Upconversion nanophosphores for bioimaging

Upconversion nanophosphore (UCNP)-based optical imaging is a promising technique in the field of bioimaging due to the unique optical properties of UCNPs. UCNPs can be excited by long wavelength light with low power density and have excellent biocompatibility. By carefully controlling the synthesis of UCNPs, near-infrared emission can also be obtained. Such NIR-to-NIR bioimaging probes show great advantages in terms of bioimaging, such as deep penetration and low autofluorescence. Moreover, UCNPs can also offer a platform for fabricating multifunctional nanocomposites for multi-modal imaging by incorporating magnetic resonance imaging, X-ray computed tomography and single-photon emission computerized tomography techniques. In this review, we briefly introduce the fundamental points of UCNPs, highlight the methods of synthesis and surface-modification strategies, and summarize recent progress. We also discuss the current issues faced by researchers.