Lanthanide-doped fluoride nanoparticles: luminescence, upconversion, and biological applications

There has recently been a surge of interest in the luminescence of lanthanide (Ln(3+)) ions doped in fluoride nanoparticles due to their potential for use in a number of diverse fields. Due to their remarkable optical properties, efforts are underway to investigate these nanoparticles for biotechnological applications, for example as fluorescent biolabels. Ln(3+)-doped fluoride nanoparticles can be excited with near-infrared (NIR) radiation and in turn, will efficiently convert the NIR light to visible, a process known as upconversion, resolving the problem of autofluorescence from biological species. In this review, we describe some selected examples of the luminescence (including upconversion) of fluoride nanoparticles doped with various Ln(3+) ions and present strategies used to improve the luminescence (core/shell). Dispersibility of the Ln(3+)-doped nanoparticles in water is vital for biological use thus we discuss the optical properties of fluoride nanoparticles that have been either synthesised in an aqueous medium, or have been surface engineered to render them water dispersible. Finally, we discuss the state of the art in biological uses of Ln(3+)-doped fluoride nanoparticles.