Membrane selection and ISFET configuration evaluation for soil nitrate sensing

Successful implementation of site-specific crop management relies on accura te quantification of spatial variation of important factors. Data collectio n on a finer spatial resolution than is feasible with manual and/or laborat ory methods is often required but cost prohibitive. Therefore, there is a n eed for the development of sensors to more accurately characterize within-f ield variability. The objective of this research was to investigate matrix membranes produced from different combinations of ligand and plasticizer ma terials using ion-selective electrode (ISE) technology, and to use selected membranes to develop a nitrate ion-selective field effect transistor (ISFE T) which might be integrated with a flow injection analysis (FIA) system fo r real-time soil analysis. Several ion-selective membranes were tested, and all of the evaluated membranes proved to be viable candidates for the deve lopment of a nitrate ISFET membranes using methyltridodecylammonium chlorid e (MTDA) as the ligand showed a better response to nitrates at low concentr ations while those using tetradodecylammonium nitrate (TDDA) ligand showed superior selectivity for the nitrate ion. A multi-ISFET nitrate sensor was successfully developed. The electrical responses of the ISFETs were consist ent and predictable. While significant difficulty was found in preparing a multi-ISFET chip with all four sensors operational, once prepared, the mult i-ISFET chips were reliable and performed through extensive tests without f ailure. The sensitivities of the nitrate ISFETs (38-46 mV/decade) were lowe r than the theoretical Nernst sensitivity. The nitrate ISFETs proved to be viable sensors for the development of a real-time soil nitrate analysis sys tem, under the conditions of our tests.