Miniaturized electronic “noses” offer intriguing prospects, such as exhaled-breath analysis and environmental air-quality monitoring with portable devices and mobile phones. Yet detecting the resultant tiny frequency shifts in a microelectromechanical system (MEMS) sensor is complicated when operated in viscous media, such as ambient air. Reporting in Nanotechnology, researchers have demonstrated a method to enhance the sensitivity of such resonant vibrating gas sensors by making use of nonlinearity in the vibrational response
A typical microelectromechanical system (MEMS) sensor is a microcantilever made of silicon. The vibrational frequency depends on mass and is affected by the specific adsorption of gas molecules on its surface. When a cantilever is driven very strongly, it executes nonlinear oscillations, where the vibrational amplitude no longer scales linearly with the applied force.
Jumping into motion
Instantaneous jumps can then occur between states with high and low vibrational amplitudes. The characteristic frequency of these jumps is extremely sensitive to the amount of gas adsorbed. Using a cantilever with electrochemically functionalized surfaces, the researchers at the Delft University of Technology in The Netherlands and Cambridge University in the UK, demonstrate an enhanced response to the selective detection of ethanol vapour as a test case. The detected frequency shift is more than three times larger than that obtained in the linear regime.The detection is reversible and reproducible, and the nonlinear scheme is straightforward to implement in existing instruments. The nonlinear cantilever can also operate as a sensitive threshold gas detector: when gas molecules are detected, the cantilever jumps from a stable rest position to a state with high-amplitude oscillations.
While nonlinear behaviour is generally to be avoided in a transducer used in a sensor device, here it is shown, in contrast, that it can be exploited. Moreover, as mechanical transducers are scaled down in size, it becomes increasingly difficult to operate them in a linear regime. Further research will be directed towards developing sensor arrays in which nonlinear resonators detect multiple gases.More information can be found in the journal Nanotechnology25 425501.