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.
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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