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ANALYTICAL CURRENTS Nanotubes as chemical sensors About two years ago, researchers discovered that carbon nanotubes—cylindrical arrangements of graphite molecules—conduct electricity at room temperature with little resistance. Now, Jing Kong, Hongjie Dai, and colleagues at Stanford University have put that capability to use for gas sensing. The researchers begin with semiconducting single-walled nanotubes, which are deposited between two metal contacts to create units that behave like ptype transistors. The conductance of these units increases when exposed to NO2 gas and decreases when exposed to NH3 gas. The researchers attribute these two responses to different mechanisms: charge transfer in the case of NO2 and interactions either with the substrate or with other species that have been preabsorbed on the nanotube, in the case of NH3. The nanotube sensors have sensitivities (the ratio of resistance after gas exposure vs before exposure) of
~100–1000 and response times of 2–10 s for 200-ppm NO2. These findings are comparable with the results for high-performance metal oxide sensors (sensitivity of ~300 and response time of ~50 s for 100 ppm NO2), but the nanotube sensors operate at room temperature instead of >250 °C. In addition, the researchers note that nanotube sensors are reusable and can be tuned to different types of molecules by adjusting the electrical gate to either a conducting or an insulating state. (Science 2000, 287, 622–625)
Current versus gate voltage before NO2 (circles), after NO2 (triangles), and after NH3 (squares). (Adapted with permission. Copyright 2000 American Association for the Advancement of Science.)
Imaging cell membrane molecules TOF-SIMS images of freeze-fractured liposomes. In each case, three types of membrane components pictured are (left to right) C3 hydrocarbons, phosphorylated headgroups, and cholesterol. (a) Two types of liposomes (DPPC/cholesterol and DPPDME/cholesterol) before mixing. (b) DPPC/cholesterol and DPPNME/cholesterol liposomes, after membrane fusion but before redistribution of membrane components. (c) DPPC/cholesterol and DPPNME/cholesterol liposomes after redistribution of membrane components.
Far from being static and ordinary, cellular membranes are now thought to play important roles in a wide range of cellular functions. Unfortunately, the tools for examining membranes have not told the whole story. Even early experiments with the
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promising technique of time-of-flight secondary ion mass spectrometry (TOF-SIMS) have been limited. Now, Andrew G. Ewing, Nicholas Winograd, and colleagues at Pennsylvania State University describe an imaging TOF-SIMS technique that may unravel complex dynamic events associated with cellular membranes. Imaging TOF-SIMS is based on
A N A LY T I C A L C H E M I S T R Y / A P R I L 1 , 2 0 0 0
static TOF-SIMS, a relatively nondestructive technique in which a focused ion beam is rastered over the surface of a specimen and mass spectra are collected at each position. This technique allows the reconstruction of molecule-specific images of particular masses. For the imaging, the researchers look at a series of samples. To prepare the samples, the researchers use fast-freeze fracturing—a technique that is related to freeze dry-
