Image: CU dots bound to immunoglobin-G antibodies attach to the surface of leukemia cells, demonstrating a possible use in biological tagging. Copyright Cornell University
"People have done superb experiments with quantum dots that were not previously possible," says Ulrich Wiesner, Cornell associate professor of materials science and engineering. "Hopefully Cornell dots will serve the same purpose and offer new possibilities." There are also some interesting physics questions about how the new dots work, he adds.
Since optical microscopes can't resolve individual molecules, and electron microscopes can't be used on living organisms, biologists often tag organic molecules with fluorescent dyes in order to track their movements through biological processes, such as the action of enzymes inside a living cell. While it can't see the molecules, an optical microscope can track the bright light given off by the dye.
Quantum dots -- which have been used for the same purpose -- are tiny particles of semiconductors such as cadmium selenide that behave as if they were individual atoms: They can absorb light energy, kicking their internal electrons up to higher energy levels, then release the energy by emitting light. A quantum dot fluoresces much more brightly than a dye molecule, making it a desirable marker.
Cornell dots, also known as CU dots, are nanoparticles consisting of a core about 2.2 nanometers (nm) in diameter containing several dye molecules, surrounded by a protective silica shell, making the entire particle about 25 nm in diameter. The researchers call this a "core-shell architecture." (A nanometer is one-billionth of a meter, about three times the diameter of a silicon atom.)
Like quantum dots, CU dots are many times brighter (20-30 times) than single dye molecules in solution and resist "photobleaching," a process by which dyes in solution rapidly lose their fluorescence. CU dots can be made with a wide variety of dyes, producing a large assortment of colors.
The manufacture of CU dots and early experiments with them are described in a paper, "Bright and Stable Core-Shell Fluorescent Silica Nanoparticles," in the journal
Nano Letters (Vol. 5 No. 1) by Wiesner and his Cornell colleagues Hooisweng Ow, Daniel R. Larson, Mamta Srivastava, Barbara A. Baird and Watt W. Webb .
Unlike quantum dots, CU dots are mostly chemically inert. The silica shell is silicon dioxide -- essentially glass. For use as biological markers,
quantum dots are encased in a polymer shell -- a process that adds to their already high manufacturing cost. Quantum dots also contain heavy metals like cadmium that can leach through the polymer shell and disrupt the chemistry being observed.
However, Wiesner says, "Silica is benign, cheap and easy to attach, and it is totally compatible with silicon manufacturing technology. That opens enormous possibilities in the life sciences and in information technology."
The Cornell researchers tested the use of CU dots as biological markers by attaching an antibody, immunoglobin E (IgE), and observing how this combination attached to cell receptors on leukemia mast cells.
The dots also offer an intriguing physics question: Why do they fluoresce so brightly? In effect, the whole is brighter than the sum of its parts. "We have this enormous brightness, and we don't know exactly where it's coming from," Wiesner says. Several explanations have been offered. One is that the silicon shell protects the dye molecules from the solvent. A second is that dye molecules floating free can lose energy by actions other than emitting photons, but in the packed core of the particle those other actions are diminished.
The dots were created by Ow, then Wiesner's graduate student. Webb, the S.B. Eckert Professor in Engineering, and Larson, a graduate student in applied and engineering physics now at Albert Einstein College of Medicine, studied their photophysical properties. Baird, director of the Cornell Nanobiotechnology Center, and Srivastava, a postdoctoral researcher, studied the dots as labels on living cells.
The research was supported by the National Science Foundation, the state of New York and Phillip Morris USA. Quantum Dot Corp. supplied quantum dots used for comparison.
Source: Cornell University
Related stories:
Electron spin and orbits in carbon nanotubes are coupled
Researchers hoping to use carbon nanotubes for quantum computing -- in which the spin of a single electron would represent a bit of data -- may have to change their approaches, according to new Cornell research.
Discovering How to Focus on Tiniest of the Very Small
If you need a good picture of a molecule, your first job is getting its atoms to pose for you, says John Silcox, Cornell's David E. Burr Professor of Engineering and an expert in the realm of the very tiny.
Scientists take the sharpest image ever made with light
(PhysOrg.com) -- A team of scientists from the Technische Universität Dresden (Germany) and the ESRF in Grenoble (France) has produced the image of an object at the highest resolution ever achieved with X-ray light. A 100-nanometre gold particle fixed on a substrate was reconstructed with 5 nanometre resolution. Contrary to other techniques, X-ray imaging works also in real-life environments like chemical processing or in the presence of high magnetic fields. The team reports its findings in the newest issue of
Phys. Rev. Lett. dated 5 September 2008 (published online 29 August 2008).
Operating quantum memory at room temperature
Quantum dots, along with quantum wires, have been attracting notice over the past decade as possible building blocks of quantum information processing. Indium arsenide quantum dots (InAs) can be used for memory operations in devices made from gallium arsenide and aluminum gallium arsenide (known as GaAs/AlGaAs devices). The problem is that at room temperature – the experiments are usually done at lower temperatures, the memory operation of these devices suffers, unless there are multiple quantum dot layers.
Fast quantum computer building block created
(PhysOrg.com) -- The fastest quantum computer bit that exploits the main advantage of the qubit over the conventional bit has been demonstrated by researchers at University of Michigan, U.S. Naval Research Laboratory and the University of California at San Diego.
Light touch: Controlling the behavior of quantum dots
Researchers from the National Institute of Standards and Technology and the Joint Quantum Institute (JQI), a collaborative center of the University of Maryland and NIST, have reported a new way to fine-tune the light coming from quantum dots by manipulating them with pairs of lasers. Their technique, published in
Physical Review Letters, could significantly improve quantum dots as a source of pairs of “entangled” photons, a property with important applications in quantum information technologies.
First reported video of cell's recognition of danger through its protein response
(PhysOrg.com) -- Cells are expected to respond defensively when an antigen lands on a cell membrane and prepares to cause mischief.
Nanoparticles + light = dead tumor cells
Medical physicists at the University of Virginia have created a novel way to kill tumor cells using nanoparticles and light. The technique, devised by Wensha Yang, an instructor in radiation oncology at the University of Virginia, and colleagues Ke Sheng, Paul W. Read, James M. Larner, and Brian P. Helmke, employs quantum dots. Quantum dots are semiconductor nanostructures, 25 billionths of a meter in diameter, which can confine electrons in three dimensions and emit light when exposed to ultraviolet radiation.
