Engineering anisotropic magnetic-plasmonic Au–Fe3O4 nano-urchins for theranostic applications

Here, we present a detailed study on the effect of the different synthetic parameters on the morphology of Au–Fe3O4@PEG nano-urchins, leveraging an innovative seed-growth method to create peculiar urchin-like nanostructure combining magnetic and plasmonic responses within the near-infrared biological transparency window. Importantly, we demonstrate the tunability of the optical response by modulating the size and the morphology of the gold domain through controlled reactants’ concentration and reaction time. The resulting particles, constituted by a small Fe3O4 nanosphere (average size 11 nm), embedded in a gold nano-urchin with extremely elongated and irregular spiky tips (average size 131 nm), displayed magnetic and optical properties exploitable as contrast agents in magnetic resonance imaging and as heat mediators in photothermal therapies. Indeed, optical spectroscopy revealed a continuous plasmonic band from 550 nm to 1200 nm covering the therapeutic optical window, where two distinct localized surface plasmon resonance bands at 664 nm and 972 nm, corresponding to the transverse and longitudinal peaks of the Au nano-urchins, respectively, can be identified. Thus, this multifunctional material not only displayed a wider plasmon band compared to the commercial gold nanourchins, but the 1H NMR relaxometry experiments displayed a relaxometric efficiency very close to that of the commercial contrast agents Endorem. Preliminary biocompatibility analysis performed on three different cell lines, RAW 264.7 murine macrophages, fibroblasts (MRC5), and endothelial cells, underscores the potential of these nano-urchins in biomedical applications.