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Fig. 1 Gold nanostructures widely applied for PTT. A
nanospheres TEM image supplied by (23). b nanorods TEM image
provided by (24). c nanoshells image supplied by (25).

Photothermal properties of gold nanostructures were tunable
by their shape. Since 1857 when Faraday firstly made colloidal gold via reduced
gold chloride with phosphors (26) and in his notes, he indicated
that the particles turned to be reddish was due to the tiny size of the
colloidal gold. One hundred years later, this protocol was simplified by using
sodium citrate (27) as reducing agents. Gold nanoparticles produced
by this method showed unique interaction with light and the mystery behind was
studied since then. One trait that made gold nanostructures a good fit to
today’s PTT treatment was gold nanoparticles absorb light strongly in both
visible and NIR region; the mechanism turned out to be their coherent oscillations
of metal conduction band electrons in strong resonance with such wavelength of
light. This trait was named SPR. SPR frequency was determined by the type of
substance, size and shape of the nanostructure and dielectric constant of the
local environment. Such findings empowered human with an optical tunability to
nano drugs’ SPR frequency, as a consequence, a therapist could use pre-designed
gold nanoparticles to reach the optimal absorbance of wavelength.

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Fig 2 Size, shape and composition dependence of absorbance of
gold nanostructures. a nanospheres, b nanorods, c nanoshells (28).

For gold nanospheres, an increased radius of gold resulted
from a redshift of max absorption (Fig 2a). Furthermore, when these nanospheres
aggregated, their absorption maximum redshifted to NIR region. Rods can be
deemed as a double-radius sphere as it had a longitudinal length and a
latitudinal length so that it was not surprising when gold nanorods were
reported to have two absorption peaks normally (29). As further
research indicated, the stronger band in the NIR was assigned to longitudinal
oscillation of electrons and the weaker peak in visible region around 530 nm
was a result of transverse electronic oscillation (30). There is
another important parameter that determined the gold nanorods’ peak location,
the aspect ratio (length/width), as such ratio increased, the stronger peak
significantly redshifted as Fig. 2b. The third structure gold nanoshell was the
studied the latest in Rice University (31). Its actual structure was
a silica core glided with a thin layer of gold and this design also absorbed
strongly in NIR region (Fig 2c). All three structures were adjustable to the
incident wavelength of light to receive the best absorption at a designated
wavelength of light.

The relationship of absorption peak versus gold
nanostructures was well developed and discussed; it was also very interesting
to know what exactly happened upon gold nanodrugs were exposed to a laser. This
question was also well studied with femtosecond transient absorption
spectroscopy (29).  Upon
exposed under a laser, the heated electron gas was cooled within one ps of
excitation time, at this point of the time, the energy brought by laser was
mainly at the nanoparticle lattice. The surroundings received such energy in
100 ps of first exposure via phonon-phonon interaction. This rapid conversion
of energy and transferring of heat could efficiently introduce precise local
overheating when the laser wavelength was close enough to its nanodrug SPR
absorption peak. Researches had demonstrated such protocol could deliver energy
from the incident light of the laser to tumour cells four to five orders in
efficiency compared with the strongest absorbing dye in clinical use in 1998 (32).
Even a 100 nJ laser input could bring the nanostructures’ electron to several
thousand kelvins. Meanwhile, its lattice could be heated to few tens of degrees
(29). This precise locally destroying treatment might be able to bring
targeted therapy into a new era. The absorption wavelength could be tuned into
NIR region upon changing shape and size of nanodrugs, and NIR region was the
optimal wavelength to penetrate human body as it was reported to have the least
absorption by human tissue and water (33). The recent paper
indicated such heat not only broke cell membrane and denatured protein, but
also formed the bubble around gold nanoparticles followed by a rapid underwater
explosion. Further study indicated that gold nanoparticle aggregation on breast
cancer could significantly enhance bubble size and resulted in serious cell
damage (34).

In 2006, the efficiency of different shape and size of gold
nanodrugs’ ability to convert their optimal wavelength of light into heat was
well established including linear chains, 2D and 3D simulations. Optimal sphere
radius was reported to be 30-40 nm and for rods turned out to be 15-70 nm while
for shell shape, the optimal diameter was 50-100 nm silica with a glided gold
thickness of 4-8 nm for NIR region of incident light (35).

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