A simple neridronate-based surface coating strategy for upconversion nanoparticles: highly colloidally stable<sup>125</sup>I-radiolabeled NaYF<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup>@PEG nanoparticles for multimodal<i>in vivo</i>tissue imaging
Nanoscale