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 (beta) 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 (alpha) 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 alpha-NaYbF4:Er3+@CaF2 core/shell UCNPs doped with varying Er3+ concentrations (5-25%), studied over a wide P range of similar to 8-10(5) 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 (phi(UC)) and the highest particle brightness were obtained for an Er3+ dopant concentration of 12%, reaching the highest phi(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 similar to 10(2)-10(5) 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.