A simple nitrate sensor for distributed in-situ soil and water nitrate monitoring

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Carol L. Baumbauer, Margaret E. Payne, Derek Wong and Ana C. Arias, Electrical Engineering and Computer Science, University of California Berkeley, Berkeley, CA, Payton J. Goodrich, Mechanical Engineering, University of California Berkeley, Berkeley, CA

Background/Question/Methods
Plant-available nitrogen, often in the form of nitrate, is an essential nutrient for plant growth, but excessive nitrate in the environment and watershed has harmful impacts on both human health and natural ecosystems. Current nitrate measurements techniques, in both soil and water quality monitoring, involve taking samples from the environment or field to a lab, where they can be analyzed with chromatography or spectrographic methods. Such measurements are highly accurate, but they are also expensive and labor-intensive, and give data for only one point in time and space. A distributed network of nitrate sensors could better quantify and monitor nitrogen in agriculture and the environment.
Here, we demonstrate a printed solid-state potentiometric nitrate sensor. This sensor consists of a working electrode and reference electrode, which are printed on flexible substrates and functionalized with polymeric membranes. The potential voltage difference between the working and reference electrode is a function of the logarithm of the nitrate concentration. Printing, which encompasses a range of solution-based processing techniques, enables simple large-scale manufacturing. This means that a large number of nitrate sensors could be distributed throughout a field site to map nitrate concentrations in time and space.
Results/Conclusions
An inkjet printed gold working electrode with a PVC membrane including a nitrate ionophore was optimized for sensor sensitivity and selectivity. In solution, the working electrode shows a voltage change of -53.3±1.1 mV for every factor of 10 increase in nitrate concentration (noted as “mV/decade”) from 3.2 to 1240 ppm nitrate, for solutions of NaNO3, KNO3, and NH4NO3. This sensitivity is near the theoretical limit governed by the Nernst equation. A set of definitive screening design experiments was used to demonstrate that common interfering species P2O5, K+, Mg2+, Cl−, NO2− and SO42−do not significantly interfere with the sensor at concentrations expected in soil. A printed reference electrode was developed with a membrane containing PVB, NaCl, and NaNO3, which provides a stable reference potential in varying ionic solutions. The sensitivity of the sensors remained above 48 mV/decade for at least three months.
The sensor does not require an on-board power supply and its output is simply a voltage, making it easy to integrate with wireless read out systems.