Article pubs.acs.org/ac
Advantages and Limitations of Reference Electrodes with an Ionic Liquid Junction and Three-Dimensionally Ordered Macroporous Carbon as Solid Contact Tiantian Zhang,†,‡ Chun-Ze Lai,‡ Melissa A. Fierke,‡ Andreas Stein,‡ and Philippe Bühlmann*,‡ †
College of Chemical Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu, Sichuan 610065, China Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
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S Supporting Information *
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.
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depend on minor variations of the salt bridge introduced more than hundred years ago.2,6 Only few approaches to this problem were proposed. It was suggested that hydrophobic anion-exchanger membranes loaded with the polyanion heparin can be used as pseudoreference electrodes because heparin passively diffuses at a low rate from those ion exchanger membranes into the sample, effectively exhibiting a poor detection limit for heparin.7,8 As a result, the potential drop across the interface of the sample and the hydrophobic membrane is sample-independent and well defined. Similarly, it was proposed not to separate the reference electrode and the sample with an aqueous salt bridge but, instead, to use water-immiscible ionic liquids, or hydrophobic polymeric membranes doped with ionic liquids9−12 or salts of various hydrophilicities.13−16 All these approaches differ from the conventional salt bridge by the fact that the interface of sample and reference electrode is not an interface of two miscible electrolyte solutions but, instead, an interface between the aqueous sample and a water-
he poor performance of reference electrodes is an often overlooked cause of error in electroanalysis1 and can seriously limit the lifetime of electrochemical sensor systems. A conventional reference electrode contacts the sample through a salt bridge filled with an aqueous electrolyte solution.2,3 Ideally, the liquid junction potential at such an interface of two miscible solutions is dominated by the high concentration of ions in the salt bridge and is, therefore, sample-independent. However, salt bridges can become contaminated by sample ions and get clogged by proteins and lipids, which causes slow responses and erratic liquid junction potentials.4,5 A limiting factor for the use of salt bridges is also the eventual loss of the electrolyte into the sample, in particular in the case of miniaturized devices or when contamination of the sample with salt bridge ions interferes with the detection of target ions at very low concentrations. Such problems can be counteracted by frequent flushing of the reference electrode with cleaning solutions, use of free-flowing liquid junctions, and regular replenishing of salt bridges.3 However, these procedures are cumbersome, even in a routine laboratory setting, and can be difficult to implement in remote long-term monitoring. Nevertheless, surprisingly little progress has been made to improve the performance of reference electrodes, and commercially available reference electrodes still © 2012 American Chemical Society
Received: May 12, 2012 Accepted: August 17, 2012 Published: August 17, 2012 7771
dx.doi.org/10.1021/ac3011507 | Anal. Chem. 2012, 84, 7771−7778
Analytical Chemistry
Article
immiscible (hydrophobic) organic phase loaded with ions that slowly but continuously leach into the sample. In the case of the ionic liquids and the salt-doped polymeric membranes as reference electrodes, the ionic liquid9−12,17 (or the salt with which the polymeric membrane is doped)13−16,18 is soluble to some extent in water. This establishes a local equilibrium distribution of the ionic liquid (or salt) between the two phases at the interface between the hydrophobic (reference) membrane and the sample, which can be described as follows: KIL =
[I+]mem [L−]mem [I+]aq [L−]aq
guarantee a satisfactory performance of these reference electrodes. Moreover, it reports the observation of zero-current transmembrane ion fluxes that can disturb the functioning of these reference electrodes unless they are properly addressed by the use of inner solid contacts or appropriately formulated inner filling solutions.
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EXPERIMENTAL SECTION Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF− HFP), with an average molecular weight of Mw = 455 000; ratio of hexafluoropropylene monomers
