Film bulk acoustic formaldehyde sensor with layer-by-layer assembled carbon nanotubes/polyethyleneimine multilayers

Abstract

Trace formaldehyde vapor was detected by a micron-scale AlN film bulk acoustic resonator based on mass-sensitive mechanism. The layer-by-layer carbon nanotubes/polyethyleneimine multilayers were assembled on the resonator surface as the sensitive coating. An almost linear decrease of the resonant frequency was observed as a function of the number of nanotubes/ polyethyleneimine periods. The multilayers showed a random and porous structure and thus provided a large specific surface area for gas adsorption and diffusion. At the same time, the amine groups in polyethyleneimine had an strong affinity to formaldehyde with excellent selectivity. When exposed to gaseous formaldehyde, the attachment of gas molecules induced a small decrease in the resonant frequency, which made the sensor easily detect formaldehyde at ppb levels with 1 min response time. A linear relationship was observed between the formaldehyde concentrations and the frequency downshift of the resonator. The layer number had an obvious influence on the absorption/desorption behavior of formaldehyde. The gas sensitivity of FBAR sensors was 1.29-1.90 kHz ppb −1 with the limit of detection of 24-38 ppb.

Review of Gravimetric Sensing of Volatile Organic Compounds

“…It has been shown that polymer coatings can decrease the quality factor of acoustic resonators, which leads to less sensitive measurements. 41,42 The mechanical properties of some sensitizers, in particular, polymers or polymer composites, can also change due to humidity or analyte adsorption, which can change the sensitivity of the device. 43−45 Aging and humidity of sensitizers like self-assembled monolayers also can affect the sensors' function.…”

“…For example, the integration of CNTs inside polymers mitigates the toxicity resulting from inhalation of CNTs, while the CNTs can provide sensitivity the pure polymer cannot match. 175 42 The PEI/MWCNT coating is shown to be selective for formaldehyde against other VOCs, and the sensing response shows little degradation due to humidity. Sayago et al used other polymers (PECH and PEUT) with multiple percentages of MWCNTs incorporated to selectively detect toluene and octane, and to improve both the limit of detection and the sensitivity of the device in comparison to the pure polymer films.…”

“…Another source of error can arise from interactions between the device and the sensitizer. It has been shown that polymer coatings can decrease the quality factor of acoustic resonators, which leads to less sensitive measurements. , The mechanical properties of some sensitizers, in particular, polymers or polymer composites, can also change due to humidity or analyte adsorption, which can change the sensitivity of the device. − Aging and humidity of sensitizers like self-assembled monolayers also can affect the sensors’ function. , These issues are materials-specific, and information about the chemistry of the analyte/sensitizer reaction should be acquired before testing to minimize these errors. Limited research has been done on the kinetics of these reactions, but several notable works are referenced here. ,− …”

“…Fabrication and handling of graphene and carbon nanotubes can be complicated because CNTs will aggregate together and graphene will stick together in sheets . Carbon nanotubes and graphene have shown high selectivity for NH 3 , NO 2 , and O 2 , and some sensitivity for CO 2 inherently, but for VOC sensing, functionalization is often necessary. ,,,− Adding carbon nanotube ”forests” to the surface of the gravimetric sensor has been shown to degrade the quality factor if its thickness is not carefully controlled, but graphene shows no indication of altering the Q of the device for thin layers. , Hybrid sensitizers where carbon nanotubes are added to other sensitizers, most commonly polymers, have been popular. ,− This approach can mitigate some of the difficulty in handling the carbon nanotubes, increase the sensitivity of the primary layer, and add selectivity to the carbon nanotubes instead of functionalization.…”

“…For example, the integration of CNTs inside polymers mitigates the toxicity resulting from inhalation of CNTs, while the CNTs can provide sensitivity the pure polymer cannot match . Several groups of researchers have used polymers with carbon nanotubes and other nanoparticles. ,− For formaldehyde sensing, MWCNTs and PEI composite films have been used by multiple groups. ,− Song et al and Wang et al both used aluminum nitride-based FBARs for their devices. , Song et al observed an LOD of 24–38 ppb, and the work by Wang et al is well characterized for sensitivity, response time, and recovery times. Figure shows both SEM images of the hybrid sensitizer used in Song et al, as well as typical real-time responses.…”