Ionic liquid-based reference electrodes for miniaturized ion sensors: What can go wrong?

Ionic liquid-based reference electrodes, especially those containing 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (MeOctIm TFSI), are increasingly used in potentiometric measurements. They replace the conventional salt bridges and establish the reference potential through local partitioning of the ions of the ionic liquid between the reference membrane and the sample solution. Unless the electrochemical cell and the measurement protocol are designed appropriately, these ions can interfere with the response of ion-selective electrodes (ISEs) with polymeric membranes. This work characterizes the effect of MeOctIm TFSI on the response of K⁺, Na⁺, and Cl⁻ ISEs with polymeric membranes. The leaching of MeOctIm TFSI from the reference membrane to the sample solution was monitored with UV–vis spectroscopy. The concentration of MeOctIm TFSI in the aqueous phase increased gradually and plateaued at approximately 1.0 mM. Concentrations as low as 100 μM of MeOctIm TFSI caused large changes to the emf (50–150 mV) of K⁺, Na⁺, and Cl⁻ ISEs. The presence of 10 μM of TFSI⁻ was enough to cause Donnan failure of K⁺ ISEs with valinomycin as ionophore (that is, co-extraction of K⁺ and TFSI⁻ into the sensing membrane). Use of less lipophilic anions such as tetrafluoroborate (BF₄-) or triflate (OTf-) postponed the onset of Donnan failure of cation-selective ISEs to higher concentrations of the anion, but decreased the stability in the reference potential and lifetime of the reference electrode. These results imply that although MeOctIm TFSI-based reference electrodes provide sample-independent and stable electrical potentials, they should be used with caution for measurements with polymeric-membrane ISEs, due to strong interference of both MeOctIm⁺ and TFSI⁻ with measured values of emf.