Solid-Contact Ion-Selective Electrodes Comprising Hydrophobic Redox Buffers

Abstract

A calibration-free measurement with an ionophore-doped polymeric membrane ion-selective electrode requires that the phase boundary potential at the sample/sensing membrane interface is controlled by the activity of the target ion in the sample of interest, while all other phase boundary potentials in the electrochemical cell are constant and long term stable. Historically, the biggest difficulty lies in establishing a reproducible phase boundary potential at the interface of the sensing membrane and the underlying electron conductor. Efforts over several decades to use conducting polymers as an interlayer between the ion-selective membrane and an underlying electron conductor, such as a metal or carbon, have had limited success. While the performance of such devices has been much improved in terms of light sensitivity and hydrophobicity of the conducting polymer layer, devices that can be considered calibration-free are still elusive. To that end, hydrophobic redox buffers have been introduced as an alternative to conducting polymers. While hydrophilic redox buffers play central roles in all living organisms, control many geological and environmental processes, and are often utilized in the laboratory, buffering of redox potentials in hydrophobic media is a topic that has in the past been overlooked. This presentation will address principles for the use of hydrophobic redox buffers, and it will discuss recent examples of hydrophobic redox buffers suitable for the fabrication of ion-selective electrode membranes that are calibration-free. (1) Redox Buffer Capacity of Ion-Selective Electrode Solid Contacts Doped with Organometallic Complexes, Zhen, X. V.; Rousseau, C. R.; Buhlmann, P. Anal. Chem., 2018, 90, 11000-11007. (2) Paper-Based All-Solid-State Ion-Sensing Platform with a Solid Contact Comprising Colloid-Imprinted Mesoporous Carbon and a Redox Buffer, Hu, J.; Zhao, W.; Bühlmann, P.; Stein, A., ACS Appl. Nano Mat. 2018, 1, 293–301.