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Adsorption behaviour of DNA bases at the Au(111) electrode |
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PhysChemComm,
Volume 5,
Issue 22,
2002,
Page 151-157
A. P. M. Camargo,
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摘要:
Paper Adsorption behaviour of DNA bases at the Au(111) electrode{ A. P. M. Camargo, H. Baumga�rtel and C. Donner* Free University of Berlin, Institute of Chemistry/Physical and Theoretical Chemistry, Takustr. 3, D-14195 Berlin, Germany. E-mail: donner@chemie.fu-berlin.de Received 20th August 2002, Accepted 27th September 2002 First published as an Advance Article on the web 15th October 2002 We report on the adsorption of adenine as well as on the coadsorption of adenine/thymine and uracil/thymine on Au(111). Adenine is chemisorbed in two different states. Mutual interaction between adenine and thymine could be detected only at negative potentials where both molecules are oriented with their plane parallel to the surface. This interaction depends on the concentration of thymine, the pH value, the temperature and the roughness of the surface.At positive potentials where thymine/adenine are oriented perpendicular to the electrode surface, from our experiments no hints to their interaction could be found. Thymine prevents the uracil adsorption and no cocondensation signal between the noncomplementary bases thymine/uracil was found. It is well known that the DNA bases undergo a twodimensional first-order phase transition on mercury and single crystal electrodes, forming well ordered condensed monolayers. This was the subject of many studies in recent years.1–6 Besides the application of these substances as corrosion inhibitors and electroplating brighteners, the DNA bases are of special interest because of their role as constituents of the nucleic acid.For thymine on gold electrodes, depending on the potential, two different, well-ordered adsorption layers can be recognized (Fig. 1a). Negative of the PZC (potential of zero charge), a first order phase transition takes place in which randomly adsorbed thymine molecules (I) form an ordered, physisorbed condensed state (II). The plane of thymine molecules is oriented nearly parallel to the electrode surface as was confirmed by STM and SNIFTIRS experiments.4,7 In the potential region positive of the PZC the physisorbed monolayer is transformed into a chemisorbed adsorption state of thymine (IV). According to in-situ STM experiments, the plane of thymine is oriented perpendicular to the electrode surface. Thymine molecules form stacks in this state with water molecules between the stacks.7,8 The formation of the chemisorbed phase is a complex process (III) which takes place simultaneously with the lifting of the reconstruction of the surface.Consequently, adsorption of thymine at the Au(111) electrode leads to three different adsorption states in different potential ranges: randomly adsorbed molecules (I) the physisorbed condensed monolayer (II) and the chemisorbed stacked monolayer (IV). Uracil adsorption on Au(111) showed different adsorption states very similar to thymine (Fig. 7a, b) i.e., a noncondensed phase at a very negative potential (I) is followed by a twodimensional physisorbed film (II). Phase (III) reflects the transition from the physisorbed into the chemisorbed phase. During the transition the molecules undergo a reorientation connected with a partial charge transfer.At very positive potentials a chemisorbed phase is formed (IV). In-situ STM measurements on Au(111) and Au(100) for the physisorbed films confirm that uracil forms a two-dimensional network of planar oriented molecules stabilized by hydrogen bonding independently of the substrate geometry. At sufficiently positive potentials the uracil molecules form a surface coordination complex resulting in a highly organized two-dimensional structure.9 The configuration and hydrogen bonding observed within the molecules were very similar to those found in the 3D crystal.10 {This paper was originally presented as a poster at the Faraday Discussion 121 meeting. Fig. 1 Cyclic voltammograms of 20 mM thymine (1a) and 2 mM adenine (1b) in 0.1 M NaClO4, pH ~ 2, T ~ 20 uC, scan rate 50 mV s21.T1/T0 1: formation and dissolution of the ordered physisorbed thymine monolayer; T2/T0 2 dissolution and formation of the ordered physisorbed thymine monolayer; T3/T0 3 formation and dissolution of the ordered chemisorbed thymine monolayer; A0 1/A0 2 and A1/A2: formation and dissolution of the charge-transfer complex between the p*-orbital of adenine and d orbitals of the gold surface; A3/A0 3 oxidation of gold in the Au–adenine complex. DOI: 10.1039/b208139c PhysChemComm, 2002, 5(22), 151–157 151 This journal is # The Royal Society of Chemistry 2002Sowerby and Petersen11 investigated the structure of 2D thymine and uracil adsorbates on MoS2 and HOPG (highly oriented pyrolytic graphite) electrodes. It was suggested that on both surfaces the two-dimensional hydrogen bonding networks are almost identical to the intermolecular configurations observed in their respective three-dimensional crystals.Up to now, the coadsorption and cocondensation behaviour of the complementary base pair thymine/adenine and noncomplementary base pair thymine/uracil was investigated under electrochemical conditions only at the mercury electrode. 12,13 Surprisingly, on mercury coadsorption leads to a destabilization of the condensed monolayers. For a critical adenine/thymine ratio and thymine/uracil ratio, respectively, the condensation region disappears completely. This means that the interaction between the base pairs in the physisorbed condensedmonolayer is less attractive or even repulsive compared with that between the unmixed bases.In the present paper the interaction between adenine/ thymine and uracil/thymine at the Au(111) electrode is reported. The paper is organized as follows: the first section contains the experimental details and the second section reports on our experimental results, including the adsorption behaviour of adenine at Au(111), which has not been studied so far. Finally, the results obtained in this work are discussed in context with relevant results reported in the literature. Experimental The measurements were performed using the Autolab Potentiostat PGSTAT 12. All data were recorded and stored in a computer. The working electrode was an Au(111) single crystal with a diameter of 5 mm.It was flame-annealed and cooled in argon atmosphere before each measurement. The electrode potentials mentioned are quoted vs. Ag/Ag1. A salt bridge filled with 0.1 M HClO4 separated the reference electrode from the cell. The salt bridge consists of Duran glass with a platinum wire molten at the tip in order to facilitate the contact between the solutions by a ring crevice between the glass and the platinum. Leaking of perchloric acid into the cell is therefore negligibly small, so that there was no change of the pH during the experiments. A gold wire was used as a counter electrode. The water was triply distilled. The supporting electrolyte NaClO4 p.A. (Merck), uracil (Fluka, purity ¢ 99%) as well as the thymine (Merck, purity w 99%) and the adenine (Aldrich, purity w 99.5%) were used as received.The cell was purged with argon before each measurement for at least 30 min and kept at constant temperature (¡0.1 uC) by a Julabo F-30 HC cryostat. Results Adenine on Au(111) The adsorption behaviour of adenine on Au(111) differs from that of thymine on Au(111) as well as that of adenine on mercury electrodes (Fig. 1a, b). Adenine adsorption on Au(111) causes three peak pairs, A1/ A0 1 A2/A0 2 and A3/A0 3 (Fig. 1b). In contrast to thymine, where the peak pairs T1/T0 1 and T2/T0 2 define the stability region for the physisorbed condensed monolayer, no corresponding needle peaks are observed in the adenine system. The position of adenine peak pairs (A1/A0 1 and A2/A0 2) depends only slightly on the bulk adenine concentration in a range between 2 and 0.08 mM.This behaviour indicates that the surface obviously is completely covered with adenine at potentials positive and negative of these peak pairs. The position of the peak pair A3/A0 3 remains unchanged by increasing the adenine concentration but their intensity becomes enhanced. Coadsorption of adenine and thymine on Au(111) As it is known from the adsorption behaviour on mercury electrodes adine is adsorbed much stronger than thymine. Therefore, we started our experiments at the Au(111) electrode at bulk concentrations of adenine (2 mM) and thymine (20 mM) in an acidic electrolyte mixture (pH ~ 2). For this concentration ratio and pH value the chemisorption of pure adenine indicated by A1/A0 1 and A2/A0 2, takes place in a potential region where pure thymine is physisorbed (Fig.1). In Fig. 2 the corresponding adsorption behaviour for the mixture is shown. The needle peaks T1/T0 1 indicating the firstorder phase transition of thymine in the physisorbed phase could not be obtained. Instead of the well resolved peaks T2/T0 2 and T3/T0 3 of pure thymine around 20.214 V (Fig. 1a), a broad signal appears (T2/3/T0 2=3) (Fig. 2). In contrast to the chemisorption peaks of adenine (A1/A0 1 and A2/A0 2) observed in a pure adenine solution (Fig. 1b), their shapes are changed strongly in the mixture (AT1/AT0 1 and AT2/AT0 2). In Fig. 2a it can be clearly distinguished between two well separated peaks in both directions, whereas in Fig. 1b these peaks are hardly resolved, but their position remains nearly unchanged in comparison to A1/A0 1 and A2/A0 2.The shape of these peaks is influenced by the concentration ratio of adenine/thymine, by the temperature and the pH value of the solution. Starting from an adenine/thymine ratio of 3 mM/20 mM a decrease of the adenine concentration to 2 mMchanges neither the position nor the shape of the peak pairs AT1/AT0 1 and AT2/AT0 2 (Fig. 2a, b). On the other hand, a decrease of the thymine concentration to 10 mM at an adenine concentration of 2 mMleads to chemisorption peaks A1/A0 1 and A2/A0 2, which Fig. 2 Cyclic voltammogram of thymine 20 mM 1 adenine 3 mM (a); thymine 20mM1 adenine 2 mM(b); thymine 10mM1 adenine 2 mM (c) and thymine 10mM1 adenine 1 mM(d) in 0.1MNaClO4, pH~2, T ~ 20 uC, scan rate 50 mV s21.AT1/AT2 and AT0 1/AT0 2 dissolution and formation of the charge-transfer complex between the p*-orbital of the A–T complex and the d orbitals of the surface. 152 PhysChemComm, 2002, 5(22), 151–157resemble the peaks of a pure adenine solution. Only the broad waves T2/3/T0 2=3 indicate the presence of thymine at the electrode surface. Following a further decrease of the adenine concentration to 1 mM, with an adenine/thymine ratio of 1 mM/10 mM (Fig. 2d), a CV was obtained that is very similar to that presented in Fig. 2a. It seems that only below a critical ratio of the surface concentration of adenine and thymine (1 mM/10 mM) can a measurable effect on the chemisorption kinetics represented by the peak pairs AT1/AT0 1 and AT2/AT0 2 be obtained. A decrease of the adenine concentration at a given thymine concentration of 20 mM (pH ~ 2) reveals new features in the CVs (Fig.3). The peak pairs AT1/AT0 1 and AT2/AT0 2 are broadened and are shifted to more positive potentials by several tens of mV, with decreasing adenine concentration. The potential variation for AT and T peaks depending on the adenine concentration in the mixed solution is shown in Fig. 4. At a concentration ratio of adenine/thymine of 0.08 mM/ 20 mM (Fig. 3b) the peak pair T1/T0 1 is observed, which clearly indicates the formation and dissolution of the pure condensed physisorbed thymine film. Finally, at very low adenine concentrations (0.001 mM, Fig. 5c) a CV results which seems to be identical with that of a pure thymine solution. But a detailed inspection of the peak pair T1/T0 1 (Fig.6) reveals that even at this very low concentration the formation and dissolution of the thymine film is influenced by adenine as can be seen by the small shifts of these peaks to positive potentials in comparison to their position in the pure thymine solution. The broad signals T2/3/T0 2=3 are correlated with the formation and dissolution of the chemisorbed and physisorbed thymine phase, respectively. With decreasing adenine concentration this peak pair splits into separated signals. Coadsorption of uracil and thymine on Au(111) Assuming that the adsorption behaviour of uracil and thymine on gold surface has similar features, the experiments with noncomplementary bases started at concentration of 6 mM uracil and 5 mM thymine in an acidic electrolyte (pH ~ 2).The peak pairs T1/T0 1 and U1/U0 1 (Fig. 7) in a negative potential region are related to the phase-transition of thymine and uracil, respectively. For the chosen temperature and concentration, the formation peaks U1/U0 2 for uracil are hardly seen. The reason is the slow formation kinetics in comparison to that for thymine. At more positive potentials the chemisorbed phase for both bases is formed as described before. The resulting CV for the mixture at the same concentration of uracil and thymine is shown in Fig. 8. The phase-transition peaks for thymine (T1/T0 1) can be clearly detected, but at lower intensity compared with that for pure thymine. Comparing the position of these needle peaks with the same peak pairs for the single bases (Fig.7), it is seen that they are shifted to more positive potentials. The peaks denoted for the transformation into the chemisorbed state look more like the peaks U2/U3 U0 2/ U0 3 (Fig. 7a) for uracil than that for thymine (Fig. 7b). It seems Fig. 3 Cyclic voltammogram of thymine 20mM1 adenine 0.4mM(a); thymine 20 mM 1 adenine 0.08 mM (b); and thymine 20 mM 1 adenine 0.06 mM (c) in 0.1 M NaClO4, pH ~ 2, T ~ 20 uC, scan rate 50 mV s21. Fig. 4 Peak potential variation with the lowering adenine concentration in the mixed solution. Thymine concentration is 20 mM, pH ~ 2, T ~ 20 uC, V ~ 50 mV s21. PhysChemComm, 2002, 5(22), 151–157 153that the transition into the chemisorbed state is influenced mostly by uracil molecules. The effect becomes more evident by lowering the concentration ratio of uracil/thymine (Fig.9). At a uracil concentration of 3 mM (uracil/thymine ratio 3 mM/5 mM) the needle peaks (T1/T0 1) shift to more negative potentials and, for a uracil concentration of 1 mM at constant thymine concentration (6 mM), the peaks T1/T0 1 become more pronounced than for the concentration ratio before. The shape of the peaks denoted for the transformation into the chemisorption region becomes closer to that for thymine the lower the uracil concentration is. The transition into the chemisorption region indicates four well ordered peaks (Fig. 9a, b). The needle peaks T2/T0 2 reveal that thymine molecules form the physisorbed condensed layers (II). We cannot decide which molecule, thymine or uracil, is responsible for the chemisorption peaks at positive potentials.A small thymine concentration (1 mM) prevents the physisorbed phase transition of uracil (6 mM). Only the transformation into the chemisorbed region can be detected (Fig. 10a). Increasing the thymine concentration (2 mM) and decreasing the uracil concentration (3 mM) leads to the same CV as before (Fig. 10b). Discussion Adenine Xiao and Che14 studied the adsorption behaviour of adenine on a gold electrode by SERS, which revealed important results. They found, in agreement with the STM studies,4 that at positive potentials (w0.4 V) a direct contact of the NH2 group and N(7) with the surface exists. At negative potentials (20.4 V) the p-system of adenine is in contact with the surface. In this state a charge transfer complex is formed in which a partial electron transfer from the gold d-orbitals to the p*-orbital of adenine is assumed. In accordance with this model we identified the broad peaks A0 1/A0 2 in Fig. 1b as the formation and the peaks A1/A2 as the dissolution of the charge transfer complex between the adsorbed protonated adenine and the gold surface. At present, only a tentative interpretation of the signals A3/A0 3 at about 1 V (vs.SCE) can be given (Fig. 1b). These peaks appear about 100 mV more negative than the gold oxidation in the pure electrolyte. We assume that in this potential range also an adenine–gold complex is formed. The adenine orientation at the electrode at this potential points to an interaction of lone pair electrons of adenine with the gold surface favorable for a partial electron transfer from adenine to gold.Coadsorption adenine/thymine Adenine is adsorbed stronger than thymine which is proved by comparing the adsorption behaviour at different concentration ratios of both bases (Fig. 2). For concentration ratios adenine/ thymine greater than or equal to 0.2 (pH~2) the obtained CV is identical with that for a pure adenine solution. For smaller ratios the broad chemisorption peak pairs of adenine A1/A0 1 and A2/A0 2 changed their shapes and one can clearly distinguish between two well resolved sharp peak pairs AT1/AT0 1 and AT2/AT0 2. It should be pointed out that the ratio of both bases is more responsible for the shape of the peaks than the absolute Fig. 5 Cyclic voltammogram of thymine 20 mM 1 adenine 0.008 mM (a); thymine 20 mM 1 adenine 0.005 mM (b); and thymine 20 mM 1 adenine 0.001 mM (c) in 0.1 M NaClO4, pH~ 2, T ~ 20 uC, scan rate 50 mV s21.Fig. 6 Needle peaks of thymine 20 mM (red); thymine 20 mM 1 0.001 mM adenine (blue); thymine 20 mM 1 0.005 adenine (green); thymine 20 mM 1 0.008 mM adenine (black) in 0.1 M NaClO4, pH ~ 2, T ~ 20 uC, scan rate 50 mV s21. 154 PhysChemComm, 2002, 5(22), 151–157concentration of one of the bases at the surface. Narrowing the peak pairs A1/A0 1 and A2/A0 2 at a defined thymine concentration may be taken as a kinetic effect caused by the interaction of adenine with thymine molecules, which leads to a faster kinetics of the adenine/Au charge-transfer complex formation. In contrast to the coadsorption behaviour of adenine and thymine on Fig.7 Cyclic voltammograms of 6 mM uracil (a) and 5 mM thymine (b) in 0.1 M NaClO4, pH~ 2, T ~ 20 uC, scan rate 50 mV s21. U1/U0 1 and U2/U0 2: formation and dissolution of the charge-transfer complex between uracil and the gold surface; T1/T0 1: formation and dissolution of the ordered physisorbed thymine monolayer; T2/T0 2 dissolution and formation of the ordered physisorbed thymine monolayer; T3/T0 3 formation and dissolution of the ordered chemisorbed thymine monolayer; Fig. 8 Cyclic voltammogram of thymine 5 mM1 uracil 6 mMin 0.1M NaClO4, pH ~ 2, T ~ 20 uC, scan rate 50 mV s21. Fig. 9 Cyclic voltammogram of thymine 5 mM 1 uracil 3 mM (a); thymine 5 mM 1 uracil 1 mM (b) in 0.1 M NaClO4, pH ~ 2, T ~ 20 uC, scan rate 50 mV s21. Fig.10 Cyclic voltammogram of thymine 1 mM 1 uracil 6 mM (a); thymine 2 mM 1 uracil 3 mM (b) in 0.1 M NaClO4, pH ~ 2, T ~ 20 uC, scan rate 50 mV s21. PhysChemComm, 2002, 5(22), 151–157 155mercury electrodes, where both bases are physisorbed without mutual interaction,12 obviously at the Au(111) electrode both bases interact in the negative potential region. STM investigations 7 show that physisorbed thymine molecules are lying flat in this potential region. The SERS experiments for the adenine molecules reveal a parallel orientation with the plane of the aromatic rings towards the electrode surface.13 Both bases are therefore oriented in a preferable position to form hydrogen bonds. It is not clear how the thymine molecules interact with adenine. Further STM experiments are planned to enlighten the structure of the charge transfer complex of Au–adenine/ thymine.The condensed physisorbed layer of thymine indicated that the needle peak pair T1/T0 1 could not be found at adenine/ thymine ratios greater than 0.02 (T ~ 20 uC, Figs. 2 and 3). At a first glance, it seems that this is in accordance with the results found for the coadsorption behaviour of adenine and thymine on mercury. But one should not forget that at potentials negative of the peak pairs AT1/AT0 1 and AT2/AT0 2 the adenine/ thymine charge transfer complex is adsorbed. The strong adsorption of adenine and its interaction with thymine indirectly leads to an immobilization of thymine. The concentration of thymine molecules being mobile at the surface is therefore considerably decreased.Obviously, at ratios of adenine/thymine larger than 0.02 the critical concentration of mobile thymine molecules necessary for the formation of the condensed thymine layer is not reached. Most of the thymine molecules at the surface are ‘‘prisoners’’ of the immobilized adenine molecules. At small adenine/thymine ratios (0.004 for 20 uC, Figs. 3, 5), sufficient ‘‘mobile’’ thymine molecules are present to form an ordered physisorbed thymine phase. This occurs at potentials negative of the peak pairs AT1/AT0 1 and AT2/AT0 2. The temperature behaviour of T1/T0 1 is in accordance with the temperature behaviour of pure condensed thymine monolayers. 7 In the potential region between the needle peak pair T1/T0 1 and the peak pairs AT1/AT0 1 and AT2/AT0 2 the appearance of domains of ordered physisorbed thymine besides domains consisting of the Au–adenine/thymine charge transfer complex may be assumed.Although at potentials positive of the peak pairs AT1/AT0 1 and AT2/AT0 2 (Fig. 2) adenine is not chemisorbed, it is so strongly adsorbed even in its physisorption state that it prevents a well defined and complete formation of the chemisorbed thymine layer. The broad wave T2/3/T0 2=3 indicates that only a small amount of thymine is adsorbed. The positive shift of the peak pairs AT1/AT0 1 and AT2/AT0 2 at adenine/thymine ratios smaller than 0.004 (Figs. 3, 5) shows that an increase of the relative surface concentration of thymine favours the formation of the charge transfer complex between the lowest unoccupied p*-orbital of adenine and the highest occupied d-orbital of Au(111).This means that the energy of the p*-orbital is lowered by the interaction between thymine and adenine. Guerra et al.15 have shown that the formation of hydrogen bonds between adenine and thymine leads to an increase of negative charge on thymine due to the different interactions between the involved s-orbitals. Although the p-electron density is not involved in the formation of hydrogen bonds, the p-system is influenced by the electrostatic potential which both bases experience from each other. The p-system wants to eliminate the charge differentiation by the s-hydrogen bonds and becomes polarized, or, in other words, the energy of the p-orbital increases. Consequently, the energy of the p*-orbital decreases.The charge-transfer complex formation between gold and adenine is therefore energetically favoured, the more hydrogen bonds between adenine and thymine are formed. Therefore, the formation process itself takes place at more positive potentials. The broad signals T2/3/T0 2=3 are correlated with the formation and dissolution of the chemisorbed thymine phase. With decreasing adenine concentration this peak pair splits into separated signals, which again shows the mutual interaction of adsorbed thymine and adenine. Coadsorption thymine/uracil The pure bases uracil and thymine show a similar adsorption behaviour. Both form four different well defined adsorption states (Fig. 7). In an electrolyte solution containing both uracil and thymine in a ratio of 6 mM/5 mM, the formation of the physisorbed condensed thymine is hindered, which is seen by the shift of the peak pairs T1/T0 1 into positive direction.Due to the coadsorption of uracil at negative potentials, the surface concentration of thymine is lowered, so that the critical concentration necessary for the phase transition cannot be reached at negative potentials, as it would be in a pure thymine solution. These results indicated that the attractive interations between uracil and thymine at the surface is less than that between the pure bases. It is confirmed by further decreasing the uracil concentration. The peak pairs T1/T0 1 shifted into the negative potential region. This behaviour is equivalent to the behaviour of the peak pairs T1/T0 1 in a pure solution by increasing the bulk concentration of thymine.On the other side, a low thymine concentration (1 mM) in a solution containing additionally 6 mM uracil prevents the formation of a condensed physisorbed layer of uracil. Because thymine is more strongly adsorbed than uracil, which can be seen by comparing the peak positions T1/T0 1 and U1/U0 1 (Fig. 7) a small amount of thymine is sufficient to hinder the formation of the physisorbed uracil monolayer (Fig. 10a). Summary Adenine is strongly adsorbed at Au(111) in the double layer region (Fig. 1b), and no first-order phase transitions equivalent to those of thymine on Au(111) were observed. At negative potentials the plane of adenine is oriented parallel to the electrode surface. This geometry allows a partial charge transfer from the electrode surface to the p*-system of the heteroaromatic molecule.Due to the planar orientation of adenine molecules, an attractive interaction between coadsorbed planar oriented thymine and adenine molecules is possible. In the positive potential range thymine is chemisorbed. The plane of the chemisorbed thymine is oriented perpendicular to the electrode surface. By SERS experiments the same orientation was found for adenine in this potential range. We found no experimental features pointing to a mutual interaction of adenine and thymine in this perpendicular orientation. Thymine adsorbs more strongly than uracil and prevents the uracil phase-transition. On the other hand, uracil hinders the formation of the condensed monolayer consisting of thymine alone, which is seen by a shift of the corresponding peak pairs T1/T0 1. No mutual interaction could be found. References 1 R. de Levie, Chem. Rev., 1988, 88, 599. 2 U. Retter, J. Electroanal. Chem., 1987, 236, 21. 3 M.H.Ho� lzle, T. Wandlowski and D. Kolb, Surf. Sci., 1995, 335, 281. 4 W. Haiss, B. Roelfs, S. N. Port, E. Bunge, H. Baumga� rtel and R. Nichols, J. Electroanal. Chem., 1998, 454, 107. 5 T. Dretschkow and T. Wandlowski, Electrochim. Acta, 1998, 43, 2991. 6 Th. Wandlowski, D. Lampner and S. M. Lindsay, J. Electroanal. Chem., 1996, 404, 215. 156 PhysChemComm, 2002, 5(22), 151–1577 B. Roelfs and H. Baumga� rtel, Ber. Bunsen-Ges. Phys. Chem., 1995, 99, 677. 8 C. Buess-Herman, Prog. Surf. Sci., 1994, 46, 335. 9 Th. Wandlowski, Langmuir, 1997, 13, 2843. 10 G. S. Parry, Acta Crystallogr., 1954, 7, 313. 11 S. J. Sowerby and G. B. Petersen, J. Electroanal. Chem., 1997, 433, 85. 12 S. Kirste and C. Donner, Phys. Chem. Chem. Phys., 2001, 3, 4384. 13 S. Kirste, PhD Thesis, FU-Berlin, 2002. 14 Y. J. Xiao and Y. F. Che, Spectrochim. Acta, Part A, 1999, 55, 1209. 15 C. F. Guerra, J. M. Bickelhaupt, J. G. Snijders and E. J. Baerends, Chem. Eur. J., 1999, 5, 3581. PhysChemComm, 2002, 5(22), 151–1
ISSN:1460-2733
DOI:10.1039/b208139c
出版商:RSC
年代:2002
数据来源: RSC
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