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Surface-enhanced Raman spectroscopy as a probe for orientation of pyridine compounds on colloidal surfaces
Journal of Molecular Structure 935 (2009) 92–96
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Surface-enhanced Raman spectroscopy as a probe for orientation of pyridine compounds on colloidal surfaces D.H. Dressler b,c,*, Y. Mastai b,c, M. Rosenbluh a,b, Y. Fleger a,b a
The Jack and Pearl Resnick Institute for Advanced Technology, Department of Physics, Bar-Ilan University, Ramat-Gan, Israel Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel c Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel b
a r t i c l e
i n f o
Article history: Received 3 May 2009 Received in revised form 28 June 2009 Accepted 29 June 2009 Available online 2 July 2009 Keywords: Raman spectroscopy Surface-enhanced Raman scattering Self assembly Aromatics Colloids
a b s t r a c t Surface-enhanced Raman spectroscopy (SERS) has been used to characterize three isomeric pyridine compounds adsorbed to silver colloids. The three structural isomers, which all adsorb in the carboxylate form, to the silver colloids, bind in two different geometries to the silver surface. The differences in the binding geometry correlate with Raman resonance shifts and intensity enhancements that can be probed by SERS. For picolinic acid, the ortho isomer, we observed blue shifts of many of the SERS peaks attributed to the pyridine ring in comparison to the normal Raman vibrations of the non-adsorbed crystalline material. For this molecule, steric hindrance between the carboxylate and nitrogen groups causes the pyridine ring and the carboxylate group to tilt from the flat geometry, and hence both groups can form r bonding to the silver surface. On the other hand, nicotinic and isonicotinic acid, the meta and para isomers (respectively), that have less steric hindrance, adsorb to the silver colloids with the pyridine ring in a flat geometry, where only the carboxylate functional group tilts in order to bind coordinatively to the silver colloids. In this case, p bonding between the pyridine ring and the metal colloids causes small red shifts between the Raman and SERS peaks. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS), which was discovered in 1970 [1,2] enables detection of even weak Raman vibrations due to the SERS enhancement. This enhancement, as many experiments show [3,4], occurs due to the presence of a metal surface, and contains two components; electromagnetic [5,6] and chemical [7–9]. Since the discovery of SERS a vast literature concerning this effect has been published using different metal (gold, silver and copper [10–14]) substrates and ways to create SERS enhancements including aggregated metal colloids [13,15], metal island films [16], electrochemically roughened electrodes and metal colloidal monolayers [13,14]. One of the most important applications of SERS has been in the spectroscopy of organic self assembled monolayers (SAMS), which have drawn much attention since they provide a useful method of tailoring physiochemical properties of surfaces to various technical applications in biosensing, nanopatterning, and molecular electronic devices [17,18]. In particular, aromatic carboxylates such as mercapto benzoic acid (MBA) have been shown to be useful
* Corresponding author. Address: Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel. Tel.: +972 3 5317877; fax: +972 3 7384053. E-mail address: [email protected] (D.H. Dressler). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.06.042
for detection of different explosives [19–21]. SERS has proved to be one of the most sensitive techniques for monitoring the adsorbates of particularly short aromatic molecules on metal substrates at the submonolayer coverage limit [5,6,22–25]. An exciting recent development is the ability to get single molecule detection or Raman enhancements of up to 1015 by creating metal nanostructures or ‘‘hot spots” [26,27]. The occurrence of shifts in the Raman lines of a SERS spectrum depends on the mode of enhancement in the SERS spectra. The electromagnetic component, which is the major SERS enhancement factor, occurs due to the excitation of surface plasmons in the metal nanocluster, while the chemical component occurs due to molecular interactions between the metal surface and the adsorbate. This chemical component causes weaker SERS enhancement but may lead to frequency shifts of some of the molecular Raman vibrations, as will be shown in this paper. In order to analyze the influence of chemical enhancement on the SERS spectra, one needs to consider two different possible interactions between the adsorbate and the metal colloid. Physical adsorption, which is the result of relatively weak interactions between the solid surface and the adsorbate with an enthalpy of adsorption higher than 25 kJ/mol, and chemical adsorption, where the adsorbed molecules are strongly bound to the surface with an enthalpy of adsorption lower than 40 kJ/mol [28,29]. In the process of chemisorption, the adsorbed molecule changes its
D.H. Dressler et al. / Journal of Molecular Structure 935 (2009) 92–96
chemical structure and symmetry due to bond formation with the metal. This stronger adsorption usually introduces frequency shifts between vibrations of adsorbed molecules in comparison to the normal Raman spectrum of these molecules as shown for instance for C60 adsorbed on noble metals [30], and for thiophenol adsorbed on gold [31]. Generally, on a silver surface, chemisorption can take place through the formation of a bond between the Ag surface and an adsorbed molecule e.g., Ag–N, Ag–O, Ag–S, or Ag–X (X = halogen) bond [32–34]. In this paper, we show that frequency shifts in the SERS spectrum can be correlated to specific molecular bonding geometries. The relationship between molecular configuration and the specific binding mechanisms inferred from the frequency shifts, enables us to identify surface geometries of three different pyridine compounds adsorbed to metallic colloids, and may be implemented to several other adsorbed molecules. SERS has been used previously for determining the orientation of molecules adsorbed on certain surfaces [35–39]. The SERS spectra of many aromatic carboxylic acids have shown that the acid is adsorbed on the surface as a carboxylate ion [36,37,40–42]. The carboxylate can bind to a metal surface in a flat orientation through the carboxylate p system, or tilted with respect to the aromatic ring, in the case where steric hindrance occurs between the carboxylate and other ring substituents [43]. The geometry of the carboxylates on the surface, can affect the binding nature of these molecules on the surface. Moreover, for ortho substituents as the pyridine nitrogen, steric hindrance causes both the pyridine ring and the carboxylate to tilt from the flat orientation, in order to bind to the surface. This geometry also implicates different binding to the surface, as will be shown in this paper. Suh and Kim [40] identified three different binding geometries for aromatic carboxylates adsorbed on surfaces, depending on the relative position between the carboxylate and other substituents on the aromatic ring. The one-legged geometry, in which the carboxylate rotates from the plane of the ring due to steric hindrance between adjacent substituents, and only one oxygen of the CO2 group binds strongly through its oxygen lone pair electrons to the surface. This geometry can be observed with ortho, meta, and para isomers, depending on the relative substituents. The two-legged geometry, where the CO2 is adsorbed to the metal surface in a tilted position through both of its oxygen atoms, and the ring is also tilted with respect to the surface. This geometry, is evident only with ortho substituents, where large steric hindrance is apparent. The third geometry, is the totally flat geometry, which is not observed in this paper, where the ring as well as the carboxylate and the other substituent lie entirely flat on the surface, and the carboxylate binds to the surface through its p-electrons. This is usually observed in para and meta substituted molecules with less steric hindrance. In their paper, they showed [40], that the different geometries can be detected by the relative intensities of the carboxylate bending/stretching vibrations. In this paper, we show, that the nature of the adsorbate–metal bonding can also be detected by the presence of frequency shifts in the SERS spectrum [44], in comparison to the bulk crystalline Raman spectrum of the same molecule. We compare the SERS and Raman spectrum of three pyridine compounds, para-, meta-, and ortho-pyridinecarboxylic acid (Fig. 1). These isomers, with the carboxylate and pyridine nitrogen at different relative positions, bind in two different geometries to the silver colloids [40], which can be detected by frequency shifts in the SERS spectrum. For the ortho isomer, picolinic acid, with the two-legged geometry [40], both the nitrogen and the carboxylic acid tilt and bind coordinatively to the metal nanoclusters. In this case, r bonding of the pyridine nitrogen and the carboxylate to the metal nanoclusters causes blue shifts [44] between its Raman and SERS spectrum. The meta, and para isomer, with a one-legged geometry, adsorb to
93
Fig. 1. Molecular structures of para-pyridinecarboxylic acid, meta-pyridinecarboxylic acid and ortho-pyridinecarboxylic acid.
the silver nanoparticles with the pyridine ring in a flat geometry. In this case, p bonding between the pyridine ring and the metal nanoparticles causes small red shifts [44] between the Raman and SERS peaks. 2. Experimental Analytical grade reagents (>99.9%) were purchased from Aldrich–Sigma and used without further purification. We prepared silver colloids in an aqueous solution by the reduction of silver nitrate with hydroxylamine hydrochloride at alkaline pH as described in the literature [45], and then adsorbed the pyridinecarboxylic acids on them. Aqueous solutions (0.1 M) of the pyridinecarboxylic acids were added to the silver colloidal solution, to a final concentration of 0.01 M. The SERS spectrum of the colloidal solutions was immediately collected under a Raman microscope. For comparison the normal Raman spectrum of the crystalline acids were measured. Optical absorptions spectra of the colloids were obtained by a Cary 500 UV–Vis–NIR spectrophotometer. Raman and SERS measurements were performed with a Jobin Yvon Micro Raman (Model HR800, k = 633 nm) using different objectives. Raman and SERS spectrum were obtained in the range of 200–3500 cm 1. In order to confirm the results Raman and SERS measurements were repeated several times (several samples and acquisitions). TEM images were acquired by a JEOL TEM using an acceleration voltage of 120 kV. The TEM samples were prepared by lifting an aliquot of the colloidal solution onto copper G-300 mesh grids. 3. Results and discussion The SERS colloids were first examined by TEM (Fig. 2) and optical absorption spectra. TEM images reveal average particle sizes of
Fig. 2. TEM image of the SERS substrate showing the silver colloids.
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50 nm. Optical absorption spectra measurements clearly show the silver plasmon resonance at 415 nm. At the molecular concentrations used in the SERS experiments (0.01 M), the ordinary Raman spectrum of all three molecules (ortho-, meta-, and para-pyridinecarboxylic acid) (Fig. 1) was completely unobservable and it is only the enhancement by the silver colloids that allows the observation of the SERS spectrum, thus implying that all three molecules are in close proximity to the silver colloids, which are responsible for enhancing the Raman scattering. Thus, the differences observed between the SERS and Raman spectra for the three molecules can only be due to the bonding nature and strength between the silver colloids and the various pyridine compounds. A comparison of the bulk Raman (crystals) and SERS spectrum of the different pyridine molecules bound to the silver colloids yields information as to the nature of the molecular surface bonding. Bulk-Raman and SERS spectra are presented in the 600 to 1800 cm 1, region which contains the main Raman peaks of these molecules. In our colloidal solutions, we maintained a neutral to basic pH, as the colloids were prepared in that range of pH (7–9), and since the pKa of the carboxylic group of all three molecules is 1, we expect that the carboxylate form is adsorbed to the silver colloids. In the SERS spectrum, of all three molecules, we can observe the carboxylate mS (COO ) band at 1390 cm 1 and the d(COO ) band at 840 cm 1 which are characteristic of the carboxylate. Moreover, no peaks of the pyridinium cation [46,47] were observed in the SERS spectrum, as expected, since the pKa for deprotonation of the cation is 5. These observations support that the molecule is bound to the surface via the adsorption of the molecular anion form to the colloids. Bulk-Raman and SERS spectra of ortho-pyridinecarboxylic acid are shown in Fig. 3. According to an X-ray diffraction study [48], ortho-pyridinecarboxylic acid in a crystalline form takes the intermediate between the neutral molecule and the zwitterion. In the bulk-Raman of the ortho-pyridinecarboxylic acid crystals we can observe the d(COO ) band at 840 cm 1, and the N+H pyridinium cation bands at 1235 and 995 cm 1, which are characteristic of the zwitterion form of the molecule [47]. Additionally, the d(COO ) of the zwitterion form of picolinic acid at 840 cm 1 is somewhat red shifted as compared to the anionic form (SERS) which appears at 846 cm 1 (Fig. 3).
Fig. 3. Bulk-Raman (black) and SERS (red) spectra for ortho-pyridinecarboxylic acid, with 633 nm excitation wavelength (For interpretation of color mentioned in this figure the reader is referred to the web version of the article.).
Table 1 Peak frequencies and their assignments for the observed Raman and SERS spectra of ortho-pyridinecarboxylic acid [47]. Raman ortho-pyridinecarboxylic acid
SERS
Assignment
613 629 674 693 799 820 840 995 1008 1036 1050 1086 1164 1235 1298 1375 – 1440 – 1573 1596 1621 1691 3029 3055 3090 3112
– 638 – 700 – 825 846 – 1016 – 1056 – 1174 – 1298 – 1385 1438 1475 1573 1596 – – – – – –
mCO + mCC + mCH, 1 mCCC, 6b msCO, 18a II CCO2 bend
p CH, 10a OCO scissoring dCOO N+H bend mCO + mCC + mCH, 1 mCCC, 12 mCCC, 12 dCCC + dCCO2 , 18b msCCO, 18a I N+H bend dCH + mCC, 3 mCC + bOH, 14 msCOO N+H bend mCC, 19a mCC, 8b mCC, 8a mCC, 8a msC@O mCCO2H, 13 mCH, 7b mCH, 2 mCH, 20b
When comparing between the bulk-Raman and SERS spectrum of the ortho isomer (Fig. 3) we observe four peaks that are blue shifted in comparison to the bulk-Raman spectrum, where all the assignments were made by Spinner [49] (Table 1) The m1 and m6b radial skeletal vibrations associated with the C–O stretching vibrations are shifted from 1008 and 629 cm 1 in the Raman spectrum to 1016 and 638 cm 1 in the SERS spectrum. The CCO2 bend is shifted from 693 cm 1 in the Raman spectrum to 700 in the SERS and the mS (CCO) in plane bending vibration is shifted from 1164 in the Raman to 1174 in the SERS spectrum. In accordance with Suh’s work we propose a two-legged geometry, where the pyridine ring on the silver colloids is tilted from a totally flat geometry, and both the pyridine nitrogen and the carboxylate can interact with the metal colloid. In this case, the carboxylate interacts with both oxygens to the metal. Herein, r bonding of the pyridine nitrogen and the carboxylate with the metal colloid occurs, instead of the more common p bonding of pyridine rings. This phenomena is known to cause blue shifts [44] between the Raman and SERS spectrum, as seen here. We can also corroborate the geometry of the ortho isomer by Suh’s [40] convention as the intensity of the mS (COO ) band at 1385 cm 1 and the d(COO ) band at 846 cm 1 are of comparable intensity, which is characteristic of the two-legged geometry. On the whole, blue shifts between the Raman and SERS, which occur due to r bonding, are proven to correlate to a standing up or tilted geometry of pyridine rings, where p bonding does not occur between the pyridine ring and the metal colloids (Fig. 6). Conversely, with meta- and para-pyridinecarboxylic acid we observe red shifts between the Raman and SERS spectrum (Figs. 4 and 5 and Tables 2 and 3). For meta-pyridinecarboxylic acid the skeletal ring vibration [47] m12 is shifted from 1040 cm 1 in the bulk-Raman spectrum to 1028 cm 1 in the SERS spectrum. The m3 in plane bending vibration is shifted from 1237 cm 1 in the Raman spectrum to 1220 cm 1 in the SERS spectrum (Fig. 4). Similarly, in the para-pyridinecarboxylic acid spectrum [46] the m1 skeletal ring vibration is shifted from 1022 cm 1 in the Raman spectrum to 1005 cm 1 in the SERS spectrum (Fig. 5).
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Table 2 Peak frequencies and their assignments for the observed Raman and SERS spectra of meta-pyridinecarboxylic acid [47]. Raman meta-pyridinecarboxylic acid
SERS
Assignment
636
654 700 814 841 910 1002 1028 1182 1220 1300 1377 1506 1538 1585 – – –
ms(CCO), 18a II CCO2 bend OCO scissoring dCOO – mCO + mCC + mCH, 1 mCCC, 12 dCH, 9a dCH + mCC, 3 msCCO msCOO
804 – 1026 1040 1182 1237 1300
Fig. 4. Bulk-Raman (black) and SERS (red) spectra for meta-pyridinecarboxylic acid, with 633 nm excitation wavelength (For interpretation of color mentioned in this figure the reader is referred to the web version of the article.).
– – 1595 1687 3075 3088
mCC, 8a msC@O mCH, 7b mCH, 20b
Table 3 Peak frequencies and assignments for the observed Raman and SERS spectra of parapyridinecarboxylic acid [46].
Fig. 5. Bulk-Raman (black) and SERS (red) spectra for para-pyridinecarboxylic acid, with 633 nm excitation wavelength (For interpretation of color mentioned in this figure the reader is referred to the web version of the article.).
According to Suh’s work [40] meta- and para-pyridinecarboxylic acid belong to the one-legged geometry, with the pyridine ring lying flat on the surface, and the carboxylate tilted in order to bind one of the oxygens coordinatively to the surface (Fig. 6). To prove this, in the SERS spectrum the intensities of the d(COO ) bands at 840 cm 1 for both isomers are stronger than the comparable mS (COO ) bands at 1375 cm 1 which is common with the one-legged geometry [40]. In this geometry, p bonding between the pyridine ring and the metal surface causes some red shifts between the Raman and SERS peaks [44]. r bonding may also have a small affect, in this geometry, as one oxygen of the carboxylate is bonded coordinatively to the silver colloid. This can be seen by a small blue
Raman para-pyridinecarboxylic acid
SERS
Assignment
656 – 1022 – – – – 1611 3056 3073 3085 3105
– 840 1005 1145 1216 1372 1550 1600 – – – –
CCO sym str + 18a, II COO scis mCO + mCC + mCH, 1 CCO sym str + 18a, I dCH, 9a COO sym str mCC, 8b mCC, 8a mCCO2H, 13 mCH, 7b mCH, 2 mCH, 20b
shift in the OCO scissoring of the meta isomer, which is shifted from 804 cm 1 in the Raman spectrum to 814 cm 1 in the SERS spectrum, typical of r bonding. Conversely, in a totally flat geometry, where no coordinative bonds are created, as with the oxygens in the one-legged geometry, shifts are less likely to be seen [39] since bonding between the pyridine ring and the surface is also considerably weaker, and mostly physical. 4. Conclusion In summary, the chemical enhancement component of the overall SERS enhancement is associated with chemical interactions between the metal and the adsorbate which can also affect some of the vibrational frequencies of the adsorbed molecules. When chemical adsorption occurs through r bonding, as with ortho-pyridinecarboxylic acid, SERS peaks are blue shifted in comparison to
Fig. 6. Illustration of the molecular orientation on the silver surface of para-pyridinecarboxylic acid, meta-pyridinecarboxylic acid and ortho-pyridinecarboxylic acid.
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the bulk-Raman spectrum. These blue shifts, which correlate to higher vibrational energies, are an indication of a strong coordinative bond, which is apparent in the tilted two-legged geometry that binds coordinatively through both the pyridinic nitrogen and both oxygens of the carboxylate group. On the other hand, when chemical adsorption takes place through p bonds, as with the meta and para isomers, SERS peaks are red shifted in comparison to their respective Raman peaks. Red shifts correlate to lower vibrational energies, which point towards delocalized p binding to the surface. This surface binding is apparent in the one-legged geometry that binds through the pyridine p system and coordinatively with only one of the carboxylate oxygens. Peaks that are blue or red shifted are usually associated with vibrations that are in close proximity to the binding sites of the molecules to the metal colloids, and are therefore most affected by it. References [1] M. Fleischmann, P.J. Hendra, A.J. McQuilan, J. Chem. Soc. Chem. Comm. 3 (1973) 80. [2] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Chem. Phys. Lett. 26 (1974) 163. [3] M.G. Albrecht, J.F. Evans, J.A. Creighton, Surf. Sci. 75 (1978) L777. [4] D.L. Jeanmaire, R.P. Van Duyne, J. Electroanal. Chem. 84 (1977) 1. [5] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [6] G.C. Schatz, R.P. Van Duyne, Handbook of Vibrational Spectroscopy, John Wiley & Sons, Inc., New York, 2002. [7] B.N.J. Persson, Chem. Phys. Lett. 82 (1981) 561. [8] P. Kambhampati, C.M. Child, M.C. Foster, A. Campion, J. Chem. Phys. 108 (1998) 5013. [9] J.R. Lombardi, R.L. Birke, T. Lu, J. Xu, J. Chem. Phys. 84 (1986) 4174. [10] T. Wang, X. Hu, S. Dong, J. Phys. Chem. B 110 (2006) 16930. [11] S.P. Mulvaney, H. Lin, M.J. Natan, C.D. Keating, J. Raman Spectrosc. 34 (2003) 163. [12] L. Rivas, S. Sanchez-Cortes, J.V. Garcia-Ramos, G. Morcillo, Langmuir 16 (2000) 9722. [13] F. Jinghuai, H. Yunxis, L. Xia, D. Xiaoming, J. Raman Spectrosc. 35 (2004) 914. [14] N.R. Jana, Analyst 128 (2003) 954. [15] K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, Chem. Rev. 99 (1999) 2957.
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Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Surface-enhanced Raman spectroscopy as a probe for orientation of pyridine compounds on colloidal surfaces D.H. Dressler b,c,*, Y. Mastai b,c, M. Rosenbluh a,b, Y. Fleger a,b a
The Jack and Pearl Resnick Institute for Advanced Technology, Department of Physics, Bar-Ilan University, Ramat-Gan, Israel Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel c Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel b
a r t i c l e
i n f o
Article history: Received 3 May 2009 Received in revised form 28 June 2009 Accepted 29 June 2009 Available online 2 July 2009 Keywords: Raman spectroscopy Surface-enhanced Raman scattering Self assembly Aromatics Colloids
a b s t r a c t Surface-enhanced Raman spectroscopy (SERS) has been used to characterize three isomeric pyridine compounds adsorbed to silver colloids. The three structural isomers, which all adsorb in the carboxylate form, to the silver colloids, bind in two different geometries to the silver surface. The differences in the binding geometry correlate with Raman resonance shifts and intensity enhancements that can be probed by SERS. For picolinic acid, the ortho isomer, we observed blue shifts of many of the SERS peaks attributed to the pyridine ring in comparison to the normal Raman vibrations of the non-adsorbed crystalline material. For this molecule, steric hindrance between the carboxylate and nitrogen groups causes the pyridine ring and the carboxylate group to tilt from the flat geometry, and hence both groups can form r bonding to the silver surface. On the other hand, nicotinic and isonicotinic acid, the meta and para isomers (respectively), that have less steric hindrance, adsorb to the silver colloids with the pyridine ring in a flat geometry, where only the carboxylate functional group tilts in order to bind coordinatively to the silver colloids. In this case, p bonding between the pyridine ring and the metal colloids causes small red shifts between the Raman and SERS peaks. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS), which was discovered in 1970 [1,2] enables detection of even weak Raman vibrations due to the SERS enhancement. This enhancement, as many experiments show [3,4], occurs due to the presence of a metal surface, and contains two components; electromagnetic [5,6] and chemical [7–9]. Since the discovery of SERS a vast literature concerning this effect has been published using different metal (gold, silver and copper [10–14]) substrates and ways to create SERS enhancements including aggregated metal colloids [13,15], metal island films [16], electrochemically roughened electrodes and metal colloidal monolayers [13,14]. One of the most important applications of SERS has been in the spectroscopy of organic self assembled monolayers (SAMS), which have drawn much attention since they provide a useful method of tailoring physiochemical properties of surfaces to various technical applications in biosensing, nanopatterning, and molecular electronic devices [17,18]. In particular, aromatic carboxylates such as mercapto benzoic acid (MBA) have been shown to be useful
* Corresponding author. Address: Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel. Tel.: +972 3 5317877; fax: +972 3 7384053. E-mail address: [email protected] (D.H. Dressler). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.06.042
for detection of different explosives [19–21]. SERS has proved to be one of the most sensitive techniques for monitoring the adsorbates of particularly short aromatic molecules on metal substrates at the submonolayer coverage limit [5,6,22–25]. An exciting recent development is the ability to get single molecule detection or Raman enhancements of up to 1015 by creating metal nanostructures or ‘‘hot spots” [26,27]. The occurrence of shifts in the Raman lines of a SERS spectrum depends on the mode of enhancement in the SERS spectra. The electromagnetic component, which is the major SERS enhancement factor, occurs due to the excitation of surface plasmons in the metal nanocluster, while the chemical component occurs due to molecular interactions between the metal surface and the adsorbate. This chemical component causes weaker SERS enhancement but may lead to frequency shifts of some of the molecular Raman vibrations, as will be shown in this paper. In order to analyze the influence of chemical enhancement on the SERS spectra, one needs to consider two different possible interactions between the adsorbate and the metal colloid. Physical adsorption, which is the result of relatively weak interactions between the solid surface and the adsorbate with an enthalpy of adsorption higher than 25 kJ/mol, and chemical adsorption, where the adsorbed molecules are strongly bound to the surface with an enthalpy of adsorption lower than 40 kJ/mol [28,29]. In the process of chemisorption, the adsorbed molecule changes its
D.H. Dressler et al. / Journal of Molecular Structure 935 (2009) 92–96
chemical structure and symmetry due to bond formation with the metal. This stronger adsorption usually introduces frequency shifts between vibrations of adsorbed molecules in comparison to the normal Raman spectrum of these molecules as shown for instance for C60 adsorbed on noble metals [30], and for thiophenol adsorbed on gold [31]. Generally, on a silver surface, chemisorption can take place through the formation of a bond between the Ag surface and an adsorbed molecule e.g., Ag–N, Ag–O, Ag–S, or Ag–X (X = halogen) bond [32–34]. In this paper, we show that frequency shifts in the SERS spectrum can be correlated to specific molecular bonding geometries. The relationship between molecular configuration and the specific binding mechanisms inferred from the frequency shifts, enables us to identify surface geometries of three different pyridine compounds adsorbed to metallic colloids, and may be implemented to several other adsorbed molecules. SERS has been used previously for determining the orientation of molecules adsorbed on certain surfaces [35–39]. The SERS spectra of many aromatic carboxylic acids have shown that the acid is adsorbed on the surface as a carboxylate ion [36,37,40–42]. The carboxylate can bind to a metal surface in a flat orientation through the carboxylate p system, or tilted with respect to the aromatic ring, in the case where steric hindrance occurs between the carboxylate and other ring substituents [43]. The geometry of the carboxylates on the surface, can affect the binding nature of these molecules on the surface. Moreover, for ortho substituents as the pyridine nitrogen, steric hindrance causes both the pyridine ring and the carboxylate to tilt from the flat orientation, in order to bind to the surface. This geometry also implicates different binding to the surface, as will be shown in this paper. Suh and Kim [40] identified three different binding geometries for aromatic carboxylates adsorbed on surfaces, depending on the relative position between the carboxylate and other substituents on the aromatic ring. The one-legged geometry, in which the carboxylate rotates from the plane of the ring due to steric hindrance between adjacent substituents, and only one oxygen of the CO2 group binds strongly through its oxygen lone pair electrons to the surface. This geometry can be observed with ortho, meta, and para isomers, depending on the relative substituents. The two-legged geometry, where the CO2 is adsorbed to the metal surface in a tilted position through both of its oxygen atoms, and the ring is also tilted with respect to the surface. This geometry, is evident only with ortho substituents, where large steric hindrance is apparent. The third geometry, is the totally flat geometry, which is not observed in this paper, where the ring as well as the carboxylate and the other substituent lie entirely flat on the surface, and the carboxylate binds to the surface through its p-electrons. This is usually observed in para and meta substituted molecules with less steric hindrance. In their paper, they showed [40], that the different geometries can be detected by the relative intensities of the carboxylate bending/stretching vibrations. In this paper, we show, that the nature of the adsorbate–metal bonding can also be detected by the presence of frequency shifts in the SERS spectrum [44], in comparison to the bulk crystalline Raman spectrum of the same molecule. We compare the SERS and Raman spectrum of three pyridine compounds, para-, meta-, and ortho-pyridinecarboxylic acid (Fig. 1). These isomers, with the carboxylate and pyridine nitrogen at different relative positions, bind in two different geometries to the silver colloids [40], which can be detected by frequency shifts in the SERS spectrum. For the ortho isomer, picolinic acid, with the two-legged geometry [40], both the nitrogen and the carboxylic acid tilt and bind coordinatively to the metal nanoclusters. In this case, r bonding of the pyridine nitrogen and the carboxylate to the metal nanoclusters causes blue shifts [44] between its Raman and SERS spectrum. The meta, and para isomer, with a one-legged geometry, adsorb to
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Fig. 1. Molecular structures of para-pyridinecarboxylic acid, meta-pyridinecarboxylic acid and ortho-pyridinecarboxylic acid.
the silver nanoparticles with the pyridine ring in a flat geometry. In this case, p bonding between the pyridine ring and the metal nanoparticles causes small red shifts [44] between the Raman and SERS peaks. 2. Experimental Analytical grade reagents (>99.9%) were purchased from Aldrich–Sigma and used without further purification. We prepared silver colloids in an aqueous solution by the reduction of silver nitrate with hydroxylamine hydrochloride at alkaline pH as described in the literature [45], and then adsorbed the pyridinecarboxylic acids on them. Aqueous solutions (0.1 M) of the pyridinecarboxylic acids were added to the silver colloidal solution, to a final concentration of 0.01 M. The SERS spectrum of the colloidal solutions was immediately collected under a Raman microscope. For comparison the normal Raman spectrum of the crystalline acids were measured. Optical absorptions spectra of the colloids were obtained by a Cary 500 UV–Vis–NIR spectrophotometer. Raman and SERS measurements were performed with a Jobin Yvon Micro Raman (Model HR800, k = 633 nm) using different objectives. Raman and SERS spectrum were obtained in the range of 200–3500 cm 1. In order to confirm the results Raman and SERS measurements were repeated several times (several samples and acquisitions). TEM images were acquired by a JEOL TEM using an acceleration voltage of 120 kV. The TEM samples were prepared by lifting an aliquot of the colloidal solution onto copper G-300 mesh grids. 3. Results and discussion The SERS colloids were first examined by TEM (Fig. 2) and optical absorption spectra. TEM images reveal average particle sizes of
Fig. 2. TEM image of the SERS substrate showing the silver colloids.
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50 nm. Optical absorption spectra measurements clearly show the silver plasmon resonance at 415 nm. At the molecular concentrations used in the SERS experiments (0.01 M), the ordinary Raman spectrum of all three molecules (ortho-, meta-, and para-pyridinecarboxylic acid) (Fig. 1) was completely unobservable and it is only the enhancement by the silver colloids that allows the observation of the SERS spectrum, thus implying that all three molecules are in close proximity to the silver colloids, which are responsible for enhancing the Raman scattering. Thus, the differences observed between the SERS and Raman spectra for the three molecules can only be due to the bonding nature and strength between the silver colloids and the various pyridine compounds. A comparison of the bulk Raman (crystals) and SERS spectrum of the different pyridine molecules bound to the silver colloids yields information as to the nature of the molecular surface bonding. Bulk-Raman and SERS spectra are presented in the 600 to 1800 cm 1, region which contains the main Raman peaks of these molecules. In our colloidal solutions, we maintained a neutral to basic pH, as the colloids were prepared in that range of pH (7–9), and since the pKa of the carboxylic group of all three molecules is 1, we expect that the carboxylate form is adsorbed to the silver colloids. In the SERS spectrum, of all three molecules, we can observe the carboxylate mS (COO ) band at 1390 cm 1 and the d(COO ) band at 840 cm 1 which are characteristic of the carboxylate. Moreover, no peaks of the pyridinium cation [46,47] were observed in the SERS spectrum, as expected, since the pKa for deprotonation of the cation is 5. These observations support that the molecule is bound to the surface via the adsorption of the molecular anion form to the colloids. Bulk-Raman and SERS spectra of ortho-pyridinecarboxylic acid are shown in Fig. 3. According to an X-ray diffraction study [48], ortho-pyridinecarboxylic acid in a crystalline form takes the intermediate between the neutral molecule and the zwitterion. In the bulk-Raman of the ortho-pyridinecarboxylic acid crystals we can observe the d(COO ) band at 840 cm 1, and the N+H pyridinium cation bands at 1235 and 995 cm 1, which are characteristic of the zwitterion form of the molecule [47]. Additionally, the d(COO ) of the zwitterion form of picolinic acid at 840 cm 1 is somewhat red shifted as compared to the anionic form (SERS) which appears at 846 cm 1 (Fig. 3).
Fig. 3. Bulk-Raman (black) and SERS (red) spectra for ortho-pyridinecarboxylic acid, with 633 nm excitation wavelength (For interpretation of color mentioned in this figure the reader is referred to the web version of the article.).
Table 1 Peak frequencies and their assignments for the observed Raman and SERS spectra of ortho-pyridinecarboxylic acid [47]. Raman ortho-pyridinecarboxylic acid
SERS
Assignment
613 629 674 693 799 820 840 995 1008 1036 1050 1086 1164 1235 1298 1375 – 1440 – 1573 1596 1621 1691 3029 3055 3090 3112
– 638 – 700 – 825 846 – 1016 – 1056 – 1174 – 1298 – 1385 1438 1475 1573 1596 – – – – – –
mCO + mCC + mCH, 1 mCCC, 6b msCO, 18a II CCO2 bend
p CH, 10a OCO scissoring dCOO N+H bend mCO + mCC + mCH, 1 mCCC, 12 mCCC, 12 dCCC + dCCO2 , 18b msCCO, 18a I N+H bend dCH + mCC, 3 mCC + bOH, 14 msCOO N+H bend mCC, 19a mCC, 8b mCC, 8a mCC, 8a msC@O mCCO2H, 13 mCH, 7b mCH, 2 mCH, 20b
When comparing between the bulk-Raman and SERS spectrum of the ortho isomer (Fig. 3) we observe four peaks that are blue shifted in comparison to the bulk-Raman spectrum, where all the assignments were made by Spinner [49] (Table 1) The m1 and m6b radial skeletal vibrations associated with the C–O stretching vibrations are shifted from 1008 and 629 cm 1 in the Raman spectrum to 1016 and 638 cm 1 in the SERS spectrum. The CCO2 bend is shifted from 693 cm 1 in the Raman spectrum to 700 in the SERS and the mS (CCO) in plane bending vibration is shifted from 1164 in the Raman to 1174 in the SERS spectrum. In accordance with Suh’s work we propose a two-legged geometry, where the pyridine ring on the silver colloids is tilted from a totally flat geometry, and both the pyridine nitrogen and the carboxylate can interact with the metal colloid. In this case, the carboxylate interacts with both oxygens to the metal. Herein, r bonding of the pyridine nitrogen and the carboxylate with the metal colloid occurs, instead of the more common p bonding of pyridine rings. This phenomena is known to cause blue shifts [44] between the Raman and SERS spectrum, as seen here. We can also corroborate the geometry of the ortho isomer by Suh’s [40] convention as the intensity of the mS (COO ) band at 1385 cm 1 and the d(COO ) band at 846 cm 1 are of comparable intensity, which is characteristic of the two-legged geometry. On the whole, blue shifts between the Raman and SERS, which occur due to r bonding, are proven to correlate to a standing up or tilted geometry of pyridine rings, where p bonding does not occur between the pyridine ring and the metal colloids (Fig. 6). Conversely, with meta- and para-pyridinecarboxylic acid we observe red shifts between the Raman and SERS spectrum (Figs. 4 and 5 and Tables 2 and 3). For meta-pyridinecarboxylic acid the skeletal ring vibration [47] m12 is shifted from 1040 cm 1 in the bulk-Raman spectrum to 1028 cm 1 in the SERS spectrum. The m3 in plane bending vibration is shifted from 1237 cm 1 in the Raman spectrum to 1220 cm 1 in the SERS spectrum (Fig. 4). Similarly, in the para-pyridinecarboxylic acid spectrum [46] the m1 skeletal ring vibration is shifted from 1022 cm 1 in the Raman spectrum to 1005 cm 1 in the SERS spectrum (Fig. 5).
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Table 2 Peak frequencies and their assignments for the observed Raman and SERS spectra of meta-pyridinecarboxylic acid [47]. Raman meta-pyridinecarboxylic acid
SERS
Assignment
636
654 700 814 841 910 1002 1028 1182 1220 1300 1377 1506 1538 1585 – – –
ms(CCO), 18a II CCO2 bend OCO scissoring dCOO – mCO + mCC + mCH, 1 mCCC, 12 dCH, 9a dCH + mCC, 3 msCCO msCOO
804 – 1026 1040 1182 1237 1300
Fig. 4. Bulk-Raman (black) and SERS (red) spectra for meta-pyridinecarboxylic acid, with 633 nm excitation wavelength (For interpretation of color mentioned in this figure the reader is referred to the web version of the article.).
– – 1595 1687 3075 3088
mCC, 8a msC@O mCH, 7b mCH, 20b
Table 3 Peak frequencies and assignments for the observed Raman and SERS spectra of parapyridinecarboxylic acid [46].
Fig. 5. Bulk-Raman (black) and SERS (red) spectra for para-pyridinecarboxylic acid, with 633 nm excitation wavelength (For interpretation of color mentioned in this figure the reader is referred to the web version of the article.).
According to Suh’s work [40] meta- and para-pyridinecarboxylic acid belong to the one-legged geometry, with the pyridine ring lying flat on the surface, and the carboxylate tilted in order to bind one of the oxygens coordinatively to the surface (Fig. 6). To prove this, in the SERS spectrum the intensities of the d(COO ) bands at 840 cm 1 for both isomers are stronger than the comparable mS (COO ) bands at 1375 cm 1 which is common with the one-legged geometry [40]. In this geometry, p bonding between the pyridine ring and the metal surface causes some red shifts between the Raman and SERS peaks [44]. r bonding may also have a small affect, in this geometry, as one oxygen of the carboxylate is bonded coordinatively to the silver colloid. This can be seen by a small blue
Raman para-pyridinecarboxylic acid
SERS
Assignment
656 – 1022 – – – – 1611 3056 3073 3085 3105
– 840 1005 1145 1216 1372 1550 1600 – – – –
CCO sym str + 18a, II COO scis mCO + mCC + mCH, 1 CCO sym str + 18a, I dCH, 9a COO sym str mCC, 8b mCC, 8a mCCO2H, 13 mCH, 7b mCH, 2 mCH, 20b
shift in the OCO scissoring of the meta isomer, which is shifted from 804 cm 1 in the Raman spectrum to 814 cm 1 in the SERS spectrum, typical of r bonding. Conversely, in a totally flat geometry, where no coordinative bonds are created, as with the oxygens in the one-legged geometry, shifts are less likely to be seen [39] since bonding between the pyridine ring and the surface is also considerably weaker, and mostly physical. 4. Conclusion In summary, the chemical enhancement component of the overall SERS enhancement is associated with chemical interactions between the metal and the adsorbate which can also affect some of the vibrational frequencies of the adsorbed molecules. When chemical adsorption occurs through r bonding, as with ortho-pyridinecarboxylic acid, SERS peaks are blue shifted in comparison to
Fig. 6. Illustration of the molecular orientation on the silver surface of para-pyridinecarboxylic acid, meta-pyridinecarboxylic acid and ortho-pyridinecarboxylic acid.
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