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. The prospects for improving photon upconversion systems by combining organic dyes and inorganic nanoparticles are reviewed.