Blinking tamed in quantum dots
Michael A. Greenwood
Quantum dots in some ways are comparable to the display lights popular during the holidays: They are bright and colorful, and they blink incessantly.
The first two qualities are highly desirable, but the blinking nature of quantum dots can cause serious problems; for example, blinking can hinder both speed and accuracy when state-of-the-art bio-assays are extended down to the single-molecule level.
Researchers chemically modified the environment of quantum dots to substantially suppress their characteristic blinking. Courtesy of David J. Nesbitt of JILA.
Although other methods have been used to tame the blinking, a novel chemical technique that strongly suppresses this characteristic flashing has been reported by a research team from JILA, a joint venture between the University of Colorado at Boulder and the National Institute of Standards and Technology, which has research facilities in both Boulder and in Gaithersburg, Md.
Researchers David J. Nesbitt and Vasiliy Fomenko quenched the blinking in quantum dots with a CdSe core and a ZnS shell (the quantum dots also were conjugated with streptavidin) without affecting their fluorescence quantum yield. This was accomplished by chemically modifying their environment.
The researchers inoculated the semiconductor nanocrystals with a propyl gallate (3,4,5-trihydroxybenzoic acid propyl ester) solution. They first deoxygenated the quantum dots and the solution by subjecting both to bubbling argon gas for 20 min.
To test the method, they excited the quantum dots at 434 nm with a PicoQuant 70-ps pulsed diode laser. The sample fluorescence was imaged with a pair of PerkinElmer avalanche photodiodes, which measured the precise time delay between excitation and emission for millions of photon events.
To its surprise, the team found that this chemical modification noticeably reduced both the radiative and nonradiative rates of the quantum dots, resulting in a near-complete suppression of blinking. In particular, the propyl gallate solution resulted in as much as a fivefold increase in the pure radiative emission rate of the quantum dot; because this medium competes more effectively with nonradiative relaxation pathways, it represents a novel mechanism for blinking suppression.
The suppression of blinking in quantum dots is not unique to propyl gallate, the researchers said. They observed similar results when using a number of simple ligands and plan to report those findings in the future.
Nano Letters, January 2008, pp. 287-293.
- quantum dots
- Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.
MORE FROM PHOTONICS MEDIA