Advanced quartz crystal microbalance techniques applied to calixarene sensing membranes

• Main Author: Holloway, Alan Franciszek
• Other Authors: Nabok, Aleksey ; Ray, Asim ; Siddiqi, Jawed ; Crowther, David ; Thompson, M.
• Published: Sheffield Hallam University 2005
• Subjects: 661.807
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.421344

Several Quartz Crystal Microbalance (QCM) measurement techniques in conjunction with a series of calix[4]resorcinarene sensing membranes have been successfully exploited for the detection of volatile organic solvents at vapour concentrations below their lower explosive level. The impedance analysis technique involves the measurement of the electrical properties of the QCM around the resonant frequencies of crystal. Subsequent fitting of the measured spectra to an equivalent circuit allows parameters directly related to mass loading and the mechanical properties (viscosity) of the film to be obtained. An experimental setup which allows the real time in situ extraction of these parameters has been developed. It has been shown that unique changes in mass loading and the films viscoelastic properties caused by the adsorption of target vapours into a calix[4]resorcinarene C15H31 sensing membrane can be detected. In some cases this facilitates both the detection and discrimination of target vapours using a single QCM sensing element. The changes in the films mechanical properties are believed to be caused by capillary condensation of vapours at values below saturated vapour pressure inside the nano-porous calix[4]resorcinarene film matrix. The work is extended by the use of the sensor array technique. In the first instance frequency only measurements are used. Four QCM have been coated with calix[4]resorcinarene compounds with different hydrocarbon chain lengths and exposed to range of organic vapours. The variation in chain length produces selectivity between the sensing membranes, and leads to the classification of all the tested organic vapours using a feed forward multilayer Artificial Neural Network. The trained network successfully classified over 98% of the test data. The additional measurement of film dissipation using impedance analysis/QCMD shows interesting phenomena. An unexpected increase in mechanical stiffness of the film is observed for small chain length C[4]RA compounds (CH3) on vapour sorption. A speculative model has been proposed relating the chain length and effective cavity size to the observed phenomena. An alternative low cost multi parameter measurement set up has also been developed using the QCMD principle. The crystal is driven from an external oscillatory source and subsequently disconnected. The resonant frequency and dissipation factor can be extracted from the decaying sinusoid signal. This approach eliminates the need for expensive network analysers. An additional multiplexing circuit has been combined with the QCMD technique and allows both the frequency and dissipation factor of several crystals to be measured in pseudo real time. This makes the system ideally suited for multi parameter array measurements. The basis for a discriminative explosive vapour sensor based on calix[4]resorcinarene membranes has been investigated and promising results for future development have been obtained. The exact adsorption mechanisms are however complex and althoughspeculative models have been proposed, further research is suggested to fully characterize the complete adsorption process and the mechanical changes taking place within the film.