The synthesis of water-dispersible nano-assemblies containing upconverting nanoparticles (UCNPs) and porphyrin molecules using a one-pot method is described. When the nano-assemblies are excited by 980 nm near-infrared light, the intensity of the 'green emission' band of the UCNP is reduced due to a combination of Föster Resonace Energy Transfer (FRET) and emission-reabsorption. This process is...