Real-Time Detection of Organic Compounds in Liquid Environments Using Polymer-Coated Thickness Shear Mode Quartz Resonators

Abstract

The selection of sensitive coatings is a critical task in the design and implementation of chemical sensors using coated thickness shear mode quartz crystal 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 partition coefficient, which is controlled by the chemical and physical properties of the coating materials, determines the inherent selectivity and sensitivity toward analyte molecules. The selection of the coatings is logically determined by the interactions between coating and target analyte molecules, but can also be made through a systematic variation of the coating's properties. The determination of the partition coefficients is only accurate if all contributions to the total measured frequency shifts, deltafs, of the coated QCR can be established. While mass loading is often assumed to be the dominant factor used in determining partition coefficients, viscoelastic effects may also contribute to deltafs. Both the effect of viscoelastic properties and the effect of mass loading on the sensor responses are investigated by using a network analyzer and oscillator circuit and by characterizing 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 (trichloroethylene, tetrachloroethylene, etc). It is seen that changes in viscoelastic properties due to analyte sorption may be significant enough to place the sensor in the nongravimetric regime. However, for most applications involving the detection of relatively low concentrations of organic compounds and the use of acoustically thin films, changes in the complex shear modulus of the coatings contribute less than 5% of the total shift in the series resonant frequency, depending on the coating. In that case, the measured deltafs and, hence, the calculated approximate classification and selection of the coatings for operation in a complex solution of water/analyte molecules.

Analysis of Liquid-Phase Chemical Detection Using Guided Shear Horizontal-Surface Acoustic Wave Sensors

“…[1][2][3][4][5] Following successful application in gas sensing, liquid sensors attracted considerable attention due to the need for real-time, rapid, and direct detection in liquid environments where the device is in direct contact with the solution for applications such as the detection of biochemical warfare agents and environmental contaminants. [6][7][8][9][10] Various types of acoustic wave devices have been developed for operation in liquids, including thickness shear mode (TSM), shear horizontal acoustic plate mode, shear horizontal surface acoustic wave (SH-SAW), and flexural plate wave devices. Significant challenges exist in effectively implementing acoustic wave devices as chemical sensors in liquid environments.…”

Kinetic study of silica gels by a new rheological ultrasonic investigation

“…According to Eq. (5) the viscoelastic parameters and the mass effect modify the frequency shift too [15][16][17].…”

Electrostatic gas sensor with a porous silicon diaphragm

“…QCR sensors, which are coated with different polymers or are etched to produce micropores into the surface of the resonator, have been studied intensively in relation to the detection of organic vapors, humidity, and organic compounds in liquids [18][19][20]. According to the operation principle of these sensors, the resonant frequency of quartz crystal is shifted because the molecules of the target gas are absorbed into the coating material or the porous structure.…”