Bradford D. Pendley

Microfabricated ISEs: critical comparison of inherently conducting polymer and hydrogel based inner contacts

A rigorous side by side comparison of miniature planar potassium-selective electrodes with hydrogel and potassium hexacyanoferrate(II)/(III) doped polypyrrole (PPy/FeCN) based inner contacts is presented. The planar electrodes were manufactured by screen printing as four- and five-site arrays on ceramic substrates. These electrode arrays were incorporated into a flow-through cell, which could accommodate nine electrode sites. Two identical flow cells were connected in series and the effect of the inner contacts on the analytical performance of the respective electrodes has been critically evaluated. The time necessary to reach steady state conditions has been determined and the effect of experimental parameters (temperature, ambient light intensity, CO(2), and O(2) concentration of the sample) on the potential stability of the electrodes was analyzed. At controlled temperature, the drift of the planar potassium electrodes with hydrogel and PPy/FeCN solid contact were 0.11+/-0.02mVh(-1) and 0.03+/-0.007mVh(-1), respectively. The experimental data proved that there is no aqueous film formation between the PPy/FeCN film and the potassium-selective solvent polymeric membrane.

A tutorial on the application of ion-selective electrode potentiometry: An analytical method with unique qualities, unexplored opportunities and potential pitfalls; Tutorial

Ion-selective potentiometry enjoys practical utility as a simple analytical technique to measure ionic constituents in complex samples. Advances in the field have improved the selectivity and decreased the detection limit of ion-selective electrodes (ISEs) by orders of magnitude such that trace analysis in micro and nanomolar concentrations is now possible with potentiometric sensors. This tutorial reviews the fundamental principles of ion-selective potentiometry, describes the practical considerations involved in the use of these sensors to measure real samples, and discusses the statistical evaluation of experimental results compared with alternative analytical techniques.

Microelectrodes as probes in low electrolyte solutions: the reduction of quinone in aqueous sulfuric acid solution

Abstract The steady-state voltammetric behavior of quinone in dilute aqueous solutions of sulfuric acid has been investigated at platinum disk microelectrodes and compared with the model presented by Oldham (J. Electroanal. Chem., 250 (1988) 1). At high concentrations of sulfuric acid, a single voltammetric wave whose limiting current is predicted by Oldhams model is observed. As the concentration of the sulfuric acid is lowered relative to that of the quinone, the single voltammetric wave splits into two waves. The first wave is due to the two-electron reduction of quinone in acidic solution and the magnitude of its limiting current agrees with that predicted by Oldhams model. The second wave also results from the reduction of quinone but occurs at a more negative potential owing to the change in pH at the electrode surface. Such a process was not considered in Oldhams model.

A chronoamperometric method to estimate ionophore loss from ion-selective electrode membranes

A novel chronoamperometric method was developed to estimate the concentration of a neutral ionophore in fixed-site, dioctyl sebacate plasticized, poly(vinyl chloride)-based, ion-selective electrode membranes. The membranes contained between 0.5 and 16 mmol/kg valinomycin. The chronoamperometric technique was used to estimate the valinomycin concentration in freshly prepared membranes and after extraction of some of the ionophore from the membranes using dioctyl sebacate. Replicate measurements indicated a relative standard deviation in the calculated valinomycin concentration of less than 10%, and these values accurately represented the true concentration of valinomycin within 10%. The method permitted an estimate of the valinomycin concentration after valinomycin was leached from a membrane. The results of preliminary experiments using heparinized dog blood suggest that blood protein adsorption does not interfere qualitatively or quantitatively with the analysis.

Equilibration Time of Solid Contact Ion-Selective Electrodes

Papers published on ion-selective electrodes (ISEs) generally report on the performance characteristics of these devices after long, extensive conditioning. Conditioning refers to the equilibration of the ion-selective electrode in an aqueous solution before the measurement of the sample. The requirement for long and repeated conditioning is a significant burden in a variety of applications, for example, single-use sensors aimed for in vivo or field applications and solid contact (SC) ISEs, which were developed to provide simple, mass-produced sensors that have the potential to be implemented without calibration and extensive conditioning. In this study we recorded the potential of SC K(+), Na(+), and H(+) ISEs as a function of time following their first contact with an aqueous electrolyte solution and used these transients to determine their equilibration times. The SC electrodes were built on Au, Pt, and glassy carbon (GC) substrates using galvanostatically deposited conductive polymer (PEDOT(PSS(-)), poly(3,4-ethylenedioxythiophene) polystyrenesulfonate) as ion-to-electron transducer (solid internal contact) between the ion-selective membrane and the substrate. The SC electrodes built on GC and Au had significantly shorter equilibration times (between 5 and 13 min) than the SC electrodes built on Pt substrates (>60 min). Such significant differences in the equilibration times of SC ISEs built on different substrate electrodes are reported here for the first time. These unexpected findings suggest that the interface between the conductive polymer and the electron-conducting substrate (EC) has significant influence on the long-term dynamic behavior of SC ISEs.

A glance into the bulk of solvent polymeric pH membranes

The pH-sensitive chromoionophores brought the dream of the ion-selective membrane scientist close to realization. With the help of these molecules, one can build pH-sensitive, ion-selective electrodes and look into the bulk of solvent polymeric membranes during potentiometric measurements (spectropotentiometry) and image concentration profiles in situ with high spatial and temporal resolution. The combination of electrochemical and optical information helped to interpret non-idealities in the potentiometric responses, suppress or tailor the undesirable transport across sensor membranes, and estimate the residual lifetime of chronically implanted sensors. These novel opportunities provide feedback in membrane optimizations and are expected to lead to sensor systems with picomolar detection limits and superb selectivities.

Determination of copper at surface-modified electrodes: Effects of competitive binding and electrode size

Abstract Electrodes modified with electropolymerized films of [Ru(v-bpy) 3 ] 2+ into which Xylenol orange, Eriochrome cyanine R, Nitroso R salt or Bathocuproine sulfonate were incorporated by ion exchange can be used to determine copper in solution with high sensitivity, excellent linearity and a dynamic range of over two orders of magnitude. The presence of iron, cobalt, nickel, chloride or bromide does not alter the response measurably. Oxalate anions produce a steady increase in the current measured at the potential for oxidation of the copper complexes. The presence of phosphate anions in the preconcentration solution also gives rise to an enhanced signal. The use of modified microelectrodes (75 μm diameter) or ultramicroelectrodes (5 μm diameter) results in an enhancement in the sensitivity, although a saturation response is obtained for copperconcentrations above 6 × 10 −6 M. Ultramicroelectrodes modified with Bathocuproine sulfonate could be used in multiple determinations with good reproducibility (±9%) and without the need for remodification of the electrode surface.

Poly(3-octylthiophene) as solid contact for ion-selective electrodes: contradictions and possibilities

The hydrophobic conductive polymer, poly(3-octylthiophene) (POT), is considered as uniquely suited to be used as an ion-to-electron transducer in solid contact (SC) ion-selective electrodes (ISEs). However, the reports on the performance characteristics of POT-based SC ISEs are quite conflicting. In this study, the potential sources of the contradicting results on the ambiguous drift and poor potential reproducibility of POT-based ISEs are compiled, and different approaches to minimize the drift and the differences in the standard potentials of POT-based SC ISEs are shown. To set the potential of the POT film, it has been loaded with a 7,7,8,8-tetracyanoquinodimethane (TCNQ/TCNQ·−) redox couple. An approximately 1:1 TCNQ/TCNQ·−ratio in the POT film has been achieved through potentiostatic control of the potential of the redox couple-loaded conductive polymer. It is hypothesized that once the POT film has a stable, highly reproducible redox potential, it will provide similarly stable and reproducible interfacial potentials between the POT film and the electron-conducting substrate and result in SC ISEs with excellent reproducibility and potential stability. Towards this goal, the potentials of Au, GC, and Pt electrodes with drop-cast POT film coatings were recorded in KCl solutions as a function of time. Some of the POT films were loaded with TCNQ and coated with a K+-selective membrane. The improvement in the potential stabilities and sensor-to-sensor reproducibility as a consequence of the incorporation of TCNQ in the POT film and the potentiostatic control of the TCNQ/TCNQ·−ratio is reported.

Solid-Contact pH Sensor without CO2 Interference with a Superhydrophobic PEDOT-C14 as Solid Contact: The Ultimate “Water Layer” Test

The aim of this study was to find a conducting polymer-based solid contact (SC) for ion-selective electrodes (ISEs) that could become the ultimate, generally applicable SC, which in combination with all kinds of ion-selective membranes (ISMs) would match the performance characteristics of conventional ISEs. We present data collected with electrodes utilizing PEDOT-C14, a highly hydrophobic derivative of poly(3,4-ethylenedioxythiophene), PEDOT, as SC and compare its performance characteristics with PEDOT-based SC ISEs. PEDOT-C14 has not been used in SC ISEs previously. The PEDOT-C14-based solid contact (SC) ion-selective electrodes (ISEs) (H+, K+, and Na+) have outstanding performance characteristics (theoretical response slope, short equilibration time, excellent potential stability, etc.). Most importantly, PEDOT-C14-based SC pH sensors have no CO2 interference, an essential pH sensors property when aimed for whole-blood analysis. The superhydrophobic properties (water contact angle: 136 ± 5°) of the PEDOT-C14 SC prevent the detachment of the ion-selective membrane (ISM) from its SC and the accumulation of an aqueous film between the ISM and the SC. The accumulation of an aqueous film between the ISM and its SC has a detrimental effect on the sensor performance. Although there is a test for the presence of an undesirable water layer, if the conditions for this test are not selected properly, it does not provide an unambiguous answer. On the other hand, recording the potential drifts of SC electrodes with pH-sensitive membranes in samples with different CO2 levels can effectively prove the presence or absence of a water layer in a short time period.

Medical Sensors for the Diagnosis and Management of Disease: The Physician Perspective

The objective of this paper is to assist developers of medical sensors to better formulate the clinically relevant design criteria and required performance characteristics of their novel sensor based on an understanding of how these devices will be used by physicians. Sensor technologies play a central role in medicine, and the most critical aspect of the sensors clinical utility relates to these design decisions. Clinically, sensors are used by health care providers to make both diagnostic and management decisions, and the sensors that aid in these decisions are evaluated by certain clinical, as well as analytical, criteria. Failure to adequately address these end-user requirements can lead to the development of sensors without clinical utility.