406
Langmuir 1989,5, 406-414
Raman and Surface-Enhanced Raman Spectroscopy of the Three Redox Forms of 4,4'-Bipyridine Tianhong Lu and Therese M. Cotton* Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0304
Ronald L. Birke* and John R. Lombardi Department of Chemistry, City College, City University of New York, New York, New York 10031 Received September 15, 1988. I n Final Form: December 19, 1988
The electrochemistry of 4,4'-bipyridine (BiPy) at a kg electrode and the normal Raman (NR), resonance Raman (RR), surface-enhanced Raman scattering (SERS), and surface-enhancedresonance Raman scattering (SERRS) spectra of the three redox forms of BiPy were studied. Voltammetric studies in conjunction with surface spectroscopy indicate that the behaviqr of BiPy is pH dependent. Two reduction peaks were observed in the cyclic voltammogram of BiPy on a polished Ag electrode in pH C6.0 solutions, while there was only one reduction peak for pH >6.0 solutions. However, the SERRS spectrum of the BiPy radical intermediate was observed on a roughened Ag electrode when the electrolyte solution pH was 16.0; thus, it is apparent that the reduction of adsorbed BiPy proceeds through two one-electron steps at this pH. In addition, the SERS/SERRS data suggest that BiPy and its reduction products are adsorbed on the Ag electrodes in several ways: through *-bonding to the aromatic rings, through the lone-pair electrons on the N atoms, or via an ion-pair mechanism with halide ions. The bonding interaction is dependent upon the solution pH and the nature of the anions in solution. The adsorbed species, corresponding to each adsorption mechanism, are identified, and their orientations with respect to the electrode surface are proposed. Finally, the potential-intensity profiles of BiPy SERS bands and the effect of self-absorption on the profiles are discussed.
Introduction The ability of 4,4'-bipyridine (BiPy) to enhance redox reactions of certain biological molecules at metal electrodes has generated considerable interest. Specifically, this compound has been used to chemically modify electrodes in order to accelerate the electron-transfer rate between the electrode and cytochrome c.l-15 The catalytic mechanism differs from that which proceeds via the usual redox mediators. In the presence of mediators, electrochemical reactions of proteins are initiated after the mediator itself has been reduced or oxidized. In contrast, BiPy does not undergo a redox reaction when it promotes redox reactions of proteins at electrodes. Therefore, this type of catalysis is convenient for investigating the electrochemical behavior of proteins without interference from mediator redox reactions. The basis for this surface acceleration is not well understood. In order to obtain more information concerning the mechanism, in situ surface spectroscopy, such as surface-enhanced Raman scattering (SERS), of adsorbed BiPy would be of value. However, such studies have been rather limited. Only three papers concerned with the SERS behavior of BiPy on Ag16J7and A d s electrodes have been published. Cotton et al.16J7studied SERS spectra of BiPy on a Ag electrode and concluded that the surface-bound BiPy is responsible for the enhancement of the electron-transfer rate between cytochrome c and the Ag electrode. Taniguchi et a1.18 reported the SERS spectra of BiPy on the Au electrode and inferred that BiPy is adsorbed with a vertical orientation on the Au electrode via one N atom of BiPy. Considerable attention has been paid to the adsorptive behavior of BiPy because the orientation of the adsorbed BiPy appears to be an important factor in accelerating protein redox reactions at electrode~.'~J~J~ Normal Raman (NR) and resonance Raman (RR) studies of the various redox forms of BiPy have also been
* Authors to whom correspondence should be addressed.
limited. To our knowledge, only Guptalg and Kihara and Gondo20 reported the NR spectrum of BiPy in the solid state. The latter authors also studied the RR spectra of the BiPy radical anion (BiPy'-) in the solid state and in an organic solvent. The NR and RR spectra of various redox forms of BiPy in aqueous solutions have not been reported.
(1)Eddowes, M. J.; Hill, H. A. 0. J . Chem. SOC.,Chem. Commun. 1977,21,771. (2)Eddowes, M.J.; Hill, H. A. 0. J . Am. Chem. SOC.1979,101,4461. (3)Eddowes, M.J.; Hill, H. A. 0.;Uosaki, K. J. Am. Chem. SOC.1979, 101,7114. Uosaki, K. Bioelectrochem. Bioe(4)Eddowes, M. J.; Hill, H. A. 0.; nerg. 1980,7,527. (5)Uosaki, K.; Hill, H. A. 0. J . Electroanal. Chem. 1981,122,321. (6) Albery,W. J.; Eddowes, M. J.; Hill, H. A. 0.; Hillman, A. R.J.Am. Chem. SOC.1981,103,3904. (7)Taniguchi, I.; Murakami, T.; Toyosawa, K.; Yamaguchi, H.; Yasukouchi, K. J. Electroanal. Chem. 1982,131,397. (8) Haladjian, J.; Pilard, R.; Bianco, P.; Serre, P.-A. Bioelectrochem. Bioenerg. 1982,9,91. (9)Bianco, P.; Haladjian, J.; Pilard, R.; Bruschi, M. J . Electroanal. Chem. 1982,136,291. (10) Eddowes, M. J.; Hill, H. A. 0.Faraday Discuss. Chem. SOC.1982, 74,331. (11) Dhesi, R.;Cotton, T. M.; Timkovich, R. J. Electroanal. Chem. 1983,154,129. (12)Haladjian, J.; Bianco, P.; Pilard, R.Electrochem. Acta 1983,28, 1823. (13)Allen, P. M.; Hill, H. A. 0.;Walton, N. J. J.Electroanal. Chem. 1984,178,69. (14)Kuznetsov, B. A.; Shumakovich, G. P.; Mutuskin, A. A. Bioelectrochem. Bioenerg. 1985,14,347. (15) Gui, Y.; Kuwana, T. J. ElectronaL Chem. 1987,226,199. (16) Cotton, T. M.; Kaddi, D.; Iorga, D. J. Am. Chem. SOC.1983,105, 7462. (17)Cotton, T.M.; Vavra, M. Chem. Phys. Lett. 1984,106,491. (18)Taniguchi, I.; Iseki, M.; Yamaguchi, H. J. Electroanal. Chem. 1985,186,299. (19)Gupta, V. P.;Ind. J. Pure Appl. Phys. 1973,11, 775. (20) Kihara, H.; Gondo, Y. J. Raman Spectrosc. 1986,17,263.
0743-746318912405-0406$01.50/0 0 1989 American Chemical Society
Langmuir, Vol. 5, No. 2, 1989 407
R a m a n Spectroscopy of Redox Forms of 4,4'-Bipyridine
The present paper describes the voltammetric behavior of BiPy at a Ag electrode and reports NR, RR, and surface Raman spectra of the three redox forms of BiPy in aqueous solutions.
Experimental Section 4,4'-Bipyridine (BiPy) from Aldrich Chemical Co. was recrystallized from hot water and sublimed under vacuum. Reagent grade Na2S04, KCl, KBr, and K I were used as the supporting electrolytes. The solutions were prepared with deionized, distilled water and deaerated before use by nitrogen purging. For the Raman and electrochemical experiments, three-electrode cells were used. A saturated calomel electrode (SCE) served as a reference electrode, and all potentials are quoted with respect to the SCE. The working electrode was silver or glassy carbon, and the counter electrode was Pt. The exposed surface was about 4.0 mm2 for the Ag electrode and 5.7 mm2 for the glassy carbon electrode. For SERS/SERRS experiments, the Ag electrode was constructed from flattened silver wire, which was sealed into glass tubing with Torr Seal (Varian Associates). The dimensions of the exposed, rectangular surface were approximately 2 x 10 mm. Pretreatment of the Ag electrode was as follows. The electrode was polished with a slurry of 0.3-km alumina powder on a mechanical polishing wheel and sonicated in distilled water before being mounted in the cell. Finally, an oxidation-reduction cycle (ORC) was applied to the Ag electrode by using a double-potential-step procedure: -0.2 to +0.3 to -0.2 V for the solution with 0.1 M KCl or KBr supporting electrolyte, -0.4 to +0.3 to -0.4 V for the solution with 0.1 M KI, and -0.2 to +0.5 to -0.2 V for the solution with 0.1 M Na2S04. The total charge passed during the oxidation step was equivalent to about 25 mC/cm2. In this paper, a Ag electrode which was not subjected to an ORC is termed a "polished Ag electrode". A "roughened Ag electrode" refers to one which had undergone the ORC. The ORC was carried out in the electrolyte solution containing the sample. In some cases (Le., in order to carry out spectral measurements of adsorbed species without interference from solution species) the cell was washed with the supporting electrolyte solution following the ORC, and the sample solution was replaced with the supporting electrolyte solution prior to spectral measurements. The following wavelengths were used in the Raman experiments: 488, 514.5 (Coherent Innova 95 Ar'), and 647.1 nm (Spectra Physics 2000 Kr+). Laser lines between 572 and 605 nm were obtained with a Spectra Physics Model 375 tunable dye laser excited by the argon ion laser. The power was between 30 and 300 mW. An Anaspec 300-S premonochromator was used to remove plasma lines. A Spex Triplemate 1877 monochromator equipped with a Model 1420 intensified SiPd detector (Princeton Applied Research Co.) and an optical multichannel analyzer (OMA-2, Princeton Applied Research Co.) or a Spex Model 1401 double monochromator equipped with an FW 130/72 photomultiplier tube detector was used to record Raman spectra. A BAS 100 electrochemical analyzer was used for the electrochemical measurements.
Results and Discussion Electrochemistry of BiPy at a Ag Electrode. The electrochemistry of BiPy at a Hg electrode has been These investigations indicated studied in the that the reduction of BiPy is pH-dependent in aqueous solutions. BiPy can be protonated in two steps, forming monoprotonated BiPy (BiPyH+) and diprotonated BiPy ( B ~ P Y H ~ ~The ' ) . acid dissociation constants of the protonated BiPy in 0.2 M ionic strength solutions are pK1 = 3.5 and pK2 = 4.9, re~pectively.~~ (21) Zahlan, A. B.;Linnel, R. H. J. A m . Chem. SOC.1955, 77, 6207. (22) Falqui, M. T.;Secci, M. Ann. Chem. 1958, 48, 1168. (23) Volke, J.; Volkova, V. Collect. Czech. Chem. Commun. 1972,37, 3686. (24) Musgrave, T.R.;Mattson, C. E. Inorg. Chem. 1968, 7, 1433.
J -1.00
-0.50
-1.60
Potential (V vs SCE) Figure 1. Cyclic voltammograms of 1 X 10" M (solid line) and 1 X low3M BiPy at a glassy carbon electrode in a p H 6.4 solution with 0.1 M KC1 as supporting electrolyte. Scan rate: 50 mV/s. Initial potential: -0.3 V. Current unit is arbitrary.
In acid solutions, pH 6.0 only one reduction peak was observed on a Hg electrode. Thus, it was considered that BiPy reduction under these conditions takes place via a single two-electron step. Volke and VolkovaZ3suggested a possible mechanism: BiPy + 2eBiPy2BiPy2- + 2Hz0
-
BiPyH2 + 20H-
The electrochemical behavior of BiPy at a polished Ag or glassy carbon electrode is similar to that a t a Hg electrode. When the pH of the solution is 1.8, the first oneelectron reduction peak is near -0.68 V in the cyclic voltammogram. The blue color of the radical in solution near the electrode surface can be observed when the electrode potential is maintained at ca. -0.7 V. The second reduction peak cannot be observed because of the reduction of hydrogen ions. However, when the pH is shifted to about 3.5, two reduction peaks are observed at -0.77 and -0.98 V. Upon increasing the pH to greater than 6, only one reduction peak is observed, and the peak potential (E,) shifts in the negative direction with increasing solution pH. For example, E, is about -1.25 V in a pH 6.4 solution and about -1.36 V in a pH 12.0 solution. The concentration of BiPy has a pronounced effect on its voltammetric behavior. A nearly reversible response M sowas observed (dotted line in Figure 1)for the lution. However, the electrochemical response was quite irreversible at higher concentrations. A single reduction peak is found near -1.30 V, and the corresponding oxidation peak is near -0.70 V (solid line in Figure 1) for a M and pH 6.4. This solution concentration of 1 x irreversible behavior may be attributed to the formation of inactive species, possibly polymeric, from the reduction product, BiPyH2:25,26 n BiPyHz (BiPyH2),
-
408 Langmuir, Vol. 5, No. 2, 1989
Lu et al.
p,w\ 1662
n
1072
1224
1290
-
c 1046
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'
1 0.0 0
-0.00
-1.10
Potential (V
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VI)
"8
24
-1.10
\
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1298 1000
Figure 2. Cyclic voltammograms of 1X W3M BiPy at a polished (dotted line) and a roughened (solid line) Ag electrode in a pH 6.4 solution with 0.1 M KCl as supporting electrolyte. Scan rate: 50 mV/s. Initial potential: -0.9 V.
Thus, high concentrations of BiPy result in an inactivation reaction. A film of (BiPyHz), is formed on the electrode surface, which blocks further electron transfer and leads to the observed irreversible cyclic voltammetry. If the concentration is lower than M, the inactivation reaction is not sufficiently fast to compete with the electron-transfer reaction, and nearly reversible voltammetric behavior is observed. The voltammetric behavior of BiPy on a roughened Ag electrode is similar to that on a polished Ag electrode except for an additional adsorption peak in the former. In Figure 2 the cyclic voltammogram of BiPy in a 0.1 M KC1, pH 6.4 solution is compared at a roughened (solid line) and a polished (dotted line) Ag electrode. Only one reduction peak at -1.36 V is present in the voltammogram obtained in the latter, whereas two reduction peaks at -1.23 and -1.41 V are present in the voltammogram from the roughened electrode. The peak a t -1.23 V is due to the two-electron reduction of adsorbed BiPy, and that at -1.41 V is due to the two-electron reduction of bulk BiPy. The reason that the peak associated with adsorbed BiPy is not apparent in the cyclic voltammogram a t the polished electrode is that the roughening procedure enhances adsorption. Also, the overall response, including the current associated with the reduction of bulk BiPy, is greater a t the roughened electrode (compare the solid curve with the dashed in Figure 2). In the case of the first reduction peak, the peak current was found to be directly proportional to scan rate, and the anodic and cathodic peak currents were equal, indicating a reversible adsorption wave. A small difference was observed between the anodic and cathodic peak potentials. This may be due to solution resistance, or it may reflect quasi-reversible electron-transfer kinetics, because the difference becomes very small when the scan rate is slow. For a reversible process, the following equation can be used to determine n, the number of electrons transferred per molecule, for an adsorbed specie^:^' n = 4RTi,/(uQF) where i, is the peak current, u is the potential scan rate, (25) Heyrovsky, M. J . Chem. SOC.,Faraday Trans. 1 1986, 82, 585. (26) Heyrovsky, M. J. Chem. Soc., Chem. Commun. 1983, 1137. (27) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods; Wiley: New York, 1980; p 522.
1084
1
I . 1000
1200
1400
1600
Raman shift (cm'')
Figure 3. NR spectra of BiPy solid and solutions of various pH values. (A) BiPy solid; (B) BiPy solution at pH 6.4; (C) BiPy solution at pH 3.5; (D) BiPy solution at pH 1.8. Excitation wavelength: 488.0 nm. Power: 30 mW for A and 300 mW for
B-D.
and Q is the charge associated with the reduction of adsorbed molecules as determined from the area under the reduction peak. The value of n obtained by using this relationship was 2.02. Therefore, the above results demonstrate that the reduction peak a t about -1.23 V is not due to the one-electron reduction of BiPy but rather to the two-electron reduction of the adsorbed BiPy molecules. The second reduction peak at -1.41 V is due to the reduction of diffusing bulk BiPy molecules, as its peak current is proportional to the square root of the scan rate. However, it should be noted that the adsorption peaks in the cyclic voltammograms of BiPy on the roughened Ag electrode are observed only in the presence of halide ions. When NaZSO4is used as electrolyte, the adsorption peaks are not seen, indicating that the adsorption process involves coadsorption of halide ion. Although Volke and VolkovaZ3have suggested a single two-electron reduction mechanism for BiPy in basic solution, the SERRS bands due to the BiPy radical are nevertheless observed on the roughened Ag electrode at potentials more negative than -1.1 V (this observation will be discussed further below). This implies that the electrochemical reduction of adsorbed BiPy in basic solution passes through the radical intermediate step. Raman Spectroscopy of BiPy. Figure 3A shows the NR spectrum of BiPy in the solid state in the frequency range 1000-1700 cm-'. Most of the strong bands are totally symmetric vibrational modes.20 The frequencies and relative intensities of the bands in this spectral range are listed in Table I. Figure 3B-D shows the effect of pH on the NR spectra of BiPy in aqueous solutions. BiPy is not protonated in pH 6.4 solutions, and, thus, the NR spectrum (Figure 3B) is similar to that of BiPy in the solid state (Figure 3A). In the pH 1.8 solution most BiPy molecules are diprotonated and the spectrum (Figure 3D) is some~ - ~ most ~ sigwhat similar to that of the v i o l o g e n ~ . ~ The (28) Feng, Q.; Cotton, T. M. J. Phys. Chem. 1986, 90,983.
Langmuir, Vol. 5, No. 2, 1989 409
Raman Spectroscopy of Redox Forms of 4,4'-Bipyridine
solid BiPy 1620 (9) 1606 (9) 1595 (m) 1510 (m) 1423 (w) 1343 (w) 1298 (vs) 1218 (m) 1100 (w) 1087 (w) 1082 (w) 1041 (w) 1000 (vs) 983 (m) 885 (w) 881 (w) 854 (m) 815 (w) 756 (m) 674 (w) 660 (m) 608 (w) 572 (m) 500 (w) 386 (m) 324 (m) 315 (m) 262 (w) 251 (w) 164 (w) 145 (w)
Table I. NR and RR Frequencies (cm-')O of the Three Redox Forms of BiPy RR bands NR bands solution radical pH 6.4 BiPy pH 1.8 BiPyH2+ BiPyHP'+ BiPy'-b 1624 (9) 1652 (vs) 1653 (s) 1606 ( 8 ) 1612 (9)
two-electronreduced form BiPyHz 1652 (s)
1514 (m)
1531 (m)
1523 (vs) 1503 (m)
1509 (s)
1538 (9)
1296 (vs)
1353 (s) 1236 (w) 1213 (w)
1350 ( 8 ) 1230 (m)
1224 (m)
1298 (vs) 1256 (m) 1220 (w)
1392 (w) 1291 (w) 1219 (w)
1080 (w)
1072 (w)
1011 (vs)
1013 (vs)
1042 (s) 999 (w)
1043 (s) 990 (m)
875 (w)
873 (w)
767 (m)
760 (w)
741 (m)
742 (m)
661 (m)
644 (m)
574 (w)
563 (w)
388 (w) 334 (w)
401 (m)
266 (w)
272 (w)
326 (m)
997 (9)
749 (m)
348 (w) 320 (w)
'vs = very strong, s = strong, m = medium, w = weak, sh = shoulder. bThe data for BiPy'- were reported in ref 20.
Table 11. SERS/SERRS Vibrational Frequencies (cm-') of the Three Redox Forms of BiPy" BiPy'- (pH 1.8) BiPy'- (pH 1.8) BiPyHz (pH 1.8) BiPyHz2+(pH BiPy (pH 1.8) BiPy (pH 6.4) 0.1 M SOL20.1 M KCl 0.1 M KI 0.1 M KI 1.8) 0.1 M KCl 0.1 M SOL2-1633(s) 1638 (vs) 1652 (s) 1612 (m) 1604 (vs) 1587 (vs) 1593 (vw) 1584 (9) 1555 (w) 1530 (m) 1518 (vs) 1513 (m) 1530 (9) 1522 (m) 1503 (m) 1500 (m, sh) 1476 (m) 1440 (m) 1446 (w) 1458 (s) 1385 (m) 1410 (w) 1331 ( 8 ) 1334 ( 8 ) 1331 (w, sh) 1295 (s) 1295 (9) 1349 ( 8 ) 1357, 1295 (m) 1293 (vs) 1295 (vs) 1251 (m) 1251 (m) 1250 (w) 1204 (s) 1214 (w) 1207 (s) 1213 ( 8 ) 1219 (w) 1214 (w) 1190 (s) 1188 (vw) 1188 (m, sh) 1163 (m) 1165 (s) 1112 (w) 1067 (w) 1070 (vw) 1072 (m) 1060 (9) 1034 (s) 1012 ( 8 ) 1008 (m) 1012 (w) 998 (w) 1012 (m) 1004 (m) 978 (m) OAbbreviations the same as in Table I.
nificant change in the NR spectrum of BiPy with variation of the pH of the solution is in the range 1600-1660 cm-'. In particular, two major changes occur in the spectrum as the pH is decreased from 6.4 to 1.8. The 1606-cm-l band disappears (Figure 3D), and the band at 1624 cm-' (Figure 3B) shifts to 1652 cm-' in the pH 1.8 spectrum (Figure 3D). (29) Lu, T.; Cotton, T. M. J . Phys. Chem. 1987, 91, 5978. (30) Forster, M.; Girling, R. B.; Hester, R. E. J . Raman Spectrosc. 1982, 12,36.
(31) Lu, T.; Birke, R. L.; Lombardi, J. R. Langmuir 1986, 2, 305.
Since the molecular structure of the viologens is similar to that of diprotonated BiPy cation and there is also a band a t 1654 cm-I in the NR spectra of v i o l o g e n ~ it, ~ ~ ~ ~ ~ can be concluded that the band at 1652 cm-' is due to the diprotonated BiPy cation. The band near 1606 cm-' is assigned to the nonprotonated BiPy molecules. The frequencies and the relative intensities of the NR bands of BiPy in the pH 6.4 and pH 1.8solutions are listed in Table I. From the electrochemistry, it is known that BiPy is not reduced at -0.2 V in aqueous solution. Therefore, following
Lu et al.
410 Langmuir, Vol. 5, No. 2, 1989
I
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1587
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Figure 4. SERS spectra of BiPy solutions (1 X M) on a roughened Ag electrode at -0.2 V at various pH values. (A) pH 12.0; (B) pH 6.4; (C) pH 3.5; (D) pH 1.8. Supporting electrolyte: 0.1 M KC1. Excitation wavelength: 488.0 nm. Power: 30 mW.
an ORC of a Ag electrode, a SERS spectrum of BiPy is obtained on the roughened Ag electrode a t -0.2 V. When the cell is washed with electrolyte solution the spectrum does not change, indicating that it is due to irreversibly adsorbed BiPy molecules. Figure 4 shows the SER spectra of BiPy at -0.2 V in 0.1 M KCl solutions at different pH values. The band frequencies are listed in Table I1 for some of the solutions. As observed in the NR spectra, the SERS spectra of BiPy are also somewhat different with variation in pH. One difference is that the intensity of the SERS bands is much lower at pH 6.0. Another difference is that the relative intensities of the SERS bands change with solution pH. For example, the band at 1633 cm-l is strong in the SERS spectrum of the pH 1.8 solution (Figure 4D), but this band is barely observed from the pH 12.0 solution, and a very strong band at 1608 cm-l is observed instead (Figure 4A). From a comparison with the shift of the band at 1654 cm-' in the NR spectra of viologen dications in aqueous solution to 1638 cm-' in the SERS spectra on a roughened Ag electrode at -0.2 V,29831it can be inferred that the band at 1633 cm-I is due to adsorbed, diprotonated BiPy molecules. The band at 1608-1612 cm-' is due to adsorbed, nonprotonated BiPy. As noted above, most of the BiPy molecules in the pH 1.8 solution are diprotonated, and there is no band at 1606 cm-I in the NR spectrum (Figure 3D). However, two bands at 1612 and 1633 cm-I are observed in the SERS spectrum of BiPy in the pH 1.8 solution with 0.1 M KC1 (Figure 4D). This suggests that both protonated and nonprotonated BiPy are adsorbed on the roughened Ag electrode in the pH 1.8 solution with 0.1 M KC1. Furthermore, two bands (1610 and 1626 cm-', Figure 4B) are also observed at pH 6.4 and -0.2 V, indicating that both species exist on the surface even though the distribution in solution favors the basic form under these conditions. The SERS spectra of BiPy were also found to depend strongly upon the anion in the solution. For example, when KBr is used as the supporting electrolyte in the pH 1.8 solution, the ratio of the intensities of the two SERS bands, 11633/11612, is higher than that for KCl. If KI is used,
-
1000
1200
1400
1800
Raman shift (cm")
M Figure 5. SER spectra of BiPy adsorbed from a 1 X solution on a roughened Ag electrode as a function of anion. (A) pH 6.4, 0.1 M KCl as the supporting electrolyte, potential -0.4 V; (B) pH 6.4,O.l M KI as the supporting electrolyte, potential -0.4 V; (C) pH 1.8,O.l M Na2S04as the supporting electrolyte, potential -0.2V. Excitation wavelength: 488.0 nm. Power: 50 mW.
in either acidic or basic solution, only the SERS band at 1638 cm-l is observed, and the band at about 1612 cm-' disappears, indicating that most of the adsorbed BiPy molecules are diprotonated. Figure 5B shows the SERS spectrum of BiPy on a roughened Ag electrode at -0.4 V in a pH 6.4 solution with 0.1 M KI. This spectrum is similar to the SERS spectra of viologen cation^.^^^^^ The potential was maintained at -0.4 V because Ag oxidizes at potentials more positive than -0.4 V in this electrolyte solution. For comparison, the SERS spectrum of BiPy on a roughened Ag electrode at -0.4 V in a pH 6.4 solution with 0.1 M KC1 is also shown (Figure 5A) where the nonprotonated BiPy surface species is dominant. When Na2S04is used as supporting electrolyte in a pH 1.8 solution, a very different SERS spectrum (Figure 5C) is observed. In this spectrum the bands that are not totally symmetric, e.g., 1060, 1334, 1458, and 1587 cm-', etc., are strong with respect to the totally symmetric bands. (Note: the 1587-cm-' band may be the totally symmetric 1633cm-I band, which is downshifted due to the interaction of BiPy with the surface.) However, at pH >6.0, the SERS spectrum obtained with Na2S04 is identical with that obtained with KC1. Upon consideration of the above results and the fact that the adsorptive ability of these anions decreases in the order: I- > Br- > C1- > S042-,three possible modes of adsorption can be postulated for BiPy on a roughened Ag electrode: (1)adsorption through the lone-pair electrons of the N atoms, (2) adsorption through the delocalized r-orbitals of the rings, or (3) adsorption through ion pairing with strongly adsorbed halide ions. These three types of adsorption interactions are depicted in Figure 6. Thus, the type of interaction depends on the pH and the anion in the solution, as well as the electrode potential, as discussed below. Most of the BiPy molecules in pH >6.0 solutions are not protonated. Nonprotonated BiPy is strongly adsorbed on the electrode surface through the lone-pair electrons on
Langmuir, Vol. 5, No. 2, 1989 411
Raman Spectroscopy of Redox Forms of 4,4'-Bipyridine 1-
1604
A
fN>
A
A
D 1004
4
1070
AS
k
cOD C
al
CI
B
C
As!
I
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Figure 6. Proposed structures for BiPy adsorbed on a Ag electrode. (A) Solution pH >6,Na2S04electrolyte; (B) solution
pH
C6, Na2S04electrolyte; (C)
all pH values, NaI electrolyte.
the N atom with an end-on orientation, as shown in Figure 6A.32 Thus, the SERS spectra of BiPy in the pH >6.0 solutions (Figure 4A,B) are intense, and most of the strong bands belong to the totally symmetric vibrational modes.20 At pH C6.0, BiPy is protonated to some extent depending upon the pH value. The protonated BiPy cations can adsorb directly on the electrode surface through abonding, as shown in Figure 6B. This flat orientation of the rings occurs in pH 1.8 solutions containing SO-: and gives rise to non-totally symmetric SERS bands, as shown in Figure 5C. Note, however, that bipyridine rings are normally twisted with respect to one another so that only one of the rings may be oriented flat with respect to the surface, unless the rings are forced to lie coplanar on the surface. In the presence of strongly adsorbed anions at pH C6, protonated BiPy can be adsorbed to the surface through ion pairing with the anion, as shown in Figure 6C.32933 For example, I- is strongly adsorbed on the surface of the roughened Ag electrode and forms a monolayer a t potentials more positive than -0.8 V.33 Therefore, the protonated BiPy dication is adsorbed through an electrostatic interaction with adsorbed I- anion, and only the SERS bands of protonated BiPy cations, such as the band at 1638 cm-l, are observed. Such ion pairs can also form with more weakly adsorbed halide such as C1- at more positive potentials. It was shown that an increase in the concentration of Cl- in the bulk solution can increase the ratio of pyridinium to pyridine on the surface of Ag as observed by SERS, even though the bulk pH is constant.% An end-on orientation is likely for the ion-paired BiPy cations judging from the strong totally symmetric modes in the spectrum (Figure 5B).35936 At low pH values (C6.0)and in the presence of C1-, the situation is more complex. It has been reported that C1is not adsorbed as strongly as I- on the roughened Ag electrode (i.e., only 50% of the electrode surface is covered by C1- a t about -0.2 V).33 Thus, a fraction of the protonated BiPy cations is adsorbed through the ion-pair mechanism at sites which are covered by the adsorbed C1-. (32) Chang, H.; Hwang, K.-C. J. Am. Chem. Soc. 1984, 106,6586. (33) Hupp, J. T.;Larkin, D.; Weaver, M. J. Surf. Sci. 1983,125,429. (34) Sun, S. C.; Bernard, I.; Birke, R. L.; Lombardi, J. R. J. EEectroanal. Chem. 1985, 196, 359. (35) Ghoshal, S.; Lu, T.;Feng, Q.; Cotton, T.M. Spectrochem. Acta. 1988,44A, 651. (36) Rogers, D. J.; Luck, S. D.; Irish, D. E.; Guzonas, D. A.; Atkinson, G. F. J . Electroanal. Chem. 1984, 167, 237.
1000
1200
1400
1600
Raman shift (Cm-')
Figure 7. RR and SERRS spectra of the BiPy radical. (A) RR spectrum of BiPy radical produced electrochemicallyon a polished Ag electrode at -0.7 V in 1 X BiPy solution at pH 1.8 with 0.1 M KCl as the supporting electrolyte. (B) SERRS spectrum of the BiPy radical on a roughened Ag electrode at -0.7 V in a pH 1.8 solution with 0.1 M KI as supporting electrolyte. (C) The same as B, except the supporting electrolyte was 0.1 M KC1. Excitation wavelength 488.0 nm. Power: 200 mW for A, 50 mW for B and C.
The remainder are adsorbed directly on the electrode surface through a-bonding and/or through the lone-pair electrons on the N atoms following proton dissociation. This explains why SERS bands are observed at both 1633 and 1612 cm-' in the pH 1.8 solution containing 0.1 M KC1 (Figure 4D) and why the relative intensities of the non totally symmetric bands (1072,1331,1476, and 1555 cm-l) are stronger than in the pH 12.0 spectrum (Figure 4A). It should be noted that even in basic solutions, when Ianions are adsorbed strongly and completely cover the electrode surface, the adsorbed I- can electrostatically attract H+ from the bulk solution, producing a more acidic solution near the electrode. The BiPy in the solution near the electrode is then protonated and adsorbed through an ion-pair mechanism. This is the reason that SERS bands of only the protonated form of BiPy are observed in solutions containing I- irrespective of the solution pH. Thus, the pH at, or near to, the electrode surface can be changed by adsorbed ions and may be quite different from that in the bulk solution. A mechanism for this type of surface pH effect involving the potential in OHP has been given previou~ly.~~ Raman Spectroscopy of the BiPy Radical. As discussed in the electrochemical section, BiPy radical can be electrochemically generated at solution pH values of C6.0. At pH 1.8 the blue color of the BiPy radical appears in the solution near the electrode surface when the potential is adjusted to -0.7 V. The RR spectrum of the BiPy radical, as observed on the surface of a polished Ag electrode, is shown in Figure 7A. Because the RR spectrum of the BiPy radical in the pH C6.0 solution (Figure 7A) is different from that of the solid BiPy radical anion, BiPy'-, reported by Kihara and Gondo,20but similar to that of viologen it is clear that the BiPy radical is protonated a t this pH (Le., BiPyH2"). The frequencies
412 Langmuir, Vol. 5 , No. 2, 1989
Lu et al.
and the relative intensities of the RR bands of BiPyH2*+ are listed in Table I. For comparison, the frequencies and relative intensities of the BiPy'- RR bands20 are also provided. The surface spectra of the BiPy radical were also examined. In order to avoid interference from RR scattering from bulk species, the BiPy solution was replaced with supporting electrolyte solution following the ORC and prior to SERS measurements. Two types of anion-dependent surface Raman spectra were observed for the adsorbed BiPy radical. The surface Raman spectrum (Figure 7B) we designate type I is observed when KI was used as supporting electrolyte, and it is similar to the RR spectrum of BiPyH6.0), the behavior is more comSimilar shifts are observed in the spectra of BiPy and its plex. If, following the ORC of a Ag electrode, the bulk reduction products (1298 cm-' in BiPy, 1353 cm-' in the solution is not replaced with supporting electrolyte soluradical, and 1392 cm-' in the two-electron-reduced BiPy), tion, a surface Raman spectrum similar to that in Figure and these bands are given the same assignments. However, 8B is observed at potentials more negative than -1.2 V. a band at -1291 cm-' in the RR spectrum of two-elecIf, on the other hand, the cell is washed with supporting tron-reduced BiPy is also observed. This band is not electrolyte solution following the ORC and then the poobserved in the RR spectra of BiPy radical (Figure 7A) and tential is adjusted to a value more negative than -1.2 V, the viologen radical^^^-^' or in the RR spectra of twoa surface Raman spectrum similar to that in Figure 8C is
-
-
Langmuir, Vol. 5, No. 2, 1989 413
Raman Spectroscopy of Redox Forms of 4,l'-Bipyridine
obtained. Comparison of the two spectra shows that bands at 996,1536, and 1652 cm-l disappear after washing. These bands may be due to RR scattering from reduced BiPy in the vicinity of the electrode, or they may also result from RR scattering from adsorbed multilayers of BiPy as suggested previously.'' In any case, if the cell is not washed with the electrolyte solution, the observed Raman spectrum (Figure 8B) is the superposition of RR and surface Raman spectra of two-electron-reduced BiPy. Figure 8C shows the surface Raman spectrum on the roughened Ag electrode at -1.4 V after washing. There are two types of bands in this spectrum: type A bands, 998,1042,1226,1336,1500 cm-'; type B bands, 1004,1070, 1213,1293,1385,1446,1530,1594cm-'. The behavior of the two types of bands is quite different. For example, comparing the Raman spectrum before (Figure 8B) and after (Figure 8C) washing shows that, except for the disappearance of the RR bands, type A bands decrease in intensity after washing. In addition, when a longer wavelength laser line, such as 647.1 nm, is used as an excitation source, type A bands are very strong. If a shorter wavelength laser line, such as 457.9 nm, is used, the type A bands almost disappear. Therefore, the two types of bands are probably due to different surface species, as discussed below. Type A bands are similar to the RR bands of solid BiPy radical, BiPy*-.20Thus, these are attributed SERRS from BiPy'- on or near the surface of the roughened Ag electrode. Usually, BiPy'- cannot exist in aqueous solution, but it may be stabilized on the roughened Ag surface. Detection of BiPy radical on the electrode under basic conditions indicates that, although there is only one reduction peak in the cyclic voltammogram of bulk BiPy at pH >6.0, the reduction of adsorbed BiPy nevertheless proceeds through the radical step. A similar phenomenon was observed for the electrode process of flavins on roughened Ag.37 An alternative explanation is that the SERRS spectrum represents a small population of adsorbed species that does not undergo the second reduction step. SERRS bands of the adsorbed radical are strong even a t electrode potentials as negative as -1.6 V. Type B bands are similar to those in the SERS spectra of two-electron-reduced v i o l ~ g e n s . ~In ~ ~addition, ~' BiPy is reduced in two one-electron steps in pH 1.8 solutions. Although the second reduction peak was not observed in the CV due to the high current for reduction of the H+ion, the strong surface Raman spectrum of fully reduced BiPy was nevertheless observed (Figure 8D), and the bands in the spectrum are similar to type B bands. Therefore, type B bands are assigned to the fully reduced BiPy. As in the case of the type I1 spectrum of the BiPy radical, type B bands are different from the bands in the RR spectrum of fully reduced BiPy (Figure 8A). Thus, the fully reduced BiPy molecules may be adsorbed with a flat orientation on the electrode surface. Potential Dependence of SERS Intensities. The potential dependence of SERS intensities has been widely studied. The potential-intensity profile is usually bellshaped and exhibits a maximum a t a specific potential ( Vmar).38939V,,, often shifts with the excitation wavelength. This response was considered evidence for the charge-transfer mechanism of SERS.4*42 Furthermore, (37) Xu, J.; Birke, R. L.; Lomdardi, J. R. J.Am. Chem. SOC.1987,109, 5645. (38) Jeanmaire, D. L.; Van Duyne, R. P. J.Electroanal. Chem. 1977, 84,1. (39) Lombardi, J. R.; Birke, R. L.; Lu, T.; Xu, J. J. Chem. Phys. 1986, 84,4174. (40) Furtak, T. E.; Macomber, S. H. Chem. Phys. Lett. 1983,95,328.
o.20
t
0.00' -0.20
t
I
I
-0.40
-0.60
-0.10
potentla1 (V vs
I
-1.00
SCE)
Figure 9. Potential-intensity profiie (solid line) for the 1295-cm-l band in the BiPy SERS spectrum on a roughened Ag electrode. Dots show the best fit of the calculated curve obtained by using a line width parameter r = 0.3 eV. Excitation wavelength: 488 nm. Power: 50mW.
0.20 -0.20
-0.40
-0.00
-0.80
- 1.oo
Potential (V vs SCE)
Figure 10. Potential-intensity profiles for the 1295-cm-I band in the BiPy RS spectrum on a roughened Ag electrode obtained by using the following excitation wavelengths: (A) 488.0, (B) 514.5, (C) 572, (D) 581, (E) 593, (F) 605, (G) 647.1 nm.
it has been demonstrated that the potential-intensity profile can be fitted with the curve calculated from the charge-transfer theory of Lombardi et al.39 The solid line in Figure 9 shows the potential-intensity profile for the 1295-cm-' SERS band of BiPy on a roughened Ag electrode in a pH 6.4 solution using 488-nm excitation. The dotted line in Figure 9 was calculated by using the charge-transfer theory of Lombardi et al.39 It can be seen that the experimental profile is a good fit to the calculated curve for this excitation wavelength. However, it was also found that the shape of the experimentally obtained potential-intensity profile changes with excitation wavelength and no longer matches the calculated curve (Figure 10C-G). In addition, V,, does not shift with excitation wavelength as predicted. Also, when 647.1-nm excitation is used, the profile is not bell-shaped (Figure 10G). One possible explanation of the lack of agreement between theory and the experimental results at longer excitation wavelengths is self-absorption of the scattered light by adsorbed BiPy molecules. Following an ORC of the Ag electrode, a multilayer of BiPy molecules is adsorbed on (41) Billman, J.; Otto, A. Solid State C O ~ M U1982, ~ . 44,105. (42) Lombardi, J. R.; Birke, R. L.; Sanchez, L. A.; Bernard, I.; Sun, S. C. Chem. Phys. Lett. 1984, 104,240.
414 Langmuir, Vol. 5, No. 2, 1989
the surface of the roughened Ag e1ectrode.l' This layer may have a strong absorption peak near 700 nm, as in the case of the pyridine-Ag system.* Under these conditions, the scattered light from the first layer of adsorbed BiPy molecules may be absorbed by succeeding layers of BiPy molecules. When blue excitation is used, self-absorption is weak, and the potential-intensity profile is not affected. The profile can be fitted with the calculated curve. However, when red excitation is used, the absorption is so strong that V,, cannot be observed, and the profile no longer follows the calculated curve. Additional experimental evidence consistent with the self-absorption explanation has been obtained from concentration studies. When the concentration of BiPy in solution is lower than lo4 M, the SERS intensity increases with increasing concentration of bulk BiPy, whereas if the concentration is higher than M, the SERS intensity decreases with increasing concentration of bulk BiPy. In addition, the SERS intensity of BiPy on the roughened Ag electrode at -0.2 V is 5-10-fold higher after washing the cell with 0.1 M KC1 solution (which should remove all but the first monolayer) than before washing. Therefore, the phenomenon of self-absorption must be considered in surface Raman spectroscopy in cases where multilayer adsorption occurs.
Conclusions Electrochemistry. The electrochemical behavior of BiPy at Ag and glassy carbon electrodes was found to be similar to that reported previously for Hg electrodes. The cyclic voltammetry response is dependent upon both solution pH and BiPy concentration. At pH 3.5, protonated BiPy is reduced in two separate one-electron steps at -0.77 and -0.98 V. At higher pH values (>6.0), a single twoelectron reduction peak of the nonprotonated form of BiPy is observed. The peak potential is pH dependent and shifts from -1.25 V at pH 6.4 to -1.36 V at pH 12.0. The cyclic voltammetry response is for BiPy solution concentrations 5 M. However, highly concentrated solutions of BiPy (1X M) at pH 6.4 behave irreversibly, probably as a result of the formation of a blocking polymer film on the electrode surface. A difference in the CV response was found for polished vs roughened Ag electrodes at pH 6.4 in the presence of halide ions. Only one peak (-1.36 V) is observed on the former, whereas a sharp adsorption prewave (-1.23 V) is present in the cyclic voltammogram obtained by using the roughened electrode. Integration of the area under the adsorption peak indicated that this is a two-electron reduction. The adsorption prewave is not observed when sulfate is used as the counterion. Thus, coadsorption of the halide ion and BiPy appears to be necessary. Raman and Surface-Enhanced Raman Spectroscopy. The Raman and RR spectra of the three redox forms of BiPy were measured on a polished Ag electrode (43) Pettinger, B.; Wenning, U.;Kolb, D. M. Ber. Bunsen-Ges Phys. Chem. 1978, 82, 1326.
Lu et al.
as a function of pH. The greatest frequency shift occurs in the band near 1620 cm-' in the neutral form of BiPy, which shifts to higher frequencies (1650 cm-') in the protonated forms. The major change observed upon reduction of the BiPy is a shift of the interring C-C stretching mode to higher frequencies, as observed previou~ly.'~ The SERS spectrum of BiPy was also found to be sensitive to pH and counterion. The data are interpreted to indicate that there are three different modes of interaction between BiPy and the surface. These include the following: 1. Bonding between the Lone-Pair Electrons on One of the N Atoms of BiPy. This type of interaction is observed with weakly adsorbed electrolyte (SO:-) at pH values greater than 6 (Le., when bulky BiPy is not protonated). The bipyridine appears to be oriented vertically with respect to the electrode. The SERS spectrum is very similar to the NR spectrum under these conditions, except for small frequency shifts and relative intensity differences. 2. Ion-Pair Bonding between Protonated BiPy and Strongly Adsorbed Anions (Cl- when E > -0.7 V; Iwhen E > -0.8 V). The aromatic rings of bipyridine are probably oriented at an angle to the surface under these conditions. The spectra of the protonated neutral and radical are very similar to those reported previously for heptyl v i o l ~ g e n . ~ ~ 3. a-Bonding between the BiPy Rings and the Electrode Surface in the Presence of Weakly Adsorbed Anions (e.g., SO:-). Under these conditions, BiPy is adsorbed flat on the electrode surface. Large differences are observed in the SERS/SERRS spectra. The intensities of non-totally symmetric modes increase significantly relative to those of the symmetric modes. The strong C=C mode is downshifted by approximately 40 cm-l as compared to the value observed for the other two types of surface-bonding interactions. The spectra of the three redox forms are quite similar to those reported for a-bonded heptyl vi~logen.*~ What do these results imply about the possible role of BiPy in facilitating electron-transfer reactions in proteins? If the orientation of BiPy is crucial in its role as an electron-transfer promoter, it is likely that counterions should play a significant role in its effectiveness. Further experiments are now underway to test the role of counterions on the reversibility of cytochrome c reactions at bipyridine-modified electrodes.
Acknowledgment. We are indebted to the Department of Energy (DE-FG02-84ER13261, T.M.C.), the National Science Foundation (DMB-8509594, T.M.C.; RTT8305241, R.L.B. and J.R.L.), the PSC-BHE Research Award Program (RF-664197,R.L.B.) of the City University of New York, and the National Institutes of Health MBRS program (RR-08168 R.L.B. and J.R.L.) for financial assistance. Registry No. BiPy, 37275-48-2; BiPyH2*+,46040-54-4; BiPyH2'+, 35862-62-5; BiPyH,, 64429-05-6; BiPy'-, 34475-11-1; Ag, 7440-22-4.
