MEMBRANE SELECTION AND ISFET CONFIGURATION EVALUATION FOR SOIL NITRATE SENSING

Abstract

Successful implementation of site-specific crop management relies on accurate quantification of spatialvariation of important factors. Data collection on a finer spatial resolution than is feasible with manual and/or laboratorymethods is often required but cost prohibitive. Therefore, there is a need for the development of sensors to moreaccurately characterize within-field variability. The objective of this research was to investigate matrix membranesproduced from different combinations of ligand and plasticizer materials using ion-selective electrode (ISE) technology,and to use selected membranes to develop a nitrate ion-selective field effect transistor (ISFET) which might be integratedwith a flow injection analysis (FIA) system for real-time soil analysis. Several ion-selective membranes were tested, andall of the evaluated membranes proved to be viable candidates for the development of a nitrate ISFET. Membranes usingmethyltridodecylammonium chloride (MTDA) as the ligand showed a better response to nitrates at low concentrationswhile those using tetradodecylammonium nitrate (TDDA) ligand showed superior selectivity for the nitrate ion. A multi-ISFETnitrate sensor was successfully developed. The electrical responses of the ISFETs were consistent and predictable.While significant difficulty was found in preparing a multi-ISFET chip with all four sensors operational, once prepared,the multi-ISFET chips were reliable and performed through extensive tests without failure. The sensitivities of the nitrateISFETs (38-46 mV/decade) were lower than the theoretical Nernst sensitivity. The nitrate ISFETs proved to be viablesensors for the development of a real-time soil nitrate analysis system, under the conditions of our tests.