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ANALYTICAL CURRENTS Moving fluids mechanically pattern to propel fluid. The valves are built by polymerizing microspheres into a “long” arm and placing the device in the channel. A passive valve simply shifts back and forth with changing fluid direction, like a gate, and halts large-diameter particles but lets smaller ones pass. An active valve is demonstrated by positioning the arm in the center of a T-junction and optically moving the gate up or down to allow flow into one junction channel or the other. These tiny mechanical systems can handle nonaqueous solvents and are easy to fabricate. And, although optical trapping was used in this study, other microsphere colloids can operate in ap-
Using colloidal silica microspheres, John Oakey, David Marr, and Alex Terray of the Colorado School of Mines have created pumps and valves the size of human red blood cells that direct fluids through microchip channels. These miniaturized devices offer the potential of extending the concept of a micro total analysis system to include microfluidic control on the chip. The researchers built their pumps and valves by using optical trapping techniques to move and direct micrometer-sized silica microspheres into various shapes. For example, they constructed a pump capable of moving fluids as fast as 1 nL/h by collecting four 3-µm microspheres into two dumbbell shapes that mesh together like two gears. As the laser optical trapping system turns the two dumbbells—one clockwise and the other counter-clockwise—this gear pump sends the fluid down a microchannel. A second pump, built with six microspheres, undulates in a snakelike
Follow the moving particle. Two sets of nanoparticles mesh together to form a gear pump. (Adapted with permission. Copyright 2002 American Association for the Advancement of Science.)
plied electric or magnetic fields. (Science 2002, 296, 1841–1844)
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New lead chemosensor Chao-Tsen Chen and Wan-Pei Huang at Nation-
high photostability and visible excitation wave-
The sensor is selective for Pb2+ over Ca2+,
al Taiwan University have developed a fluo-
length. The chemosensor was prepared by the
Zn2+, Cd2+, Fe2+, Mn2+, and Hg2+. This selectiv-
condensation of 4-
ity is important because Pb2+ targets Ca2+-
(N,N-diethylamino)-
and Zn2+-binding sites in vivo, and Cd2+, Fe2+,
salicylaldehyde with
Mn2+, and Hg2+ frequently interfere with Pb2+
-ketoester append-
analysis. Furthermore, the sensor exhibits 40-,
ed with 15-monoaza-
12-, and 18-fold fluorescence enhancements
crown-5 ether in the
for Pb2+, Ba2+, and Cu2+, respectively. In aque-
presence of piperi-
ous systems, the stability of the sensor–Pb2+
dine. The data col-
complex is attenuated by 2 orders of magni-
lected so far sug-
tude, but the binding strength and the magni-
The researchers chose ke-
gests that the sensor
tude of fluorescence enhancement remain
toaminocoumarin as a signal-
forms a 2:2 complex
unaffected. (J. Am. Chem. Soc. 2002, 124,
rescent sensor that detects 2+
Pb selectively with a large fluorescence enhancement.
N N
The researchers hope that their new fluorescent chemosensor
2+
monitor Pb concentrations in contaminated sources.
transducing unit because of its
O O
O
O O
will help explain the cellular role of lead ions in vivo and
O
Pb O
Pb2+
O O
2+
O O
O
O O
N N
A proposed 2:2 complex formed between the chemosensor and Pb2+.
2+
with Pb .
6246–6247) A U G U S T 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
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