The influence of spontaneous charging/discharging of conducting polymer ion-to-electron transducer on potentiometric responses of all-solid-state calcium-selective electrodes

Fast and sensitive coulometric signal transduction for ion-selective electrodes by utilizing a two-compartment cell

Here, we propose a fast and sensitive coulometric signal transduction method for ion-selective electrodes (ISEs) by utilizing a two-compartment cell. A potassium ion-selective electrode (K⁺-ISE) was connected as reference electrode (RE) and placed in the sample compartment. A glassy carbon (GC) electrode coated with poly(3,4-ethylenedioxythiophene) (GC/PEDOT), or reduced graphene oxide (GC/RGO), was connected as working electrode (WE) and placed in the detection compartment together with a counter electrode (CE). The two compartments were connected with an Ag/AgCl wire. The measured cumulated charge was amplified by increasing the capacitance of the WE. The observed slope of the cumulated charge with respect to the change of the logarithm of the K⁺ ion activity was linearly proportional to the capacitance of the GC/PEDOT and GC/RGO, estimated from impedance spectra. Furthermore, the sensitivity of the coulometric signal transduction using a commercial K⁺-ISE with internal filling solution as RE and GC/RGO as WE allowed to decrease the response time while still being able to detect a 0.2% change in K⁺ concentration. The coulometric method utilizing a two-compartment cell was found to be feasible for the determination of K⁺ concentrations in serum. The advantage of this two-compartment approach, compared to the coulometric transduction described earlier, was that no current passed through the K⁺-ISE that was connected as RE. Therefore, current-induced polarization of the K⁺-ISE was avoided. Furthermore, since the GCE/PEDOT and GCE/RGO (used as WE) had a low impedance, the response time of the coulometric response decreased from minutes to seconds.

Electropolymerized hydrophobic polyazulene as solid-contacts in potassium-selective electrodes

Electropolymerized hydrophobic polyazulene based solid-contact potassium-selective electrodes have been characterized in terms of their suitability for potassium measurements in serum.

Improved potentiometric response of all-solid-state Pb(2+)-selective electrode

Zero-current ion-flux has a great influence on the characteristics of the ion-selective electrodes. In this work the improvement of analytical performance of all-solid-state Pb(2+)-selective membrane electrodes was demonstrated by adjusting the transmembrane ion flux. The study is focused on the relationship between the conditioning solution and the linear working range of the obtained electrodes for different sample matrixes. Results show that the electrode with appropriate conditioning keeps good reproducibility within linear working range. The utility of the electrode has been tested by successfully determining Pb(2+) concentration in real water samples.

Determination of unbiased selectivity coefficients using pulsed chronopotentiometric polymeric membrane ion sensors

A new procedure for the determination of selectivity coefficients of neutral carriers using pulsed chronopotentiometric ion selective sensors (pulstrodes) is established. Pulstrode membrane which lacks an ion-exchanger suppresses the zero current ion flux, allowing a Nernstian response slope for even highly discriminated ions. Unlike previously developed methods, unbiased selectivity remains unaltered even with the exposure to the primary ion solution for prolonged time. Studies with potassium-, silver-, and calcium-selective electrodes reveal that pulstrodes yield the same or slightly favorable unbiased selectivity coefficients than reported earlier. In contrast to alternative methods for the determination of unbiased selectivity, this technique offers a unique simplicity and reliability. Therefore the new procedure promises to be a valuable additional tool for the characterization of unbiased selectivity coefficients for the ISEs.

Composite polyacrylate-poly(3,4- ethylenedioxythiophene) membranes for improved all-solid-state ion-selective sensors

A novel type of self-plasticizing polyacrylate-based membrane was developed for all-solid-state ion-selective potentiometric electrodes. The membrane composition contains a conducting polymer (CP): poly(3,4-ethylenedioxythiophene) end capped with methacrylate groups, chemically grafted with the membrane during the photopolymerization step. This composition results in ion-selective membranes with the following advantages: lower electrical resistance compared to the CP-free membrane, facile ion-to-electron transduction between the membrane and the electrode support, controlled low activity of analyte ions, and high concentration of interferent ions (incorporated with the CP) within the membrane, potentially resulting in improved analytical parameters. Ca2+- and K+-selective membranes were chosen as model systems to study the effect of pretreatment and CP content on the potentiometric sensor's characteristics. For Ca2+ sensors, reproducible and stable Nernstian characteristics were obtained within the range from 0.1 to 10(-9) M CaCl2, without a time-consuming preconditioning step. For K+-selective sensors, the influence on Nernstian response range was observed for varying KCl concentrations in the conditioning solution, with the lowest detection limit found close to 10(-8) M KCl. Mass spectrometry coupled with laser ablation studies of the membranes revealed that in this case the detection limit is not related to primary ion content in the membrane contacting a sample solution, but is affected by interfering ion concentration close to the membrane surface.

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.

Effect of interferents present in the internal solution or in the conducting polymer transducer on the responses of ion-selective electrodes

The effect of interferents present on the opposite side of the Pb2+-selective membrane has been studied for both internal solution and all-solid-state sensors with a conducting polymer (CP) transducer. For interferents with moderate selectivity coefficients (sodium cations) present in the internal solution or in the CP transducer phase, super-Nernstian responses were obtained. For sensors containing strongly discriminated interferents (lithium ions), however, responses typical of conventional electrodes are observed, despite the low activity of primary ions on the opposite side of the membrane. This effect is attributed to hindered incorporation of interfering ions into the membrane, which also impairs the long term stability of the potential. Because of the relatively small absolute amounts of interferents in the transducer of all-solid-state sensors, their exchange for primary ions occurs quickly. Thus, transformation of the sensor to one with a micromolar detection limit and high potential stability is observed.

Optimizing the analytical performance and construction of ion-selective electrodes with conducting polymer-based ion-to-electron transducers

All-solid-state ion-selective electrodes that use a conducting polymer as the ion-to-electron transducer have emerged as one of the most promising classes of all-solid-state potentiometric sensors in recent years. This is largely because it has many analytical advantages, including high response stability, which is unique in the field of internal-solution-free ion-selective electrodes. This paper reviews the considerable progress that has been made in this area of sensing in recent years, in terms of detection limits, selectivity coefficients and novel construction methods.