Future and challenges for hybrid upconversion nanosystems

To improve the efficiency of photon upconversion, a hybrid approach of combining organic dyes and inorganic nanoparticles is proving successful, especially in the form of dye-sensitized lanthanide-doped upconversion nanoparticles, nanoparticle-sensitized molecular triplet-triplet annihilation systems and metal-organic-framework nanoparticles. In this Review, we survey the latest advances and examine the key factors affecting upconversion performance, such as spectral overlap, core-shell design and the management of triplet excitons and quenchers at the interface between materials. Although issues such as stability, triplet-state quenching, concentration quenching and reabsorption must still be overcome, smart designs of hybrid nanosystems offer exciting opportunities for applications such as solar photovoltaic devices, deep-tissue biomedical imaging, optogenetics and nanomedicine among others.