Removing the Obstacle of Dye‐Sensitized Upconversion Luminescence in Aqueous Phase to Achieve High‐Contrast Deep Imaging In Vivo

Abstract Although upconversion nanoparticles (UCNPs) have drawn increasing attention for their unique photophysical characteristics, they suffer from a bottleneck of low luminescence efficiency due to the poor absorption coefficient of Ln 3+ . Dye sensitization has provided thousands‐fold enhancement of upconversion luminescence (UCL) in organic solvents because of the remarkably improved light absorption ability, but the sensitization of UCL in aqueous phase is only less than 20 folds by far, with unknown restrictive factors. Herein, the aggregation‐caused quenching (ACQ) of dyes is revealed as the most important reason limiting dye sensitization in aqueous phase, and the problem is circumvented through delicately modulating the physical properties of dyes and their assembly manner with UCNPs. By further alleviating energy back transfer (EBT) from Ln 3+ to dyes, more than 600‐fold enhancement of UCL is achieved in aqueous phase. The as‐obtained dyes modified UCNPs show good biocompatibility and high signal contrast when applied for deep in vivo imaging.