Screen-printed ion selective electrodes for soil ion detection

Abstract

Rural Indian farmers lack an available, affordable, usable, and actionable means to diagnose soil health, which is a prerequisite for judicious application of fertilizer. Although government labs and mobile kits are available, these fail to meet the aforementioned qualities; as a result fewer than 5% of rural Indian farmers have ever performed a soil test. This thesis details the design and characterization of a point-of- use soil testing device, comprising a set of disposable ion-selective electrode (ISE) strips and a handheld electrochemical reader, which together transduce soil analyte concentrations into a numeric input for a mobile-phone based fertilizer recommendation engine. As a proof-of-concept, potassium (K+) selective electrodes are studied, because K+ is essential to plant growth and reproduction. The strips employ a simple two electrode system, wherein carbon nanotube and silver chloride inks are the electrode contacts, plasticizer-free butyl acrylate ion selective membranes are located on the working electrodes, and sodium-chloride doped polyvinyl butyral membranes provide stable reference potentials. The electrodes were fabricated by a combined screen printing and drop casting process, and the protocol, extraction solution, and reference potential stability were tuned to maximize ISE performance and repeatability. Using a mathematical model and supporting experiments, we verified that hydrogen selectivity determines the detection limit, and furthermore, that scientists can tune the ionophore-ionic site ratio to minimize this effect. Using the final electrode fabrication process, a set of 20 full ISE devices in the presence of 0.02M SrCl2 extraction solution were shown to have sub-uM detection limits and near-Nernstian average sensitivity (61 mV/decade). When compared to ICP data for a set of 30 soil samples using the final reader, the sensors demonstrated a 0.898 correlation factor. Workshops in India were conducted to determine user needs and corresponding product attributes, as well as a "usability index" for the system assessed across farmer demographics. The favorable system performance proves that solid-state ISEs are a promising technology for soil diagnostics in resource-constrained environments.