A voltammetric sensor developed for in situ trace metal analysis in natural waters is presented. It consists of an array of 100 mercury-plated, iridium-based microdisk electrodes, coated with a 300−600-μm-thick 1.5% agarose gel membrane. This membrane acts as a dialysis membrane by allowing the diffusion of metal ions and complexes and by hindering the diffusion of colloids and macromolecules. Chronoamperometry and square wave anodic stripping voltammetry (SWASV) have been used to characterize the diffusion of hexacyanoferrate(III), lead, and cadmium in the agarose gel. For these species, the diffusion coefficients have been found to be half of the diffusion coefficient in free solution, and the time necessary for complete equilibration with the test solution varied with the gel thickness in accordance with the theory and can be lowered to 5 min for a gel thickness of 300 μm. The same techniques have been used to demonstrate the efficiency of the membrane against fouling and convection. Pressures in the range 1−600 bar have been found to have no effect on the sensor response. In contrast, variations in temperature in the range 4−22 °C considerably affected diffusion and charge-transfer kinetics, the resulting currents obeying a simple Arrhenius equation. These results confirm the suitability of the voltammetric sensor for in situ analysis of heavy metals in natural waters.

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Electrochemiluminescence (ECL) of Ru(bpy)(3)(2+) in water only, without any added electrolyte or reducing agents, has been obtained at carbon interdigitated microelectrode arrays (C-IDAs) of 2 μm width and spacing. In a generation/collection biasing mode, ECL can be clearly seen with the naked eye in normal room lighting at concentrations greater than 1 mM. Using a conventional photomultiplier tube (PMT), a detection limit of 10(-)(7) M Ru(bpy)(3)(2+) has been achieved for an electrode area of 0.25 mm(2). In comparison, the ECL intensity produced at Pt-IDA of the same geometry, under identical experimental conditions, was more than 300 times less. The ECL obtained at C-IDAs is attributed to the annihilation reaction of the reduced and oxidized forms of the Ru(bpy)(3)(2+) made possible due to the small electrode spacing.

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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.

Thin-film technology provides definite advantages for producing small-dimension electrodes and electrochemical transducers. The aim of this review article is to describe the main technological steps of microfabrication. Three selected examples, derived from research in our laboratory, are used to illustrate basic concepts as well as some nonstandard processes available for the fabrication of thin-film Pt, Au, Ir and C microelectrodes.

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.