MIMICKING the way sea snails make mother-of-pearl has led to a new way of
making 鈥渜uantum dots鈥�, tiny clusters of semiconducting material that could
create a new generation of lasers.
Quantum dots can trap just one or two electrons. These confined electrons can
easily be persuaded to emit light.
But making structures tiny enough to imprison just a few electrons is
tough鈥攖hough it comes naturally to the abalone sea snail. The creatures
make mother-of-pearl by secreting proteins that stack grains of calcium
carbonate in a super-strong brick-like orientation that is 3000 times stronger
than the calcium carbonate formed without the help of proteins.
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Replace the calcium carbonate with a semiconducting material and each grain
should make a perfect quantum dot, says Angela Belcher, a materials chemist at
the University of Texas at Austin. At a conference on nanotechnology in
California last week, she described how she 鈥渆volved鈥� peptides analogous to the
abalone proteins. These can grab cadmium and sulphide from a surrounding
solution and put them together to create quantum dots.
Belcher鈥檚 team synthesised a billion different peptides and tested them to
see how well they would stick to the surface of a semiconductor crystal. They
collected the peptides that bound most strongly to the semiconductor and
rearranged their subunits to see if they could make the peptides even stickier.
By repeating this process several times, Belcher and her colleague Roz Sweeney
and Christine Flynn evolved a bunch of peptides that would bind to
semiconductors.
One of them stuck fast to the semiconductor cadmium sulphide, forming a
quantum dot. Belcher tested the peptide by inserting it into the outer membrane
of a bacterial cell using a technique developed by University of Texas
colleagues Brent Iverson and George Georgiou. She then put the bacterial cells
into a Petri dish with a source of cadmium and sulphide. As she had hoped,
quantum dots of cadmium sulphide formed on the surface of the cells, and they
glowed when lit by a laser.
鈥淲e can select for bacteria that can grow quantum dots in solution,鈥� says
Belcher. The technology could lead to a new generation of quantum dot lasers
that could be buried inside superfast optical microchips.
鈥淚t is really quite exciting,鈥� says chemist Paul Alivisatos at the University
of California at Berkeley, one of the pioneers of quantum dot research.
鈥淏elcher鈥檚 technique opens up all sorts of new possibilities.鈥�