Single-particle counting of quantum dots

Simultaneous detection of 605QDs (blue) and 700QDs (red) in a mixture by single-particle counting.

Lawrence Johnson and Chun-yang Zhang at York College of the City University of New York have developed a method to quantify quantum dot (QD) concentration via single-particle counting. QD concentration is usually measured using either absorption or fluorescence spectroscopy, but the absorption coefficient and quantum yield values for QDs vary with conditions such as pH and ionic strength. For a successful quantification by these means, the absorption coefficient or quantum yield must be determined separately under each set of conditions by laborious and time-consuming techniques.

In Johnson and Zhang's method, each QD is detected by the fluorescence that it emits. The QDs are passed through a 50 µm capillary tube where they are excited with a laser and their emitted photons are detected by photodiodes.

As a proof-of-principle experiment, the researchers looked at 605-nm-emitting streptavidin-coated CdSe/ZnS core–shell QDs (605QDs). They found that they could achieve an S/N of up to 200, which allowed them to unambiguously identify each 605QD as it moved through their system. Similar results were obtained with QDs that emitted at other wavelengths.

The researchers say that advantages of their system include a sample size 5 orders of magnitude smaller than that of conventional spectrometric techniques and the ability to quickly and easily assay QDs in a variety of buffer solutions and organic solvents. They also note that their system allows them to simultaneously count more than one type of QD in a mixture, which is not possible with conventional techniques. (J. Am. Chem. Soc. 2008, 130, 3750–3751)