June 10, 2005, Ithaca, NY--Move over, quantum dots. Make way for the new kids on the block--brightly glowing nanoparticles dubbed 'Cornell dots.' By surrounding fluorescent dyes with a protective silica shell, Cornell University researchers have created fluorescent nanoparticles with possible applications in displays, biological imaging, optical computing, sensors and microarrays such as DNA chips.
These are all applications for which quantum dots have been used or are being considered. But the new Cornell nanoparticles offer an appealing alternative because of their greater chemical inertness and reduced cost.
Quantum dots 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 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." 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.
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.
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.