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Surface-Enhanced Raman Spectroscopic Studies of Dissociative Adsorption of Amino Acids on Platinum and Gold Electrodes in Alkaline Solutions Xiao-Yin Xiao,†,‡ Shi-Gang Sun,*,† Jian-Lin Yao,† Qi-Hui Wu,† and Zhong-Qun Tian† State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen, 361005, People’s Republic of China, and Xiangtan Normal University, Xiangtan, 41110, People’s Republic of China Received April 9, 2002. In Final Form: May 23, 2002 The dissociative adsorption of amino acids on Pt and Au electrodes in 0.1 M NaOH solutions was studied by cyclic voltammetry and surface-enhanced Raman spectroscopy (SERS). The intermediate species has been determined as adsorbed cyanide, which is designated by a potential-dependent vibration band around 2110 cm-1 on both Pt and Au surfaces. The dissociation of glycine can be observed on Pt surface in a wide potential region to form cyanide, while the dissociation of serine and threonine occurs at relatively high potentials along with the oxidation of their functional groups. The onset potential of dissociation of amino acids on the Pt surface increases in the order glycine < threonine < serine. It has been revealed that the self-inhibition of amino acid oxidation is originated from the strongly adsorbed cyanide, which is oxidized at potentials above 0.2 V vs SCE. On gold surfaces, cyanide species can be formed only from anodic oxidation of amino acids. The present study reveals characteristic interactions between amino acid molecules and metallic electrode surfaces, as well as the role of amine group in the adsorption configuration.
Introduction The interaction of small organic molecules with electrode surface is a key step involved in electrocatalytic processes and has received extensive attention in studies concerning direct fuel cells.1 The most frequently employed electrocatalysts in direct fuel cells are platinum group metals and alloys.2 Infrared spectroscopy has played an important role in investigating the interaction of small organic molecules with Pt-based electrocatalysts,3-5 while Raman spectroscopy has less often been applied to such studies.6,7 This is because (1) highly reliable sensitivity of Raman spectral signal could be obtained only on roughened surfaces of coinage metals up to the end of the last century, and (2) low enhancement factors were found up to now only on rough platinum surfaces.8 However, surfaceenhanced Raman spectroscopy (SERS) is going forward with its capability of delivering specific chemical identification to more and more applications related to catalytic processes.6-8 Recent studies revealed the possibility of extending SERS on platinum surfaces to identifying molecules in the submonolayer range. Intermediate * Corresponding author: Tel +86 592 2180181; fax +86 592 2183047; e-mail
[email protected]. † Xiamen University. ‡ Xiangtan Normal University. (1) Sun, S. G. In Electrocatalysis; Frontiers of Electrochemistry Series, Vol. 4; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; Chapt. 6, pp 243-290. (2) Carrette, L.; Friedrich, K. A.; Stimming, U. Chem. Phys. Chem. 2000, 1, 162-193. (3) Wasmus, S.; Kuver, A. J. Electroanal. Chem. 1999, 461, 14-31. (4) Lin, W. F.; Wang, J. T.; Savinell, R. F. J. Electrochem. Soc. 1997, 144, 1917-1922. (5) Iwasita, T.; Hoster, H.; John-Anacker, A.; Lin, W. F.; Vielstich, W. Langmuir 2000, 16, 522-529. (6) Tian, Z. Q.; Gao, J. S.; Li, X. Q.; Ren, B.; Huang, Q. J.; Cai, W. B.; Liu, F. M.; Mao, B. W. J. Raman Spectrosc. 1998, 29, 703-711. (7) Mrozek, M. F.; Weaver, M. J. J. Am. Chem. Soc. 2000, 122, 150155. (8) Cai, W. B.; Ren, B.; Li, X. Q.; She, C. X.; Liu, F. M.; Cai, X. W.; Tian, Z. Q. Surf. Sci. 1998, 406, 9-22.
adsorbates from adsorption of small organic molecules, for example, methanol and ethylene, have been successfully examined by SERS.9-11 Amino acids contain different functional groups (-COOH, -OH, -NH2, -CHx, etc.) and provide the convenience to investigate the interaction of these functional groups with electrocatalytic surfaces. Their properties, and their adsorption characteristics as well, depend strongly on the acidity (pH value) of electrolyte.12,13 The weakly adsorbed species in acid media were examined as deprotonated amino acid molecules,14-17 of which the adsorption behavior was similar to those of their relative aliphatic acids (e.g., acetic acid).18,19 However, in alkaline media, Ogura et al.20-22 proposed the same weak adsorption model on Pt surface as that in acid media, which was in disagree(9) Ren, B.; Li, X. Q.; She, C. X.; Wu, D. Y.; Tian, Z. Q. Electrochim. Acta 2000, 46, 193-205. (10) Gu, R. A.; Cao, W. D.; Cao, P. G.; Sun, Y. H.; Yao, J. L.; Ren, B.; Tian, Z. Q. Acta Chim. Sin. 2001, 59, 356-359. (11) Mrozek, M. F.; Weaver, M. J. J. Phys. Chem. B 2001, 105, 89318937. (12) Horanyi, G.; Rizmayer, E. M. J. Electroanal. Chem. Interfacial Electrochem. 1986, 393-400. (13) Xiao, X. Y.; Sun, S. G.; Wu, Q. H.; Zhou, Z. Y.; Chen, S. P. Chem. J. Chin. University-Chin. 2000, 21, 1288-1292. (14) Huerta, F.; Morallon, E.; Cases, F.; Rodes, A.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1997, 421, 179-185. (15) Huerta, F.; Morallon, E.; Cases, F.; Rodes, A.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1997, 431, 269-275. (16) Huerta, F.; Morallon, E.; Vazquez, J. L.; Perez, J. M.; Aldaz, A. J. Electroanal. Chem. 1998, 445, 155-164. (17) Huerta, F.; Morallon, E.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1999, 475, 38-45. (18) Rodes, A.; Pastor, E.; Iwasita, T. J. Electroanal. Chem. 1994, 376, 109. (19) Pastor, E.; Rodes, A.; Iwasita, T. J. Electroanal. Chem. 1996, 404, 61. (20) Ogura, K.; Kobayashi, M.; Nakayama, M.; Miho, Y. J. Electroanal. Chem. 1998, 449, 101-109. (21) Ogura, K.; Kobayashi, M.; Nakayama, M.; Miho, Y. J. Electroanal. Chem. 1999, 463, 218-223. (22) Ogura, K.; Nakayama, M.; Nakaoka, K.; Nishihata, Y. J. Electroanal. Chem. 2000, 482, 32-39.
10.1021/la025817f CCC: $22.00 © 2002 American Chemical Society Published on Web 07/11/2002
Adsorption of Amino Acids on Pt and Au Electrodes
Figure 1. Cyclic voltammograms of Pt electrode in 0.1 M NaOH (a) and 0.1 M NaOH + 5 mM glycine (b, c) solutions: (b) the first two cycles; (c) the steady profile. Sweep rate: 50 mV/s.
ment with the model suggested by Horanyi et al.12 In the present work, we studied the adsorption and oxidation of amino acids with different configuration of R-group, such as glycine [CH2(NH2)COOH], serine [CH2(OH)CH(NH2)COOH], and threonine [CH3CH(OH)CH(NH2)COOH] on platinum and gold electrode surfaces by SERS. The molecular and surface structural effects in the dissociative adsorption were investigated in a wide potential range from hydrogen adsorption potential region up to oxygen evolution. Experimental Section Raman spectra were obtained on a confocal microprobe Raman system (LabRam I), with a holographic notch filter and a CCD detector. The excitation line was 632.8 nm from an air-cooled He-Ne laser that delivers a power of 10 mW at the sample point. A more detailed description of the spectroelectrochemical measurement was given elsewhere.8 Solutions were prepared from Millipore water and deaerated by nitrogen gas before immersion of the working electrodes. Amino acids of biochemical grade and sodium hydrate of analytical grade were used as received. The reference electrode was a saturated calomel electrode (SCE). All experiments were performed at room temperature. The working electrode of Au or Pt was a rod with a geometric surface area of 0.1 cm2 embedded in a Teflon sheath. The Au electrode used for SERS measurement was roughened by a procedure similar to that reported by Weaver and co-workers.23 The Pt electrode was roughened with a method reported by Tian and co-workers,8 e.g., the Pt electrode was first mechanically polished with 0.3 and 0.05 µm alumina powder to a mirror finish, ultrasonically cleaned with triply distilled water, and then roughened in 0.5 M H2SO4 for 5-10 min by applying a square wave of 1.5 kHz with upper and lower potentials of 2.4 and -0.2 V. The electrode was then subjected to potential cycles between -0.25 and 1.2 V at a scan rate of 0.5 V/s until all unstable atoms or clusters were removed and reproducible hydrogen adsorptiondesorption peaks were obtained. Finally, the electrode was rinsed thoroughly and transferred to the spectroelectrochemical cell for measurement. The electrodes in cyclic voltammetric studies are Pt and Au beads, which were flame-annealed before immersion in electrolyte at a controlled potential.
Results Cyclic Voltammetry of Amino Acids on Pt Electrode. Figure 1a shows the cyclic voltammogram of a Pt electrode in 0.1 M NaOH solution. Two pairs of oxidation/ reduction peaks at around -0.72 and -0.58 V were observed and ascribed to the adsorption/desorption of hydrogen. The current that is increased above -0.30 V is (23) Gao, P.; Gosztola, D.; Weaver, M. J. J. Phys. Chem. 1988, 92, 7122.
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ascribed to the adsorption of oxygen. Figure 1b shows the voltammograms of a Pt electrode in 0.1 M NaOH solution containing 5 mM glycine. The electrode was immersed in the solution at -0.8 V, and the electrode potential was scanned at first in the negative direction going to -0.92 V and then swept positively up to 0.0 V. Three anodic peaks appear at, approximately, -0.74, -0.64, and -0.30 V in the first anodic sweep. The first two peaks correspond to hydrogen desorption with peak potentials slightly negative-shifted. The peak at -0.30 V is ascribed to oxidation of adsorbed glycine, because the adsorption of oxygen is inhibited, and is indicated by the disappearance of the reduction current of oxygenated species in the reverse potential sweep. These anodic currents decrease rapidly in the subsequent potential cycles between -0.92 and 0.0 V, and finally, the voltammogram reaches a steady profile shown in Figure 1c, in which the hydrogen adsorption and glycine oxidation are close to fully inhibited. It is evident that some poisonous species are left at the surface and block all surface reactions involved in this potential region. It has confirmed also that the oxidation of the poisonous species occurs at potentials above 0.2 V. To see the dependence of the formation of poisonous species on electrode potential, three testing experiments were conducted. (1) Effect of immersion potentials: The results show that the voltammetric features in the first potential cycle are strongly influenced by the immersion potentials. The adsorption charge of hydrogen and the oxidation charge of glycine decrease with the increase of the immersion potential. At immersion potentials more positive than -0.4 V, only one hydrogen desorption peak can be observed. (2) Effect of potential cycling in the hydrogen region: It was found that the hydrogen adsorption charge decreased with the increase of the number of potential cycling between -0.95 and -0.6 V, i.e., along with potential cycling in this potential region, more and more dissociative adsorbates were formed and accumulated on electrode surface. (3) Effect of adsorption in 0.1 M NaOH + 5 mM glycine solution at open circuit: In this case, either the adsorption state was checked in the same solution containing 5 mM glycine or the electrode was transferred into 0.1 M NaOH solution after the adsorption. It was found that, in both solutions for an adsorption at open circuit for about 3 min, the hydrogen adsorption is largely inhibited in comparison with a freshly prepared surface, indicating that the dissociative adsorption can take place spontaneously. These results illustrated that the poisonous species can be formed spontaneously or in the potential region of hydrogen adsorption and that the kinetics of the formation of poisonous species depends on electrode potential applied. The same measurements were also performed at Pt surfaces in 0.1 M NaOH solutions containing threonine or serine. The oxidation of threonine and serine in the first potential cycle gives rise to a large current peak at around -0.6 V. The larger oxidation current in both cases may be attributed to the bulk oxidation of their functional groups (CHOH), since we cannot observe such a large oxidation current in the case of glycine. A large decrease of the oxidation current is observed in the subsequent potential sweep. The results show that the hydrogen adsorption is strongly inhibited in the first potential cycle, and further blockage can be obtained by potential cycling between -0.95 and 0.0 V. The steady cyclic voltammetric profile of threonine and serine is similar to the one described in Figure 1c. As an example, the voltammogram of the first two potential cycles of threonine at a Pt surface is shown in
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Figure 2. Voltammograms of the first two anodic sweeps of threonine at Pt surface in 0.1 M NaOH + 20 mM threonine solution. Einitial, -0.90 V; sweep rate, 100 mV/s.
Figure 3. Steady cyclic voltammetric profiles at Pt surfaces. Electrolyte: (a) 0.1 M NaOH; (b) 0.1 M NaOH + 20 mM glycine; (c) 0.1 M NaOH + 20 mM serine; (d) 0.1 M NaOH + 20 mM threonine. Sweep rate: 100 mV/s.
Figure 2. In the first anodic potential sweep, the large oxidation current starts to appear at -0.7 V, which can be ascribed to the oxidation of the secondary alcohol in threonine to the corresponding carbonyl compounds. This reaction may be immediately followed by further dissociation, yielding poisonous species adsorbed at the surface. This is indicated by a peak shoulder in the first sweep and the large decrease of the oxidation current in the second potential sweep. Further gradual decrease of the hydrogen adsorption and the threonine oxidation occurs in the subsequent potential sweeps. However, when the upper potential limit was increased to 0.60 V, the surface blockage was not as large and fast as that described above. The steady CV profiles could be reached in about three or four cycles. Figure 3 shows their steady profiles at Pt surfaces with the upper potential limit of 0.60 V. Figure 3b is in agreement with that reported by Marangoni et al.24 It is shown that both the hydrogen and the oxygen region are modified upon the introduction of amino acids; i.e., hydrogen adsorption is partially inhibited, and oxygen adsorption is shifted to the positive potential direction with an additional oxidation current above 0.2 V. It is important to note that, in each case, hydrogen adsorption current and amino acid oxidation current can be observed in these steady profiles, which is different from that observed in Figure 1c. This implies that the poisonous species would be partially oxidized at more positive potentials. Cyclic Voltammetry of Glycine on an Au Electrode. Figure 4b shows voltammograms of Au electrode in 0.1 M NaOH + 5 mM glycine solution. The electrode
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Figure 4. Cyclic voltammograms of a poly-Au surface in 0.1 M NaOH (a) and 0.1 M NaOH + 5 mM glycine (b) solutions. Sweep rate: 50 mV/s.
was immersed in the solution at -0.6 V and the polarization program started in the negative potential direction. Upon going positively at a scan rate of 50 mV/s, no characteristic current can be observed up to 0.1 V. The first anodic current peak appears at 0.22 V. The second one appears at 0.45 V, which is 200 mV more positive than that for oxidation of the Au surface when glycine is absent in the solution (cf. Figure 4a). In the reverse potential sweep, the broad reduction peak at 0.12 V is consistent with that of reduction of oxygenated Au species. Therefore, the large oxidation current observed in the presence of glycine can be attributed to the oxidation of glycine. In the subsequent potential sweep, the first peak current decreases, while the second one increases, and the total reduction of oxygenated species in the reverse potential sweep is nearly independent of the number of the potential cycling. If the upper potential limit was set at 0.30 V, the first peak could be decreased completely to the baseline in about eight cycles, indicating that the surface was fully blocked. The results demonstrate that, on one hand, the formation of adsorbed poisonous species occurred after the oxidation of glycine and oxidative desorption of them occurred at potentials above 0.3 V, and on the other hand, the observed two oxidation peaks in Figure 4b can be ascribed respectively to the oxidation of glycine (the first peak) and the oxidation of Au surface, which is superimposed with oxidation of glycine and its intermediate oxidation products. It is necessary to mention that potential cycling between -0.8 and 0.0 V did not make much difference on the observed first voltammetric profile shown in Figure 4b. This may indicates that no characteristic reactions take place in this potential region. SERS on Pt Electrodes. Figure 5 illustrates the timedependent Raman spectra recorded on a roughened Pt electrode in glycine-containing solution together with the spectrum obtained in pure NaOH solution at -0.8 V. The Pt-H vibration yields a rather weak and broad band at around 2030 cm-1 in pure NaOH solution as shown in Figure 5a.25 The intensity of this peak did not change with time, unless glycine was afterward added into the solution at the same potential. After glycine was added, a narrow and strong peak appeared at 2040 cm-1 (spectrum b1 in Figure 5). Then the band frequency and intensity gradually increased with time (spectrum b2 in Figure 5). It is well-known that the Raman band located at the frequency region around 2100 cm-1 can only be assigned to Pt-H, CO, and CN vibration mode. In the present case, (24) Marangoni, D. G.; Smith, R. S.; Roscoe, S. G. Can. J. Chem. 1989, 67, 921. (25) Ren, B.; Xu, X.; Li, X. Q.; Cai, W. B.; Tian, Z. Q. Surf. Sci. 1999, 428, 157-161.
Adsorption of Amino Acids on Pt and Au Electrodes
Figure 5. In situ SERS spectra obtained on the Pt electrode at -0.8 V in 0.1 M NaOH (a) and 0.1 M NaOH + 5 mM glycine (b1, b2) solutions, respectively. Time interval: 100 s.
Figure 6. In situ SERS spectra of Pt electrode in 0.1 M NaOH + 5 mM glycine solution at the indicated applied potentials. Time interval: 100 s.
this band can be therefore attributed to the formation of cyanide on the Pt surface,26 because this band can be consistently observed at potentials up to 0.6 V, while CO adsorption at the Pt surface is not stable when the potential is more positive than -0.5 V in alkaline solutions.27,28 The slow increase in the intensity indicates the dissociative adsorption of glycine is a slow surface process at -0.8 V. Another evidence is the increase in frequency with time, which may be due to the increase of dipole-dipole coupling as the coverage of the cyanide increases. The band intensity increases much faster when the immersion potential is set positively, especially at potentials around -0.2 V, which is in an agreement with the results of cyclic voltammetry. Figure 6 shows further changes in the spectrum when the applied potential was increased positively. The band intensity seems to increase to its maximum when the potential was increased from -0.8 to -0.6 V and keeps almost constant at potentials below 0.0 V. Therefore, a maximum coverage of adsorbed cyanide has been achieved at -0.6 V. With the potential further going positively above 0.0 V, the band becomes broad and the intensity starts to decrease and then vanishes at 0.6 V. The decrease in intensity in the potential region above 0.2 V is due to either the further oxidation of cyanide to form solution (26) Ren, B.; Li, X. Q.; Wu, D. Y.; Yao, J. L.; Xie, Y.; Tian, Z. Q. Chem. Phys. Lett. 2000, 322, 561-566. (27) Santos, E.; Giordano, M. C. J. Electroanal. Chem. Interfacial Electrochem. 1984, 201-210. (28) Sun, S. G.; Chen, A. C. J. Electroanal. Chem. 1992, 323, 319328.
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Figure 7. In situ SERS spectra of the Pt electrode in 0.1 M NaOH + 20 mM threonine solution at the indicated applied potentials. Einitial, -0.90 V; time interval, 100 s.
species, i.e., cyanate (OCN-) and carbonate (CO32-), or the oxidation of the Pt surface, which led to the deactivation of Pt surface to Raman sensitivity. But here we mostly attribute such a decrease of intensity to the decrease of the coverage of cyanide, because it is consistent with our previous Fourier transform infrared (FTIR) results of the formation of OCN- at potentials above 0.2 V, in which a band assigned to OCN- at 2168 cm-1 has been detected.13 The band frequency shifts from 2042 cm-1 at -0.8 V to 2123 cm-1 at 0.2 V, which is considered to be mainly due to the electrochemical Stark effect for adsorbed surface species.26,29 Raman spectra obtained on the Pt electrode in threonine-containing solution are shown in Figure 7. To clearly observe the dissociation process of threonine, the immersion potential was set at -0.9 V when threonine was added into 0.1 M NaOH solution and then increased by every 50 mV. No characteristic Raman band between 1600 and 2600 cm-1 can be observed at potentials below -0.70 V. The formation of cyanide starts at -0.70 V, which is the onset potential of threonine oxidation, indicating that the dissociation of threonine starts as soon as the oxidation process takes place. It may be inferred that dissociation of the oxidation products is easier than that of threonine itself. The increase of the intensity shown in Figure 7 indicates a slow increase of the coverage of adsorbed cyanide in the potential region below -0.5 V. However, such a process has already accompanied by a large oxidation current as observed in the cyclic voltammograms. The onset potential for cyanide formation in serinecontaining solution was measured at -0.6 V, also at the potential when its bulk oxidation starts. It is clear that dissociation of glycine takes place in a wide potential region including the hydrogen potential range, while the dissociation of other amino acids occurs at more positive potentials, probably only at potentials when oxidation processes take place. Cyanide formed as an intermediate is strongly adsorbed at Pt surface below 0.2 V, and then blocks the surface reactions as a poison. At potentials above 0.2 V, further cyanide oxidation releases surface sites for hydrogen adsorption, amino acid oxidation, and oxygen adsorption as well. SERS on Au Electrodes. Figure 8 shows the SERS on an Au electrode in the glycine-containing solution. There is no characteristic Raman peak between 1600 and 2600 cm-1 at potentials below 0.0 V. A peak at 2120∼2135 cm-1 (29) Zou, S. Z.; Weaver, M. J. J. Phys. Chem. 1996, 100, 4237-4242.
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Figure 9. Proposed adsorption models of amino acid molecule on metallic surface. The crosses in the figure (×) indicate the bond broken in the dissociative reactions. (See text for details.)
Figure 8. In situ SERS spectra of the Au electrode in 0.1 M NaOH + 5 mM glycine solution at the indicated applied potentials. Einitial, -0.80 V; time interval, 20 s.
appeared at potentials above 0.1 V, and its intensity increased when potential was set at 0.2 V for more than 20 s. Obviously, this potential-dependent vibration can be also assigned to the adsorbed cyanide derived from the oxidation of glycine. The measurements performed in a serine- or threoninecontaining alkaline solution also show that the formation of cyanide occurs along with a large amount of oxidation current being observed. Discussion The reactivity of the dissociation of amino acids to form cyanide is dependent on their molecular structures: glycine is more easily dissociated to form cyanide, while the dissociation of serine and threonine occurs along with the oxidation of their -CHOH group. At potentials above 0.2 V, the amino acid oxidation proceeds rather smoothly because the electrode surface is partially released by further oxidation of cyanide to solution species OCN- and CO32-. Meanwhile, the dissociation takes place more easily at a Pt than at an Au surface. At a Pt surface, adsorbed cyanide can be detected in a wide potential region including hydrogen adsorption region, whereas a more positive potential is needed for the formation of a cyanide adlayer at an Au electrode. This is evidence for a stronger interaction between amino acid molecules and a Pt surface. The inhibition of hydrogen adsorption in the first potential cycle in the presence of amino acids is probably due to (1) the strongly adsorbed cyanide formed from the dissociation of amino acids and (2) the chemisorption of the amino acid molecules. Strong chemisorption can then lead to the dissociation of these molecules. However, it is shown in SERS that the dissociative adsorption of these amino acid molecules to cyanide is a rather slow surface process, especially in the cases of threonine and serine; the strong inhibition of the hydrogen adsorption therefore contributes to the chemisorption of amino acid molecules itself. We did not observe such a strong inhibition of hydrogen adsorption in acidic solutions, where we can clearly observe two reversible hydrogen adsorption peaks with slight shifts of the peak potentials in the case of glycine. Also, in aliphatic carboxyl compounds containing alkaline solutions, for example, acetic acid solution, such a strong inhibition is also not visible.18,19 The above comparison indicates that the strong inhibition of hydrogen is mostly due to the presence of the neutral amine group in amino acid molecules in alkaline solutions. Figure 9 shows the possible adsorption models of these simple amino acid molecules on metallic electrode surfaces. Figure 9a is the adsorption model proposed for acidic solutions in the literature.14-17 It was also suggested for
alkaline solutions by Ogura et al.,20-22 based on the FTIR measurement. In their reports they described an IR shift around 1390 cm-1 as evidence for the original bonding between amino acid molecules and the Pt surface through a carboxyl group. However, it is problematic that this band is always overlapped with that of carbonate ions and aliphatic acid (e.g., acetic acid ions), which are formed in the dissociative oxidation at positive potentials; and on the other hand, such a bond would not strongly influence the hydrogen adsorption as described above. Figure 9b is the one suggested by Horanyi and Rizmayer12 on the basis of the irreversible adsorption behavior observed by radiotrace measurements. It is assumed that these amino acids are chemically adsorbed on the Pt surface by anchorage of the molecules to the surface by both the nitrogen and the adjacent carbon atom. It is reasonable in this model that the different configuration of the R-group would strongly influence the bond to the Pt surface. Glycine is the simplest amino acid and the one adsorbed most strongly at the Pt surface. In contrast, serine and threonine contain rather big functional groups, which leads to a weaker bond to the Pt surface. According to the current results, we might propose another adsorption model shown in Figure 9c. The amino acid molecule is adsorbed on the metal surface through both one of its carboxyl bonds and the amine group. In this model, the role of the amine group has been taken into account, and it fits to the coordination chemistry, considering the amino acid molecule as a biligand.30 Among these possible adsorption configurations, model a is favorable in strong acidic solutions due to the positively charged amine group. It was found that such a bond becomes weak at negative potentials and therefore only slightly influences the hydrogen adsorption.14-17 In the strong alkaline solutions, the hydrogen adsorption is strongly inhibited, depending on both the immersion potentials and the adsorption time, which is an indication of a strong bond of the molecules to the surface. Such a strong bond then leads to the dissociative adsorption, which is accelerated by increasing the electrode potential or positive charge density of the surface. This behavior is mainly a consequence of the neutral amine group involved in the adsorption configurations. It is our intention to distinguish model b from c by SERS, relying on the lowfrequency Pt-C, Pt-N, and Pt-O vibrations. From our results, it might be evident that no distinct vibration band between 200 and 600 cm-1 was observed at -0.8 V and even at potentials below -0.2 V, an indication of no direct interaction between carbon and Pt atoms.26 The band around 400 cm-1 could be observed only at more positive potentials, where a large amount of cyanide signal has been detected. This band is therefore ascribed to the interaction between Pt and the adsorbed cyanide. A strong spectroscopic identification should be obtained at the (30) Zou, J.; Guo, Z. J.; Parkinson, J. A.; Chen, Y.; Sadler, P. J. Chem. Commun. 1999, 1359-1360.
Adsorption of Amino Acids on Pt and Au Electrodes
beginning of immersion and at negative potentials for the sake of the formation of cyanide in the dissociative adsorption. Since serine and threonine have relatively larger space resistance than glycine due to the presence of an R-group instead of hydrogen, the adsorption of them is weaker, and then the dissociation takes place at potentials more positive than in the case of glycine. However, these molecules are getting easier to be dissociated due to the oxidation of their >CHOH group to a >CdO group, because on one hand, changing from 4-fold to 3-fold bonding decreases their space resistances; and on the other hand, the bond from the molecule to the surface becomes stronger as the potential is increased positively. The formation of the dissociative products such as cyanide/ cyanate and carbonate implies the cleavage of the C-C bond but not the C-N bond. The possible position of the molecular breakage in the dissociative reactions is also shown in Figure 9.
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Summary The adsorption and oxidation of the amino acids were studied by cyclic voltammetry and surface-enhanced Raman spectroscopy. The hydrogen adsorption is strongly inhibited by the adsorption of amino acid molecules. With the increase of the electrode potential, the adsorption bond becomes stronger and leads to the dissociation of these adsorbed molecules. The dissociation of glycine starts in the hydrogen potential region, while that of serine and threonine molecules starts along with the oxidation of their CHOH hydroxyl groups. The dissociative adsorption leads to another adsorbed species, cyanide. The strong chemisorption model has been discussed by taking the role of the amine group into account. Acknowledgment. Financial support from grants of Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC) is gratefully acknowledged. LA025817F
