K. Liu, X. Xue, V. Sukhotskiy, E.P. Furlani
University at Buffalo,
Keywords: localized surface plasmon resonance (LSPR), plasmonic nanocages, photothermal energy conversion, LSPR-induced optical absorption, pulsed-laser photothermal heating, photothermal therapy, plasmonic nanobubble cancer treatment
Summary:We introduce a bottom-up method for the fabrication of nanostructured photonic media and demonstrate proof-of-principle using a multiphysics modeling approach. The method involves magnetic field-directed self-assembly of magnetic-plasmonic core-shell particles (e.g. Fe3O4@Au) into extended particle superstructures that have a desired photonic functionality. The superparamagnetic core enables adaptive magnetophoretic control of the particles and provides interparticle coupling that drives the assembly process. The plasmonic shell provides unique optical and photothermal properties of assembled structures. Monte Carlo analysis and full-wave field theory are combined to investigate the self-assembly of the particles and optical properties of the assembled structures. We demonstrate the method for two different Fe3O4@Au particle assemblies: heptamer particle structures and 1D particle chains. The former supports Fano resonance behavior, while the latter exhibits extraordinary field enhancement and focused photothermal transduction. We analyze the Fano resonance of the heptamer nanostructures in the absorption spectrum. The narrowband Fano absorption peak in the background of the broadband absorption dip exhibits dependence on a variety of distinct factors including particle dimensions, interparticle spacing and coupling conditions, and the refractive index of the surrounding medium. The analysis demonstrates flexible spectral tunability of the optical response of the structure and its potential for biosensing applications. We performed a similar analysis of self-assembled 1D chain structures. The analysis shows that the LSPR-enhanced absorption peak of a chain red-shifts as it forms and asymptotically approaches that of a finite chain. In addition, a thermodynamic analysis reveals advantageous profiles of heat generation and dissipation, which are beneficial for biomedical therapy applications. Last, an absorption dip was observed for the first time in the magnetic-plasmonic chain structures. It possesses a stronger sensitivity to the change of the environment than the absorption peak, which makes it extremely useful for biosensing modalities.