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.

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.

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, silica is benign, cheap and easy to attach, and compatible with silicon manufacturing technology.

Cornell news release