Rare-earth-doped ceramic nanophosphor (RED-CNP) materials are promising near-infrared (NIR) fluorescence bioimaging (FBI) agents that can overcome problems of currently used organic dyes including photobleaching, phototoxicity, and light scattering. Here, we report a NIR–NIR bioimaging system by using NIR emission at 1550 nm under 980 nm excitation which can allow a deeper penetration depth into biological tissues than ultraviolet or visible light excitation. In this study, erbium-doped yttrium oxide nanoparticles (Er3+:Y2O3) with an average particle size of 100 and 500 nm were synthesized by surfactant-assisted homogeneous precipitation method. NIR emission properties of Er3+:Y2O3 were investigated under 980 nm excitation. The surface of Er3+:Y2O3 was electrostatically PEGylated using poly (ethylene glycol)-b-poly(acrylic acid) (PEG-b-PAAc) block copolymer to improve the chemical durability and dispersion stability of Er3+:Y2O3 under physiological conditions. In vitro cytotoxic effects of bare and PEG-b-PAAc-modified Er3+:Y2O3 were investigated by incubation with mouse macrophage cells (J774). Microscopic and macroscopic FBI were demonstrated in vivo by injection of bare or PEG-b-PAAc-modified Er3+:Y2O3 into C57BL/6 mice. The NIR fluorescence images showed that PEG-b-PAAc modification significantly reduced the agglomeration of Er3+:Y2O3 in mice and enhanced the distribution of Er3+:Y2O3.
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