Cholesterol-Driven Guest Exchange Tunes Plasmon Damping in Single β-Cyclodextrin-Modified Gold Nanorods

Abstract

Chemical interface damping (CID) in gold nanorods (AuNRs) originates from additional non-radiative decay pathways associated with molecule–metal electronic interactions. While CID has been extensively investigated, strategies for its tunability at the single-particle level remain limited. Herein, we demonstrate a host–guest chemistry–based approach to modulate CID in single AuNRs using a thiolated β-cyclodextrin (SH-βCD) platform with methylene blue (MB) and cholesterol (Chol) as competing guest molecules. CID modulation is achieved through competitive binding and guest-exchange processes. Molecular dynamics simulations show that although the AuNR@βCD/MB complex is thermodynamically stable, Chol exhibits a stronger binding affinity, enabling efficient displacement of MB from the βCD cavity. Complementary UV–Vis absorption, surface-enhanced Raman spectroscopy, and photoluminescence measurements confirm the release of encapsulated MB, MB dimerization, and competitive inclusion of Chol. Single-particle dark-field spectroscopy further reveals systematic changes in localized surface plasmon resonance peak positions and linewidths upon guest exchange. Although MB does not form strong covalent bonds with the AuNR surface after displacement, the released MB molecules reside closer to the nanoparticle surface, enhancing molecule–metal electronic interactions and increasing CID, as reflected by linewidth broadening. Collectively, these results establish βCD-based host–guest chemistry as an effective strategy for systematically tuning CID in single AuNRs.