Rare earth spectrocopy for biophotonics

Luminescence is now extensively used in biology since it constitutes as a very sensitive way of investigations. Several applications of fluorescence concern immunoassays, DNA hybridization, protein interactions, cell imaging, Lab-on-chip etc.;; As non biological fluorescent probes, organic dye molecules and semiconductors quantum dots are the most popular. However, they suffer from drawbacks which are linked to the physical processes giving rise to their optical properties. In both cases, the luminescence is unstable with time and photobleaching quickly occurs, then they are not fully adapted for lengthy observations. Furthermore, quantum dots exhibit intermittent photon emission. If this "blinking" phenomena is exciting from a fundamental point of view, this results in signal intensity fluctuations in ultrasensitive detection required for single particle tracking experiments. Quantitative analysis are also difficult even if this problem can be partially overcome by coating the nanoparticules and reducing the excitation light. Optical properties of rare earth ions which are used in a wide range of applications, are not very popular for biological purposes (except if we consider new laser sources for medical applications). Up to now, rare earth luminescence is mostly used in LRET (Luminescence Resonant Energy Transfer) for immunofluorescence purposes. Very few studies have been performed on rare earth doped materials to realize biological probes based on lanthanide direct fluorescence.