news
MEETING NEWS Nanoparticles instead of redox agents? When Jed Harrison and graduate student Jian-Bin Bao at the University of Alberta (Canada) took on the challenge of trying to make an on-chip magnetohydrodynamic (MHD) pump, they never imagined what the ultimate payoff would be. They found that nanoparticles in solution can have a dc conductivity of >100 mA/mm2, which suggests that one could use small particulates instead of the usual redox-active agents to get conductive solutions. “One of the things that’s always frustrating about fluids and electrochemistry is that you’re stuck having to add a significant concentration of redox couple if you want to get any kind of conductivity and current,” Harrison says. The problem is that the redox couple can interfere with the rest of the chemistry in the experiment. Particulates would be much more benign, he notes, and would be easier to remove from the solution, because they
could be filtered out in many cases. But finding an alternative to redox agents wasn’t the problem Bao was trying to solve. He was trying to address some big issues associated with MHD on a chip. The concept is attractive because it yields pressure-driven flow without moving parts, but the hitch, Harrison says, is that MHD “scales in the wrong direction for microfluidics.” The flow velocity is inversely proportional to the square of the channel dimension, so as the channels get smaller, the pump gets weaker. Bao optimized the channel design and fabricated electrodes that ran the full length of the channel to obtain the highest flow velocity. Then he faced MHD’s lose–lose choice: Use a dc system and contend with bubbles in the channel generated by electrolysis, or use an ac system and wind up with heat induced by the eddy currents. He opted for dc. To avoid the plague of bubbles, Bao switched to a highly dc-conductive fluid. He first tried 1 M KNO3. Then he started making solutions of gold
nanoparticles. A typical 0.01% solution was not conductive, but at concentrations of a few percent, he observed conductivity as high as a few hundred mA/mm2 with 300 mV applied. Although Bao has only tried gold so far, any conductive nanoparticles should work, Harrison says. The researchers are still trying to pinpoint the mechanism behind this effect. One possibility is the kind of particle charging and subsequent electrophoretic mobility that Royce Murray described a few years ago, says Harrison. Another is an electrorheological effect that lines up the particles to make short-lived wires. Harrison speculates that this phenomenon might reduce resistive losses in batteries and air-based fuel cells—if one could circumvent the possibility of a short circuit. Many other applications are out there, even if they can’t be predicted right now, he says. “I think it’s one of those observations that people will latch on to and run off with in a hundred different directions.”
joint meeting of the American Electrophoresis Society and the American Institute of Chemical Engineers—San Francisco, Calif. Rex Graham reports from the
14 A
(a) Relative fluorescence units
A team of Northwestern University researchers has developed microchip electrophoresis (ME) technology that can sensitively detect mutant forms of p53, the most commonly mutated gene in human cancers. Graduate student Christa Hestekin reports that the method uses smaller amounts of reagents and takes
