Small but bright: origin of the enhanced luminescence of ultrasmall ZnGaO:Cr 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 Zn 2+ , and an interaction between Zn 2+ in CZGO and the MSN matrix was identified. Mesoporous silica plays an active role in improving the persistence luminescence properties of sub-5 nm ZnGa 2 O 4 :Cr 3+ nanoparticles.