Philippe Bühlmann

Polymer Membrane Ion-Selective Electrodes–What are the Limits?

This article reviews recent advances in the field of potentiometric solvent polymeric membrane electrodes. These sensors have found widespread applications in a variety of fields, especially in the area of clinical diagnostics. Emphasis is given on the discussion of the theoretical and practical limits of ionophore-based ion-selective electrodes, with a special focus on electrode sensitivities, characterization of selectivities and dramatic improvements in detection limits. Advances in ionophore design and in the underlying model assumptions are also discussed. It is shown that a multitude of exciting new research possibilities have recently emerged in this well—established field.

13C NMR Spectroscopy

The characterization of an organometallic complex involves obtaining a complete understanding of the same right from its identification to the assessment of its purity content, to even elucidation of its stereochemical features. Detailed structural understanding of the organometallic compounds is critical for obtaining an insight on its properties and which is achieved based on the structure-property paradigm.

Strong hydrogen bond-mediated complexation of H2PO4− by neutral bis-thiourea hosts

Abstract Highly preorganized bis-thiourea receptors based on a xanthene spacer selectively (H2PO4− > CH3COO− > Cl−) bind dihyrogenphosphate via multitopic hydrogen bonding, giving stronger complexes with H2PO4− neutral receptor known so far. The high complexation strengths are rationalized by the hydrogen bond donor strength of the thiourea groups and by host preorganization. The hydrogen acceptor strengths of the guest anions and, for small ions, guest solvation explain the observed selectivity of complexation in dimethyl sulfoxide (DMSO).

Anion recognition by urea and thiourea groups: Remarkably simple neutral receptors for dihydrogenphosphate

A bis-urea and a bis-thiourea host, both derived in only one step from 1,3-bis(aminomethyl)benzene, are shown to bind dihydrogenphosphate selectively over various other anions (H2PO4− > CH3COO− > Cl− > HSO4− > NO3− > ClO4−). The much stronger binding of H2PO4− by the bis-thiourea is rationalized by the stronger H-bond donor strength of the thiourea groups and the binding selectivity is explained in terms of the complex geometry and the basicity of the guest anions. The lack of self-association and the changes in the UV spectrum upon complexation make bis-thiourea hosts a promising new class of neutral receptors for dihydrogenphosphate.

The phase-boundary potential model.

The response of ion-selective electrodes (ISEs) can be described on the basis of two different theoretical approaches. On one hand, the phase-boundary model is based on the assumption of local equilibria at the aqueous/organic interface. The phase-boundary model allows the description of all practically relevant cases of steady state and even transient responses with sufficient accuracy. Moreover, it has the advantage of relating simple thermodynamic parameters to the response function of the electrodes and hence allowing an intuitive interpretation of many observed facts. On the other hand, the comprehensive but quite involved dynamic model requires knowledge of mobilities and ion transfer rate constants. It has never been applied to ionophore-based electrodes in its full complexity. Both models were first suggested decades ago but have been recently extended to explain so far poorly understood aspects of ionophore-based ISEs. Due to space restrictions, only the most important original references are given in this paper, which summarizes the major assumptions of the phase-boundary potential model and discusses the usefulness and limits of this approach. Recent applications are discussed towards understanding sensor selectivity, upper and lower detection limits (even when concentration polarizations are relevant), the so-called sandwich membrane method to determine thermodynamic parameters, apparently non-Nernstian responses, potential drifts with solid contact electrodes, polyion sensors, and galvanostatically controlled ion sensors.

Effects of Humic and Fulvic Acids on Silver Nanoparticle Stability, Dissolution, and Toxicity

The colloidal stability of silver nanoparticles (AgNPs) in natural aquatic environments influences their transport and environmental persistence, while their dissolution to Ag(+) influences their toxicity to organisms. Here, we characterize the colloidal stability, dissolution behavior, and toxicity of two industrially relevant classes of AgNPs (i.e., AgNPs stabilized by citrate or polyvinylpyrrolidone) after exposure to natural organic matter (NOM, i.e., Suwannee River Humic and Fulvic Acid Standards and Pony Lake Fulvic Acid Reference). We show that NOM interaction with the nanoparticle surface depends on (i) the NOMs chemical composition, where sulfur- and nitrogen-rich NOM more significantly increases colloidal stability, and (ii) the affinity of the capping agent for the AgNP surface, where nanoparticles with loosely bound capping agents are more effectively stabilized by NOM. Adsorption of NOM is shown to have little effect on AgNP dissolution under most experimental conditions, the exception being when the NOM is rich in sulfur and nitrogen. Similarly, the toxicity of AgNPs to a bacterial model (Shewanella oneidensis MR-1) decreases most significantly in the presence of sulfur- and nitrogen-rich NOM. Our data suggest that the rate of AgNP aggregation and dissolution in aquatic environments containing NOM will depend on the chemical composition of the NOM, and that the toxicity of AgNPs to aquatic microorganisms is controlled primarily by the extent of nanoparticle dissolution.

Potentiometric selectivity coefficients of ion-selective electrodes. Part II. Inorganic anions (IUPAC Technical Report)

Potentiometric selectivity coefficients, KA,Bpot have been collected for ionophore-based ion-selective electrodes (ISEs) for inorganic anions reported during 1988-1998. In addition to the actual numerical values of KA,Bpot together with the methods and conditions for their determination, response slopes, linear concentration (activity) ranges, chemical compositions, and ionophore structures for the ISE membranes are tabulated.

Electrochemical Detection of a One‐Base Mismatch in an Oligonucleotide Using Ion‐Channel Sensors with Self‐Assembled PNA Monolayers

Mixed monolayers of peptide nucleic acid (PNA) and 6-mercapto-1-hexanol on gold disk electrodes were used to detect complementary oligonucleotides at a micromolar level with the ion-channel sensor technique. In contrast, no responses to 100 times more concentrated (i.e., 100 µM) solutions of a one-base mismatch oligonucleotide were observed. Measurements for the mismatch oligonucleotides (dA)10 and (dT)10 provided no responses either. Binding of the complementary oligonucleotide to the PNA probe monolayer increases the negative charge at the surface of the electrode, which results in electrostatic repulsion between the redox marker [Fe(CN)6]4–/3– in the solution and the monolayer, thereby hindering the redox reaction of the marker. This allows the indirect detection of the complementary oligonucleotide.

An Ion-Selective Electrode for Acetate Based on a Urea-Functionalized Porphyrin as a Hydrogen-Bonding Ionophore

An ion-selective electrode for acetate based on (α,α,α,α)-5,10,15,20-tetrakis[2-(4-fluorophenylureylene)phenyl]porphyrin as an ionophore that has no metal center and forms hydrogen bonds to the analyte is described. At pH 7.0 (0.1 M HEPES-NaOH buffer), the electrode based on this ionophore and cationic sites (50 mol % relative to the ionophore) responds to acetate in a linear range from 1.58 × 10(-)(4) to 1.58 × 10(-)(2) M with a slope of -54.8 ± 0.8 mV/decade and a detection limit of (3.06 ± 1.15) × 10(-)(5) M. Selectivity coefficients determined with the separate solution method (SSM) indicate that interferences of hydrophobic inorganic anions are relatively small (log[Formula: see text] (SSM):  NO(3)(-), +0.68; SCN(-), +0.60; NO(2)(-), +0.22; I(-), +0.20; ClO(4)(-), +0.12; Br(-), -0.13). Responses to anions that are good hydrogen bond acceptors, i.e., Cl(-), HSO(3)(-), and HCO(3)(-), were Nernstian and were weaker than the response to acetate (log[Formula: see text] (SSM): -0.54, -0.56, and -1.34, respectively). Negligibly small responses were observed for very hydrophilic anions, i.e., F(-), SO(4)(2)(-), and H(2)PO(4)(-)/HPO(4)(2)(-). While aliphatic carboxylates such as formate, propanoate, pyruvate, and lactate gave Nernstian responses similar to acetate, interferences of salicylate and benzoate were considerably decreased in comparison with electrodes based on cationic sites only. Concentrations of acetic acid in vinegar samples were determined by direct potentiometry and agreed with values determined by a standard enzymatic method.

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.