Overcoming surface quenching in persistent luminescence nanoparticles with organic ligand coordination: Implications for enhanced optical imaging

Persistent luminescence nanoparticles (PLNPs) have been considered as excellent luminescent tools for sensitive optical imaging. However, the persistent luminescence (PL) properties of PLNPs are usually suppressed due to severe surface quenching caused by nanoscale size. Despite considerable efforts, the understanding of surface quenching mechanisms remains a challenge. Here we report a surface ligand coordination method to enhance the PL properties of ZnGa₂O₄:Al,Cr (ZGO:AC) PLNPs. ZGO:AC PLNPs coated with organic ligands containing carboxyl groups show enhanced PL intensity and prolonged decay time. Among the six ligands, the phthalic acid (PA) ligands yielded the largest luminescence enhancement effect, on which the PL intensity and decay time increased 3 and 2 times compared with ligand-free ZGO:AC PLNPs, respectively. Density functional theory (DFT) calculations and detailed characterizations demonstrated that surface quenching is suppressed due to the coordination of non-saturated Ga ions and -COO^- of ligands. Finally, ZGO:AC@PA PLNPs were applied in autofluorescence-free bioimaging and exhibited high imaging sensitivity and contrast ratio. These findings provide insights into the surface quenching mechanisms of PLNPs and offer a platform for developing highly emissive PLNPs on nanoscale.