Smaller, more sensitive sensors revolutionize public safety, medicine

The relentless progress in microsensor science is driving the growth in the fast-growing, fast-shrinking field of single-molecule detection. The growing sensitivity of detection devices now allows some of them to spot substances in the parts-per-trillion range. Washington Post’s Curt Suplee writes that sensors known as “e-noses” function as artificial snouts that can identify the barest trace of compounds in the air, while microfluidic “lab on a chip” sensors can flag individual DNA strands and other entities in liquids.

Only a few of these devices are commercialized. The trend, however, is unmistaken and it portends a revolution in public safety , according to Stephen Semancik, who heads the Chemical Microsensor Program at the National Institute of Standards and Technology (NIST). “What we can’t smell can hurt us,” he says, citing dangers from carbon monoxide to spoiled food, low-level industrial toxins, water contaminants, and building fires where “five minutes can be life or death.” The smaller the amount you can detect, the earlier your warning. For example, in the event that terrorists use chemical weapons, you really do not want to wait until concentrations reach levels at which you can smell the chemical. At that point it may be too late.

Suplee writes that whether in medical application or public safety, the next of microsensors will be small, highly automated, and sufficiently sensitive to operate in the chemical cacophony of the real world. There are numerous designs.

• Mechanical systems. In a typical mechanical system, airborne molecules stick to the end of a microscopic cantilever. Just as a diving board vibrates differently if it is occupied by your 6-year-old or a bigger adult, the vibrational frequency of the tiny cantilever changes in response to the mass of the molecule trapped on it and sounds the alarm when its target compound shows up. Other mechanical systems use nanoscale tubes (that is, with dimensions in the range of billionths of a meter) to snag pathogens by size. University of Missouri scientists, for example, recently created a glass “nanopore” system custom-tailored to trap single molecules of ricin, a poison extracted from castor beans.
• Optical systems. Optical systems sense changes in the properties of transmitted or reflected light that occur when the molecule of interest binds to another molecule or undergoes a chemical reaction. David Walt of Tufts University devised a novel variation: “My lab recently demonstrated that we could isolate single molecules into very tiny wells etched into the end of a fiber optic bundle,” Walt wrote Suplee in an e-mail. The bundle had tens of thousands of such wells, each just big enough for one target DNA molecule coated with special compounds that would react with the DNA to produce fluorescence. When a solution containing the molecules flowed across the bundle, wells that trapped the molecules lit up. Counting the ratio of dark to light fibers in the bundle, Walt’s team was able to determine the concentration of molecules in the solution to unprecedented accuracy.

The ultimate goal is to get out of the lab. “I believe that the focus of this field is moving from ‘sensors’ to ‘point of care technologies’ (POCT) that bring bioanalytic methods from the laboratory to the point of need, whether it is the bedside, emergency room or for preventative medicine such as cholesterol monitoring,” Jerome Schultz, chair of the Department of Bioengineering at the University of California at Riverside, told Suplee. “This type of device would have wide applications for water quality, especially for third world countries, food contamination and bioterrorism.”

If such sensors can be made sufficiently small, sensitive and cheap, the public itself could become an army of roving detectors. “We all have cellphones now,” Semancik says. “Suppose we put a chemical sensor in every phone?” (see “Day of Americans Serving as Mobile Chemicals Sensors Nears,” 6 November 2009 HSNW)If it encountered a threat, it would send out a signal. “If the monitoring system started getting lots of the same signals from one place (as determined by the phones’ GPS locators), responders could home in on the threat,” Suplee rites.