J. Phys. Chem. 1987, 91, 1289-1292 enzyme for an electrochemically detectable current to flow to gold, platinum, or carbon electrodes because the distance between the FAD/FADH2 centers and the metal electrodes is excessive (Figure 3, curve a). Reaction 3 does, however, proceed rapidly and a significant current does flow, after an average of 12 ferrocene/ ferrocinium centers are bound to the enzyme molecules by amide links between the ferrocenecarboxylic acid and protein amine groups. To establish electrical communication between an enzyme and a metal electrode, it is necessary that the density of relays be adequate, i.e., that the electron-tunneling distances involved in communicating between the redox center of the enzyme (FAD/FADH2 in the case of glucose oxidase) and the electrode be substantially shortened by incorporation of the relays (Figure 1). Furthermore, in the case of glucose oxidase, the redox potential of the relays must be oxidizing relative to the FAD/FADH2 couple. The +OS1 V (SHE) potential' of the ferrocene carboxylate/ferrocinium carboxylate couple assures the oxidation of FADH2 to FAD, because the redox potential of the FAD/ FADH2 couple is 0.05 V (SHE) at pH 7.2.42 The greatest difficulty that we encountered in this work was, not unexpectedly, covalently attaching a large number of relay molecules to the inner protein part of the enzyme without causing a deactivating structural change. In a very large number of experiments, involving different redox couples and bonding reactions, we were unable to reproducibly attach the electron-transfer relays to the enzyme's protein. In most cases our hopes of success, raised by the observation of electrochemical glucose oxidation, were dashed following the gel filtration step, which readily differentiates between electrochemical oxidation involving diffusing mediators (known already to Thunberg and to T h e 0 r e 1 1 ~ and ~~~) electrochemical oxidation based on covalently attached nondif(42) Fieser, L. F.; Fieser, M. Organic Chemistry; Reinhold: New York, 1956; p 460. (43) Thunberg, T. Skand. Arch. Physiol. 1925, 46, 339. (44) Theorell. H. Biochem. 2. 1935. 278, 263. (45) Theorel1,'H. In Sumner, J. B.; Myrback, K. The Enzymes; Academic: New York, 1951.
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fusing electron-transfer relays. (Because the diffusing mediators are small molecules, they are retained longer on the gel filtration columns. The large enzyme molecules do not penetrate the column's pores and, therefore, move rapidly.) Our early failures taught us that the polysaccharide envelope of the enzyme can prevent reaction between the 0-acylisourea (111) and the inner protein in some batches of the enzyme. The results became, however, reproducible when we used 3 M urea solutions, where the inner protein was just sufficiently exposed and unfolded for proper relay attachment, yet the FAD was not lost and the enzyme could recover its structure in a urea-free aqueous buffer. Thus we obtained an enzyme having to 2 / 3 of the activity of the unmodified enzyme in catalyzing the classical reaction of glucose oxidase (eq 4), yet able to communicate with metal electrodes. 8-D-glucose
+ O2
-
6-D-gluconolactone
+ H202
(4)
Direct electrical communication between electron-relay modified enzymes and metal electodes opens a new route to electrochemical and bioelectronic sensors and to the direct electrochemical synthesis of biochemicals. Work in progress in our laboratory shows that there are several families of useful electron-transfer relays and that these can be attached to different functional groups of enzyme proteins. Although the ferrocene relays are at this time the most effective, we find that R ~ " ( N H ~ ) ~ ~ ~ , ~ ' relays, - b a s eattached d to the histidine functions of the protein of glucose oxidase, are short-lived but also effective. Furthermore, we find that the establishment of electrical communication between enzymes and metal or graphite electrodes is not limited to glucose oxidase but is feasible also in other enzymes such as amino acid oxidase.
Acknowledgment. We are grateful to Robert Bittman of Queens College, CUNY, for sharing with us his insight into conformational changes in proteins, to Anthony M. Williams for iron assays by atomic absorption spectroscopy, and to Tetsuo Yamane for many enlightening discussions. (46) Matthews, C. R.; Erickson, P. M.; Van Vliet, D. L.; Petersheim, M. J . Am. Chem. SOC.1978, 100, 2260. (47) Sandberg, R. J.; Gupta, G. Bioinorg. Chem. 1973, 3, 39.
Optical Studies of Silver Clusters and Larger Surface Structure Evolution on a Silver Electrode during Electrochemical Cycling C. D. Marshall and G. M. Korenowski* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 121 80-3590 (Received: September 1 1 , 1986; In Final Form: January 5, 1987)
Optical second-harmonic generation and laser-induced luminescence are used to follow the formation and removal of silver electrode surface structure during an oxidation-reduction cycle in a NaF electrolyte. Small silver clusters, detected by luminescence, form during surface reorganization following dissolution and deposition of silver. The clusters slowly evolve into larger structures that enhance second harmonic generation. The changing character of the surface is found to persist for a few minutes following silver deposition. These studies point out the potential importance of metal clusters and evolving surface structure in silver electrode chemistry and a method of studying this structure.
Introduction both macroAn oxidation-reduction cycle (ORC) can scopic and atomic scale roughness on the silver electrode surface. This roughness can have major effects on the physical and chemical behavior of the electrode. MacroscoDic roughness of 1-100-nm size is known to give rise to a classical electromagnetic
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enhancement of optical fields and is a source of enhancement for optical processes such as surface-enhanced Raman scattering (SERS) and reflected optical second-harmonic generation (SHG).'" Atomic scale roughness or surface silver clusters can (1) Gersten, J.; Nitzan, A. J . Chem. Phys. 1980, 73, 3023.
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1290 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
also increase the interaction cross sections for surface optical events. Surface silver clusters have been proposed as the “active sites” observed in certain SERS experiment^.^,' Small clusters, such as the four-atom silver cluster, were recently found to be responsible for low-frequency bands in SERS spectra.* Evidence for the chemical activity of electrode surface clusters was obtained in recent S H G and laser-induced luminescence (LIL) studies performed in this l a b o r a t ~ r y . ~ It J ~was established that a cluster (or clusters) of six or fewer silver atoms forms a complex with AgCl during its reduction on the electrode. The clusters are surface sites for adsorption and chemical complex formation and as such can be the origin of important electrode surface chemistry. Increases in optical cross sections at these atomic level sites result from resonances that are either internal to the silver clusters or resulting from chemical complex formation with adsorbates. There is currently little information on the formation dynamics of these important atomic scale and macroscopic surface structures or their evolution during an ORC. In this work, we use a combination of LIL and S H G as “in situ” probes to follow the formation and disappearance of both atomic scale and macroscopic electrode roughness during the events of an ORC. The results of these experiments show the silver electrode surface as a highly mobile surface requiring substantial times to reach equilibrium structure following either dissolution or deposition of silver. During this time of surface mobility, the electrode presents to the solution a variety of slowly evolving surface structures ranging from molecular clusters to macroscopic roughness.
Experimental Section Details of the experimental apparatus are given in a previous paper.’O The working silver electrode was a Teflon-encased polycrystalline silver cylinder (99.999% purity) with an exposed area of 1.27 cm2. The counter electrode was platinum and all potentials were referenced against a saturated calomel electrode (SCE). An argon-purged 0.1 M NaF solution prepared from triply distilled water served as the electrolyte solution. The high solubility of AgF in aqueous solution ensured that the electrode surface would remain free of this oxidation product, leaving a “clean” electrode surface for the study of surface structure evolution. In the experiment, the optical signals originating from the working electrode were followed as a function of a cyclic electrochemical potential scan. One complete ORC consisted of a linear potential sweep at a rate of 2.5 mV/s from -0.15 V (vs. the SCE) to +0.7 V and back to -0.15 V. Optical signals, SHG and LIL, from the working electrode were generated by the 1064-nm fundamental of a Q-switched Nd:YAG laser. The p-polarized laser light was incident at 45’ to the working electrode surface and optical signals were detected in the direction of specular reflection. The 532-nm S H G signal was generated by using an incident laser energy of 8.2 mJ cm-2 pulse-’. LIL was induced by a multiphoton absorption using 39.0 mJ cm-2 pulse-’ of the 1064-nm light and detected over the spectral region from 380 to 420 nm. Use of the 1064-nm fundamental to induce the cluster luminescence ensured that the same region of the electrode surface was probed in both the S H G and LIL experiment. This also enabled us to compare these LIL results with those of our previous study in which the same excitation source was used. In addition multiphoton excitation with 1064-nm light ~~
(2) Jha, S.; Kirtley, J.; Tsang, J. Phys. Reu. B Condens. Mutter 1980, 22, 3973. ( 3 ) Kirtley, J.; Jha, S.; Tsang, J. Solid State Commun. 1980, 35, 509. (4) Barber, P. W.; Chang, R. K.; Massoudi, H Phys. Rev. B Condens. Mutter 1983, 27, 7251. ( 5 ) Chen, C. K.; deCastro, A. R. B.; Shen, Y. R.Phys. Reu. Lett. 1981, 46, 145. (6) Pettinger, B.; Wenning, U.; Kolb, D. M. Ber. Bunsen-Ges.Phys. Chem. 1978,82, 1326. (7) Plieth, W. J. J . Phys. Chem. 1982, 86, 3166. (8) Roy, D.; Furtak, T. E. Chem. Phys. Lett. 1986, 124, 299.
(9) Marshall, C. D.; Korenowski, G. M. In Advances in Laser Science; Stwalley, W. C., Lapp, M., Eds.; American Institute of Physics: New York, 1986, Vol. 1, p 716. (10) Marshall, C. D.; Korenowski, G. M. J . Chem. Phys. 1986,85,4172.
Letters
-I
-15 - 1 - 4 - 7 -4 - 1 -15 APPLIED VOLTAGE (Volts vs SCE)
Figure 1. The upper third of the figure is the voltammogram for one
oxidation-reduction cycle of the silver electrode in an aqueous NaF electrolyte. Arrows A-D indicate important potentials in the electrochemical cycle and are described in the text. Second harmonic generation at 532 nm from the silver electrode surface is shown as a function of the electrochemistry in the middle of the figure. Laser-induced silver cluster luminescence (420-380 nm) from the silver electrode is shown as a function of the electrochemical cycle in the bottom portion of the figure. vs. one-photon excitation with the 355-nm laser line provided a much higher signal-to-noise ratio in the LIL detection of silver clusters. An electrochemically roughened electrode provided a reproducibly rough surface from cycle to cycle, which made possible a comparison of SHG and LIL from successive ORCs. Beginning with a polished electrode, S H G from the electrode surface was monitored for each ORC until a reproducible signal strength and a consistent pattern of signal variation with applied potential were observed in successive cycles. The first few ORCs on the polished electrode showed only small monotonic variations of SHG intensity with applied potential. The larger and distinctly different S H G signal variations observed during later cycles were consequently attributed to electrochemically induced changes in surface roughness. After a roughening sequence of about 10 ORCs the signal variations remained consistent for subsequent cycles and for those following a cycle in which LIL was recorded with higher incident laser energies. On the basis of the size of the electromagnetic enhancement for SHG, it was assumed that following the roughening sequence the electrode surface was reproducibly rough on the 1-100-nm scale from cycle to cycle.
Discussion of Results Results of the experiment are summarized in Figure 1. The upper portion of the figure is the electrochemical record for a representative ORC. The middle and bottom sections of the figure are, respectively, the S H G and LIL signals as a function of the electrochemical cycle. The potential scan direction is from the left- to the right-hand side in the figure. Near the potential of arrow A in Figure 1, the working electrode begins to oxidize with dissolution of silver as Ag’. As the applied potential is made more positive, the oxidation current increases, reaching its maximum at the + 0.7-V upper potential limit of the scan. This is marked by arrow B. The oxidation current decreases during the reverse potential sweep and terminates at the potential of arrow C. At C and continuing throughout the remainder of the ORC, Ag+ begins to be reduced and deposited as neutral silver on the electrode. The maximum rate of silver deposition corresponds with the maximum reduction current as marked by arrow D. The smaller reduction vs. oxidation current is indicative that reduction is a diffusion-limited process. Return of Ag’ from the solution near the electrode surface accounts for the vast majority of the reduction current with most of the oxidation-produced Ag+ being lost to the bulk solution. The Ag+ in the bulk solution, however, does result in a minute flux of Ag’ which over time slowly returns to the electrode surface. It was determined that this
Letters residual reduction current provides a negligible contribution to surface feature formation in these experiments. At the start of an ORC, the silver electrode surface is highly roughened on the macroscopic size scale. These structures result in a large SHG signal at the beginning of each ORC as is observed from Figure 1. A small increase for S H G is evident as the electrode potential is made more positive. This increase is attributed to a greater polarization of the electrode-electrolyte interface with the attraction of the strongly hydrated fluoride ions at the more positive potentials just preceding the start of oxidation. The result is an increase in the optical nonlinearity of the interface and larger SHG from the electrode surface. This effect has been previously noted and used to study anion interaction with electrode surfaces."J2 At the start of oxidation, S H G from the electrode surface rapidly falls to a minimum value. This indicates that the macroscopic roughness that provides the large enhancement for SHG is rapidly oxidized and removed from the electrode surface. A preferential oxidation of these protruding surface structures could be expected in view of the electric field gradients at such structures and the less restrictive conditions they present for diffusion to and from the bulk solution. After the removal of the majority of the large scale roughness, the surface is undoubtedly still rough on the macroscopic scale. The amount of this roughness, however, has been substantially reduced by oxidation. During this time, the electrode surface must be in a steady state with respect to the amount of macroscopic roughness as observed from the long minimum in the S H G signal. Structures that are destroyed by oxidation are constantly regenerated by silver dissolution and surface rearrangement. The minimum in the S H G signal persists until well after the reduction current has reached its maximum (arrow D) and subsided. Following the major silver deposition, the S H G signal begins to increase, indicating the formation of additional macroscopic surface roughness. This regrowth of macroscopic roughness is slow, requiring almost the remainder of the cyclic scan before returning to a maximum value of the same strength as that observed at the beginning of the ORC. Holding the potential at the cycle-terminating potential (-0.15 V) for a few minutes following the scan end does not result in any further increase in SHG. If the electrode is held at the terminating potential for longer times of 10-15 min a slight increase for SHG is observed. These results indicate the residual reduction current contributes little to the roughness formation observed in these experiments. The macroscopic roughness formation observed in these experiments is apparently the result of a slow rearrangement and restructuring of the electrode surface following the major silver deposition event. Equilibrium surface structure is reached only after the surprisingly long time of a few minutes. The induced luminescence in the bottom part of Figure 1 is from silver clusters at the electrode surface. The spectral bandwidth of this emission, which is centered between 380 and 420 nm in these experiments, is narrower than the broad absorption and emission bands associated with the plasmon resonances of silver clusters consisting of seven or more silver atoms and silver micr~particles.l~-'~ Also, the emission spectrum did not resemble the multiphoton-induced luminescence which has been observed from both smooth and roughened silver surfaces." In fact, the transient emission observed in these experiments occurs at a wavelength where there is a minimum in the multiphoton-induced luminescence from a roughened silver surface. Although a detailed spectrum of the transient emission signal could not be obtained with our present instrumentation, the luminescence does appear to possess the character of molecular absorptions and emissions Richmond, G. L. Chem. Phys. Lett. 1984, 1 1 0 , 571. Richmond, G. L. Langmuir 1986, 2, 132. Schulze, W.; Becker, H . V.; Abe, H . Chem. Phys. 1978, 35, 177. Welker, T.;Martin, T.P. J . Chem. Phys. 1979, 70,5683. Ozin, G . A . Faraday Symp. Chem. Soc. 1980, 1 4 , 7. Schulze, W.; Abe, H . Faraday Symp. Chem. Soc. 1980, 1 4 , 87. (17) Boyd, G. T.; Yu, 2. H.; Shen, Y. R. Phys. Rev. B Condens. Matter 1986, 33, 7923. (11) (12) (13) (14) (15) (16)
The Journal ofPhysica1 Chemistry, Vol. 91, No. 6, 1987 1291 identified with clusters of six or fewer silver atoms and is so assigned. The lack of spectroscopic informaiton on these size silver clusters in aqueous solution let alone at a highly electrified interface does not permit a more specific assignment of the signal. Absorption spectra of silver clusters in a solid water matrix at cryogenic temperatures show the three-photon excitation with 1064-nm laser light would match with an absorption band of the four-atom cluster and is in proximity to absorption bands of other small clusters in this size range.15 This serves to demonstrate that the excitation energy is in the appropriate energy region for stimulation of small cluster emission. It was not determined, though, whether excitation is via a simultaneous or sequential absorption of three photons or if luminescence results from energy transfer between the silver surface and the clusters. As previously mentioned, excitation with 355-nm light was tried but a broad wavelength, background signal from the bulk silver surface and the optical-grade quartz window made it more difficult to observe the emission signal from transient clusters. It is not until well into the oxidation portion of the ORC that cluster luminescence is observed. The lack of LIL in the early stages of oxidation is additional evidence that emission is from molecular clusters rather than microparticles of silver. As the macroscopic silver structures oxidize, they undergo a size reduction process that accelerates as the structures become smaller. During this oxidation, the surface features progress through a range of sizes until cluster size is reached, at which time the small silver clusters would oxidize rapidly and could not be detected. The lack of emission during the early stages of oxidation indicates that the signal is not from silver microparticles smaller than those which give an enhancement to SHG. Cluster emission is observed during oxidation but only reaches its maximum strength after the oxidation current is decreasing in the reverse potential scan. With oxidation of the bulk surface and the slow rearrangement of the surface after silver dissolution, the maximum number of oxidation-produced clusters is observed only after their rate of oxidation has decreased and the surface has had time to restructure to form the clusters. This is consistent with the fact that surface diffusion and restructuring appear to be slow at the solution-electrode interface. Similar results were also observed in previous studies using a chloride s o l u t i ~ n . ~ - ~ ~ After the cluster emission reaches its maximum value during oxidation, it decreases with the decreasing oxidation current. The LIL continues to decrease even into the reduction region of the cycle. Well after maximum silver deposition has occurred but definitely before an increase in SHG is observed, a sharp rise in cluster luminescence is detected. This is the result of reduction-produced clusters forming on the electrode surface. The emission reaches its maximum and begins to fall before the formation of macroscopic roughness is observed through the SHG signal. Decaying more rapidly than the rise of the SHG, the cluster emission decays away before there is a substantial enhancement of S H G by newly forming macroscopic roughness. The observed delay between the reduction-generated cluster signal and reduction current maximum is a definite indication that atom-level reconstruction is slow at the interface and that the signal is not from silver adatoms and probably not from the smaller clusters like diatomics. It is most likely that the LIL is from the larger molecular clusters of four to six atoms. We cannot at this time rule out the possibility that the light-absorbing species could be even larger molecular clusters that fragment into smaller clusters whose emission is observed. Photon-assisted diffusion of adatoms and clusters is undoubtedly also taking place during these experiments. Studies are presently being conducted to determine the magnitude of this perturbation on the observed rates of cluster and macroscopic surface roughness formation on the silver electrode. Summary The combination of optical SHG and LIL studies have been demonstrated as a means of following macroscopic- and atomic-level surface structure evolution on the silver electrode during an ORC. These studies demonstrate that the silver electrode
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surface is a surprisingly mobile surface whose reorganization appears to be impeded by the interfacial solution. During both oxidation and reduction of silver, molecular clusters of silver are formed through a slow relaxation of the electrode surface following silver dissolution or deposition. Surface clusters, detected through LIL, and growth of macroscopic silver surface structures, detected with SHG, are found to continue into times well after the reduction of silver had reached its maximum and decreased to a negligible amount, with the clusters appearing first and then followed by the growth of macroscopic roughness. These results indicate that the silver electrode surface presents an evolving variety of atomic-level and macroscopic surface features for chemical interaction
with adsorbates or solution-phase species during and following and ORC. Consequently, silver cluster chemistry and evolving surface structure must be considered in describing the chemical behavior of the electrode and its interaction with solution-phase species. These studies emphasize the need for real time, in situ studies of electrode surface structure to determine its relation to electrode surface chemistry. In addition, this work opens the possibility of further temporal studies of atomic-level and macroscopic surface reconstruction events at the solid-liquid interface.
Acknowledgment. We thank R. L. Mortensen for his gift of laser equipment which made these studies possible.
Appllcations of the Quartz Crystal Microbalance to Electrochemistry. Measurement of Ion and Solvent Populations in Thin Films of Poly(vinylferrocene) as Functions of Redox State Pierre T. Varineau and Daniel A. Buttry* Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071 (Received: October 20, 1986; In Final Form: January 15, 1987)
The quartz crsytal microbalance (QCM) technique is applied to a study of ion and solvent content within poly(viny1ferrocene) (PVF) as a function of redox state in various supporting electrolytes. It is concluded that in C10; and PF6- containingelectrolytes the oxidation of the film occurs with little and no change in solvent content of the film, respectively. These findings are in agreement with previously postulated phaselike behavior for this system. Multiple peaks are observed in the voltammetry of the PVF film in C1- containing electrolytes. Based on the concurrent QCM measurements the structure in the voltammetric response is proposed to result from the electrochemically induced dissolution or delamination of the film and a new charge transport (diffusional) situation which results from loss of the film from the surface. The QCM technique is shown to be a powerful tool for the study of processes which result in mass changes at solid/liquid interfaces, especially electrode surfaces.
Introduction
Electrodes coated with polymers are of practical and fundamental interest both because of the possibility of studying the redox chemistry (and associated processes) of the immobilized polymer and because of the control over electrode processes which can often be exerted through manipulation of surface structure. The many potential uses of such modified electrodes have been adequately identified,' and more will undoubtedly be found. Of the processes which influence their electrochemical response, the transport of solvent and ionic species within these structures is thought to have great impact on the kinetics and thermodynamics of the redox event. However, very little is presently known about even the qualitative aspects of these processes. The relative absence of adequate techniques is largely responsible for this situation. Several research groups have recently reported on the use of the quartz crystal microbalance (QCM) to study electrode surface processes.* The Q C M is a piezoelectric device widely used in the vacuum community for the determination of the mass of thin films deposited on its s ~ r f a c e . Under ~ some conditions it can be used to probe mass changes of the electrode surface (or structures (1) Murray, R. W. In Electroanalyiical Chemistry, Vol. 13, Bard, A. J., Ed.; Marcel Dekker: New York, 1984; p 191. (2) (a) Kaufman, J. H.; Kanazawa, K. K.; Street, G. B. Phys. Rev. Lett. 1984,53,2461. (b) Bruckenstein, S.; Shay, M. J . Electroanal. Chem. 1985, 188, 131. (c) Bruckenstein, S.; Swathirajan, S.J . Electrochim. Acta. 1985, 30, 851. (d) Melroy, 0.; Kanazawa, K.; Gordon 11, J. G.; Buttry, D Langmuir 1986, 2, 697. (3) Applicaiiom of Piezoelectric Quartz Crystal Microbalances. Methods and Phenomena, Vol. 7, Lu, C., Czanderna, A. W., Eds.; Elsevier: New York, 1984.
attached to the surface) which may (or may not) be electrochemically induced. It has two significant attributes. One is its ability to make the mass determination in situ, in conjunction with the electrochemical measurements. The second is its excellent sensitivity, being capable of measuring mass changes corresponding to submonolayer adsorption and desorption. We have undertaken to apply this powerful new tool to the elucidation of the redox processes which occur within microstructures on electrodes, with particular emphasis on ionic and solvent transport. In this Letter we report on some initial findings which speak to the question of the degree of solvent transport which occurs during oxidation of thin films of poly(viny1ferrocene) (PVF) immobilized on the electrode surface, and which reveal the nature of the processes responsible for the unusual multiple peaks often seen in the voltammograms of this (and other) system(s). Experimental Section A schematic of the apparatus used for the QCM/electrochemical experiment is shown in Figure 1. The microbalance
is comprised of a 5-MHz AT-cut quartz crystal which is driven at its resonant frequency with a feedback oscillator. The crystal is sandwiched between two vacuum deposited gold electrodes by using a standard keyhole electrode c~nfiguration.~One electrode is kept out of the solution by using an O-ring mounting, and the other is used as the working electrode. The piezoelectrically and electrochemically active areas are 0.28 and 0.34 cm2, respectively. The signal from the oscillator is sent to a Philips PM6654 fre(4) Bruckensiein, S.; Shay, M. Electrochim. Acta. 1985, 30, 1295
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