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 (NaYF₄) materials, the upconversion luminescence (UCL) of which saturates at power densities (P) of several hundred W cm^-2 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-NaYbF₄:Er³⁺@CaF₂ core/shell UCNPs doped with varying Er³⁺ concentrations (5-25%), studied over a wide P range of ∼8-10⁵ W cm^-2, which emit intense UCL even at a low P of 10 W cm^-2 and also saturate at relatively low P. The highest upconversion quantum yield (Φ_UC) and the highest particle brightness were obtained for an Er³⁺ dopant concentration of 12%, reaching the highest Φ_UC of 0.77% at a saturation power density (P_sat) of 110 W cm^-2. These 12%Er³⁺-doped core/shell UCNPs were also the brightest UCNPs among this series under microscopic conditions at high P of ∼10²-10⁵ W cm^-2 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.