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RESEARCH PROFILE SAW chip sniffs out cocaine This IDT spacing allows the researchers to reduce SAW reflections within the resonant cavity, which interfere with resonance and degrade the performance of the device. The researchers chose an ST-X quartz substrate because this cut provides good stability near room temperature. The antibodies were immobilized using Protein A as the cross-linker, and a 3-µmthin layer of hydrogel kept the antibodies in their proper conformation and Reference minimized humidity effects. Anti-BZE + gel To simulate cocaine in Anti-FITC + gel the atmosphere, a vapor Gel only generator delivered ~1 ng of cocaine vapor as 5-s pulses in an airstream to a flow cell containing four SAW resonators. Because the flow rate, temperature, and pressure remained conscribe a highly specific, –4 stant with the addition of real-time vapor sensor – 4.5 20 40 60 80 100 120 140 160 180 200 the pulse, the researchers that detects cocaine Time (s) concluded that the obplumes in the atmosserved frequency shifts phere in spite of the were the result of cocaine diversionary tactics of Responses of a device coated with anti-benzoylecgonine (BZE) antibody molecules perturbing the smugglers. Their techand a protective hydrogel (green line) and two negative control devices surface of the chip. Comnology, unlike previous (red and blue lines) to pulses of cocaine vapor from a vapor generator. pared with uncoated deinstruments for this purvices and antifluorescein isothiocyanatepose, uses a surface acoustic wave (SAW) gen do a sort of dance when they bind, coated devices, which were used as and it is that dance that we are listening quartz resonator chip. negative controls, the sensor showed a to,” says Hunt. Other molecules will The researchers say that previous 40-fold stronger response to cocaine. not interact in quite the same way with vapor sensors were easily confused A newer version of the device has three the antibody, and the difference can be because of a lack of specificity. “The sensors on each chip, one to detect exdetected electronically. problem is that other electronic nose plosives, one to detect cocaine, and one SAW devices typically have gratings technologies generally rely on pattern with metal fingers spaced at equal widths. to detect heroin, says Hunt. recognition and learning algorithms to Because the technology can be packThe metal fingers convert the applied discern what analytes are present,” says aged in handheld units and because of voltage to an acoustic wave. Between Hunt. Their device singles out the coits specificity and ability to give real-time the gratings lie interdigital transducers caine molecules with a monolayer of results, Hunt, Stubbs, and Lee believe (IDTs), which act as the input and outantibodies to benzoylecgonine, the put for Rayleigh acoustic waves. But un- their new vapor-sensing device will allow major metabolite of cocaine found in police officials, customs agents, and borlike traditional IDTs, which often have the bloodstream. Its structure differs der patrols to replace a technology that the same finger width as the gratings from that of cocaine by just a methyl has literally gone to the dogs, while at paired with them, the research group’s group. The researchers say that only the same time tightening the leash on design uses a split-finger configuration molecules that have the same epitope today’s drug traffickers. a with an IDT finger width of 1.5 µm the antibody is looking for will confuse —Wilder D. Smith and a grating finger width of 3.1 µm. their sensor. “But then if this occurs, it probably is cocaine, under the guise of another molecule,” says Hunt. In the SAW quartz device, when the antigen binds to the antibody, the researchers measure and compare the shift in the oscillator frequency caused by the alteration in the acoustic velocity. “To put it colloquially, the antibody and anti-
Frequency shift (Hz)
Trained dogs, or rather their noses, are a critical and well-known tool for detecting illegal drugs and explosives. But there simply are not enough sniffers to cover the borders. Electronic noses have been proposed as an alternative (Anal. Chem. 2003, 75, 98 A–105 A). However, drug smugglers have become savvier about eluding sensor detection with tricks such as 1 packing their contraband0.5 filled crates with a variety 0 of other chemicals to confuse the sensor and pro– 0.5 duce a high number of –1 false-positive readings. In the November 15 – 1.5 issue of Analytical Chem–2 istry (pp 6231–6235), – 2.5 William Hunt, Desmond Stubbs, and Sang-Hun –3 Lee at the Georgia Insti– 3.5 tute of Technology de-
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