Real-time detection of organic compounds in liquid environments using polymer-coated thickness shear mode quartz resonators

The selection of sensitive coatings is a critical task in the design and im plementation of chemical sensors using coated thickness shear mode quartz c rystal resonators (QCRs) for detection in liquid environments, This design or selection is performed through a study of the sorption process in terms of the partition coefficients of the analytes in the coatings, The partitio n coefficient, which is controlled by the chemical and physical properties of the coating materials, determines the inherent selectivity and sensitivi ty toward analyte molecules. The selection of the coatings is logically det ermined by the interactions between coating and target analyte molecules, b ut can also be made through a systematic variation of the coating's propert ies. The determination of the partition coefficients is only accurate if al l contributions to the total measured frequency shifts, Delta f(s), of the coated QCR can be established. While mass loading is often assumed to be th e dominant factor used in determining partition coefficients, viscoelastic effects may also contribute to Delta f(s). Both the effect of viscoelastic properties and the effect of mass loading on the sensor responses are inves tigated by using a network analyzer and oscillator circuit and by character izing the total mechanical impedance of the loaded sensor. Different types of coatings including rubbery and glassy polymers are investigated, and the targeted analytes include classes of polar compounds (methanol), nonpolar compounds (toluene, xylenes), and chlorinated hydrocarbons (trichloroethyle ne, tetrachloroethylene, etc). It is seen that changes in viscoelastic prop erties due to analyte sorption may be significant enough to place the senso r in the nongravimetric regime. However, for most applications involving th e detection of relatively low concentrations of organic compounds and the u se of acoustically thin films, changes in the complex shear modulus of the coatings contribute less than 546 of the total shift in the series resonant frequency, depending on the coating. In that case, the measured Delta f(s) and, hence, the calculated sensitivity/partition coefficients can still be used for an approximate classification and selection of the coatings for o peration in a complex solution of water/analyte molecules.