Report
Color Images for Fast-Scan CV
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any cells communicate by secreting small molecules that diffuse to target cells containing specific receptors for the particular small molecule. Receptors are membrane-bound proteins that recognize a specific substance. When molecules bind to receptors on a cell, they cause a physiological change. For example, neurons (specialized cells that pass and integrate information) secrete neurotransmitters to communicate. For many neurotransmitters, the length of time to diffuse and to bind to their respective receptors is governed by transporter proteins. These proteins, located in the membranes of the nerve terminals of the neurons originating the secretion, uptake the neurotransmitters and place them back in an intraneuronal compartment. Thus, the independent processes of release and uptake gate the time during which there is sufficient concentration for a transmitter to interact with its extracellular receptors and relay its particular chemical message If these molecules could be measured while they do their job, the mechanisms that regulate them could be better understood. In particular, because of the important role of neurotransmitters in the normal functioning of the brain, measuring the concentration dynamics of these molecules has long been of interest to analytical chemists (1-3). To monitor the gating of neurotransmitter concentrations in real
Color plotsallowexamination on all the cyclic voltammetry data simultaneously and provide an overview of chemical and temporal changes. time, chemical sensors that respond rapidly and do not perturb the local environment are required. Carbon-fiber microelectrodes have been shown to meet these requirements for easily oxidized neurotransmitters, in the extracellular space of the brain and at the surface of single cells in culture (4). Their chief attribute is their small size (radii of commercially available fibers range from 2.5 to 15 um), which allows them to be placed directly adjacent to microscopic sites of secretion. Measurements with carbon-fiber electrodes are not chemically selective because they respond to any substance that can be electrolyzed at the applied potential. Membrane coatings such as ion exchangers can be used to restrict access of certain molecules to the electrode. Nevertheless, because an additional degree of selectivity is required, repetitive-scanning voltammetric techniques have been developed for use with these electrodes to monitor the extracellular environment. Fast-scan cyclic voltammetry (CV) is particularly useful because it has sufficient temporal resolution so that individual secretory be ob-
served (2) In addition the neuronal uptake and release processes can be resolved and examined individually More importantlv approach is such that University of North Carolina at Chapel Hill
Darren Michael Eric R. Travis R. Mark Wightman
586 A Analytical Chemistry News & Features, September 1, 1998
the low concentrations of neurotransmitters can be monitored. In fast-scan CV, individual voltammograms are acquired at relatively fast scan rates (e.g., 300 V/s) so that a single voltage scan is completed in a few milliseconds. The electrode potential is then held at its initial value for a sufficient length of time to allow the diffusion layer to relax and come into equilibrium with the local environment. At the end of the delay period, another voltage scan is initiated. Thus, each cyclic voltammogram contains information on the instantaneous chemical composition of the local environment at the electrode tip In just a few seconds, much data on the detected species and any temporal changes in concentration are obtained. This information is crucial to interpreting these measurements in the complex and dynamic chemical environment of the extracellular space around biological cells. In this Report, a procedure is presented that allows a large amount of recorded information to be rapidly evaluated. As with any electrochemical technique, care must be taken to distinguish the Faradaic current arising from the solution components of interest from other sources of current. This issue is particularly important
Measurements in Biological Systems
with high-speed methods, such as fast-scan CV, because of the large current that arises from double-layer charging. At carbon surfaces another contributor is oxidation/reduction of various surface functional groups, which include phenols and quinones as well as lactones, esters, and carboxylic acids (5). To remove the contribution of these two sources, scans are taken when chemical changes are not anticipated, such as during neural inactivity. These scans are used as background and are subtracted from the remainder of the data set (6).
The background-subtracted result contains the desired information concerning the chemical changes that occurred in the spatial environment surrounding the electrode. However, it is a three-dimensional record comprising applied potential, elapsed time, and current. Typical file size is 3 megabytes. Traditionally, practitioners have examined only portions of this data set in two dimensions. Typically, the background-subtracted voltammogram recorded at the maximal concentration change is used to identify the monitored species.
To examine the temporal concentration profile, the current at the peak potential of the species of interest is monitored in successive voltammograms. Although this approach may be satisfactory when only a single species changes in concentration, applications of this technique in more complex situations require that the complete data set be examined to determine the contributions of all of the species detected. Failure to produce a high degree of chemical selectivity with in vivo electrochemical measurements has led to considerable confusion in the literature (7-9). Previously, the major use of repetitivescanning electrochemical techniques has been with electrochemical chromatographic detectors. As with the use of repetitive scanning in vivo, large quantities of three-dimensional data are produced that are time-consuming to analyze by examining each voltammogram. One approach to this display problem is to use wire plots to provide the illusion of three dimensions. For example, Osteryoung et al. used this approach to represent square-wave voltammograms obtained at the end of a conventional chromatographic column (10), and, more recently, the same approach was used for CE separations (11). Similar plots were used by White et al. to evaluate voltammograms obtained with carbon-fiber electrodes placed at the end of capillary LC columns (12). Although providing considerable information, this approach has the disadvantage that large peaks can obscure portions of the data. Two-dimensional contour plots allow all of the data to be seen, but noise may confuse their interpretation. Alterna-
Analytical Chemistry News & Features, September 1, 1998 5 8 7 A
Report three-dimensional plots presented here, time increases along the abscissa whereas electrode potential is plotted along the ordinate. Current is encoded according to the color bar below each figure. The approach is illustrated in Figure 1, which shows data collected from a flow-injection system. Data are obtained with a cylindrical (2.5-um radius) carbon-fiber electrode connected to a three-electrode potentiostat and computer data-acquisition system (14). Dopamine (1 uM) was introduced through the loop injector for 10 s the electrode potential was swept from -0 4 V to 10 V and back to - 0 4 V at 300 V / s and the scans were repeated at 100-ms intervals Fntially spaced
contours near each of the peaks are superimDosed The voltammetric currents were low nass filtered at ? kHz and digitized To Figure 1 . CV of dopamine. In all color images, the asterisk indicates the starting point for the voltammetric sweep. The tracing above the image is the average current response at the oxidative peak (600 mV). Inset is the average of five voltammograms from the plateau of the tracing.
tively, gray-scale images combine the continuity of wire diagrams with the planar nature of contour plots. These have been used to reveal the chemical changes that can be monitored by fast-scan CV at the surface of a mast cell during secretion (13). These plots revealed that chemical secretion occurred as a series of sharp spikes and further revealed that two substances were secreted simultaneously. Color representation
Recently, the software necessary for color representation of multidimensional data has become commonplace, as have color printers suitable for use with laboratory personal computers. This allows substitution of the gray-scale approach with a colorcoded representation of the data. In this Report, we demonstrate that this type of representation of the large amounts of data obtained with fast-scan CV is particularly useful, especially when contour plots are superimposed on the graphs. Distinct patterns of individual analytes are readily apparent and large amounts of information can be evaluated rapidly The variables for repetitive CV are electrode potential, current, and time. In the 588 A
reve l the
l ti rha p-ps the voltammn, , , ,, c ' n u
grams recorded for the first 3 s were subtraded from the remainder of the data set. In all the figures shown, a similar background subtraction was done. Two large features are readily apparent in the color representation of Figure 1. On the positive scan, a negative (oxidative) current occurs with a maximum at 600 mV; on the negative scan, a positive (reductive) current occurs with a maximum at -250 mV The full width of the oxidative peak is about twice as wide as the reductive peak (800 mV vs. 400 mV)) These features arise from the oxidation of dopamine to its o-quinone, followed by reduction of the latter species on the negative scan. Identical characteristics are also apparent in the voltammogram taken from the plateau region of the signal The advantage of the color representation is that ti allows observatton of fhe voltammograms during the whole recording period. It makes clear that the voltammetric characteristics of dopamine remain constant during the whole exposure. Above the color plot, the current at 600 mV is plotted as a function of ttme. The color scheme chosen was nonlinear and designed to enhance transitions. It also avoids extensive use of colors that are not apparent to those who are red-green colorblind. The superimposed contour lines further enhance the patterns that are apparent in the voltammetric data. The program is limited to 8-bit color resolution, whereas
Analytical Chemistry News & Features, September 1, 1998
the data were obtained with a 12-bit analogto-digital converter. This means that there is some degree of smoothing inherent to the color plots. When necessary, the threedimensional data were further smoothed using a nearest-neighbor scheme. The three-dimensional representation of data clearly allows more information to be obtained than do the line plots. In addition to the major redox processes, several subtle features can be observed. First, it is apparent that the largest noise is at the initial and switching potentials. This noise is caused by a small jitter, on the order of a few microseconds, in the trigger to the triangular waveform generator located in the potentiostat and at the start of data collection. Thii jitter roccrs sa thetimesin the voltammogram when the changes in background current are maximal and leads to artifacts after subtraction. Second, a small, broad increase in current between -400 mV and 0 mV on the positive scan is apparent during exposure to dopamine, which is attributed to the adsorption of dopamine prior to its oxidation (4). It has been established that the amplitude of the dopamine oxidation wave partially comprises adsorbed dopamine, which accumulates on the surface while the electrode is held at -400 mV between scans. Apparently, this adsorption decreases the double-layer capacitance in the region where the broad feature is seen. As the potential is swept more positively and dopamine is oxidized, the effect is removed. Responding to other substances In addition to responding to Faradaically active species, fast-scan CV at carbon-fiber electrodes also shows a response to changes in solution pH (15-17). This arises because a change in the local pH alters the peak potentials of the carbon surface waves. When voltammograms before and after pH changes are subtracted, the responses have what appear to be a voltammetric signature. Figure 2a shows a color representation of the currents caused by an acidic change measured with a cylindrical electrode. A solution 0.26 pH units more acidic than the initial buffer was introduced into the flowinjection apparatus. There are two peaks
(-250 mV and 350 mV) on the positive scan, and the sign of the current for both is the same as for an oxidation. All of the peaks are broader than those found for dopamine, which leads to contour lines that are somewhat farther apart. Thus, the three-dimensional patterns of colors and contours readily allow distinction between dopamine and pH changes. In addition to allowing the identification of electroactive species, the threedimensional plots allow time-dependent electrochemical reactions to be resolved more easily than do traditional twodimensional plots of current versus time. Figure 2b shows the fast-scan CV response obtained during a 10-s exposure of a carbon-fiber electrode to 2 uM 5-hydroxytryptamine, another neurotransmitter. When oxidized this neurotransmitter forms products that gradually adsorb to the electrode surface (18). When viewed in two dimensions at the potential where 5-hydroxytryptamine is oxidized (600 mV), the current shows an initial increase when exposed to this substance, like that seen for dopamine. But the current grows during continued exposure, behavior quite unlike that for most Faradaically active species. Furthermore, when 5-hydroxytryptamine is removed from the electrode, the current does not return to baseline. A background-subtracted cyclic voltammogram obtained ~5 s after the initial exposure has two oxidation waves: one well-resolved reduction wave and, at the end of the negative scan, what appears to be the beginning of a second reduction wave. This behavior can readily be understood when the current is viewed in three dimensions. Upon exposure to 5-hyd^oxytryptarnine, the current initially grows at only two locations (oxidative peak at 600 mV, reductive peak at -50 mV) . As the scanning is continued, however, a second redox couple appears (oxidation peak at 250 mV, foot of reductive peak at -400 mV)) The later appearance of the second set of waves clearly reveals that these are caused by formation of a new species that adsorbs to the electrode and is formed as a result of the electrooxidation of 5-hyydoxyttyptamine. Thii wave eontributes to the current measured at 600 mV, leading to the time-dependent increase in
current seen at that potential. Examination of cal subcellular compartments in which molethe three-dimensional data set also reveals cules that are to be secreted are stored. In that when 5-hydroxytryptamine is removed mast cells isolated from the mouse peritofrom the solution, the adsorbed species reneal cavity, the primary small molecule semains, leading to the failure of the current to creted is histamine (13)) 5-Hydroxytryptamine return to baseline. is also secreted but at considerably lower The voltammetric behavior of 5-hyconcentrations. Secretion is normally trigdroxytryptamine depends on the initial pogered by the presence of an antigen that tential and applied waveform. For example, binds to an antibody on the cell surface. when an initial potential of 200 mV is used, This binding triggers a series of biochemithe electrogenerated, adsorbed product is cal events that ends with the vesicle fusing no longer apparent (18). As wiil be shown with the cell membrane and dumping its in the following section, the voltammocontents into the extracellular space. This grams of 5-hydroxytryptamine are also difrelease mechanism is termed exocytosis, ferent when the switching potential is very and it is thought to be the predominant positive. mode of secretion at neurons as well. A small number of molecules escapes from a single vesicle, and therefore high temporal Measuring secretions from resolution is required to examine this mast cells response. Mast cells are found throughout the body and are part of the immune system. Like Because histamine can also be electrooxneurons they contain vesicles, small spheriidized at carbon-fiber electrodes, secretions
Figure 2. CV of (a) an acidic change and (b) 5-hydroxytryptamine. (a) The trace above the color plot was created using the average current near the major oxidative peak (350 mV). Inset is a cyclic voltammogram from the plateau of this plot, (b) The trace above the color plot was created using the average current centered at 600 mV. Inset is the average of 20 cyclic voltammograms from the peak in the current. All other conditions are identical to those in Figure 1. Analytical Chemistry News & Features, September 1, 1998 5 8 9 A
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from these cells were examined to determine if histamine and 5-hydroxytryptamine are co-secreted from the same vesicles (Figure 3). The electrode potential was swept from 0.1 V to 1.4 V and back to 0.. V. To achieve the high temporal resolution required, a scan rate of 800 V/s was used with a repetition rate of 33.3 ms. Isolated mast cells adhering to a plastic culture dish in physiological buffer were viewed with an inverted stage microscope. A single disk-shaped carbon-fiber electrode was placed directly adjacent to an individual cell with a piezoelectric micropositioner. A disk electrode is prefered to minimize the electroactive areas with respect to the sites on the cell surface where secretion occurs. The color plot (without contours) shows the exocytotic response of an isolated mast cell stimulated with Ca2+ ionophore. The histamine (upper) and 5-hydroxytryptamine (lower) traces were obtained from the average voltammetric current around the respective peak potentials and converted to concentration using
flow-injection calibration data. The coincident concentration spikes indicate that histamine and 5-hydroxytryptamine are being co-released from individual secretory vesicles. The color representation allows rapid inspection of all the data and clearly reveals that both analytes come from the same exocytotic secretory event. With the potential waveform used in this application, 5-hydroxytryptamine exhibits a peak at 550 mV on the positive scan, whereas the oxidation of histamine occurs at the anodic limit. In addition, oxidation of histamine results in a species that contributes in subsequent scans to the presence of another oxidation wave at 850 mV. More detailed information concerning the kinetics of release of the two substances can be achieved by examining the currents at the potentials where each of the substances is oxidized. Because the peak for 5-hydroxytryptamine is sharp and symmetrical, indicating an oxidation process involving adsorbed 5-hydroxytryptamine, the current at the potential where hista-
mine is oxidized does not have any contribution from this species. The data in Figure 3 are of much higher quality than in our original report of the measurement of co-secretion from mast cells (13). The major reason is that the computer digital-to-analog converter was used to generate the waveform, rather than the waveform generator in the potentiostat. This eliminates jitter artifacts that occur at the switching potential, such as those observed in Figure 1, which is particularly important in this application because the oxidation wave for histamine occurs at the anodic limit. In addition, the representation of the current in false colors, rather than as a gray scale, makes the features much more apparent. Dopamine measurements in vivo
The three-dimensional representation of data is particularly useful for understanding measurements made in vivo. In our experiments, a carbon-fiber electrode is inserted into a specific region of the brain of an anesthetized rat. Stainless-steel electrodes capable of exciting neurons are placed in a region of the brain that contains axons that project to the measurement region. Neurons are stimulated, and the concurrent processes of neurotransmitter release and uptake are measured (2,14). A typical response at a carbon-fiber electrode following electrical stimulation of the medial forebrain bundle, a region that contains dopamine axons, is shown in Figure 4. In this experiment, the electrode was placed in the caudate nucleus, a region with a high density of dopamine terminals. The color pattern and contours demonstrate that there is a rapid increase in the dopamine concentration during the stimulation interval, and, following stimulation, the dopamine concentration falls to its original level. The release is evidenced by the increase in color at the peak potential for dopamine oxidation (600 mV) with the concurrent aDoearance of its associated reductive peak (—250 mAA In this example no other changes are seen at any other times in the data
Figure 3. Co-secretion of (upper) histamine and (lower) 5-hydroxytryptamine from a mast cell. 590 A
Analytical Chemistry News & Features, September 1, 1998
set When the current at 600 mV is plotted against time as shown above the color plot, the rapid concentration changes of dopa-
mine are clearly revealed. Postexperiment calibration of the electrode in authentic dopamine solutions allows the current to be expressed in terms of concentration. The single background-subtracted voltammogram shown was taken near the peak response (five voltammograms were averaged) and shows the presence of dopamine, but only at that time. The processes governing the temporal concentration change are now quite well understood (19). During stimulation, the signal increases rapidly with the release of dopamine from the nerve terminals and its diffusion into the extracellular space adjacent to the electrode. When the stimulation is terminated, dopamine is actively transported out of the extracellular environment into neighboring cells by the dopamine transporter. Indeed, in the brains of mice that have been genetically altered so that they do not synthesize the transporter, the disappearance of dopamine after stimulation was 300x slower than in normal mice (20). However, with some electrode placements, different behavior is observed. In Figure 5, the current at the potential anticipated for dopamine oxidation (600 mV, the dotted trace) rapidly increases during stimulation. After stimulation, the signal rapidly decreases at first, but then is followed by a transition to a slower decay. Examination of all of the data in the three-dimensional color plot reveals that there are, in fact, two analytes contributing to the signal. During stimulation, there is a rapid increase in current with the closely spaced contours. This pattern indicates the presence of a species with characteristics identical to dopamine in both the oxidative and reductive portions of the figure. As time progresses after stimulation, however, the width of the oxidative current feature in the color representation increases, and the oxidative feature shifts to a slightly more negative potential. A similar feature is seen in the reductive current, only the voltammetric feature shifts to a slightly more positive value. As iilustrated in Figure 2, these are the characteristics of an acidic change at the carbon-fiber tip. Individual voltammograms examined at different times in this experiment reveal similar information (Figure 5). The first
Figure 4. CV response measured in vivo following stimulation of dopamine neurons.
voltammogram on the far left, obtained during the stimulation, is clearly that of dopamine with the characteristic sharp peaks in both the oxidative and reductive currents. Note, however, that the second voltammogram, obtained a few seconds after the stimulation, contains features characteristic of both dopamine and pH changes. The peaks in both the oxidative and reductive currents are fairly sharp yet they differ from those in the previous voltammogram. Each peak is beginning to broaden at its base as would be expected if both a pH and dopamine change were occurring simultaneously. The third voltammogram reveals that, astimepasses, the signal is entirely due to a change that is similar to that for pH—the peaks are broad and characteristic of those seen for an acidic change. As this example clearly indicates, the color plots do not remove the need to examine individual sections of the data in two dimensions. The ability to examine all of the data simultaneously, however, makes it clearer which regions to select. Confirming in vivo pH changes
Changes of pH in vivo suggested by the carbon-fiber microelectrode responses
have been confirmed with ion-selective pHsensitive microelectrodes (21) placed adjacent to the carbon-fiber electrodes. Both the pH and carbon-fiber microelectrodes report similar pH variations in vivo. The response of the pH microelectrode was generally slower and smaller than that of the carbonfiber electrode suggesting that the carbonfiber electrode may be a better sensor for detecting relatively fast pH changes. Both basic and acidic changes were seen following stimulation in different animals. Regulating pH in the brain is complex and is influenced by local blood flow which is affected by neuronal stimulation and other local metabolic mechanisms (22) Studies to unravee flie ^nprific oricrin of thf* n\i chantrps observed here are in progress Separating pH and dopamine signals
Changes in pH interfere with detecting dopamine because their voltammetric waves overlap (compare the responses for dopamine in Figure 1 with those for pH changes in Figure 2). A signal-separation technique is thus needed to independently monitor the dopamine change. To understand how this can be accomplished, first examine the voltammetric changes that
Analytical Chemistry News & Features, September 1, 1998 591 A
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Figure 5. Dopamine and pH changes in rat brain. The dashed line represents the average current occurring near the peak oxidative potential for dopamine (600 mV). The solid line below the dashed line represents the dopamine concentration following a normalized subtraction of the currents as described in the text. The solid line just above the color plot represents the average oxidative current between 150 and 250 mV. Stimulation occured at the horizontal bar.
occur when a cndiige in pH occurs. Bare carbon-iiDer electrodes respond linearly to pH changes that occur over a broad range of potentials (Figure 2). Therefore, during a prl change, current versus time traces taken from different potential regions have identical shapes but differ only in their amplitude. To obtain the same magnitude, each of the curves must be normalized to the same time. iiow consider wiidi happens when dopamine and pH change simultaneously, as illustrated in Figure 5. By comparing the voltammograms for pH (Figure 2) and dopamine (Figure 1), it is clear that the current at some potentials will reflect both processes (e.g., 600 mV), whereas others will reflect only prhchanges (e.g., 250 mV). Unfortunately, a potential does not exist that is sensitive to dopamine but not to pH. Resolving the dopamine change requires subtracting the signal that reflects only pH changes from one that contains both dopamine and pH changes. riowever, the two signals cannot be sub592 A
tracted directly because of the differing sensitivities of the potentials to pH changes. Thus, before subtraction, the signals must be normalized to the same point in time. To accomplish this, the time point must be one at which the dopamine concentration has returned to its original level. The voltammogram at the far right has this characteristic because it indicates only that a pH change is occurring. This is expected because of the rapid rate at which dopamine is taken up by the dopamine transporter following release. Thus, if the current traces from successive measurements at 600 mV and 250 mV are normalized to their respective values at the far right voltammogram and subsequently subtracted, the remainder reflects the dopamine change. The normalization process must be reversed to obtain the correct magnitude for the current caused by the presence of dopamine. To accomplish this, the remainder was multiplied by the value at the far right voltammogram before it was converted to dopamine concentration using the calibration data. The result removes the contribution of pH, as is shown in Figure 5. Conclusion Three-dimensional false-color imaging is a useful approach in complex situations, in which more than one voltammetnc feature is changing during observation. In themselves, the images provide only a qualitative view of the data, but they provide the crucial overview that is necessary for further data processing. One draiA'back of the color plots is limited color resolution, wmen arises because the software has a range of zoo colors, diminishing the detail of the original 12-bit data. For this reason, the added contour lines are superimposed on the color images to aid visual identification. A select range of narrowly spaced contours accentuates the peaks that occur in cyclic voltammograms. However, to avoid clutter, the data near the noise level are not assigned to the contour plots, thereby providing a clear and detailed map of the oxidative and reductive peaks of interest.
References (1) Adams, R N.Anal. Chem. .976,48,1128 A. (2) Wightman, R. M.; May, L J.; Michael, A CC Anal. Chem. 1988, 60,769 A (3) Stamford, J. A; Justice, J. B. Anall Chem. 1996, 68,359 A (4) Cahill, P. S.; Walker, Q. D.; Finnegan, J. M.; Mickelson, G. E.. Travis, E. R; Wightman, R M.Anal. Chem. 1196,68, 3180. (5) McCreery, R L. Electroanalytical Chemistry; Bard, A J.. Ed.. Marcel Dekkerr New York, 1991; pp 221-374. (6) Millar, J; Stamford, J. A; Kruk, Z. L; Wightman, R M. Eur. J. Pharm. 1985, 109,341. (7) Marsden, C. A; Joseph, M. H.. Kruk, Z. L; Maidment, N. T.; O'Neill, R D.; Schenck, J. O.; Stamford, J. A Neuroscience 1988, 25,389. (8) Blaha, C. D.; Phillips, A G. Behavioural Pharmacology 1996, 6,675. (9) Joseph, M. H. Behavioural Pharmacology 1996, 7, 709. (10) Osteryoung, J.; O'Dea, J.. Samuellson, R Anal. Chem. .980,52,2215. (11) Gerhardt, G. C; Cassidy, R M.; Baranskii A S.Anal. Chem. 1198, 70, 2167. (12) White, J. G.; St. Claire III, R I.; ;orgenson, i. Vf.Anal. Chem. .986,55,293. (13) Pihel, K; Hsieh, S.; Jorgenson, J. W.; Wightman, R M.Anal. Chem. .195567, 4514. (14) Kawagoe, K. T.; Zimmerman, J. B.; Wightt man, R M.J. Neurosci. Methods 1993,48, 225. (15) Kawagoe, KT.;Garris, P. A; Wightman, R M.J. Electroanal. Chem. 1993,359, 193. (16) Jones, S. R; Mickelson, G. E.; Collins, L B.; Kawagoe, K T.; Wightman, R M. /. Neurosci. Methods ds94,52,1. (17) Rice, M. E.; Nicholson, C. Anal. Chem. 1989, 61,1805. (18) Jackson, B. P.; Dietz, S. M.; Wightman, R M. Anal. Chem. 1995, 67,1115. (19) Garris, P. A; Ciolkowski, E. L; Pastore, P.; Wightman, R M.J. Neurosci. 1994,14, 6084. (20) Giros, B.; Jaber, M.; Jones, S.; Wightman R. M.; Caron, M. Nature 1996,379, 606. (21) Chen, J. C; Chesler, M. Proc. Natl. Acad. Sci. USA 1199289, ,786. (22) Chesler, M. Prog. Neurobiol. 1990,34, 401.
Darren Michael and Eric Travis are graduate students under the direction of Mark Wightman, professor of chemistry at the University of North Carolina-Chapel Hill. Wightman's research group focuses on microsensors and neurochemical applications. Address correspondence about this article et Wightman at the Department of Chemistry, Researcc in this area eas supported oy isiri (JVO University of North Carolina, Chapel Hilll NC 27599-3290. I0o41).
Analytical Chemistry News & Features, September 1, 1998
