Near Real-Time Monitoring of Organophosphate Pesticides in the Aqueous-Phase Using SH-SAW Sensors Including Estimation-Based Signal Analysis

The sensor response times for the absorption of organophosphate pesticides (phosmet and parathion) from aqueous solution into partially selective coatings [poly(epichlorohydrin) (PECH) and polyurethane (PU)], are investigated using guided shear horizontal surface acoustic wave (SH-SAW) devices on LiTaO₃ as the sensing platform. A study of the response time (absorption time constant) reveals that it is possible to decrease the sensor response time by increasing temperature and/or decreasing film thickness. However, these approaches reduced device sensitivity. A second approach involving nonlinear estimation-based sensor signal analysis is also investigated in an attempt to decrease the time required for identification and quantification without decreasing sensitivity. Specifically, the extended Kalman filter is employed for online analysis of the sensor data during the detection process. To achieve this, 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 sensor response and absorption time constant to be extracted online well before the steady-state is reached, thus reducing the time required for quantification. Extracting the absorption time, which is often unique to a class of analyte-coating pairs, will make it possible to improve analyte recognition in sensor array design.