Resonant Nanocantilever Chemical Vapor Sensors

Chemical vapor sensors are used in a wide variety of fields such as security, environmental monitoring, the food and beverage industry, and healthcare to detect disease biomarkers on exhaled breath. An electronic nose is composed of an array of cross-responsive chemical vapor sensors, in which every sensor responds to a varying degree to each chemical vapor, creating a "fingerprint" for that vapor. Incorporating an electronic nose into a highly-miniaturized vapor detection system, capable of bringing near laboratory-quality analysis into the field, requires the use of extremely small, fast, and sensitive sensors. One option is resonant nanocantilevers, which respond to changes in mass and stiffness by shifts in resonant frequency, and are capable of detecting mass-loading at the attogram (10-18 g) level in ambient conditions. To determine whether nanocantilevers can be used in an electronic nose, an array of five nanocantilevers, wherein each sensor was coated with a different dropcast polymer film (2-10 nm thick), were exposed to seven chemical vapors with a range of functional groups. The array successfully discriminated between all vapors, indicating that sensor responses were dominated by vapor absorption into polymer films, and not by non-specific physisorption. The thinness of the polymer film, combined with the small vapor capture area of the nanocantilevers, resulted in lower sensitivity than desired, limiting their effectiveness. To overcome this challenge, surface initiated atom transfer radical polymerization (SI-ATRP) was used to grow a 100 nm thick, uniform films of poly(methylmethacrylate) (PMMA), poly(methyl acrylate) (PMA), and poly(n-butyl methacrylate) (PBMA) on nanocantilevers. The thick polymer films absorbed more vapor, significantly increasing nanocantilever sensitivity. To determine the relative roles of mass loading and stiffness change on nanocantilever sensor response, SI-ATRP was combined with chromium masking, enabling polymer film growth to be localized to either the clamped end (sensitive to stiffness) or the free end (sensitive to mass-loading) of the nanocantilevers. These experiments revealed that changes in stiffness, induced by vapor absorption into the polymer films, dominated the sensor responses, and not mass-loading as was initially assumed. This work demonstrated that an array resonant nanocantilevers can be successfully used a sensitive, nanoscale electronic nose.