Unexpected enhancement of Förster Resonant Energy Transfer thanks to quantum dots aggregation

Here we investigate Förster Resonant Energy Transfer (FRET) occuring 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 the suitability
of QDs 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. It is obviously necessary
to distinguish hetero-FRET (between QDs and FSA) from homo-FRET (between QDs within a
same aggregate) in order to avoid misleading interpretations.