Sensors drop-planar

The most salient feature of flow-through sensors is the way in which the sample is brought into contact with the sensitive microzone (see Fig. 1.14), which distinguishes them from probe and drop-planar sensors. In fact, the liquid (or gaseous) sample is passed over the microzone rather than dropped onto it or used to immerse the probe [1]. 

For a voltammetric sensor, the current or potential peak shift that may relate to the concentration of the sensing species is an important measurement. In a dynamic situation in which polarization characteristics are obtained, it is essential that the mass transfer characteristics are reproducible for both calibration and actual measurements. In the case of a stationary planar sensor, stagnant solution or steady flow conditions in a flow cell provides good reproducibility. Or in another case, a sufficiently high concentration of an electrolyte is used to maintain a constant ohmic drop in the cell, regardless of the concentration of the pertinent sensing component. Under these conditions, the mass transfer can be purely diffusional and adequately described by Pick s law of diffusion. 

Amperometric sensors as well as most electrocatalytic and battery electrodes operate under diffusion-controlled conditions, and thus their response depends on the flux of electroactive solutes (or their transformation products) to the conductive electrode surface, be it a film-coated electrode or the three-dimensional conductive network of porous electrodes. In order to examine the qualitative factors that influence the flux to such electrodes, it is valuable to examine a simple conceptual model. We examine here the diffusion-limited current to a test case planar electrode of area. A, coated by a thin flhn of thickness, d. Diffusion-limited conditions frequently accompany a high overpotential operation such that the concentration of the key analyte is negligible at the surface of the electrode. Under these conditions, the concentration drop across the membrane determines the faradaic current ... 

The above results show that curing of the assembled sensor was readily accomplished at 120 °C without seriously affecting performance. Planar electrodes coated with enzyme and Nafion show a drop in activity consistent with that... 

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