Optical studies and cell imaging with

individual semiconductor quantum dots

Maxime Dahan

Laboratoire Kastler Brossel

Physics department, l'Ecole normale supÈrieure

 Paris, France

            Semiconductor quantum dots (QDs) have emerged as fascinating systems at the nanoscale with a wide range of applications extending from optoelectronic devices to biological detection. Their spectral properties combined to their superior photostability have in particular enabled new experiments at the single molecule level. I will present our effort to develop single-QD imaging techniques in live cells and apply them to the study of the dynamics of biological processes.

            In this interdisciplinary approach, it has been essential to account for different aspects of the optical, colloidal and biochemical properties of QDs. I will first discuss some of our results on the fluorescence emission of individual nanoparticles. We have in particular introduced a description of the QD intermittency in terms of Lévy flights that led to an explicit demonstration of statistical aging and non-ergodicity in the fluorescence and shed a new light on an issue rarely discussed in single molecule studies: the relationship between ensemble and time-averaging [1]. We have also analyzed the emission properties of single QDs close to a dielectric interface and demonstrated a new method to determine their radiative and non-radiative recombination rates at the single molecule level [2].

            The fluorescence properties of QDs are not only of subject of investigation but can also be efficiently used to detect single proteins in live cells. We have shown that the motion of individual QD-tagged receptors in the neuronal membrane could be tracked with high spatial and temporal resolution over unprecedented durations [3]. Using these ultrasensitive imaging techniques, we have studied mechanisms by which a cell can control and modify the spatial distribution of proteins in its plasma membrane during important biophysical and biological processes, such as chemotaxis and synaptic regulation. I will illustrate the potential of this approach with recent results on the membrane organization of GABA receptors during axonal guidance.

            Apparently very different, the optical and biophysical experiments discussed above have in fact often raised conceptually similar questions, related to the dynamics of a system in a complex and fluctuating environment. These questions suggest some directions and requirements for future studies of individual molecules in live cells.

[1] X. Brokmann, J.P. Hermier, G. Messin, P. Desbiolles, J.P. Bouchaud, M. Dahan, Phys. Rev. Lett. 90, 120601-1 (2003).

[2] X. Brokmann, L. Coolen, M. Dahan and J.P. Hermier, Phys. Rev. Lett. 93, 107403 (2004).

[3] M.Dahan, S. Lévi, C. Luccardini, P. Rostaing, B. Riveau, and A. Triller, Science 302, 442 (2003).