Potentiometric Nitrate Sensors with Inkjet Printed Nafion Films

Nitrates are a key nutrient for crop growth in agriculture, but excessive use of fertilizers can cause nitrate leaching, resulting in contamination of groundwater and damage to the environment. Thus, there is a need for a fast, reliable, and accurate system for the detection of nitrates in soil. Drop-on-demand (DoD) inkjet printing is a mask-free printing technique that allows for the deposition of solution-processable materials onto a substrate using a piezoelectric actuator, resulting in greater control and precision for patterning sensors whilst minimizing costs at the same time. While printed electrochemical sensors offer great advantages for the in-situ detection of nitrates, chloride leaching from silver/ silver chloride (Ag/AgCl) reference electrodes remains a key issue that affects sensor stability.

In this work, we have developed an inkjet-printed potentiometric nitrate sensor that consists of a reference electrode encapsulated with Nafion, and a working electrode coated with a nitrate ion-selective membrane (ISM). The Nafion film prevents chloride ions from leaching out of the reference electrode, making the sensor more reliable for nitrate detection. First, we developed a Nafion ink through rheological modification that is suitable as a protective layer for Ag/AgCl reference electrodes on Kapton substrates. We optimized the jetting waveform of the printing nozzle, the drop spacing, and the number of layers required to obtain a uniform and consistent Nafion film across the reference electrode. A 20-layer print of a 10% Nafion and 1-propanol solution at 15 μm drop spacing was experimentally determined to be the optimum Nafion film for nitrate sensing.

Next, the performance of the sensor with printed Nafion protective films was compared to sensors with dropcast Nafion films and bare Ag/AgCl electrodes by carrying out open-circuit potentiometry. The printed Nafion reference electrode showed greater stability in different pH conditions than its dropcast and bare electrode counterparts, having a variation of open-circuit potential of only 1.68 mV/pH unit, compared to 3.27 mV/pH unit and 1.91 mV/pH unit for dropcast and bare electrodes, respectively. The printed Nafion electrodes also performed better in terms of nitrate sensitivity compared to dropcast Nafion electrodes, having an average sensitivity of -58.31 mV/dec, which is close to the theoretical Nernst limit. Finally, Energy Dispersive X-ray (EDX) spectroscopy showed that both the dropcast and the printed Nafion films exhibited significantly lower chloride leaching compared to bare Ag/AgCl electrodes, thus facilitating the electrodes to remain stable for a longer time in nitrate solutions.

In conclusion, this work shows that the usage of Nafion films as a protective layer in reference electrodes of potentiometric sensors enables precise and accurate measurements of nitrate concentration in aqueous environments. Additionally, there are other benefits in terms of spatial control in patterning for mass production of sensors. In particular, the stability and sensitivity of sensors made with printed Nafion films make inkjet printing a lucrative option for the fabrication of potentiometric nitrate sensors. These sensors are cost- and resource-efficient, enabling future use in agricultural and environmental applications for measuring nitrate concentrations in soil.