When it comes to rapidly detecting toxins and small molecules in soil, water and food, luminescent semiconductor quantum dots could hit the spot. A team from the US Naval Research Laboratory in Washington and from MIT in Cambridge, Mass., has shown that quantum dots combined with Förster resonance energy transfer (FRET) can detect biomolecules in a rapid and sensitive multiplexed assay. The technique has applications in the health care and food industries.The group looked at several configurations and determined that the best arrangement in terms of implementation, data collection and analysis is to use multiple dots with one type of acceptor, said Hedi Mattoussi, a research physicist at the naval laboratory. In FRET, a close-range interaction between donor and acceptor fluorophores changes the spectral characteristics of the pair. The method can be combined with organic dyes to generate independent signal channels for a multiplexed assay to detect target molecules. However, the need for multiple excitation sources and overlapping spectral emissions makes this difficult to implement.The emission from quantum dots is narrow and can be tuned by adjusting their size, with larger dots emitting more toward the red than smaller ones. Moreover, quantum dots work across a range of excitation wavelengths and are resistant to chemical degradation and photo-bleaching. Problems with quantum dot synthesis and with functionalizing the surface for a given task, however, have held back the use of these materials. The researchers built quantum dots with a ZnS shell surrounding a CdSe core and then self-assembled a protein layer on the surface into which they inserted dye acceptors. The quantum dots acted as donors in one of two configurations.In the first, a single dot was paired with various dye acceptors. In the second setup, different sizes — and therefore different colors — of dots were paired with one acceptor. The group found that the latter arrangement yielded the better results because it was easier to implement and because it was easier to interpret the data.The investigators are applying the technique to parallel sensing arrangements in which as many as 15 molecular-sensing channels could span the spectral window from 490 to 630 nm. Implementing this method for an in vivo sensor is an important goal.A challenge is that FRET efficiency falls steeply as quantum dot size increases, so moving to a larger size dot to get emission in the red could present a problem. This is not an insurmountable drawback, however, because the efficiency can be improved by increasing the number of fluorophores near the center of the quantum dot, Mattoussi said.Journal of the American Chemical Society, online Dec. 3, 2005, doi:10.1021/ ja054630i.