Functionalized upconversion nanoparticles: New strategy towards FRET-based luminescence bio-sensing

Functionalized upconversion nanoparticles (UCNPs) have perceived massive success in the area of bio-photonic applications. UCNPs are extensively used as a successful emitter in terms of unique luminescent properties, minimum auto-fluorescence, high chemical &thermal photo-stability, deep tissue penetration, excellent biocompatibility, and low toxicity. The relatively high emission efficiency of the UCNPs creates them greatly attractive and encouraging materials for widespread usages in biomedical and clinical sciences. High color purity, easy surface functionalization, and good colloidal stability are the advantageous features of the UCNPs. Widespread efforts are dedicated to emerging operational approaches for the preparation and surface functionalization/engineering of luminescent UCNPs of varying morphologies, simplifying the advancement of this auspicious material for fluorescent resonance energy transfer (FRET) based luminescence biosensing applications. We widely review the recent advancement of UCNPs in optical bio-sensing applications, with importance on the synthesis, surface functionalization, emission efficiency enhancement factors, and their successful use in the detection of pathogenic diseases, free radicals, and biomarkers. A detailed overview of functionalization techniques for UCNPs has also been presented, with a special emphasis on organic ligands-functionalization, doping, and methods of biological species-encapsulation/conjugation. Based on the current scientific achievements, traditional optical applications such as bio-sensing, immunosensing, biomarkers are now added. Finally, as a roadmap scheme for study in this area, future trends and recent problems are emphasized.