A novel core–shell structured upconversion nanorod as a multimodal bioimaging and photothermal ablation agent for cancer theranostics

Abstract

A multifunctional core–shell nanocomposite based on noble metal plasmons coated with upconversion material has emerged as a promising cancer theranostics nanoplatform that integrates properties such as multimodal imaging, photothermal effects, good biocompatibility, and efficient therapy. However, a reasonable combination of plasmons and upconversion materials, as well as increased penetration depth, has always challenged the anti-cancer efficiency. Here, a unique kind of fluorescent thermal-magnetic resonance core–shell upconversion nanostructure has been designed and fabricated to simultaneously achieve photothermal therapy (PTT) and multimodal imaging. Gold nanorods (GNRs) are used as the plasmon cores and NaGdF₄ with rare-earth Yb³⁺/Er³⁺ ions co-doping are used as the upconversion luminescence (UCL) shells, merging into upconversion nanorods (UCNRs) of GNRs@NaGdF₄:Yb³⁺,Er³⁺. An NaGdF₄ shell synthesized by a hydrothermal method can substitute for the cetyltrimethylammonium bromide (CTAB) on the surface of GNRs, which offers the benefits of reducing toxicity and increasing biocompatibility. More significantly, the red and green emission of Yb³⁺/Er³⁺ couples convert near-infrared (NIR) into visible light, appropriately overlapping with absorbance of GNRs, which improves the photothermal conversion efficiency. Meanwhile, we designed small and low-aspect-ratio GNR cores for the absorption of UCNRs in vivo. Verification with evidence from in vivo and in vitro assays shows that these core–shell UCNRs exhibit a talented potential application in multimodal bioimaging and PTT.