Brightening Upconversion Nanoparticles

ConspectusLanthanide-doped upconversion nanoparticles (UCNPs) represent a distinctive class of luminescent nanomaterials capable of converting low-energy near-infrared (NIR) photons into higher-energy visible or ultraviolet emission through multiphoton processes. This anti-Stokes luminescence underpins a broad spectrum of applications, including deep-tissue bioimaging, biological therapy, high-sensitivity biosensing, nanothermometry, and super-resolution microscopy. However, the upconversion brightness of UCNPs─defined as the number of photons emitted per particle per second at a given power density─remains intrinsically low, thereby constraining their widespread implementation.The weak upconversion emission brightness of UCNPs arises from a combination of intrinsic and extrinsic factors of the lanthanide ions. Intrinsically, the parity-forbidden 4f-4f electronic transitions of lanthanide ions result in low absorption cross sections and low radiative decay rates, rendering both photon absorption and emission inherently inefficient. Extrinsically, the large surface-to-volume ratio of UCNPs amplifies nonradiative energy losses through surface-related quenching, while high lanthanide dopant concentrations induce luminescence concentration quenching by nonradiative depopulation of excited states via cross-relaxation and back-energy-transfer pathways. The concentration quenching effect also precludes the possibility of reducing lanthanide interionic distances in the host lattice to accelerate energy transfer from sensitizer to emitter ions, thereby constraining the upconversion quantum yields (UCQYs). Moreover, because upconversion is a multiphoton process, the emission brightness is highly sensitive to the excitation power density, leading to a marked decrease in UCQYs under low excitation light irradiance. Additionally, uncontrolled energy migration can divert excitation energy to nonemissive sites such as crystal defects or surface quenching centers, further diminishing the luminescence efficiency. Acting individually or synergistically, these factors collectively suppress the emission brightness of UCNPs.In this Account, we review recent advances in the study of upconversion processes in lanthanide-doped nanoparticles, with a particular focus on our group's research progress over the past several years. We present a systematic framework for enhancing upconversion brightness through both intrinsic and extrinsic engineering strategies. Intrinsically, coupling UCNPs with NIR-absorbing dye sensitizers has significantly improved light harvesting, as dye molecules possess absorption cross sections approximately 4 orders of magnitude higher than those of lanthanide ions. Such sensitization has even enabled lanthanide-mediated excitation of perovskite nanocrystals under low irradiance, achieving emission enhancements exceeding 4 orders of magnitude. In parallel, plasmonic coupling accelerates radiative decay via the Purcell effect, enabling tunable emission amplification across several orders of magnitude. Extrinsically, core-ultrathick-shell architectures that leverage newly identified size-dependent lanthanide energy-transfer pathways have achieved UCQYs of up to 13%, approximately 4 times higher than their bulk counterparts. Furthermore, spatial separation of sensitizer and activator ions through core-shell nanostructuring mitigates back energy transfer and increases the concentration-quenching threshold of activator ions to as high as 50%, thereby substantially enhancing upconversion brightness by increasing the number of emitter ions. Meanwhile, lanthanide-mediated photon-avalanche processes typically introduce ultrahigh-order nonlinearities (order > 20) and yield exceptional upconversion emission intensities suitable for super-resolution imaging. Collectively, these developments deepen our understanding of nanoscale energy transfer dynamics and guide the rational design of next-generation bright UCNPs.