Hollow upconversion nanoparticles: Synthesis and luminescence in comparison with their solid counterparts

Upconversion nanoparticles (UCNPs) have attracted a lot of interest in recent years due to their unique optical properties and potential for use in many different applications. However, direct synthesis of UCNPs with hollow structure at room temperature remains challenging. Here, ultra-facile one-pot synthesis of hollow Yb/Er co-doped NaBiF₄ UCNPs at room temperature is reported. The NaBiF₄:Yb,Er nanoparticles exhibit bright upconversion luminescence (UCL) upon near infrared excitation at 980 nm. Benefiting from the inherent advantages of the hollow structure (large surface permeability and large internal surface area), NaBiF₄:Yb,Er hollow nanoparticles have superior luminescence properties than their solid counterparts. The UCL intensity of NaBiF₄:Yb,Er hollow nanoparticles is 6.8 times that of solid counterparts, and its upconverison quantum efficiency is almost 10 times that of solid counterparts. Successful synthesis of these hollow UCNPs at room temperature in one-pot makes large-scale production possible due to the simplicity and low temperature nature of the synthesis and opens new ways to load sensitizer dyes or other materials for enhanced upconversion emission or multiplexed assays/imaging, or loading of photosensitive drugs for therapeutic applications. These finding will encourage researchers to further explore hollow structural materials with excellent UCL properties in the future. © 2021 Elsevier B.V.