Photocatalysis, Anti-counterfeiting and Optical Thermometry Applications of Upconversion Nanoparticles

Lanthanide-doped upconversion nanoparticles (UCNPs) have been the subject of extensive investigation owing to their ability to convert low-energy infrared photons to high-energy visible and ultraviolet photons (anti-stokes emissions). Well-established synthesis protocols for the UCNPs have already been in place via which UCNPs can be easily prepared with full control over their size, crystal structure, phase, morphology, etc. Lanthanide-doped UNCPs offer low background autofluorescence, sharp emission bandwidths and large anti-stokes shifts, high photostability, high penetration depth, and low toxicity. The UCNPs have therefore found multifunctional applications in various fields of photonics, energy conversion, solar cells, photocatalysis, biomedicine, imaging, sensing, therapy, anti-counterfeiting, etc. Despite progress, the upconversion emission efficiency of the UCNPs remains very low as compared to their bulk counterparts. Therefore, the need for novel strategies for the effective enhancement of the upconversion efficiency of the UCNPs for any meaningful practical application is urgent. In this chapter, we will present recent developments on the applications of the UCNPs in three specific fields viz photocatalysis, anti-counterfeiting, and optical thermometry. We will first briefly present the fundamentals of upconversion in lanthanides and the selection of hosts and activators for the efficient upconversion process. In the following sections, emphasis will then be given to the mechanisms involved for each of the specific applications of the UCNPs, the available state of the arts, current challenges, and future perspectives.