Upconversion nanoparticles (UCNPs) offer a myriad of opportunities for bioanalytical applications, well beyond those of their down-converting counterparts (QDs, organic dyes, etc.). It is due to their several unique photophysical attributes of UCNPs, including: large anti-Stokes shift, improved penetration depth in biological tissues, resistance to photobleaching, low autofluorescence background, remarkable chemical stability, and low toxicity. Plenty of articles have provided details on UCNP synthesis, photophysical characteristics, and their use in therapeutics. Herein, we provide a comprehensive description on their analytical applications in light of the surge in UCNP research over last few years. Beginning with a brief overview on concepts related to the luminescent properties of UCNPs, it leads into the essential surface-functionalization strategies that have been adopted thus far in designing UCNP probes. Subsequently, a comprehensive detail of recent advancements in UCNP probe development is provided. Finally, current challenges in UCNP research and future prospects have been discussed.
