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Adsorption of acridine on silver electrode: SERS spectra potential dependence as a probe of adsorbate state
Journal of Molecular Structure 1034 (2013) 19–21
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Adsorption of acridine on silver electrode: SERS spectra potential dependence as a probe of adsorbate state Elena V. Solovyeva, Liubov A. Myund, Evgeniya M. Dem’yanchuk, Artiom A. Makarov, Anna S. Denisova ⇑ Chemistry Department, Saint-Petersburg State University, Universitetsky Pr. 26, Stary Peterhof, Saint-Petersburg 198504, Russian Federation
h i g h l i g h t s +
" Desorption of AcrH occurs at shifting the Ag electrode potential to negative range. " Chloride anions co-adsorption provides the acridine adsorption. +
" The equilibrium between Acr and AcrH on the silver electrode surface is reversible.
a r t i c l e
i n f o
Article history: Received 20 July 2012 Accepted 3 September 2012 Available online 11 September 2012 Keywords: SERS Acridine Silver electrode Potential dependence
a b s t r a c t This work investigates acridine adsorption on the silver electrode surface. The dependence of the acridine SERS spectra on the electrode potential proved to be quite different for azaheterocycle molecules, while the pH effect as expected. The changes in the acridine SERS spectrum caused by the double electric layer (DEL) rearrangement can be explained by sorption/desorption rather than the adsorbate molecule reorientation. The presence of chloride anions close to the silver surface is important not only for the SERSactive properties but for the formation of the stabilised surface complexes of the protonated acridine as well. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Surface Enhanced Raman Spectroscopy (SERS) is one of the few optical methods allowing to obtain the information on the processes on the metal-solution border. To investigate the adsorption of molecules by the SERS method substrates are used of various types. When the surface of the working electrode in the electrochemical cell is used as an SERS-active substrate, the experimental setup allows one to vary the potential on the surface of the metalsolution border which is important for SERS spectrum formation [1]. On one hand, the surface potential affects the double electric layer (DEL), while on the other, the surface complex orientation. The effect of the electrode potential on adsorption has already been investigated for a number of various compounds. General features for SERS spectra potential dependence can be found for azaheterocycles containing plane aromatic fragments interacting with the silver electrode via nitrogen lone pair. By shifting the electrode potential to the more negative range not below the point of zero charge (PZC) the intensity of symmetric in-plane vibrations growth is observed for such molecules as pyridine, phenanthroline, ⇑ Corresponding author. Tel.: +7 812 428 4062; fax: +7 812 428 6733. E-mail address: [email protected] (A.S. Denisova). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.09.001
bipyridile and their derivatives [2–5]. One of the explanations of such an interrelation between the electrode potential and SERS intensity is given according to the surface selection rule: flat or tilt molecule is reoriented nearly normal to surface [6,7]. The acridine molecule is sensitive to the electrolytic surrounding of the metal surface in solutions. Studies of acridine SERS spectra pH-dependence allowed the authors [8,9] to obtain characteristic features of protonated and non-protonated molecules. The SERS spectra of acridine have earlier been obtained on silver colloids [8,10–12], substrates obtained by laser ablation [13], nanodroplets prepared by ultrasonic levitation [14]. This work studies the acridine SERS spectra on the silver electrode surface. By changing the electrode potential DEL rearrangement could be controlled, which in its turn affects adsorption. 2. Experimental The SERS spectra were recorded using a DFS-52 (LOMO, USSR) spectrometer controlled by a personal computer. The 488 nm line of Ar+ laser LGN-503 was used to excite spectrum. The spectra were recorded by FEU-79 photomultiplier in the photon counting mode. The input and output slits of the monochromator were
20
E.V. Solovyeva et al. / Journal of Molecular Structure 1034 (2013) 19–21
200 lm. The accuracy of the wave number reproduction was 0.1 cm 1. The silver electrode had been etched with the strong nitric acid and carefully rinsed with distilled water. The surface relief had been obtained as a result of 5 oxidation – reduction cycles (ORCs) in the three-electrode electrochemical cell. The conditions are analogous to those described in [15]. All the potentials are referred to the saturated Ag|AgCl electrode. In all cases the acridine had been adsorbed from the solution 10 6 AcrH+ on the background of 10 2 KCl by 5 ORC. The subsequent changes of the pH solution, the electrode potential and the KCl concentration were introduced in the limits of one life experiment without any additional electrochemical electrode treatment.
The effect of pH changing on the SERS spectra of the acridine adsorbed on the silver electrode surface is shown in Fig. 1. As seen, the main pH-depending changes are observed in the ranges of 1260–1290 cm 1 and 1560–1590 cm 1 which agrees with the results obtained for the protolytic equilibrium of the acridine on the other SERS-active substrates [8,9]. The acidification of solution leads to the growth of the relative intensity of the bands 1276 cm 1 and 1584 cm 1 compared to the 1266 cm 1 and 1565 cm 1 components, respectively. Thus, the 1266 cm 1 and 1565 cm 1 components are characteristic for the non-protonated acridine form on the surface, while 1276 cm 1 and 1584 cm 1 bands – for protonated acridine form on the surface. The SERS signal is more intensive for the protonated form in the range 1000– 1800 cm 1. The addition of alkali to the acidic solution up to pH 12 (see curve c in Fig. 1) causes opposite changes in the acridine SERS spectrum. The total intensity (both spectrum and background) is decreased after the contact of the silver surface with the alkali solution, which is probably caused by the loss of SERS-active properties of the electrode surface. After the addition of acid to the alkali solution up to pH 3 the spectral bandshape of the protonated acridine becomes restored (see curve d in Fig. 1). Thus, the
pH-dependence of the SERS spectra shown in Fig. 1 indicates the reversibility of the two forms of the adsorbed acridine transformation. The dependence of the acridine SERS spectra on the electrode potential is shown in Fig. 2. As seen in Fig. 2, at the initial value of the potential 150 mV both forms of the adsorbed acridine (protonated and non-protonated) are observed in the SERS spectrum. The analysis of the potential dependence of the acridine SERS spectra shows that the total intensity of the registered signal decreases at the more negative range of the potential while it increases when the values are more positive. The shift of the potential into the more negative range results in the decrease of the bands intensity of the protonated form, while the intensity of the components of the non-protonated form remains nearly unchanged. Presented in the spectrum at 550 mV are the bands characteristic for the non-protonated form only. At the potential turned to 150 mV after 550 mV the intensity of the spectrum does not reach the initial value, but the spectral bandshape is fully restored. The analogous changes of the SERS spectra are also observed when the negative potential shift is repeated again. The loss of the intensity after the negative potentials were applied is probably connected with the stepwise decrease of the surface activity. The non-protonated form of the adsorbed acridine is not sensitive to the electrode potential. The dependence of the SERS intensity on the electrode potential was earlier discussed for a number of the heterocycles containing plane aromatic fragments and interacting with the silver surface via nitrogen atom. For such substances at the negative shifting of the electrode potential not below the PZC the growth of the SERS spectrum intensity is expected. According to the surface selection rule this type of the dependence is usually explained by the reorientation of the tilt molecule to the normal position [6,7]. Fig. 2 illustrates a thoroughly different tendency that could be interpreted by sorption/desorption of the protonated form of acridine AcrH+. The stepwise desorption of AcrH+ followed by the potential negative shift is probably explained by the way in which the particle forms the surface complex. Since the nitrogen lone pair
Fig. 1. Evolution of SERS spectra of acridine adsorbed on the silver electrode by pH changing from a–d (acridine 10 6 M, 200 mV). Spectra b–d are shifted on the ordinate axis.
Fig. 2. SERS spectra of acridine adsorbed on the silver electrode by stepwise electrode potential changing from bottom to top (acridine 10 6 M). All spectra are shifted on the ordinate axis.
3. Results and discussion
E.V. Solovyeva et al. / Journal of Molecular Structure 1034 (2013) 19–21
21
as well. Thus, increasing the concentration of chloride anions in the near-electrode space stabilises the protonated acridine surface complexes. This study of SERS spectra of acridine adsorbed on silver electrode brings, along with the potential dependence, some new information on the state of adsorbed acridine molecules. For adsorbed acridine molecules there is the reversible equilibrium between protonared and non-protonated form on the surface of the silver electrode. The protonated form of acridine is most sensitive to the surface potential change: desorption of AcrH+ occurs when the potential shifts to the negative range. Chloride anions coadsorption provides the reverse process. 4. Conclusions
Fig. 3. SERS spectra of acridine adsorbed on the silver electrode at different concentration of Cl anions, acridine 10 6 M, 200 mV: solid line – 10 2 M KCl; dotted line – 5 10 1 M KCl.
of the protonated acridine molecule is occupied by the proton, the interaction of AcrH+ with the silver surface may occur via the aromatic system and/or by means of the hydrogen bonds formation with the adsorbed chloride anions. The assumption that in case the protonated acridine can form the intermolecular hydrogen bond with chloride anion N–H Cl is correlated with the results obtained in the research of the crystal structure of the complex AcrHClH2O [16]. Thus, AcrH+ desorption occurs as a result of the following factors: the repulsion of the p-system from the negative surface and from the chloride anions desorption. At the potentials lower than the zero charge potential ( 540 mV for the polycrystalline silver surface [17]) the composition and structure of DEL are changed. The water molecules turn toward the surface with hydrogen atoms, chloride anions migrate to the outer layer of DEL, H-bonded with chloride anions AcrH+ also leave the surface. When the more positive electrode potential returns, AcrH+ approaches the surface together with the chloride anions. The role of adsorbed on the silver surface halide ions is widely enough discussed in literature [18–21]. Their presence is considered to be necessary for the active sites (adatoms) to be stabilised on the surface. The effect of chloride anions concentration on acridine SERS spectra shown in Fig. 3 indicate the importance of chloride anions co-adsorption not only for the surface properties but for the adsorbed molecules as well. As seen, increasing the concentration of chloride anions in the solution provides the considerable growth of the SERS spectrum total intensity. Besides, the intensity of the bands of protonated form of acridine (1276 cm 1 and 1584 cm 1) grows more in comparison with the components of the non-protonated form (1266 cm 1 and 1565 cm 1). This indicates that adsorbed chloride ions stabilise not only SERS-active adatoms, but protonated acridine molecules
This study of SERS spectra of acridine adsorbed on silver electrode brings, along with the potential dependence, some new information on the state of adsorbed acridine molecules. For adsorbed acridine molecules there is the reversible equilibrium between protonated and non-protonated form on the surface of the silver electrode. The protonated form of acridine is most sensitive to the surface potential change: desorption of AcrH+ occurs when the potential shifts to the negative range. Chloride anions co-adsorption provides the reverse process. Acknowledgment The authors acknowledge Saint-Petersburg State University for the research Grant 12.38.17.2011. References [1] B. Pettinger, in: J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1992, p. 285 (Chapter 6). [2] A.A. Makarov, A.S. Denisova, L.A. Myund, Russ. J. Appl. Chem. 71 (1998) 1135. [3] R. Livingstone, J.R. Lombardi, J. Raman Spectrosc. 42 (2011) 1945. [4] D.A. Carter, J.E. Pemberton, K.J. Woelfel, J. Phys. Chem. B 102 (1998) 9870. [5] M. Takahashi, M. Ito, Chem. Phys. Lett. 103 (1984) 512. [6] M. Moskovits, J. Chem. Phys. 77 (1982) 4408. [7] J.R. Lombardi, R.L. Birke, J. Phys. Chem. C 112 (2008) 5605. [8] S.T. Oh, K. Kim, M.S. Kim, J. Phys. Chem. 95 (1991) 8844. [9] R. Brayner, R. Iglesias, S. Truong, Z. Beji, N. Feligj, F. Fievet, J. Aubard, Langmuir 26 (2010) 17465. [10] N. Felidj, G. Levi, J. Pantigny, J. Aubard, New J. Chem. 22 (1998) 725. [11] D.H. Jeong, J.S. Suh, M. Moskovits, J. Phys. Chem. B 104 (2000) 7462. [12] G. Levi, J. Pantigny, J.P. Marsault, J. Aubard, J. Raman Spectrosc. 24 (1993) 745. [13] S.L. Truong, G. Levi, F. Bozon-Verduraz, A.V. Petrovskaya, A.V. Simakin, G.A. Shafeev, Appl. Surf. Sci. 254 (2007) 1236. [14] N. Leopold, M. Haberkorn, T. Laurell, J. Nilsson, J.R. Baena, J. Frank, B. Lendl, Anal. Chem. 75 (2003) 2166. [15] E.V. Potapkina, A.S. Denisova, L.A. Myund, A.A. Makarov, E.M. Dem’yanchuk, J. Mol. Struct. 996 (2011) 128. [16] K. Ha, Z. Kristallogr. New Cryst. Struct. 226 (2011) 321. [17] D.D. Bode Jr., J. Phys. Chem. 71 (1967) 792. [18] R.M. Lazorenko-Manevich, Russ. J. Electrochem. 41 (2005) 799. [19] A. Otto, A. Bruckbauer, Y.X. Chen, J. Mol. Struct. 661–662 (2003) 501. [20] H. Wetzel, H. Gerischer, B. Pettinger, Chem. Phys. Lett. 78 (1981) 392. [21] M. Muniz-Miranda, J. Phys. Chem. A 104 (2000) 7803.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Adsorption of acridine on silver electrode: SERS spectra potential dependence as a probe of adsorbate state Elena V. Solovyeva, Liubov A. Myund, Evgeniya M. Dem’yanchuk, Artiom A. Makarov, Anna S. Denisova ⇑ Chemistry Department, Saint-Petersburg State University, Universitetsky Pr. 26, Stary Peterhof, Saint-Petersburg 198504, Russian Federation
h i g h l i g h t s +
" Desorption of AcrH occurs at shifting the Ag electrode potential to negative range. " Chloride anions co-adsorption provides the acridine adsorption. +
" The equilibrium between Acr and AcrH on the silver electrode surface is reversible.
a r t i c l e
i n f o
Article history: Received 20 July 2012 Accepted 3 September 2012 Available online 11 September 2012 Keywords: SERS Acridine Silver electrode Potential dependence
a b s t r a c t This work investigates acridine adsorption on the silver electrode surface. The dependence of the acridine SERS spectra on the electrode potential proved to be quite different for azaheterocycle molecules, while the pH effect as expected. The changes in the acridine SERS spectrum caused by the double electric layer (DEL) rearrangement can be explained by sorption/desorption rather than the adsorbate molecule reorientation. The presence of chloride anions close to the silver surface is important not only for the SERSactive properties but for the formation of the stabilised surface complexes of the protonated acridine as well. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Surface Enhanced Raman Spectroscopy (SERS) is one of the few optical methods allowing to obtain the information on the processes on the metal-solution border. To investigate the adsorption of molecules by the SERS method substrates are used of various types. When the surface of the working electrode in the electrochemical cell is used as an SERS-active substrate, the experimental setup allows one to vary the potential on the surface of the metalsolution border which is important for SERS spectrum formation [1]. On one hand, the surface potential affects the double electric layer (DEL), while on the other, the surface complex orientation. The effect of the electrode potential on adsorption has already been investigated for a number of various compounds. General features for SERS spectra potential dependence can be found for azaheterocycles containing plane aromatic fragments interacting with the silver electrode via nitrogen lone pair. By shifting the electrode potential to the more negative range not below the point of zero charge (PZC) the intensity of symmetric in-plane vibrations growth is observed for such molecules as pyridine, phenanthroline, ⇑ Corresponding author. Tel.: +7 812 428 4062; fax: +7 812 428 6733. E-mail address: [email protected] (A.S. Denisova). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.09.001
bipyridile and their derivatives [2–5]. One of the explanations of such an interrelation between the electrode potential and SERS intensity is given according to the surface selection rule: flat or tilt molecule is reoriented nearly normal to surface [6,7]. The acridine molecule is sensitive to the electrolytic surrounding of the metal surface in solutions. Studies of acridine SERS spectra pH-dependence allowed the authors [8,9] to obtain characteristic features of protonated and non-protonated molecules. The SERS spectra of acridine have earlier been obtained on silver colloids [8,10–12], substrates obtained by laser ablation [13], nanodroplets prepared by ultrasonic levitation [14]. This work studies the acridine SERS spectra on the silver electrode surface. By changing the electrode potential DEL rearrangement could be controlled, which in its turn affects adsorption. 2. Experimental The SERS spectra were recorded using a DFS-52 (LOMO, USSR) spectrometer controlled by a personal computer. The 488 nm line of Ar+ laser LGN-503 was used to excite spectrum. The spectra were recorded by FEU-79 photomultiplier in the photon counting mode. The input and output slits of the monochromator were
20
E.V. Solovyeva et al. / Journal of Molecular Structure 1034 (2013) 19–21
200 lm. The accuracy of the wave number reproduction was 0.1 cm 1. The silver electrode had been etched with the strong nitric acid and carefully rinsed with distilled water. The surface relief had been obtained as a result of 5 oxidation – reduction cycles (ORCs) in the three-electrode electrochemical cell. The conditions are analogous to those described in [15]. All the potentials are referred to the saturated Ag|AgCl electrode. In all cases the acridine had been adsorbed from the solution 10 6 AcrH+ on the background of 10 2 KCl by 5 ORC. The subsequent changes of the pH solution, the electrode potential and the KCl concentration were introduced in the limits of one life experiment without any additional electrochemical electrode treatment.
The effect of pH changing on the SERS spectra of the acridine adsorbed on the silver electrode surface is shown in Fig. 1. As seen, the main pH-depending changes are observed in the ranges of 1260–1290 cm 1 and 1560–1590 cm 1 which agrees with the results obtained for the protolytic equilibrium of the acridine on the other SERS-active substrates [8,9]. The acidification of solution leads to the growth of the relative intensity of the bands 1276 cm 1 and 1584 cm 1 compared to the 1266 cm 1 and 1565 cm 1 components, respectively. Thus, the 1266 cm 1 and 1565 cm 1 components are characteristic for the non-protonated acridine form on the surface, while 1276 cm 1 and 1584 cm 1 bands – for protonated acridine form on the surface. The SERS signal is more intensive for the protonated form in the range 1000– 1800 cm 1. The addition of alkali to the acidic solution up to pH 12 (see curve c in Fig. 1) causes opposite changes in the acridine SERS spectrum. The total intensity (both spectrum and background) is decreased after the contact of the silver surface with the alkali solution, which is probably caused by the loss of SERS-active properties of the electrode surface. After the addition of acid to the alkali solution up to pH 3 the spectral bandshape of the protonated acridine becomes restored (see curve d in Fig. 1). Thus, the
pH-dependence of the SERS spectra shown in Fig. 1 indicates the reversibility of the two forms of the adsorbed acridine transformation. The dependence of the acridine SERS spectra on the electrode potential is shown in Fig. 2. As seen in Fig. 2, at the initial value of the potential 150 mV both forms of the adsorbed acridine (protonated and non-protonated) are observed in the SERS spectrum. The analysis of the potential dependence of the acridine SERS spectra shows that the total intensity of the registered signal decreases at the more negative range of the potential while it increases when the values are more positive. The shift of the potential into the more negative range results in the decrease of the bands intensity of the protonated form, while the intensity of the components of the non-protonated form remains nearly unchanged. Presented in the spectrum at 550 mV are the bands characteristic for the non-protonated form only. At the potential turned to 150 mV after 550 mV the intensity of the spectrum does not reach the initial value, but the spectral bandshape is fully restored. The analogous changes of the SERS spectra are also observed when the negative potential shift is repeated again. The loss of the intensity after the negative potentials were applied is probably connected with the stepwise decrease of the surface activity. The non-protonated form of the adsorbed acridine is not sensitive to the electrode potential. The dependence of the SERS intensity on the electrode potential was earlier discussed for a number of the heterocycles containing plane aromatic fragments and interacting with the silver surface via nitrogen atom. For such substances at the negative shifting of the electrode potential not below the PZC the growth of the SERS spectrum intensity is expected. According to the surface selection rule this type of the dependence is usually explained by the reorientation of the tilt molecule to the normal position [6,7]. Fig. 2 illustrates a thoroughly different tendency that could be interpreted by sorption/desorption of the protonated form of acridine AcrH+. The stepwise desorption of AcrH+ followed by the potential negative shift is probably explained by the way in which the particle forms the surface complex. Since the nitrogen lone pair
Fig. 1. Evolution of SERS spectra of acridine adsorbed on the silver electrode by pH changing from a–d (acridine 10 6 M, 200 mV). Spectra b–d are shifted on the ordinate axis.
Fig. 2. SERS spectra of acridine adsorbed on the silver electrode by stepwise electrode potential changing from bottom to top (acridine 10 6 M). All spectra are shifted on the ordinate axis.
3. Results and discussion
E.V. Solovyeva et al. / Journal of Molecular Structure 1034 (2013) 19–21
21
as well. Thus, increasing the concentration of chloride anions in the near-electrode space stabilises the protonated acridine surface complexes. This study of SERS spectra of acridine adsorbed on silver electrode brings, along with the potential dependence, some new information on the state of adsorbed acridine molecules. For adsorbed acridine molecules there is the reversible equilibrium between protonared and non-protonated form on the surface of the silver electrode. The protonated form of acridine is most sensitive to the surface potential change: desorption of AcrH+ occurs when the potential shifts to the negative range. Chloride anions coadsorption provides the reverse process. 4. Conclusions
Fig. 3. SERS spectra of acridine adsorbed on the silver electrode at different concentration of Cl anions, acridine 10 6 M, 200 mV: solid line – 10 2 M KCl; dotted line – 5 10 1 M KCl.
of the protonated acridine molecule is occupied by the proton, the interaction of AcrH+ with the silver surface may occur via the aromatic system and/or by means of the hydrogen bonds formation with the adsorbed chloride anions. The assumption that in case the protonated acridine can form the intermolecular hydrogen bond with chloride anion N–H Cl is correlated with the results obtained in the research of the crystal structure of the complex AcrHClH2O [16]. Thus, AcrH+ desorption occurs as a result of the following factors: the repulsion of the p-system from the negative surface and from the chloride anions desorption. At the potentials lower than the zero charge potential ( 540 mV for the polycrystalline silver surface [17]) the composition and structure of DEL are changed. The water molecules turn toward the surface with hydrogen atoms, chloride anions migrate to the outer layer of DEL, H-bonded with chloride anions AcrH+ also leave the surface. When the more positive electrode potential returns, AcrH+ approaches the surface together with the chloride anions. The role of adsorbed on the silver surface halide ions is widely enough discussed in literature [18–21]. Their presence is considered to be necessary for the active sites (adatoms) to be stabilised on the surface. The effect of chloride anions concentration on acridine SERS spectra shown in Fig. 3 indicate the importance of chloride anions co-adsorption not only for the surface properties but for the adsorbed molecules as well. As seen, increasing the concentration of chloride anions in the solution provides the considerable growth of the SERS spectrum total intensity. Besides, the intensity of the bands of protonated form of acridine (1276 cm 1 and 1584 cm 1) grows more in comparison with the components of the non-protonated form (1266 cm 1 and 1565 cm 1). This indicates that adsorbed chloride ions stabilise not only SERS-active adatoms, but protonated acridine molecules
This study of SERS spectra of acridine adsorbed on silver electrode brings, along with the potential dependence, some new information on the state of adsorbed acridine molecules. For adsorbed acridine molecules there is the reversible equilibrium between protonated and non-protonated form on the surface of the silver electrode. The protonated form of acridine is most sensitive to the surface potential change: desorption of AcrH+ occurs when the potential shifts to the negative range. Chloride anions co-adsorption provides the reverse process. Acknowledgment The authors acknowledge Saint-Petersburg State University for the research Grant 12.38.17.2011. References [1] B. Pettinger, in: J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1992, p. 285 (Chapter 6). [2] A.A. Makarov, A.S. Denisova, L.A. Myund, Russ. J. Appl. Chem. 71 (1998) 1135. [3] R. Livingstone, J.R. Lombardi, J. Raman Spectrosc. 42 (2011) 1945. [4] D.A. Carter, J.E. Pemberton, K.J. Woelfel, J. Phys. Chem. B 102 (1998) 9870. [5] M. Takahashi, M. Ito, Chem. Phys. Lett. 103 (1984) 512. [6] M. Moskovits, J. Chem. Phys. 77 (1982) 4408. [7] J.R. Lombardi, R.L. Birke, J. Phys. Chem. C 112 (2008) 5605. [8] S.T. Oh, K. Kim, M.S. Kim, J. Phys. Chem. 95 (1991) 8844. [9] R. Brayner, R. Iglesias, S. Truong, Z. Beji, N. Feligj, F. Fievet, J. Aubard, Langmuir 26 (2010) 17465. [10] N. Felidj, G. Levi, J. Pantigny, J. Aubard, New J. Chem. 22 (1998) 725. [11] D.H. Jeong, J.S. Suh, M. Moskovits, J. Phys. Chem. B 104 (2000) 7462. [12] G. Levi, J. Pantigny, J.P. Marsault, J. Aubard, J. Raman Spectrosc. 24 (1993) 745. [13] S.L. Truong, G. Levi, F. Bozon-Verduraz, A.V. Petrovskaya, A.V. Simakin, G.A. Shafeev, Appl. Surf. Sci. 254 (2007) 1236. [14] N. Leopold, M. Haberkorn, T. Laurell, J. Nilsson, J.R. Baena, J. Frank, B. Lendl, Anal. Chem. 75 (2003) 2166. [15] E.V. Potapkina, A.S. Denisova, L.A. Myund, A.A. Makarov, E.M. Dem’yanchuk, J. Mol. Struct. 996 (2011) 128. [16] K. Ha, Z. Kristallogr. New Cryst. Struct. 226 (2011) 321. [17] D.D. Bode Jr., J. Phys. Chem. 71 (1967) 792. [18] R.M. Lazorenko-Manevich, Russ. J. Electrochem. 41 (2005) 799. [19] A. Otto, A. Bruckbauer, Y.X. Chen, J. Mol. Struct. 661–662 (2003) 501. [20] H. Wetzel, H. Gerischer, B. Pettinger, Chem. Phys. Lett. 78 (1981) 392. [21] M. Muniz-Miranda, J. Phys. Chem. A 104 (2000) 7803.