To obtain a quantitative guide for selection of nanoparticles
for light-scattering and absorption-based applications in biomedicine,
a systematic study of the trends in the optical
resonance wavelength, the extinction cross-section, and the
relative contribution of scattering to the extinction with changes
in the nanoparticle dimensions, was undertaken for three
different classes of nanoparticles viz. silica-gold nanoshells,
gold nanospheres, and gold nanorods. It was clearly evident
from the calculated spectra that the optical properties of
nanoparticles were highly dependent on the nanoparticle size,
shape, and core-shell composition. All three nanoparticle types
had optical cross-sections a few orders of magnitude higher than
those of conventional dyes. For all three nanoparticle types, the
increase in the size resulted in an increase in the extinction crosssection
as well as the relative contribution of scattering.
However, nanoshells and nanorods were found more favorable
for in vivo applications due to their tunable optical resonance
in the NIR region. Moreover, their relative scattering to
absorption contribution could be easily tuned by a change in
their dimensions. For the comparison of the optical properties
of nanoparticles across a range of sizes, size-normalized crosssections
were calculated. From the numerical comparison, the
gold nanorods are seen to offer the most superior NIR absorption
and scattering at much smaller particle sizes. Smaller sized
nanorods may also offer better cell uptake31 as compared to
the larger nanoshells and nanospheres. This, in addition to the
potential noncytotoxicity92 of the gold material, easy optical
tunability, and facile synthesis, makes gold nanorods the most
promising nanoparticle agents for use in biomedical imaging
and photothermal therapy applications.