Rapid detection of organophosphates in aqueous solution using a hybrid organic/inorganic coating on SH-SAW devices

Rapid detection of organophosphates pesticides (OPs) in groundwater is necessary to allow for real-time monitoring and cleanup. Detection of OPs in the liquid phase has already been demonstrated using poly(epichlorohydrin) [PECH] and polyurethane as the sensing layer. However, the response times are relatively long, on the order of hours. In this work, a hybrid organic/inorganic chemically sensitive layer [bisphenol A-hexamethyltrisiloxane (BPA-HMTS)] is synthesized and investigated for the rapid detection and analysis of organophosphate pesticides. Direct chemical sensing in aqueous solutions is performed using the guided shear horizontal surface acoustic wave sensor platform on 36° rotated Y-cut LiTaO₃. It is shown that, for the same coating thickness, a 60% reduction in sensor response time is achieved without a significant reduction in sensitivity when compared with PECH. Considering the glass transition temperature, T_g, for the polymers, it is seen that the faster sensor response exhibited by the BPA-HMTS coating is due to the porous siloxane backbone, HMTS. Furthermore, sensor signal analysis in the form of the extended Kalman filter (EKF) is employed on-line during the detection process. This allows for the steady-state sensor response and absorption time constant to be extracted on-line well before equilibrium, thus further reducing the time required for analyte identification and quantification. 500 μg/L of parathion has been detected and a limit of detection of 20 μg/L (ppb) for parathion and 100 μg/L (ppb) of paraoxon is reported for the present non-optimized sensor.