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Lowering Detection Limits Toward Target Ions Using QuasiSymmetric Polymeric Ion-Selective Membranes Combined with Amperometric Measurements Xénia Nagy and Lajos Höfler* Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szt. Gellért tér 4, Budapest, 1111, Hungary
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ABSTRACT: An amperometric method is reported that compensates for the interference from marginally discriminated interfering ions when using traditional polymeric ion-selective membrane (ISM) electrodes. The concept involves utilizing two ISMs in a threecompartment electrochemical cell configuration. The two ISMs are identical in composition except for the addition of an ionophore to one of the membranes. Initially, all three compartments contain the same concentration of interfering ion and the membrane does not contain primary ions. Reference electrodes are placed into each of the two outer compartments. At this point, there is no potential difference between the two reference electrodes. We show experimentally and theoretically that, when the concentration of an interfering species is increased in the sample compartment, the phase-boundary potentials of both sample solution|ISMs change similarly. However, when the primary ion is added to the sample, an asymmetry will emerge, and the membrane with the ionophore will exhibit a larger phase-boundary potential change. At low concentrations, the difference in membrane potentials can be too small for reliable potentiometric detection. Current, which can be routinely measured on pA levels, can be used instead to detect the small primary ion concentration changes with a significant lowering of detection limits. The theory of this method is described by Nernst-Planck-Poisson finite element simulations, and both amperometric and potentiometric experimental verification is demonstrated using ammonium ISM. It is shown that amperometric measurements enable 200 nM ammonium to be detected in the presence of 0.1 mM of potassium, detection capability that is not possible via conventional potentiometry.
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Currently, use of classical potentiometric measurements with available ammonium membrane electrodes would make the above biomedical applications impossible. However, application of current has been extensively researched to achieve enhanced analytical properties for ion-selective membranes. For instance, external current has been used, coupled with classical potentiometry, ,to neutralize the trans-membrane flux of primary ions, resulting in a lower detection limit. 7−9 Monitoring the trans-membrane flux of ions leads to an important direction in potentiometry, the backside calibration potentiometry, where the disappearance of concentration gradients across the membrane is used to determine the sample composition by the knowledge about the solution composition at the backside of the membrane.10−12 Direct amperometric13−17 and coulometric18−21 detection of ions with ion-selective membranes has been also reported in the literature. One major advantage of the amperometric methods over classical potentiometry is that current can be routinely measured on pA levels. According to Ohm’s law, sufficiently large currents are expected when the resistance of the ion-
n recent decades, many ionophores with high selectivity for their preferred ion have been reported for use in polymeric ion-selective membrane electrodes. With the utilization of these ionophores, selective, subnanomolar detection of many ions became possible with ion-selective electrodes (ISEs) with little or no sample preparation.1−3 For several biologically significant ions, such as chloride, fluoride, nitrate, and ammonium, available ionophores often lack the required selectivity to provide adequate sensitivity when significant levels of interfering ions are present. For example, direct detection of elevated blood levels of ammonium can indicate liver dysfunction, such as cirrhosis or hepatitis.4 Unfortunately, the selectivity of the classical ammonium ionophores (e.g., nonactin) over potassium are less than 2 orders of magnitude because of the very similar radii of the ammonium and potassium ions.5 Indeed, the high levels of potassium in blood interfere with the potentiometric detection of ammonium using nonactin-based membrane electrodes. Further, there is a need for more selective detection of ammonium in environmental samples as well. Ammonium can be found in groundwater due to anthropogenic sources, such as sewage and agricultural wastes; detecting elevated levels of ammonium is critical for identifying the source of the pollution and limiting the environmental impact.6 © 2016 American Chemical Society
Received: August 5, 2016 Accepted: September 16, 2016 Published: September 16, 2016 9850
DOI: 10.1021/acs.analchem.6b03043 Anal. Chem. 2016, 88, 9850−9855
Article
Analytical Chemistry
were mounted between the compartments of the cell (Figure 1) after overnight evaporation of THF. Experimental Setup. Measurements were carried out in a three-compartment poly(methyl methacrylate) cell setup (Figures 1 and S1). The left and the right compartments, each containing 3 mL of solution, were separated by membranes from the sample solution (3 mL). Thicknesses of the membranes were calculated from the disk weight to be 80 μm, and the diameter in direct contact with the solutions was 9 mm. Both outer compartments contained Ag/AgCl electrode. Electrochemical Measurements. Potentials were monitored by a Lawson Laboratories 16 channel EMF meter (Pennsylvania, USA). Currents were registered by an Autolab PGSTAT10 amperometric workstation (ECO Chemie, Netherlands). The sampling frequency in both cases was 1 Hz. The galvanic cell was Ag/AgCl/left solution (0.1 mM KCl)/ISE membrane with ionophore/sample solution/ISE membrane without ionophore/right solution (0.1 mM KCl)/AgCl/Ag. Electrochemical impedance spectroscopy measurements (Gamry Instruments, Reference 600, Warminster, PA, USA) were used to assess the bulk resistances of the membranes. The impedance spectra were recorded in the frequency range of 1 MHz to 1 Hz by using a sinusoidal excitation signal with amplitude of 10 mV and DC potential of 0 V. All compartments contained 10 mM KCl.
selective membrane (ISM) is
