Efficient sub-15 nm cubic-phase core/shell upconversion nanoparticles as reporters for ensemble and single particle studies.

Single particle imaging of upconversion nanoparticles (UCNPs) has typically been realized using hexagonal (β) phase lanthanide-doped sodium yttrium fluoride (NaYF) materials, the upconversion luminescence (UCL) of which saturates at power densities (P) of several hundred W cm under 980 nm near-infrared (NIR) excitation. Cubic (α) phase UCNPs have been mostly neglected because of their commonly observed lower UCL efficiency at comparable P in ensemble level studies. Here, we describe a set of sub-15 nm ytterbium-enriched α-NaYbF:Er@CaF core/shell UCNPs doped with varying Er concentrations (5-25%), studied over a wide P range of ∼8-10 W cm, which emit intense UCL even at a low P of 10 W cm and also saturate at relatively low P. The highest upconversion quantum yield (Φ) and the highest particle brightness were obtained for an Er dopant concentration of 12%, reaching the highest Φ of 0.77% at a saturation power density (P) of 110 W cm. These 12%Er-doped core/shell UCNPs were also the brightest UCNPs among this series under microscopic conditions at high P of ∼10-10 W cm as demonstrated by imaging studies at the single particle level. Our results underline the potential applicability of the described sub-15 nm cubic-phase core/shell UCNPs for ensemble- and single particle-level bioimaging.