Optimising the design of magnetic-plasmonic nanoparticles for use in nanotheranostic applications

Individually, magnetic and plasmonic nanostructures have demonstrated efficacy in a wide range of applications, perhaps most notably in medical applications as contrast and hyperthermia agents. In recent years, multifunctional nanomaterials are at the forefront owing to their suitability in nanotheranostics (combined therapeutics and diagnostics), opening up exciting new possibilities in personalised medicine. To achieve nanostructures that are suitable for a wide range of applications, careful design is required. Hybrid nanostructures that consist of two or more materials are of particular interest, as it is hoped that the hybridisation will combine the favourable characteristics of each material into a superior, hybrid nanostructure. An example of a popular hybrid nanostructure is magnetic-plasmonic nanoparticles, which can be interacted with using both light and magnetic fields for stimuli-responsive applications. Such structures have been explored in diagnostics and therapeutics, in-vitro and in-vivo. However, the impact of hybridisation on the nanostructures physical properties and the efficacy of the nanostructures in various applications has not been fully explored.

Herein, superparamagnetic iron oxide nanoparticles are coated with varying levels of gold, from a core-satellite structure using gold nanoseeds, to a full thick gold shell (five stages in total). These multistage magnetic-plasmonic nanostructures are subsequently characterised structurally and optically, and assessed in a number of applications to more systematically understand the effects of the gold shell.

First, the optical properties of the multistage nanoparticles are explored, exposing a spectral drift (separation in optical scattering and extinction) for moderate thicknesses of gold coverage. Photothermal activity and scatter imaging of the nanoparticles is studied in the two distinct spectral regions, showing enhanced photothermal activity for thinner gold coatings.

Next, the efficacy of the multistage nanoparticle in magnetic-based applications is explored. The magnetic manipulability is seen to decrease exponentially with increased gold coverage, as is the MRI contrasting and alternating magnetic field induced heating. Thus, it is preferable for magnetic-based applications to use thinner gold shells.

We explore the use of magnetic-plasmonic nanostructures as contrast agents in coherent anti-Stokes Raman scattering (CARS) microscopy and laser dark-field microscopy, to allow visualisation of nanoparticle uptake in cells. Furthermore, the nonlinear optical properties of the multistage magnetic-plasmonic nanoparticles are studied by the open aperture z-scan technique. Lastly, the good biocompatibility of the nanostructures in human pancreatic cancer cells is shown.

Lastly, Multiphysics simulations to understand further the physical properties and applications of magnetic-plasmonic nanoparticles with varying gold shell thickness and seed coating radii are shown. In these models, the effects of an intermediate silica layer on the plasmonic activity, the spectral drift, the local electric field enhancement and photothermal activity are studied. Furthermore, the electric and magnetic field absorption and scattering are studied, along with the Kerr rotation and ellipticity enhancements associated with the plasmonic shell.