We investigate the effects of the concentration of CdTe quantum dots (QDs) on their fluorescence in water. The emission spectra, acquired in right angle geometry, exhibit highly variable shapes. The measurements evidence a critical value of the concentration beyond which the intensity and the spectral bandwidth decrease and the fluorescence maximum is redshifted. To account for these observations, we develop a model based on the primary and secondary inner filter effects. The accuracy of the model ensures that the concentration dependent behaviour of QD fluorescence is completely due to inner filter effects. This result is all the more interesting because it precludes the assumption of dynamic quenching. As a matter of fact, the decrease of the emission intensity is not a consequence of collisional quenching but an effect of competition between fluorescence and absorption. We even show that this phenomenon is linked not only to the QD concentration but also to the geometric configuration of the setup. Hence, our analytical model can be easily adapted to every fluorescence spectroscopy configuration to quantitatively predict or correct inner filter effects in the case of QDs or any fluorophore, and thus improve the handling of fluorescence spectroscopy for highly concentrated solutions.

As luminescent quantum dots (QDs) are known to aggregate themselves through their chemical activation by carbodiimide chemistry and their functionalization with biotin molecules, we investigate both effects on the fluorescence properties of CdTe QDs and their impact on Förster Resonant Energy Transfer (FRET) occurring with fluorescent streptavidin molecules (FA). First, the QDs fluorescence spectrum undergoes significant changes during the activation step which are explained thanks to an original analytical model based on QDs intra-aggregate screening and inter-QDs FRET. We also highlight the strong influence of biotin in solution on FRET efficiency, and define the experimental conditions maximizing the FRET. Finally, a free-QD-based system and an aggregated-QD-based system are studied in order to compare their detection threshold. The results show a minimum concentration limit of 80 nM in FA for the former while it is equal to 5 nM for the latter, favouring monitored aggregation in the design of QDs-based biosensors.

The manufacturing of silica platforms functionalized by CdTe quantum dots (QDs) of 3.4 nm diameter through (3aminopropy)triethoxysilane (APTES) aliphatic organosilanes is performed to preserve QDs excitonic properties after their transfer from colloidal solutions to surfaces at ambient air. In these conditions, the chemical stability and the structural homogeneity of monolayers are monitored and attested by probing their optical efficiency through UV-Visible spectroscopy (absorption), time-resolved fluorescence spectroscopy and microscopy (emission). The grafting of the aliphatic organosilanes on silicon is examined by XPS measurements that show that a 0.9 nm sublayer thickness is electrostatically stabilized between SiO2 substrates and QDs layers without EDC-NHS (1-ethyl-3-(3-dimethyl-aminopropyl)carbodiimide, N-hydroxysuccinimide) activation. Surprisingly, in the latter case, the optical absorption of the QD layer does not vary beyond 10 days while it degrades in one day if QDs are activated. Finally, SFG spectroscopy evidences a vibroelectronic coupling between the QDs and APTES monolayers constituting the platforms.

As fluorescent nanocrystals, colloidal quantum dots (QDs) are increasingly used for biosensing thanks to their ability to perform Forster resonant energy transfer (FRET). Especially, all-QD-based donor− acceptor systems offer promising approaches for the design of FRET biosensors. But contrary to molecular fluorophores, QD emission properties are highly conditioned by the size distribution of QDs, so it is possible to observe energy transfers between QDs coming from the same monodisperse population, when packed into clusters. Here, we characterize such homogeneous resonant energy transfer (homo-FRET) processes occurring between CdTe QDs clustered within an organosilane polymer matrix and develop a mathematical model to account for their efficiencies. We evidence the critical role of the statistical donor−acceptor polarization of the QD population and provide tools to quantify it. Interestingly, as QD−QD homo-FRET proves to only depend on size dispersion and Stokes shift, the conclusions of our study can be extended to any kind of QDs and our model can be used to predict their own homo-FRET efficiency.