The preparation, characterization and application of NaYF4:Yb3+, Tm3+–NaYF4:Yb3+, Er3+ core–shell upconversion nanocrystals (UCNPs) with multiple emission peaks (e.g. 539, 654 and 802 nm) have been demonstrated in this work. The monodisperse nanocrystals were prepared via a modified thermal decomposition synthesis. The resulting UCNPs were ∼31 nm in diameter with the lanthanide ions Tm3+ and Er3+ doped in the core and the shell, respectively. Under the laser diode excitation at 980 nm, these core–shell nanocrystals give strong upconversion emissions from the visible to near-infrared (NIR) region. By coating a PEG–phospholipid (PP) layer on the surface of the nanocrystals, the as-prepared UCNPs were favorably endowed with good water solubility for the potential biological applications. Here, a photosensitizer drug of Chlorin e6 (Ce6), which has maximum absorption that overlaps with the red emission of UCNPs, was loaded on these PP-coated UCNPs (UCNP@PP) by physical adsorption. The activity of the Ce6-loaded UCNP@PP (UCNP@PP–Ce6) in photodynamic therapy of cancer cells in vitro has been fully investigated in this work. Our results indicated that these multifunctional UCNP@PP–Ce6 nanoparticles have efficient NIR-to-NIR upconversion luminescence and photodynamic therapy capabilities, which could be potentially employed as a theranostic platform for cancer treatment.
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