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ANALYTICAL CURRENTS Raman from a distance Close, but not too close. That’s the concept behind the mini-Raman lidar system developed by Mark D. Ray, Arthur Sedlacek, and Ming Wu at Brookhaven National Laboratory. The system is portable, yet it nondestructively analyzes samples from distances as far away as 50 m. As such, it can help emergency-scene personnel or soldiers quickly screen a potentially hazardous site or object. This proof-of-concept system uses a 266-nm Nd:YAG laser emitting a 200mW/cm2 beam to probe surface samples from a distance. The returning light is collected by a 6-in.-diam f/4 Newtonian telescope equipped with a chargecoupled device camera. A sharp-cut edge filter rejects 266-nm light that bounces back. Spectral resolution is only 22 cm21 full-width at half-maximum, which the
5” primary
Laser range Finder (635 nm)
730 A
5 mW Secondary
Motor driven translation
Return signal
Edge filter Detector
Quartz viewpoint
Spectrometer
Diagram of the mini-Raman lidar. (Adapted with permission. Copyright 2000 American Institute of Physics.)
researchers hope to improve in the future. However, high resolution is not needed for rapid screening.
Mutation detection by electrocatalysis Studies suggest that long-range charge transport in DNA is extremely sensitive to base-stacking perturbations, such as mismatches. Now, Jacqueline K. Barton, Michael G. Hill, and colleagues at the California Institute of Technology are harnessing that phenomenon to detect single-base mismatches and other DNA lesions. To begin, preassembled DNA duplexes are attached to a gold electrode using thiol tethers. Unlike traditional hybridization, this method detects the charge transport through a DNA duplex as current flows from a redoxactive DNA intercalator (methylene blue) to a redox species (ferricyanide). When the complementary bases of a duplex are paired, the intercalator system yields a well-resolved electrochemical signal. However, if a base is mispaired, the signal diminishes substantially. The signal is not perturbed if the DNA is purely single-stranded; instead, it is as if no DNA is present.
100 mW
Nd: YAG laser (266 nm)
All eight of the possible single-base mismatches were tested. Although the technique could not differentiate one mismatch from another, it could detect the elusive GA mismatch, which is often missed by traditional methods and, sometimes, even by the cellular mismatch repair system. In addition, the method detected mismatches that occur naturally in two mutational “hot spots” in the p53 tumor suppressor gene, and it detected several common DNA lesions. Finally, the researchers fabricated chips with 18 separately addressable gold electrodes. Single-stranded oligonucleotides were attached to the electrodes, and mismatched sequences were allowed to bind. The mismatches were again detected, and the researchers noted that differences in the line shapes for the single- and doublestranded monolayers suggested different mechanisms for catalysis. (Nat. Biotechnol. 2000, 18, 1096–1100)
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In laboratory experiments, the system identified cyclohexane on topsoil and a film of acetonitrile on a Teflon sheet. The results show that the system can identify films several micrometers thick at distances of meters and “puddles” of bulk liquids at tens of meters. Acquisition times are
