Fabrication and Optimization of Electrochemical Detector for HPLC by Use of Microhole Array Electrodes

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Microhole array electrodes, which can be fabricated by electrochemical etching of a carbon fiber-epoxy composite, were employed as electrochemical detectors for HPLC or flow injection analysis. Fabrication and optimization of the flow cells were pursued. Examination with a commercially available flow cell in combination with a microhole array electrode embedded in an epoxy disk plate revealed that steady state current responses which are independent of the flow rate are obtained for hexacyanoferrate ions, indicating that direct determination of analyte is possible from the current value without any calibration curves if the diffusion coefficient value is known. It was shown that the detectors can be used for electroactive organic species like catecholamines such as dopamine. However, gradual depression of current values was observed for the use of the cell for a long duration. In order to increase the sensitivity of the detector, fabrication of microhole array electrodes consisting of tens of or a hundred of a thousand carbon fibers was tried. It was found that potential controlled etching is preferable for uniform etching of these carbon fibers to galvanostatic etching. Though a relation between electrode potential for etching, electrolysis time and depth of the microholes has been obtained, further study for getting uniform microholes would be required. Use of pulse voltammetry for the detection of species was also investigated but it was found that contribution of double layer charging current to the total current is large and surface conditioning of the etched carbon must be studied. It was also found that immobilization of some enzyme to the microhole array electrode makes possible a selective detection of specified species in the flow cell.