Small but bright: origin of the enhanced luminescence of ultrasmall ZnGa2O4:Cr3+ in mesoporous silica nanoparticles.

Chromium(III)-doped zinc gallate (CZGO) is one of the representative persistent luminescent phosphors emitting in the near-infrared (NIR) region. The emission wavelength it covers falls in the tissue-transparent window, making CZGO a promising optical probe for various biomedical applications. The PersL mechanism dictates that such a phenomenon is only profound in large crystals, so the preparation of CZGO with sizes small enough for biological applications while maintaining its luminescence remains a challenging task. Recent attempts to use mesoporous silica nanoparticles (MSN) as a template for growing nanosized CZGO have been successful. MSN is also a well-studied drug carrier, and incorporating CZGO in MSN further expands its potential in imaging-guided therapeutics. Despite the interest, it is unclear of how the addition of MSN would affect the luminescence properties of CZGO. In this work, we observed that forming a CZGO@MSN nanocomposite could enhance the luminescence intensity and extend the PersL lifetime of CZGO. X-ray absorption fine structure (XAFS) analysis was conducted to investigate the local structure of Zn2+, and an interaction between Zn2+ in CZGO and the MSN matrix was identified.

Synthesis and characterization of a near-infrared persistent luminescent Cr-doped zinc gallate-calcium phosphate composite.

Near-infrared (NIR)-emitting persistent luminescence (PersL) nanoparticles have attracted great attention as a novel optical probe for bioimaging and biosensing applications. These nanoparticles emit long-lasting luminescence after the removal of the excitation source, which effectively eliminates the interference from tissue autofluorescence. Cr-doped zinc gallate (ZnGa2O4:Cr3+, CZGO) is a representative NIR-emitting PersL material. On the other hand, amorphous calcium phosphate (ACP) is a widely used drug carrier due to its high biocompatibility. In this work, we present a design of an ACP-based drug carrier with PersL properties, by forming a CZGO-ACP composite. The PersL properties of CZGO were preserved by composite formation, while it is found that the Zn2+ could migrate from CZGO to ACP during composite formation, leading to different luminescence mechanisms between pure CZGO and the CZGO-ACP composite. The electronic structure of the composite was analyzed by synchrotron X-ray absorption spectroscopy, and a structure-luminescence correlation was proposed.