Mass-producible disposable needle-type ion-selective electrodes for plant research

Easily mass-producible needle-type Na⁺ and K⁺ ion-selective electrodes (ISEs) were developed for the direct and indirect measurement of Na⁺ and K⁺ ion concentrations in live plants. A polyimide strip with a silver layer on one side and Ag/AgCl formed at one end was used to construct two types of ISEs. For the type I ISE, an electrolyte layer was formed on the layer of silver and Ag/AgCl, which was then covered with a protecting layer. Subsequently, an ion-selective membrane (ISM) was formed at the truncated end with Ag/AgCl. For the type II ISE, a syringe needle was used as a container and an ISM was formed at the sharp end. Then, the polyimide strip with Ag/AgCl at one end was inserted and an electrolyte solution was injected to complete the ISE. Reference electrodes (REs) with similar structures were fabricated by forming a liquid junction instead of the ISM. The electrode responses and the relationship between the ISE potential and the Na⁺/K⁺ ion concentration agreed with those predicted by the Nernst equation. The Na⁺ and K⁺ ion concentrations in different parts of the rice plant (Oryza sativa L.) were measured using the Na⁺ and K⁺ ISEs, respectively. The results obtained using these devices agreed well with those obtained using inductively coupled plasma atomic emission spectrometry, thus confirming the practical applicability of the developed ISEs.