Lanthanide ions exhibit unique luminescent properties, including the ability to convert near infrared long-wavelength excitation radiation into shorter visible wavelengths through a process known as photon upconversion. In recent years lanthanide-doped upconversion nanocrystals have been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays and medical imaging. These techniques offer low autofluorescence background, large anti-Stokes shifts, sharp emission bandwidths, high resistance to photobleaching, and high penetration depth and temporal resolution. Such techniques also show potential for improving the selectivity and sensitivity of conventional methods. They also pave the way for high throughput screening and miniaturization. This tutorial review focuses on the recent development of various synthetic approaches and possibilities for chemical tuning of upconversion properties, as well as giving an overview of biological applications of these luminescent nanocrystals.

Upconversion refers to non-linear optical processes that convert two or more low-energy pump photons to a higher-energy output photon. After being recognized in the mid-1960s, upconversion has attracted significant research interest for its applications in optical devices such as infrared quantum counter detectors and compact solid-state lasers. Over the past decade, upconversion has become more prominent in biological sciences as the preparation of high-quality lanthanide-doped nanoparticles has become increasingly routine. Owing to their small physical dimensions and biocompatibility, upconversion nanoparticles can be easily coupled to proteins or other biological macromolecular systems and used in a variety of assay formats ranging from bio-detection to cancer therapy. In addition, intense visible emission from these nanoparticles under near-infrared excitation, which is less harmful to biological samples and has greater sample penetration depths than conventional ultraviolet excitation, enhances their prospects as luminescent stains in bio-imaging. In this article, we review recent developments in optical biolabeling and bio-imaging involving upconversion nanoparticles, simultaneously bringing to the forefront the desirable characteristics, strengths and weaknesses of these luminescent nanomaterials.