Karekin D. Esmeryan

Manipulated wettability of a superhydrophobic quartz crystal microbalance through electrowetting

The liquid phase response of quartz crystal microbalances (QCMs) with a thin coating (~9 µm) of epoxy resin with and without a carbon nanoparticles top layer is reported. The nanoparticles convert the epoxy surface to a superhydrophobic one with a high static contact angle (~151°–155°) and low contact angle hysteresis (~1°–3.7°) where droplets of water are in the suspended Cassie–Baxter state. The frequency decrease of the fully immersed QCM with the superhydrophobic surface is less than with only epoxy layer, thus indicating a decoupling of the QCM response. A wettability transition to a liquid penetrating into the surface roughness state (for droplets a high contact angle hysteresis Wenzel state) was triggered using a molarity-of-ethanol droplet test (MED) and electrowetting; the MED approach caused some surface damage. The electrowetting-induced transition caused a frequency decrease of 739 Hz at a critical voltage of ~100 V compared to the QCM in air. This critical voltage correlates to a contact angle decrease of 26° and a high contact angle hysteresis state in droplet experiments. These experiments provide a proof-of-concept that QCMs can be used to sense wetting state transitions and not only mass attachments or changes in viscosity–density products of liquids.

Durable superhydrophobic carbon soot coatings for sensor applications

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.

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.

Single-step flame synthesis of carbon nanoparticles with tunable structure and chemical reactivity

A novel method for the flame synthesis of carbon nanoparticles with controllable fraction of amorphous, graphitic-like and diamond-like phases is reported. The structure of nanoparticles was tailored using a conical chimney with an adjustable air-inlet opening. The opening was used to manipulate the combustion of an inflamed wick soaked in rapeseed oil, establishing three distinct combustion regimes at fully-open, half-open and fully-closed opening. Each regime led to the formation of carbon coatings with diverse structure and chemical reactivity through a facile, single-step process. In particular, the fully-closed opening suppressed most of the inlet air, causing an increased fuel/oxygen ratio and decreased flame temperature. In turn, the nucleation rate of soot nanoparticles was enhanced, triggering the precipitation of some of them as diamond-like carbon (DLC). Surface characterization analyses using Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron spectroscopy confirmed this hypothesis, indicating a short-range ordered nanocrystalline structure and ∼80% sp3 bonds in the coatings deposited at fully-closed opening. Furthermore, three groups of 5 MHz Quartz Crystal Microbalances (QCMs) coated with soot and DLC, corresponding to each of the three combustion regimes, showed different frequency responses to aqueous ethanol and isopropanol solutions in the concentration range of 0–12.5 wt%. The DLC coated QCMs exhibited relatively constant frequency shift of ∼2250 Hz regardless of the chemical, while the response of soot coated counterparts was influenced by the quantity of heteroatoms in the film. Our method can be applied in chemical sensing for the development of piezoresonance liquid sensors with tunable sensitivity.

Temperature Frequency Characteristics of Hexamethyldisiloxane (HMDSO) Polymer Coated Rayleigh Surface Acoustic Wave (SAW) Resonators for Gas-Phase Sensor Applications

Temperature induced frequency shifts may compromise the sensor response of polymer coated acoustic wave gas-phase sensors operating in environments of variable temperature. To correct the sensor data with the temperature response of the sensor the latter must be known. This study presents and discusses temperature frequency characteristics (TFCs) of solid hexamethyldisiloxane (HMDSO) polymer coated sensor resonators using the Rayleigh surface acoustic wave (RSAW) mode on ST-cut quartz. Using a RF-plasma polymerization process, RSAW sensor resonators optimized for maximum gas sensitivity have been coated with chemosensitive HMDSO films at 4 different thicknesses: 50, 100, 150 and 250 nm. Their TFCs have been measured over a (−100 to +110) °C temperature range and compared to the TFC of an uncoated device. An exponential 2,500 ppm downshift of the resonant frequency and a 40 K downshift of the sensor’s turn-over temperature (TOT) are observed when the HMDSO thickness increases from 0 to 250 nm. A partial temperature compensation effect caused by the film is also observed. A third order polynomial fit provides excellent agreement with the experimental TFC curve. The frequency downshift due to mass loading by the film, the TOT and the temperature coefficients are unambiguously related to each other.