Nanoparticles Enhanced Hyperthermia - Intracellular Delivery: Fundamentals and Applications - page 581

the same diameter results in a red-shift and this resonance frequency can be tuned
by changing the shell thickness to core diameter ratio. By adjusting the proportion
of core-shell, the range of optical resonance frequency can cover visible to infrared
region including near infrared region (700-1,300 nm) with the best penetrability for
biological tissue.

3.2.3 Gold Nanorods

Gold nanorods, because of their adjustable aspect ratio, have chemical and optical
anisotropy. As a new class of nanoparticles, nanorods have unique optical pro-
perties compared with other structure. They have both transverse and longitudinal
SPR double-peaks. The transverse SPR, which is due to an electronic oscillation
across the width of the rod, is effectively of the same nature as the plasma reso-
nance of simple gold nanospheres. However, the longitudinal one, which is due to
oscillation of electrons in the long direction of the rod, provides a much larger
extinction coefficient. The longitudinal SPR is red-shifted relative to the transverse
one and can be tuned by controlling aspect ratio. Therefore, the gold nanorods
biological probe can match NIR laser wavelength and allow the near infrared light
to penetrate the deep tissue.

3.2.4 Gold Nanocages

Nanocages are hollow porous GNPs ranging in size from 10 to over 150 nm. They are
created by reacting silver nanoparticles with chloroauric acid in an aqueous solution
(Lu et al. 2007 ; Chen et al. 2006 ). Gold nanocages also absorb light and heat up,
killing surrounding cancer cells. Importantly, compared with other gold nanostruc-
tures, the gold nanocages have a more conveniently and precisely fine-tunable SPR
peak by varying the wall thickness relative to the overall dimension that extends into
the NIR where the optical attenuation caused by blood and soft tissue is essentially
negligible (Skrabalak et al. 2007 ). Skrabalak et al. have found that changing the
amount of metal precursor added to the suspension of Ag nanocubes is a simple means
of tuning both the composition and the localized surface plasma resonance of the
metal nanocages (Skrabalak et al. 2008 ). Besides, they are also biocompatible and
present a well-established surface for easy functionalization.
The key to PTT is the gold nanostructures' ability to efficiently absorb light and
convert it to heat. Compared with nanoshell, both the nanorods and nanocages are
stronger light absorbers due to their higher ratio of light absorption to scattering.

3.2.5 Carbon Nanotubes

CNTs are made from sheets of carbon, usually one-atom thick, and then folded
into tubular structures. These impossibly tiny cylinders are extremely efficient