Cylindrically Etched Carbon-Fiber Microelectrodes for Low-Noise Amperometric Recording of Cellular Secretion

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Abstract
The most important sources of noise with disk-shaped carbon-fiber microelectrodes (CFMEs) are the exposed cut disk face of the fiber itself and the seal region between the carbon fiber and the applied insulating layer. To reduce noise and to fabricate simple, reproducible low-noise CFMEs, we sealed commercially available carbon fibers in pulled glass pipets and then we performed cylindrical etching of the fiber extending beyond the glass sheath, followed by insulation with anodic electrophoretic deposition of paint. The resulting CFMEs had electroactive carbon disks with radii as small as ∼0.5 μm. The noise of such electrodes was minimized by virtue of a design that ensures a good seal between the carbon fiber and its insulation and a reduced diameter of the exposed carbon. In contrast to CFMEs made of conically etched carbon fibers, cylindrically etched CFMEs offer the significant advantage that they can be easily reused: The cylindrically etched region extends over several hundreds of micrometers and, therefore, can be cut back repeatedly to expose a fresh carbon surface of uniform diameter. The low noise and small size of these electrodes make them ideal for the high-sensitivity measurements demanded in studies of single-vesicle transmitter release from secretory cells. Furthermore, the small cross-sectional diameter of the tips allows them to be used in restricted spaces, such as inside the tapering micrometer-diameter tips of melted and pulled glass microcapillaries (e.g., patch pipets).