  Upconversion persistent luminescence (UCPL) phosphors that can be directly
charged by near-infrared (NIR) light have gained considerable attention due to
their promising applications ranging from photonics to biomedicine. However,
current lanthanide-based UCPL phosphors show small absorption cross-sections
and low upconversion charging efficiency. The development of UCPL phosphors
faces challenges of lacking flexible upconversion charging pathways and poor
design flexibility. Herein, we discovered a new lattice defect-mediated
broadband photon upconversion process and the accompanied NIR-to-NIR UCPL in
Cr-doped zinc gallate nanoparticles. The zinc gallate nanoparticles can be
directly activated by broadband NIR light in the 700-1000 nm range to produce
persistent luminescence at about 700 nm, which is also readily enhanced by
rationally tailoring the lattice defects in the phosphors. This proposed UCPL
phosphors achieved a signal-to-background ratio of over 200 in bioimaging by
efficiently avoiding interference from autofluorescence and light scattering.
Our findings reported the lattice defect-mediated photon upconversion for the
first time, which significantly expanded the horizons for the flexible design
of NIR-to-NIR UCPL phosphors toward broad applications.
