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.

A novel approach for the fabrication of durable superhydrophobic (SH) carbon soot coatings used in quartz crystal microbalance (QCM) based gas or liquid sensors is reported. The method uses modification of the carbon soot through polymerization of hexamethyldisiloxane (HMDSO) by means of glow discharge RF plasma. The surface characterization shows a fractal-like network of carbon nanoparticles with diameter of ~50 nm. These particles form islands and cavities in the nanometer range, between which the plasma polymerized hexamethyldisiloxane (PPHMDSO) embeds and binds to the carbon chains and QCM surface. Such modified surface structure retains the hydrophobic nature of the soot and enhances its robustness upon water droplet interactions. Moreover, it significantly reduces the insertion loss and dynamic resistance of the QCM compared to the commonly used carbon soot/epoxy resin approach. Furthermore, the PPHMDSO/carbon soot coating demonstrates durability and no aging after more than 40 probing cycles in water based liquid environments. In addition, the surface layer keeps its superhydrophobicity even upon thermal annealing up to 540 °C. These experiments reveal an opportunity for the development of soot based SH QCMs with improved electrical characteristics, as required for high-resolution gas or liquid measurements.