Towards mass-production of ion-selective electrodes by spotting: Optimization of membrane composition and real-time tracking of membrane drying

Abstract

Solid-contact ion-selective electrodes present tools for rapid assessment of charged species within a vast number of samples. Commercially available point-of-care electrochemical sensing systems are most often produced by screen-printing. However, this method is not applicable to polymeric membrane deposition, due to the mismatch of the membrane physical characteristics to those intended for screen printing. Herein, an industrial automated dispensing machine was used for fast and controlled deposition of small volumes of ion-selective membrane. Compared to the previously published literature, the dry solute membrane content was not altered. Rather, we optimized the solvents for the ISM preparation, thus significantly lowering the required overall number of membrane deposition steps. It was shown that dry membrane uniformity strongly depends on the solvent carrier system, with the best uniformity for the membrane prepared in a solvent mixture consisting of equal volumes of THF and cyclohexanone. The volume of a single membrane deposit (spot) was estimated based on colorimetric absorbance measurements to be 0.20 μL. Already with a single spot of the ion-selective membrane, an adequate potentiometric response to potassium ions was obtained, supporting the efficiency of the production technique. Evaporative membrane drying was investigated for the first time using time-resolved impedance spectroscopy, giving useful information on the influence of the solvent composition to the characteristics of the evaporative time profiles. With this approach, the overall production and drying of the described devices takes less than 30 min.