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Upconversion nanoparticles possess the ability to convert low energy photons to high energy photons, with the advantages of no autofluorescence, reduced photodamage, deeper tissue penetration, and extended photostability. Therefore, upconversion nanoparticles are considered ideal probes for framing potential theranostic agents. Generally, oleic acid has been used as a capping agent to fabricate high-quality upconversion nanoparticles. However, the hydrophobic oleic acid ligands on the surface of the nanoparticles continue to be a barrier when used in biological applications. Herein, we modified the surface of oleic acid coated upconversion (OA-NaGdF 4 :Yb/Er) nanoparticles to be hydrophilic using a novel one-step solvent assisted mechanochemical (SAM) milling ligand exchange strategy. Normal ligand exchange processes are tedious and take one to two days to complete, but the SAM ligand exchange strategy presented here is facile, rapid, and takes less than 1 h. This surface modified citric acid coated upconversion (CA-NaGdF 4 :Yb/Er) nanoparticles further ensures dispersibility in water and good biocompatibility, as revealed by electron microscopy, ATR-FTIR spectroscopy, zeta potential measurement, upconversion luminescence studies, and cytotoxicity assessment. Besides, the size and shape of the nanoparticles were retained after surface modification. Moreover, the monodispersed CA-NaGdF 4 :Yb/Er nanoparticles exhibit intrinsic paramagnetic behavior which makes them suitable to be used as a contrast enhancer for  T   1 -weighted MRI. In addition, the measured CT numbers (in HU) increased linearly with increasing the concentration of the surface modified CA-NaGdF 4 :Yb/Er nanoparticles, indicating their plausibility as a CT contrast agent. The current findings suggest that the SAM ligand exchange strategy could be used to fabricate hydrophilic and biocompatible upconversion nanoparticles for bioimaging applications.

Rapid and one-step mechanochemical ligand exchange of Yb 3+ /Er 3+ co-doped NaGdF 4 upconversion nanoparticles for efficient MR and CT imaging