Recent Advances in Rare-Earth Based Persistent Luminescent Probes

Over the past few decades, lanthanide-based nanoparticles (NPs) have found extensive industrial applications due to their excellent photoluminescence as well as phosphorescent behaviour. Their unique photostability owes its origin to the shielding effect of f-levels. It is known that while some rare-earth (RE) elements display downconversion (DC) luminescence, others exhibit exquisite upconversion (UC) luminescence. To name a few, RENPs find widely ranged commercial usage in screens, lights, glasses, laser hosts etc. DC-RENPs also have utility as fluorescent agents in various immunoassays, providing better sensitivity as compared to the conventional techniques. However, due to better penetrability in biological systems, low energy near-infrared excited UC RENPs have also drawn the attention of the researchers. Interestingly, both UC and DC-RENPs can be utilized simultaneously in LED devices as multicolour components based on their emission responses. Lately, lanthanide resonance energy transfer has been exploited to augment the luminescent property of RENPs utilized as labels in biosensing devices. In this context, RENPs exhibiting persistent luminescence (PerL) need special mention as it has a great potential in biosensing because of their delayed photoemission responses (up to several days in some cases). This chapter highlights the mechanisms involved in the various emission processes describing the importance of RE-based PerL nanoprobes, specifically in the field of biosensing. A comprehensive discussion on the recent progress in this domain as well as in the combined use of the technique with other existing methods overcoming the limitations and increasing the efficacy has been made.