Magnetic field-induced plasmonic enhancement of near infrared fluorescence from a Magnetoplasmonic nanoplatform for bioimaging applications.

A phenomenon of plasmon-enhanced fluorescence (PEF) arises from interactions between fluorophores and metal nanostructures, leading to a substantial amplification of the fluorescence signal. Herein, we report a magnetic field (MF) induced on-demand PEF from the magnetoplasmonic nanoplatform and demonstrate its application in near infrared (NIR) bioimaging. The developed magnetoplasmonic nanoparticles (~ 50 nm diameter) feature a core-shell-satellite architecture comprising a Fe₃O₄ magnetic core, a mesoporous silica (mSiO₂) shell housing IR775-silane NIR dye, and surface-anchored gold (Au) seeds (satellites). Application of an external MF causes the magnetophoretic movement and aggregation of the nanoparticles (NPs), resulting in a formation of localized plasmonic hotspots and, consequently, in a plasmonic enhancement of NIR fluorescence from IR775 dye molecules. Correspondingly, a substantial reduction of the fluorescence lifetime in the MF-treated area was observed, in addition to the enhanced fluorescence intensity. In vivo studies with NPs subcutaneously injected into mice revealed MF-activated amplification of NiR fluorescence. At 6 h post-injection, the injected region treated by MF exhibited 2.1-fold stronger NIR fluorescence signal than the MF-untreated one; the fluorescence enhancement correlated with the reduction of the emission lifetime (from 0.68 ns to 0.47 ns). At 96 h post-injection, the MF-treated region exhibited 6.8-fold more intense NIR fluorescence. Histological analysis showed absence of toxicity from the injected NPs, revealing their biocompatibility. Hence, a considerable potential of MF-induced PEF with the magnetoplasmonic nanoplatform for targeted NIR fluorescence bioimaging was demonstrated. This work also introduces MF-induced PEF as a powerful strategy for spatiotemporal control of optical signals, offering new opportunities for targeted imaging and sensing.