Eric Bakker

Lipophilic and immobilized anionic additives in solvent polymeric membranes of cation-selective chemical sensors

Abstract Lipophilic borate salts are frequently used as anionic additives in potentiometric and optical cation-selective sensors based on solvent polymeric membranes. The lifetime of such membranes may be limited owing to chemical decomposition and leaching of the components. Borate salts, in particular, are decomposed in the presence of acids in the membrane. Adequately substituted borate salts and sulphonic acids, such as sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate and dinonylnaphthalenesulphonic acid (DNSS), are shown to be sufficiently stable as membrane additives. Furthermore, lipoholic mobile or immoblizied sulphonic acids [DNNS or poly(2-acrylamido-2-methyl-1-propanesulphonic acid-co-styrene), respectively were also tested as anionic additives. Their influence on the selectivity behaviour of the sensor is attributed to their strong association with positively charged species in the membrane phase. It may be kept small by choosing ionophores that from stable complexes with the analyte.

ANION-SELECTIVE MEMBRANE ELECTRODES BASED ON METALLOPORPHYRINS: THE INFLUENCE OF LIPOPHILIC ANIONIC AND CATIONIC SITES ON POTENTIOMETRIC SELECTIVITY

The role of lipophilic anionic and cationic additives on the potentiometric anion selectivities of polymer membrane electrodes prepared with various metalloporphyrins as anion selective ionophores is examined. The presence of lipophilic anionic sites (e.g. tetraphenylborate derivatives) is shown to enhance the non-Hofmeister anion selectivities of membranes doped with In(III) and Sn(IV) porphyrins. In contrast, membranes containing Co(III) porphyrins require the addition of lipophilic cationic sites (e.g. tridodecylmethylammonium ions) in order to achieve optimal anion selectivity (for nitrite and thiocyanate) as well as rapid and reversible Nernstian response toward these anionic species. These experimental results coupled with appropriate theoretical models that predict the effect of lipophilic anion and cation sites on the selectivities of membranes doped with either neutral or charged carrier type ionophores may be used to determine the operative ionophore mechanism of each metalloporphyrin complex within the organic membrane phase.

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.

Aptamer-based potentiometric measurements of proteins using ion-selective microelectrodes.

We here report on the first example of an aptamer-based potentiometric sandwich assay of proteins. The measurements are based on CdS quantum dot labels of the secondary aptamer, which were determined with a novel solid-contact Cd2+-selective polymer membrane electrode after dissolution with hydrogen peroxide. The electrode exhibited cadmium ion detection limits of 100 pM in 100 mL samples and of 1 nM in 200 microL microwells, using a calcium-selective electrode as a pseudoreference electrode. As a prototype example, thrombin was measured in 200 microL samples with a lower detection limit of 0.14 nM corresponding to 28 fmol of analyte. The results show great promise for the potentiometric determination of proteins at very low concentrations in microliter samples.

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.

Synthesis and characterization of neutral hydrogen ion-selective chromoionophores for use in bulk optodes

Abstract The type of ion-selective sensors discussed is based on a solvent polymeric membrane with optical transduction (bulk optode), and requires a reversible mass transfer of analyte ions into and from the bulk of the organic phase. Apart from the ion to be analysed, such sensors simultaneously measure a reference ion. For analytical purposes, an H + -selective carrier absorbing light in the visible range (chromoionophore) is incorporated in the membrane together with an ionophore as known from ion-selective electrodes. A series of neutral H + -selective chromoionophores, synthesized with a view to their possible use in bulk optodes, are described. The results of various studies on the basicity in different solvents, lipophilicity, chemical stability, selectivity, molar absorption coefficients and absorbance maxima are given and, in part, discussed theoretically. It is shown that the chromoionophores described are highly selective and offer a wide range of basicities. Their lipophilicity usually guarantees a long lifetime for applications in both aqueous solutions and diluted blood and serum, but for measurements in undiluted blood samples they should be covalently immobilized to the polymer of the optode membrane. Their chemical stability is usually sufficient, only direct sunlight causing decomposition.

Ion sensors: current limits and new trends

The current status of ion sensors in their main application, clinical chemistry, is highlighted. The reasons for the practical success of sensors in this particular area are discussed together with the expected influence of novel technical possibilities for the next generation of clinical ion analyzers. A series of recent research results, including the improvement of lower detection limits, the establishment of selectivities that are much better than reported so far, and new types of reference electrodes will very likely open up numerous new fields of applications of these sensors.

Optimum composition of neutral carrier based pH electrodes

Abstract A simple formalism for the quantitative description of the upper and lower detection limit of pH selective solvent polymeric membrane electrodes containing a neutral carrier and a lipophilic anionic additive is presented, which is based on the consideration of phase transfer equilibria at the sample/membrane interface. It is shown that the lower and upper detection limits are controlled by the activity and lipophilicity of the interfering ions and by the basicity of the ionophore. When interfering cations or anions have completely penetrated the organic phase boundary layer through ion exchange or coextraction equilibria, respectively, the electrode response is expected to be a Nernstian function of the interfering ion activity alone. The measuring range may be shifted by incorporating ionophores of different basicity in the membrane, but cannot be extended with this approach. Instead, a high concentration of ionophore, together with 50 mol-% anionic additive relative to the ionophore, and, most importantly, a more hydrophobic membrane matrix with less cation binding characteristics has to be chosen for achieving a maximum measuring range of the potentiometric sensor.

Potentiometry at trace levels

Recently, it was realized that polymeric membrane ion-selective electrodes can be optimized to show dramatically improved detection limits over traditional values. This discovery has initiated much excitement in the chemical sensor community, since it appears that the applicability of ion-selective electrodes in chemical analysis has been very much underestimated in the past. While this is a rather new research area, some significant progress has already been made to understand important basic processes that dictate the response behavior of ISEs under dilute sample conditions. This review summarizes these developments from basic considerations to practical applications. It outlines the current state of the art of this research and the long-term promises that this technology holds.

Determination of complex formation constants of 18 neutral alkali and alkaline earth metal ionophores in poly(vinyl chloride) sensing membranes plasticized with bis(2-ethylhexyl)sebacate and o-nitrophenyloctylether

A segmented sandwich membrane method is used to determine complex formation constants of 18 electrically neutral ionophores in situ in solvent polymeric sensing membranes. These ionophores are commonly used in potentiometric and optical sensors, and knowledge of such binding information is important for ionophore and sensor design. In this method, two membrane segments are fused together, with only one containing the ionophore, to give a concentration-polarized sandwich membrane. Unlike other approaches, this method does not require the use of a reference ion in the sample and/or a second ionophore in the membrane, and is typically pH insensitive. The following ionophores responsive for the common cations lithium, sodium, potassium, magnesium and calcium are characterized and discussed: valinomycin, BME-44, bis[(benzo-15-crown-5)-4′-ylmethyl]pimelate, ETH 157, ETH 2120, bis[(12-crown-4)methyl]dodecylmethylmalonate], 4-tert-butylcalix [4] arene tetraacetic acid tetraethyl ester, ETH 149, ETH 1644, ETH 1810, 6,6-dibenzyl-14-crown-4, N,N,N′,N′,N″,N″-hexacyclohexyl-4,4′,4″-propylidyne tris(3-oxabutyramide), ETH 1117, ETH 4030, ETH 1001, ETH 129, ETH 5234, and A23187. The logarithmic complex formation constants range from 4.4 to 29 and compare well to published data for ionophores that were characterized earlier. From the observed complex formation constants, maximum possible selectivities are calculated that would be expected if interfering ions show no binding affinity to the ionophore, and the values are compared with experimental findings. Each ionophore is characterized in poly(vinyl chloride) membranes plasticized either with a polar (NPOE) or a nonpolar plasticizer (DOS). Membranes based on NPOE always show larger complex formation constants of the embedded ionophore.