Upconversion nanoparticles and their potential in the realm of biomedical sciences and theranostics

Interdisciplinary sciences have paved the way for the creation of new technologies that could potentially alter how things were conventionally done. Biotechnology and nanotechnology have resulted in the creation of various techniques, one of which is upconversion nanoparticles (UCNPs), which can be conjugated with biomaterials for added functionality. UCNPs work based on optical phenomena referred to as anti-Stoke shift, wherein the nanoparticle absorbs light of a high wavelength (lower energy) and emits light having a lower wavelength (higher energy). This ability to provide luminescent signals when irradiated with near-infrared (NIR) light sources is particularly useful as a consequence of its minimal toxicity and high depth of penetration. When coupled with the appropriate biomaterials, the UCNPs can conjugate with the target under study and via quenching/luminescence recovery and quantitative and qualitative tests can be performed relating to the target molecule, in vivo. These UCNP-nanomaterial structures have proven to be very useful in the in vivo image detection, diagnostic, therapeutic as well as combined approaches for disease treatment, drug delivery, disease diagnosis etc. This review aims at providing an in-depth explanation about the origins of UCNPs, how they may be synthesized, their mechanisms of upconversion (UC), their adaptability and flexibility under in vivo conditions, and the future of the field. Graphical Abstract Overall representation of Upconverting Nanoparticle (UCNP) synthesis followed by the currently researched/applied modes of utilization for the same. The drug delivery aspect involves loading the porous layer over the UCNP with the required drug while the bio-imaging and therapeutic application (via photothermal therapy and/or photodynamic therapy) involves irradiation of the nanoparticles by tissue-penetrating near-infrared radiation (NIR).