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86 FROM THE LABS
TECHNOLOGY REVIEW /
NEXT STEPS: The researchers are
exploring fabrication techniques that
can decrease the gap between the rods---
and the spot size---to 20 nanometers.
They are also exploring alter natives to
the gold that coats the rods; silver, say,
could focus light more e ciently than
gold at the wavelengths used in the
consumer electronics industry.
A practical method for nanowire-
based CMOS circuits
SOU RCE: "Complementary Symmetry
Silicon Nanowire Logic: Power-Efficient
Inverters with Gain"
Dunwei Wang et al.
Small 2(10): 1153--1158
RESULTS: Caltech researchers have
made silicon-nanowire-based logic
circuits similar to the complementary
metal-oxide semiconductor circuits
used in computer chips. Such cir-
cuits combine two kinds of transistors
that respond in opposite ways to elec-
tronic signals---a useful ar rangement
for energy-e cient chips. Because the
new method can produce both types of
transistors on a single surface, it could
be suitable for mass production.
WHY IT MATTERS: Because of their
small size and excellent electronic
properties, silicon nanowires could
enable ultrasensitive handheld sen-
sors for detecting cancer or identifying
biological hazards. What s more, the
nanowires could lead to more power-
ful, energy-e cient computer chips.
But previous prototypes of nanowire-
based circuits were made using tech-
niques that don t lend themselves to
batch processing. The new methods
could make nanowire circuits practi-
cal to manufacture.
METHODS: To make p- and n-type
transistors, the two types needed in
CMOS circuits, researchers rst cre-
ated a checkerboard pattern of the
p- and n-type silicon: they doped
adjacent squares with di erent dop-
ants, using photolithography-produced
masks. Then, using a method they d
previously developed, the researchers
selectively etched away silicon to form
orderly arrays of nanowires. Finally,
they connected these nanowires using
e-beam lithography to form transistors
and a fundamental type of logic circuit
called an inverter.
NEXT STEPS: For mass production,
the researchers will replace the e-beam
lithography with the faster method of
photolithography. They also need to
demonstrate that an experimental pro-
cess for making batches of nanowire
arrays, called nano imprinting, will
work in large-scale manufacturing.
Nanoparticles could signal when
they are inside specific types of
cells, leading to new diagnostic
and treatment methods
SOU RC E: "Toward Intelligent Nanosize
Bioreactors: A pH-Switchable,
Channel-Equipped, Functional Polymer
Pavel Broz et al.
Nano Letters 6(10): 2349--2353
RESULTS: Researchers in Switzerland
have made 200-nanometer-wide con-
tainers dotted with pores whose walls
are made of bacterial proteins. They
demonstrated that these nano contain-
ers can control the location and dura-
tion of a uorescent signal---lighting up
only when the acidity of their environ-
ment matches that inside cell str uctures
called lysosomes, which digest foreign
materials that enter a cell.
WHY IT MATTERS: The work shows
that nanoparticles using active pores
can respond to environmental cues,
such as acidity, to perform useful func-
tions. In one application, pH-sensitive
nano carriers would light up only once
they encountered lysosomes, ensuring
that they d reached the inside of cells.
The researchers earlier showed that the
carriers can latch onto particular types
of cells, such as macrophages, suggest-
ing that such a system could be used to
identify speci c cells in a lab sample.
With some modi cations, it could also
be used to release a drug only inside
targeted cells, making dr ug treatment
more e ective and reducing side e ects
by protecting nearby tissue.
METHODS: Specially designed poly-
mers combined with bacterial proteins
self-assemble to form the containers,
while added enzymes that break down
certain compounds, causing them to
fluoresce, are trapped inside. The
pores size prevents the enzymes from
escaping but lets compounds gradu-
ally enter the container to be broken
down, creating a long-lasting signal
that is con ned to the containers. The
pH sensitivity is a result of two fac-
tors: the enzymes work best at lyso-
somal acidities, and the pores, which
are open in most conditions, close at
acid concentrations that are too high.
NEXT STEPS: The research requires
further tests to confir m that the
nanoparticles work in living subjects.
For potential dr ug-delivery applica-
tions, the researchers will pair dr ugs
with specific cellular targets and
develop a release mechanism; it could
be based on synthetic pores that stay
closed in neutral and alkaline environ-
ments as well as highly acidic ones,
opening only in the particular pH
range of the inside of a lysosome.
COURTESY OF DUNWEI WANG
This nanowire-based CMOS circuit (the
nanowires are too small to see) could lead
to smaller, more powerful computers.
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