How Quantum Dots Aggregation Enhances Förster Resonant Energy Transfer.

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.

A global method for handling fluorescence spectra at high concentration derived from the competition between emission and absorption of colloidal CdTe quantum dots.

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.

All-quantum dot based Förster resonant energy transfer: key parameters for high-efficiency biosensing.

While colloidal quantum dots (QDs) are commonly used as fluorescent donors within biosensors based on Förster resonant energy transfer (FRET), they are hesitantly employed as acceptors. On the sole basis of Förster theory and the well-known behaviour of organic dyes, it is often argued that the QD absorption band over the UV-visible range is too wide. Discarding these preconceptions inherited from classical fluorophores, we experimentally examine the FRET process occurring between donor and acceptor CdTe QDs and provide a mathematical description of it. We evidence that the specific features of QDs unexpectedly lead to the enhancement of acceptors' emission (up to +400%), and are thus suitable for the design of highly efficient all-QD based FRET sensors. Our model enables us to identify the critical parameters maximizing the contrast between positive and negative biosensing readouts: the concentrations of donors and acceptors, their spectral overlap, the densities of their excitonic states, their dissipative coupling with the medium and the statistics of QD-QD chemical pairing emerge as subtle and determinant parameters. We relate them quantitatively to the measured QD-QD FRET efficiency and discuss how they must be optimized for biosensing applications.

Quenched or alive quantum dots: The leading roles of ligand adsorption and photoinduced protonation.

HYPOTHESIS: The fluorescence emission of water-soluble CdTe quantum dots (QDs) capped with mercaptocarboxylic acids (MCAs) is known to be pH-dependent. However, this behaviour is quite different from a study to another, so that literature suffers from a lack of coherence. Here we assume that the QD fluorescence efficiency is actually driven by the acid-base equilibrium of MCA thiol groups, and that light-excited QDs open a non-radiative relaxation path through photoinduced protonation. EXPERIMENTS: We address this issue by examining colloidal CdTe QDs with (time-resolved) fluorescence spectroscopy under various conditions of acidity and light excitation. FINDINGS: It appears that the emission of QDs is quenched below a critical pH value of 6.87, and that light excitation power strengthens this quenching. We thus demonstrate the existence of an additional photochemical process and developed a mathematical modeling accounting for all our experimental results. With only three parameters, it is possible to accurately predict the fluorescence decay of QDs over time, at any pH. Further, we also related the critical pH value of 6.87 to QD surface properties, explaining why observations may differ from a study to another and making the literature much more coherent.