J . Phys. Chem. 1992,96, 5641-5652 (50) Schroeder, J.; Thoe, J. Chem. Phys. Lett. 1985, 116, 453. (51) Otto, 8.; Schroeder, J.; Troe, J. J . Chem. Phys. 1984,81, 202. (52) Betts, T.A.; Bright, F. V. Appl. Spectrosc. 1990, 44, 1196. (53) Betts, T. A.; Bright, F. V. Appl. Spectrosc. 1990, 44, 1204. (54) Bright, F. V.; Betts, T.A,; Litwiler, K. S . C.R.C. Crit. Reu. Anal. Chem. 1990,21, 389. ( 5 5 ) Lakowicz, J. R.; Lackzo, G.; Gryczynski, I.; Szmacinski, H.; Wiczk, W. J. Photochem. Phorobiol. E : Eiol. 1988, 2, 295. (56) Jameson, D. M.; Gratton, E.; Hall, R. D. Appl. Spectrosc. Rev. 1984, 20, 55.
(57) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. ( 5 8 ) Demas, J. N. Excited Stare Lifetime Measurements; Academic Res: New York, 1983. (59) Alcala, J. R.; Gratton, E.; Prendergast, F. G. Eiophys. J . 1987, 51, 587. (60) James, D. J.; Ware, W. R. Chem. Phys. Lett. 1985, 120, 455. (61) James, D. J.; Ware, W. R. Chem. Phys. Lett. 1986, 126, 7. (62) Gryczynski, I.; Wiczk, W.; Johnson, M. L.; Lakowicz, J. R. Eiophys. Chem. 1988. 32. 173. (63) Lakowicz, J. R.; Cherek, H.; Gryczynski, I.; Joshi, N.; Johnson, M. L. Eiophys. Chem. 1987, 28, 35. (64) Eftink, M.; Ghiron, C. A. Eiophys. J . 1987, 52, 467. (65) Bright, F. V.; Catena, G. C.; Huang, J. J . Am. Chem. Soc. 1990,112, 1344. (66) Huang, J.; Bright, F. V. J . Phys. Chem. 1990, 94, 8457. (67) Catena, G. C.; Bright, F. V. J . Fluor. 1991, I , 31. (68) Younglove, B. A.; Ely, J. F. J. Phys. Chem. Ref. Data 1987,16, 577. (69) Lakowicz, J. R.; Cherek, H.; Baker, A. J. Eiochem. Eiophys. Methods 1981, 5, 131.
5641
(70) Litwiler, K. S.;Huang, J.; Bright, F. V. Anal. Chem. 1990,62,471. (71) Spencer, R. D.; Weber, G. J. Chcm. Phys. 1970,52, 1654. (72) Beechem, J. M.; Gratton, E. In Time-Resolved Laser Spectroscopy; Lakowicz, J. R., Ed.; Prw. SPIE 909, 1988; p 70. (73) Swaid, I.; Nickel, D.; Schneider, G. M. Fluid Phase Equil. 1985,21, 95. (74) Rbsling, G. L.; Franck, E. U.Eer. Bunsen-Ges. Phys. Chem. 1983, 87, 882. (75) Ebeling. H.; Franck, E. U.Eer. Bunsen-Ges. Phys. Chem. 1984,88, 863. (76) Kosower, E. M.; Dudiuk, H. J. 1. Phys. Chem. 1978,82, 2012. (77) Seliskar, C. J.; Brand, L. J. Am. Chem. Soc. 1971, 93, 5405. (78) Grellman, K. H.; Schmitt, U. 1.Am. Chem. Soc. 1982. 104.6267. ( 7 8 Jobe, D. J.; Verral, R. E.; Palepu, R.; Reisenborough, V.'C. Ji Phys. Chem. 1988, 92, 3582. (80) Kosower, E. M.: Kanetv. H.: Dodiuk H.: Striker. G.: Jovin. T.: Boni. H.;'Huppert, D. J. Phys. Chem. 1983,87, 2479. (81) Wong, M.; Gratzel, M.; Thomas, J. K. J . Am. Chem. Soc. 1976,98, 2391. (82) Catena, G. C.; Bright, F. V. Anal. Chcm. 1989, 61, 905. (83) Chou, S.-H.; Wirth, M. J. J . Phys. Chem. 1989, 93, 7694. (84) Mantsch, H. H.; Wong, T. T. Vib. Spectrosc. 1990, 1, 151. ( 8 5 ) Knutson, J. R.; Walbridge, D. G.; Brand, L. Biochemistry 1982, 21, 4671. (86) Willis, K. J.; Szabo, A. G.; Drew, J.; Zuker, M.; Ridgeway, J. M. Eiophys. J . 1990, 57, 183. (87) Further analysis of thcse data in terms of uniform or Gaussian distributions always resulted in poorer fits. In general, the x2 value was 50% greater for these other distribution models.
Electrochemistry of Organic Conducting Salt Electrodes. A Unified Mechanistic Description Shishan Zhao, Ulrich Korell, Louis Cuccia, and R. Bruce Lennox* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, P.Q., Canada H3A 2K6 (Received: October 1 , 1991; In Final Form: February 18, 1992)
A detailed,unified description of electrocatalyticmechanisms at organic conducting salt (OCS) paste electrodes ('lTF-TCNQ, tetrathiafulvalene-p-tetracyanoquinodimethane;HMTTeF-TCNQ, hexamethylenetetratellurofulvalentp-tetracyanoquinodimethane) has been presented. A wide spectrum of behavior is observed depending on the electrochemical and kinetic characteristics of the analyte of interest, the nature of the OCS,the applied potential, and the electrolytecomposition. These electrodes behave like metal electrodes in the presence of an analyte (Le. Fe(CN),3-/+) whose Ed is greater than the E( of either the donor or the acceptor molecule or if competing indirect, mediated pathways are not kinetically viable. Current-potential and current-concentration studies are consistent with ascorbate (pH 8.3) and NADH oxidation occurring via a homogeneous mediation mechanism. Rotating disk electrode behavior is consistent with a homogeneous mechanism by invoking the concept of replacing an electrochemical mechanism involving homogeneous kinetics with a "heterogeneous equivalent". It is concluded that electrooxidation of a reduced enzyme (glucose oxidase) occurs via a homogeneous mechanism on both electrode materials. A detailed examination of background currents of OCS electrodes is shown to be very useful in understanding their intrinsic redox activity and their solubilities. Evidence for Faradaic chemistry and potential-induced solubility of componentsof the OCS electrodes well within the stable limits of the material shows that surface redox phenomena differ considerably from bulk or lattice redox phenomena. This "background" generation of soluble mediators is shown to be the source of the homogeneous mediators present in bioelectrocatalysis experiments.
Introduction Many organic conducting salts (OCS), such as tetrathiafulvalene-p-tetracyanoquinodimethane(TTF-TCNQ), have been shown to be excellent materials for bioelectrocatalysis'-" applications. Electrocatalytic oxidations, in particular, readily proceed for important analytes such as fla~oenzymes,~-~ redox-active enzyme cofactor^,^+'^ and biologically important molecules such as ascorbate and dopamine.YJl Despite this broad applicability to a number of analytes which are otherwise difficult to access at *Author to whom correspondence should be addressed.
solid electrodes, the mechanism of electron transfer has not been clearly established. It is our feeling that widespread acceptance and application of these very useful materials has been hampered by confwion about their mode of action. A variety of mechanisms for electron transfer between an analyte and the electrode are possible including direct and indirect mechanisms. Important progress as to the mechanism of flavoenzyme oxidation by one of these OCS materials (TTF-TCNQ) has recently been de~ c r i b e d . ~These ~ J ~ reports suggest that an indirect mechanism, possibly combining both homogeneous and heterogeneous mediation processes, is in effect for glucose oxidase/TTF-TCNQ glucose electrodes.
0022-3654/92/2096-564 1%03.00/0 0 1992 American Chemical Society
Zhao et al.
5642 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
i
In this work, we have endeavored to place the Faradaic electrochemistry of simple redox couples (Le. Fe(CN)63-/e), complex redox couples (i.e. 8-NADH, ascorbic acid), and enzymes (Le. glucose oxidase, methanol dehydrogenase) all within a coherent mechanistic description based on chemical and electrode kinetics. Primarily using two different OCS materials and an array of analytes, we have been able to describe a mechanistic spectrum which is self-consistentwith the results presented here as well as the large body of previously published work. The resulting detailed understanding of electron-transfer mechanisms at OCS electrodes provides a basis for the design of new electrode materials for bioelectrocatalysis applications.
r -0.4pA
Experimental Section Preparation of Organic Conducting Salt (OCS) Materials. TTF-TCNQ and NMP-TCNQ were prepared by literature procedures.isJ6 TTF, NMP+MeSO,-, and TCNQ were obtained from Aldrich (Milwaukee, WI) and were used as received. Elemental analysis of recrystallized materials (hot CH3CN) were satisfactory. Both recrystallized and nonrecrystallized materials were found to be equivalent in terms of intrinsic electrochemical properties. Hexamethylenetetratellurofulvalene-p-tetracyanoquinodimethane (HMTTeF-TCNQ) was prepared using a modification of the TTF-TCNQ procedure. Poorly soluble HMTTeF (Aldrich) was dissolved in CH3CN to which TCNQ (1.05-fold excess) in CH$N was added. The resulting black precipitate was filtered, washed with cold CH3CN,and dried under vacuum (25 "C). The elemental analysis of the resulting material indicates that the donor molecule is in slight excess compared to the acceptor. Despite thii nonstoichiometry, the material was a good electrical conductor and the slight donor excess did not lead to any characteristic peaks in the cyclic voltamograms of the material in its stable window. EIectrochemicalMea"&. Electrochemical measurements were performed using an Oxford Electrodes (Oxford, England) potentiostat (Model OEPP2) and rotator (Model MCI). All measurements were made with respect to a Ag/AgCl reference electrode (BAS, West Lafayette, USA). A Pt wire coil (Aldrich) served as a counter electrode. Pt and Au microdisk electrodes were from BAS. Experiments were performed at ambient temperature (22 f 2 "C) under a water-saturated N2 atmosphere except where noted. Two similar electrodes were used for OCS studies. One electrode, which was capable of being rotated, had a l-mm-diameter Pt disk permanently recessed in PTFE to give a 05"deep cavity. The other electrode combined a BAS Pt electrode (1.9." diameter) and a PTFE doughnut pressed over it. The resulting cavity was 0.5 mm deep. OCS/Si oil paste was pressed into either PTFE cavity to constitute a working electrode. The paste electrode was made by thoroughly mixing high temperature Si oil (Aldrich) with dry OCS. The ratio of oil to OCS was not critical for electrochemical measurements, and ratios of 1:1.6 (wt/wt) were common. A similar electrode was constructed by using polystyrene (PS,106 MW, Polysciences, USA) as the matrix instead of Si oil. Approximately 50 pL of a viscous solution of PS in CH2C12was added to -100 mg of TTF-TCNQ. The resulting slurry was thoroughly mixed and cast as a thick film on a Pt electrode surface; when cured (2 h), the electrode was matte black and hard. A (pseudo) ring disk electrode was prepared by depositing a thin layer of TTF-TCNQ paste onto the surface of a 6-mm-diameter Pt disk electrode (Oxford Electrodes). This electrode was surrounded by an annular 0.5-mm-thick Pt wire electrode (Aldrich). This rather awkward, nonhydrodynamically controlled tweelectrode codiguration proved to be adequate to detect soluble electroactive species originating from the disk. We resorted to this configuration because of our inability to construct a reliable rotating ring disk electrode where the disk could accommodate the OCS paste. A homemade bipotentiostat (generously loaned by Dr. S. McClintock) controlled both disk and ring potentials
Figure 1. Cyclic voltamograms of a TTF-TCNQ/Si oil paste electrode. pH 7.4, 0.1 M potassium phosphate, 0.1 M KCI. Scan rate = 20 mV/s.
simultaneously. Output was recorded on chart recorders. Membrane electrode experiments involved retaining a thin (-10 pm) layer of buffer over a TTF-TCNQ cavity paste electrode using a SpectroPor type 2 dialysis membrane (12-14K MW cutoff, Spectrum Ind., Los Angeles, CA). The membrane was held tightly over the electrode with a PTFE O-ring. Enzyme electrodes used this same configuration, except that the entrapped solution contained ca. 0.1 unit of glucose oxidase (EC 1.1.3.4, Boehringer Mannheim, FRG (grade II,20 800 units/g of solid)). Electrolytes (KCl, NaBr, etc.) were ACS reagent grade or better (Aldrich, BDH). Tris (ultrapure grade, Aldrich), Tris-HC1 (reagent grade, Sigma), sodium ascorbate (+99%, Aldrich), sodium pyrophosphate decahydrate (ACS, Aldrich), mono- and dihydrogen sodium phosphate (ACS, Aldrich), and @-NADH (Sigma, Grade 11) were all used as received. All solutions were prepared with deionized water (18 MQ, MilliQ) which had been passed through an organic removal cartridge. Tris solutions (up to 0.100 M) were prepared from Tris.HCI and adjusted to pH 8.3 using NaOH while 0.5 M Tris solutions were prepared from combining Tris/Tris.HCl solutions. Pyrophosphate solutions were pH adjusted using HCl. Ascorbic acid solutions were freshly prepared and stored in N2-degassed buffer.
Results I. Characteristics of 'ITF-TCNQ, HMTTeF-TCNQ, and NMP-TCNQ Electrodes in the Absence of Redox Active Compouuds. The electrochemical properties of TTF-TCNQ electrodes has been investigated in detail using cyclic voltammetry by Jaeger and Bard,17.'sel Kacemi and Lamache,I9 and Brajter-Toth and Because both the matrix in which the OCS material and the manner in which it is pretreated20~2i evidently affect the instrinsic electrochemical properties of the OCS, it is useful to begin this discussion by presenting representative cyclic voltammograms (cv's) of TTF-TCNQ/Si oil paste electrodes used in this study (Figure 1). As is observed for pressed pellet electrodes1' and single-crystal electrodes,14TTF-TCNQ paste electrodes exhibit relatively small capacitive currents and relatively constant currents between -100 and +350 mV. Excursions past +350 mV result in a large anodic current attributed to lattice oxidation. This oxidation is coupled to the appearance of a cathodic peak at +lo0 mV. This latter peak has been assigned to the reduction of TCNQ to TCNQ'-, where the TCNQ is initially produced via the lattice oxidation process (Le. TTF-TCNQ TTF+ + TCNQ + e-). Subsequent cycling leads to an anodic peak at +310 mV (corresponding to the oxidation of MTCNQ; M+ = Na+, K+, etc.) and a cathodic peak at +lo0 mV. These two peaks typically diminish in mag-
-
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5643
Organic Conducting Salt Electrodes
A
-I5 -25
v' 0
I
50
100
200
150
250
300 0
Eappl(mV vs. Ag/AgCI)
-
TTF-TCNQ f
E F
+
TCNO
A
7
TTF+
Removal of TTF+ from the electrode surface facilitates further oxidation. Increased rotation results in enhanced mass transport
2
3
4
5
6
5
6
7
flR(HzlR)
Figure 2. Background current (iB)-Enwlprofile of a TTF-TCNQ/Si oil paste electrode under constant rotation conditions: (0) 0 Hz;(A)10 Hz; (0) 40 Hz. pH 7.4, 0.1 M potassium phosphate, 0.1 M KCI. nitude only after hours of repeated cycling between -100 and +350 mV. Excursions more negative than -200 mV lead to a large cathodic signal in the cv, which has been assigned to lattice reduction. The products of the lattice reduction are observable during subsequent cycling between -150 and +350 mV, with the appearance of the MTCNQ/TCNQ couple at +310/+100 mV. The investigation of electrode processes in the window of potential stability is however difficult using cv, because polycrystalline electrodes have capacitive currents (- 50 nA at 5 mV/s at 150 mV) which are much greater than steady-state background currents (--1 to 1 nA at 150 mV). A very slow sweep technique has been used to reduce the capacitive current contribution to measured currents.19 On the other hand, we have used fixed potential, steady-state currents at OCS electrodes to probe the Faradaic processes intrinsic to these electrodes in the "stable" potential region. An i-V plot constructed from a series of individual background current (iB) measurements reveals a measurable increase in anodic current on moving from 0 to +300 mV which is not readily apparent using potential sweeps at polycrystalline electrodes because their capacitance currents are so large. The electrode on which these experiments were performed had not been subjected to previous potential extremes (i.e. 350 mV) and did not have any voltammetric peaks associated with TCNQ or MTCNQ. The currents in Figure 2, all in the nanoamp range, are therefore associated with intrinsic redox processes of the OCS material. It is notable that at 150 mV, iBis = 0 nA. Rotating disk electrode (RDE) experiments were performed in order to determine whether the background current is affected by the hydrodynamics at the electrode surface. As is shown in Figure 3A, the background current is in fact very dependent upon the rotation rate-with Levich behavior observed throughout. Levich behavior in this context indicates that a mass transportlimited dissolution of a component of the OCS leads to larger quantities of both oxidizable and reducible species being produced as the rotation rate increases. In order to establish that the electrode matrix (Si oil) does not induce these phenomena, we have also performed RDE i B studies with a TTF-TCNQ/polystyrene paste electrodesimilar to the polymer paste electrodes used by other investigators (Figure 3B). We find that the same trends in terms of i B as a function off (i.e. linear i vsfll2 plots) are observed using polystyrene and Si oil as matrices. Positive Levich slopes for E,,,! > 150 mV are consistent with a scheme wherein neutral dissociation of TTF-TCNQ22 results in oxidation of the donor. Using a notation where an underlined species is surface adsorbed, then
1
B 15
-
10
d
5
d
o
5
-5
v
j
1
"
-10
- 15 -20
'
0
1 1
2
3
4
7
flR(Hz")
Figure 3. (A) Plot of background current ( i s ) vs j'I2 of a TTFTCNQ/Si oil electrode for different Enppl.(B) Plot of background current (iB)V S ~ ' / ~of a TTF-TCNQ/polystyrene electrode for different E, (A)0 mV; (0) 50 mV; (0)100 mV; (0)130 mV; (A) 160 mV; (05 200 mV; (e) 250 mV; (m) 300 mV. pH 7.4, 0.1 M potassium phosphate, 0.1 M KCI.
of TTF+ away from the electrode surface. This mechanism requires that TTFX species formed in solution have a finite if not significant aqueous solubility. That this is the case in the presence of phosphate buffer has been shown recently." Cathodic Levich behavior on the other hand is consistent with a scheme where surface-adsorbed TCNQ is reduced to a soluble TCNQ'- salt. TTF-TCNQ
+
fTTJ
TCNO
I TCN0'-
lMt
t
MTCNQ
Removal of TCNQ'- therefore occurs via simple escape from the diffusion layer into the bulk solution. The background current is insensitive to the rotation rate at 150 mV. The two Faradaic processes are both minimal but equal at +150 mV so that any TTF+ and TCNQ'- released can recombine to form the OCS. These studies are corroborated by a series of experiments using an annulart'F electrode as a detector of soluble species originating from the disk electrode (Figure 4). The current at the detector electrode (E, = +380 mV) was monitored as a function of disk potential (-2'Bb to +350 mV). Oxidizable species are detected when the disk potential is >130 and +300 mV. RDE measurements at fmed potentials between -100 to +250 mV reveal virtually no measurable affect of rotation rate on is whereas at >300 mV i B increases slightly withf, albeit in neither a simple Levich nor a Koutecky-Levich manner (Figure 8). These observations are consistent with the rate of electrode dissolution rather than mass transport of electrode components away from the electrode being current-controlled between -100 and +250
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5645
Organic Conducting Salt Electrodes
20
I
O L
0.00 0.50
1.00
1.50
2.00
2.50
3.00 3.50
flO(HP)
Figure 9. Plot of background current (in)V S ~ "for ~ an NMP-TCNQ/Si oil paste electrode (pH 7.4, 0.1 M potassium phosphate, 0.1 M KCI) poised at -60 mV (A),0 mV (a), +lo0 mV (e),and +250 mV (W).
studies (-60 to +250 mV) do indicate however that an electrooxidation product is dissolved in the diffusion layer and that increased rotation serves to increase the rate of its removal from the electrode surface region. This is consistent with an anodic i B arising from NMP-TCNQ
Figure 7. Cyclic voltammograms of an HMTTeF-TCNQ/Si oil paste electrode. (a) Scan starts at 0 mV, proceeds cathodically, and reverses at -320 mV. (b) Scan starts at 0 mV, proceeds anodically, and reverses at +480 mV. (c) Scan starts at 0 mV, proceeds cathodically,and reverses at -450 mV. Sweep rate = IO mV/s; pH 7.4,O.l potassium phosphate buffer, 0.1 M KCI. 2-pA scale marker applies to a, b, c. (d) Steady-state i,,; same solution conditions as per a-c. Note vertical scale of 10 nA.
+
TCNQ
NMP+
N M P is very soluble in H20which is consistent with the overall trends in anodic background current where (IB)NMP > (iB)TTF > (~B~HMTT~F.
150 130 110
90
70 50 30 i1"n
-lo& 0.00
f NMP
'
-
-
~
- ' I
1.50
'-
3.00
- ~~
4.50
' -
'
6.00
Figure 8. Plot of background current ( i n ) vs f for an HMTTeFTCNQ/Si oil paste electrode poised at designated E,,,: ( 0 )-100 mV; (A)-50 mv; ( 0 )0-150 mV inclusive; (0) 250 mv; ( 0 )300 mv; (V) 350 mV; (+) 400 mV; (W) 450 mV.
mV. When lattice oxidation is initiated there is a measurable quantity of a diffusable species apparent a t this electrode. The HMTTeF-TCNQ electrode therefore complements the TTFTCNQ electrode in that they both appear to have similar electrochemical properties, yet HMTTeF-TCNQ exhibits much lower electrochemically-induced solubility. NMP-TCNQ has been frequently used in bioelectrochemistry studiesI3J0and its cation is much more soluble than TTF+ or HMTTeF+. Although we will not deal with the use of NMPTCNQ with analytes here, a brief discussion of the i B properties is relevant to the two OCS electrodes of interest. Rotation rate has a significant effect on iBfor NMP-TCNQ paste electrodes. Increasing iBresults from increasing f a t E, > 0 mV (Figure 9). Unlike the TTF-TCNQ case, neither &ear-cut Levich nor Koutecky-Levich behavior is observed at any potential studied. Moreover, rotation rate data was acquired only up to 10 Hz because the material was visibly unstable at higher$ The RDE
These data, particularly the steady-state voltammetry under hydrodynamically controlled conditions, clearly show that all three materials undergo Faradaic chemistry even within their regions of "potential stability". The extent of this "background" electrochemistry varies with donor species, with NMP-TCNQ evidently producing substantial quantities of soluble species while HMTTeF-TCNQ produces extremely small quantities of soluble species. Of particular importance to discussions of electrocatalysis mechanisms is the observation that iB,and by inference, electrochemically induced solubility, is enhanced by common buffers (phosphate," Tris). Examination of TTF-TCNQ electrodes by voltammetry (Figure 5 ) further establishes that a Si oil paste electrode poised within the stable window accumulates a nonsoluble component of the OCS which we tentatively assign to TCNQ species. This assignment is supported by Raman work by Bard and co-workers25 which showed that TCNQ accumulates on TTF-TCNQ electrode surfaces after lattice oxidation. 11. Properties of OCS Electrodes in the Presence of Redox Couples. Redox couples whose E [ s vary from +275 to +lo0 mV and which are known to be reversible or quasireversible a t solid electrodes are also reversible or quasireversible at TTF-TCNQ. Whereas we can readily confirm the apparent quasireversibility observed for Fe(CN)63-/4-,17C U ~ + / C U +and , ~ ~TMDB (tetramethyldiaminobenzene),26we observe no voltammetric peaks for Fe(edta)(H20)22-/'-( E l l 2= -130 mV on glassy carbon). As a guideline, metal electrode-like behavior is evidently observed for fast couples whose Ed > 0 mV and a more complex, indirect electron transfer process is apparent if Ed < 0 mV. RDE data for Fe(CN)6e exhibits Levich behavior at a potential well positive (360 mV) of the Ed of this couple (data not shown). Reduction of Fe(CN6)'- a t 0 mV exhibits similar behavior except that a t higher concentrations, downwards deviation from Levich behavior is observed (data not shown). This deviation is likely due to increasing electrode areas induced by high rotation rates as it is in the opposite sense to that caused by rate-limiting electroil transfer. Comparison of HMTTeF-TCNQ electrode properties with the above is interesting. Fe(cN)b3-I4- at this electrode yields a cv with 60-mV peak separation, an E I 1 2of 290 mV, and excellent Levich behavior over a range of concentrations (0.01-0.1 M) at
Zhao et al.
5646 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 A
90
-
70
3
50
4
d
E L W
2
'"W 0 -100
0
100
30
0
10 -10' 0
200
"
"
2
"
6
4
"-50 10
8
[Ascl(mM)
Eapp1(mV vs. AgIAgCI)
Figure 10. Cyclic voltammograms of a TTF-TCNQ/Si oil paste electrode in 1.0 mM sodium ascorbate (0.10 M sodium pyrophosphate) at various pH values: (a) pH 6.6; (b) pH 7.6; (c) pH 8.6. Scan begins at -100 mV. Scan rate = 20 mV/s.
both 0 and +350 mV (data not shown). Excellent Levich behavior complements the iB/E,,, data (Figure 7), again suggesting that HTTeF-TCNQ is less soluble than TTF-TCNQ under fixed potential conditions. Unlike the TTF-TCNQ electrode case however, Fe(edta)(H20)22-/'- electrochemistry a t the HMTTeF-TCNQ electrode exhibits a reversible cv, with a peak separation of 60 mV and an Ellzof -130 mV. Ascorbic Acid. Ascorbic acid electrochemistry has been reported a t TTF-TCNQ electrode^,"-^^*^^ and very recently a mechanism has been proposed for its electrooxidation at these electrodes.*O Ascorbate is an interesting species to study in this regard because its redox chemistry is well-studiedzs and because it has been the target of a number of electrocatalytic systems. This interest in electrocatalysis of ascorbate arises because it is electrochemically irreversible at Pt for example, yet it is an important analytical target. Cv's as a function of pH are very interesting at TTF-TCNQ. As seen in Figure 10, an anodic peak (+150 mV) is observed at pH 6.6. However, no corresponding cathodic peak is observed. The anodic maximum shifts to -+120 mV at pH 7.6. No maximum is however observed a t pH 8.6 where the maximum is replaced by a broad plateau which begins at -0 mV. The transition from a peak to a plateau response is consistent with the mechanism of oxidation shifting from some form of direct electron transfer to some form of mediated process on increasing pH. The voltammetric maximum and its pH-dependent shift is therefore consistent with a quasireversible, direct electron transfer into the electrode material for pH 6.6 and 7.6and an indirect mechanism when pH > 8.0. The absence of a cathodic signal on sweep reversal is most likely is due to competitive degradation of the unstable dehydroascorbate species. RDE results at pH 8.3 over a wide range of concentrations show i increasing asf increases, but the i vs f /2 plots are highly curved. The corresponding Koutecky-Levich plots are, however, linear over a wide potential range (-100 to +200 mV) and over large concentration ranges (5-40 mM). Current-concentration profiles fall into two distinct classes depending on the E, (Figure 11). At -100 mV, i a [Asc] up to >50 mM, after wiich it obeys an i a [ASC]'/~ relationship. If the Eappl is greater than -75 mV, the i vs [Asc] curve is again biphasic, except that the linear region spans from 0 to -2.5 mM and the square root region arises at >2.5 mM. We have gone to considerable effort to experimentally resolve these transition concentrations ([SI,) in the i vs [Asc] curves, particularly at low potential, because.of their mechanistic and analytical importance.27 As shown in Figure 11, these data exhibit a distinct break from first-order to half-order dependence at concentrations which are dependent upon EaPpl.Attempts to plot these data in terms of a simple Langmuir isotherm leads to a poor fit, particularly at high concentrations. The linear region therefore does not cor-
"
B 5
0
10
25
20
15
30
70
-2
50
5
30
- 350 . 250
4
.
L. L
1 V
10
-
150
50
'
-10' ' ' ' ' ' ' -50 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 '
'
'
'
'
'
'
[ A s c ] ' ~(mM) I n
Figure 11. (A) Plots of current vs [Asc], 0.100 M Tris, pH 8.3 for a TTF-TCNQ/Si oil paste electrode. Equal increments of the ascorbate stock solution in 0.100 M Tris were added. Eappl= -100 mV (l), -75 mV (2), -50 mV (3), 100 mV (4, note scale change). Constant stirring throughout. (B) Plots of current vs [Asc]'i2 for data in A.
respond to the initial ulinearnregion of a Langmuir isotherm, and the high concentration regions clearly do not exhibit the requisite saturation phenomenon. This is readily apparent from data at -100 mV, where the electrode response does not suggest that saturation will set in, even a t 500 mM ascorbate.27 The role played by buffer concentration in determining Faradaic currents was discovered after initial dfliculties with reproducibility were traced to a lack of strict control over buffer concentration. Surprisingly, [Tris] has a large effect on the ascorbate-associated with increasing [Tris] leading to increasing anodic current at constant [Asc]. This effect appears to saturate at large [Tris] (Le. 0.5 M), yet the relative current increase is approximately the same a t the three potentials examined. Furthermore, the [SI, at each potential is independent of the [Tris]; parallel experiments to those reported in Figure 11 where [Tris] = 0.50 M instead of 0.10 M lead to larger currents but identical i vs [ A x ] relationships. These observations provide further evidence that a complex, solubility-dependent mechanism is in effect and that interactions between buffer components and electrode components lead to an increased concentration of mediator molecules at the electrode surface. This buffer concentration effect, coupled with the B i sensitivity to pH, also underscores the necessity of carefully defining and maintaining conditions when using OCS electrode materials in electrocatalysis applications. &NADH. @-NADH is an extremely important analytical targetMfor biosensor applications and has been the subject of many electrocatalytic strategies (refs 10, 3 1, and 32 and references therein). OCS electrodes, particularly NMP-TCNQ,9 are excellent materials for the electrooxidation of NADH in that stable, reproducible signals are produced at potentials considerably less than is necessary on Pt or glassy carbon (0-1 50 mV cf. 500-800 mV32). The redox chemistry of 0-NADH is, however, complex,
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5647
Organic Conducting Salt Electrodes 35, 28
,
i
I
2.00
100
0
zoo
100
0.00 0.00
300
Eappl(mV VS. Ag/AgCI)
A
1.50
0.5
1.30 .
*,,
5.0 .*,
I
0.90
0.70
10 25
1
I
0.50
0.00
1.60
3.20
4.80
6.40
8.00
f 'I2(Hz ' I 2 ) B 1.50
/ 5.0
'
0.00 0.00
Figure 14. Plots of relative current vs j ' / 2for a HMTTeF-TCNQ electrode (pH 8.0, 0.1 M potassium phosphate, 0.1 M KCI) poised at Relative +250 mV. [NADH] = 0.1 mM ( O ) , 0.5 mM (A),5 mM (0). current is defined by i,if-o-*.
-
at 1-4 Hz, while at high [NADH], the minimum disappears and apparently "normal" RDE phenomena are observed. The "inverse rotation rate" behavior further manifests itself in a peculiar manner in that particular potential and [NADH] combinations lead tof-independent currents. Needless to say, alternative forms of these data, such as a double reciprocal representation (i.e. Koutecky-Levich) do not lead to a linearization of the data and therefore do not fit standard iff dependences. The HMTTeF-TCNQ electrode appears to exhibit the same phenomena (Figure 14) where similar patterns are observed except that the [NADH], E, and f combinations leading to a given curve differ compare$ to TTF-TCNQ. Reflecting these complexities is the nature of the i vs [NADH] curves (not shown). The current at a TTF-TCNQ electrode is highly curved as a function of [NADH]. A linear region, if it exists, is not manifested at the concentrations (> 1, i a [SI'/', and when XK-lXD I1, i 0: [SI.The value of [SI,is dependent upon k,,the rate constant for reaction of [MI and [SI. For a given pair of M and S, [SI,should be potential independent. For small values of k,,the linear regime will extend to relatively high [SI whereas for large values of k,,very small linear [SIregimes and extensive regimes will be observed. Equation 1 also predicts that i will decrease with increasingf (i.e. "inverse rotation" behavior) when XKis not jindircct will this situation be observed. This in fact arises with HMTTeF-TCNQ and Fe(edta)(H20)*'-I2-, where reversibility is observed at potentials less than the Ed of the electrode components. RDE data (Figure 8) clearly show that the flux of mediator dissolving in the solution is very small indeed for this electrode material. The net conclusion is that the homogeneous mechanism is not competent and direct electron transfer occurs instead. The distinction between direct and indirect electrochemistry is an important one and is relevant to a number of previous mechanistic discussions. Because of its ability to produce substantial quantities of soluble mediators, we believe that indirect electrochemistry at TTF-TCNQ will usually be operative if the E,' of the analyte is
