Lipophilic Redox Buffers for Polymeric Solid-Contact Electrodes

Abstract

A calibration-free measurement with an ion-selective membrane requires that only the phase boundary potential at the sample/sensing membrane interface is variable, i.e., either dependent on the activity of the target ion in the sample, as in a potentiometric experiment, or determined by an applied potential, as in a voltammetric experiment. All the other phase boundary potentials in the electrochemical cell are constant. Unfortunately, one of the biggest challenges in the fabrication of devices comprising solid-contact membranes has been to establish a reproducible phase boundary potential at the interface of the hydrophobic ion-selective membrane and the underlying electron conductor. This talk will address our use of ion-selective membranes doped with lipophilic redox buffers, both in view of potentiometry and voltammetry. Solid contact ion-selective electrodes (ISEs) typically have an intermediate layer between the ion-selective membrane and the underlying solid electron conductor that reduces the irreproducibility and instability of the measured electromotive force (emf). Nevertheless, the electrode-to-electrode reproducibility of the emf of current solid-contact electrodes in a zero-current measurement is widely considered to be unsatisfactory. To address this problem, we reported on lipophilic redox buffers consisting of the Co(III) and Co(II) complexes with hydrophobic ligands such as 1,10-phenanthroline (e.g., [Co(phen)3]3+/2+) paired with tetrakis(pentafluorophenyl)borate as counterion [1,2]. The resulting electrodes exhibit zero-current emf values with an electrode-to-electrode standard deviation as low as 1.7 mV after conditioning of freshly prepared electrodes for 1 h. While many prior examples of solid contact ISEs also used intermediate layers that contained redox-active species, the importance of selecting a balanced ratio of the reduced and oxidized species has typically been difficult and was often ignored, contributing to emf irreproducibility. The ease of the control of the [Co(phen)3]3+ / [Co(phen)3]2+ ratio explains the high emf reproducibility, as confirmed by the emf decrease of 58 mV per tenfold increase in the ratio of the reduced and oxidized redox buffer species. The result is an electrode-to-electrode standard deviation of Eº of 0.7 mV after 1 week of exposure to KCl solution. The use of this redox buffer in combination with colloid-imprinted mesoporous carbon [3,4] will also be discussed. [1] Zou, X. U.; Cheong, J. H.; Taitt, B. J.; Bühlmann, P., Anal. Chem. 2013, 85, 9350–9355. [2] Zou, X. U.; Zhen, X. V.; Cheong, J. H.; Bühlmann, P. Anal. Chem., 2014, 86, 8687–8692. [3] Hu, J.; Ho, K. T.; Zou, X. U.; Smyrl, W. H.; Stein, A.; Bühlmann, P. Anal. Chem., 2015, 87, 2981–2987 [4] Hu, J.; Stein, A.; Bühlmann, P. Angew. Chem. Int. Ed., in press (DOI: 10.1002/anie.201603017).