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

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.

Arrays of microelectrodes: technologies for environmental investigations

Within this work it is our intention to provide an overview of the use of arrays or microelectrodes in the characterisation of environmental samples. Electrochemical methods are often a relatively simple and inexpensive alternative to spectroscopic or chromatographic methods for the analysis of a wide range of analytes. Arrays of microelectrodes display a number of advantages over simple planar macroelectrodes and the reasons for this will be detailed within this work. We will also describe some of the most common methods for constructing microarrays. The application of these arrays for analysis of environmental samples such as soil and water for heavy metal contamination has been the major focus of research in this field and comprises much of this review. However other systems will also be detailed such as determination of various anions or other samples such as pesticides.

Microelectrode arrays for electrochemistry: approaches to fabrication

Microelectrode arrays have unique electrochemical properties such as small capacitive-charging currents, reduced iR drop, and steady-state diffusion currents. These properties enable the use of microelectrode arrays and have captured much interest in the field of electrochemistry. Techniques for the fabrication of such arrays are reviewed. The relative features and merits of different techniques are also discussed.

Marine chemical technology and sensors for marine waters: potentials and limits

A significant need exists for in situ sensors that can measure chemical species involved in the major processes of primary production (photosynthesis and chemosynthesis) and respiration. Some key chemical species are O2, nutrients (N and P), micronutrients (metals), pCO2, dissolved inorganic carbon (DIC), pH, and sulfide. Sensors need to have excellent detection limits, precision, selectivity, response time, a large dynamic concentration range, low power consumption, robustness, and less variation of instrument response with temperature and pressure, as well as be free from fouling problems (biological, physical, and chemical). Here we review the principles of operation of most sensors used in marine waters. We also show that some sensors can be used in several different oceanic environments to detect the target chemical species, whereas others are useful in only one environment because of various limitations. Several sensors can be used truly in situ, whereas many others involve water brought into a flow cell via tubing to the analyzer in the environment or aboard ship. Multi-element sensors that measure many chemical species in the same water mass should be targeted for further development.

Individually addressable gel-integrated voltammetric microelectrode array for high-resolution measurement of concentration profiles at interfaces

The application of a novel voltammetric probe, based on an individually addressable gel-integrated microelectrode array (IA-GIME), for real-time, high-spatial resolution concentration profile measurements at interfaces is described. Reliability and validity of steep metal concentration gradients obtained with this novel system have been demonstrated by performing systematic tests at well-controlled liquid-liquid and liquid-solid interfaces. The liquid-liquid interface was formed by two layers of aqueous solutions with different components; only one layer contained trace metal ions (Pb(II) and Cd(II)); the individually addressable microelectrode array was placed at the interface of the liquid-liquid system; the concentration profiles were recorded as function of time; and the effective diffusion coefficients were calculated. The liquid-"solid" interface was formed from an aqueous solution layer overlying a bed of silica particles saturated with an aqueous solution. The sensor array has been used to monitor the diffusion processes of Tl(I) or Pb(II) from the liquid phase to the "solid" phase. The influences of porosity, geometry of the porous media, and complexation between metal ion and silica, on the diffusion processes, have been studied. All these results show that correct diffusion profiles of metal ions at interfaces can be obtained with 200-microm resolution with the IA-GIME. They also demonstrate that, for measurements in "solid" phase, the aforementioned factors must be considered carefully for correct calibration of any electrodes and the gel-integrated microelectrodes are unique tools to enable calibration of the sensors with synthetic solutions.