The Influence of Surface Hydroxylation Media on the Luminescence and Electronic Structure of Cr³⁺-Doped Zinc Gallate Nanoparticles

Near-infrared (NIR) persistent luminescent (PersL) nanomaterials, such as Cr3+-doped zinc gallate (CZGO), have shown strong potential for biologically based imaging owing to their unique optical properties. For intravenous injection, maintaining high dispersibility in aqueous media is important, and surface hydroxylation is a common strategy to improve colloidal stability. Prior studies claimed that both acidic and basic treatments can hydroxylate CZGO surfaces. It is unclear why hydroxylation can be achieved in aqueous media with drastically different chemical environments and whether these different hydroxylation routes lead to comparable optical performances and long-term colloidal stability. In this work, CZGO nanoparticles were hydroxylated in acidic, neutral, and basic conditions, where their morphology, dispersibility and luminescence were compared. We found that a stable colloidal dispersion can be successfully produced in all media used, but acid-treated samples exhibited the strongest and longest-lasting luminescence. X-ray photoelectron spectroscopy (XPS) revealed distinct surface chemistry depending on the hydroxylation medium, enabling direct correlation between the treatment environment and luminescence behaviors. These findings clarified the unsolved questions about how hydroxylation conditions influence both surface chemistry and optical performance, which provided insights for optimizing PersL nanoparticles for biomedical applications.