Low-cost in-field determination of soil ion concentration using a portable 3D printed device based on ion-selective electrodes and textile threads

Population keeps growing so as the need for food production. The increase in energy prices is putting a lot of pressure in energy intensive industrial processes such as the production of fertilizers. Farmers need to fine-tune the amount of fertilizer needed by the soil, so that they do not add in excess, elevating costs and polluting the environment, or do not fall short, suffering sub-optimal crop yields.

This work reports the fabrication and characterization of a low-cost device for the continuous monitoring of the concentration of plant nutrients based on ion-selective electrodes and textile threads that work in direct contact with soils. Here, as proof of concept, we developed a thread-based, microfluidic sensor platform. We utilized traditional polymer membrane-based ion-selective electrodes (ISEs) for potassium, nitrate, ammonium and pH were drop-casted directly on top of a miniaturized, 3D-printed holder. Electrical contact is established via graphite-based contacts link to the electrochemical signal reader via electrical wires. The sensor platform was enhanced by the addition of five 30 cm long textile threads connected to an absorption pad on the opposite side. This is the key innovation as these threads mimic the roots and via capillary action wick the moisture from the soil to the sensing area. The entire sensor platform contained 4 ISEs for each chemical species and one reference electrode and was encased into a 3D printed housing. The device is placed next to the soil that is going to be analysed inserting the threads in the soil sampling area.

Preliminary results show that thread-based sensor system is reproducible and consistently provides a near-Nernstian sensitivity of 55±5 and 50±3 for potassium, -58±1 and -63±2 for nitrate, and 60±1 and 60±12 (mV/decade) for ammonium between 2.8x10^-6 and 1.3x10^-2 M without (directly in solution) and with textile threads respectively. Analysis of soil samples with different soil moisture content (100%, 75%, 50% and 40%) using our low cost device gave a correlation coefficient of R² = 0.91 for potassium and R² = 0.92 for ammonium when compared to the values measured using traditional methods such as inductively coupled plasma optical emission spectroscopy (ICP-OES) and flow injection analysis (FIA), respectively. The promising performance of this low-cost device is encouraging towards its use as an extended network to measure soil ion concentration at high temporal and spatial resolution.