Andrzej Lewenstam

Potentiometric Ion Sensors

Åbo Akademi University, Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Biskopsgatan 8, FI-20500 Turku-Åbo, Finland; Faculty of Material Science and Ceramics, AGH-University of Science and Technology, Al. Mickiewicza 30, PL-30059 Cracow, Poland; and Åbo Akademi University, Process Chemistry Centre, c/o Center for Process Analytical Chemistry and Sensor Technology (ProSens), Biskopsgatan 8, FI-20500 Turku-Åbo, Finland

Electrochemical impedance spectroscopy of oxidized poly(3,4-ethylenedioxythiophene) film electrodes in aqueous solutions

The electrochemical properties of oxidized (p-doped) poly(3,4-ethylenedioxythiophene) (PEDOT) film electrodes in aqueous solutions were investigated by electrochemical impedance spectroscopy (EIS). PEDOT was electrochemically deposited on platinum from aqueous solutions containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.1 M supporting electrolyte: KCl, NaCl or poly(sodium 4-styrenesulfonate) (NaPSS). Impedance spectra were obtained for Pt/PEDOT electrodes at dc potentials where PEDOT is in the oxidized (p-doped) state. Electrodes with PEDOT films of different thickness, containing different doping ions, were investigated in contact with different aqueous supporting electrolyte solutions. The EIS data were fitted to an equivalent electrical circuit in order to characterize the electrochemical properties of the Pt/PEDOT film electrodes. Best fits to the experimental impedance data were obtained for an equivalent circuit where the total bulk (redox) capacitance of the polymer film is composed of the diffusional pseudocapacitance in series with a second bulk capacitance. The results imply that the PEDOT film contains an excess of supporting electrolyte, which facilitates ion diffusion and gives rise to a large diffusional pseudocapacitance.

Approved IFCC recommendation on reporting results for blood glucose

Abstract In current clinical practice, plasma and blood glucose are used interchangeably with a consequent risk of clinical misinterpretation. In human blood, glucose is distributed, like water, between erythrocytes and plasma. The molality of glucose (amount of glucose per unit water mass) is the same throughout the sample, but the concentration is higher in plasma, because the concentration of water and therefore glucose is higher in plasma than in erythrocytes. Different devices for the measurement of glucose may detect and report fundamentally different quantities. Different water concentrations in the calibrator, plasma, and erythrocyte fluid can explain some of the differences. Results for glucose measurements depend on the sample type and on whether the method requires sample dilution or uses biosensors in undiluted samples. If the results are mixed up or used indiscriminately, the differences may exceed the maximum allowable error for glucose determinations for diagnosing and monitoring diabetes mellitus, thus complicating patient treatment. The goal of the International Federation of Clinical Chemistry and Laboratory Medicine, Scientific Division, Working Group on Selective Electrodes and Point of Care Testing (IFCC-SD-WG-SEPOCT) is to reach a global consensus on reporting results. The document recommends reporting the concentration of glucose in plasma (in the unit mmol/L), irrespective of sample type or measurement technique. A constant factor of 1.11 is used to convert concentration in whole blood to the equivalent concentration in plasma. The conversion will provide harmonized results, facilitating the classification and care of patients and leading to fewer therapeutic misjudgments. Clin Chem Lab Med 2006;44:1486–90.

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.

Conducting polymer-based ion-selective electrodes

Single-piece, conducting polymer-based, potentiometric sensors with enhanced cationic sensitivity were obtained by doping a polypyrrole with metal-complexing, multivalent anions. Sulphosalycilic acid, Tiron, Eriochrome Black T and Kalces have been used as doping ions. Predominant sensitivity for copper was observed for the first two dopants, whereas for the two remaining ones, induced response for magnesium and calcium was found, respectively. Interestingly, in all sensors studied the metal-binding properties of the specific ligands known from polar solvents were retained. Hence, the specific reagents can be deliberately utilized for the construction of conducting polymer-based ion-selective sensors.

Influence of oxygen and carbon dioxide on the electrochemical stability of poly(3,4-ethylenedioxythiophene) used as ion-to-electron transducer in all-solid-state ion-selective electrodes

Abstract The electrochemical stability of poly(3,4-ethylenedioxythiophene) (PEDOT) is studied in view of its use as ion-to-electron transducer (solid contact) in all-solid-state ion-selective electrodes (ISEs). PEDOT is electrochemically deposited on glassy carbon (GC) and the resulting GC/PEDOT electrodes are studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and potentiometry. Valinomycin-based all-solid-state K + -ISEs are constructed by placing a K + -selective poly(vinyl chloride) (PVC)-based membrane on the GC/PEDOT electrode (solid contact). The influence of dissolved O 2 and CO 2 on the potential of the GC/PEDOT electrodes and of all-solid-state K + -ISEs is studied. PEDOT is compared with polypyrrole (PPy) as the solid contact material. A significant difference between the two conducting polymers (CPs) is that PEDOT is less sensitive to O 2 and CO 2 (pH) than PPy. Therefore, PEDOT is a promising new candidate as ion-to-electron transducer (solid contact) in all-solid-state ISEs based on solvent polymeric membranes that are permeable to O 2 and CO 2 .

Mechanism of ionic and redox sensitivity of p-type conducting polymers: Part 1. Theory

Equations have been derived to describe the open-circuit potentiometric response of a p-type conducting polymer membrane. The final model is derived step by step from the model of a single-metal electrode by using the concept of solvated electron and from the model of a solid-state ion-selective and/or electron- selective electrode containing a sparingly soluble metal-rich salt in the membrane. All electrodes considered, i.e. the metal, ion-selective and polymer electrodes, are presented as systems based on an ionic salt of the M+e− type. The ionic and redox sensitivities of a p-type conducting polymer are described by assuming two possible equilibration pathways. In the first approach the ion-electron exchange process at the polymer surface without a change of the oxidation state of the polymer is discussed and in the second the metathesis of the polymer surface with a concurrent change in the oxidation state of the polymer is considered. The equations derived allow us to interpret the anionic, cationic and redox sensitivities and to discuss the observed changes in the standard potentials.

All solid-state poly(vinyl chloride) membrane ion-selective electrodes with poly(3-octylthiophene) solid internal contact

All-solid-state ion-selective electrodes were prepared by using poly(3-octylthiophene)(POT) as solid contact material. A film of POT (thickness approximately 0.25 µm) was deposited on a solid substrate (platinum, gold or glassy carbon) by electrochemical polymerization of 3-octylthiophene. The POT layer was subsequently coated with an ion-selective membrane (ISM) to produce a solid-contact ion-selective electrode (SCISE), SCISEs for several ions (Li+, Ca2+ and Cl–) were prepared and investigated. The compositions of the ion-selective membranes were the same as normally used for the same ions in poly(vinyl chloride)(PVC)-based ion-selective electrodes (ISEs) with internal filling solution. The potentiometric response of the SCISEs was studied and compared with that of coated-wire electrodes (CWEs) prepared by coating the bare substrate with the same ion-selective membrane. The potentiometric slopes, limits of detection and response times of the SCISEs were similar to those of the corresponding CWEs, but the long-term stability of the potential was different for the two type of electrodes. The SCISEs exhibited a more stable electrode potential than the corresponding CWEs. However, the stability of the SCISEs was found to be influenced by the substrate material and this was studied in detail for the Ca-SCISE and Ca-CWE. For comparison, a Ca-ISE with internal filling solution was also used. By using glassy carbon as the substrate it was possible to obtain a Ca-SCISE exhibiting a standard potential that was almost as stable (ESCISE= 259.3 ± 1.3 mV, drift = 0.23 mV d–1) as for the conventional Ca-ISE (EISE= 61.4 ± 0.5 mV, drift = 0.16 mV d–) and significantly more stable than for the Ca-CWE, during a time period of 8 d. The most stable Ca-CWE, prepared by using glassy carbon as substrate, showed a potential drift of –3.8 mV d–1(ECWE= 269.6 ± 10.2 mV) during testing for 8 d. Electrochemical impedance spectrometry was used to understand the charge-transfer mechanisms of the different types of ion-selective electrodes studied. The impedance response of the electrodes was modelled by equivalent electrical circuits.

A polypyrrole-based amperometric ammonia sensor

An ammonia sensor is described in this work. The sensing membrane is a thin layer of oxidized polypyrrole (PPy) on a platinum substrate. This sensor is used as the working electrode in a conventional three-electrode system for amperometric measurement of ammonia in aqueous solutions in the potential range of + 0.2 to + 0.4 V (vs. Ag/AgCl). Contact with ammonia causes a current to flow through the electrode. This current is proportional to the concentration of free ammonia in the solution and ammonium ions do not contribute to the measured signal. The signal is due to reduction of PPy by ammonia with subsequent oxidation of PPy by the external voltage source. The sensor is able to detect ammonia reproducibly at the muM level. The main interference is the doping effect of small anions such as Cl(-) and NO(3)(-), also giving a response on PPy at the mM level. This anionic response can, to a certain degree, be reduced by covering the polymer surface with dodecyl sulfate. The sensor gradually loses its activity when exposed to ammonia concentrations greater than 1 mM. The sensor has been tested by the flow injection analysis technique.

Kinetics of electron transfer between Fe(CN)63−/4− and poly(3,4-ethylenedioxythiophene) studied by electrochemical impedance spectroscopy

The electron transfer between the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and the Fe(CN)63−/4− redox couple in aqueous solution was investigated by electrochemical impedance spectroscopy (EIS). PEDOT was electrochemically deposited on platinum from aqueous solutions containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.1 M poly(sodium 4-styrenesulfonate) (NaPSS) as supporting electrolyte. Pt/PEDOT(PSS) electrodes with polymer films of different thickness were investigated at different concentrations of the redox couple in 0.1 M KCl background electrolyte solution. Impedance spectra were obtained at the dc-potential corresponding to the formal redox potential of Fe(CN)63−/4− (E°′≈220 mV) where the polymer is in the oxidized and electrically conducting state. The EIS data for the electrodes were fitted to an equivalent electrical circuit. The standard rate constant (ko) for electron-transfer between Pt/PEDOT(PSS) and Fe(CN)63−/4− was determined by calculations based on the Butler–Volmer equation. The diffusion coefficient (D) of the redox couple was also calculated from the EIS data.