A microfluidic-chip-based system for the determination of nutrient ion concentrations in hydroponic solutions by means of ion-selective electrodes (ISEs)

Abstract:
Within this paper microfluidic-chip-based measuring technology is introduced, based on miniaturized solid contact liquid membrane ion-selective electrodes, implemented on an electronic board for direct potentiometric measurement of different ion types in aqueous solution. Ion-selective electrodes are prepared on top of gold electrodes on an electronic board, forming double-layered films, consisting of a graphite ion-to-electron conversion layer and a PVC layer containing an ionophore, being specific for a certain type of ion. The electronic board is covered with a microfluidic chip, connecting the hydroponic solution to the different ion-selective electrodes. This procedure allows for minimized sample volumes, producing low waste and for a compact setup, suitable for installation on site of a hydroponic plant. Ions under investigation were ammonium, nitrate, calcium, potassium and phosphate. For all ion types suitable ionophores were found and tested. The measured potential differences against a commercial standard silver/silverchloride (Ag/AgCl) electrode show a nearly Nernstian behaviour with a slope close to the theoretical limit. NO3 --ISEs were tested in mixed solutions of K3PO4/NH4NO3(aq) (constant ratio of ions), showing that the sensitivity for NO3 --concentration is identical for K3PO4/NH4NO3(aq) solutions and pure KNO3(aq) solutions, but the offset is shifted by -15.4 mV.