SCIENCE & TECHNOLOGY DESIGNERS Crooks (left) and Zhan create electrochemical logic circuits that can be used as smart sensors.
ITS ONLY LOGICAL Device combines electrochemistry and microfluidics to create logic gate for sensors
R
ICHARD M. CROOKS A N D H I S
group at Texas A&M University had been working on ways to integrate electrochemistry with a microfluidic platform when it occurred to graduate student Wei Zhan that there's a key difference between doing electrochemistry in beakers and doing it in a microfluidic platform. In a microfluidic context, "you can have lots and lots of channels and lots and lots of electrodes all communicating with each other in a pretty straightforward way," Crooks says, "and you can set this up using lithography, a tool of the electronics industry." Crooks and Zhan set out to design a chip that combines microfluidics and electrochemistry, where what happens in one channel affects what happens in the others. The result is a chip that acts as a logic gate. The most complicated device they have designed is a nine-channel, seven-electrode NAND gate \J. Am. Cbem. Soc., 125, 9934 (2003)}. A NAND gate combines the functions AND and NOT. In addition to detecting signals elec-
trochemically, they can also use electrogenerated chemiluminescence. "The nice thing is that you can detect light in parallel," Crooks says. "%u not only could have individual devices that have many channels and many electrodes, but you could have many of those on a single chip. %u could have a small photodetector that could read all of the chips simultaneously." No physical limitations prevent Crooks and Zhanfrommaking devices with more channels. However, more complicated circuits make it difficult to visualize how individual changes will affect the overall response of the system. "It's like a puzzle," Crooks says. "Presumably, a clever person could write a computer algorithm to design parallel-processing microfluidic systems that would have many channels and many electrodes—many more than weVe looked at here. I would say that nine channels was probably at the practical limit of complexity for a blackboard and a piece of chalk." The device opens up the prospect of parallel sensing, but not just in the sense of one channel for each analyte. For instance,
More complicated circuits make it difficult to visualize how individual changes will affect the overall response of the system.
30
C&EN / SEPTEMBER
1, 2003
it can handle situations where analytes are of interest only as they relate to the presence or absence of other species. "Rather than doing separate detection events, you only have to do one, because you can hook this up as a logic gate," Crooks says. For example, organophosphates can be used both as pesticides and as chemical weapons, and "you might only be interested in organophosphates if there are other things also in the environment associated with making weapons," Crooks says. "You're only really interested in an organophosphate if three other chemicals are present as well—or maybe one chemical for sure and one of two other chemicals. That would be an AND and an OR type of logic process." Such devices would serve as alarm systems rather than quantitative measures because, regardless of the number of inputs, each circuit has only a single output. "Logic gates are digital. The/re either yes or the/re no," Crooks says.
CLEAR CHANNELS Devices are made of poly(dimethylsiloxane) channels and indium tin oxide electrodes patterned on glass. They can be run off a 1.5-V battery.
"The paper by Zhan and Crooks is an important fundamental step along the path to creating intelligent instrumentation for chemical analysis and manipulation of chemical processes," says Paul W. Bohn, a chemistry professor at the University of Illinois, Urbana-Champaign. "By demonstrating diode, OR, and NAND outputs, they open the door to complex fluidic systems in which the spatial positioning or processing of a packet of chemical information is dependent in a sophisticated way on the relationship of a number of externally controlled inputs."—CELIA HENRY HTTP://WWW.CEN-ONLINE.ORG
