Phosphorus dendrimers decorated with 2PA chromophores (Organic Nanodots)

Organic Nanodots

Semiconductor quantum dots (QDs) have been shown to provide a particularly effective approach towards fluorescent nano-objects for biological imaging. Indeed, these inorganic nanoparticles exhibit large one- and two-photon absorption cross-sections, reasonable fluorescence quantum yields, broad excitation but narrow emission bands, and high photostability. These properties make them of particular interest for in vitro and in vivo imaging, and they have found applications in specific labelling of cells and tissues. In addition, their electronic characteristics are closely related to the size and shape of the individual crystal, therefore their emission spectra can be tuned by playing on their size and composition, so they can be used for multicolor imaging. They have also found applications in two-photon excited fluorescence (2PEF) microscopy, which has gained widespread popularity in the biology community due to the many advantages it provides for biological microscopic imaging, including intrinsic three-dimensional resolution and increased penetration depth in tissues. However, these inorganic nanocrystals suffer from several drawbacks such as biological toxicity (due in particular to the presence of heavy metals such as cadmium), polydispersity and blinking phenomenon that diminishes their fluorescence. These nano-objects also raise a number of questions with respect to environmental issues. Pioneering work by Mongin et al consisted in a new approach towards brilliant nanoprobes, mimicking the confinement effect of QDs by using quadrupolar fluorophores grafted to an eco-friendly phosphorus based dendritic scaffold, developed by Jean Pierre Majoral group. In this work it was demonstrated that the covalent confinement of optimized chromophores within hierarchical architectures in which interactions are controlled, led to tuneable nano-objects showing record one- (e x F) and two-photon brightness (s2 x F). As a result, the maximum values (7·106 M-1 cm-1 at 386 nm and 55 900 GM at 705 nm) of this dendritic structures so-called organic nanodots (ONDs) are comparable to the s2 value measured by Webb and coworkers. Furthermore, if the fluorophores are injected in the blood stream without any organization, the brightness in a localized volume tends to be limited to the diffusion range, a limitation that is solved with this dendritic structures.

Within this context, we have focused our attention in quasi-symmetrical conjugated orthogonal dyes which are known to display better response in 2PA than push-pull systems. However, if the fluorophores are injected in the blood stream without any organization, the brightness in a localized volume tends to be limited to the diffusion range. To overcome this drawback, the 2PA dyes are confined in a well-defined structure allowing a more uniform dispersion and overcoming the π-stacking. Thus, taking advantage of dendritic architectures, which are highly ordered three-dimensional molecules with activated ending groups; these rod-like chromophores can be grafted into the periphery of the scaffold leading to the particular architecture so-call organic nanodot (OND). This disposition allows a larger number of fluorophores to be activated within the focal plane compared with the out-focused region, resulting in an increase signal/noise ratio.