Recent Development of Upconversion Nanocrystal–Based Heterostructures: Synthetic Strategies and Biomedical Applications

Upconversion nanocrystals (UCNC) as energy converters have garnered significant attention due to their exceptional luminescent properties. However, the limited performance capabilities of single UCNC fail to meet the demands of increasing application–oriented research. To integrate multiple functionalities, UCNC–based heterostructures have been explored. These heterostructures, comprising UCNC and other functional components (transition metals, semiconductors, quantum dots, metal–organic frameworks, SiO 2 , etc.), present an intriguing system in which the morphology and physicochemical properties are significantly influenced by the combination of functional units. As multifunctional hybrid architectures, UCNC–based heterostructures surmount the intrinsic limitations of individual UCNC configurations, exhibiting synergistically enhanced properties. Nevertheless, due to the chemical composition discrepancy and large lattice mismatches, the synthesis of UCNC–based heterostructures remains challenging. To date, most UCNC–based heterostructures have been fabricated through nonepitaxial growth methods, while epitaxial growth connections remain relatively limited. In this review, recent advancements in the field of UCNC–based heterostructures are summarized and trends in their biomedical applications are outlined. Finally, the challenges and potential opportunities in this field are discussed.