Modulation of Plasmon Damping and Fluorescence Activity in Dumbbell-Shaped Gold Nanorods Decorated with Platinum Islands via Thiolated β-Cyclodextrin-Based Host–Guest Chemistry

Bimetallic gold nanorods decorated with platinum islands (AuNRs@Pt) are widely recognized as promising photothermal agents due to their localized surface plasmon resonance (LSPR) effects, which enable localized heat generation for photothermal therapy (PTT). Additionally, photodynamic therapy (PDT) using photosensitizers (PSs) offers synergistic potential for light-activated cancer treatment. This study investigates the fluorescence behavior and plasmon energy transfer mechanisms of dumbbell-shaped AuNRs@Pt with varying Pt coverages, functionalized with thiolated β-cyclodextrin (SH-βCD) as a host for methylene blue (MB), which serves as a PS. The synthesis and functionalization of AuNRs@Pt revealed tunable plasmon damping pathways, including chemical interface damping and metal interface damping, modulated by Pt deposition and host-guest supramolecular interactions. MB attachment induces plasmon damping, with LSPR damping increasing as the Pt content increases. Functionalization with SH-βCD alters the dielectric environment, while energy dissipation facilitated by Pt islands further amplifies LSPR damping. Molecular dynamics simulations revealed the preferential host-guest interaction of MB with SH-βCD rather than direct interaction with the nanoparticle surface. Meanwhile, MB inclusion increased the local dielectric constant, resulting in redshifts. Notably, host-guest inclusion reduced fluorescence quenching and Förster resonance energy transfer (FRET)-based nonradiative decay, enhancing MB fluorescence and optimizing its emission properties. Additionally, Pt content modulated radiative decay rates and fluorescence quantum yield, with Pt islands amplifying electromagnetic enhancements via the Purcell effect while simultaneously reducing FRET quenching. Therefore, this work lays the foundation for developing advanced theranostic nanoplatforms that leverage the synergy between plasmonic nanoparticles and fluorescence-active molecules for targeted cancer therapies.