Heterostructures Combining Upconversion Nanoparticles and Metal–Organic Framework: Fundamental, Classification, and Theranostic Applications

Integrating biocompatible metal–organic frameworks (MOFs) with upconversion nanoparticles (UCNPs), capable of converting near-infrared (NIR) light into visible or ultraviolet emissions, affords unique UCNPs@MOFs heterostructures for nanomedicine. Alongside inherited advantages from parent UCNPs on imaging and parent MOFs with porous structure on chemodrugs delivery, upconversion Förster resonance energy transfer (FRET) from UCNPs to molecular moieties in MOFs offers unique abilities to probe or modulate intracellular and tumor microenvironments at high spatiotemporal precision. As a result, UCNPs@MOFs heterostructures allow to detect hint number of biomolecules, regulate intracellular ion concentrations for tumor cell killing and lamellipodia-induced cell metastasis, on-demand release NO gas molecules for antibacterial therapy and neurotherapy, and perform tumor microenvironment-responsive multimodal therapy, all achieved possible using tissue-penetrating NIR light stimulation of low photon flux. In this work, recent progress on the construction and applications of UCNPs@MOFs heterostructures toward these regards is surveyed. An emphasis is placed on the classification of heterostructures (Janus, core@satellite, core@shell) with distinct geometric shapes as well as the fundamental upconversion FRET mechanism underpinning all their theranostic applications. Cutting-edge applications on combined therapies, drug delivery, multimodal imaging, and biosensing are also highlighted, along with provided perspectives on the future development.