This invention provides a groundbreaking approach to PLNPs and their preparation. In particular, the synthetic methodology disclosed herein fundamentally differs from the traditional solid-state annealing reactions that require extreme and harsh reaction conditions. In one unique aspect of the invention, a simple, one-step mesoporous template method utilizing mesoporous silica nanoparticles (MSNs) is disclosed that affords in vivo rechargeable NIR-emitting mesoporous PLNPs with uniform size and morphology. In another unique aspect of the invention, the novel synthetic approach is based on aqueous-phase chemical reactions conducted in mild conditions, resulting in uniform and homogeneous PLNPs with desired size control (e.g., sub-10 nm).