Maria Cuartero

Paper-Based Thin-Layer Coulometric Sensor for Halide Determination

We report on a paper-based analytical device (PAD) for the exhaustive, and therefore absolute, determination of halides in a range of diverse water samples and food supplements. A mixture of chloride, bromide, and iodide ions is assessed in a wide range of concentrations, specifically, from 10(-4.8) to 0.1 M for bromide and iodide and from 10(-4.5) to 0.6 M for chloride, with a limit of detection of 10(-5) M. As a result of a careful optimization of the electrochemical cell, a thin layer made of cellulose paper (75-μm thickness), a cation-exchange Donnan exclusion membrane (FKL), and a silver-foil working electrode were selected as optimum materials. Cyclic voltammetry (from 0 to 0.8 V) was chosen as the interrogation technique to impose the exhaustive oxidative plating and re-reduction of halides on the silver element, accompanied by outward and inward counterion fluxes. The scan rate plays an important role in the ability of the technique to resolve mixtures of ions. Moderate scan rates (10 mV s(-1)) provide a suitable compromise between sensitivity, limit of detection, and resolution. This paper-based microfluidic device is extremely simple in terms of manipulation, cost, and contamination risk. Paper is an excellent basis for the establishment of a confined thin aqueous layer, the construction of disposable halide sensors, and portability for measuring outside the controlled laboratory environment. A discussion of the relevant analytical characteristics is presented herein, followed by a demonstration of halide assessment in water samples (sea, tap, river, and mineral waters) and food supplements enriched with iodide and chloride as early examples.

All-Solid-State Potentiometric Sensors with a Multiwalled Carbon Nanotube Inner Transducing Layer for Anion Detection in Environmental Samples

While ion to electron transducing layers for the fabrication of potentiometric membrane electrodes for the detection of cations have been well established, similar progress for the sensing of anions has not yet been realized. We report for this reason on a novel approach for the development of all-solid-state anion selective electrodes using lipophilic multiwalled carbon nanotubes (f-MWCNTs) as the inner ion to electron transducing layer. This material can be solvent cast, as it conveniently dissolves in tetrahydrofuran (THF), an important advantage to develop uniform films without the need for using surfactants that might deteriorate the performance of the electrode. Solid contact sensors for carbonate, nitrate, nitrite, and dihydrogen phosphate are fabricated and characterized, and all exhibit comparable analytical characteristics to the inner liquid electrodes. For example, the carbonate sensor exhibits a Nernstian slope of 27.2 ± 0.8 mV·dec(-1), a LOD = 2.3 μM, a response time of 1 s, a linear range of four logarithmic units, and a medium-term stability of 0.04 mV·h(-1) is obtained in a pH 8.6 buffered solution. Water layer test, reversibility, and selectivity for chloride, nitrate, and hydroxide are also reported. The excellent properties of f-MWCNTs as a transducer are contrasted to the deficient performance of poly(3-octyl-thiophene) (POT) for carbonate detection. This is evidenced both with a significant drift in the potentiometric measures as well as a pronounced sensitivity to light (either sunlight or artificial light). This latter aspect may compromise its potential for environmental in situ measurements (night/day cycles). The concentration of carbonate is determined in a river sample (Arve river, Geneva) and compared to a reference method (automatic titrator with potentiometric pH detection). The results suggest that nanostructured materials such as f-MWCNTs are an attractive platform as a general ion-to-electron transducer for anion-selective electrodes.

Thin Layer Ionophore-Based Membrane for Multianalyte Ion Activity Detection

A concept is introduced that allows one to detect the activity of multiple ions simultaneously and selectively with a single ion-selective membrane. This is demonstrated with ∼300 nm thin plasticized PVC membranes containing up to two ionophores in addition to a lipophilic cation-exchanger, overlaid on an electropolymerized poly-3-octylthiophene (POT) film as the electron to ion transducer. The ion-selective membranes are formulated under ionophore depleted conditions (avoiding excess of ionophore over ion-exchanger), which is purposely different from common practice with ion-selective electrodes. Cyclic voltammetry is used to interrogate the films. An anodic scan partially oxidizes the POT underlayer, which results in the expulsion of cations from the membrane at an appropriate potential. During the scan of a membrane containing multiple ionophores, the least bound ion is expelled first, giving distinct Gaussian peak shaped ion transfer voltammetric waves that are analyzed in terms of their peak potential. These potentials are found to change with the logarithm of the ion activity, in complete analogy to ion-selective electrodes, and multiple such waves are observed with multiple ionophores that exhibit no obvious interference from the other ionophores present in the membrane. The concept is established with lithium and calcium ionophores and accompanied by a response model that assumes complete equilibration of the membrane at every applied potential. On the basis of the model, diffusion coefficients in the membrane or aqueous phase bear no influence on the peak potentials as long as thin layer behavior is observed, further confirming the analogy to a potentiometric experiment. Idealized ion transfer waves are narrower than experimental findings, which is explained by a broader than expected anodic peak for the oxidation of conducting polymer. The correspondence between experiment and theory is otherwise excellent in terms of thin layer behavior and Nernstian shift of the peaks with analyte concentration.

Exhaustive thin-layer cyclic voltammetry for absolute multianalyte halide detection.

Water analysis is one of the greatest challenges in the field of environmental analysis. In particular, seawater analysis is often difficult because a large amount of NaCl may mask the determination of other ions, i.e., nutrients, halides, and carbonate species. We demonstrate here the use of thin-layer samples controlled by cyclic voltammetry to analyze water samples for chloride, bromide, and iodide. The fabrication of a microfluidic electrochemical cell based on a Ag/AgX wire (working electrode) inserted into a tubular Nafion membrane is described, which confines the sample solution layer to less than 15 μm. By increasing the applied potential, halide ions present in the thin-layer sample (X(-)) are electrodeposited on the working electrode as AgX, while their respective counterions are transported across the perm-selective membrane to an outer solution. Thin-layer cyclic voltammetry allows us to obtain separated peaks in mixed samples of these three halides, finding a linear relationship between the halide concentration and the corresponding peak area from about 10(-5) to 0.1 M for bromide and iodide and from 10(-4) to 0.6 M for chloride. This technique was successfully applied for the halide analysis in tap, mineral, and river water as well as seawater. The proposed methodology is absolute and potentially calibration-free, as evidenced by an observed 2.5% RSD cell to cell reproducibility and independence from the operating temperature.

Ionophore-Based Voltammetric Ion Activity Sensing with Thin Layer Membranes

As shown in recent work, thin layer ion-selective multi-ionophore membranes can be interrogated by cyclic voltammetry to detect the ion activity of multiple species simultaneously and selectively. Additional fundamental evidence is put forward on ion discrimination with thin multi-ionophore-based membranes with thicknesses of 200 ± 25 nm and backside contacted with poly-3-octylthiophene (POT). An anodic potential scan partially oxidizes the POT film (to POT(+)), thereby initiating the release of hydrophilic cations from the membrane phase to the sample solution at a characteristic potential. Varying concentration of added cation-exchanger demonstrates that it limits the ion transfer charge and not the deposited POT film. Voltammograms with multiple peaks are observed with each associated with the transfer of one type of ion (lithium, potassium, and sodium). Experimental conditions (thickness and composition of the membrane and concentration of the sample) are chosen that allow one to describe the system by a thermodynamic rather than kinetic model. As a consequence, apparent stability constants for sodium, potassium, and lithium (assuming 1:1 stoichiometry) with their respective ionophores are calculated and agree well with the values obtained by the potentiometric sandwich membrane technique. As an analytical application, a membrane containing three ionophores was used to determine lithium, sodium, and potassium in artificial samples at the same location and within a single voltammetric scan. Lithium and potassium were also determined in undiluted human plasma in the therapeutic concentration range.

Tandem Electrochemical Desalination-Potentiometric Nitrate Sensing for Seawater Analysis

We report on a methodology for the direct potentiometric determination of nitrate in seawater by in-line coupling to an electrochemical desalination module. A microfluidic custom-fabricated thin layer flat cell allows one to electrochemically reduce the chloride concentration of seawater more than 100-fold, from 600 mM down to ∼2.8 mM. The desalinator operates by the exhaustive electrochemical plating of the halides from the thin layer sample onto a silver element as silver chloride, which is coupled to the transfer of the counter cations across a permselective ion-exchange membrane to an outer solution. As a consequence of suppressing the major interference of an ion-exchanger based membrane, the 80 μL desalinated sample plug is passed to a potentiometric flow cell of 13 μL volume. The potentiometric sensor is composed of an all-solid-state nitrate selective electrode based on lipophilic carbon nanotubes (f-MWCNTs) as an ion-to-electron transducer (slope of -58.9 mV dec(-1), limit of detection of 5 × 10(-7) M, and response time of 5 s in batch mode) and a miniaturized reference electrode. Nitrate is successfully determined in desalinated seawater using ion chromatography as the reference method. It is anticipated that this concept may form an attractive platform for in situ environmental analysis of a variety of ions that normally suffer from interference by the high saline level of seawater.

Polyurethane Ionophore-Based Thin Layer Membranes for Voltammetric Ion Activity Sensing

We report on a plasticized polyurethane ionophore-based thin film material (of hundreds of nanometer thickness) for simultaneous voltammetric multianalyte ion activity detection triggered by the oxidation/reduction of an underlying poly(3-octylthiophene) film. This material provides excellent mechanical, physical, and chemical robustness compared to other polymers. Polyurethane films did not exhibit leaching of lipophilic additives after rinsing with a direct water jet and exhibited resistance to detachment from the underlying electrode surface, resulting in a voltammetric current response with less than <1.5% RSD variation (n = 50). In contrast, plasticized poly(vinyl chloride), polystyrene, and poly(acrylate) ionophore-based membranes of the same thickness and composition exhibited a significant deterioration of the signal after identical treatment. While previously reported works emphasized fundamental advancement of multi-ion detection with multi-ionophore-based thin films, polyurethane thin membranes allow one to achieve real world measurements without sacrificing analytical performance. Indeed, polyurethane membranes are demonstrated to be useful for the simultaneous determination of potassium and lithium in undiluted human serum and blood with attractive precision.

Electrochemical Ion Transfer with Thin Films of Poly(3-octylthiophene)

We report on the limiting conditions for ion-transfer voltammetry between an ion-exchanger doped and plasticized poly(vinyl chloride) (PVC) membrane and an electrolyte solution that was triggered via the oxidation of a poly(3-octylthiophene) (POT) solid-contact (SC), which was unexpectedly related to the thickness of the POT SC. An electropolymerized 60 nm thick film of POT coated with a plasticized PVC membrane exhibited a significant sodium transfer voltammetric signal whereas a thicker film (180 nm) did not display a measurable level of ion transfer due to a lack of oxidation of thick POT beneath the membrane film. In contrast, this peculiar phenomenon was not observed when the POT film was in direct contact with an organic solvent-based electrolyte. This evidence is indicative of three key points: (i) the coated membrane imposes a degree of rigidity to the system, which restricts the swelling of the POT film and its concomitant p-doping; (ii) this phenomenon is exacerbated with thicker POT films due to an initial morphology (rougher comprising a network of large POT nanoparticles), which gives rise to a diminished surface area and electrochemical reactivity in the POT SC; (iii) the rate of sodium transfer is higher with a thin POT film due to a smoother surface morphology made up of a network of smaller POT nanoparticles with an increased surface area and electrochemical reactivity. A variety of techniques including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), ellipsometry, scanning electron microscopy (SEM), atomic force microscopy (AFM), and synchrotron radiation-X-ray photoelectron spectroscopy (SR-XPS) were used to elucidate the mechanism of the POT thickness/POT surface roughness dependency on the electrochemical reactivity of the PVC/POT SC system.

Voltammetric Thin-Layer Ionophore-Based Films: Part 1. Experimental Evidence and Numerical Simulations

Voltammetric thin layer (∼200 nm) ionophore-based polymeric films of defined ion-exchange capacity have recently emerged as a promising approach to acquire multi-ion information about the sample, in analogy to performing multiple potentiometric measurements with individual membranes. They behave under two different regimes that are dependent on the ion concentration. A thin layer control (no mass transport limitation of the polymer film or solution) is identified for ion concentrations of >10 μM, in which case the peak potential serves as the readout signal, in analogy to a potentiometric sensor. On the other hand, ion transfer at lower concentrations is chiefly controlled by diffusional mass transport from the solution to the sensing film, resulting in an increase of peak current with ion concentration. This concentration range is suitable for electrochemical ion transfer stripping analysis. Here, the transition between the two mentioned scenarios is explored experimentally, using a highly silver-selective membrane as a proof-of-concept under different conditions (variation of ion concentration in the sample from 0.1 μM to 1 mM, scan rate from 25 mV s-1 to 200 mV s-1, and angular frequency from 100 rpm to 6400 rpm). Apart from experimental evidence, a numerical simulation is developed that considers an idealized conducting polymer behavior and permits one to predict experimental behavior under diffusion or thin-layer control.

In-Line Acidification for Potentiometric Sensing of Nitrite in Natural Waters

We report on a novel approach for in-line sample acidification that results in a significant improvement in the limit of detection of potentiometric anion-selective electrodes aiming at determining nutrients in natural waters. The working principle of the developed acidification module relies on the cation-exchange process between the sample and an ion-exchange Donnan exclusion membrane in its protonated form. The resulting in-line acidification of natural waters with millimolar sodium chloride level (freshwater, drinking water, and aquarium water, as well as dechloridized seawater) decreases the pH down to ∼5. By using the acidification module, the limit of detection of nitrite-selective electrodes significantly improves by more than 2 orders of magnitude with respect to that observed at environmental pH. The originality of the proposed flow cell lies in the possibility to adjust the pH of the sample by modifying its exposure time with the membrane by varying the volumetric flow rate. Facile coupling with a detection technique of choice, miniaturized configuration and simple implementation for long-term monitoring with submersible probes for environmental analysis are possible analytical configurations. This approach was here successfully applied for the potentiometric detection of nitrite in aquarium and dechloridized seawater samples.