Self-Assembled Upconversion Nanoparticle Clusters for NIR-controlled Drug Release and Synergistic Therapy after Conjugation with Gold Nanoparticles

Abstract

Fabricated three-dimensional (3D) upconversion nanoclusters (abbreviated as EBSUCNPs) are obtained via an emulsion-based bottom-up self-assembly of NaGdF₄:Yb/Er@NaGdF₄ nanoparticles (abbreviated as UCNPs), which comprise a NaGdF₄:Yb/Er core and a NaGdF₄ shell. The EBSUCNPs were then coated with a thin mesoporous amino-functionalized SiO₂ shell (resulting in EBSUCNPs@SiO₂ precursor) and further conjugated with gold nanoparticles to give the novel EBSUCNPs@SiO₂@Au material. Finally, EBSUCNPs@SiO₂@Au was applied as a biocompatible and efficient drug carrier for doxorubicin (DOX), thus giving rise to a multifunctional EBSUCNPs@SiO₂-DOX@Au nanocomposite. This final material, EBSUCNPs@SiO₂-DOX@Au, and the precursor nanoparticles, EBSUCNPs@SiO₂@Au, were both fully characterized and their luminescence was investigated in detail. In addition, the drug release properties and photothermal effects of EBSUCNPs@SiO₂-DOX@Au were also discussed. Interestingly, when under NIR irradiation, an increasing DOX release was achieved owing to the thermal effect of the Au NPs after absorbing the green light from the upconversion nanoclusters based on the fluorescence resonance energy transfer (FRET) effect. Thus, a near-infrared (NIR)-controlled "on-off" pattern of drug release behavior can be achieved. Moreover, compared with a single therapy method, the assembled nanocomposites exhibit a good synergistic therapy against cancer cells that combines chemotherapy with photothermal therapy. In addition, the in vitro fluorescence microscopy images of EBSUCNPs@SiO₂-DOX@Au show a higher enhancement in the red region due to the loading of DOX molecules with respect to EBSUCNPs@SiO₂@Au. Therefore, this novel multifunctional 3D cluster architecture can be used in the biomedical field after modification and may pave a new way in other application areas of UCNPs clusters.