title: Solid contact ion-selective electrodes with a well-controlled Co(II)/Co(III) redox buffer layer.
abstract: Solid contact ion-selective electrodes (ISEs) typically have an intermediate layer between the ion-selective membrane and the underlying solid electron conductor that is designed to reduce the irreproducibility and instability of the measured electromotive force (emf). Nevertheless, the electrode-to-electrode reproducibility of the emf of current solid contact ISEs is widely considered to be unsatisfactory. To address this problem, we report here a new method of constructing this intermediate layer based on the lipophilic redox buffer consisting of the Co(III) and Co(II) complexes of 1,10-phenanthroline ([Co(phen)3](3+/2+)) paired with tetrakis(pentafluorophenyl)borate as counterion. The resulting electrodes exhibit 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 selection of a balanced ratio of the reduced and oxidized species has typically been difficult and was often ignored, contributing to the 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 10-fold increase in the ratio of the reduced and oxidized redox buffer species. Use of a gold electrode modified with a self-assembled 1-hexanethiol monolayer as underlying electron conductor suppresses the formation of a water layer and results in an electrode-to-electrode standard deviation of E° of 1.0 mV after 2 weeks of exposure to KCl solution.
authors.affiliation: Department of Chemistry, University of Minnesota , 207 Pleasant St. SE, Minneapolis, Minnesota 55455, United States.
authors.forename: Xu U
authors.initials: XU
authors.lastname: Zou
authors.forename: Jia H
authors.initials: JH
authors.lastname: Cheong
authors.forename: Brandon J
authors.initials: BJ
authors.lastname: Taitt
authors.forename: Philippe
authors.initials: P
authors.lastname: Bühlmann
mesh_terms: D003035:Cobalt / Q000737:chemistry*; D004566:Electrodes; D006046:Gold / Q000737:chemistry; D007477:Ions / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010618:Phenanthrolines / Q000737:chemistry; D011143:Polyvinyl Chloride / Q000737:chemistry*; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D007477:Ions; D009942:Organometallic Compounds; D010618:Phenanthrolines; D013438:Sulfhydryl Compounds; D014867:Water; D003035:Cobalt; D006046:Gold; D011143:Polyvinyl Chloride; C025205:1,10-phenanthroline