Plasticized Polymer Coatings for SH-SAW Sensors for High Sensitivity and Long-Term Monitoring of BTEX Analytes in Liquid Phase

BTEX compounds (benzene, toluene, ethylbenzene, and xylene) are constituents of crude oil and hazardous to human health. Among them, benzene has the lowest maximum contaminant level for drinking water because of its carcinogenicity. Spills or leakage from underground storage tanks or hazardous waste sites can contaminate nearby groundwater with these volatile organic compounds. Therefore, it is very important to detect the presence of BTEX contamination as early as possible in order to start the remediation process and maintain a healthy environment. To develop an in-situ continuous monitoring sensor system, shear horizontal surface acoustic wave (SH-SAW) sensor devices are being investigated and have shown promising results with the use of suitable coatings for BTEX analyte sorption. However, commercially available polymers that can be used as suitable coatings for BTEX detection directly in the aqueous phase are limited in sensitivity and long-term stability. To improve the sensitivity of a suitable polymer, the addition of a plasticizer is a convenient means to lower the glass transition temperature and thus increase sensitivity. The best coatings for acoustic-wave chemical sensors will be those which are rubbery in the low-frequency range, resulting in good analyte sorption, but glassy at the operating frequency of the sensor device, resulting in low acoustic-wave attenuation. Plasticized polymer coatings allow adjustment of the shear modulus of the coating by varying the polymer-plasticizer mixing ratio; this enables the use of thicker coatings with larger analyte sorption capacity and, ultimately, higher sensitivity. This work investigates polymer-plasticizer blends as sensor coatings for detection of BTEX in water at low concentrations (parts per billion range). Two polymers and two plasticizers were studied. For each polymer-plasticizer combination, the influence of the mixing ratio of the blend on the sensor response was investigated. The sensitivity to benzene for each polymer-plasticizer blend was compared with commercially available polymers that had been used for BTEX detection in previous work. The highest sensitivity and lowest detection limit for benzene were found for a 1.25 μm-thick sensor coating of 17.5% diisooctyl azelate-polystyrene. This work demonstrates that by varying type of plasticizer, mixing ratio and coating thickness, the properties of the coating can be conveniently tailored for BTEX analyte sorption. Thus, the addition of plasticizers increases significantly the number of suitable coatings available for use with a single sensor or a sensor array