Bi³⁺-Doped BaYF₅:Yb,Er Upconversion Nanoparticles with Enhanced Luminescence and Application Case for X‑ray Computed Tomography Imaging

In this work, BaYF₅:20%Yb³⁺/2%Er³⁺/x%Bi³⁺ (abbreviated as BaYF₅:Yb,Er,Bi_x, where x = 0–3.0) upconversion nanoparticles (UCNPs) with various doping concentrations of Bi³⁺ were synthesized through a simple hydrothermal method. The influence of the doping amount of Bi³⁺ on the microstructures and upconversion luminescence (UCL) properties of the BaYF₅:Yb,Er,Bi_x UCNPs was studied in detail. The doping concentration of Bi³⁺ has little influence on the microstructures of the UCNPs but significantly impacts their UCL intensities. Under excitation of a 980 nm near-IR laser, the observed UCL intensities for the BaYF₅:Yb,Er,Bi_x UCNPs display first an increasing trend and then a decreasing trend with an increase in the ratio x, giving a maximum at x = 2.5. A possible energy-transfer process and simplified energy levels of the BaYF₅:Yb,Er,Bi_x UCNPs were proposed. The potential of the BaYF₅:Yb,Er,Bi_x UCNPs as contrast agents for computerized tomography (CT) imaging was successfully demonstrated. An obvious accumulation of BaYF₅:Yb,Er,Bi_x in tumor sites was achieved because of high passive targeting by the enhanced permeability and retention effect and relatively low uptake by a reticuloendothelial system such as liver and spleen. This work paves a new route for the design of luminescence-enhanced UNCPs as promising bioimaging agents for cancer theranostics.