Investigation of quantum dot FRET in the far-red spectral region

Colloidal quantum dots (QDs) are now commercially available in a bio-functionalized form and Förster resonance energy transfer (FRET) between bioconjugated dots and fluorophores within the visible range has been observed by several groups of researchers. We are particularly interested in the far-red region, as from a biological perspective, there are benefits in pushing to ∼700 nm to minimize optical absorption (ABS) within tissue and avoiding cell autofluorescence. We report on FRET between streptavidin (STV) conjugated CdTe quantum dots, Qdot705-STV, with biotinylated Dy731-Bio fluorescent molecules in a donor-acceptor assay. We also highlight an unusual change in Dy731-Bio absorptivity during the streptavidin-biotin binding process that can be attributed to the structural reorientation. In moving to wavelengths beyond 700 nm, different alloy compositions are required for the quantum dot core and these introduce associated changes in the physical shape. These changes directly affect the fluorescence decay dynamics producing a marked biexponential decay with an extremely long lifetime component, a lifetime in excess of 100 ns. We compare and contrast the influence of the two QD relaxation processes upon the FRET dynamics in the presence of Dy731-Bio.