Advances in single upconverting nanoparticle sensing

Lanthanide doped upconversion nanoparticles (UCNPs) that convert near infrared photons to visible/ultraviolet emissions have been widely used in sensing applications due to high chemical/photostability without bleaching and blinking, large anti-Stokes shift with low autofluorescence background, sharp and tunable emission bands, etc. Normally, these prominent achievements are accomplished by using UCNP ensembles with statistical average luminescence. However, the nanoparticle ensembles neglect the discrepancy between nanoparticles, especially for the interactions between nanoparticles and in situ status. Benefiting from uniform UCNP construction and single particle spectroscopy, the investigations of upconverting sensing have been expanded to single nanoparticle level. These facilitated the revelation of photophysics and photochemistry variations in micro/nano regions, leading to efficient and sensitive in situ detection, tracking, and sensing. Herein, we present a systematic review on the recent advances in single upconverting nanoparticle sensing, including the strategies to obtain uniform and sensitive upconversion nanoprobes, optical detection systems and emerging single UCNP applications in ions, molecules and in situ microenvironment sensing. Then, current challenges and future potentials of single UCNP sensing with high sensitivity and spatial resolution are discussed. This review is expected to inspire more thorough investigations of high throughput single upconverting nanoparticle sensing with high spatial and temporal resolution.