An in Vivo Probe Based on Mechanically Strong but Structurally Small Carbon Electrodes with an Appreciable Surface Area Michael McNally and Danny K. Y. Wong* Department of Chemistry, Macquarie University, Sydney, New South Wales 2109, Australia Physically small carbon electrodes were fabricated by pyrolyzing acetylene in a nitrogen atmosphere using pulled quartz capillaries as the supporting substrate. A carbon disk geometry was obtained when a parallel flow of acetylene (5 0 kPa) and nitrogen (1 0 mL min -1 ) was introduced into the system. Further, carbon was found to deposit at the tip and on the shank of the quartz capillaries when the nitrogen flow rate was increased (8 0 mL min -1 ), yielding an approximately cylindrical geom- etry. A series of electrochemical and spectroscopic analy- ses was carried out to examine the type of carbon surface obtained by pyrolysis of acetylene. The results suggested that a surface consisting of an almost defect-free highly oriented pyrolytic graphite type structure was formed by the pyrolyzed acetylene. However, this contradicts the kinetically reversible electron transfer observed for dopam- ine oxidation at these electrodes. Meanwhile, the nonpolar and relatively oxygen-free characteristics indicate that these electrodes also behave similarly to a hydrogenated carbon surface. The formation of a hydrogenated carbon- type surface may be plausible as a result of the attack on the carbon surface by a surplus of hydrogen produced by the pyrolysis of acetylene to form graphitic carbon. These characteristics are expected to aid in reducing electrode fouling, which is often encountered in electrochemical detection of neurotransmitters in vivo. In conjunction with a miniature physical dimension, their appreciable surface area and enhanced mechanical strength make these carbon electrodes well suited to the detection of neuro- transmitters in vivo. There are several neurotransmitters (dopamine, noradrenaline, serotonin, etc.) known to be easily oxidized at an electrode surface. Thus, coupled with anatomical, physiological, and pharmacological evidence, electrochemistry is increasingly becoming a sensitive, real-time detection method for neurotransmitters. 1,2 For example, on the basis of amperometry, Chen et al. measured the release of catecholamines from the dopamine neuron of the pond snail, Planorbis corneus, which contains vesicles with a mean content of 800 000 molecules, 3 and from PC12 cells, which have an approximately 115 000 molecules/ vesicle. 4 We have used continu- ous amperometry 5 to monitor spontaneous and electrically evoked norepinephrine release from postganglionic sympathetic nerve terminals supplying rat mesenteric arteries. In such work, ∼160 000 norepinephrine molecules were detected, indicating a release likely caused by the exocytosis of large dense-cored vesicles. Recently, upon chemical stimulations by several secre- tagogues, Hochstetler et al. 6 reported an amperometric method capable of detecting (8-170) × 10 -21 M dopamine released from neurons isolated from the retina of genetically modified mice, which corresponds to the estimated dopamine concentration released in a synapse. Evidently, applications of electrochemistry in neuroscience continue to provide significant contributions to the fundamental understanding of various aspects of neurotrans- mission such as the mechanisms involved in the biological process exocytosis and the type of neurotransmitters taking part in the process. Microelectrodes are commonly known to exhibit several advantages including enhanced mass transport, reduced ohmic drop, and double-layer charging effects, making them well suited to use in detection of neurotransmitters in vivo. Further, the small size of microelectrodes not only causes minimal physical damage in living tissues while they are being implanted into the specimen but also permits a careful selection of neuronal region to be investigated. In this respect, there is still a continued effort in the manufacture of electrodes with characteristically small dimensions even after more than 15 years of development of microelectrodes. 7-9 To date, submicrometer-sized electrodes with different geometries including disk, band, and ring have been prepared in such common insulators as glass, epoxy, varnish, and copolymers (see references quoted in ref 10). Further, emerging technologies these * Corresponding author: (e-mail) Danny.Wong@ mq.edu.au; (phone) 61-2- 9850-8300; (fax) 61-2-9850-8313. (1) Michael, D. J.; Wightman, R. M. J. Pharm. Biomed. Anal. 1999 , 19, 33- 46. (2) Stamford, J. A. Trends Neurosci. 1989 , 12, 407-412. (3) Chen, G.; Gavin, P. F.; Luo, G.; Ewing, A. G. J. Neurosci. 1995 , 15, 7747- 7755. (4) Chen, T. K.; Luo, G.; Ewing, A. G. Anal. Chem. 1994 , 66, 3031-3035. (5) Brock, J. A.; Dunn, W. R.; Boyd, N. S. F.; Wong, D. K. Y. Br. J. Pharm. 2000 , 131, 1507-1511. (6) Hochstetler, S. E.; Puopolo, M.; Gustincich, S.; Raviola, E.; Wightman, R. M. Anal. Chem. 2000 , 72, 489-496. (7) Wightman, R. M.; Wipf, D. O. Voltammetry at ultramicroelectrodes. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1989; Vol. 15, pp 267-353. (8) Wightman, R. M. Science 1988 , 240, 415-420. (9) Zoski, C. G. Steady-state voltammetry at microelectrodes. In Modern Techniques in Electroanalysis; Vany ˜ sek, P., Ed.; John Wiley & Sons: 1996; pp 241-312. (10) Conyers, J. L.; White, H. S. Anal. Chem. 2000 , 72, 4441-4446. Anal. Chem. 2001, 73, 4793-4800 10.1021/ac0104532 CCC: $20.00 © 2001 American Chemical Society Analytical Chemistry, Vol. 73, No. 20, October 15, 2001 4793 Published on Web 09/13/2001