Limitations of current polarization for lowering the detection limit of potentiometric polymeric membrane sensors.

Ion fluxes across polymeric ion-selective membranes are a decisive parameter dictating the lower detection limit of potentiometric ion sensors. An applied current was earlier proposed to counteract such fluxes and reduce the detection limit to ultratrace levels. So far, however, the method has not been used in practical situations since the correct current amplitude requires prior knowledge of the sample composition. This paper explores the use of the stir effect to evaluate the optimal current by theory and experiments. It is shown that the traditionally used steady-state model assuming a uniform distribution of ion exchanger in the membrane, fixed with time, violates the electroneutrality condition. A modified steady-state model is introduced that allows for a concentration tilt of the ion exchanger and predicts that a stir effect can indeed be utilized to find the optimal current. Ideally, by choosing the optimal current and very long measurement times, the thermodynamic detection limit might be obtained. However, in practice the stir effect declines at low concentrations and the conditions are far from steady state. Therefore, the improvement of the lower detection limit achievable by galvanostatic control is only about 1 order of magnitude. A numerical finite-difference approximation is shown to trace the experimental potential responses of silver-selective electrodes well and to reproduce the stir effect adequately, even for different conditioning protocols. The stir effect is successfully used to improve the detection limit of electrodes with ill-optimized inner solutions; however, significant improvements beyond what is commonly feasible by chemical optimization does not seem to be easily achievable. The results indicate that with conventional membranes the possibility of improving the detection limit by current polarization is much more limited than assumed so far.

Potentiometric immunoassay with quantum dot labels.

Potentiometric sensors based on polymer membrane electrodes, if properly optimized, are useful for measurements at trace levels. The expected independence of the electrochemical signal of the sample size makes them extremely attractive for measurements in small volumes. Here, we report on electrodes for the potentiometric detection of cadmium ions that reach a detection limit of 6 nM and utilize a Na(+)-selective electrode as pseudoreference in order to facilitate measurements in 150-microL samples. A potentiometric immunoassay of mouse IgG is performed via CdSe quantum dot labels on a secondary antibody according to a sandwich immunoassay protocol in a microtiter plate format. The CdSe quantum dots are found to be easily dissolved/oxidized in a matter of minutes with hydrogen peroxide, allowing us to maintain the pH at a near-neutral value. The potentiometric protein immunoassay exhibits a log-linear response ranging from 0.15 to 4.0 pmol of IgG, with a detection limit of <10 fmol in 150-microL sample wells.

Potentiometry at trace levels in confined samples: ion-selective electrodes with subfemtomole detection limits.

We explore here for the first time the direct potentiometric detectability of calcium, lead, and silver ions in amounts on the order of 300 attomoles at 100 picomolar concentrations without any preconcentration, analyte recycling, or electrocatalytic signal enhancement. The results presented here place zero-current potentiometry among the most sensitive electrochemical methods available.

Novel potentiometric and optical silver ion-selective sensors with subnanomolar detection limits.

Ten Ag+-selective ionophores have been characterized in terms of their potentiometric selectivities and complex formation constants in solvent polymeric membranes. The compounds with pi-coordination show much weaker interactions than those with thioether or thiocarbamate groups as the coordinating sites. Long-term studies with the best ionophores show that the lower detection limit of the best Ag+ sensors can be maintained in the subnanomolar range for at least 1 month. The best ionophores have also been characterized in fluorescent microspheres. The so far best lower detection limits of 3 x 10(-11) M (potentiometrically) and 2 x 10(-11) M Ag+ (optically) are found with bridged thiacalixarenes.

Approaches to Improving the Lower Detection Limit of Polymeric Membrane Ion-Selective Electrodes.

More than ten different approaches for improving the lower detection limit of polymeric membrane ion-selective electrodes have been suggested during the recent years. In this contribution, their principles are briefly summarized with a focus to their general practical applicability. The methods that are the most rugged and the easiest to implement in a routine laboratory will be highlighted.

Improving the response behavior of Cd-selective polymeric membrane electrodes by incorporated lipophilic particles.

The influence of incorporated lipophilic particles on the potentiometric response behavior of two Cd(2+)-selective membranes is described. The particles show only minor influence on the selectivity behavior of the membranes. As described earlier for a similar Ca(2+)-selective membrane, the flux of primary ions from the sample to the membrane can be successfully suppressed by the incorporation of lipophilic particles.

Ion-selective supported liquid membranes placed under steady-state diffusion control.

Supported liquid membranes are used here to establish steady-state concentration profiles across ion-selective membranes rapidly and reproducibly. This opens up new avenues in the area of nonequilibrium potentiometry, where reproducible accumulation and depletion processes at ion-selective membranes may be used to gain valuable analytical information about the sample. Until today, drifting signals originating from a slowly developing concentration profile across the ion-selective membrane made such approaches impractical in zero current potentiometry. Here, calcium- and silver-selective membranes were placed between two identical aqueous electrolyte solutions, and the open circuit potential was monitored upon changing the composition of one solution. Steady state was reached in approximately 1 min with 25-microm porous polypropylene membranes filled with bis(2-ethylhexyl) sebacate doped with ionophore and lipophilic ion exchanger. Ion transport across the membrane resulted on the basis of nonsymmetric ion-exchange processes at both membrane sides. The steady-state potential was calculated as the sum of the two membrane phase boundary potentials, and good correspondence to experiment was observed. Concentration polarizations in the contacting aqueous phases were confirmed with stirring experiments. It was found that interferences (barium in the case of calcium electrodes and potassium with silver electrodes) induce a larger potential change than expected with the Nicolsky equation because they influence the level of polarization of the primary ion (calcium or silver) that remains potential determining.

Monolithic capillary-based ion-selective electrodes.

Poly(styrene-co-divinylbenzene)-based monolithic capillaries of an inner diameter of 200 mum and a length of 2-5 mm have been used to construct Ca2+-, Ag+-, and Na+-selective electrodes. The membranes consist of a solution of ionophore and ion exchanger in bis(2-ethylhexyl) sebacate or 2-nitrophenyl octyl ether, which are used as plasticizers in conventional PVC-based membranes. With capillaries of low porosity, the potentiometric responses down to 10(-8)-10(-9) M solutions do not depend on the composition of the internal solution, which indicates a strong suppression of transmembrane ion fluxes. Thus, no tedious optimization of the inner solution is required with monolith ISEs. The lower detection limits of Ag+- and Ca2+-ISEs are comparable to the best ones obtained earlier with optimized inner solutions. Additionally, a monolithic Na+-selective ISE has been obtained exhibiting a lower detection limit of 3 x 10(-8) M Na+. With monolithic capillaries of higher porosity and fused-silica GC capillaries, the transmembrane flux effects are noticeable but still significantly smaller than with conventional PVC membranes.

Biorecognition-modulated ion fluxes through functionalized gold nanotubules as a novel label-free biosensing approach.

A novel biosensing principle is presented, based on the potentiometric monitoring of an indicator ion such as Ca2+, whose zero-current flux through chemically modified nanochannels is altered by biorecognition events.