Tunable Dual Visible and Near-Infrared Persistent Luminescence in Doped Zinc Gallogermanate Nanoparticles for Simultaneous Photosensitization and Bioimaging

Zinc gallogermanate nanoparticles have been synthesized by a sol-gel process with adjustment of Zn²⁺, Ga³⁺, and Ge⁴⁺ ratios and subsequent sintering at 1000 °C. The resulting X-ray diffractograms can be simulated by variation of the ratios α, β, and γin the standard cards of αZnGa₂O₄-βZn₂GeO₄-γGeO₂. Similarly, the FT-IR and Raman vibrational spectra are sums of the spectra of the individual components. The major difference in our synthesis of the ratio α:β:γ= 1:1:1 compound, ZGGO, from previous studies is the contribution of Zn₂GeO₄ in the rhombohedral lattice system and not the cubic phase. Photoluminescence and electron spin resonance spectra supply sensitive analyses of our samples and show the presence of zinc and oxygen vacancies, with the latter playing an important role in emission. In particular, a tunable visible emission band is discovered which can be deconvoluted into several Gaussian components. The lowest energy band, at ∼520 nm, gives the strongest persistent luminescence, and it is tunable by adjusting the ionic ratios in the samples. The persistent luminescence decay can be fitted by biexponential functions with five free parameters, but the normalized persistent luminescence decay can also be fitted by a hyperbolic function with one free parameter, representing an average rate constant. This implies a fairly unimodal distribution of trap-acceptor distances and/or orientations. The rate constant decreases with measurement time because deeper traps are being emptied. The fitted average rate constants, when monitoring persistent luminescence at different energies, display the persistent luminescence spectrum. Doping the zinc gallogermanates with chromium enables the relative intensity of green/NIR persistent luminescence bands to be tuned by adjusting the Cr³⁺ content. A potential application of the dual persistent luminescence is in simultaneous photosensitization and bioimaging.