Portland, Ore - Now that it has proved the concept of a "small, vibrating nose" built as a silicon microelectromechanical system, Oak Ridge National Laboratory in Tennessee has set itself the task of seeing how small an amount of substance the MEMS design can detect. "From our calculations, we believe that we can make it sensitive to the mass change of a single molecule," said researcher Panos Datskos.
Oak Ridge has claimed a world's record by detecting just 5.5 femtograms with its silicon MEMS sensor (see June 16, page 8). Measuring 2 microns long by 50 nanometers thick, the device's silicon cantilevers-like the teeth of a comb-were vibrated by an inexpensive diode laser. Measurements of the frequency of oscillation confirmed that the sensor had detected 5.5 femtograms.
"It's basically a small vibrating nose, but doing a better job than a dog can do," said Datskos, who worked with fellow researcher Nickolay Lavrik to claim the record. Indeed, Datskos said his team is working on a "universal" sensor that functions like a gas chromatograph to identify any substance. The device uses 10 different types of cantilevers but will be small enough to be handheld. To achieve his goal of detecting a single molecule, Datskos plans to up the sensitivity of his MEMS sensors by increasing the resonance frequency from the current 2 MHz to 50 MHz and by making the cantilevers smaller and stiffer.
Datskos' group coated one side of the silicon cantilevers with a monolayer of a well-known substance, such as gold or aluminum, then illuminated the cantilevers with a laser beam calculated to match the thermal time constant. The flash heating makes the cantilevers bend, since one side expands more than the other depending upon its coating. In this manner, the laser essentially plucks the teeth of the comb (cantilever array), and subsequent measurements of the frequency of vibration of each separate cantilever indicate how much mass of the sensed material was attracted to the cantilever's surface.
"We can control very precisely the effect of the laser, and not only did we detect this small mass, but we did so under ambient conditions," said Datskos. "People can probably do this very easily in a vacuum, but to do it in air and in the presence of friction-because the cantilevers have to displace air to vibrate, so friction increases-[has presented] great difficulty."
Datskos described the laser used as "not a big ugly funky laser, but the same kind that is used inside your CD player."
Datskos claimed that all the working mechanisms of a single-molecule, universal substance detector-including the sensor chip, the laser and the electronics to measure and communicate the results-could, with a few years' engineering effort, fit inside a handheld unit.
"We need to put down very uniform coatings, reproducibly and not contaminating the adjacent members of the array-all very important technical challenges," he said. "But now that the principle has been proven, we are much surer of achieving our goal."Under the hood
Oak Ridge's basic technique is to "measure the resonant frequency of a MEMS cantilever," said Datskos. "When we add more mass to it, the resonant frequency will shift, and from that shift we will know how much mass was deposited." Thus, coating one side of the cantilever with a substance that can only bond to what should be detected creates an ultrasensitive MEMS sensor technology.
To calibrate the device, Datskos deposited a monolayer of a carbon-based substance of known molecular weight, enabling him to calculate what the mass of the cantilever should be afterward and compare that against what the MEMS sensor was reading out.
"We measured how much frequency shift there was, and calculated how much the mass change should have been," he said. "It happened to be very close to what we calculated that we put on the cantilever." The researchers deposited 5.5 femtograms on the cantilever, and that is what the MEMS sensor read out. But during calibration it became obvious that even a cantilever with a 10 percent coating-little more than half a femtogram-could be read out by the sensor, Datskos said.
"Next we want to make the cantilevers even higher-frequency," he said. They are 2 MHz today, but Datskos' group hopes to raise that to 50 MHz. "Higher frequency makes them more sensitive, so we can go from femtograms today to attograms or even less. A nice milestone would be to be able to detect a single protein molecule in a liquid environment."
By building a vast array of cantilevers on a silicon chip, many varying kinds of substances could be simultaneously tested. Using the model of the gas chromatograph, which can identify almost any substance, Datskos has calculated how many types of cantilevers would be needed to make a handheld device that could identify a single molecule of any substance.
"Measuring the mass is one thing; [identifying the substance itself] is another problem. In order to do this you need to have differently coated cantilevers-seven, eight or maybe 10 different types that have affinities for different molecules. That way you don't just get a change in frequency, but you get a pattern, a signature," said Datskos.
The gas chromatograph community has already accumulated a database of such signature patterns, making it possible to identify any substance with a finite number of different coatings for the gas columns-which translate to the same coatings for the microscopic cantilevers made by Datskos' group.
Datskos holds three U.S. patents on MEMS sensors, and several others are pending.