The effects of the amount of Ge4+ doped in Zn2Ga3.98-4x/3GexO8:Cr0.02 nanoparticles on size distribution, NIR afterglow imaging and temperature sensing

Abstract

Single spinel-phased Zn2Ga3.98-4x/3GexO8:Cr0.02 (ZGGO:Cr) near infrared (NIR) persistent luminescent nanoparticles (PLNPs) with various Ge4+ concentrations (0.45 ≤ x ≤ 0.90) were prepared via an initial hydrothermal synthesis and a subsequent vacuum annealing. Unlike the case of co-substitution of Ge4+ and Zn2+ for Ga3+ via a one-step hydrothermal method, the increase of Ge4+ substitution concentration from 0.45 to 0.90 can lead to the decrease in average particle size from 51.7 to 45.4 nm. Meanwhile, more anti-site Zn′Ga−Ga°Zn, GeGa° and VGa′′′ defects are introduced into ZGGO host lattice after Ge4+ substitution and it results in the shallowed trap depths, narrowed band gap and increased NIR afterglow intensity around 700 nm, which derives from the 2E, 4T2 → 4A2 transitions of surface and interior Cr3+ ions. In our case, not only afterglow imaging but also temperature sensing can be simultaneously realized by constructing a modified ratiometric afterglow nanothermometer based on the temperature-dependent afterglow intensity ratios of 4T2 → 4A2 to 2E → 4A2 emissions of surface and interior Cr3+ doped in ZGGO:Cr nanoparticles. Although the decrease in absolute thermal sensitivity from 0.50 to 0.41 K-1 is found at 310 K with increasing Ge4+ substitution concentration from 0.45 to 0.90, the relative thermal sensitivity exhibits an increasing trend, and the maximum relative sensitivity reaches 1.0% K−1 within the physiological temperature window (295–328 K) when x = 0.90. Our results suggest that ZGGO:Cr nanoparticles with an amount of Ge4+ concentration ranging from 0.75 to 0.90 have potential applications in bioimaging and biosensing.