Self-Assembly Strategies in Upconversion Nanoparticle-Based Nanocomposites: Structure Designs and Applications.

Self-assembly has emerged as a powerful bottom-up strategy for the construction of multifunctional nanocomposites based on upconversion nanoparticles (UCNPs). In contrast to epitaxial shell growth, self-assembly enables the modular integration of UCNPs with a broad spectrum of other functional nanomaterials. This characteristic makes it particularly attractive for various practical applications. This review provides a comprehensive overview of self-assembly methodologies for UCNP-based nanocomposites, including electrostatic interactions, hydrophobic interactions, covalent coupling, and specific biorecognition. The resultant nanohybrids exhibit a wide range of morphologies and functionalities, making them suitable for various applications, including multimodal imaging, bioimaging, advanced biosensing, smart nanocarriers for controlled molecular delivery, and orthogonal photoactivation for programmable therapy. Key recent studies are highlighted to elucidate the structure-function relationships and technological potential of these materials. Additionally, the current challenges, such as stability, reproducibility, and functional integration, and proposed future directions for the development of UCNP-based nanocomposites are further discussed.