  Longer near-infrared wavelengths provide better penetration depth in
biological tissues, so these are useful for plasmonic photothermal cancer
therapeutics. In the context of nanoparticles for such applications, the
absorption can be tuned for longer NIR wavelengths. However, on increasing the
size of the nanoparticle, the scattering is enhanced and thus is not suitable
for plasmonic therapeutics. Therefore, to overcome this issue, different types
of small gold nanorods were synthesized and converted into stable aggregates to
red-shift the plasmonic resonance wavelength to longer near-infrared
wavelengths. The gold nanorod aggregates were embedded into the agarose gel
phantoms mimicking the tumor-tissue-like structure. The photothermal response
was measured through the prepared phantoms using a broadband near-infrared
light source. It was shown that even in an extremely dilute concentration of
gold nanorods, the photothermal heat generation could increase after the
aggregation and also give gives deeper penetration of thermal energy. The
observed photothermal response was also verified through numerical simulation.
The current study shows better performance by increasing the plasmonic
coupling, reducing the mismatch issue of plasmonic resonance shift in the
second biological therapeutic window for the aggregates without increasing the
size of individual nanoparticles. The aggregates provide better light
penetration at deeper tissue by red-shifting the absorbance resonance
wavelength which is useful for plasmonic photothermal cancer therapy.
