Calibration-Free Ionophore-Based Ion-Selective Electrodes With a Co(II)/Co(III) Redox Couple-Based Solid Contact

Abstract

A high electrode-to-electrode reproducibility of the emf response of solid contact ion-selective electrodes (SC-ISEs) requires a precise control of the phase boundary potential between the ion-selective membrane (ISM) and the underlying electron conductor. To achieve this, we introduced previously ionophore-free ion exchanger membranes doped with a well controlled ratio of oxidized and reduced species of a redox couple as redox buffer and used them to make SC-ISEs that exhibited highly reproducible electrode-to-electrode potentials. Unfortunately, ionophores were found to promote the loss of insufficiently lipophilic species from the ionophore-doped ISMs into aqueous samples. Here we report on an improved redox buffer platform based on equimolar amounts of the much less hydrophilic Co(III) and Co(II) complexes of 4,4'-dinonyl-2,2'-bipyridyl, which makes it possible to extend the redox buffer approach to ionophore-based ISEs. For example, K(+)-selective electrodes based on the ionophore valinomycin exhibit electrode-to-electrode standard deviations as low as 0.7 mV after exposure of freshly prepared electrodes for 1 h to aqueous solutions. Exposure of freshly prepared ISE membranes to humidity prior to their first contact to electrolyte solution minimizes the initial (reproducible) emf drift. This redox buffer has also been successfully applied to sodium, potassium, calcium, hydrogen, and carbonate ion-selective electrodes, which all exhibit the high selectivity over interfering ions as expected for ionophore-doped ISE membranes.

Pre-Polarized Hydrophobic Conducting Polymer Solid-Contact Ion-Selective Electrodes with Improved Potential Reproducibility

“…The selectivity coefficients ( + pot K ,J log K ) of the K + -SCISEs for most relevant interfering ions in biological samples was found to be between -4.5 to -5.6 (except of J=NH 4 + ) as shown in Table S-2. 72 The optimized K + -SCISEs showed in the best case a very good E 0 reproducibility of only 501.0±0.7 mV (n=4) with a SD that is equal to that obtained for the redox buffer based SCISEs (±0.7 mV) 55 and which surpasses the SDs of the state of the art ECP based SCISEs (usually ca. ±20 mV).…”

“…Indeed, recently it was shown that the standard potential of ECP-based SCISEs can be adjusted by polarizing or short cutting the respective SCISEs against a reference electrode. 54 However, the potentials of the SCISE are usually drifting after these treatments and the reproducibility of the standard potential lags much behind the state of the art SCISEs having 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 redox buffer 55 and graphene/carbon black-fluorinated acrylic copolymers 42 as SC with standard deviations (SD) in the best case as low as 0.7 mV and 0.1 mV, respectively.…”

All-Solid-State Reference Electrodes Based on Colloid-Imprinted Mesoporous Carbon and Their Application in Disposable Paper-based Potentiometric Sensing Devices

“…The reference membranes are doped with the ionic liquid 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide ([C 8 min + ][C 1 C 1 N -]) to define the potential at the reference membrane/sample interface, 19 as well as a hydrophobic redox buffer consisting of [Co(C 9 ,C 9 -bipy) 3 ](TPFPB) 2 and [Co(C 9 ,C 9 -bipy) 3 ](TPFPB) 3 to define the potential at the CIM carbon/reference membrane interface. 35 Benefiting from the high capacitance of the interface between the CIM carbon and the reference membrane, outstanding potential stability is observed, with a potential drift as low as 1.7 µV/h over 110 h, making CIM carbon-based reference electrodes the most stable all-solid-state reference electrodes reported so far. To demonstrate their compatibility with miniaturized potentiometric systems, CIM carbon-based reference electrodes are integrated into disposable paper-based Cl -sensing devices to replace the conventional Ag/AgCl reference electrodes, eliminating the reference electrolyte and the associated liquid junction potentials.…”

Highly hydrophobic solid contact based on graphene-hybrid nanocomposites for all solid state potentiometric sensors with well-formulated phase boundary potentials

“…Although the incorporation of Ag + ion carrier into ISM [24] can fix the phase boundary at ISM/silver epoxy-based CS interface via reversible redox reaction, namely AgL + + e M Ag + L (where L refers to the Ag + ion carrier), the selectivity of the ISM containing mixed ionophores is challenging. SCs consisting of a redox couple, e.g., mixed-valent gold nanoclusters [25], cobalt [26,27], or osmium salts [28,29], have also been proposed for providing thermodynamically defined phase boundary potentials.…”