Upconversion Spectroscopic Investigation of Single Nanoparticles

Upconversion nanoparticles (UCNPs) have made a significant and valuable contribution to photophysics and biomedicine, due to their specific spectroscopic characteristics. However, the ensemble spectroscopy of UCNPs is limited for the electronic behavior in average effect, which ignores the fact that the nanoparticles are heterogeneous. Towards the research focus on heterogeneous intrinsic structure, unique photophysical phenomena, and advanced applications, the optical characterization of single UCNPs are promoted to a frontier breakthrough of UCNPs community. Electronic behavior detection aimed at a single nanoparticle displays signals from the micro-structure of nanoparticles, while single nanoparticle spectroscopy offers clear insight into the interplay between intrinsic and extrinsic influences without noise, and subsequently gives instructions for the high quality preparation of UCNPs. Single nanoparticle optical characterization possesses a powerful capacity to explore the crystalline structure anisotropy related optical differences, or some unexpected unique optical phenomena at sub-micron and even nano-scale. In this review, the importance of single UCNPs characterization and single particle detection methods are overviewed, in which the considerable emphasis is placed on the specific spectroscopic study of single UCNPs Showing fantastic photophysical phenomena beyond ensemble measurement. Finally, this review identifies