Plasmonic Photothermal Therapy in Third and Fourth Biological Windows

The recently reported 3rd and 4th biological transparency windows located respectively at 1.6-1.9um and 2.1-2.3um promise deeper tissue penetration and reduced collateral photodamage, yet they haven't been utilized in photothermal therapy applications. Nanoparticle based plasmonic photothermal therapy poses a nontrivial optimization problem in which the light absorption efficiency of the nanoparticle has to be maximized subject to various constraints that are imposed by application environment. Upscaling the typical absorber-dominant nanoparticle designs (rod, sphere etc.) that operate in the 1st and 2nd transparency windows is not a viable option as their size gets prohibitively large for cell intrusion and they become scatterer-dominant. The present study addresses this issue and suggests a versatile approach for designing both lithography based and self-assembling absorber dominant nanostructures for the new transparency windows, while keeping their size relatively small. The proposed nanoparticles demonstrate up to 40% size reduction and 2-fold increase in absorption efficiency compared to the conventional nanobar design. The overall photothermal performance per nanoparticle in the 4th window is boosted up by 250% compared to the 2nd window.