Chemically Modified Electrodes n n U
for Electroanalysis Royce W.Murray
Kenan Laboratories 01 Chemistry Unlversity 01 North Carolina Chapel Hill. N.C. 27514
Andrew G. Ewlng
Department of Chemistry Pennsylvanla State University University Park, Pa. 16802
Richard A. Dust
Center For Analytical Chemistry National Bureau of Standards Gaithersbug. Md. 20899 Electrochemical methods traditionally have found important applications in sample analysis and organic and inorganic synthesis. The electrode surface itself can be a powerful tool. By controlling the electrode potential, the chemist can use it as a variable free energy source (or sink) of electrons. In addition, electrons crossing the electrode-solution interface can he deter-
mined with great sensitivity hy measuring current. As with most measurement tools, however, electrodes encounter specific phenomena that reduce their applicability to analytical and synthetic schemes. Chief among these are fouling of the electrode by unwanted precipitation or adsorption processes and the slow electrochemical reaction rates of some species that require application of an overpotential to cause the desired reaction to occur. These phenomena often can be controlled by manipulating the chemical nature of the electrode surface. Until the mid-19708, however, the repertoire of electrodes available to the electroanalytical chemist was confined to mate' rials such as C, Au, Hg, and Pt. The concept of chemically modified electrodes (CMEs) was in part borne out of the frustrated electrochemist's desire to seize direct control of the chemical nature of the electrode surface. By deliberately attaching chemical reagents to it, one hoped that the electrode surface would take on the chemical properties of the attached re0003-2700/87/0359-379A~$O1Sot0 @ 1987 American Chemical Society
Flgure 1. Schematic lllustratlonsof modifled electrodes used for electrocataiysls. (a)mawlayers and (b) miiimoll)w!arpolymer lay= ofcatalyst snea 0 and a:(c)prsconoemtlon 01 polyanionic polymer film: (d) membrsne barrl~r(3to undesired anions wing dectroactlve catlms polyanionic films: and (e)electrorslss~lng of munlerlons lntD solUtlOn bv reduction by poWIlmlC eIeclroacUve polymer.
agents. If the proper reagent were chosen, desirable properties such as reagent-based control of the rates and selectivities of electrochemical reactions (Le., electrocatalysis), freedom
from adsorptive and coatlng effects, and special optical or excited state features might he obtained. This appealing concept of rational molecular design of electrode surfaces, although am-
ANALYTICAL CHEMISTRY. VOL. 59. NO. 5, MARCH 1. 1987
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Table 1. Electrode modification schemes Monomolecular layers Chemisorptlon of reagent On platinum surface (3 On carbon surface (10) On mercwy surface ( 18) Au surface ( 19) Formation of wvalent bond between eiectmde and electroactive reagent At metal oxide surfaces (20) At carbon surfaces (21) At semiconductors (5) Of electroinactlve. chiral substances (2.27 Multimolecular layers, polymer film coatings on electrodes Redox polymers (23-25) Ion exchange-electrostatically trapped ( 11,267 Electronically wnducting polymers (27) lonically conducting polymers (17) Crown ether or wmplexing agent (28) Electroinactive chiral polymers (29) Heterogeneous multimolecular layers [integrated systems (3@] Modifying agent mixed with carbon pasta (30,4 0 Clay modified (31) Zeolite modified (3.27 Elecboactive particles in electroactive polymer ( 5 )
o=c
c=o
Flgure 2. Representative immobilized chemical reagents on electrode surfaces. (a) dicabaii wlacial porphyrin dloxygen reduction elecuocatalyst chemisorbed on oarboon: (b) and (d) Mydroxybenzene derivatives anached lo carbon surface by a surface amide and psndanl to a polymer. rsspktively; (c) and (0 lermcene derivatives anached to elenrDdes by metal-axygenailiwn bonda: (e) elsctrochemicallypolymtwirable pynole-subsIiMed wbalt Ietraphenylpuphyrin.
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ANALYTICAL CHEMISTRY. VOL. 59. NO. 5. MARCH 1. 1987
bitious, has enjoyed considerable success and stimulated much research (I, 2) since its inception ( 3 , 4 ) .Interest in CMEs has spread in varied directions, including synthetic design of electrochemically reactive polymers; basic studies of electrocatalysis, electron transfer kinetics, and membrane permeation; electrochromics; photoelectrochemistry; molecular electronics; and electroanalysis. This REPORT deals with CMEs in electroanalysis; selected reviews that include the other topics are available (I, 2,5-9). There are currently five main ideas underlying uses of modified electrodes in analysis: electrocatalysis, preconcentration, membrane barriers, electroreleasing, and microstructures. These ideas are schematically depicted in Figure 1. Modified electrodes uniformly dependon acapabilityto immobilize chemical species on electrodes; thus we will begin with a brief discussion of the general approaches to immobilization. Attaching chemicals to dectrode surfaces Methods for immobilizing chemical reagents on electrode surfaces are s u n marized in Table I along with illustrative references. Figure 2 s h o w representative structures. The immobilized reagents are usually electroactive [oxidizable or reducible, e.g., the electrocatalyst redox labelled D l 0 in sections (a) and (b) of Figure 11. In the case of polymer films, clectroactivity is sometimes unnecessary because the electrode is designed to selectively preconcentrate or transport substrates through the film to the electrode, based on membrane partitioning or permeability effects [sections (e) and (d) of Figure 11. The earliest attachment research involved irreversibly adsorbing monolayers or submonolayers of electroactive reagents onto the electrode material. Lane and Hubbard (3) demonstrated this pathway in their pioneering experiments with quinone-bearing olefins chemisorbed on platinum electrodes. Carbon electrodes are especially effective a t chemisorbing reagents that have extended *-bond systems. A significant example of this wa8 the chemisorption of a dicobalt cofacial porphyrin electrocatalyst [ C O ~ F T F ~ , Figure 2, section (a)] (IO)onto pyrolytic carbon to create an electrode that carries out the four-electron reduction of dioxygen. Mercury electrodes strongly adsorb reagents with mercaptide groups, and this behavior has been exploited (18). The concept ( 4 ) of using chemical functionalities of electrode surfaces as anchoring groups to attach reagents by definable covalent bonds followed the early chemisorption work. This ap-
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proach offered substantial synthetic diversity and has been extensively developed (1-9) for attaching monomolecular and multimolecular layers of electroactive substances to semiconductor, metal oxide, and carbon electrodes (Table I). An example of a suitably reactive metal oxide surface [with organosilane reagents, see Figure 2, section (c)] is Pt, which readily forms a layer or two of platinum oxide terminated by Pt-OH groups. Carbon surfaces are readily oxidized to produce a high density of surface carboxyl groups (15) with which amide bonds can be formed (21) [Figure 2, section (b)]. Electroactive polymer, multimolecular layer films are popular today ( 2 ) because they are technically easier to apply to electrodes than are covalently bonded monolayers. In addition, they contain up to 105 monomolecular layers of electroactive sites so that their electrochemistry is more easily observed. In some circumstances, their electrocatalytic efficacy may be enhanced. Electroactive or electroinactive polymer films also can serve as preconcentrating media [Figure 1, section (c)] or as transport barriers [Figure 1, section (d)], whereas monolayer films generally are not reliable for such uses. Polymer films can be applied to electrodes by evaporating solutions of preformed polymer (23) such as that in Figure 2, section (d), by electrochemical precipitation of a preformed polymer (33) or of a hydrolytically reactive monomer ( 2 ) ,or by electrochemically polymerizing an electroactive monomer (25, 34) such as the pyrrole-substituted porphyrin in Figure 2, section (e). Polymer films generally adhere satisfactorily to electrodes simply by chemisorption forces or by being insoluble in the contacting solvent, but they can also be surface-bonded, as in the case of polymerized organosilane reagents (24).In general, polymer films are more stable than monolayer films, and film stability is, of course, important for analytical sensor applications. Electroactive polymer films are intrinsically conductive of both ions and electrons ( 2 ) .There are three kinds: redox polymers, electronically conducting polymers, and ion exchange polymers. Polymer films with redox sites (localized electronic states), called redox polymers (35), conduct electricity via electrons hopping between oxidized and reduced sites [electron self-exchanges, see Figure 1,section (b)]. This is the case with the o-quinone (23),pyrrole-substituted porphyrin (25), and ferrocene polymers (24) in Figure 2 , sections (d), (e), and (f). So-called organic metals, or electronically conducting polymers, conduct electricity more efficiently than do redox polymers via delocalized, metal-like band structures. One of these, polypyrrole, is quite pop382A
ular because it is easily formed by electrochemical polymerization (27),and it can be used to entrap electrocatalysts such as phthalocyanines (36) and enzymes (37).Ion exchange polymer films are made electroactive by exchange of some of their charge-compensating counterions for electroactive ones (11, 13, 26). An example is exchanging Fe(CN)63- for the Clod- counterion of a protonated polyvinylpyridine film (13).Because the ion exchange equilibrium can be slowly reversed when the polymer is used in an electrolyte solution free of the electroactive counterion, the electroactivity of ion-exchanged polymer films is less permanent than that of fixed-site, redox polymers. On the other hand, developing the chemistry of a given polyelectrolyte film such as Nafion (26)or polyL-lysine copolymer (II ) offers flexibility because many different electroactive counterions can be accommodated. In the ion exchange films, electrons are transported in part by physical diffusion of electroactive ions, which may also undergo self-exchange as part of the transport process (38). Understanding the electrical properties of electroactive polymer films on electrodes is an active research subject, as are the processes by which they electrocatalytically mediate electron transfers between the electrode and reactive substrates in the contacting solution ( 2 ) . Ionic conductivities of the films have been less fully investigated but need to be better understood, because for electroneutrality, a change in the film’s oxidation state requires movement of charge-compensating counterions in the film. Integrated systems (39) (Table I) refer to heterogeneous films on electrodes designed to contain a mixture of constituents with different functions. The incorporation of PtO microparticles into viologen polymer films on p-Si photocathodes exemplifies (5, 40) the integrated system approach; the PtO provides a catalytic surface for Hz generation following photoreduction of the viologen film by band gap absorption in the semiconductor space charge layer. Other examples are adding constituents such as catalyst monomers (41) and metal complexing agents (30) to carbon paste electrodes. Polymer films containing entrapped particles with ordered lattices, such as zeolite (32) or clay (31) particles, are especially intriguing integrated systems possibilities. For instance, the size-selective trapping of redox species in zeolite particles near the electrode surface has been demonstrated by trapping dioxygen in 3-A sieves (32).The heterogeneity of the zeolite and clay-containing films poses the challenging electrode design question of how electrons transfer from the electrode to electroactive
* ANALYTICAL CHEMISTRY, VOL. 59, NO.
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guests in the particles. These films appear to offer novel opportunities to fashion selective catalytic or photosensitive reaction sites on electrode surfaces. Although methods for immobilization of chemicals on electrode surfaces undoubtedly will continue to evolve and important discoveries will be made, this is already a powerful topic. The statement “chemical immobilization on demand or your money back” is not an overly presumptuous advertisement. The hard part remains the design of the chemical system, such as an electrocatalyst, that you wish to immobilize.
Electrocatalysis An important motivation for modifying electrode surfaces is electrocatalysis of the electrode reaction of an analytically desired substrate. It is often observed that the electrode kinetics of an analyte at a naked electrode surface are slow, so that oxidation or reduction occurs a t a potential that is much more positive or negative, respectively, than the expected thermodynamic potential (Le., an activation overpotential exists). Figure 1, sections (a) and (b), illustrates the idea of decreasing this overpotential by accelerating the desired reaction with an immobilized mediator catalyst. The oxidized form of the mediator catalyst (0in Figure 1)is rapidly reduced by the electrode, and then its reduced form ( 0 )reacts with the substrate (SI or analyte species in solution. There are three important characteristics of mediated electrocatalysis. First, the catalyzed reaction occurs near the formal potential of the mediator catalyst couple ( O / O ) unless a catalyst-substrate adduct is formed, in which case reaction occurs at the potential for the adduct. Second, the mediator catalyst and substrate formal potentials should be similar. Because this lowers the 0 S reaction free energy, however, the choice is also subject to maintaining a satisfactorily fast reaction rate. Finally, a successfully catalyzed reaction of S occurs a t less negative or positive potential for reduction or oxidation, respectively, than the naked electrode reaction of S would require. Such ideas are part of formulating a detailed theory for mediated electrocatalysis (42), which for polymer films also includes considering the rate of permeation of substrate into the film and the rate of regeneration of mediator sites by the electrode. These processes, shown schematically in Figure 1, section (b), in turn strongly influence the fraction of the total population of catalyst sites in the polymer films that can undergo reaction with the substrate (11). For instance, if the sub-
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Flgure 3. Catalytic Oxidation of NADH on q u i n o n e polymer electrode. (a) naked carbon electrcde with 0.5 mM NADH: (b) s i e ~ t r ~ dcoated e wim quinone poiymer at coverage 7.5 x 10-10 mollcm2 01 quinone sites (about eight monolayers); (c) reduction 01 0.5 mM NADH at coated elecbode Of ibl. iAdaDted f r m Reference 23.)
strate permeates the film very slowly, then only the outermost monolayer of catalyst sites a t the polymer-solution interface (12) may participate in the reaction. Based on schemes like those illustrated in Figure 1,sections (a) and (h), mediated electrocatalysis represents an opportunity and a challenge for the electrochemist to molecularly engineer electrode surfaces that will consume reaction substrates rapidly and selectively. If the reaction substrate is an analyte, the electrocatalysis becomes a h a sis for an electroanalytical sensor. One of the beauties of the CME idea is that it leads the chemist to rationally draw “paper schemes” designed to produce certain types of electrode behavior, using energetic and dynamic ground rules no more complicated than those of spectroscopy and photochemistry. The current challenge is the fahrication of chemical materials that have the desired energetic and dynamic properties needed for the scheme. This especially applies to the chemicals used as electrocatalysts [Le., 0/0 in Figure 1, sections (a) and (b)]. There have already been some remarkahle successes in designed mediated electrocatalysis (ZO), and this should continue to be a fruitful area for the future. Interesting examples of using electrocatalysis in analytically important 384A
electrode reactions involve the electrooxidizable, physiologically important analytes dopamine, ascorbic acid, and NADH at carbon electrodes. NADH is slowly oxidized at carbon electrodes, and CME catalysis of its oxidation via immobilized mediators has been sought by a number of investigators. Another problem with NADH, which has been less successfully dealt with, is that its oxidation product(s) tend to foul the electrode. The analytical problem with ascorbic acid is to observe the electrochemistry of the neurotransmitter dopamine in the presence of a large excess of ascorbic acid (an interferent); a differential electrocatalysis of the reactions of these two substances is desired. In an important study, Kuwana and co-workers (21.43) took a molecularly systematic approach to designing electrode surfaces for electrocatalysis of ascorbic acid and NADH oxidations. They first examined homogeneous solution reaction kinetics, using a series of quinones as oxidants. The most potent were o-quinones. They then immobilized monolayers of 3.4-dihydroxybenzylamine on carbon electrodes [see Figure 2, section (b)], and as predicted from the homogeneous solution data, electro-oxidation of this substance led to mediated oxidation of both ascorhicacidand NADH atsignificantly less positive potentials than on naked carbon electrodes. This work has provided an apt illustration of the power of molecularly designing electrode surfaces. Subsequently, Miller and co-workers (23) incorporated the same o-quinone moiety into a polymer film electrode coating designed for NADH oxidation. The cyclic voltammetry they observed for oxidizing and re-reducing the dopamine-containing film is shown in Figure 3, curve (b). When NADH is added to the solution contacting the film, NADH becomes oxidized near the o-quinone potential on the modified electrode [Figure 3, curve (c)]. The NADH reaction in curve c occurs a t a potential about 250 mV less positive than that observed a t a naked electrode (curve a), signaling effective electrocatalysis of the reaction. We should point out that other forms of designed CME electrocatalysis are possible in addition to the electron transfer mediation of Figure 1,sections (a) and (b). New electrode surfaces can be generated using electrically conducting polymers (27) and, in fact, polypyrrole films have been observed to enhance the rate of ascorbic acid oxidation (44). Chemical reactions often exhibit Bronsted or Lewis acid-base catalysis or are catalyzed by partition into hydrophobic environments (phase transfer catalysis). These tactics have not yet been intensively investigated in
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1. 1987
CMEs, hut useful electroanalysis could result where such effects are coupled to electron transfer processes. Pracomentration into CMEs and membrane barrier CMEs Preconcentrating CMEs hear formal analogy to trace analysis by the electrochemical technique called anodic stripping voltammetry. The sample species is partitioned from a dilute solution into the CME layer [preconcentrated, Figure 1, section (e)] and is suhsequently reduced or oxidized by an electrode potential sweep or step. The partitioning and measurement steps are analogous to the deposition and stripping steps in stripping voltammetry. The preconcentration chemistry may be based on procedures such as those (1) by which ion-exchanged electroactive films are prepared (Table I). T o a certain extent, the rules of conventional ion exchange selectivity apply to the electroactive species and the electrolyte counterions (45) that exchange places between solution and film. Highly charged electroactive counterions can be scavenged from quite dilute solutions (13).For some polyionic films such as Nafion, however, special phasesegregated hydrophobic effects can combine with ion exchange principles to produce enormous (104-10’) partitioning selectivities (46),especially for electroactive counterions that have some hydrophobic character. It is even possible to use the hydrophobic effect of Nafion to immobilize neutral species, as Osa (47) showed recently with a-cyclodextrin. For metal ion samples with labile coordination shells, preconcentrating CME films can he designed with metal coordinating or chelating properties. Guadalupe and Abruiia (28)drew upon the well-developed metal coordination chemistry of 2,2’-bipyridine (bpy) in fashioning polymer films with strong metal-binding properties. Exposure of electrodes coated with bpy-containing polymers to aqueous 5 X M Fe(II1) solutions allowed them to obtain voltammograms of the [Fe(bpy)J2t’3t complex upon transfer of the CME to acetonitrile solvent. This experiment demonstrated both the preconcentration capability of the bpy film and the internal mobility of the polymeric bpy sites required for their assembly into a stable complex. In another adaptation of known coordination chemistry, Baldwin et al. (30) coated the carbon particles of a carbon paste electrode with dimethylglyoxime and used i t to detect Ni(I1) ions a t the 50 ppb level. These two studies again show that it is indeed possible to plan and achieve molecular design of electrode surface behavior. Sometimes one would like to prevent contact between the electrode and a
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species in the sample solution that ii adsorbable or electroactive in an unde sirable way, yet permit transport of the desired sample species to the electrode surface. Polymer films used for this purpose can be called membrane barriers, which can be produced using the same polymer film-making techniques that are employed to make electroactive films on CMEs. Exclusion of the undesired substance from the polymer film can, for cross-linked, pinhole-free films, be based on molecular size effects (48). and for polyionic films, on charge (Donnan) exclusion effects (14, 15) [see Figure 1,section (d)]. It is possible to combine preconcentration and membrane barrier effects in a CME if the desired and undesired sample species are counterions and coions, respectively, of the CME film. This was done in an important experiment by Martin and A d a m (14, 15) that dealt with the ascorbic acid-dopamine problem. Films of Nafion were coated onto carbon electrodes and used to monitor dopamine in the presence of the interferants ascorbic acid and dihydroxyphenylacetic acid in the complex medium of the rat brain. Ascorbic acid and dihydroxyphenylacetic acid are anions a t physiological pH, and because they are eo-ions of the polyanionic Nafion, they are excluded from the Nafion film and give no electrochemical response. On the other hand, dopamine, a cation, is readily partitioned into and transported through the film. An approximately ZOO-fold selectivity for dopamine over ascorbic acid was obtained by this simple approach.
Eleetroraleasingfilms There are many situations in which one would like to release (or incorporate) some species or reagent in a controlled microdosage and by externally initiated electrical command, and this is possible using C M b . One way to release a microdose from a CME is to bind the desired reagent to a polymer film by an electrochemically cleavable bond. This was done by Miller et al. (23),who in a chemical analogue of a nerve synapse released dopamine from films of the polymer shown in Figure 2, section (d) by reducing and cleaving the amide bond. Gamma-amino butyric acid and glutamic acid could be released (49) in a similar manner, but in all cases the quantities involved were very small because the bond-cleavage electrochemistry also destroyed the ability of the polymer film to transport electrons. The result was an insulating, “usedup” layer of film separating the electrode and the unused film. Improving the CME film design in this case clearly required some built-in capability to continue to transport electrons to the outer reaches of the film. 388A
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I Flgure 4. Mlcrostructured electrode made from ionically conducting polymer and microelectrode for solid-state voltammetry and bathing gas sensor.
Polypyrrole, a popular conducting polymer that is easily made by electropolymerization (27), has the property of being polycationic (and thus an anionic exchanger) in its oxidized, conducting form and more nearly neutral in its reduced, less conducting form. In an early approach to electroreleasing CMEs, Burgmayer (50) showed that applying reducing or oxidizing potentials to a porous Au electrode embedded in a polypyrrole membrane interposed between two electrolyte solutions could be used to turn the flow of anions between those solutions on and off. This arrangement was called an ion gate, and as such is a type of molecular electronic device (2). Polypyrrole was subsequently adapted by Miller (16) in an electroreleasing scheme that avoided the electron transport problem. As illustrated in Figure 1,section (e), if an anion (glutamate) is the counterion of a polycationic polymer film (oxidized polypyrrole), then steady or pulsed reduction of the film releases the glutamate anion, for reasons of electroneutrality, in a steady or pulsed flux into the contacting solution. This appears now to be a promising approach for microdosage of reagents and has been reported by Martin (51) in another form based on a ferrocene polymer. The electroneutrality game with polypyrrole can also be employed to turn a reduced polypyrrole CME into a flow injection analysis (FIA) ion detector (52).If the solvent medium passing the polypyrrole surface contains no anionic species that will readily serve as a counterion to the film, little or no anodic current flows, even though the electrode potential is a t a sufficiently positive value to oxidize the film. When a band of anions passes in the FIA experiment, a peak of current then results as some of the anions are used as
ANALYTICAL CHEMISTRY. VOL. 59, NO. 5. MARCH 1, 1987
counterions in polypyrrole oxidation. The advantage gained here is the ability to use amperometric electrodes to detect nonelectroactive ions; a new set of analytes amenable to determination by CMEs thus is added.
Micrcwtructuedelectrodes These CMEs represent both a physical-spatial and a chemical design aimed at evoking some special electrochemical, chemical, or optical property. The ion gate (50) was one example of a microstructured electrode; others have been recently reviewed (7)in the context of macromolecular electronics. It seems clear that some useful analytical sensor capabilities will emerge from this rapidly moving new venture; indeed, proposals already have been made for use of “molecular transitors” as sensors (53).These transistors can be viewed as molecular analogues of CHEMFET detectors, a field that has begun adopting chemically active films from the CME area for gas sensors (541, seeking improved response specificity. Microstructured electrodes also permit the application of CMEs to study and use electrochemical reactions where no liquid solvent is employed. We believe the resulting capability for solid-state voltammetry will have farreaching implications. Figure 4 shows a recent example (1 7)of how an ionically conducting polymer film [about 1pm thick semisolid solution of lithium triflate in poly(ethy1eneoxide)] coated on a 10 pm Pt disk microelectrode can be used as a gas sensor. The diffusion rates of electroactive solutes in the polymer (such as the metal complex (O~(phen)~]*+/”), and thus in the current flowing a t the microelectrode, are strongly affected by the composition of the bathing gas over the film. Organic components of the bathing gas seem to partition rapidly in and out of the thin
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matic reaction to produce a burst of current at the electrode. This scheme offers the molecular specificity of the antibody-antigen reaction as well as a considerable potential chemical amplification. All of the elements in this scheme are physically reasonable, and there are precedents for each chemical step, so it will not be surprising to see it demonstrated in some future publication. This scheme is just one of many presently conceivable designs for future electrochemical biosensors. Preparation of this REPORT was supported in part by a grant (RWM) from the National Science Foundation. Helpful suggestions were made by Reginaldo Saracen0 during manuscript preparation.
Figure 5. Postulated CME for measuremonl of me analyte A, an antigen to me membrane-bound antibody. by release of me encapsulated enzyme SDH mat Iums on the NAD+/NADH elecbocatalysls. Symools: Y = amibody: C = complemem: E = actlvmBd complmnl; S = su~smu): A,Y = anliwa m body mmplex.
polymer film and plasticize it to enhance the microvoltammetric currents. Future scheming with CMEs We hope that this article has clarified some of the ground rules for designing CMEs for different analytical purposes. The extent w which future specific applications involving electrocatalysis or selective membrane permeation with CMEs will he made rests substantially on successes in the chemical part of the design. We will finish this REPORT with an illustration (Figure 5) of a paper design of a new CME 155) that has not yet been brought to pass. We do this to emphasize again how the elements of rational molecular and spatial design of CME electrode surfaces and the ideas of integrated systems (37) might be applied in ana988,.
lytical chemistry. The scheme in Figure 5 is based on using immunologically sensitized liposomes (56, $7) or red blood cell ghosts (58)into which some electrochemically detectable species or a substance capable of producing such a species has been encapsulated. For thedouble-amplification process illustrated in the figure, substrate dehydrogenase (SDH), whose enzymatic consumption of substrate requires an electroactive cofactor such as NAD' or ferrocene, is the encapsulated species. If NAD' and liposome are coimmobilized on the CME, and the liposomes have further been sensitized for release of SDH by incorporation of antibody Y, then the presence of the analyte (antigen A) triggers the perforation of the liposome, release of SDH. and the enzy-
ANALYTICAL CHEMISTRY. VOL. 59, NO. 5, MARCH 1. 1987
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(26) White, H. S.; Leddy. J.; Bard. A. J. J. Am. Chem. Soc. 1983.104,4811. (27) Kanazawa. K. K.; Dim, A. F.; Geiss, R.
H.;Gill, W. D.; Kuak, J. F.; Lo an J A f i b l t , J. F.; Street, G . B. J. c$leA. SO:
Chem. Commun. 1979,854. 128) Guadaluoe. A. R.: Abruna. H. D. Anal. ' Chem. 198i.57.142.' (29) Komori. T.; Nonaka, T. J. Am. Chem. Soe. 1983,105,5690. (30) Baldwin, R. P.; Christenen, J. K.; Kryger, L. Anal. Chem. 1986.58,1790. (31) Ghosh. P. K.; Bard,A. J. J. Am. Chem. Soe. 1983,105,5691. (32) Murray, C. G.; Nowak, R. J.; Rolison.. D. R. J. Eleetroanal. Chem. 1984, 164. mc.
(3i)"kerz,A.;Bard,A. J. J.Am. Chem.Soc.
1978.100.3222. (34) Calvert. J. M.; Schmehl. R. H.; Sullivan, B. P.; Facei, J. S.; Meyer, T. J.; Murray, R. W. lnor Chem. 1983.22,2151. (35) Pickup, P. Murray, R. W. J. Am. Chem. Sac. 1983,105,4510. (36)Bull, R. A.; Fan, F. R.; Bard, A. J. J. Electrochem. SOC.1984,131,681. (37) Umana, M.; Waller. J. AM^. Chem..in
8:;
press.
(38) Buttry, D. A.; Anaon, F. C. J. Am. Chem. Sac. 1982.104.4824. (39) Krishnan. M.; White, J. R.;F0x.M. A.; Bard, A. J. J. Am. Chem. Soe. 19&1,105, llM"
,WL.
(40) Bookbinder, D. C.; Bruce. J. A,;
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miney. R. N.; hwin. N. S.; Wrighton, M. S. hoe. Nat. Acad. Sei. USA 1980, 77,
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M.; Saveant, J. M. J. ElectroaMl. Chem.
1982.131.1. (43) Udea, C.; T e , D.C.-S.; Kuwana, T. Anal. Chem. 1982.54.850. (44) Soraceno, R. A,; Pack, J. G.; Ewing, A. G.J. Eleetroanal. Chem. 1986,197.265. (45) Schneider, J. R.; Murray, R. W. Anal. Chem. 1982.54.1508. (46) Espenscheid, M. W.; Ghatek-Roy, A.
R.; Moore. R. 9.; Penner, R. M.; Szentirmay, M. N.;Martin C. R. J. Chem. SOC., Faraday Trans. 1 1986.82,1051. (47) Matsue, T.; Akiba, U.;Om. T. Anal. Chem. 1986.58,2096. (48) Ikeda, T.; Schmehl, R.;Deniaevich, P.; Willman. K.; Murray, R.W. J. Am. Chem. Soe. 1982,104,2683. (49) Lau. A.N.K.;Miller,L.L.;Zinger,B. J. Am. Chem. Sac. 1983.105,5278. (50) Burgmayer. P.; Murray. R. W. J. Am. Chem. Soe. 1982,104,6139. (51) Espenscheid, M. W.; Martin, C. R. J. Eleetroonal. Chem. 1985.188,73. (52) Ikariyama, Y.;Heineman. W. R. Anal. Chem. 1986.58.1843. (53) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985,89,1441. (54) Josowicz. M.: Janata. J. Anal. Chem. 1986,58,514. (55) Durst. R. A. Presented at the Fourth Scientific Session on Ion-Selective Electrodes, Matrafured. Hungary, October 10,
.""-.
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(561 Shiba,K.; Umnawa, Y.; Watnnak,T.; 0gawa.S.; Fujiwara.S. Anal. Chem. 1¶80. 52,1610. (57) Umezawa, Y.; Sofue. S.;Takamoto, Y. Anal. Lett. 1982.15.135. (58) D'Orazio, P.; Rechnitz, C . A. Anal. Chem. 1971.49.2083.
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Royce (left) received his B.S. in 1957from Birmingham Southern College and his Ph.D. in 1960from Northwestern Uniuersity. He then joined the faculty of the Uniuersity of North Carolina at Chapel Hill, where he is now Kenan Professor. His fundamentally oriented research interests are in analytical chemistry, electrochemistry, and the chemistry of surfaces. His recent research activities haue included synthesis. electron transfer chemistry, and electrochemistry of electroactiue polymers, microstructured polymer films. and macromolecular electronics. Andrew G.Ewing (center)is assistant professor of chemistry at the Pennsyluania State Uniuersity. He received his B.S. from St. Lawrence Uniuersity i n 1979 and his Ph.D. in analytical chemistry from Indiana Uniuersity in 1983. He then spent 13 months as a postdoctoral research associate in
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Professor Murray's laboratories. His research interests include chemically modified electrodes, ultra small electrodes, and ultra small-scale separations. all applied to neurochemical analysis and determination of chemical species i n single cells. Richard A. Durst (right) received his B.S. in chemistry in 1960 from the Uniuersity of Rhode Island and his Ph.D. in analytical chemistry in 1963 from the Massachusetts Institute of Technology. After teaching at Pomona College and Boston College, he joined the National Bureau of Standards, where he has serued as chief of the electrochemical analysis section, group leader in organic electrochemistry, and deputy director of the Center for Analytical Chemistry. His current research interests include bio-organic electrochemistry,spectroelectrochemistry, and chemically modified electrodes.
