Investigation of sensing membranes for QCM devices in gas sensing applications

The standard Quartz Crystal Resonator (QCR) and network analysis based methods in conjunction with curve fitting were used to investigate the sensing capability and characterize the properties of phthalocyanine films on vapour exposure. The measurement of frequency shift and resistance change (mass loading and film damping), caused by adsorption of organic vapour namely, Benzene, Hexane, Ethanol and Toluene were investigated. Confirmation of film properties using supplementary methods such as AFM, Ellipsometry and UV-visible spectrometer was also performed to provide a full characterization of the sensing membranes. The extracted values of Δƒ and ΔR from subsequent fitting of the spectra to the BVD model are observed on vapour exposure. A frequency shift (Δƒ) and change in magnitude (as related to ΔR of the BVD equivalent circuit) indicate changes in the films viscoelastic properties for the increasing concentrations of tested vapours. The sensitivity of the coating has been estimated from the slope of fitted trend line and gives values below LEL thresholds (the Lower explosive limit) and IDLH thresholds (Immediately Dangerous to Life or Health) for the ZnPcs films. The experimental results of the study demonstrate selected sensing membranes are easily applied through spin coating techniques evident from definitive shifts in resonance. Additionally when exposed to the target vapours tested, the film(s) exhibit fast and consistent responses, consequently giving significant potential for gas/vapour sensing applications. Changes in the film parameters have also been observed through the measurement of the admittance spectra. Shifts in both frequency and resistance are observed on exposures which indicate mass loading and changes in film viscosity caused by ad/absorption of the vapour. Response times appear to be quick and full recovery is observed. From the tested vapours, toluene gives the most significant frequency shift exhibiting the highest sensitivity for this compound; this can be attributed to relatively high saturated vapour pressure as compared to the other analytes. In addition, the film parameters extracted from this work were used to estimate the shear modulus parameters. It was found the shear modulus of viscous material (coating film) extracted electrical equivalent circuit parameters are dependent on film properties, thickness and analyte ad/absorption. Consequently, the QCR sensor can act iv as a gravimetric and non gravimetric sensitive device for thin film depending on load and adsorption characteristics. In most instances the studied film behaviour demonstrates a rubbery regime that was indicated from increase in resistance for the coating film at series resonant frequency typically. Consequently the calculation of change in film mass from frequency shift (Sauerbrey equation) is inaccurate except for suitably thin rigid films. A range of Phthalocyanine sensing membranes have been successfully evaluated; selected variants (mainly ZnPc) have given promising results to their viability as gas sensing membrane to detect a range of organic solvents at vapour concentrations below their lower explosive level, It was found suitably sensitive with detection limits in the low parts-per million ranges for the selected analytes. Furthermore, a comparison of gas sensor responses for the selected materials is included, and consequently a particular type of substituent is proposed as a suitable sensor coating for Quartz Crystal Resonator (QCR) gas sensor applications. Other phthalocyanine materials initially chosen proved less successful; demonstrating limited responsiveness to analytes ad/absorption and giving inconsistent results over the tested concentration range. Factors range from non-homogenous film surfaces to the structure and consequent suitability of the synthesised film(s). Moreover, further research is suggested to fully characterize the complete adsorption process with wide range of phthalocyanine material and various organic analytes.