Influence on the Apparent Luminescent Lifetime of Rare-Earth Upconversion Nanoparticles by Quenching the Sensitizer's Excited State for Hypochlorous Acid Detection and Bioimaging.

Lanthanide-ion-doped upconversion materials have been widely used in biological detection, bioimaging, displays, and anticounterfeiting due to their abilities of real-time readings, high spatial resolution, and deep tissue penetration. The typically long fluorescence lifetimes of rare-earth nanoparticles, in the microsecond to millisecond range, make them useful in interference-free lifetime detection imaging. Most detection systems are accompanied by fluorescence resonance energy transfer (FRET), in which the lifetime of the luminescence center can be used as a signal to reveal the degree of FRET. Due to the complex energy level structure and complex energy transfer processes, the apparent lifetimes of upconversion nanoparticles (UCNPs) do not simply equal the decay time of the corresponding energy level, inducing an insignificant lifetime change in the upconversion detection system. In this study, the relationship between the apparent luminescence lifetime of upconversion and the decay rate of each energy level was studied by numerical simulations. It was proved that the apparent lifetime of the emission at 540 nm was mainly affected by the decay rate of Yb. We then constructed a nanocomposite with Rh1000 fluorophores loaded onto the surface of UCNPs to quench the sensitizer Yb. We found that the lifetime of the emission at 540 nm from Er was affected to a large extent by the number of attached Rh1000 molecules, proving the greater influence on the apparent luminescent lifetime of Er at 540 nm caused by quenching the Yb excited state. The qualitative detection of hypochlorous acid (HClO) was also achieved using the luminescent lifetime as the signal.