Nanoegg Unveiled By Rice Scientists

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" Nanoegg " Unveiled By Rice Scientists

http://www.medicalnewstoday.com/medicalnews.php?newsid=47841

Researchers at Rice University's Laboratory for Nanophotonics (LANP)

have unveiled the " nanoegg, " the latest addition to their family

ultrasmall, light-focusing particles. A cousin of the versatile

nanoshell, nanoeggs are asymmetric specks of matter whose striking

optical properties can be harnessed for molecular imaging, medical

diagnostics, chemical sensing and more.

Nanoeggs are described in the July 18 issue of the Proceedings of the

National Academy of Sciences.

Like nanoshells, nanoeggs are about 20 times smaller than a red blood

cell, and they can be tuned to focus light on small regions of space.

But each nanoegg interacts with more light - about five times the

number of wavelengths - than their nanoshell cousins, and their

asymmetric structure also allows them to focus more energy on a

particular spot.

" The field of nanophotonics is undergoing explosive growth, as

researchers gain greater and greater sophistication in the design and

manipulation of light-active nanostructures, " said LANP Director

Naomi Halas, the Stanley C. Professor of Electrical and

Computer Engineering and professor of chemistry. " The addition of

nanoeggs and, earlier this year, nanorice to LANP's family of optical

nanoparticles is a direct result of our increased understanding of

the interaction between light and matter in this critical size

regime. "

Like nanoshells, nanoeggs have a spherical, non-conducting core

that's covered with a thin metal shell. But where the casing on a

nanoshell has a uniform thickness - like the peel covering an orange -

the nanoegg's covering is thicker on one side than the other - in

much the same way that a hard-boiled egg white is thick in some

places and thin in others.

The off-center core in the nanoegg radically changes its electrical

properties, said co-author and theoretical physicist

Nordlander, professor of physics and astronomy. The reasons for this

have to do with the odd and often counterintuitive rules that govern

how light interacts with electrons at the nanoscale.

" All metal particles have a sea of free electrons flowing

continuously over their surface called plasmons, " Nordlander

said. " These plasmons slosh around constantly, just like waves in the

ocean. Light also travels in waves, and when the wavelength of

incoming light matches the wavelength of the plasmon, the amplitude

of their sloshing gets bigger and bigger, much like the waves in a

bathtub when a child rhythmically sloshes bathwater until it spills

out of the tub. "

In order for plasmons to be excited by light, the electrons on a

particle's surface must behave in such a way as to create a 'dipole

moment,' a state marked by two equal but opposite poles, one positive

and the other negative - much like a magnet that attracts on one end

and repels on the other.

" Without a dipole moment, there is no 'handle' for light to grab hold

of, " Nordlander said. " In symmetric nanoshells, most of the light

energy is lost to these 'dark modes.' With symmetry breaking, we are

able to make these dark modes bright by providing dipole moments for

more of the incoming light. "

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Co-authors on the paper include Hafner, assistant professor of

physics and astronomy and of chemistry, and graduate students Hui

Wang, Yanpeng Wu, Britt Lassiter and Colleen Nehl. The research was

supported by the U.S. Army Research Office, the National Science

Foundation and the Welch Foundation.