Recent work in the field of ionophore‐based ion‐selective electrodes (ISEs) has greatly improved our understanding of the thermodynamics and kinetics that describe the response and selectivity of these sensors. These efforts resulted in the lowering of detection limits from micro‐ to subpicomolar concentrations, improvements of selectivities by many orders of magnitude, and major advancements in biocompatibility and long‐term stabilities. This chapter summarizes the current state of the art for an audience that is new to the field, introducing the basic concepts of ISE theory that replaced in recent years the empirical approach of the early ISE history. It illustrates, with specific examples, the design principles of host‐guest chemistry that have been used to develop ionophores for ISEs, and describes with a minimum of equations the recently developed sophisticated concepts for the efficient use of these ionophores in ISEs. Importantly, this chapter shows not only how ionophores are used in modern potentiometry to develop new ISEs but also illustrates how ionophore‐based potentiometry can support host‐guest chemistry by providing tools to determine thermodynamic properties of ionophores, such as stoichiometries and stabilities of their complexes, using only a minimum amount of ionophore.
Ion-selective electrodes (ISEs) with fluorous anion-exchanger membranes for the potentiometric detection of perfluorooctanoate (PFO(-)) and perfluorooctanesulfonate (PFOS(-)) were developed. Use of an anion-exchanger membrane doped with the tetraalkylphosphonium derivative (Rf8(CH2)2)(Rf6(CH2)2)3P(+) and an optimized measurement protocol resulted in detection limits of 2.3 × 10(-9) M (1.0 ppb) for PFO(-) and 8.6 × 10(-10) M (0.43 ppb) for PFOS(-). With their higher selectivity for PFO(-) over OH(-), membranes containing the alternative anion exchanger (Rf6(CH2)3)3PN(+)P((CH2)3Rf6)3 with a bis(phosphoranylidene)ammonium group further improved the detection limit for PFO(-) to 1.7 × 10(-10) M (0.070 ppb). These values are comparable with results obtained using well-established techniques such as gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and liquid chromatography-tandem mass spectrometry (LC-MS-MS), but the measurement with ISEs avoids lengthy sample preconcentration, can be performed in situ, and is less costly. Even when eventual spectrometric confirmation of analyte identity is required, prescreening of large numbers of samples or in situ monitoring with ISEs may be of substantial benefit. To demonstrate a real-life application of these electrodes, in situ measurements were performed of the adsorption of PFOS(-) onto Ottawa sand, which is a standard sample often used in environmental sciences. The results obtained are consistent with those from an earlier LC-MS study, validating the usefulness of these sensors for environmental studies. Moreover, PFOS(-) was successfully measured in a background of water from Carnegie Lake.
Manganese(III) complexes of three fluorophilic salen derivatives were used to prepare ion-selective electrodes (ISEs) with ionophore-doped fluorous sensing membranes. Because of their extremely low polarity and polarizability, fluorous media are not only chemically very inert but also solvate potentially interfering ions poorly, resulting in a much improved discrimination of such ions. Indeed, the new ISEs exhibited selectivities for CO32− that exceed those of previously reported ISEs based on non-fluorous membranes by several orders of magnitude. In particular, the interference from chloride and salicylate was reduced by two and six orders of magnitude, respectively. To achieve this, the selectivities of these ISEs were fine-tuned by addition of non-coordinating hydrophobic ions (i.e., ionic sites) into the sensing membranes. Stability constants of the anion–ionophore complexes were determined from the dependence of the potentiometric selectivities on the charge sign of the ionic sites and the molar ratio of ionic sites and the ionophore. For this purpose, a previously introduced fluorophilic tetraphenylborate and a novel fluorophilic cation with a bis(triphenylphosphoranylidene)ammonium group, (Rf6(CH2)3)3PN+P(Rf6(CH2)3)3, were utilized. The optimum CO32− selectivities were found for sensing membranes composed of anionic sites and ionophore in a 1:4 molar ratio, which results in the formation of 2:1 complexes with CO32− with stability constants up to 4.1 × 1015. As predicted by established theory, the site-to-ionophore ratios that provide optimum potentiometric selectivity depend on the stoichiometries of the complexes of both the primary and the interfering ions. However, the ionophores used in this study give examples of charges and stoichiometries previously neither explicitly predicted by theory nor shown by experiment. The exceptional selectivity of fluorous membranes doped with these carbonate ionophores suggests their use not only for potentiometric sensing but also for other types of sensors, such as the selective separation of carbonate from other anions and the sequestration of carbon dioxide.
Silver nanoparticle (Ag NP) dissolution, or ionization from Ag(0) to Ag + , is an important determinant of the nanoparticles' toxicity as silver ions are considered to be a major contributor to Ag NP cytotoxicity. In this work, we characterize ion dissolution from Ag NPs using a selective and dynamic technique, Ag + -selective electrodes (ISEs) with ionophore-doped fluorous sensing membranes. We examined dissolution of various concentrations of Ag NPs (0.3, 3, and 15 mg mL À1 ) in water and bacterial growth medium in real-time. A decrease in the concentration of free Ag + was observed as a result of complexation with components of the growth medium. Overall, a greater percentage of the nanoparticles dissolve in growth medium than water (28% vs. 13%). Individual chemical components of the growth medium were examined for their complexation capability, and it was determined that ammonia-silver complexes are the predominant species of dissolved Ag + , with 8.9% occurring as AgNH 3 + , 87.8% occurring as Ag(NH 3 ) 2 + , and only 3.3% occurring as free Ag + . After characterizing Ag NP dissolution in growth medium, the viability and growth of Shewanella oneidensis, a ubiquitous beneficial bacterium, were monitored upon exposure to the known in situ levels of Ag + and Ag NPs. Ag + and Ag NPs both caused a dose-dependent decrease in bacterial viability and growth rate, though the growth and viability changes upon Ag NP exposure did not correlate with the ISE-measured Ag + . Using ISEs to monitor Ag NP dissolution in the presence of S. oneidensis revealed that the presence of the organisms influences the nanoparticle dissolution profile, a result not previously reported that has significant implications for understanding nanotoxicity. This work lays the foundation for the use of fluorous-phase ISEs as an in situ nanoparticle characterization tool, addressing a critical technology gap in the field of nanoparticle toxicology.
Solid‐contact reference electrodes that comprise a polymeric reference membrane doped with an ionic liquid are an alternative to conventional reference electrodes with a salt bridge. To enhance the electrode‐to‐electrode reproducibility of solid‐contact reference electrodes, a hydrophobic redox buffer consisting of the Co(III) and Co(II) complexes of 1,10‐phenanthroline ([Co(phen)3]3+/2+) was incorporated into these reference membranes, providing redox buffer capacity at the interface of the reference membrane and the underlying solid electron conductor. As required for proper functioning, the E° of these reference electrodes is not sample dependent and exhibits a good electrode‐to‐electrode reproducibility, with a standard deviation as low as 2.1 mV.
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