Résumé
Semiconductor quantum dots (QDs) constitute very promising candidates as light emitters for numerous applications in the field of biotechnology, including cell labeling, in vivo imaging and diagnostics.[1] For such applications, semiconductor QDs represent an attractive alternative to classic organic fluorophores as they exhibit a higher brightness thanks to their large absorption cross-sections and high photoluminescence quantum yields. Nevertheless, QDs usually suffer from higly oxidative environments, such as water, which can cause a dramatic decrease of their photoluminescent quantum yield but also can result in the realease of toxic elements. In this contribution we present a new generation of QD@SiO2 nanoparticles based on newly developped core-shell QDs that mostly overcome these limitations, resulting in efficient nanoprobes for long term cell labeling.
Among the numerous QDs being reported, core-shell heterostructures such as CdSe/CdS QDs with relatively thick CdS shells, are of particular interest as they 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 as ‘flash’ CdSe/CdS, which can feature up to 20 CdS monolayers after only 3 minutes of reaction.[2] They 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 ‘flash’ CdSe/CdS QDs were encapsulated in silica nanoparticles through a water-in-oil microemulsion process, which allows a high control on the morphology of the resulting QD@SiO2 nanoparticles. All the nanoparticles contain one single QD located in its center (Fig. 1) and the thickness of the silica shell can be varied from only a few nanometers up to several tens of nanometers. The silica matrix provides the QDs with enhanced colloidal stability in polar solvents, but also with enhanced photo-physical and photo-chemical stability under continuous irradiation. More importantly, the QD@SiO2 nanoparticles based on ‘flash’ CdSe/CdS QDs fully retain their photoluminescence quantum yield even after a year of storage in water (Fig. 1), 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 (Fig. 1). 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, with up to 9 cell divisions being tracked. 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.