While infrared upconversion imaging using halide nanoparticles are so common the search for a very efficient halide free upconverting phosphors is still lacking. In this article we report Gd2O2S:Yb/Er,YbHo,YbTm systems as a very efficient alternative phosphors that show upconversion efficiency comparable or even higher than existing halide phosphors. While the majority of rare earth dopants provide the necessary features for optical imaging, the paramagnetic Gd ion also contributes to the magnetic imaging,thereby resulting in a system with bimodal imaging features. Results from imaging of the nanoparticles together with aggregates of cultured cells have suggested that imaging of the particles in living animals may be possible. In vitro tests revealed no signficant toxicity because no cell death was observed when the nanoparticles were in the presence of growing cells in culture. Measurement of the magnetization of the phosphor shows that the particles are strongly magnetic, thus making them suitable as an MRI agent.
Here we report the synthesis, characterization and application of a multifunctional surface functionalized GdF3:Nd3+ nanophosphor that exhibits efficient near infrared (NIR) fluorescence as well as magnetic properties, which can be utilized for bimodal imaging in medical applications. The nanoparticles are small with an average size of 5 nm and form stable colloids that last for several weeks without settling, enabling the use for several biomedical and photonic applications. Their excellent NIR properties, such as nearly 11 % quantum yield of the 1064 nm emission, make them ideal contrast agents and biomarkers for in vitro and in vivo NIR optical bioimaging. The nanophosphors which were coated with poly(maleic anhydride- alt-1-octadicene) (PMAO) were implemented in cellular imaging and show no significant cellular toxicity for concentrations up to 200 μg ml−1. Furthermore, the incorporation of Gd into the nanocrystalline structure supplies exceptional magnetic properties, making them ideal for use as magnetic resonance imaging (MRI) contrast agents. The utility of these NIR emitting nanoparticles in infrared bioimaging and as contrast agent in magnetic resonance imaging was demonstrated by confocal imaging, magnetic resonance and tissue experiments.
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