Mary-Lou Tercier-Waeber

Direct Optical Carbon Dioxide Sensing Based on a Polymeric Film Doped with a Selective Molecular Tweezer-Type Ionophore

A novel optical method for the determination of CO(2) concentration in aqueous and gaseous samples of plasticized PVC film is presented. The detection principle makes use of a direct molecular recognition of the carbonate ion by a molecular tweezer-type ionophore, which has previously been demonstrated to exhibit excellent carbonate selectivity. The carbonate ion is extracted together with hydrogen ions into a polymeric film that contains the anion exchanger tridodecylmethylammonium chloride, a lipophilic, electrically charged, and highly basic pH indicator, which is used for the readout in absorbance mode, in addition to the lipophilic carbonate ionophore. According to known bulk optode principles, such an optical sensor responds to the product of the carbonate ion activity and the square of hydrogen ion activity. This quantity is thermodynamically linked to the activity of carbon dioxide. This allows one to realize a direct carbon dioxide sensor that does not make use of the traditional Severinghaus sensing principle of measuring a pH change upon CO(2) equilibration across a membrane. A selectivity analysis shows that common ions such as chloride are sufficiently suppressed for direct PCO(2) measurements in freshwater samples at pH 8. Chloride interference, however, is too severe for direct seawater measurements at the same pH. This may be overcome by placing a gas-permeable membrane over the optode sensing film. This is conceptually confirmed by establishing that the sensor is equally useful for gas-phase PCO(2) measurements. As expected, humid air samples are required for proper sensor functioning, as dry CO(2) gas will not cause any signal change. The sensor showed acceptable response times and good reproducibility under both conditions.

Submersible voltammetric probes for in situ real-time trace element measurements in surface water, groundwater and sediment-water interface

A summary of the state of the art in the development of two submersible voltammetric probes performed by us to allow continuous, real-time monitoring of trace elements (Cu(II), Pb(II), Cd(II), Zn(II) and Mn(II), Fe(II)) in natural aquatic ecosystems is given. The first one, called the voltammetric in situ (VIP) profiling system, allowed in situ measurements in surface water and groundwater down to 500 m. Its construction required the development of: (i) a gel-integrated, either single or interconnected, array microsensor, (ii) a submersible probe and (iii) hardware, firmware and software for control of the whole system: i.e. data transmission and acquisition, data processing and maintenance operations. The second system, called the sediment-water interface voltammetric in situ profiling (SIVIP) system, has been developed to allow real-time, high spatial resolution trace elements concentration profile measurements at the sediment-water interface. Its construction required the development of: (i) a gel-integrated microsensor array with 64 individually addressable lines, (ii) a voltammetric probe based on powerful double multiplexing system and single potentiostat allowing simultaneous measurements over the 64 sensor lines, and (iii) hardware, firmware and software for control of the whole system. A general description of both systems as well as examples of laboratory characterization and/or field applications are reported.

A Novel Voltammetric Probe with Individually Addressable Gel-Integrated Microsensor Arrays for Real-Time High Spatial Resolution Concentration Profile Measurements

In this article, the development of a novel voltammetric system for real-time, high spatial resolution trace element concentration profile measurements at the sediment-water interface, or other interfaces, is reported. The heart of this system is a novel gel-integrated Hg-plated Ir-based 64 individually addressable microsensor arrays. The gel membrane minimizes fouling problems and insures high stability of the Hg semidrops. Measurements were performed simultaneously over the 64 lines using a powerful multiplexing system and a single potentiostat. The voltammetric probe and sensor have been described in detail. This voltammetric system has been characterized by running both cyclic voltammetry (CV) in hexacyanoferrate(III) solution using the gel-integrated Ir-based microsensor and square-wave anodic stripping voltammetry (SWASV) in Pb(II) and Cd(II) solution using the gel-integrated Hg-plated Ir-based microsensor. The results showed that reproducible and reliable simultaneous measurements can be obtained over the 64 individually addressable lines even with fast dynamic techniques such as SWASV. Moreover, theoretical and experimental data were in good agreement. Profiling capability has also been demonstrated.

Decrease of labile Zn and Cd in the rhizosphere of hyperaccumulating Thlaspi caerulescens with time

By using a rhizobox micro-suction cup technique we studied in-situ mobilization and complexation of Zn and Cd in the rhizosphere of non-hyperaccumulating Thlaspi perfoliatum and two different Thlaspi caerulescens ecotypes, one of them hyperaccumulating Zn, the other Zn and Cd. The dynamic fraction (free metal ions and small labile complexes) of Zn and Cd decreased with time in the rhizosphere solution of the respective hyperaccumulating T. caerulescens ecotypes, and at the end of the experiment, it was significantly smaller than in the other treatments. Furthermore, the rhizosphere solutions of the T. caerulescens ecotypes exhibited a higher UV absorptivity than the solution of the T. perfoliatum rhizosphere and the plant-free soil. Based on our findings we suggest that mobile and labile metal-dissolved soil organic matter complexes play a key role in the rapid replenishment of available metal pools in the rhizosphere of hyperaccumulating T. caerulescens ecotypes, postulated earlier.

On-Line Coupling of Flow Through Voltammetric Microcell to Hollow Fiber Permeation Liquid Membrane Device for Subnanomolar Trace Metal Speciation Measurements

A novel microliter volume flow through electrochemical cell incorporating a Hg-Ir microelectrode for on-line coupling to hollow fiber permeation liquid membrane (PLM) for trace metal separation and preconcentration probe to determine metal speciation in natural water is described. The permeation liquid membrane contained 0.1 mol L−1 didecyl diaza-crown-ether and 0.1 mol L−1 lauric acid dissolved in toluene/phenylhexane mixture. The membrane acts as a barrier between the sample solution and a strip solution containing a complexant into which the extracted metals are back extracted. This membrane transports selectively the trace metal ions that are capable of binding to the carrier, in particular Cu, Pb, Cd and Zn. High preconcentration factors are obtained using small volumes of strip solution and large sample volume. Combination of this chemical sensor to voltammetric detectors allows one to determine free metal concentration in real time and eases automation. The difficulties linked with the use of a microliter voltammetric cell for the application sought, the optimal cell design as well as optimal conditions for permitting on-line determination of Pb and Cu are described. The advantage of using PLM-ASV coupling with respect to ASV for the determination of trace metals in natural waters are that matrix effects, which are often a problem in ASV, are minimized and the sensitivity of the method is increased because of the additional preconcentration from the PLM prior to ASV determination. After testing the system with a synthetic solution, applicability for speciation of lead and copper ion in river waters were assessed. Comparison of the results with off-line metal determination by graphite furnace atomic absorption spectrometry has also been reported. Preconcentration factors of ca. 800 and ca. 300 in 20 minutes for Pb and Cu, respectively, were obtained using 50 μL strip and 250 mL source solutions. Under the working conditions used, e.g., with 20 minutes PLM preconcentration time, detection limits of ca. 0.02 nmol L−1 can be achieved.

Trace metal speciation at the sediment–water interface of Vidy Bay: influence of contrasting sediment characteristics

Abstract Trace metal analysis and speciation were performed at the sediment–water interface of Vidy Bay (Lake Geneva, Switzerland). This bay is impacted by hazardous compounds released via the sewage effluent of a major wastewater treatment plant (WWTP). Sediment cores and overlying water were sampled simultaneously at 12 sites characterized by contrasting sediment surface characteristics (color, methanogenic activity, bacterial mat) using corers deployed from a MIR submarine or research boat. The concentrations of trace metals in particulate form in the sediment and dissolved in the interstitial water, as well as the particulate, colloidal and dynamic fractions of trace metals in the overlying water were determined by combining an in situ and laboratory multi-method analytical approach. The results indicate differences in trace metal speciation in the sediment and overlying water at the 12 investigated sites. The observed differences were found to be more correlated to bacterial community, abundance, type and activity than to distance from the WWTP sewage outlet.

Antifouling membrane integrated renewable gold microelectrode for in situ detection of As(III)

Arsenic in the environment is of a global concern because of the widespread, chronic poisoning found in a number of countries and affecting large populations. Robust and sensitive analytical tools capable of direct, continuous in situ As(III) sensing is therefore of prime interest for As health risk assessment. For this purpose, we have developed here a microelectrode consisting of a gel integrated renewable gold nanoparticles plated iridium-based microelectrode (Au-GIME). The gel minimizes fouling problems by hindering diffusion of organic matters and inorganic colloids/macromolecules toward the sensor surface. Square Wave Anodic Stripping Voltammetry (SWASV) was used to characterize (i) the kinetics of As(III) diffusion in the agarose gel as a function of the gel thickness, (ii) the analytical performance of the sensor in synthetic and natural waters, and (iii) the influence of the temperature on the arsenic stripping peak current intensities. An evaluation of direct environmental application was performed in freshly collected lake water samples and validated by ICP-MS after separating As(V) and As(III) with an ion-exchange resin. The results reveal that the Au-GIME fulfills the requirements for direct measurements of (oxy)anions in freshwaters, i.e.: a reproducible mass transport of arsenite species in the gel; a gel equilibration time varying with the thickness of the gel in accordance with theory; a temperature effect on the SWASV As(III) signal intensities following a Arrhenius behavior and thus readily corrected using a factor defined in laboratory; and a nanomolar detection limit at pH 8.

Voltammetric Detection of Hg2+ Using Peptide-Functionalized Polymer Brushes

Polymer brushes grafted by surface-initiated atom transfer radical polymerization (SI-ATRP) from the surface of Ir-based microelectrode arrays are explored as a platform for the fabrication of sensory coatings for the voltammetric detection of Hg2+. The polymer brush coatings are post-modified with a metallothionein derived peptide to enable the selective detection of Hg2+. The performance of the polymer brush modified microelectrode arrays was evaluated using both cyclic voltammetry (CV) as well as square-wave anodic stripping voltammetry (SWASV) experiments. These studies revealed that the polymer brush based coatings allowed the selective detection of Hg2+ with detection limits in the subnanomolar range.

Agarose hydrogel containing immobilized pH buffer microemulsion without increasing permselectivity

A heterogeneous pH buffer based on a colloidal emulsion containing ion-exchanger and lipophilic base is described that can be integrated into hydrogels without affecting their ion-exchange properties. Each sphere works on the basis of reversible ion-exchange of hydrogen ions with solution cations, acting as a pH buffer while staying removed from solution in the nonpolar core of the spheres. The ion-exchange mechanism is supported by titration experiments in aqueous emulsion, showing that the nature and concentration of the exchanging solution cations influences the buffer action, with increasing lipophilicity moving the equilibrium to lower pH values. Agarose gels with entrapped pH buffer emulsions and mounted in a transport cell are shown by zero current potentiometry to exhibit negligible permselective properties above an ionic strength of 1mM, a behavior no different from unmodified agarose, with an observed ion-exchanger concentration of 7mM in dry agarose. This suggests that such pH buffers do not give rise to substantial ion-exchange properties of the gel material. In a first attempt to control the pH in the vicinity of an electrode surface by this approach, the emulsion was entrapped in an agarose gel in direct contact with a pH electrode, demonstrating the ability to buffer such gel films.

In Situ Detection of Macronutrients and Chloride in Seawater by Submersible Electrochemical Sensors

A new submersible probe for the in situ detection of nitrate, nitrite, and chloride in seawater is presented. Inline coupling of a desalination unit, an acidification unit, and a sensing flow cell containing all-solid-state membrane electrodes allows for the potentiometric detection of nitrate and nitrite after removal of the key interfering ions in seawater, chloride and hydroxide. Thus, the electrodes exhibited attractive analytical performances for the potentiometric detection of nitrate and nitrite in desalinated and acidified seawater: fast response time ( t95 < 12 s), excellent stability (long-term drifts of <0.5 mV h-1), good reproducibility (calibration parameter deviation of <3%), and satisfactory accuracy (uncertainties <8%Diff compared to reference technique). The desalination cell, which can be repetitively used for about 30 times, may additionally be used as an exhaustive, and therefore calibration-free, electrochemical sensor for chloride and indirect salinity detection. The detection of these two parameters together with nitrate and nitrite may be useful for the correlation of relative changes in macronutrient levels with salinity cycles, which is of special interest in recessed coastal water bodies. The system is capable of autonomous operation during deployment, with routines for repetitive measurements (every 2 h), data storage and management, and computer visualization of the data in real time. In situ temporal profiles observed in the Arcachon Bay (France) showed valuable environmental information concerning tide-dependent cycles of nitrate and chloride levels in the lagoon, which are here observed for the first time using direct in situ measurements. The submersible probe based on membrane electrodes presented herein may facilitate the study of biogeochemical processes occurring in marine ecosystems by the direct monitoring of nitrate and nitrite levels, which are key chemical targets in coastal waters.