certain nanocrystals possess exceptional optical properties that may make them valuable probes for biological imaging , but rendering these nanoparticles biocompatible requires that they be small enough not to perturb cellular systems . this invention describes a phosphorescent upconverting sub - 10 nm nanoparticle comprising a lanthanide - doped hexagonal β - phase nayf 4 nanocrystal and methods for making the same .