Proc. SPIE. 11659, Colloidal Nanoparticles for Biomedical Applications XVI
KEYWORDS: Semiconductors, Luminescence, Molecules, Energy transfer, Biosensors, Quantum dots, Resonance energy transfer, Fluorescence resonance energy transfer, Molecular aggregates, Molecular energy transfer
Here we investigate Förster Resonant Energy Transfer (FRET) occurring between luminescent colloidal semiconductor quantum dots (QDs) and fluorescent streptavidin molecules (FSA), chemically coupled thanks to N-hydroxysuccinimide (NHS) and biotin. In some conditions, QDs are known to form agglomerates throughout their functionalization by NHS and biotin molecules. Thus, we wondered how this collective aggregation could influence the efficiency of FRET. Interestingly, this proves to enhance the energy transfer from QDs to FSA. In terms of detection threshold, aggregated-QD-based systems lead to a limit down to 5 nM, while it is up to 80 nM for non-aggregated ones. Therefore, these unexpected results evidence our ability to exploit QDs aggregation for the design of biosensing systems with lower and lower molecular detection thresholds. Unlike the common beliefs, QD agglomeration is an asset that we can benefit from in order to improve the performance of QD-based biosensors. As a counterpart, this requires a fine monitoring of the emission spectrum of QDs while they are aggregating. This is why we provide a complete characterization of the QD fluorescence throughout their chemical funtionalization with NHS and biotin, supporting that such precautions are mandatory. Further, we show it is necessary to distinguish hetero-FRET (between QDs and FSA) from homo-FRET (between QDs within a same aggregate) in order to avoid misleading interpretations.
Colloidal semiconductor quantum dots (QDs) constitute zero-dimension excitonic materials characterized by quantized states and able to emit fluorescence. QDs are promising materials for catalysis, molecular recognition and biosensing. In this context, our work consists in the design of a new class of biochemical sensors based on QD-grafted chips to benefit from their high opto-electronic activity in the visible range. In order to achieve this, we functionalize silica and CaF2 susbtrates with 4 nm-diameter CdTe QDs and organic molecules (e.g. phenylamine). The originality of our work then lies in two-colour sum-frequency generation nonlinear optical spectroscopy, mixing Raman and IR spectroscopies. This technique enables to probe and to quantify the coupling between the excitonic properties of the QDs and the vibrational response of their molecular environment: two tunable visible and IR laser beams are mixed on the QD-grafted chips to electronically excite the QDs while the vibrational spectroscopy of the surrounding organic species is performed. Through this approach, we clearly demonstrated a correlation between QDs and molecules. Especially, the vibrational response of the molecules is maximized when the first excitonic state of the CdTe QDs is pumped by the visible beam, which means it is possible to enhance the detection of given biomolecules thanks to QDs. Considering that confined excitons transfer their energy to molecules through dipolar interaction, the model we developed accounts indeed for such a behaviour.