PORTLAND, Ore Oak Ridge National Laboratory in Tennessee is claiming a new world's record by detecting just 5.5 femtograms with its silicon micro-electro-mechanical-system (MEMS) sensor.
Measuring just 2 microns long by 50 nanometers thick, the silicon cantileverslike the teeth of a combwere vibrated by an inexpensive diode laser. Measurements of the frequency of oscillation confirmed that the sensor had detected just 5.5 femtograms.
"Its basically a small vibrating nose, but doing a better job than a dog can do," said ORNL researcher Panos Datskos, who worked with fellow researcher Nickolay Lavrik to claim the record.
By coating the cantilevers with various materials sensitive to proteins, cells or trace amounts of almost any chemical contaminant, the researchers said nearly any substance can theoretically be detected with a MEMS sensor.
Datskos said his team is also working on a "universal" sensor
that works like a gas chromatograph to identify any substance. The device uses 10 different types of cantilevers but will be small enough to be handheld.
Datskos also plans to up the sensitivity of his MEMS sensors to achieve his ultimate goal of detecting a single molecule. This can be accomplished by increasing the resonance frequency from its current 2 MHz to 50 MHz and by correspondingly 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 whatever it was sensing was attracted to its surface.
"We can control very precisely the affect of the laser, and not only did we detect this small mass, but we did so under ambient conditions. 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 increasespeople have had great difficultly so far trying to achieve that," said Datskos.
"The laser we used, it wasn't a big ugly funky laser, but was the same kind that is used inside your CD player," he added.