Anal. Chem. 2001, 73, 1048-1052
A Method for the Fabrication of Low-Noise Carbon Fiber Nanoelectrodes Wei-Hua Huang, Dai-Wen Pang, Hua Tong, Zong-Li Wang, and Jie-Ke Cheng*
Department of Chemistry, Wuhan University, Wuhan 430072, China
A new and facile method has been developed for the fabrication of low-noise carbon fiber microelectrodes (CFMEs) and carbon fiber nanoelectrodes (CFNEs). The carbon fiber was flame-fuse sealed in the tip of the glass capillary. The CFMEs were made by cutting the protruding carbon fiber to the desired length, and the CFNEs were achieved by etching the protruding carbon on the flame to form a nanometer-scale tip. The tip of CFNEs can be controlled within the range from 100 to 300 nm. Thus, no epoxy wax was involved in the CFMEs and CFNEs. The experimental results of inspecting CFMEs and CFNEs by scanning electron microscopy demonstrated that the surface of the electrodes and the glass/fiber interface are very smooth. Therefore, the noise caused by the glass/ fiber of these electrodes is much lower than that of the electrodes fabricated conventionlly. The electrodes were characterized by ferricyanide, catecholamine (dopamine,DA), norepinephrine (NE), and epinephrine (E)) and 5-hydroxytryptamine (5-HT) neurotransmitters using CV, LSV, DPV, and FSCV. The results showed that the CFMEs and CFNEs have very excellent electrochemical behavior and high sensitivity. The CV and DPV detection limits of DA, NE, and E are 7.6 × 10-8, 7.0 × 10-8, and 5.0 × 10-8 mol/L, and the DPV detection limits of DA, NE, and E are 4.0 × 10-8, 1.0 × 10-7, and 2.2 × 10-7 mol/L, respectively. This experiment offers a new and facile method for the fabrication of CFMEs and CFNEs of very high sensitivity and low noise. Due to their intrinsic characteristics, microelctrodes have evoked widespread research interest and have been studied in many fields comprehensively and intensively. The very small dimensions of the microelectrodes facilitate measurements in biological microenvironments1-3 and allow them to be coupled with microcolumns to serve as detectors in separating compounds.4-7 The low iR drop enables a two-electrode system to be (1) Wightman, R. M.; May, L. J.; Michael, A. C. Anal. Chem. 1988, 60, 769A779A. (2) Wightman, R. M.; Jankowski, J. A.; Kennedy, R. T.; Kawagoe, K. T.; Schroeder, T. J.; Leszczyszyn, D. J.; Near, J. A.; Diliberto, E. J.; Viveros, O. H. Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 10754-10758. (3) Chow, R. H.; Von Ruden, L.; Near, E. Nature 1992, 356, 60-63. (4) Kennedy, R. T.; Oates, M. D.; Cooper, B. R.; Nickerson, B.; Jorgenson, J. W. Science 1989, 246, 57-63. (5) Cooper, B. R.; Wightman, R. M.; Jorgenson, J. W. J. Chromatogr., B 1994, 653, 25-34. (6) Ewing, A. G.; Mesaros, J. M.; Gavin, P. F. Anal. Chem. 1994, 66, 527A537A.
1048 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001
used. In addition, the low charged currents of the microelectrodes can increase the ratio of signal/noise and allow the high scan rate which are the requirements on monitoring fast electrochemical processes. Carbon fiber microelectrodes (CFMEs) are the important portion in the whole microelectrode family. Ever since Adams’ group detected the amine neurotransmitters in the brain with CFMEs, the microelectrode’s voltammetry has been considered to be a powerful technique to investigate the role of neurotransmitters in the brain,8-9 and CFMEs have played a significant role in the research of biological fields. Wightman, Ewing, Kennedy, Chow, and others monitored the secretion of neurotransmitters and hormones at the single-cell level with CFMEs and carbon fiber-modified microelectrodes (CFMMEs).3,10-17 The conventional method reported for fabrication of CFMEs is as follows. The carbon fiber is aspirated into the glass capillary, which then is pulled to the dimensions of the fiber with a vertical puller. The fiber is then sealed in the glass capillary with epoxy, and the electrical junction is made by back-filling the capillary with colloidal graphite and inserting a chromel wire.6-7,10,13,14,18 In this method, the glass/fiber interface contains epoxy, which is the main factor influencing the characteristics of the electrodes, and the unavoidable bad sealing and leakage of the epoxy consequentially result in the high noise, low sensitivity, and short life of the electrodes and sometimes may lead the solution detected to be polluted. In addition, it is difficult to ensure that the back-fill procedure with colloidal graphite is successful every time, so the survival rate of fabrication is low. On the other hand, since the epoxy can solute in some organic solvent, it is impossible to determine and modify these electrodes in organic solvent. (7) Hu, S.; Pang, D. W.; Wang, Z. L.; Cheng, J. K. Anal. Chem. (Chinese) 1998, 26, 752-757. (8) Adams, R. N. Anal. Chem. 1976, 48, 1128A-1137A (9) Rice, M. E.; Oke, A. F.; Bradberry, C. W.; Adams. R. N. Brain Res. 1985, 340, 151-155. (10) Jankowaski, J. A.; Schroeder, T. J.; Holz, R. W.; Wightman, R. M. J. Biol. Chem. 1992, 267, 18329-18335. (11) Wightman, R. M.; Finnegan, J. M.; Pihel, K. Tr. Anal. Chem. 1995, 14, 154-158. (12) Xin, Q.; Wightman, R. M. Anal. Chem. 1998, 70, 1677-1681. (13) Chen, T. K.; Luo, G.; Ewing, A. G. Anal. Chem. 1994, 66, 3031-3035. (14) Kozminski, K. D.; Gutman, D. A.; Davila, V.; Suler, D.; Ewing, A. G. Anal. Chem. 1998, 70, 3123-3130. (15) Kennedy, R. T.; Huang, L.; Atkinson, M. A.; Dush, P. Anal. Chem. 1993, 65, 1882-1887. (16) Huang, L.; Kennedy, R. T. Trends Anal. Chem. 1995, 14, 158-164 (17) Kennedy, R. T.; Huang, L.; Aspinwall, C. A. J. Am. Chem. Soc. 1996, 118, 1795-1796. (18) Kelly, R. S.; Wightman, R. M. Anal. Chim. Acta 1986, 187, 79-87. 10.1021/ac0008183 CCC: $20.00
© 2001 American Chemical Society Published on Web 01/25/2001
Carbon fiber nanoelectrodes (CFNEs) of even smaller size than CFMEs are suitable for the measurement of ultrasmall sample volumes, such as monitoring the cell release with high spatial and temporal resolution, analysis of the inside of cells, direct measurements of the single exocytotic events. and probing into the synaptic cleft. In recent years, efforts have been made to exploit an efficient and facile method for the fabrication of the CFNEs. Ewing19 and Malinski20 pioneered flame etching to fabricate CFNEs; however, the epoxy had been involved in the process of sealing the cabon fiber in the glass tip before etching, which increased the difficulty of etching and resulted in a hard to control etching process. The surface of the electrodes was not very smooth and therefore influenced their electrochemical characteristics. In addition, the overall tip of the nanoelectrode was not very small, the minimal size of which was 400 nm. The electrochemical etching method of the carbon fiber instead of flame etching method has been employed by Wightman21 and Schute,22 and the minimal size of the overall tip was not smaller than 500 nm. Zhang et al.23 put forward the argon ion beam etching method for fabrication of CFNEs with overall tip dimensions ranging from 50 to 500 nm in diameter. The surface of the electrodes was very smooth; however, this method is costly in terms of time and money and is not suitable for widespread application. In this paper, we report a new and facile method for the fabrication of the CFMEs and CFNEs. Without epoxy involved, the carbon fiber was flame-fuse sealed in the tip of the capillary which had been pulled earlier to form a tip with an inner diameter of ∼20 µm. This method offers a very tight seal for the CFMEs, and the problems mentioned above are avoided. Since there is no epoxy involved in the sealing process, the glass tip and the protruding carbon fiber can be put into the flame together and the carbon fiber is easily etched to form CFNEs with overall tip dimensions ranging from 100 to 300 nm in diameter. Moreover, the surface of the electrode is quite smooth, which results in excellent electrochemical characteristics and high sensitivity in the electrochemical detection. The potential application area will focus on monitoring the secretion from cells with high spatial and temporal resolution and direct measurement of the single secretion vesicles. EXPERIMENTAL SECTION Apparatus. Cyclic, fast-scan cyclic, linear sweep, and differential pulse voltammetric methods were all carried out using a CHI660A electrochemical workstation (CH instruments, Shanghai, China) in conjunction with a Pentium 350 computer. A twoelectrode system was employed. A Ag/AgCl reference electrode was used throughout the experiment. A scanning electron microscope (SEM ×650, Hitachi, Tokyo, Japan) was used for observation of the electrodes. Fabrication of the CFMEs and CFNEs. Carbon fiber (7 µm in diameter, Goodfellow Co., Oxford, U.K.) was initially cleaned by sonication for 5 min sequencely in acetone, alcohol, and doubly (19) Strein, T. G.; Ewing, E. W. Anal. Chem. 1992, 64, 1368-1373. (20) Malinski, T.; Taha, Z. Nature 1992, 358, 676-678. (21) Kawagoe, K. T.; Jankowski, J. A.; Wightman, R. M. Anal. Chem. 1991, 63, 1589-1594. (22) Schuite, A.; Chow, R. H. Anal. Chem. 1998, 70, 985-990. (23) Zhang, X. J.; Zhang, W. M.; Zhou, X. Y.; Ogorevc, B. Anal. Chem. 1996, 68, 3338-3343.
Figure 1. Schematic drawing of a CFNE: 1, copper wire; 2, epoxy resin; 3 glass capillary; 4, joint of copper and carbon fiber (silver print conductive paint); 5, carbon fiber; 6, glass/fiber interface; 7, flameetched carbon fiber tip. (See details of (6) and (7) in Figure 2.)
Figure 2. Scanning electron microscopy pictures of a CFNE: (a) the general view of CFNE, (b) the interface of carbon fiber and capillary, (c) the EDX map of the interface presented in (b), and (d) the tip of the CFNE
distilled water. One end of the carbon fiber was connected to a copper wire with silver print conductive paint (GC Thorsen, 1801 Morgan St., Rockford, IL). The carbon fiber-copper wire was inserted from the other end of the glass capillary into the glass capillary tip with an inner diameter of ∼20 µm. A 1-cm length of the carbon fiber was exposed from the tip then the tip was placed on the outer flame of the gas lamp. We found that the carbon fibers did not burn out as soon as they were put into the flame. In a very short period of time (
