Humidity Tolerant Organic Vapor Detection Using a Superhydrophobic Quartz Crystal Microbalance

The organic vapor sensitivity of a quartz crystal microbalance (QCM) with an epoxy resin-carbon soot coating designed to be tolerant to humidity is reported. This is achieved using nonengulfed, but attached, hydrophobic carbon soot nanoparticles, coated in an irregular surface topography composed of islands and cavities. The root mean square roughness (Rrms) of 130 nm, together with the hydrophobic soot, convert the epoxy surface to a superhydrophobic (SH) one with high static contact angle (~151°) and low contact angle hysteresis (~1.1°). The frequency shift of the SH QCM at 100% relative humidity is ~7 times lower compared with an uncoated device, and thus indicating low water vapor adsorption due to superhydrophobicity. In addition, the SH QCM shows between three and six times higher gas sensitivity and lower detection limit compared with the conventional polymer coated QCMs. These results correlate well with the Rrms and surface topography of the coating, which ensure enhanced sensing area. Furthermore, the sensor demonstrates reproducibility, reversibility, and fast response-recovery time (~10 s) to ethanol, methanol, and isopropanol vapor. These experiments reveal that superhydrophobicity increases the organic vapor sorption at the expense of water vapor sorption, and thus allowing operation of the QCM gas sensors in an uncontrolled humidity environment.