Lanthanide-Doped Nanocrystals Serve as Single-Molecule Imaging Probes
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As reported in the June 25 issue of Nature (DOI: 10.1038/nature08115; p. 1110), the research team used high-speed photography to measure nanoscale forces that cause droplet formation. The researchers observed falling 100-μm-diameter glass beads, or streaming sand, and found that forces as much as 100,000 times smaller than those that produce surface tension in ordinary liquids could cause droplet formation in granular streams and cause these dry streams to behave like an ultralow surface-tension liquid.
Royer, a graduate student in physics at the University of Chicago, who developed the apparatus, and his colleagues also directly measured grain-to-grain interactions with an atomic force microscope. “At first we thought grain-grain interactions would be far too weak to influence the granular stream,” said Royer. “The atomic force microscopy surprised us by demonstrating that small changes in these interactions could have a large impact on the break-up of the stream, conclusively showing that these interactions were actu-
ally controlling the droplet formation.” “Our experiments ask two questions for which currently there is no established answer,” said Jaeger. “Both questions are about how a liquid breaks apart. How does the break-up proceed in the ultra-low surface-tension limit and what happens in the ultra-low temperature limit when particles cease to move relative to each other? “It is quite remarkable that a granular stream consisting of macroscopic particles provides a model system to explore it.”
Figure 1. Confocal upconverted luminescent image of individual upconverting nanoparticles (NaYF4: Yb3+/ Er 3+) on a silicon nitride membrane. Laser power density is ~3 × 106 W/cm2, and dwell time per pixel is 10 ms. (Inset) The transmission mode-scanning electron microscope image taken at the upper left corner region of the optical image shows that the individual diffraction-limited luminescent spots are emitted from individual UCNPs. The five individual nanocrystals are labeled as A–E; an impurity is labeled as F. Reproduced by permission from Proceedings of the National Academy of Sciences 106 (27) (2009) DOI: 10.1073/ pnas.0904792106, p. 10917. ©2009 by the National Academy of Sciences.
light (known as “upconversion”) when illuminated by light from a 980-nm continuous wave, near-infrared laser (see Figure 1). Biological tissues are more transparent to near-infrared light, making these nanocrystals well suited for imaging living systems with minimal damage or light scatter. To study how these probes might behave in a real biological system, the research team wrapped them with low molecular weight amphiphilic polymers and incubated the nanocrystals with embryonic mouse fibroblasts, cells crucial to the development of connective tissue, allowing the nanocrystals to be taken up into the interior of the cell. Live-cell imaging using the same near-infrared laser showed similarly strong luminescence from the nanocrystals within the mouse cell, without any measurable blinking or photobleaching. The researchers reporte
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