Organophosphate pesticides (OPs) have been found as contaminants in surface and ground waters, soil, and agricultural products. Because OPs are toxic compounds, rapid detection/monitoring of OPs in groundwater is necessary to allow for real-time remediation. Detection of OPs in water has already been demonstrated using poly(epichlorohydrin) [PECH] and polyurethane as the sensing layers. However, the response times were relatively long, hindering real-time monitoring. In this work, a hybrid organic/inorganic chemically sensitive layer [bisphenol A-hexamethyltrisiloxane (BPA-HMTS)] that shows a high degree of partial selectivity for OPs is synthesized, characterized (in terms of the glass transition temperature, Tg, water stability, sensitivity, selectivity, detection limit, and absorption/response time) for the rapid detection of organophosphate pesticides. Direct chemical sensing in aqueous solutions is performed using guided shear horizontal surface acoustic wave sensor platforms on 36° rotated Y-cut LiTaO3 and 42.75° rotated Y-cut Quartz, respectively. It is shown that, for the same coating thickness, a 60% reduction in sensor response time is achieved without reduction in sensitivity compared to PECH. Considering the Tg, for the polymers, it is seen that the faster response shown by BPA-HMTS is due to the porous siloxane backbone, HMTS. Kinetic studies for the absorption of OPs (parathion-methyl, parathion, and paraoxon) from aqueous solutions into the BPA-HMTS coating are conducted. The data are analyzed within the context of two absorption models: penetration-limited and diffusion-limited absorptions. It is shown that the absorption process is rate limited by penetration with a concentration independent absorption time constant or mass transfer coefficient. The absorption time constants for parathion-methyl, parathion, and paraoxon are calculated. A limit of detection of 60, 20 and 100 μg/L (ppb) for parathion-methyl, parathion, and paraoxon, respectively, is calculated for the present non-optimized sensor. Concentrations as low as 500 μg/L (ppb) parathion are actually measured. This is much lower than the typical concentrations found on agricultural produce (≥ 10 ppm).
Furthermore, sensor signal analysis in the form of the extended Kalman filter (EKF) is employed on-line during the detection process. The sensor response was first represented by a state-space model which includes all relevant contributions to the polymer-coated device response. This allows for the steady-state response and absorption time constant to be extracted on-line well before the steady-state is reached, thus further reducing the time for analyte identification and quantification. It is noted the absorption time constant, often unique to a class of analyte-coating pairs, can be used to improve analyte recognition
