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The 20th century coincided with the rise andmaturation of electroanalytical chemistry, withvoltammetry at the core of the field. Within a

span of just over 75 years, a large number of differentmethods were developed, instruments were designed andconstructed, and a rigorous theoretical and mathematicalframework was established. This Report traces the devel-opment of voltammetry and some of its many applications.Our coverage of the historical aspects are brief (we proba-bly overemphasize U.S. contributions), and we apologizeto those who have contributed to this field to whom wehave failed to give adequate recognition. Detailed ac-counts of the history of electrochemical methods can befound in references 1 and 2.Voltammetry involves the application of a potential that

varies with time and the measurement of the correspon-ding current that flows between the working and referenceelectrodes (Figure 1). One can think of the potential asthe intensive variable (energy in V or J/C) applied to the

working electrode, and one can thinkof the current as the extensive variable

(current in A or C/s) corresponding to therate of an electrode reaction in response to the potentialperturbation. Thus, the resulting voltammogram of cur-rent versus potential may be either transient or steady-state(Figure 2), in which a potential ramp serves as the poten-tial perturbation.Voltammetry can therefore be broadly defined as the

exploration of the three-dimensional space that interrelatespotential (E), current (i), and time (t) (Figure 3). Both ofthe voltammograms in Figure 2 feature a plateau or limit-ing current that corresponds to the point at which theconcentration of electroactive species at the electrode sur-face drops to zero and the flux to the electrode surface(and hence the current) reaches a maximum value. Be-cause the limiting current is controlled entirely by diffu-sion, it can be used to determine any variable that con-tributes to its magnitude. Most often this process involves

Allen J. BardUniversity of Texas–Austin

Cynthia G. ZoskiUniversity of Rhode Island

M AY 1 , 2 0 0 0 / A N A LY T I C A L C H E M I S T R Y 3 4 7 A

determining thebulk concentration,

but it is sometimes usefulto determine electrode area or

radius, the number of electronstransferred in the electrode reaction, or

the diffusion coefficient. Thus, voltammetryhas important analytical applications, some of

which are highlighted in this article.The potential at which the wave occurs, reported as the

value at one-half of the limiting current (E1/2), is charac-teristic of the electrode reaction and hence of the speciesundergoing the electron-transfer reaction. The positionand shape of both the transient and steady-state voltam-mograms are also highly dependent on the presence ofhomogeneous and heterogeneous kinetic limitations.Thus, voltammetry is a very popular technique for initialelectrochemical studies of new systems and has provenvery useful in obtaining infor-mation about complicatedelectrode reactions.

Early voltammetry andpolarographyAlthough considerable re-search in electrochemistrydates back to Faraday’s workin the early 19th century withimportant contributions tothe understanding of masstransfer by Cottrell and Sandin the early 20th century,applying these techniques tochemical problems was rare.The birth of voltammetryreally occurred with Hey-rovsky’s experiments inCzechoslovakia in 1922 (3).Measuring the surface tensionof mercury had been of inter-est since Lippman’s work inthe late 19th century becausethese measurements providedinformation about the natureof the liquid–metal interface(and because the capillaryelectrometer could also serveas a sensitive null-point detec-tor for balancing potentiomet-ric bridges). Kucera inCzechoslovakia had beenworking on such electrocapil-lary measurements with thedropping mercury electrode

(DME), in which a bulb containing mercury is attached tonarrow capillary tubing. The mercury drops that fall fromthe capillary were weighed as a function of potential.Heyrovsky realized that by measuring the current while

the potential of the electrode was changed, he could ob-tain information about the nature of the species in solu-tion that were reduced at the mercury drop. Followingthis discovery, he and his colleagues worked out the the-ory for the processes that occur during electrolysis at theDME. The earliest measurements of i–E curves were donemanually by applying a potential with a potentiometer andmeasuring the microamp-level currents with a light-beamgalvanometer, a tedious process. In 1925, Heyrovsky andShikata developed an automatic instrument to photo-graphically record i–E curves (Figure 2a) and called it apolarograph. This term now means voltammetry at theDME. The polarograph was one of the first automatedrecording analytical instruments and ushered in the field

of instrumental analysis.The classic DME is rarely

used now, having been re-placed by the static mercurydrop electrode. However, theDME had an enormous im-pact on the development ofvoltammetry until the early1960s. Because mercury hasan atomically smooth surface,a high hydrogen overpoten-tial (which allows a large neg-ative potential range in aque-ous solutions), a continuouslyrenewable surface as it drops(which means less contamina-tion and highly reproduciblebehavior), polarography waswidely used for studyingmetal ions in solution. It istoo bad that the prevailing—almost pathologic—fear ofmercury has greatly decreasedthe use of these electrodes.

Voltammetry comes tothe United StatesInterest in polarography in theUnited States largely beganfollowing a visit by Heyrovskyin 1933. Kolthoff, Furman,and their respective groups didthe initial work in the UnitedStates. They constructedpolarographs and initiatedstudies on analytical applica-

Powersupply

i

V

Eappl

Workingelectrode

Referenceelectrodevs. ref

(a)

(b)Powersupply

Referenceelectrode

Workingelectrode

Counterelectrode

i

V

Ewk

FIGURE 1. Basic voltammetry.

(a) Apparatus for voltammetry with a two-electrode cell, appropriate for use insolutions of low resistance and microelectrodes. (b) Apparatus for voltammetrywith a three-electrode cell. In practice a potentiostat that automatically controlsthe potential of the working electrode with respect to a reference electrode isused.

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tions, especially metal ions. The term“voltammetry” was first introduced in1940 (4) to describe experiments inwhich the current as a function of po-tential at a solid working electrode ismeasured. The first and second editionof the classic monograph Polarographyby Kolthoff and Lingane helped spreadthe technique rapidly during the 1940sand early 1950s. During this sameperiod, work in polarography was beingdone in several European countriesother than Czechoslovakia, and inJapan and Australia.In these early days, voltammetric

work with solid electrodes made ofplatinum, gold, or carbon was second-ary compared with studies involvingthe DME. Even in the second editionof Polarography in 1952, the chapteron solid electrodes is only 21 pageslong. Commercial instruments firstbecame available from D. V. and J. Nejedly in Czechoslo-vakia, and they began selling their instruments in theUnited States in 1939. The trademark name “polarograph”was licensed to Sargent, who produced manual (the ModelIII) and then photographic (Model XI) recording models.Other instruments, such as the Fisher Instruments Elec-dropode, also appeared. The advent of electronic stripchart recorders led to more convenient automated instru-ments, such as the Sargent Model XXI and the Leeds &Northrup Electrochemograph in the early 1950s.

Variations on a themeMany variations of the basic polarographic method weredeveloped during the 1940s and early 1950s. These inves-tigations were motivated largely by a desire to improve thesensitivity of polarography by decreasing the contributionof the capacitive (charging) current and increasing thespeed of the experiments. Charging current is associatedwith charging the electrical double-layer capacitance thatexists at the interface between the electrode and the solu-tion as the electrode potential is changed. This current isnonfaradaic (e.g., does not cause a chemical change at theelectrode surface) and produces a baseline current thatmust be subtracted in voltammetric measurements. It isparticularly troublesome at a DME, where the electrodearea, and hence the capacitance, changes with time.In the early 1950s, Barker built an electronic pulse

polarograph. In this technique, the potential is changedfrom a fixed potential to a new value for a short period of~50 ms. The current is measured just before the pulseends. Because the charging current decays more rapidlythan the faradaic current, the measured current at the

end of the pulse is largely faradaic.Normal pulse and differential pulsepolarographies are among the mostsensitive techniques for directly meas-uring trace concentrations.Breyer and co-workers in Australia

developed ac polarography in which asmall ac potential is applied to the dcramp, with detection of the ac cur-rent. Extensive work in this techniquewas later done by Smith and co-work-ers in the United States and by theSluyters in The Netherlands. In thelate 1940s, fast linear potential sweepswere applied at the DME, so that thecomplete potential sweep occurredduring a single drop as the voltammo-gram was recorded on an oscilloscope.The Czechoslovakian group investi-gated a related technique, oscillo-graphic polarography, in which a largeamplitude sinusoidal potential excita-

tion was applied during a single drop.Square-wave polarography was developed by Barker in

the early 1950s. In its earliest form, the potential wave-form consisted of a slowly changing dc potential modu-lated by a small amplitude square wave. The potentialwaveform used now, developed by the Osteryoungs, con-sists of a large amplitude symmetrical square-wave pertur-bation, with each cycle of the square wave coinciding withone cycle of an underlying staircase. Current is sampled onboth the first and second half of the cycle and can be dis-played as a forward current, a reverse current, or the dif-ference between the two measured currents.These techniques, however, were not widely used at the

time because the electronic instrumentation, such as thesquare-wave polarograph marketed by Mervyn Instru-ments, was complicated, required considerable operatorskill, and was expensive. In addition, strip chart recorderswere slow and x–y recorders had not yet appeared. Thus,for techniques such as linear potential sweep and triangu-lar wave polarographies, the i–E curves had to be recordedon an oscilloscope, which limited the precision of themeasurements. It was also very inconvenient because thetraces had to be photographed with a 35-mm camera.By the late 1950s and early 1960s, however, the situa-

tion was changing. Interest in chronopotentiometry (alsocalled “constant current voltammetry”), in which a con-stant current is applied to the electrochemical cell and thepotential–time response recorded, became widespread,spurred by the work of Delahay and his monograph NewInstrumental Methods in Electrochemistry. In chronopoten-tiometry, the constant current applied to the electrodecauses an electroactive reactant to be reduced to product

E, V vs. SCE

(a)

(b)

-0.8 -1.0 -1.2 -1.4 -1.6

2 nA0.6 0.4 0.2

1 µA

FIGURE 2. Steady-state voltammograms.

(a) Typical polarogram for a reduction reactionobtained at a DME. (b) Typical voltammogram for anoxidation obtained at an RDE or UME.

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at a constant rate. The potential of the electrode attainsvalues that are characteristic of the electroactive coupleand varies with time as the concentration ratio of the elec-troactive couple changes at the electrode surface. One canthink of the process as a titration of the electroactive reac-tant in the vicinity of the electrode by a continuous flux ofelectrons. Eventually, the concentration of the electroac-tive reactant drops to zero at the electrode surface and theflux of the reactant to the surface is insufficient to acceptall of the electrons being forced across the elec-trode–solution interface. The potential of the electrodethen rapidly changes toward more negative values until anew, second reduction process can start. The period afterapplication of the constant current for this potential transi-tion to occuris called the “transition time”. This time is related to theconcentration and diffusion coefficient of the reactant andis the chronopotentiometric analog of the peak or limitingcurrent in controlled potential experiments. The shape ofthe potential–time curve is governed by the heterogeneousrate constant of the electrode reaction.This technique involved simple instrumentation and the-

oretical treatments, and was further developed by Reilley,Murray, Reinmuth, Bard, and others in the early 1960s.This work led to a new understanding of electrochemicalkinetics and mathematical methods of treating these sys-tems. Tafel plotters began to give way to Laplace trans-formers! Moreover, it encouraged applying electroanalyticaltechniques to chemical problems beyond analyzing metalions, such as kinetic studies of short-lived species in homo-geneous reactions. Methods were proposed in which a con-stant charge was rapidly injected into an electrode and thepotential–time response was monitored. However, suchcoulostatic methods never became popular.Advances in electrochemical methods were also driven

by new instrumentation.Operational amplifiers, firstbased on vacuum tubes andlater on transistors andintegrated circuits, wereused for constructing elec-trochemical instruments.Polaroid Land cameras andx–y recorders for recordingoscilloscope traces becameavailable. These develop-ments led the second waveof instrumentation, inwhich the earlier mechani-cal approach to generatingwaveforms was replaced bytransistorized instrumentscapable of complex experi-ments, such as pulse po-

larography. Typical instruments included those producedby Princeton Applied Research, for example, Models 170,173, and 174. Interest in solid electrodes, especially plat-inum and carbon, grew for studying the oxidations oforganic molecules as described in Adam’s Electrochemistryat Solid Electrodes, published in 1969.An important advance in solid electrode voltammetry

was the introduction of the rotating disk electrode (RDE)and later the rotating ring-disk electrode (RRDE) byLevich and co-workers in the former Union of SovietSocialist Republics. Although steady-state voltammogramshad previously been obtained for stirred solutions withmany different electrode configurations, most of thesevoltammograms were not amenable to rigorous mathemat-ical treatments. The RDE, however, showed hydrody-namic behavior that could be treated mathematically,which allowed the RDE to be applied to solution andkinetic studies. The RDE and RRDE could also be usedto study rapid homogeneous reactions under steady-stateconditions in which the time variable is associated with theangular velocity or rotation rate of the electrode. Brucken-stein, Miller, Albery, and others developed these electrodesand applied them to electrochemical problems.Many other related methods were described. For exam-

ple, further increases in sensitivity could be obtained bydepositing a metal in a stationary mercury drop, a thin mer-cury film, or a solid electrode surface for an extended timeand then measuring the current when anodically stripping itback into solution. Early work in stripping analysis was doneby Rogers and Shain in the early 1950s. The major advan-tage of stripping analysis, compared with direct voltammetricanalysis of the original solution, is that the material to beanalyzed is preconcentrated 100- to 1000-fold or more sothat the voltammetric (stripping) current is less perturbed bythe charging current. Stripping analysis is especially useful

for analyzing very dilutesolutions down to picomolarconcentrations. It is mostfrequently used for deter-mining metal ions (e.g., Bi,Cd, Cu, In, Pb, and Zn),either alone or in mixtures,by cathodic deposition fol-lowed by anodic strippingwith a linear potential scan(anodic stripping voltamme-try). An increase in sensitiv-ity can be obtained by usingpulse polarographic, square-wave, or coulostatic tech-niques during the strippingstep.Linear scan and cyclic

voltammetry at stationary

(a) (b)

i i

t t

E E

FIGURE 3. Three-dimensional voltammetry.

Representation of the three-dimensional i–t–E surface for a Nernstian reaction. Thetypical steady-state voltammogram represents a cut parallel to the i–E plane. (b) Acut representing a linear potential sweep across this surface. (Adapted from Ref. 6.)

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mercury drops and solid electrodes became popular tech-niques, spurred largely by Nicholson, Shain, Savéant, andVianello in the early 1960s(Figure 4). In these tech-niques, the potential isramped linearly at rates of0.01–105 V/s, with theresulting current recordedas a function of potential(which is equivalent torecording current versustime). In a qualitative way,this technique amounts totraversing the three-dimen-sional i–t–E realm (Figure3). It is the interplay ofthese three parameters thatleads to the complexity inthe theory of linear andcyclic voltammetry. Experi-mentally, the potential scanis begun at a potentialwell-positive of the standard potential for a reduction.When the electrode potential reaches the vicinity of thestandard potential, reduction begins and current starts toflow. As the potential continues to grow more negative,the surface concentration of the oxidized species mustdrop and the flux to the surface (and hence the current)increases. As the potential moves past the standard poten-tial, the surface concentration drops to near zero andmass transfer of the oxidized species to the surfacereaches a maximum rate, followed by a decline as thedepletion effect sets in. A peaked current–potential curveis observed. If the experiment is stopped at this point, alinear scan voltammogram is recorded. If the direction ofthe potential scan is reversed in a positive direction backto the initial potential, an anodic current flows as theelectrochemically generated reduced species is reoxidized,leading to the recording of a cyclic voltammogram (Fig-ure 4).Cyclic voltammetry, despite its theoretical complexity,

quickly became one of the most popular electrochemicalmethods and was widely used in fields outside of analyticalchemistry to obtain thermodynamic and kinetic informa-tion about organic, organometallic, and inorganic species.Chronocoulometric methods, in which a charge is recordedafter a potential step, were introduced by Anson in themid-1960s and were especially useful for studying the ad-sorption of electroactive species on an electrode surface, atopic of continuous interest and sometimes annoyance. Atabout the same time, investigations began on voltammetryin thin-layer cells, in which a thin film of liquid (2–50 µm)is held between two electrodes or between an electrodeand an insulating wall. These cells, which contain microliter

volumes, were among the earliest analytical techniques thatexamined such small volumes.

In the late 1950s, the useof aprotic solvents, such asacetonitrile, extended therange of voltammetric meas-urements to much morepositive and negative poten-tials than were available inwater, even with platinumand gold electrodes, and tospecies insoluble in water.Researchers began to usespectroscopic methods tostudy reactions in electro-chemical cells. In the early1960s, Geske and Makiintroduced electron spin res-onance (ESR) to study elec-trogenerated species by plac-ing the electrode directlyinto a cell in the cavity of the

spectrometer (although physicists advised that it wouldn’twork). This technique became a favorite for studying radicalions generated at electrodes. Work in the early 1970s byGoldberg and Bard showed how the high-resistance effectsof the usual ESR cells could be ameliorated and how cellscould be constructed so that cyclic voltammograms could berun and ESR signal intensity recorded as a function ofpotential. In the mid-to-late 1960s, Kuwana experimentedwith transparent indium tin oxide electrodes and Murray andHeineman with minigrids to monitor the optical absorbanceof electrogenerated species. In these spectroelectrochemicalexperiments, the optical absorbance response could be plot-ted as a function of potential or time.The availability of electrochemical techniques that could

probe the solution chemistry of species with short life-times and the growing use of aprotic solvents with vacuumline cells or inert atmosphere boxes led to studies by manygroups on organic systems. These studies, coupled withESR measurements, demonstrated that electrode reactionswere dominated by one-electron transfers and that earlierwork indicating irreversible two-electron reactions werethe result of homogeneous reactions following the firstelectron transfer, often involving water (so-called ECE-reaction schemes).These ESR and electrochemical studies pointed to the

importance of radical ions in organic reactions at a timewhen most organic chemistry was still being described interms of “pushing electron pairs”. In fact, many aromaticcompounds were shown to form quite stable radical anionsand cations. Interest in generating these species led Chan-dross and Visco at Bell Labs, Hercules at MIT, and Bardand co-workers at University of Texas to almost simultane-

0.4 0.2 0

10 nA

FIGURE 4. Typical cyclic voltammogram.

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ously study the annihilation reaction between radical cationsand anions generated at an electrode. This reaction pro-duced excited states and light emission in the phenomenoncalled electrogenerated chemiluminescence (ECL). Now, involtammetric experiments, the electrodes emitted light.

Computers leave their markThe first impact of mainframe computers in the early1960s was in computation—Herman and Bard even man-aged to get a computer program printed in AnalyticalChemistry in an article about cyclic chronopotentiometry,but that practice was short-lived. However, what did havea major impact were digital simulation methods, mainlythe work of Feldberg. This approach allowed complexelectrochemical mechanisms to be treated, even when themathematics was daunting. Development of these methodscontinued, and they became more efficient. When smallcomputers became more powerful and less expensive,commercial programs such as DigiSim were used to simu-late the cyclic voltammetric response. Today, problems canbe treated with a few keystrokes that would have beenimpossible to deal with 30 years earlier.Computers also had a major impact on electrochemical

instrumentation. When minicomputers, such as the PDP8,became available, they were used to generate the poten-tial programs, for example, in pulse polarography and to

record the resulting responses usingA/D and D/A converters (5).However, the power of these com-puters was limited and they werecostly. Moreover, considerable pro-gramming effort, often in assemblylanguage, was needed to run anexperiment. The situation changedwith the advent of microcomputers,and computer-controlled electro-

chemical instruments became a reality. Thus, in the mid-1980s, He and Faulkner described an electrochemicalinstrument that could perform many voltammetric experi-ments by selecting from a menu on the computer screen.This approach has evolved into the third wave of moderninstrumentation based on computer control and record-ing, with analog-integrated circuit interfaces for potentio-static control, such as those produced by Bioanalytical Sys-

tems, CH Instruments, Eco Chemie, Cypress Instruments,and many others.What’s been going on latelyFollowing the burst of activity in the 1960s and early1970s, interest began to turn to the electrodes themselvesand to a better understanding of how the surfaces could bemodified to carry out desired processes. Electroanalyticalchemists started talking about designing and engineeringmodified electrodes. Initial efforts involved covalentlyattaching monolayers to electrodes, which was done byHubbard, Kuwana, Murray, and others, followed by thecontrolled formation of polymer layers on electrodes byMiller, Bard, Murray, and co-workers in the late 1970s.This work led to a large number of investigations of differ-ent types of polymers, including poly(vinylferrocene),polylelectrolytes (Nafion), and electronically conductingpolymers (polypyrrole). Savéant, Andrieux, and co-workersmade important contributions to understanding thevoltammetry of these modified electrodes that alloweddetermining the various processes that control the rates.More recently, a whole array of different electrode modifi-cation strategies, including coatings made of clays, zeolites,inorganic crystals, enzyme layers, organic metals, and com-posites, has been investigated. Enzyme-modified electrodeshave been especially important for designing sensors andhave led to commercial products for glucose determination.Instrumentation that allowed measurement of currents in

the nanoamp, picoamp, and even femtoamp range made itpossible to do studies with micrometer-sized (or smaller)electrodes called ultramicroelectrodes (UMEs) or microelec-trodes. Microelectrodes were originally developed for biolog-ical and medical research, and it was not until the late 1970sthat electrochemists began investigating the advantageousproperties of these small electrodes. Fleischman initiatedmuch of this activity because he sought to understand elec-trode mechanisms under conditions of high current density.

At the same time, carbon-based microelec-trodes were developed separately by Adamsand Gonon for in vivo determinations of cat-echolamines. In 1981, Wightman wrote thefirst comprehensive review on the properties

and prospective applications ofmicroelectrodes. The large impactof microelectrodes is rooted intheir ability to support very usefulextensions of electrochemicalmethodology into the domains oftime, media, and space that werepreviously either completely orpractically inaccessible. Advanta-geous properties of microelec-trodes include a relative immunityto the effects of solution resist-ance, a dramatically reduced dou-

Today, problems can be treated

with a few keystrokes that would have been

impossible to deal with 30 years earlier.

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ble-layer capacitance, and increased current density. Newdomains that have been explored include submicrosecondelectrochemistry, electrochemistry in low-conductivity media,and electrochemistry under time-independent conditions.Ultramicroelectrodes combined with scanning probe

techniques showed how electrodes could be spatially manip-ulated with very high resolution to study electrode surfaces.In the late 1980s, Wightman, Engstrom, and co-workersshowed how a concentration profile at an electrode couldbe mapped with a UME. Kwak, Fan, Lev, and Bard intro-duced scanning electrochemical microscopy (SECM) andthe concept of scanning a surface and determining informa-tion about surface chemistry and topography from the feed-back current. The SECM has since been used for many dif-ferent types of surfaces and for kinetic studies of rather fastheterogeneous and homogeneous reactions. CH Instru-ments introduced a commercial SECM in 1998.Work also continued on the interaction of light with elec-

trodes in a field broadly known as photoelectrochemistry.Early work in this area in Czechoslovakia, the former Unionof Soviet Socialist Republics, the United Kingdom, and theUnited States mainly concerned photoelectron ejection frommetal electrodes. Later work centered on semiconductormaterials as electrodes, which also greatly increased therepertoire of solid electrodes that could be used in voltam-metry; more importantly, it led to new insights into thenature of the electrode/solution interface and the electron-transfer process there. Recording the i–E behavior in thedark and under irradiation was useful in mapping the energystates at the semiconductor/solution interface. In the samearea of light and electrochemistry, ECL also advanced and,with the discovery of co-reactants, led to reactions carriedout in aqueous solutions. There are now commercial ECLinstruments for immunoassays and DNA analysis.Another area of growth is voltammetric detectors for

HPLC and CZE. Electrochemical detectors, incorporatingmicroelectrodes in an on-column configuration, were firstintroduced by Manz and Simon in 1983 for HPLC, and byWallingford and Ewing in 1987 for CZE. At that time,amperometry was the electrochemical mode most com-monly used because of its sensitivity and simplicity. How-ever, this mode was soon found to be limiting because thepotential of the electrode is held at one value, and onlythose species that are oxidized or reduced at that potentialare detected. To extend the detectable range of analytes asthey elute from a column, fast-scan linear sweep and cyclicvoltammetry were introduced to HPLC during the 1980sby Jorgenson and to CZE in 1996 by Ewing. Baranskiused square-wave voltammetric detection for CZE in 1998.HPLC and CZE with fast-scan voltammetry have led toexciting investigations of single cells in which femtomolarconcentrations of neurotransmitters have been measured.

A look ahead

A large number of methods have been described forexploring the i–E–t domain, though many, such as oscillo-graphic polarography and coulostatic methods, have fallenby the wayside and it is unlikely that new approaches withsignificant advantages will be introduced. Chemistry hasbeen moving in the direction of higher speed and smallersize, and voltammetry will also evolve in this direction.Thus, the development of spatially resolved electrochemicaltechniques for analyzing materials in small domains on sur-faces and channels in membranes can be expected, and thiswill have a huge impact in biological studies. One shouldnote that electrochemistry does not suffer from size limita-tions caused by diffraction that are typical of optical tech-niques. However, it is difficult to see how electrochemicaltechniques will ever approach the picosecond and femto-second domains routinely studied by laser methods.Electrochemical sensors will continue to develop, and

sensor arrays, where voltammetry can simultaneously beperformed on many different microelectrode pads, appearto be on the horizon. These sensors and arrays could formthe basis of immunoassays, DNA probes, and enzyme-based arrays. Electrochemistry is easily integrated intosmall areas, so if chemistry on a chip becomes a reality,electrochemical schemes for synthesis and analysis shouldbe right there, in the forefront.

Suggested readingBard, A. J.; Faulkner, L. R. Electrochemical Methods: Principles and Applications,

2nd ed.; John Wiley and Sons: New York, 2000.Kissinger, P. T.; Heineman, W. R. Laboratory Techniques in Electroanalytical Chem-

istry, 2nd ed.; Marcel Dekker: New York, 1996.

References(1) Electrochemistry, Past and Present; Stock, J. T., Orna, M. V., Eds.; ACS Sym-

posium Series 390; American Chemical Society: Washington, DC, 1989.(2) A History of Analytical Chemistry; Laitinen, H. A., Ewing, G. W., Eds.; Divi-

sion of Analytical Chemistry; American Chemical Society: Washing-ton, DC, 1977.

(3) Heyrovsky, J. Chem. Listy 1922, 16, 256.(4) Kolthoff, I. M.; Laitinen, H. A. Science 1940, 92, 2381.(5) Computers in Chemistry and Instrumentation;Mattson, J. S., Mark, H. B.,

MacDonald, H. C., Eds., Vol. 2; Marcel Dekker: New York, 1972.(6) Reinmuth, W. H. Anal. Chem. 1960, 32, 1509.

Allen J. Bard is a professor at the University of Texas–Austin andthe Editor of the Journal of the American Chemical Society. His re-search interests include applying electrochemical methods, such asphotoelectrochemistry, ECL, and electroanalytical methods, to chemi-cal problems. Cynthia G. Zoski is associate professor at the Univer-sity of Rhode Island. Her research interests include transport pro-cesses to microelectrode geometries, double-layer phenomena atelectrodes approaching molecular dimensions, and scanning electro-chemical microscopy. Address correspondence about this article to