Majid Ghahraman Afshar

Reversible Sensing of the Anticoagulant Heparin with Protamine Permselective Membranes

A permselective membrane electrode allows the rapid and operationally reversible detection of the polycationic polypeptide protamine in physiological samples. Anticoagulant levels of heparin can be measured in undiluted whole blood by adding a known excess of its antidote protamine to discrete blood samples.

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.

Potentiometric sensors with ion-exchange Donnan exclusion membranes.

Potentiometric sensors that exhibit a non-Hofmeister selectivity sequence are normally designed by selective chemical recognition elements in the membrane. In other situations, when used as detectors in separation science, for example, membranes that respond equally to most ions are preferred. With so-called liquid membranes, a low selectivity is difficult to accomplish since these membranes are intrinsically responsive to lipophilic species. Instead, the high solubility of sample lipids in an ionophore-free sensing matrix results in a deterioration of the response. We explore here potentiometric sensors on the basis of ion-exchange membranes commonly used in fuel cell applications and electrodialysis, which have so far not found their way into the field of ion-selective electrodes. These membranes act as Donnan exclusion membranes as the ions are not stripped of their hydration shell as they interact with the membrane. Because of this, lipophilic ions are no longer preferred over hydrophilic ones, making them promising candidates for the detection of abundant ions in the presence of lipophilic ones or as detectors in separation science. Two types of cation-exchanger membranes and one anion-exchange membrane were characterized, and potentiometric measuring ranges were found to be Nernstian over a wide range down to about 10 μM concentrations. Depending on the specific membrane, lipophilic ions gave equal response to hydrophilic ones or were even somewhat discriminated. The medium and long-term stability and reproducibility of the electrode signals were found to be promising when evaluated in synthetic and whole blood samples.

Direct Detection of Acidity, Alkalinity, and pH with Membrane Electrodes

An electrochemical sensing protocol based on supported liquid ion-selective membranes for the direct detection of total alkalinity of a sample that contains a weak base such as Tris (pK(a) = 8.2) is presented here for the first time. Alkalinity is determined by imposing a defined flux of hydrogen ions from the membrane to the sample with an applied current. The transition time at which the base species at the membrane-sample interface depletes owing to diffusion limitation is related to sample alkalinity in this chronopotentiometric detection mode. The same membrane is shown to detect pH (by zero current potentiometry) and acidity and alkalinity (by chronopotentiometry at different current polarity). This principle may become a welcome tool for the in situ determination of these characteristics in complex samples such as natural waters.

Direct Ion Speciation Analysis with Ion-Selective Membranes Operated in a Sequential Potentiometric/Time Resolved Chronopotentiometric Sensing Mode

Ion-selective membranes based on porous polypropylene membranes doped with an ionophore and a lipophilic cation-exchanger are used here in a new tandem measurement mode that combines dynamic electrochemistry and zero current potentiometry into a single protocol. Open circuit potential measurements yield near-nernstian response slopes in complete analogy to established ion-selective electrode methodology. Such measurements are well established to give direct information on the so-called free ion concentration (strictly, activity) in the sample. The same membrane is here also operated in a constant current mode, in which the localized ion depletion at a transition time is visualized by chronopotentiometry. This dynamic electrochemistry methodology gives information on the labile ion concentration in the sample. The sequential protocol is established on potassium and calcium ion-selective membranes. An increase of the ionophore concentration in the membrane to 180 mM makes it possible to determine calcium concentrations as high as 3 mM by chronopotentiometry, thereby making it possible to directly detect total calcium in undiluted blood samples. Recovery times after current perturbation depend on the current amplitude but can be kept to below 1 min for the polypropylene based ion-selective membranes studied here. Plasticized PVC as membrane material is less suited for this protocol, especially when the measurement at elevated concentrations is desired. An analysis of current amplitudes, transition times, and concentrations shows that the data are described by the Sand equation and that migration effects are insignificant. A numerical model describes the experimental findings with good agreement and gives guidance on the required selectivity in order to observe a well-resolved transition time and on the expected errors due to insufficient selectivity. The simulations suggest that the methodology compares well to that of open circuit potentiometry, despite giving complementary information about the sample. The tandem methodology is demonstrated in a titration of calcium with nitrilotriacetic acid (NTA) and in the direct detection of calcium in undiluted heparinized and citrated blood.

Using a new ligand for solid phase extraction of mercury.

The octadecyl silica cartridge as a sorbent and 4-bp db (1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a ligand is a simple, rapid and reliable method for extracting and preconcentrating of Hg(II) in real samples prior to cold vapor atomic absorption spectrometry. Sample solutions were passed through a column at pH 4.5 then retained mercury ions on the column were eluted with minimal amount of 0.01 M nitric acid with 3 mL min(-1) flow rate. The effect of pH, type of buffer, flow rate of sample and eluent, type and volume of the eluent were investigated and optimized. At optimum effective parameters, concentration factor and detection limit were achieved 128 and 1.87 ng L(-1), respectively.

Chronopotentiometric Carbonate Detection with All-Solid-State Ionophore-Based Electrodes

We present here for the first time an all-solid-state chronopotentiometric ion sensing system based on selective ionophores, specifically for the carbonate anion. A chronopotentiometric readout is attractive because it may allow one to obtain complementary information on the sample speciation compared to zero-current potentiometry and detect the sum of labile carbonate species instead of only ion activity. Ferrocene covalently attached to the PVC polymeric chain acts as an ion-to-electron transducer and provides the driving force to initiate the sensing process at the membrane-sample interface. The incorporation of a selective ionophore for carbonate allows one to determine this anion in a background electrolyte. Various inner electrolyte and all-solid-state-membrane configurations are explored, and localized carbonate depletion is only observed for systems that do not contain ion-exchanger additives. The square root of the transition times extracted from the inflection point of the chronopotentiograms as a function of carbonate specie concentration follows a linear relationship. The observed linear range is 0.03-0.35 mM in a pH range of 9.50-10.05. By applying the Sand equation, the diffusion coefficient of carbonate is calculated as (9.03 ± 0.91) 10(-6) cm(2) s(-1), which corresponds to the established value. The reproducibility of assessed carbonate is better than 1%. Additionally, carbonate is monitored during titrimetric analysis as a precursor to an in situ environmental determination. Based on these results, Fc-PVC membranes doped with ionophores may form the basis of a new family of passive/active all-solid-state ion selective electrodes interrogated by a current pulse.

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.

Thin-Layer Chemical Modulations by a Combined Selective Proton Pump and pH Probe for Direct Alkalinity Detection†

We report a general concept based on a selective electrochemical ion pump used for creating concentration perturbations in thin layer samples (∼40 μL). As a first example, hydrogen ions are released from a selective polymeric membrane (proton pump) and the resulting pH is assessed potentiometrically with a second membrane placed directly opposite. By applying a constant potential modulation for 30 s, an induced proton concentration of up to 350 mM may be realized. This concept may become an attractive tool for in situ titrations without the need for sampling, because the thin layer eventually re-equilibrates with the contacting bulk sample. Acid-base titrations of NaOH and Na2 CO3 are demonstrated. The determination of total alkalinity in a river water sample is carried out, giving levels (23.1 mM) comparable to that obtained by standard methods (23.6 mM). The concept may be easily extended to other ions (cations, anions, polyions) and may become attractive for environmental and clinical applications.

Direct alkalinity detection with ion-selective chronopotentiometry.

We explore the possibility to directly measure pH and alkalinity in the sample with the same sensor by imposing an outward flux of hydrogen ions from an ion-selective membrane to the sample solution by an applied current. The membrane consists of a polypropylene-supported liquid membrane doped with a hydrogen ionophore (chromoionophore I), ion exchanger (KTFBP), and lipophilic electrolyte (ETH 500). While the sample pH is measured at zero current, alkalinity is assessed by chronopotentiometry at anodic current. Hydrogen ions expelled from the membrane undergo acid-base solution chemistry and protonate available base in the diffusion layer. With time, base species start to be depleted owing to the constant imposed hydrogen ion flux from the membrane, and a local pH change occurs at a transition time. This pH change (potential readout) is correlated to the concentration of the base in solution. As in traditional chronopotentiometry, the observed square root of transition time (τ) was found to be linear in the concentration range of 0.1 mM to 1 mM, using the bases tris(hydroxymethyl)aminomethane, ammonia, carbonate, hydroxide, hydrogen phosphate, and borate. Numerical simulations were used to predict the concentration profiles and the chronopotentiograms, allowing the discussion of possible limitations of the proposed method and its comparison with volumetric titrations of alkalinity. Finally, the P-alkalinity level is measured in a river sample to demonstrate the analytical usefulness of the proposed method. As a result of these preliminary results, we believe that this approach may become useful for the in situ determination of P-alkalinity in a range of matrixes.