- Email: [email protected]
Insight into vibration mode-resolved plasmon enhanced Raman optical activity
Journal of Colloid and Interface Science 415 (2014) 165–168
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Insight into vibration mode-resolved plasmon enhanced Raman optical activity Yongqing Li a,b,⇑,1, ShaoXiang Sheng b,1, Zhenglong Zhang b,c,e,⇑, Liwei Liu d, Mengtao Sun b,⇑ a
Department of Physics, Liaoning University, Shenyang, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-146, Beijing 100190, PR China c Physical Chemistry, Friedrich-Schiller University Jena, Helmholtzweg, 07743 Jena, Germany d Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, PR China e Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany b
a r t i c l e
i n f o
Article history: Received 16 August 2013 Accepted 13 October 2013 Available online 29 October 2013 Keywords: Plasmon enhancement Raman optical activities Vibration mode-resolved
a b s t r a c t Vibration mode-resolved plasmon enhanced Raman optical activities (PE-ROA) are investigated experimentally and theoretically. It is found that the profile of PE-ROA is significantly different from that of ROA. Our experimental results revealed that PE-ROA is not a universal method for the enhancement of ROA. It should be very carefully for the using of plasmon enhancement for ROA. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Raman optical activity (ROA) is a vibrational spectroscopic technique that is reliant on the difference in intensity of Raman scattered right and left circularly polarized light due to molecular chirality. Barron and Buckingham developed the definitive theory of ROA in 1971 [1], and made the first observations in 1973 [2]. Due to its sensitivity to chirality, Raman optical activity is a useful probe of biomolecular structure and behavior in aqueous solution. It has been used to study protein, nucleic acid, carbohydrate and virus structures [3–11]. However, ROA signals are even weaker 103 than the Raman signals, and it is very hard to be detected, especially at low and high frequencies, due to the limitation of instrument for chiral Raman (for example, there is no suitable optical grating and cannot resolve the signal of chiral Raman). Also, the measurements on the molecular chirality need large molecular concentration and longtime accumulation (several hours). Surface enhanced Raman scattering (SERS) has been widely used for the ultrasensitive chemical analysis [12,13], even at single molecular level [14], where the Raman signals can be enhanced up ⇑ Corresponding authors. Addresses: Department of Physics, Liaoning University, Shenyang, PR China (Y.Q. Li), Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-146, Beijing 100190, PR China (M. Sun and Z. Zhang). E-mail addresses: [email protected] (Y. Li), [email protected] (Z. Zhang), [email protected] (M. Sun). 1 These authors contributed equally to this work. 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.022
to 1011 by the electromagnetic mechanism [14], as well as the chemical enhancement [15]. Following SERS method, the ROA signals can also be strongly enhanced by surface plasmon resonance [16–21]. It is expected that plasmon enhanced ROA (PE-ROA) can resolve above intrinsic weaknesses of ROA. However, it is a great challenge that how to correctly understanding the plasmon enhanced ROA, due to the very weak chiral Raman signals and the influence of chemical enhancement as well as the substrate, which might change molecular chirality. The new stereo structure introduced by substrate may break molecular intrinsic stereo center. New symmetric minor, induced new chiral center by metal surface, and then the orientation of molecular charge is reversed, the chirality is changed. Chiral molecular vibrational motion induced electric dipole coupling with the magnetic dipole, which is the excited electronic charges, relaxed during Raman process. In this communication, we try to insight into vibration moderesolved PE-ROA of L-alanine and D-alanine experimentally and theoretically at high, low and middle frequencies, respectively. Lalanine and D-alanine are of the opposite chirality. So, they are the best molecules to study the influence of plasmon enhancement for the chirality of PE-ROA. If the plasmon can enhanced ROA spectra and the their chirality can be kept during the plasmon enhancement. We can conclude the plasmon enhancement is a valid and effective method for the enhancement of ROA by plasmon. The influences of charge transfer between molecule and metal, the new chemical bond between molecule and substrate, and the adsorption status of molecule on metal were comprehensively investigated. It is found that PE-ROA is not a universal method
166
Y. Li et al. / Journal of Colloid and Interface Science 415 (2014) 165–168
for the enhancement of ROA. It should be very carefully for the using of plasmon enhancement for POA. The solid L-alanine and D-alanine of over 99% purity were purchased from Aldrich Chemical Co., Ltd. and used without further purification. Aqueous solution of L-alanine was prepared with a concentration of 100 mg/ml. The ROA spectra were measured with the backscattering SCP Biotools l-ChiralRAMAN-2X instrument with 532 nm excitation laser [22]. Laser power was set to 400 mW, with total acquisition time of 4 h. The SERS active silver colloid was prepared by citrate reduction of AgNO3 [23]. AgNO3 (90 mg) was dissolved in 650 mL of quartz distilled water. Five hundred milliliters of this solution was brought to boiling and then a solution of 1% sodium citrate (10 mL) was added. Thirty minutes later the remaining 150 mL AgNO3 solution was added 3 times every 15 min. The solution was kept on boiling about 1.5 h. The final Ag nanoparticle concentration was estimated to be 35 pM with an average radius size of about 40 nm [24]. The SERS measurements were performed with left and right polarized lights, respectively, in the confocal Raman spectroscopic system (Renishaw, Invia) through an objective of 100 with a numerical aperture (NA) of 0.85. The wavelength of laser is 632.8 nm, and the laser power is about 7 mW. Lastly, the obtained difference of SERS spectra excited with left and right polarized lights is the experimental PE-ROA spectra. For comparison, the molecular ROA spectra of L-alanine at high frequencies was simulated, using density functional theory [25], B3LYP functional [26] and 6–31 ++g(2d,p) basis set. To theoretically interpret the PE-ROA, the vibrational modes of molecule adsorbed on the Ag20 cluster were theoretically calculated, using DFT, B3LYP functional and 6–31 g(d) basis set for C, N, O and H, and LanL2DZ basis set [27] for Ag. The optimized geometry can be seen from Supporting information. All the quantum chemical calculations were done with Gaussian 09 software [28]. Fig. 1(A) and (B) are the experimental PE-ROA of L-alanine and simulated ROA of L-alanine, respectively. It is found that PE-ROA peaks can be well observed experimentally, and interpreted with the simulated ROA spectrum. Note that the simulated ROA spectrum does not consider metal in the calculation, which results in
the difference of ROA intensities. The vibrational modes a-d in Fig. 1(A) can be seen from Fig. 2. For peak a, it is symmetric vibrations of 3 H atoms on C(21) as well as symmetric H’s vibration on C(22). For peak b, it is symmetric vibrations of 3 H atoms on C(21) as well as asymmetric H’s vibration on C(22). For peak c, it is stretching mode of H(28) on C(21). For peak d, it is stretching mode of H(30) on C(21). So, the PE-ROA at high frequencies can overcome the instrument limitation of chiral Raman for ROA. For comparison, we also measured the PE-ROA spectra of D-alanine at high vibrational frequencies (see Fig. 3(a)). Comparing Fig. 3(a) and (b), we found that their PE-ROA spectra cannot be completely reversed, and strongly influenced by the introduced metal substrate. So, for the plasmon enhancement for D-alanine is not a good choice. Secondly, we measured the PE-ROA spectrum of L-alanine and compared with the ROA spectrum of L-alanine, see Fig. 4. It is found that the profiles of them are significantly different below 600 cm1.
(a)
c
(A) PEROA (a.u.)
Fig. 2. The vibrational modes of ROA peaks (a-d) of L-alanine in Fig. 1(a).
D-alanine
b
PE-ROA
a
d
(b)
L-alanine
ROA (a.u.)
(B)
2800
2900
3000
3100
Wavenumber / cm-1 Fig. 1. (A) and (B) are the experimental PE-ROA of L-alanine and simulated ROA spectra of L-alanine, respectively.
2900
2950
3000
3050
3100
Wavenumber / cm-1 Fig. 3. (a) and (b) are the experimental PE-ROA and simulated ROA spectra, respectively.
ROA (a.u.)
Y. Li et al. / Journal of Colloid and Interface Science 415 (2014) 165–168
p
(A)
j
o 400
PEROA (a.u.)
167
600
800
1000
(B)
1200
g
i
1400
1600
h j’ k
m Fig. 7. Vibrational modes g and h in of L-alanine Fig. 4(b).
l e
f
400
600
800
1000
1200
1400
1600
Wavenumber / cm-1 Fig. 4. Experimental ROA of L-alanine (a) and experimental PE-ROA of L-alanine (b).
Fig. 5. Vibrational modes e and f in Fig. 4.
Fig. 8. Vibrational modes i, j, k and m in of L-alanine Fig. 3(b).
PE-ROA peaks e and f are strongly enhanced, due to the interaction between molecule and metal, and charge transfer results in the further chemical enhancement. Fig. 5 revealed that these PE-ROA peaks also include the vibration of new chemical bonds between O atoms and metal. Furthermore, the large amounts of charge transfer between O atoms and metal (see Fig. 6) can result in the chemical enhancement for PE-ROA. So, some of PE-ROA peaks can be chemical enhanced at low frequencies, especially for the new chemical bonds between molecule and metal, when the molecule chemically adsorbed on the metal substrate. Thirdly, we interpret the reason that the PE-ROA peaks g and h of L-alanine were strongly selected enhanced. Fig. 7 revealed that for these two PE-ROA peaks, the new stereo structure introduced
by substrate may break molecular intrinsic stereo center, and there is a new symmetric face that is perpendicular to the substrate. The C@O stretching vibrations were also involved in these two vibrational modes, as we know the chemical bond between substrate and O atoms can also influence the intensities of PE-ROA peaks (see Fig. 8). For the PE-ROA peaks i, j0 , k, l and m of in Fig. 4(b), the chirality was reversed, compared with their ROA peaks of L-alanine in Fig. 4(a). These vibrational modes can be seen from Fig. 7. New symmetric minor by the substrate and new chiral center by metal surface, can results in the orientation of molecular charge is re-
Fig. 6. The distribution of Mulliken charge distribution of L-alanine.
Y. Li et al. / Journal of Colloid and Interface Science 415 (2014) 165–168
PE-ROA (a.u.)
168
Basic Research Project of China (Grants 2009CB930701), the Program of Shenyang Key Laboratory of Optoelectronic Materials and Technology (Grant No. F12-254-1-00).
D-alanine
Appendix A. Supplementary material 500
1000
1500
PEROA (a.u.)
L-alanine
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.022. References
200
400
600
800
1000
1200
1400
1600
Wavenumber / cm-1 Fig. 9. Experimental PE-ROA of D-alanine and L-alanine.
versed, the chirality is changed. Chiral molecular vibraional motion induced electric dipole coupling with the magnetic dipole is also reversed, when excited electronic charges relaxed during Raman processes. Lastly, to study the substrate influence of the substrate and plasmon, we measured the PE-ROA spectra of D-alanine, see Fig. 9. It is found that the molecular chirality cannot be realized at all. Though L-alanine and D-alanine have the reversed ROA, but they have almost the same chirality for PE-ROA, except for the ROA peak at 850 cm1. So, the plasmon enhanced effect is not valid for our studied chiral molecules. In summary, vibration mode-resolved PE-ROA of L-alanine and D-alanine were investigated experimentally and theoretically. It is found that the plasmon can strongly enhanced ROA spectra, but the their chirality cannot be kept during the plasmon enhancement. We can conclude the plasmon enhancement is not a valid and effective method for the enhancement of ROA by plasmon. Our experimental results revealed that PE-ROA is not a universal method for the enhancement of ROA. It should be very carefully for the using of plasmon enhancement for POA. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grants 11374353 and 11274149), the National
[1] L.D. Barron, A.D. Buckingham, Mol. Phys. 20 (1971) 1111. [2] L.D. Barron, M.P. Bogaard, A.D. Buckingham, J. Am. Chem. Soc. 95 (1973) 603. [3] J. Costante, L. Hecht, P.L. Polavarapu, A. Collet, L.D. Barron, Ang. Chem. Int. Ed. Engl. 36 (1997) 885. [4] L.D. Barron, L. Hecht, E.W. Blanch, A.F. Bell, Prog. Biophys. Mol. Biol. 73 (2000) 1. [5] E.W. Blanch, L.A. Morozova-Roche, D.A.E. Cochran, A.J. Doig, L. Hecht, L.D. Barron, J. Mol. Biol. 301 (2000) 553. [6] C.D. Syme, E.W. Blanch, C. Holt, R. Jakes, M. Goedert, L. Hecht, L.D. Barron, Eur. J. Biochem. 269 (2002) 148. [7] E.W. Blanch, D.J. Robinson, L. Hecht, C.D. Syme, K. Nielsen, L.D. Barron, J. Gen. Virol. 83 (2002) 241–246. [8] C.D. Syme, V. Volpetti, G.P. Lomonossoff, K. Nielsen, L.D. Barron, J. Gen. Virol. 83 (2002) 2593. [9] L.D. Barron, E.W. Blanch, L. Hecht, Adv. Prot. Chem. 62 (2002) 51. [10] L.D. Barron, E.W. Blanch, I.H. McColl, C.D. Syme, L. Hecht, K. Nielsen, Spectroscopy 17 (2003) 101. [11] I.H. McColl, E.W. Blanch, A.C. Gill, A.G.O. Rhie, M.A. Ritchie, L. Hecht, K. Nielsen, L.D. Barron, J. Am. Chem. Soc. 125 (2003) 10019–10026. [12] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Chem. Phys. Lett. 26 (1974) 163. [13] D.L. Jeanmaire, R.P. Van Duyne, J. Electroanal. Chem. Interfacial Electrochem. 84 (1977) 1. [14] H.X. Xu, E.J. Bjerneld, M. Kail, L. Borjesson, Phys. Rev. Lett. 83 (1999) 4357. [15] M.T. Sun, S. Liu, M. Chen, H.X. Xu, J. Raman Spectrosc. 40 (2009) 137. [16] S. Efrima, Chem. Phys. Lett. 102 (1983) 79. [17] P.G. Etchegoin, C. Galloway, E.C. Le Ru, Phys. Chem. Chem. Phys. 8 (2006) 2624–2648. [18] B.G. Janesko, G.E. Scuseria, J. Chem. Phys. 125 (2006) 124704. [19] S. Abdalia, E.W. Blanch, Chem. Soc. Rev. 37 (2008) 980. [20] V. Novak, J. Sebestík, P. Bour, J. Chem. Theory Comput. 8 (2012) 1714. [21] M.T. Sun, Z.L. Zhang, P.J. Wang, Q. Li, F.C. Ma, H.X. Xu, Light: Sci. Appl. 2 (2013) e112. http://dx.doi.org/10.1038/lsa.2013.68. [22] W. Hug, G.A. Hangartner, J. Raman Spectrosc. 30 (1999) 841. [23] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391. [24] Y. Fang, Y. Li, H.X. Xu, M.T. Sun, Langmuir 26 (2010) 7737. [25] P. Hohenberg, W. Kohn, Phys. Rev. 136 (1964) B864. [26] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [27] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270. [28] M.J. Frisch, et al., Gaussian 09, Revision A.02; Gaussian, Inc., Wallingford, CT, 2009.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Insight into vibration mode-resolved plasmon enhanced Raman optical activity Yongqing Li a,b,⇑,1, ShaoXiang Sheng b,1, Zhenglong Zhang b,c,e,⇑, Liwei Liu d, Mengtao Sun b,⇑ a
Department of Physics, Liaoning University, Shenyang, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-146, Beijing 100190, PR China c Physical Chemistry, Friedrich-Schiller University Jena, Helmholtzweg, 07743 Jena, Germany d Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, PR China e Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany b
a r t i c l e
i n f o
Article history: Received 16 August 2013 Accepted 13 October 2013 Available online 29 October 2013 Keywords: Plasmon enhancement Raman optical activities Vibration mode-resolved
a b s t r a c t Vibration mode-resolved plasmon enhanced Raman optical activities (PE-ROA) are investigated experimentally and theoretically. It is found that the profile of PE-ROA is significantly different from that of ROA. Our experimental results revealed that PE-ROA is not a universal method for the enhancement of ROA. It should be very carefully for the using of plasmon enhancement for ROA. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Raman optical activity (ROA) is a vibrational spectroscopic technique that is reliant on the difference in intensity of Raman scattered right and left circularly polarized light due to molecular chirality. Barron and Buckingham developed the definitive theory of ROA in 1971 [1], and made the first observations in 1973 [2]. Due to its sensitivity to chirality, Raman optical activity is a useful probe of biomolecular structure and behavior in aqueous solution. It has been used to study protein, nucleic acid, carbohydrate and virus structures [3–11]. However, ROA signals are even weaker 103 than the Raman signals, and it is very hard to be detected, especially at low and high frequencies, due to the limitation of instrument for chiral Raman (for example, there is no suitable optical grating and cannot resolve the signal of chiral Raman). Also, the measurements on the molecular chirality need large molecular concentration and longtime accumulation (several hours). Surface enhanced Raman scattering (SERS) has been widely used for the ultrasensitive chemical analysis [12,13], even at single molecular level [14], where the Raman signals can be enhanced up ⇑ Corresponding authors. Addresses: Department of Physics, Liaoning University, Shenyang, PR China (Y.Q. Li), Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-146, Beijing 100190, PR China (M. Sun and Z. Zhang). E-mail addresses: [email protected] (Y. Li), [email protected] (Z. Zhang), [email protected] (M. Sun). 1 These authors contributed equally to this work. 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.022
to 1011 by the electromagnetic mechanism [14], as well as the chemical enhancement [15]. Following SERS method, the ROA signals can also be strongly enhanced by surface plasmon resonance [16–21]. It is expected that plasmon enhanced ROA (PE-ROA) can resolve above intrinsic weaknesses of ROA. However, it is a great challenge that how to correctly understanding the plasmon enhanced ROA, due to the very weak chiral Raman signals and the influence of chemical enhancement as well as the substrate, which might change molecular chirality. The new stereo structure introduced by substrate may break molecular intrinsic stereo center. New symmetric minor, induced new chiral center by metal surface, and then the orientation of molecular charge is reversed, the chirality is changed. Chiral molecular vibrational motion induced electric dipole coupling with the magnetic dipole, which is the excited electronic charges, relaxed during Raman process. In this communication, we try to insight into vibration moderesolved PE-ROA of L-alanine and D-alanine experimentally and theoretically at high, low and middle frequencies, respectively. Lalanine and D-alanine are of the opposite chirality. So, they are the best molecules to study the influence of plasmon enhancement for the chirality of PE-ROA. If the plasmon can enhanced ROA spectra and the their chirality can be kept during the plasmon enhancement. We can conclude the plasmon enhancement is a valid and effective method for the enhancement of ROA by plasmon. The influences of charge transfer between molecule and metal, the new chemical bond between molecule and substrate, and the adsorption status of molecule on metal were comprehensively investigated. It is found that PE-ROA is not a universal method
166
Y. Li et al. / Journal of Colloid and Interface Science 415 (2014) 165–168
for the enhancement of ROA. It should be very carefully for the using of plasmon enhancement for POA. The solid L-alanine and D-alanine of over 99% purity were purchased from Aldrich Chemical Co., Ltd. and used without further purification. Aqueous solution of L-alanine was prepared with a concentration of 100 mg/ml. The ROA spectra were measured with the backscattering SCP Biotools l-ChiralRAMAN-2X instrument with 532 nm excitation laser [22]. Laser power was set to 400 mW, with total acquisition time of 4 h. The SERS active silver colloid was prepared by citrate reduction of AgNO3 [23]. AgNO3 (90 mg) was dissolved in 650 mL of quartz distilled water. Five hundred milliliters of this solution was brought to boiling and then a solution of 1% sodium citrate (10 mL) was added. Thirty minutes later the remaining 150 mL AgNO3 solution was added 3 times every 15 min. The solution was kept on boiling about 1.5 h. The final Ag nanoparticle concentration was estimated to be 35 pM with an average radius size of about 40 nm [24]. The SERS measurements were performed with left and right polarized lights, respectively, in the confocal Raman spectroscopic system (Renishaw, Invia) through an objective of 100 with a numerical aperture (NA) of 0.85. The wavelength of laser is 632.8 nm, and the laser power is about 7 mW. Lastly, the obtained difference of SERS spectra excited with left and right polarized lights is the experimental PE-ROA spectra. For comparison, the molecular ROA spectra of L-alanine at high frequencies was simulated, using density functional theory [25], B3LYP functional [26] and 6–31 ++g(2d,p) basis set. To theoretically interpret the PE-ROA, the vibrational modes of molecule adsorbed on the Ag20 cluster were theoretically calculated, using DFT, B3LYP functional and 6–31 g(d) basis set for C, N, O and H, and LanL2DZ basis set [27] for Ag. The optimized geometry can be seen from Supporting information. All the quantum chemical calculations were done with Gaussian 09 software [28]. Fig. 1(A) and (B) are the experimental PE-ROA of L-alanine and simulated ROA of L-alanine, respectively. It is found that PE-ROA peaks can be well observed experimentally, and interpreted with the simulated ROA spectrum. Note that the simulated ROA spectrum does not consider metal in the calculation, which results in
the difference of ROA intensities. The vibrational modes a-d in Fig. 1(A) can be seen from Fig. 2. For peak a, it is symmetric vibrations of 3 H atoms on C(21) as well as symmetric H’s vibration on C(22). For peak b, it is symmetric vibrations of 3 H atoms on C(21) as well as asymmetric H’s vibration on C(22). For peak c, it is stretching mode of H(28) on C(21). For peak d, it is stretching mode of H(30) on C(21). So, the PE-ROA at high frequencies can overcome the instrument limitation of chiral Raman for ROA. For comparison, we also measured the PE-ROA spectra of D-alanine at high vibrational frequencies (see Fig. 3(a)). Comparing Fig. 3(a) and (b), we found that their PE-ROA spectra cannot be completely reversed, and strongly influenced by the introduced metal substrate. So, for the plasmon enhancement for D-alanine is not a good choice. Secondly, we measured the PE-ROA spectrum of L-alanine and compared with the ROA spectrum of L-alanine, see Fig. 4. It is found that the profiles of them are significantly different below 600 cm1.
(a)
c
(A) PEROA (a.u.)
Fig. 2. The vibrational modes of ROA peaks (a-d) of L-alanine in Fig. 1(a).
D-alanine
b
PE-ROA
a
d
(b)
L-alanine
ROA (a.u.)
(B)
2800
2900
3000
3100
Wavenumber / cm-1 Fig. 1. (A) and (B) are the experimental PE-ROA of L-alanine and simulated ROA spectra of L-alanine, respectively.
2900
2950
3000
3050
3100
Wavenumber / cm-1 Fig. 3. (a) and (b) are the experimental PE-ROA and simulated ROA spectra, respectively.
ROA (a.u.)
Y. Li et al. / Journal of Colloid and Interface Science 415 (2014) 165–168
p
(A)
j
o 400
PEROA (a.u.)
167
600
800
1000
(B)
1200
g
i
1400
1600
h j’ k
m Fig. 7. Vibrational modes g and h in of L-alanine Fig. 4(b).
l e
f
400
600
800
1000
1200
1400
1600
Wavenumber / cm-1 Fig. 4. Experimental ROA of L-alanine (a) and experimental PE-ROA of L-alanine (b).
Fig. 5. Vibrational modes e and f in Fig. 4.
Fig. 8. Vibrational modes i, j, k and m in of L-alanine Fig. 3(b).
PE-ROA peaks e and f are strongly enhanced, due to the interaction between molecule and metal, and charge transfer results in the further chemical enhancement. Fig. 5 revealed that these PE-ROA peaks also include the vibration of new chemical bonds between O atoms and metal. Furthermore, the large amounts of charge transfer between O atoms and metal (see Fig. 6) can result in the chemical enhancement for PE-ROA. So, some of PE-ROA peaks can be chemical enhanced at low frequencies, especially for the new chemical bonds between molecule and metal, when the molecule chemically adsorbed on the metal substrate. Thirdly, we interpret the reason that the PE-ROA peaks g and h of L-alanine were strongly selected enhanced. Fig. 7 revealed that for these two PE-ROA peaks, the new stereo structure introduced
by substrate may break molecular intrinsic stereo center, and there is a new symmetric face that is perpendicular to the substrate. The C@O stretching vibrations were also involved in these two vibrational modes, as we know the chemical bond between substrate and O atoms can also influence the intensities of PE-ROA peaks (see Fig. 8). For the PE-ROA peaks i, j0 , k, l and m of in Fig. 4(b), the chirality was reversed, compared with their ROA peaks of L-alanine in Fig. 4(a). These vibrational modes can be seen from Fig. 7. New symmetric minor by the substrate and new chiral center by metal surface, can results in the orientation of molecular charge is re-
Fig. 6. The distribution of Mulliken charge distribution of L-alanine.
Y. Li et al. / Journal of Colloid and Interface Science 415 (2014) 165–168
PE-ROA (a.u.)
168
Basic Research Project of China (Grants 2009CB930701), the Program of Shenyang Key Laboratory of Optoelectronic Materials and Technology (Grant No. F12-254-1-00).
D-alanine
Appendix A. Supplementary material 500
1000
1500
PEROA (a.u.)
L-alanine
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.022. References
200
400
600
800
1000
1200
1400
1600
Wavenumber / cm-1 Fig. 9. Experimental PE-ROA of D-alanine and L-alanine.
versed, the chirality is changed. Chiral molecular vibraional motion induced electric dipole coupling with the magnetic dipole is also reversed, when excited electronic charges relaxed during Raman processes. Lastly, to study the substrate influence of the substrate and plasmon, we measured the PE-ROA spectra of D-alanine, see Fig. 9. It is found that the molecular chirality cannot be realized at all. Though L-alanine and D-alanine have the reversed ROA, but they have almost the same chirality for PE-ROA, except for the ROA peak at 850 cm1. So, the plasmon enhanced effect is not valid for our studied chiral molecules. In summary, vibration mode-resolved PE-ROA of L-alanine and D-alanine were investigated experimentally and theoretically. It is found that the plasmon can strongly enhanced ROA spectra, but the their chirality cannot be kept during the plasmon enhancement. We can conclude the plasmon enhancement is not a valid and effective method for the enhancement of ROA by plasmon. Our experimental results revealed that PE-ROA is not a universal method for the enhancement of ROA. It should be very carefully for the using of plasmon enhancement for POA. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grants 11374353 and 11274149), the National
[1] L.D. Barron, A.D. Buckingham, Mol. Phys. 20 (1971) 1111. [2] L.D. Barron, M.P. Bogaard, A.D. Buckingham, J. Am. Chem. Soc. 95 (1973) 603. [3] J. Costante, L. Hecht, P.L. Polavarapu, A. Collet, L.D. Barron, Ang. Chem. Int. Ed. Engl. 36 (1997) 885. [4] L.D. Barron, L. Hecht, E.W. Blanch, A.F. Bell, Prog. Biophys. Mol. Biol. 73 (2000) 1. [5] E.W. Blanch, L.A. Morozova-Roche, D.A.E. Cochran, A.J. Doig, L. Hecht, L.D. Barron, J. Mol. Biol. 301 (2000) 553. [6] C.D. Syme, E.W. Blanch, C. Holt, R. Jakes, M. Goedert, L. Hecht, L.D. Barron, Eur. J. Biochem. 269 (2002) 148. [7] E.W. Blanch, D.J. Robinson, L. Hecht, C.D. Syme, K. Nielsen, L.D. Barron, J. Gen. Virol. 83 (2002) 241–246. [8] C.D. Syme, V. Volpetti, G.P. Lomonossoff, K. Nielsen, L.D. Barron, J. Gen. Virol. 83 (2002) 2593. [9] L.D. Barron, E.W. Blanch, L. Hecht, Adv. Prot. Chem. 62 (2002) 51. [10] L.D. Barron, E.W. Blanch, I.H. McColl, C.D. Syme, L. Hecht, K. Nielsen, Spectroscopy 17 (2003) 101. [11] I.H. McColl, E.W. Blanch, A.C. Gill, A.G.O. Rhie, M.A. Ritchie, L. Hecht, K. Nielsen, L.D. Barron, J. Am. Chem. Soc. 125 (2003) 10019–10026. [12] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Chem. Phys. Lett. 26 (1974) 163. [13] D.L. Jeanmaire, R.P. Van Duyne, J. Electroanal. Chem. Interfacial Electrochem. 84 (1977) 1. [14] H.X. Xu, E.J. Bjerneld, M. Kail, L. Borjesson, Phys. Rev. Lett. 83 (1999) 4357. [15] M.T. Sun, S. Liu, M. Chen, H.X. Xu, J. Raman Spectrosc. 40 (2009) 137. [16] S. Efrima, Chem. Phys. Lett. 102 (1983) 79. [17] P.G. Etchegoin, C. Galloway, E.C. Le Ru, Phys. Chem. Chem. Phys. 8 (2006) 2624–2648. [18] B.G. Janesko, G.E. Scuseria, J. Chem. Phys. 125 (2006) 124704. [19] S. Abdalia, E.W. Blanch, Chem. Soc. Rev. 37 (2008) 980. [20] V. Novak, J. Sebestík, P. Bour, J. Chem. Theory Comput. 8 (2012) 1714. [21] M.T. Sun, Z.L. Zhang, P.J. Wang, Q. Li, F.C. Ma, H.X. Xu, Light: Sci. Appl. 2 (2013) e112. http://dx.doi.org/10.1038/lsa.2013.68. [22] W. Hug, G.A. Hangartner, J. Raman Spectrosc. 30 (1999) 841. [23] P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391. [24] Y. Fang, Y. Li, H.X. Xu, M.T. Sun, Langmuir 26 (2010) 7737. [25] P. Hohenberg, W. Kohn, Phys. Rev. 136 (1964) B864. [26] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [27] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270. [28] M.J. Frisch, et al., Gaussian 09, Revision A.02; Gaussian, Inc., Wallingford, CT, 2009.