8 BIOGEOCHEMICAL MICROSENSORS FOR BOUNDARY LAYER STUDIES

Interfacial processes in sediments, biofilms, and other benthic aquatic systems can be strongly affected by the presence of hydrodynamic boundary layers. The boundary layer (BL) can limit mass transfer between the overlying water and the solid matrix, thus affecting biological processes. The BL region adjacent to reactive surfaces that contains significant chemical gradients is known as the diffusive boundary layer (DBL). The DBL thickness is dependent on the flow velocity, surface topography and porosity of the solid matrix, and it ranges from <0.1 mm to several millimeters in thickness (see also chapters 5, 9, 14 and 15). As the interface is approached, mass transfer of solutes in the DBL becomes less dependent on advective motions (turbulence) and increasingly dependent on diffusion. Any net solute consumption or production within the matrix relative to the surrounding water, therefore, leads to the formation of concentration gradients over the DBL as well as within the matrix. The sediment-water interface thus can be the site of steep gradients of physicochemical variables over distances of <0.1 mm, and methods for studying these variables near the interface must resolve variations over very small distances without significant disturbance to these gradients. Noninvasive optical analysis, by direct microscopic observation, analysis of water current with laser-Doppler anemometry, nuclear magnetic resonance imaging (Wieland, 2000) or positron emission tomography (Khalili et al., 1998) fulfills these requirements, but most methods with high spatial resolution are based on microsensors. Different microsensors for environmental use, based on a wide variety of principles, have been developed during the last couple of decades. This development seems to be accelerating as microscale construction makes it possible to apply principles that would not work in macroscale analogs. The latter observation is directly connected to the fact that diffusion is a fast transport process in microscale. As most electrochemical sensor principles rely on diffusion-dependent electrode reactions, scaling down the active surface area and diffusion distances leads to better signal stability, faster response, and practical independence of the microsensor signal on stirring of the external medium. Such measuring characteristics can normally not be realized with macrosensors. For electrochemical microsensors these advantageous measuring characteristics occur when the characteristic sensor tip dimension (i.e., the diameter of the sensor membrane or the diameter of the reactive surface) becomes so small that the diffusion to and from the sensor changes from a planar to a hemispherical geometry. It is now generally agreed on, that this happens with tip dimensions of <20 µm. An excellent review on the principles and advantages of electrochemical microsensors was given by Heintze (1993). Confusion exists about the term microsensor in both analytical chemistry and aquatic sciences, where terms such as ultramicroelectrode and nanode are also used. Here, we use a rather broad definition of microsensors as being any sensor that allows for measurements with a spatial resolution of ≤0.1 mm. In most cases, the actual spatial resolution is equal to the tip diameter of the sensor, if the sensor does not consume so much of the analyte that it disturbs the analyte gradient to be measured. Sensors allowing measurements at a millimeter scale are termed minisensors.