Oxygen electrode microelectrode

To the scanning system of the scanning vibrating electrode (SVE), an oxygen microelectrode and a pH microelectrode were added. The three microelectrodes (DO, pH, and SVE) were positioned about 140 pm above the sample and 700 pm apart along a line parallel to the x scan axis. By this setup the concentration of oxygen, the pH, and the ionic current density can be measured almost simultaneously at the same spot. Since local anodic currents should lead to a... 

Parthasarathy A, Srinivasan S, Appleby AJ, et al. 1992a. Temperature dependence of the electrode kinetics of oxygen reduction at the platinum/Nafion interface—A microelectrode investigation. J Electrochem Soc 139 2530-2537. 

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

Either the CO2 formation is followed potemiometrically (243) or the O2 consumption is measured amperometrically at an oxygen electrode (245). In the first method, the enzyme is physically immobilized with a dialysis membrane. The response is linear in the range 5-300 pg/mL of salicylate. The second technique uses chemically immobilized enzyme (GA -F BSA) attached to a pig intestine mounted on the tip of the O 2 electrode. Samples containing from 10 pM to 2 mM salicylate were analyzed. An elegant microelectrode (244) has the enzyme and the cofactor immobilized in the electrode matrix (carbon paste) and the catechol formation is monitored at -F 300 mV versus Ag/AgCl. The electrode consists of a disposable strip, allowing measurements to be made on a drop of blood within 1 min. 

The first important paper about the use of solid electrodes was published by Davies and Brink [47]. These authors described two types of platinum microelectrodes for determining oxygen tension in various body fluids and living tissues in vitro and in vivo. 

The so-called open type of platinum microelectrode is made by a simple platinum contact with the whole investigated medium. The results obtained with this type of electrode are not always quite reproducible, as the surface active substances can inactivate the electrode. Such an electrode can serve as an oxygen monitor measuring the decrease or increase of oxygen content in biological medium only. 

Microelectrodes are less sensitive than larger electrodes to fall off in current at very high oxygen tensions, but they tend to be somewhat unstable. It is not possible to relate the current-voltage polarogram with overall stability (6). 

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