Advantages and Limitations of Reference Electrodes with an Ionic Liquid Junction and Three-Dimensionally Ordered Macroporous Carbon as Solid Contact

Abstract

Liquid-junction-free reference electrodes that contact the sample through an ionic-liquid-doped, hydrophobic polymer membrane have attracted attention because they offer an alternative to reference electrodes with conventional salt bridges. In this work, liquid-junction-free reference electrodes were developed using plasticized poly(vinyl chloride) membranes doped with the ionic liquid (IL) 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide. Three-dimensionally ordered macroporous (3DOM) carbon substrates infused with this ionic liquid phase were used as solid contacts for these reference membranes. As in prior work with ionophore-based 3DOM carbon-contact ion-selective electrodes, the long-term stability of the liquid-junction-free reference electrodes was excellent, with potential drifts as low as 42 μV/h over 26 days. Successful measurements of pH in milk were performed and, to the best of our knowledge, are the first example of the use of liquid-junction-free reference electrodes in complex real-life samples. A thorough analysis of their performance at low pH revealed protonation of the ionic liquid anion (L(-)) and formation of LHL(-) type of associates in the reference electrode membrane, effects not observed in prior work. Also, when reference membranes were mounted into conventional electrode bodies with inner filling solutions that contained no ionic liquid ions, zero-current ion fluxes across the sample/membrane interface occurred, as previously only seen for ionophore-doped ion-selective membranes. Understanding these effects will be crucial to the design of liquid-junction-free reference electrodes suitable for other applications.

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

“…This affects the phase boundary potential at the reference membrane-sample interface, as we have reported previously. 28 This effect, combined with fluctuations in the stir rate, may have affected the noise level of the data shown in Figure 1. To eliminate this effect and make it possible to study the influence of CO 2 on the CIM carbon-reference membrane interface, a phosphate buffer (pH = 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 7.5) was used, stabilizing the pH of the sample solution.…”

“…The 2 mm diameter gold disk electrodes (CH Instruments, Austin, TX) were polished over polishing cloths with aqueous dispersions of alumina (0.3 and 0.05 µm, Buehler, Lake Bluff, IL). They were cleaned by ultrasonication in water and ethanol Precursor solutions for reference membranes were prepared by dissolving in 2 mL of freshly distilled THF 60 mg the ionic liquid [C 8 min + ][C 1 C 1 N -], 120 mg of PVC as polymeric matrix, and 120 mg of o-NPOE as plasticizer, as described by Zhang et al 28 Moreover, 1.4 mmol/kg each of [Co(C 9 ,C 9 -bipy) 3 ](TPFPB) 2 and [Co(C 9 ,C 9 -bipy) 3 ](TPFPB) 3 was added to this solutions as an internal reference. The precursor solutions were stirred for 2 h to ensure complete dissolution.…”

Reference Electrodes with Salt Bridges Contained in Nanoporous Glass: An Underappreciated Source of Error

“…1 Limiting factors for the use of these salt bridges are the loss of electrolyte into the sample and contamination of the salt bridge by sample components. 2 To minimize these effects and to obtain low and stable junction potentials, porous plugs filled with electrolyte solution are often used, separating the reference from the sample solution while maintaining electrical contact between the two. These porous plugs must have a low electrical resistance while avoiding excessive transfer of electrolyte and sample components through the junction.…”

Properties and Customization of Sensor Materials for Biomedical Applications

“…As novel designs for reference electrodes that, for example, do not rely on an inner-filling solution, are much needed, IL related developments emerging from materials science research may offer advantages. For instance, Cicmil et al (115,116) reported on the use of ILs in polyvinyl chloride (PVC) matrices for the preparation of disposable reference electrodes in which the partitioning of the ions into the liquid phase defines the interfacial potential. After comparing several ILs, the authors found that reference electrodes prepared with 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [C 2 mim][NTf 2 ] were least sensitive to changes in the concentration of common alkali metal chlorides.…”