Biomedical Engineering Reference
In-Depth Information
of conduction-band electrons, plasmons, upon irradiation with light of
appropriate wavelengths. h ese plasmons underlie the intense absorption
and elastic scattering of light, which in turn forms the basis for many bio-
logical sensing and imaging applications of gold nanoparticles [33-46].
h e elastic light-scattering properties of gold nanoparticles are sui cient to
detect individual nanoparticles in a visible light microscope with approxi-
mately 10(2) nm spatial resolution.
Jain
et al.
have used Mie theory and discrete dipole approximation
method to calculate absorption and scattering ei ciencies and optical reso-
nance wavelengths for three commonly used classes of nanoparticles: gold
nanospheres, silica-gold nanoshells, and gold nanorods [47]. By increasing
the size of gold nanospheres from 20 to 80 nm, the magnitude of extinction
as well as the relative contribution of scattering to the extinction rapidly
increases. Gold nanospheres in the size range commonly employed show
an absorption cross-section 5 orders higher than conventional absorbing
dyes, while the magnitude of light scattering by 80-nm gold nanospheres is
5 orders higher than the light emission from strongly l uorescing dyes. Jain
et al.
have stated that the variation in the plasmon wavelength maximum
of nanospheres, i.e., from approximately 520 to 550 nm, is too limited to
be useful for
in vivo
applications. h ey have found that gold nanoshells
have optical cross-sections comparable to and even higher than that of the
nanospheres. Also, gold nanorods show optical cross-sections comparable
to nanospheres and nanoshells, however, at a much smaller ef ective size.
h e growth of gold nanoparticles by reduction by citrate and ascorbic
acid has been examined in detail by Kimling
et al.
[48]. h ey have exam-
ined the growth of gold nanopaticles to explore the parameter space of
reaction conditions. It is found that gold particles can be produced in a
wide range of sizes, from 9 to 120 nm, with dei ned size distribution, fol-
lowing the earlier work of Turkevich and Frens [43, 49-50]. h e reaction
is initiated thermally or in comparison to UV irradiation, which results in
similar i nal products. h e kinetics of the extinction spectra show the mul-
tiple steps of primary and secondary clustering leading to polycrystallites.
Subrata
et al.
have produced cubic gold nanoparticles under UV pho-
toactivation by using a chiral reagent, 2-naphthol, under alkaline solution
as a reductant for HAuCl(4) in CTAB micelle [51]. Prolonged irradia-
tion helped the digestion of the primarily evolved spherical particles into
smaller gold nanocubes, which then act as tiny cubic seeds, leading to the
formation of larger nanocubes [51].
Alivisatos
et al.
have described a strategy for the synthesis of “nanocrys-
tal molecules,” in which discrete numbers of gold nanocrystals are orga-
nized into spatially dei ned structures based on Watson-Crick base-pairing
