Monday 21 November 2011

Long-lasting near-infrared emitter

Via various web sites I've been led to a paper published in Nature Materials [abstract].

Zhengwei Pan, associate professor of physics and engineering at the University of Georgia, Franklin College of Arts and Sciences, and his team have developed a material which will fluoresce in a narrow band around 700nm when excited by any visible wavelength including fluorescent lighting. OK, we all know that 700nm is the boundary between red and near-infrared but the eye's sensitivity to that wavelength is so low that we will usually refer to this as near-infrared. You can see the excitation and emission spectra in the Nature abstract.

What is exciting about this work is that, after a short excitation, the material will continue to emit for a long time ... 'seconds to minutes' will result in more than 360 hours output according to the paper. The material can be fabricated into nano-particles which could bind to cancer cells, enabling visual location of small cancers in the body, it can be made into ceramic discs or even paint in order to provide illumination visible only to people using near-IR sensitive devices. I can see an application to detect whether something has been exposed to light recently.

Apparently the starting point is the trivalent chromium ion, a well-known IR emitter when its electrons return to their ground state after excitation by visible light. Usually the effect lasts only a few milliseconds but this new material embeds the chromium in such a way that the emitted light is trapped and releases the energy more slowly. [More info at EurekaAlert for those of us without a Nature subscription.]

This photo shows Zhengwei Pan (left) and postdoctoral researcher Feng Liu in a darkened room, using only their infrared-emitting ceramic discs as a source of illumination. The phosphorescent material was also mixed into the paint that was used to create the University of Georgia logo behind them. You can just see the 'five-o'clock-shadow' on their faces, a result of near-infrared skin penetration. [Credit: Zhengwei Pan/UGA]