A new fibre microfluidic soil pore water sampling device for NH4+-N sensing using ion-selective electrode sensors (ISEs)

Several climate change scenarios predict extreme precipitation and irrigation, leading to saturated soil conditions. In this paper, we present a new fibre microfluidic device coupled to ion-selective electrode sensors (ISEs) to sense soil ammonium-nitrogen (NH₄⁺-N) under these saturated soil conditions. The strength of fibre microfluidics in ISE sensors lies in its ability to integrate electrochemical sensing with microfluidic fluid control in a flexible, miniaturized format. This technology enables miniaturization, flexibility, integrated microfluidic control for enhanced ionic selectivity, improved stability and longevity, as well as scalable and cost-effective manufacturing. The ISEs were applied to monitor NH₄⁺-N concentrations in soil pore water, which were drawn by the deployed fibre. The water wicked by the microfluidic fibre passed through an array of NH₄⁺-N ISE ionophores for real-time sensing over six days. The water was also collected for laboratory analysis of NH₄⁺-N through colourimetry to assess the ISE sensing performance. Our results indicate that the calibration slopes of the fibre microfluidic ISEs, ranging from 45.80 to 60.40 ​mV per decade, are generally acceptable, as the theoretical slope ideally stands at 59 ​mV per order of magnitude. Our sensor can be used to for real-time monitoring of soil NH₄⁺-N levels in fertilized grassland and arable soils over four to six days after installation. The fibre microfluidic ISE overestimated soil NH₄⁺-N concentrations, with deviations ranging from −61% to 248% in grassland soil and −80%–370% in arable soil. This significant range of deviation may be attributed to soil particles wicked by the microfluidic fibre, which subsequently adhered to the sensor membrane. The ISE readings were compared with the soil pore water NH₄⁺-N concentrations determined by colourimetry and the measured values were found to be within similar concentration ranges; however, there was high variability between ISE results and the directly measured soil pore water. Whilst real time responses are more variable, it nevertheless points to the highly dynamic nature of soil nitrogen cycling. Therefore, the technology has the potential for further miniaturization and fine tuning to assist optimizing soil fertilizer use for crop production while preventing environmental pollution through the avoidance of excessive fertilizer application.