Nafion Protective Membrane Enables Using Ruthenium Oxide Electrodes for pH Measurement in Milk

Abstract

The application of conventional glass electrodes for pH measurement in food samples has a serious drawback: glass is fragile and should be handled with care in order to prevent breaking and thus contaminating the food with dangerous shattered fragments. The implementation of all-solid-state sensors allows for pH measurements without this contamination risk but their application in food samples is scarce due to their inability to be used in complex food matrices that contain fats, proteins, yeasts, etc. We can solve this problem by coating solid-state RuO2 electrodes with a semi-permeable protective NafionTM membrane layer. We show that covering screen printed potentiometric RuO2 electrodes with Nafion membrane using a drop-casting technique does not alter the performance of the electrodes in milk samples and provides similar results to the conventional glass electrode. Furthermore, we discovered that thermal treatment of Nafion-layers at 80 °C for 2 h after each layer deposition optimises the performance of the electrodes, makes them usable even in heated aqueous solutions, and increases their lifetime.