Two-Electrode Screen-Printed pH Sensors for Monitoring Soil and Other Growing Media

Monitoring soil pH is important for optimizing plant biomass production, understanding nutrient cycling, predicting and mitigating greenhouse gas emissions, and improving soil health. Soil pH can vary significantly over time and space, and high-spatiotemporal-resolution data are needed to enable informed land management decisions. A potential method to address this need is to use large numbers of easily distributed pH sensor systems that can be placed at high density in soils. An approach for fabricating such sensors is described here using a voltammetric screen-printed pH sensor with a working electrode that incorporates Alizarin (a pH-sensitive dye) throughout its bulk. The use of a Nafion membrane on top of the working electrode and a salt membrane over the reference electrode provides near-Nernstian sensitivity and enhanced device stability in buffered media, soil, and hydroponic fluid. Critically, these devices can function effectively in a novel 2-electrode (working and reference) configuration with a sensitivity of 56.38 mV/pH and a resolution of 0.28 pH (n =3), enabling the use of simple, low-cost readout electronics for evaluating the sensor state. These results show that a printed pH sensor paired with affordable electronics can be used to monitor the pH of unbuffered media, providing a potential path to high-spatial density real-time measurement of pH in soils and hydroponics.