The Hindu : Opinion / Leader Page Articles : Tiny is beautiful

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Wednesday, Feb 23, 2005

Opinion

Opinion - Leader Page Articles

Tiny is beautiful

By Kenneth Chang

Nanoparticles offer promise in medicine for sensitive diagnostic tests and novel treatments.

IN THE hip science of ultra small nanotechnology, fantastical future possibilities such as rampaging nanorobots capture the most attention. But the first fruits of the field have been more mundane: tiny bits of mostly ordinary stuff that just sit there. Yet these bits — nanoparticles — gain wondrous new capabilities simply because they are so small.

Nanoparticles of various sorts are already found in products such as sunscreen, paint and inkjet paper. More exotic varieties offer promise in medicine for sensitive diagnostic tests and novel treatments: the detection of Alzheimer's disease by finding a protein in spinal fluid, for instance, or nanoparticles that heat up and kill cancer cells. Some nanoparticles are not even on the cutting edge.

Nanotechnology, nanoparticles and all of the other nano-words derive from nanometre, a billionth of a metre, or about one 25-millionth of an inch. That is far smaller than the world of everyday objects described by Newton's laws of motion, but bigger than an atom or a simple molecule, particles ruled by quantum mechanics.

A nanoparticle, an object with a width of a few nanometres to a few hundred, contains tens to thousands of atoms and exists in a realm that straddles the quantum and the Newtonian.

At those sizes, said Chad A. Mirkin, a director of Northwestern University's Institute for Nanotechnology, "everything, regardless of what it is, has new properties."

This in-between realm gives rise to an unusual physics where the properties of a material change depending on its size. At the quantum level, one gold atom acts like any other gold atom, and a nugget of gold large enough to hold has the same chemical and electrical properties as another nugget.

But two nanoparticles, both made of pure gold, can exhibit markedly different behaviour — different melting temperature, different electrical conductivity, different colour — if one is larger than the other. "That creates a new way to control the properties of materials," said A. Paul Alivisatos, a professor of chemistry at the University of California, Berkeley. "Instead of changing composition, you can change size."

Prof. Alivisatos works with nanoparticles known as quantum dots, made of semiconductors such as silicon and gallium arsenide. The edges of the small particles influence the motion of electrons in the semiconductor, and the shape and size of quantum dots can be tailored to fluoresce specific colours. Current dyes used for lighting up protein and DNA fade quickly but quantum dots could allow tracking of biological reactions in living cells for days or longer.

Other applications of nanoparticles take advantage of the fact that more surface area is exposed when material is broken down to smaller sizes. For magnetic nanoparticles, the lack of blemishes produces magnetic fields remarkably strong considering the size of the particles. Nanoparticles are also so small that in most of them, the atoms line up in perfect crystals without a single blemish.

For the medieval stained glass, the gold nanoparticles are simple spheres about 25 nanometres in diameter. At those small sizes, gold no longer glitters gold. Electrons at the surface of the nanoparticles slosh back and forth in unison, absorbing blue and yellow light.

But longer wavelength red light reflects off the particles and passes through the window. Similarly, nanoparticles of silver in stained glass give a bright yellow hue. With much more sophisticated tools, scientists today can make nanoparticles of many more different shapes and sizes.

Yi Lu, a chemistry professor at the University of Illinois, takes advantage of the colour of nanoparticles for a test for detecting hazardous levels of lead. He attaches DNA molecules to gold nanoparticles, which tangle with other specially designed pieces of DNA into clumps that appear blue. The presence of lead causes the connecting DNA to fall apart. That cuts loose the individual gold nanoparticles and changes the colour from blue to red.

Dr. Mirkin also uses gold nanoparticles as a connecting point, to build sensors of a different sort, for disease. A common technique for a diagnostic test consists of an antibody attached to a fluorescent molecule. When the antibody attaches to a protein associated with the disease, the fluorescent molecule lights up under ultraviolet light.

Dr. Mirkin and his colleagues reported this month in The Proceedings of the National Academy of Science that this approach had produced a test to detect the onset of Alzheimer's by measuring minuscule amounts of a protein in spinal fluid associated with the disease.

"To me, that's just a beautiful example of the power of this particular approach," Dr. Mirkin said. He has started a company, Nanosphere Inc., to bring his techniques to the market. Nanoparticles may help cure disease as well as detect it.

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