Multiplex measurement system for the detection of nutrients in hydroponics using ion-selective electrodes

This paper presents a microfluidic-cartridge-based measurement technology utilizing miniaturized solid-contact liquid membrane ion-selective electrodes (ISEs) for application in hydroponics. The ion-selective electrodes are integrated on printed circuit boards (PCBs) for direct potentiometric analysis of various ions in aqueous solutions. The electrodes are fabricated on top of the PCBs gold electrodes, creating double-layer films, comprising an ion-to-electron conversion layer and a polyvinylchloride (PVC) layer, containing a specific ionophore for target ion detection. A microfluidic cartridge is affixed to the electronic board, facilitating the fluidic connection of hydroponic solutions to the different electrodes. This approach minimizes sample volume, reduces waste, and enables a compact setup that is ideal for on-site deployment in hydroponic systems. Within qualification in the laboratory, a total recovery of 96.9 % (potassium), 90.6 % (hydrogen phosphate), 83.3 % (ammonium), 92.7 % (nitrate), and 92.7 % (calcium), with corresponding mean errors of 7.8 % (potassium), 4.4 % (hydrogen phosphate), 4.2 % (ammonium), 5.5 % (nitrate), and 16.0 % (calcium), could be achieved. For the first time, it could be shown that our system allows on-site measurements within a hydroponic greenhouse, with excellent correlation of the expected values for ammonium, nitrate, calcium, potassium, and (hydrogen)phosphate concentrations.