news SECM sees double
structure of the membrane with diffusion across the membrane, they say. “[But] we’ve been able to image the topogIt has long been a goal in scanning electrochemical raphy of the pore and see [its] activity on a scale 20–50 microscopy (SECM) to “see double”—that is, to get electimes better than before,” says Unwin. trochemical and topographical information at the same time. This increased resolution allowed closer examination of In recent years, scientists have edged diffusional transport. For example, a ever closer to that mark. Now, Julie V. marked heterogeneity in the current Macpherson and Patrick R. Unwin at associated with the pores in a synthe University of Warwick have put all thetic membrane might be due the pieces together and developed a to variations in pore size. But the combined SECM and atomic force researchers did not find a correlamicroscopy (AFM) instrument, which tion between pore size and measis described in the January 15th issue ured current, so they concluded that of Analytical Chemistry (pp 276–285). the variations in current more likely The instrument is based on a comreflect variations in diffusional actimercial AFM and is equipped with a vity across the pores. special tip, which allowed both EC and Now that the instrument’s basic topographical data to be recorded. capabilities have been demonstrated, “It’s a very simple idea, really,” says Macpherson and Unwin are ready to Macpherson. “[We just] take an etched move into studies of biological sysplatinum wire . . . and insulate it with a tems. Because the instrument can special paint to make a very tiny elecimage samples in fluid, it is well suittrode area. That’s the electrochemical ed for these applications. One possisensor. Then we flatten part of the wire bility is exploring a particular kind of to make a cantilever, and that’s the membrane transport, the transdertopographical sensor.” Imaging diffusion of an electroactive species mal delivery of drugs, says Unwin. through a membrane using an SECM–AFM. In this way, the new instrument “What we’ve looked at so far is just (a) Schematic. (b) The resulting current response obtains both types of information the tip of the iceberg,” he adds. and topographical data. simultaneously but records them sepaThe researchers also want to add rately. In SECMs, on the other hand, a “noncontact” mode to the instruan electrode is scanned over a surface, ment. The tip would be scanned over and the current that flows depends on diffusion, which, in the surface initially to record the substrate’s topography. Then, turn, depends on both the distance from the surface and the with the tip retracted, the EC response would be monitored. reactivity. Thus, the two effects are difficult to separate. This approach would allow the researchers to study conductCoating sharp tips is an old trick the researchers boring and semiconducting surfaces, in addition to insulating rowed from electrochemical scanning tunneling microscopy, ones. “Clearly, there’s more work to be done . . . in terms of says Unwin. “Basically, what we’re doing is using the same methodology and applications,” Unwin says. “But we think we kind of coatings and showing that they’ll work on an AFM have taken a good step forward . . . and we’re encouraged by tip.” That was not a foregone conclusion, he adds, because what we’ve achieved so far.” Elizabeth Zubritsky the geometry of AFM tips is much more irregular. In addition to being dual-purpose, the new tips are tiny. They begin as platinum wires ~50 µm in diameter, which are GOVERNMENT AND SOCIETY etched to a very fine point. Then, the wires are coated, and Gifts for U.K. scientists as the paint cures, it retracts, exposing an area on the order of tens or hundreds of nanometers. “It’s a natural process,” After a nail-biting week for U.K. scientists, the government finally says Unwin, “that just [happens to] help us make these very small tips.” To demonstrate the combined instrument’s capability, the announced the results of its university infrastructure-funding competition researchers imaged dissolution of crystal surfaces and diffuin December 1999. Despite the delay, Christmas came early for 45 research sional transport across synthetic membranes. Both processes have previously been studied with SECM, and it has been possible to obtain some complementary topographical infor- teams around the country, who will benefit from a share of the biggest mation. However, those studies were not as revealing as researchers would have liked, Macpherson and Unwin say. handout to science infrastructure in 40 years, amounting to approximately For example, in SECM-only studies of diffusional transport, it has been difficult to observe the pores and correlate the £320 million (more than U.S. $500 million). F E B R U A R Y 1 , 2 0 0 0 / A N A LY T I C A L C H E M I S T R Y
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