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August 25, 2010
DOI: 10.1021/CEN081210124126

A New Route To Germ-Killing Photons

ACS Meeting News: Lanthanide nanomaterials convert visible light into disinfecting ultraviolet light

Aaron A. Rowe

KILLER DISCS The fused silica disk on the right is coated with a material that partially converts visible light into UVC radiation. Ezra Cates
KILLER DISCS The fused silica disk on the right is coated with a material that partially converts visible light into UVC radiation.
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When you flip the switch on a fluorescent light, a coating in the bulb turns ultraviolet radiation into visible light. A team of engineers at Georgia Tech are developing nanomaterials that have the exact opposite effect. At the American Chemical Society Meeting in Boston, the researchers explained how transforming visible light into ultraviolet emissions could make these new coatings efficient germ killers.

The engineers designed the materials so that when bright indoor lighting or in direct sunlight strikes them, they emit ultraviolet radiation that can damage pathogens' DNA. Engineers Jaehong Kim and Ezra Cates say that someday coating surfaces with these substances may help disinfect drinking water, sterilize hospitals, protect packaged foods, and keep bathrooms clean.

Kim and Cates' nanomaterials perform their visible-to-UV transformation through a phenomenon called upconversion photoluminescence. The process starts when a photon of blue light enters the material and excites an electron from a lanthanide atom. Next, if that excited state lives long enough, a second blue photon arrives and promotes the electron further. Then when the electron relaxes back to the ground state, it releases a single high energy photon of UV light.

Solid state physicists first reported upconversion in lanthanide-doped crystals during the mid 1960s, Cates says, but since then nobody has designed materials that convert visible light to UV for a practical purpose.

Upconversion readily occurs in lanthanides, because their electronic configuration allows excited states to last longer. A set of filled s and p orbitals sit farther from the nucleus than the f orbitals that contain the photon-excited electrons. The s and p orbitals shield these electrons from the environment, which increases the likelihood that a second photon will excite them further.

Fluorescent lightbulbs often contain the lanthanides europium or terbium. But the engineers doped these new nanomaterials with different lanthanides such as praseodymium, Cates says, because "only a handful of the lanthanides have energy levels that are spaced properly." He says that for upconversion to work, atoms must have electronic states separated by the energy of a blue photon.

These UV-emitting coatings may an advantage over other light-activated, disinfecting nanomaterials, says Dionysios Dionysiou, an environmental materials expert at the University of Cincinnati. When struck by light, these other substances catalyze the generation of reactive oxygen species and radicals that destroy pathogens. But, Dionysiou says, organic materials in river water can scavenge many of these radicals, leaving only a few to kill germs. Still he points out that UV light can only inactivate microbes, while radicals tear them apart.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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