Diamond microelectrodes for use in biological environments

The response sensitivity, precision and stability of a diamond microelectrode for the measurement of catecholamines is reported on and compared with a carbon fiber microelectrode. The comparison was made (i) before and after exposure of the bare microelectrode to the laboratory atmosphere and to biological tissue, and (ii) during the in vitro measurement of norepinephrine (NE)-release from a test animal's mesenteric artery. The surface-sensitive redox system, Fe(CN)6-3/-4, was also used to probe the electrode response. The diamond microelectrode exhibited a low, stable and pH-independent background current over a wide potential range; good sensitivity and response reproducibility for NE; and resistance to deactivation and fouling during exposure to the laboratory atmosphere and biological tissue. In contrast, the carbon fiber microelectrode exhibited a pH-dependent background current response with evidence for electroactive surface carbon-oxygen functional groups, and deactivated irreversibly during laboratory air exposure and contact with tissue. The same diamond electrode properties that are responsible for the deactivation and fouling resistance (e.g., the non-polar, H-terminated surface with no extended π electron system) are also responsible for the more sluggish catecholamine electrode reaction kinetics (e.g., absence of molecular adsorption on the hydrogen-terminated sp3-bonded carbon surface). In addition, the bare diamond microelectrode provided excellent response characteristics during the in vitro measurement of NE-release from the mesenteric artery of a laboratory test animal. The results confirm that this new microelectrode is useful for sensitive, reproducible and stable electroanalytical measurements in complex biological environments.