Résumé
Semiconductor quantum dots (QDs) constitute very promising candidates as light emitters for numerous applications in the field of biotechnology, such as cell labeling or other bioimaging techniques. For such applications, semiconductor QDs represent an attractive alternative to classic organic fluorophores as they exhibit a far superior photostability by several orders of magnitude and a higher brightness thanks to large absorption cross-sections. Within this family of materials, core-shell heterostructures such as CdSe/CdS QDs are especially of interest. In particular CdSe/CdS QDs with relatively thick CdS shells offer several properties essential to biolabeling, including high photoluminescence quantum yields, low blinking behavior and robustness towards aggressive environments. We recently developed a new, fast and very efficient method for the synthesis of such QDs, denoted ‘flash’ CdSe/CdS, which can feature up to 20 monolayers of CdS after no more than 3 minutes of synthesis. These ‘flash’ CdSe/CdS QDs show state-of-the-art optical properties (sharp emission spectra, high photoluminescence quantum yields, low blinking behavior), and the CdS shell thickness can be easily controlled thanks to the full chemical yield of the reaction. These QDs were encapsulated in silica nanoparticles through a water-in-oil microemulsion process, a technique that allows a high control on the morphology of the resulting QD@SiO2 nanoparticles. All the nanoparticles contain one single QD located in its center and the thickness of the silica shell can be varied from only a few nanometers up to several tens of nanometers. The silica matrix provided the QDs with enhanced colloidal stability in polar solvents, but also enhanced photo-physical and chemical stability under irradiation. More importantly, QD@SiO2 nanoparticles based on ‘flash’ CdSe/CdS QDs fully retain their photoluminescence quantum yield even after more than a year of storage in water, whereas QD@SiO2 nanoparticles based on ‘classical’ SILAR grown core-shell QDs typically lose their luminescence after a few weeks or even days. Thereafter, these ‘flash’ CdSe/CdS@SiO2 nanoparticles have proven to be very promising nanoprobes for bioimaging techniques. Indeed, the rapid uptake of high levels of these nanoparticles by live cells was evidenced by confocal fluorescence microscopy. Furthermore, thanks to the high stability of their optical properties but also to their low toxicity after silica encapsulation, these nanoparticles are particularly appropriate for long term cell labeling and tracking. Thus, in this contribution we will report from the synthesis and characterization of these ‘flash’ CdSe/CdS@SiO2, all the way to the study of their toxicity and their application to cell labeling.