Bi-Doped BaYF:Yb,Er Upconversion Nanoparticles with Enhanced Luminescence and Application Case for X-ray Computed Tomography Imaging.

In this work, BaYF:20%Yb/2%Er/%Bi (abbreviated as BaYF:Yb,Er,Bi, where = 0-3.0) upconversion nanoparticles (UCNPs) with various doping concentrations of Bi were synthesized through a simple hydrothermal method. The influence of the doping amount of Bi on the microstructures and upconversion luminescence (UCL) properties of the BaYF:Yb,Er,Bi UCNPs was studied in detail. The doping concentration of Bi 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 BaYF:Yb,Er,Bi UCNPs display first an increasing trend and then a decreasing trend with an increase in the ratio , giving a maximum at = 2.5. A possible energy-transfer process and simplified energy levels of the BaYF:Yb,Er,Bi UCNPs were proposed. The potential of the BaYF:Yb,Er,Bi UCNPs as contrast agents for computerized tomography (CT) imaging was successfully demonstrated. An obvious accumulation of BaYF:Yb,Er,Bi 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.