Lowering Detection Limits Toward Target Ions Using Quasi-Symmetric Polymeric Ion-Selective Membranes Combined with Amperometric Measurements

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 three-compartment 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

Detection of 3′-End RNA Uridylation with a Protein Nanopore

Post-transcriptional modifications of the 3′-ends of RNA molecules have a profound impact on their stability and processing in the cell. Uridylation, the addition of uridines to 3′-ends, has recently been found to be an important regulatory signal to stabilize the tagged molecules or to direct them toward degradation. Simple and cost-effective methods for the detection of this post-transcriptional modification are not yet available. Here, we demonstrate the selective and transient binding of 3′-uridylated ssRNAs inside the β barrel of the staphylococcal α-hemolysin (αHL) nanopore and investigate the molecular basis of uridine recognition by the pore. We show the discrimination of 3′-oligouridine tails on the basis of their lengths and propose the αHL nanopore as a useful sensor for this biologically relevant RNA modification

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

Electrically conducting polymers (ECPs) are one of the most popular types of materials to interface ion-selective membranes (ISMs) with electron-conducting substrates to construct solid-contact ion-selective electrodes (SCISEs). For optimal ion-to-electron transduction and potential stability, the p-doped ECPs with low oxidation potentials such as PPy need to be generally in their conducting form along with providing a sufficiently hydrophobic interface to counteract the aqueous layer formation. The first criterion requires that the ECPs are in their oxidized state, but the high charge density of this state is detrimental for the prevention of the aqueous layer formation. We offer here a solution to this paradox by implementing a highly hydrophobic perfluorinated anion (perfluorooctanesulfonate, PFOS^–) as doping ion by which the oxidized form of the ECP becomes hydrophobic. The proof of concept is shown by using polypyrrole (PPy) films doped with PFOS^– (PPy-PFOS) as the solid contact in K⁺-selective SCISEs (K⁺-SCISE). Prior to applying the plasticized poly­(vinyl chloride) ISM, the oxidation state of the electrodeposited PPy-PFOS was adjusted by polarization to the known open-circuit potential of the solid contact in 0.1 M KCl. We show that the prepolarization results in a hydrophobic PPy-PFOS film with a water contact angle of 97 ± 5°, which effectively prevents the aqueous layer formation under the ISM. Under optimal conditions the K⁺-SCISEs had a very low standard deviation of <i>E</i>⁰ of only 501.0 ± 0.7 mV that is the best <i>E</i>⁰ reproducibility reported for ECP-based SCISEs