All-solid-state reference electrodes based on colloid-imprinted mesoporous carbon and their application in disposable paper-based potentiometric sensing devices.

Reference electrodes are used in almost every electroanalytical measurement. Here, all-solid-state reference electrodes are described that employ colloid-imprinted mesoporous (CIM) carbon as solid contact and a poly(vinyl chloride) reference membrane to contact the sample. Such a reference membrane is doped with a moderately hydrophilic ionic liquid and a hydrophobic redox couple, leading to well-defined constant potentials at the interfaces of this membrane to the sample and to the solid contact, respectively. Due to the intrinsic properties of CIM carbon, reference electrodes with a CIM carbon solid contact exhibit excellent resistance to common interfering agents such as light and O2, with outstanding potential stability in continuous potentiometric measurements. The potential drift of CIM carbon-based reference electrodes without redox couple is as low as 1.7 µV/h over 110 h, making them the most stable all-solid-state reference electrodes reported so far. To demonstrate the compatibility of CIM carbon-based reference electrodes with miniaturized potentiometric systems, these reference electrodes were integrated into paper-based potentiometric sensing devices, successfully replacing the conventional reference electrode with its reference electrolyte solution. As a proof of concept, disposable paper-based Cl(-) sensing devices that contain stencil-printed Ag/AgCl-based Cl(-) selective electrodes and CIM carbon-based reference electrodes were constructed. These sensing devices are inexpensive, easy to use, and offer highly reproducible Cl(-) measurements with sample volumes as low as 10 µL.

Paper-based potentiometric ion sensing.

This paper describes the design and fabrication of ion-sensing electrochemical paper-based analytical devices (EPADs) in which a miniaturized paper reference electrode is integrated with a small ion-selective paper electrode (ISPE) for potentiometric measurements. Ion-sensing EPADs use printed wax barriers to define electrochemical sample and reference zones. Single-layer EPADs for sensing of chloride ions include wax-defined sample and reference zones that each incorporate a Ag/AgCl electrode. In EPADs developed for other electrolytes (potassium, sodium, and calcium ions), a PVC-based ion-selective membrane is added to separate the sample zone from a paper indicator electrode. After the addition of a small volume (less than 10 µL) of sample and reference solutions to different zones, ion-sensing EPADs exhibit a linear response, over 3 orders of magnitude, in ranges of electrolyte concentrations that are relevant to a variety of applications, with a slope close to the theoretical value (59.2/z mV). Ion-selective EPADs provide a portable, inexpensive, and disposable way of measuring concentrations of electrolyte ions in aqueous solutions.

Calibration-free ionophore-based ion-selective electrodes with a Co(II)/Co(III) redox couple-based solid contact.

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.

Ion-selective electrodes with colloid-imprinted mesoporous carbon as solid contact.

A new type of solid-contact ion-selective electrode (SC-ISE) has been developed that uses colloid-imprinted mesoporous (CIM) carbon with 24 nm diameter, interconnected mesopores as the intermediate layer between a gold electrode and an ionophore-doped ISE membrane. For a demonstration, valinomycin was used as K(+) ionophore, and a good Nernstian response with a slope of 59.5 mV/decade in the range from 10(-5.2) to 10(-1.0) M was observed. The high purity, low content of redox-active surface functional groups and intrinsic hydrophobic characteristics of CIM carbon prepared from mesophase pitch lead to outstanding performance of these sensors, with excellent resistance to the formation of a water layer and no interference caused by light, O2, and CO2. When a redox couple is introduced as an internal reference species, calibration-free SC-ISEs can be made with a standard deviation of E° as low as 0.7 mV. Moreover, the interconnected mesopore structure of ISE membrane-infused CIM carbon facilitates both ion and electron conduction and provides a large interfacial area with good ion-to-electron transduction. Because of the large double layer capacitance of CIM carbon, CIM carbon-based SC-ISEs exhibit excellent potential stability, as shown by chronopotentiometry and continuous potentiometric measurements. The capacitance of these electrodes as determined by chronopotentiometry is 1.0 mF, and the emf drift over 70 h is as low as 1.3 µV/h, making these electrodes the most stable SC-ISEs reported so far.

Solid contact ion-selective electrodes with a well-controlled Co(II)/Co(III) redox buffer layer.

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.

Current pulse based reference electrodes without liquid junctions.

Reference electrodes with hydrophobic ion-doped polymeric membranes offer a promising alternative to reference electrodes with conventional salt bridges. This letter introduces the current pulse operation of liquid junction free reference electrodes. A brief current pulse is applied to a cation exchanger membrane, releasing a well-controlled amount of cations from the membrane into the sample and, thereby, determining the boundary potential of the sample/reference membrane interface. Measurements with ion-selective electrodes (ISEs) are performed relative to these reference electrodes immediately after the current pulse and exhibit the same Nernstian responses as typical for potentiometry, which is demonstrated with Cl(-) ISE measurements. The use of current pulse based reference electrodes avoids the sample sensitivity and clogging of conventional salt bridges and the often poorly controlled loss of bridge electrolyte into samples. Current mode measurements in serum over 2 days showed a much higher stability of the reference potential than conventional potentiometric measurements with the same ion-doped polymeric membranes.

Cyanide-selective electrode based on Zn(II) tetraphenylporphyrin as ionophore.

Receptors that exhibit high selectivity are essential for potentiometric cyanide sensors. Therefore, CN­ binding to metallotetraphenylporphyrins with different metal centers (i.e., Co(II), Co(III), Zn(II), Ni(II), Cu(II), and Fe(III)) was investigated. All these metalloporphyrins were found to function as neutral ionophores. Co(III) and Fe(III) tetraphenylporphyrins with their positive charges seemed likely to bind up to two axial CN­ ligands, but only the Co(III) porphyrin was found to strongly bind a second CN­ ligand. The electrode membranes doped with Zn(II) tetraphenylporphyrin provided the highest selectivity over chloride (logK(CN­,Cl­)(pot) = −3.71, as opposed to −0.36 for an ionophore-free ISE) and were optimized by adjusting the site-to-ionophore ratio to achieve the highest CN­ selectivity, with special consideration of interfering ions present in gold mining applications. The Zn(II) tetraphenylporphyrin-based CN(­)-selective electrodes exhibited the best discrimination of OH­; no pH effect was observed even at pH 11 (logK(CN­,OH­)(pot) = −3.42). The response slopes and unbiased selectivities of the ionophore-free and the ionophore-based electrodes with 25 mol % and 71 mol % cationic sites relative to ionophore showed that the Zn(II) tetraphenylporphyrin forms a 1:1 complex with the target ion CN­ and 2:1 and 1:1 complexes with the interfering ions OH­ and S(2­), respectively. The CN­ binding constant was 2.3 × 10(6) (mol/kg)(­1), which is slightly bigger but of the same order of magnitude as for binding of Zn(II) tetraphenylporphyrin to CN­ in dichloroethane.