Application-specific ISFET-based measurement ap-proaches for nutrient analysis in soil and hydroponic systems

Ion-selective field-effect transistors (ISFETs) have great potential as microsensors for the real-time monitoring of ions in aqueous solutions, such as pH value and nutrients in extracts or nutrient solutions. This article presents application-specific examples of ISFET-based measurement systems that have been implemented in two agricultural applications: in-field measuring of soil nutrients and determining nutrient concentration in hydroponic systems. To this end, commercially available ISFETs have been incorporated into multi-sensor modules (lab-on-chip) or as standalone sensors to detect nutrients available to plants, such as H₂PO₄⁻, NO₃⁻, K⁺, NH₄⁺ and pH. In addition, relevant temperatures were measured for calibration purposes. The ISFETs used were selected according to the application's specific requirements in terms of measurement dynamics. A laboratory test bench was set up for validation purposes. Using suitable reference measurements, the sensors' output signals were analysed in the laboratory with regard to stability against disturbance variables, reproducibility, and transient behaviour. These findings were then incorporated into the development of new readout electronics. In addition to long-term stability, the conditioning and calibration procedures of the ISFET were identified as important factors. A baseline drift correction in the form of continuous single-point calibration was also developed and successfully implemented. ISFET-based measurement systems were integrated into newly developed, application-specific mechatronic handling systems and process sequences. This article presents the implementation and experimental validation of these systems, including the first measurement results from field trials and indoor hydroponic systems.