Broadband Near-Infrared Photothermal Response of Tissue-Mimicking Phantoms Embedded with Plasmonic Gold Nanorod Aggregates: Implications for the Non-invasive Eradication of Malignant Tumors

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 near-infrared wavelengths. However, with increasing size of the nanoparticle, the scattering is enhanced and thus is not suitable for plasmonic photothermal 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 by 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 a deeper penetration of thermal energy. The observed photothermal response was also verified through a numerical simulation. The current study shows better performance by increasing the plasmonic coupling, reducing the mismatch issue of the 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.