Design of a new nanostructure for theranostic applications

Medical applications of metal nanoparticles are the subject of intense research due to their unique properties which make them suitable for both diagnostic and therapeutic use. One such property is the Surface Plasmon Resonance (SPR) which results in strong enhancement of the absorption and scattering of electromagnetic radiation. The combination of metal type, size, and shape characteristics provides unique tunability of a nanostructure´s optical properties. Several types of nanoparticles have been explored for medical and biological applications. Here we present a theoretical investigation of a novel metal nanostructure which has the unique property of distinct absorption and scattering plasmon bands. This could be beneficial for combined diagnostic and therapeutic applications since the diagnostic and therapeutic laser wavelengths can be decoupled for increased efficacy and safety. For this purpose, it is desirable to have the most intense scattering, with minimal absorption, in the near-infrared for imaging and the opposite in the red, for therapy. The efficiency factor for various metals, shapes and sizes was first calculated using the Discrete Dipole Approximation (DDA) method. From the results, nanostructures consisting of combinations of cubes and small spheres were considered to have the desired property and were thoroughly investigated. The number (diameter) and material (silver or gold) of the nanospheres were varied in order to obtain the optimum nanostructure with distinct absorption and scattering plasmon band. Given its properties, this nanostructure have the potential to be used for enhancement of various imaging and therapeutic methods.