Direct and two-stage data analysis procedures based on PCA, PLS-DA and ANN for ISE-based electronic tongue-Effect of supervised feature extraction

A novel strategy of data analysis for artificial taste and odour systems is presented in this work. It is demonstrated that using a supervised method also in feature extraction phase enhances fruit juice classification capability of sensor array developed at Warsaw University of Technology. Comparison of direct processing (raw data processed by Artificial Neural Network (ANN), raw data processed by Partial Least Squares-Discriminant Analysis (PLS-DA)) and two-stage processing (Principal Components Analysis (PCA) outputs processed by ANN, PLS-DA outputs processed by ANN) is presented. It is shown that considerable increase of classification capability occurred in the case of the new method proposed by the authors.     

A hybrid genetic algorithm for estimating the equilibrium potential of an ion-selective electrode

Non-linear equations can be used to model the measured potential of ion-selective electrodes (ISEs) as a function of time. This can be done by using non-linear least squares regression to fit parameters of non-linear equations to an ISE response curve. In iterative non-linear least squares regression (which can be considered as local optimisers), the determination of starting parameter estimates that yield convergence to the global optimum can be difficult. Starting values away from the global optimum can lead to either abortive divergence or convergence to a local optimum. To address this issue, a global optimisation technique was used to find initial parameter estimates near the global optimum for subsequent further refinement to the absolute optimum. A genetic algorithm has been applied to two non-linear equations relating the measured potential from selected ISEs to time. The parameter estimates found from the genetic algorithm were used as starting values for non-linear least squares regression, and subsequent refinement to the absolute optimum. This approach was successfully used for both expressions with measured data from three different ISEs; namely, calcium, chloride and lead ISEs.     

Kinetic response of an oxygen-selective electrode evaluated for the quantification of oxygen

An evaluation of the transient response of a membrane-based amperometric electrode for the quantification of oxygen in aqueous and gaseous phases is described. Data collected during the early part of the responses are used to compute the steady-state signals that would be measured if the responses were monitored to steady state. Both first-order and variable-order models were used successfully to fit data for aqueous samples; only the variable-order model worked successfully with data for gaseous samples. Because the shapes of response curves did not permit steady-state signals to be measured, comparisons are based on measured values of current at fixed times in each response. The slopes of computed vs. measured signals varied from 0.90 to 1.03 for fits of data over 3–5 half-lives with both models. Calibration graphs of computed current vs. concentrations were linear for similar fitting ranges and the least-squares parameters were similar to those for current measured at different fixed points in time.     

Fully automated potentiometric determination of ionized magnesium in blood serum

Basic observations leading to the successful application of magnesium ion-selective electrodes for the automated determination of ionized magnesium in undiluted serum are discussed. The principles of the method are described and the first numerical results are reported.     

Development and application of a fully automated,battery-operated,computerized field-based fluoride monitor

An automated, portable, battery-operated, computerized field-based monitor for the determination of fluoride based on the use of ion-selective electrode (ISE) potentiometry has been developed with the aid of low-powered complementary metal oxide semiconductor (CMOS) devices. The whole analytical cycle involving the rinsing of the cell, sampling, stirring, dosing of standards to the sample and data acquisition and manipulation is under microprocessor control. The modular instrument consists of pumps, valves, a flow-through cell containing a reference electrode, a fluoride ISE, a temperature probe and a stirrer, a microprocessor with a real-time clock, a pump-valve-stirrer interface, a portable terminal and a 12-V lead-acid battery to power all the instrumentation. The software for the application and monitoring functions for the instrument is written at assembler level and programmed into a CMOS erasable programmable read only memory (EPROM). The instrument is currently designed to determine fluoride in natural and fluoridated waters and is based on a double standard addition method, although the monitor can be modified easily to suit other appropriate ISE systems. Instrumental performance was evaluated with synthetic and real water samples both in the batch and continuous modes. The monitor can be used to carry out on-line fluoride determinations of water samples continuously for 1 week without anyone being attendance.     

Reproterol plastic membrane ion-selective electrodes based on its individual and mixed ion-exchangers with phosphotungstic and/or phosphomolybdic acids

Reproterol hydrochloride (RpCl), selective PVC membranes based on ion associates of reproterolium-phosphotungstate (Rp-PTA); reproterolium-phospho-molybdate (Rp-PMA) or a mixture of both (Rp-PTA/PMA) were prepared. The electrodes displayed a linear response over the concentration range of 6.3×10⁻⁶–1.0×10⁻¹ mol dm⁻³ RpCl. The working pH ranges of the above electrodes were 2.5–9.0, 2.5–8.5 and 2.0–9.0 and their isothermal temperature coefficients were 0.00014, 0.00090 and 0.00103 V/°C, respectively. The electrodes showed good selectivity to the reproterolium ion with respect to many inorganic cations, sugars and amino acids. The standard additions and potentiometric titration methods were used to determine RpCl in pure solutions and in its pharmaceutical preparations with high accuracy and precision.     

Potassium ion-selective electrode based on a cobalt(II)-hexacyanoferrate film-modified electrode

A potassium ion-selective electrode based on a cobalt(II)-hexacyanoferrate(III) (CHCF) film-modified glassy carbon electrode is proposed. The electroactive film is introduced onto the glassy carbon electrode surface by electrodeposition of cobalt, which forms a thin CHCF film on subsequent anodic scanning in KClHCl solution (pH 5.0–5.5) containing K₃Fe(CN)₆. The thickness of the film on the electrode surface can be controlled by changing the electrodeposition time and the concentrations of cobalt(II) and Fe(CN)^3?₆ ions. The modified electrode exhibits a linear response in the concentration range 1 × 10^?1 ?3 × 10^?5 M potassium ion activity, with a near-Nernstian slope (48–54 mV per decade) at 25 ± 1°C. The detection limit is 1 × 10^?5 M. The stability, response time and selectivity were investigated. The electrode exhibits good selectivity for potassium ion with the twelve cations investigated. The relative standard deviation is 1.5% (n=10). The effects of the thickness of the electroactive film and the pH of the solution on the electrode response were also investigated.     

Sulphonamides as ionophores for ion-selective electrodes. I

A series of new compounds: di-sulphonamides and mono-sulpho-mono-carboxy-di-amides has been synthesized. Comparison of the ionophoric properties of these compounds in liquid membrane electrodes was performed and the correlation between the chemical structure and potentiometric ionselectivity has been studied.     

Differential ion-selective membrane electrode-based potentiometric gas-sensing cells with enhanced gas sensitivity

A novel design for devising static and automated flow-through type potentiometric gas-sensing systems with enhanced response slopes is described. The approach involves the use of two working gas-sensing electrode half-cells in a differential measurement arrangement. One of the half-cells employs a pH-sensitive polymeric membrane electrode to sense pH changes from diffusing analyte gas within a suitable inner electrolyte solution housed behind an outer gas-permeable membrane. The second working half-cell is fabricated with an anion- or cation-selective membrane electrode that responds selectively to the conjugate acid or base ionic form of the analyte gas trapped within an inner buffer solution housed behind a similar gas-permeable membrane. When the two internal solutions of the half-cells are in electrolytic contact, the differential response of the resulting gas-sensing scheme is significantly enhanced compared with the response of a conventional single working electrode/reference electrode type gas cell. For the model analyte gas ammonia, response slopes observed for both static and flow-through measurement schemes approach the 118 mV decade^?1 predicted by theory. To demonstrate its analytical utility, the flow-through arrangement was used to determine ammonia-N concentrations in bioreactor media with good correlation with conventional electrode and enzymatic methods. The prospects of fabricating similar differential detection arrangements for CO₂, NO₂ and SO₂ are discussed.     

Photolithographically patternable nitrate-sensitive acrylate-based membrane

A photocurable acrylate matrix nitrate-sensitive membrane containing 2-nitrophenyl n-octyl ether as mediator and tetraalkylammonium nitrate as an active compound is described. The photocuring was achieved by the use of photoinitiators containing diaryliodonium chloride. This acrylate membrane is patternable via a photolithographic process with a only slight loss of electrochemical characteristics.     

Sterically encumbered hexakis(alkylseleno)benzenes: conformational behavior of hexakis(iso-propylselenomethyl)benzene toward Hg ions on selective recognition

An efficient synthesis and structural aspects of a novel class of hexakis(alkylseleno)benzenes [(RSeCH₂)₆C₆] (R = Me, ⁱPr, ⁿBu, ^sBu, ^tBu, ⁿPn, ⁿHx, ⁿOct, 1-methylnaphthalene) by the reaction of hexakis(bromomethyl)benzene with RSe⁻ ions is demonstrated. Preliminary data on ion-sensing properties reveal that these species may act as selective ionophores for Hg²⁺ ions.     

Plasticizer-free microspheres for ionophore-based sensing and extraction based on a methyl methacrylate-decyl methacrylate copolymer matrix

Plasticizer-free methyl methacrylate-decyl methacrylate (MMA-DMA) microspheres were prepared under mild, non-reactive conditions using a high-throughput particle generator. The particles were perfectly smooth and monodisperse, with a particle diameter of approximately 10.0 μm. In order to evaluate the suitability of the polymer as a matrix for bulk extraction processes, lipophilic sensing components were incorporated into the particles. Particles contained either a H⁺-selective chromoionophore (ETH 5294) only (type 1), or a K⁺-selective ionophore (BME-44), anionic sites (NaTFPB), and ETH 5294 (type 2). Type 1 particles responded according to an anion–proton coextraction mechanism and demonstrated Hofmeister selectivity by showing a preference for more lipophilic sample anions (ClO₄⁻>NO₃⁻>Cl⁻). Particles of type 2 functioned by way of an ion-exchange equilibrium and demonstrated a functional response for K⁺, with a dynamic range from 10⁻¹–10⁻⁴ M K⁺. These particles also exhibited selectivity comparable to that previously reported for analogous particles made from bis(2-ethylhexyl sebacate) (DOS)-plasticized poly(vinyl chloride) (PVC) and DMA-DOS. In addition, the behavior of both types1 and 2 particles was in agreement with analogous thin film optical sensors (optodes) prepared from MMA-DMA. With the advent of ionophore-based plasticizer-free microspheres a wide variety of ions may potentially be assessed using various popular bead-based sensing strategies, such as lab-on-a-chip technologies, bundled optical fiber arrays, and flow cytometry, without experiencing the deleterious effects resultant of plasticizer leaching.     

Light sensitivity and potential stability of electrically conducting polymers commonly used in solid contact ion-selective electrodes

The results of a systematic study of the light sensitivity and long-term potential stability (30 days) of poly(pyrrole) (PPy), poly(3-octylthiophene) (POT), poly(3,4-ethylenedioxythiophene) (PEDOT), poly(aniline) (PANI) and plasticised poly(vinyl chloride) (PVC) containing 20% (m/m) PANI are reported. Thin films were prepared either electrochemically or by the solution casting technique. This fundamental study is of importance because conducting polymers (CP) are commonly used as ion-to-electron transduction materials in all-solid-state solid contact ion-selective electrodes. The potential stability test done in 0.1 M KCl (pH 7.5) simulates the extreme situation when the CP-based SC becomes in direct contact with water. Films prepared of a nanodispersion of PANI showed both good potential stability and insensitivity to light even under illumination with very intensive light (>10⁵ lx). In contrary, it was observed that POT is very light-sensitive. Upon illumination with intensive light, the potential responses of POT films prepared by solution casting and electropolymerisation were 315 and 590 mV, respectively. A room light sensitivity of approximately −10 to −15 mV was observed for these films. The other CPs in this study were insensitive to room light (∼150 lx), but were light-sensitive under illumination with intensive light. The potential drift of PPy(Cl) is below −10 μV/h (3–30 days), whereas the other most stable CPs in this study had a slightly higher potential drift.     

Prussian blue solid-state films and membranes as potassium ion-selective electrodes

The use of thin films of Prussian blue and heterogeneous Prussian blue membranes as potassium ion-selective electrodes was investigated. All of the heavier group I cations and NH⁺₄ interfere strongly but there is relatively good selectivity towards Na⁺ with a selectivity coefficient of ca. 5 × 10^?3. The thin-film measurements, based on Prussian blue deposited on platinum, involve conditioning the electrode to a fixed potential according to the method used by Engel and Grabner for copper hexacyanoferrate(III) films. The membrane electrodes were based on mixing Prussian blue with polymeric supporting films such as polystyrene and epoxy. A particularly simple practical configuration involves Prussian blue membranes deposited directly on copper conductors where one membrane serves as a reference electrode. A reversible cell, without liquid junction, is formed with Prussian blue and Ag/AgCl electrodes and this serves as a means for determining an accurate value for the standard reduction potential of Prussian blue, which is found to be 0.238 V vs. Ag/AgCl at 25 °C.     

Potentiometric measurements in sequential injection analysis lab-on-valve (SIA-LOV) flow-system

Advantages of using sequential injection analysis lab-on-valve (SIA-LOV) in potentiometric measurements are studied with Ca²⁺ sensitive solid-contact ion-selective electrodes (SC-ISE) and pH electrode based on polyaniline (PANI). Experiments show that Ca²⁺-SC-ISE requires a stopped flow mode to be used in order to get longer equilibration time. On the other hand, PANI based pH electrode gives better results under flow conditions. SIA-LOV was found to be a flexible solution handling system for potentiometric measurements. The technique gives a possibility to fine-tune the calibration range or even recondition the electrode before every measurement without losing in linear range of calibration.     

A copper ion-selective electrode with high selectivity prepared by sol-gel and coated wire techniques

A sol-gel electrode and a coated wire ion-selective poly(vinyl chloride) membrane, based on thiosemicarbazone as a neutral carrier, were successfully developed for the detection of Cu (II) in aqueous solutions. The sol-gel electrode and coated electrode exhibited linear response with Nernstian slopes of 29.2 and 28.1 mV per decade respectively, within the copper ion concentration ranges 1.0×10^–5–1.0×10^–1 M and 6.0×10^–6–1.0×10^–1 M for coated and sol-gel sensors. The coated and sol-gel electrodes show detection limits of 3.0×10^–6 and 6.0×10^–6 M respectively. The electrodes exhibited good selectivities for a number of alkali, alkaline earth, transition and heavy metal ions. The proposed electrodes have response times ranging from 10–50 s to achieve a 95% steady potential for Cu²⁺ concentration. The electrodes are suitable for use in aqueous solutions over a wide pH range (4–7.5). Applications of these electrodes for the determination of copper in real samples, and as an indicator electrode for potentiometric titration of Cu²⁺ ion using EDTA, are reported. The lifetimes of the electrodes were tested over a period of six months to investigate their stability. No significant change in the performance of the sol-gel electrode was observed over this period, but after two months the coated wire copper-selective electrode exhibited a gradual decrease in the slope. The selectivity of the sol-gel electrode was found to be better than that of the coated wire copper-selective electrode. Based on these results, a novel sol-gel copper-selective electrode is proposed for the determination of copper, and applied to real sample assays.     

Measuring the solubilities of ionic liquids in water using ion-selective electrodes

Polyvinyl chloride-plasticized membrane ion-selective electrodes (ISE) based on conventional ion-exchangers have been proposed as a cheap universal tool to measure the solubilities of ionic liquids (ILs) in water. They are applicable for ILs with a wide range of solubilities in water, since the linear range of a potentiometric response spans several orders of magnitude. As an example, we have fabricated and tested ISEs for widely used alkylimidazolium ionic liquids. The aqueous solubilities of four typical ILs have been determined at 21 °C: 0.075±0.001 mol l^–1 (1-butyl-3-methylimidazolium, BMIm, hexafluorophosphate); 0.018±0.001 mol l^–1 (BMIm bis(triflylimide)); 0.054±0.007 mol l^–1 (1-butyl-2,3-dimethylimidazolium, BDMIm, hexafluorophosphate); 0.014±0.001 mol l^–1 (BDMIm bis(triflylimide)).     

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.     

四种双冠醚的铯离子选择电极

以合成的四种双（苯并－１８－冠－６）为中性载体制成了ＰＶＣ膜铯离子电极。研究了载体结构，电极膜组成和内充液浓度与电极性能的关系，以邻苯二甲酸二辛酯增塑的４＇，４＂－（邻－亚苯基二氧亚甲基）双（苯并－１８－冠－６）铯离子电极的选择性较好，Ｋ＾ｐ＾ｏ＾ｔＣｓ，ｋ＝２．４×１０＾－＾２，２０℃时，该电极的线性响应范围为５×１０＾－＾５－５×１０＾－＾２ｍｏｌ／Ｌ（ＣｓＣｌ），斜率接近理论值。     

Potentiometric responses of ion-selective electrodes after galvanostatically controlled incorporation of primary ions

A new method of quantitative incorporation of primary cations into ion-selective membrane by means of galvanostatic cathodic polarization/conditioning, before measurement step, was proposed and tested on the example of potassium-selective electrode with ionophore - valinomycin in poly(vinyl chloride) based membrane and with polypyrrole solid contact. Open circuit potential values recorded after polarization can be quantitatively explained by changes of primary cations and ionophore concentration in the surface part of the membrane. The influence of potassium ions concentration in the membrane (in relation to ion exchange sites amount) on the shape of potentiometric calibration plots was also observed. Improved characteristics, with extended linear range, can be obtained for membrane of minor loading with primary cations (around 25%), the responses are relatively stable in course of following calibrations.