Fabrication of Tip-Protected Polymer-Coated Carbon-Fiber Ultramicroelectrodes and pH Ultramicroelectrodes

This work presents a method to easily construct nanometer and micrometer size electrodes and pH ultramicroelectrodes, which are important tools for scientists who desire to examine electrochemical events at small scales. Consistent construction of poly(oxyphenylene)-coated carbon fiber ultramicroelectrodes (CF-UMEs) with tip diameters less than 1.5 mu m proceeds by epoxy-sealing a carbon-fiber filament within a pulled-glass capillary, electrochemical etching of the protruding fiber of an CF into a conical tip and insulating the protruding fiber by electropolymerized poly(oxyphenylene) while the conical tip is protected within a conductive silicon rubber membrane. The CF-UMEs were characterized by cyclic voltammetry and successfully employed as an amperometric SECM tip to study the correlation between crystal structure orientation of an iridium/iridium oxide substrate and its electrocatalytic activity for oxygen evolution reaction. To prepare a pH ultramicroelectrode (pH-UME), a thin film of IrOx is electrochemically deposited on the surface of a CF-UME. The pH-UME shows a response time of < 5 s over the pH range of 2-12 with a sensitivity of 65.3 mV/pH. As a potentiometric SECM-tip, the pH-UME was successfully used to map the pH changes near the surface of a negatively biased Au disk electrode in buffered solutions. Results indicate that these pH-UME tips are well-suited for SECM studies requiring high-resolution pH imaging. (C) The Author(s) 2019. Published by ECS.