Surface Dye-Coordination for Efficient Upconversion Nanosystems
Lanthanide-doped upconversion nanoparticles (UCNPs) exhibit distinctive optical characteristics, including excellent photostability, large anti-Stokes shifts, narrow emission bands, and tunable luminescence lifetimes. Despite their advantages, UCNPs suffer from inherently weak light absorption because of the 4f-4f transitions of lanthanide ions. Near-infrared dye-sensitization has emerged as an effective strategy to enhance their absorption, yet the photoconversion performance remains constrained by photobleaching and interfacial energy losses. In this review, we systematically analyze the surface coordination environments and energy transfer pathways that govern dye-sensitized UCNPs. We evaluate critical molecular parameters, such as dye frameworks, surface binding affinity, and triplet-state energy alignment, in conjunction with nanoparticle structural features, including dopant concentration, core-shell architectures, and surface electronic configurations. By providing a fundamental assessment of these photophysical and photochemical processes, we propose targeted optimization strategies to enhance the performance and stability of these hybrid materials for advanced applications.