction cross-section, and the ratio ofscattering to absorption, on the nanoparticle dimensions. A systematic quantitative study of the various trendsis presented. By increasing the size of gold nanospheres from 20 to 80 nm, the magnitude of extinction aswell as the relative contribution of scattering to the extinction rapidly increases. Gold nanospheres in the sizerange commonly employed (∼40 nm) show an absorption cross-section 5 orders higher than conventionalabsorbing dyes, while the magnitude of light scattering by 80-nm gold nanospheres is 5 orders higher thanthe light emission from strongly fluorescing dyes. The variation in the plasmon wavelength maximum ofnanospheres, i.e., from ∼520 to 550 nm, is however too limited to be useful for in vivo applications. Goldnanoshells are found to have optical cross-sections comparable to and even higher than the nanospheres.Additionally, their optical resonances lie favorably in the near-infrared region. The resonance wavelengthcan be rapidly increased by either increasing the total nanoshell size or increasing the ratio of the core-to-shell radius. The total extinction of nanoshells shows a linear dependence on their total size, however, it isindependent of the core/shell radius ratio. The relative scattering contribution to the extinction can be rapidlyincreased by increasing the nanoshell size or decreasing the ratio of the core/shell radius. Gold nanorodsshow optical cross-sections comparable to nanospheres and nanoshells, however, at much smaller effectivesize. Their optical resonance can be linearly tuned across the near-infrared region by changing either theeffective size or the aspect ratio of the nanorods. The total extinction as well as the relative scattering contribution increases rapidly with the effective size, however, they are independent of the aspect ratio. To comparethe effectiveness of nanoparticles of different sizes for real biomedical applications, size-normalized opticalcross-sections or per micron coefficients are calculated. Gold nanorods show per micron absorption andscattering coefficients that are an order of magnitude higher than those for nanoshells and nanospheres. Whilenanorods with a higher aspect ratio along with a smaller effective radius are the best photoabsorbingnanoparticles, the highest scattering contrast for imaging applications is obtained from nanorods of high aspectratio with a larger effective radius.
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