Preparation And Luminescent Mechanism Of ZGGO:Cr³⁺ Nanoparticles With Near-infrared Super Long Persistent Luminescence And Their Applications

Since near-infrared light locating in the biological transparency window(650-950 nm)possesses less absorption, deep penetration and low light damage through tissue, more studies are focused on bioimaging, disease diagnosis and therapy by using near infrared super long persistent luminescent nanoparticles(PLNPs) and they show potential applications in the fields of physiology and pathology. At present, it is essential to prepare near infrared(NIR) super long persistent luminescent nanoparticles PLNPs with uniform and small size and long afterglow time and to develop new applications, such as temperature detection and disease diagnosis and therapy. In this paper, NIR super long PLNPS Cr3+doped zinc gallogermanate(ZGGO:Cr3+) have been prepared via a hydrothermal method in combination with a subsequent annealing in a vacuum.Meanwhile the NIR persistent luminescence mechanism of ZGGO:Cr3+nanoparticles has been studied and their applications in bioimaging and temperature detection have been performed. The main research results are shown as follows:To give a insight into the understanding of how Ge doping influences the microstructure and afterglow properties of ZGGO:Cr3+, sub-10 nm Cr3+doped zinc gallate(Zn Ga2O4:Cr3+) nanoparticles have been prepared by a hydrothermal synthesis route. After the as-prepared sample was irradiated for 5 min under UV light, the NIR persistent luminesence peaked at 692 nm attributed to the 2E â†' 4A2 transition were observed and more than 15 min of the afterglow time can be obtained. For the samples annealed in a vacuum, the afterglow time can be reached to more than 6 h. Meanwhile,nanoparticles show larger particle size after the thermal-annealing. The persistent luminescence mechanism of Zn Ga2O4:Cr3+ nanoparticles has been investigated based on the analysises of optical properties, afterglow decay and luminescence kinetics. Our results show that the persistent luminscent mechanism might be dominated by tunneling process. For the annealed Zn Ga2O4:Cr3+samples with deferent Zn/Ga ratio, their afterglow properties have been compared with each other. It can be inferred that the electron traps might be related to anti-site defects(Zn Ga′-Cr3+*-Ga Zn o) and the deficiency of zinc due to the high temperature annealing might facilitate the generation of electron traps.To obtain smaller particle size and longer afterglow time, ZGGO:Cr3+ nanoparticles with excellent NIR persistent luminescent performce and average particle size of 80 nm have been prepared by a hydrothermal synthesis route in combination with a subseqent vacuum annealing. The persitent luminescence spectra show a superposition of a narrowband emission at 697 nm(2Eâ†'4A2) and a broadband emission at 712 nm(4T2â†'4A2). For the optimized sample annealed at 800 o C through tuning Ga/Ge ratio, the afterglow time can be reached to more than 15 h(320 nm UV light irradiationc for 5min). After the annealing, the introduced Ge4+ ions might restrain the increase of the particle size compared with that of Zn Ga2O4:Cr3+. Meanwhile, our results show that the afterglow properies of the annealed ZGGO:Cr3+ nanoparticles can be improved by decreasing the Ga3+/Ge4+ ratio.Besides the processes of thermal activation, the tunneling processes might have a contribution to the persistent luminescence of the annealed ZGGO:Cr3+ nanoparticles with different Ga3+/Ge4+ ratio on the basis of thermoluminescence and luminescence kinetics analysises. Furthermore the contribution of the tunneling process to NIR persistent luminescence might be relate to the concentration of electron traps.Meanwhile the Ge4+ doping and higher temperature annealing might facilitate the evopration of Zn, Ge and O and the generations of more anti-site defects Zn Ga′-Ga Zn o and more V Ge-Cr3+-V O cluster defects in the annealed ZGGO:Cr3+ nanoparticles and enhance their persistent luminescence. Furthermore, the contributions of surface and interior Cr3+ in nanoparticles to the NIR persistent luminescence have been studied by using luminescence kinetics technique. Our results indicate that the relative contribution of surface to interior Cr3+ ions in nanoparticles to the persistent luminescence does not show an obvious change after the annealing at 800 o C. However, more surface Cr3+ ions in nanoparticles occupy low-symmetry sites after the annealing above 880 o C.When applied the ZGGO:Cr3+ PLNPS to human serum albumin(HSA) solution, its observed bioimaging with naked eyes can reach to 2 min after stopping UV light irradiation for 15 min. In addition, a ratiometric persistent luminescence detection for temperature sensing has been proposed by using the ratio of the 4T2â†'4A2 to 2Eâ†'4A2(4 22/T E I I) persistent luminescence intensity. In particular, the temperature detection method based on the persistent luminescence from the interior Cr3+ in nanoparticles exhibits higher reliability and sensitivity(0.05275 K-1). In the 298-325 K(25-52 o C)region, the sensitivity is in the range of 0.0434-0.0470 K-1 and is about one order of magnitude higher than that using upconversion luminescence method.