Plasmonic Photothermal Therapy in Third and Fourth Biological Windows

The recently reported third and fourth biological transparency windows located respectively at 1.6-1.9 pm and 2.1-2.3 mu m promise deeper light penetration in many tissue types, yet they have not been utilized in photothermal therapy applications. Nanoparticle-assisted 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 the application environment. Upscaling the typical absorber dominant nanoparticle designs (e.g., rod, sphere) that operate in the first and second transparency windows is not a viable option as they become increasingly inefficient absorbers, and their size can get prohibitively large for internalization into certain cell strains. The present study addresses this issue and suggests a versatile approach for designing both monolithic and self-assembling absorber dominant nanostructures for the new transparency windows. These nanoparticles are lithographically fabricatable; additionally, they are easily adaptable to low-cost, mass production compatible chemical growth methods. We demonstrate up to 40% size reduction and 2-fold increase in absorption efficiency compared to the conventional nanobar design. The overall photothermal performance in third and fourth windows is improved up to 55% per mass and 17-fold per nanoparticle compared to the second window.