A bioprobe based on surface-modified single-phase BaGdF5:Yb/Er upconversion nanoparticles (UCNPs) for multi-modal bioimaging of fluorescent, magnetic resonance imaging (MRI) and computed X-ray tomography (CT) is disclosed herein. The modified UCNPs of the present invention are synthesized by a facile one-pot hydrothermal method with simultaneous surface modification of the nanoparticles. The surface-modified UCNPs of the present invention are useful in a variety of biomedical application fields due to their advantages in in vitro and in vivo multi-modal bioimaging such as small particle size up to 15 nm, substantially free of autofluorescence, low cytotoxicity, capable of being excited at near-infrared (NIR) wavelength, ability to deep cell penetration, long-lasting signal and long circulation time in vivo, different X-ray absorption coefficients at different photon energy levels between Ba and Gd, large magnetic moment, etc.
