Investigation on the adsorption characteristics of anserine on the surface of colloidal silver nanoparticles

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 27–32

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Investigation on the adsorption characteristics of anserine on the surface of colloidal silver nanoparticles S. Thomas a,⇑, N. Maiti b, T. Mukherjee b, S. Kapoor b,⇑ a b

High Pressure & Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Anserine is a potent anti-oxidant

The surface adsorption characteristics of anserine (an anti-oxidant) with silver nanoparticles were investigated from SERS studies. Optimized structure of the silver complex of anserine explains the enhanced Raman modes in the SERS spectrum.

present in animals.  Optimized structure of silver complex of anserine explains the enhanced Raman modes.  Concentration dependent SERS studies show changes in the orientation of anserine.

a r t i c l e

i n f o

Article history: Received 17 October 2012 Received in revised form 3 April 2013 Accepted 10 April 2013 Available online 18 April 2013 Keywords: Anserine Surface-enhanced Raman scattering DFT Adsorption

a b s t r a c t The surface-enhanced Raman scattering (SERS) studies of anserine (beta-alanyl-N-methylhistidine) was carried out on colloidal silver nanoparticles to understand its adsorption characteristics. The experimentally observed Raman bands were assigned based on the results of DFT calculations. The studies suggest that the interaction of anserine is primarily through the carboxylate group with the imidazole ring in an upright position with respect to the silver surface. Concentration dependent SERS studies suggest a change in orientation at sub-monolayer concentration. Ó 2013 Elsevier B.V. All rights reserved.

Introduction Surface-enhanced Raman scattering (SERS) is an effective technique in studying surface-interfacial properties and has been used at molecular level to investigate and characterize the interaction between the substrate and the adsorbate [1–5]. The interaction be⇑ Corresponding authors. Tel.: +91 22 25590339. E-mail addresses: [email protected] (S. Thomas), [email protected] (S. Kapoor). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.04.047

tween bio-molecules and metal surfaces has recently given rise to a large number of investigations due to challenging technological applications in the field of biomaterials, biosensors and biocatalysis. Metal nanoparticles have been exploited for the delivery of drugs [6]. Information such as molecular identity, structure, orientation and nature of bonding of the surface-adsorbed species may provide essential clues on the efficiency of these processes. The difference in the spectra of the adsorbed molecule on the metal surface in terms of the shifts in vibrational frequencies and

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S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 27–32

relative intensities with respect to the molecules in solution provides information on the relative proximity of different parts of the adsorbed molecule to the surface. The ‘‘surface selection rules’’ of Moskovits and Suh [7–11] based on image dipole field theory [12] helps in estimating the orientation of the molecules adsorbed on the metal surface. In the present article, we report the SERS studies of anserine (beta-alanyl-N-methylhistidine), a dipeptide of the amino acid beta-alanine and amino acid derivative methyl histidine. Anserine is present in high concentrations in the muscle and brain of many animals and acts as an anti-oxidant [13]. The imidazole moiety of histidine and its derivatives confers the antioxidant activity. The Raman spectra of solid anserine, aqueous solution of anserine and the SERS spectrum in silver colloid was recorded in order to investigate the nature of binding of the molecule with the silver surface and the probable orientation the molecule assumes on the surface. The experimental Raman data is supported with DFT calculations using B3LYP functional and LANL2DZ basis set. The vibrational frequencies of the molecule and its silver complex were computed at the optimized geometry and compared with the experimental values. Experimental Aqueous silver colloid was prepared by the reduction of silver nitrate with sodium borohydride using the method of Creighton et al. [14]. Anserine was obtained from Aldrich and used without further purification. The UV–visible absorption spectra of the silver colloid with and without anserine were recorded using a Jasco V650 spectrophotometer. Raman spectra of anserine (solid and aqueous solution) and anserine in silver colloid were recorded at room temperature using the 532 nm line from a diode pumped Nd3+: YAG laser (SUWTECH laser, model G-SLM-020 from Shanghai uniwave Technology Co. Ltd.). The laser power used for the Raman measurements was 25 mW. The Raman scattered light was collected at the back-scattered geometry and detected using a CCD based home-built monochromator [15]. Computational details Geometry optimization was performed for the anserine and its silver complex using the density functional theory (DFT) with B3LYP functional [16] with the LANL2DZ basis set using Gaussian 98 program [17]. The DFT calculations using the LANL2DZ pseudo potentials are an accurate descriptor of the Ag/Au cluster chemistry [18–27]. No symmetry restriction was applied during geometry optimization. The vibrational frequencies for anserine, its anion and its silver complex were computed on the geometry-optimized structures using an analytical Hessian program. The absence of imaginary frequencies confirmed that the ground states of anserine, its anion and its silver complex correspond to a local minimum on the potential energy surface and not to a saddle point. The computed vibrations at the optimized geometry were compared with the experimentally obtained normal Raman and SERS spectra. Fig. 1a and b shows the optimized geometries of anserine and its silver complex. Binding energy of the silver complex of anserine was found to be 154.6 kcal mol 1. Results and discussion UV–visible absorption spectrum The UV–visible absorption spectrum of the silver colloid before and after the addition of different concentrations of anserine (10 6, 10 5, 10 4 and 10 3 M) is shown in Fig. 2a–e. The absorption

Fig. 1. The optimized structures of (a) anserine and (b) silver complex of anserine.

spectrum of the silver colloid showed a single sharp molar extinction maximum at 380 nm, which is due to the surface plasmon resonant excitation [28]. On addition of anserine to the silver colloid, the absorption spectrum of silver colloid showed an additional band around 530 nm with a decrease in the absorbance of the 380 nm band. The appearance of the red-shifted peak is attributed to a charge transfer/aggregate band [29,30] induced by adsorption of the anserine on the silver surface. Raman spectra of solid anserine and its solution The normal Raman spectrum of solid anserine is shown in Fig. 3. The experimental values are compared with the calculated frequencies and the Raman spectrum of solid anserine is tabulated in Table 1. Anserine is a dipeptide made up of beta-histidine (hist) and Lalanine (ala) and the bands arise from the methylene groups, carboxyl group, amino group, peptide group, imidazole ring (rg) and methyl group. The very intense band in the Raman spectrum is 1042 cm 1 attributed to [rg deformation (def), CH2 (hist) rock, CH (hist)ACH2 (hist) stretch (str)]. Most of the remaining bands

S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 27–32 Table 1 Assignment of normal Raman and calculated vibrations of anserine (solid) in cm

Absorbance, arb. units

a

b

c

d

e

300

400

500

600

700

Wavelength, nm Fig. 2. UV–visible spectrum of (a) silver colloid, (b) with added anserine, 10 10 5 M, (d) 10 4 M and (e) 10 3 M.

1042

800

1000

1200

1400

Wavenumber, cm-1 Fig. 3. Normal Raman spectrum of anserine (solid).

1613

1496

1263 1344

1384

600

1104 1167

864

683 290

400

M, (c)

a

202(m) 231(w) 246(w) 290(w) 314(w) 346(w) 364(w) 400(w) 422(w) 541(w) 615(w) 633(w) 643(w) 683(m) 699(w) 711(w) 768(w) 864(m) 957(m) 977(w) 996(w) 1042(vs) 1104(m) 1128(w) 1143(w) 1167(m) 1223(w) 1263(m) 1289(m) 1330(w) 1344(m) 1384(m) 1398(sh) 1442(m) 1474(w) 1496(m) 1576(w) 1613(m) 1633(w) 1650(w)

219 232 264 287 304 332 393 407 431 529 618 623 645 671 687 704 728 854 920 974 992 1024 1080 1131 1140 1170 1235 1265 1286 1297 1350 1374 1395 1462 1495 1506 1582 1649 1688 1708

Calc.

1

.

Assignments rg tor rg tor skel tor skel tor skel tor, rg tor skel tor, rg tor skel tor rg rock, CH3 rock skel tor, rg tor skel tor amide NH oop def skel tor, rg tor rg tor, OH oop def rg tor rg NACH3 str rg tor COO def, skel tor rg CH oop def, CH2 (hist) rock CACOO str, NH2 rock, rg def, ala skel str NH2 rock, CH2(ala)ACH2(ala) str CH (hist)ACH2(hist) str rg def, CH2 (hist) rock, CH (hist)ACH2(hist) str CH3 rock, rg CN str rg CN str, rg CHdef, CANH2 str rg CH def, CANH2 str CH3 rock rg CH def, CH2 (hist) tw amide NH def, rg CH def rg CH def, rg CN str, CH2 (hist) tw rg str CH2 (hist) wag, CH (hist) def,CO str of COOH CH2 (hist) wag, rg CN str, CH3 CO str of COH, CH (hist) def, CH2 (ala) wag, CH2 (hist) wag rg CN str, rg CH def, CH3 def CH2 (ala) sciss CH3def, CH2 (hist) sciss rg CC str amide CO str, NH def NH2 sciss CO str of COOH

a B3LYP/LANL2DZ calculated frequencies in vaccum: w: weak, m: medium, s: strong, br: broad, sh: shoulder, str.: stretch, sym.: symmetric, asym.: asymmetric: oop: out of plane, sciss: scissoring, def:deformation, tw: twist, tor: torsion, rg: imidazole ring ala: alanine, hist: histidine.

Intensity 200

6

Raman

29

1600

are medium intense bands. The bands arising due to rg modes are 1576 cm 1 (rg CAC str), 1104, 1128, 1442 cm 1 all due to rg CAN str and 1143, 1223, 1263, 1289, 1297 cm 1 all due to rg CAH def. A band at 957 cm 1 corresponding to CACOO stretch along with rg def is seen. Since there are three CH2 groups (one in the histidine moiety and two in the alanine unit) there are several vibrations related to those groups. They show vibrations including scissoring, wagging, twisting and rocking modes in the region  1400, 1300, 1100–1200 cm 1 and less than 1000 cm 1 and are listed in the table. Amide C@O stretch is seen at 1613 cm 1. Several weak bands attributed to ring torsions and skeletal torsions are seen in the region < 700 cm 1. The band at 768 cm 1 is attributed to COO def mode. Weak bands at 1633 cm 1 and 1650 cm 1 are due to NH2 scissoring and C@O stretch modes respectively. Fig. 4A(a–d) shows the SERS spectra of anserine in silver colloid at different concentrations of anserine (10 6, 10 5, 10 4 and 10 3 M) and Fig. 4B(a–d) shows the SERS spectra with the deconvoluted peaks in the range 1500–1750 cm 1. The SERS spectra are different from the normal Raman spectrum of anserine which clearly indicates chemical interaction of the molecule with the silver surface. Maximum SERS enhancement was observed for the concentration of 10 5 M of anserine, which is evident from the Fig. 4A(b). The concentration of 10 4 M also showed almost similar enhancement. The concentration effects of anserine are discussed in a different

S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 27–32

1572

a

c

b

1638 1670

1523

Intensity

1572 1638

1378 1238

1040 1105

941 613

Intensity

b

(B)

1445

1331

a

(A) 1392

1366 1378

232

30

c

1046

d d

e 200

400

600

800

1000

1200

Wavenumber, cm-1

1400

1600

1500

1600

1700

Wavenumber, cm-1

Fig. 4. (A) SERS spectra of anserine (a) 10 6 M, (b) 10 5 M, (c) 10 4 M and (d) 10 3 M in silver colloid. In the inset is shown the deconvoluted peaks (for 10 5 M) in the spectral region, 1300–1500 cm 1. (e) Normal Raman spectrum of 1 M aqueous solution of anserine. (B) Deconvoluted peaks in the region 1500–1750 cm 1 for the SERS spectra of concentrations (a) 10 6 M, (b) 10 5 M, (c) 10 4 M and (d) 10 3 M.

Table 2 Assignment of Raman vibrations of the silver complex of anserine and calculated vibrations in cm 1. SERS

a

232(s) 613(w) 657(w) 683(w) 715(w) 737(w) 800(w) 872(w) 941(m) 950(w) 994(w) 1040(w) 1090(w) 1105(m) 1120(w) 1210(w) 1238(m) 1268(w) 1331(w) 1366(s) 1378(s)

300 616 638 666 692 708 807 852 929 924 977 1026 1070 1080 1134 1170 1234 1263 1297 1369 1390

1392(m) 1445(w) 1523(w) 1572(m) 1638(w) 1670(w)

1406 1458 1504 1582 1643 1685

Calc.

Assignments AgO str NH2 wag, amide NH oop def ala tor amide NH oop def, rg CH oop def rg NACH3 str, rg tor rg tor, rg CH oop def COO def, rg CH oop def CH2 (hist) rock,CH2ACO (ala) str, rg CH oop def CACOO- str, rg def, CH2 (hist) rock rg def NH2 rock, rg def CH2 (hist) rock, rg def,CHACH2 (hist) str CANH2 str, CH2 (ala) rock CH3 def, rg CN str rg CH, NH def, rg CN str CH3 rock CH2 (hist) tw, rg CH def, rg CN str rg CH def, NH2 tw rg str CH2 (ala)wag, CH2 (hist) wag, rg CN str COO sym str, rg CN str, CH3 def, CH2 (ala) wag, CH2(hist) wag rg CN str, rg CHdef, CH2 (hist) tw, CH3 def COO asym str, rg CH def, CH3 def CH2 (hist) sciss, CH3 def rg CC str, rg CH def, CH3 def, COO asym str amide CO str, NH def NH2 sciss

a B3LYP/LANL2DZ calculated frequencies in vaccum: w: weak, m: medium, s: strong, br: broad, sh: shoulder, str.: stretch, sym.: symmetric, asym.: asymmetric: oop: out of plane, sciss: scissoring, def:deformation, tw: twist, tor: torsion, rg: imidazole ring ala: alanine, hist: histidine.

section. The experimental SERS frequencies of 10 5 M anserine are compared with the calculated frequencies and are tabulated in Table 2. The Raman spectrum of 1 M aqueous solution of anserine shows only a single weak peak at 1046 cm 1 arising due to rg def, CH2 (hist) rock and CH (hist)ACH2 (hist) str as shown in Fig. 4A(e). The intense SERS band appears at 1378 cm 1 (inset of Fig. 4A) and is attributed primarily due to COO symmetric stretch with contributions from rg CAN str, CH3 def mode, CH2 (ala) wag and CH2 (hist) wag. The peak at 1366 cm 1 is assigned to CH2 (hist) wag, CH2 (ala) wag, rg CN str and CH3 def. A medium intensity band mainly due to CACOO appears at 941 cm 1. A low frequency mode at 232 cm 1 is observed in the SERS spectrum which is attributed to the AgAO stretch. A weak band due to COO def is observed at 800 cm 1. Various rg modes (941, 950, 994, 1105, 1120, 1238, 1366, 1392, 1445, 1572 cm 1) also appear in the SERS spectrum. SERS bands associated with the CH3 def modes appear at 1105, 1210, 1378, 1445, 1523 and 1572 cm 1. NH2 wag (613 cm 1), CANH2 stretch (1090 cm 1), NH2 rock (994 cm 1) are seen as weak bands in the spectrum. Several weak bands attributed to ring torsions and skeletal torsions are seen in the region < 900 cm 1. Amide CO str with NH def is seen as a weak band at 1638 cm 1 and the NH2 sciss is observed at 1670 cm 1. Interpretation of SERS spectra The most intense band observed in the SERS spectrum is 1378 cm 1 which arise mainly due to the carboxylate stretch, rg CAN str and CH3 def mode. The mode at 1366 cm 1 due to the CH2 (hist) wag, CH2 (ala) wag and rg CN str is also an intense band. The low frequency band at 232 cm 1 attributed to AgAO str, and the medium intensity band of CACOO str observed at 941 cm 1 indicates that the molecule binds to the surface through the

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31

Fig. 5. Schematic of the orientation of anserine at (a) sub-monolayer and (b) monolayer coverage.

carboxylate group with the imidazole ring in an upright position. According to ‘surface selection rules’ normal modes of adsorbed molecules involving changes in molecular polarizability with a component perpendicular to the surface are subject to the greatest enhancement [7–11]. The enhancement of other ring modes also suggests that the ring has a vertical orientation with respect to the silver surface. This is also well supported by the optimized structure shown in Fig. 1b which shows that the imidazole ring of the histidine moiety is in an upright position. Effect of concentration of anserine on the SERS spectra Freshly prepared silver colloid is added to anserine at different concentrations and all the spectra are recorded under identical conditions. Fig. 4A(a–d) shows the SERS spectra of anserine at concentrations 10 6, 10 5, 10 4 and 10 3 M and Fig. 4B(a–d) shows the deconvoluted peaks in the range 1500–1750 cm 1. As mentioned above, increased Raman signal was observed for 10 5 and 10 4 M concentrations. The spectra was weak for the concentrations 10 3 and 10 6 M of anserine. Maximum enhancement (10 5 M) is attributed to monolayer formation on the surface and also to a favorable orientation of the polarizability of the adsorbed molecule [31,32]. SERS signal attains a maximum at monolayer coverage and the ‘first layer effect’ contributes to the maximum enhancement [33]. At lower concentration (10 6 M), enhancement decreases due to a lower number of available scattering centers (submonolayer coverage) and at higher concentration (10 3 M) the formation of multilayers leads to reduction in the SERS enhancement. At 10 6 M concentration of anserine, SERS spectrum showed significant change in their features when compared to 10 4 M and 10 5 M. This indicates that there is an orientation change of anserine at sub-monolayer coverage. Orientation changes with concentration are reported in the literature [7,34–37]. At 10 6 M, the ratio of the intensity of 1378 cm 1 band to 1572 cm 1 is drastically reduced and the 1638 cm 1 band shows increase in intensity than the 1572 cm 1 band (Fig. 4A and B). From the above intensity variations, the probable orientation at sub-monolayer coverage is shown in the schematic Fig. 5a while the orientation at monolayer coverage (10 4 and 10 5 M concentrations) is shown in Fig. 5b. At sub-monolayer coverage, the carboxylate group lie nearly flat on the surface with the ring no longer in the vertical position and C@O and NH2 lying close to the surface. The molecule in this configuration can also interact through oxygen of amide C@O and N of NH2 group apart from the carboxylate p electrons. The calculated Mulliken charges for O of amide C@O and N of NH2 are 0.375206 and 0.082133 respectively and it is likely that the interaction is more with O than with N. This could be the reason that the amide C@O peak is stronger than the neighboring peaks as shown in Fig. 4B(a). At higher concentration (10 3 M),

the SERS spectrum arises due to multi-layer coverage and hence the orientation becomes random leading to a reduced Raman intensity. Conclusion SERS studies of anserine were carried out in silver colloid. The vibrational assignments of anserine and its silver complex have been carried out using DFT calculations. The interpretation of the observed features in terms of the enhancements, and shifts of the various vibrations and the application of ‘surface selection rules’ seems to strongly suggest the interaction of anserine is primarily through the oxygen of the COO group with the imidazole ring in an upright position to the silver surface. The structure of the silver complex of anserine as optimized by the B3LYP/LANL2DZ explains the enhanced modes of COO , CACOO , the imidazole ring modes with the CH3 deformation modes. SERS studies showed a change in orientation of anserine at sub-monolayer concentration. Acknowledgement The authors thank Prof. K. Maiti of TIFR, Mumbai for providing the Gaussian 98 computational facilities and Dr. S. M. Sharma, Head, HP&SRPD, BARC for the support and encouragement. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.04.047. References [1] A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 (1998) 241–250. [2] M. Moskovits, J. Raman Spectrosc. 36 (2005) 485–496. [3] K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, J. Phys. Condens. Matter 14 (2002) R597–R624. [4] B. Schrader (Ed.), Infrared and Raman Spectroscopy: Methods and Applications, Wiley, Chichester, 1995. [5] X.M. Qian, S.M. Nie, Chem. Soc. Rev. 37 (2008) 912–920. [6] A. Kulak, S.R. Hall, S. Mann, Chem. Commun. (2004) 576–577. [7] M. Moskovits, J.S. Suh, J. Phys. Chem. 92 (1988) 6327–6329. [8] M. Moskovits, J.S. Suh, J. Phys. Chem. 88 (1984) 5526–5530. [9] M. Moskovits, J.S. Suh, J. Phys. Chem. 88 (1984) 1293–1298. [10] M. Moskovits, J.S. Suh, J. Chem. Phys. 77 (1982) 4408–4416. [11] M. Moskovits, J.S. Suh, J. Am. Chem. Soc. 107 (1985) 6826–6829. [12] C.S. Allen, R.P. Van Duyne, Chem. Phys. Lett. 63 (1979) 455–459. [13] R. Kohen, Y. Yamomoto, K.C. Cundy, B.N. Ames, Proc. Nat. Acad. Sci. USA 85 (1988) 3175–3179. [14] J.A. Creighton, C.G. Blatchford, M.G. Albrecht, J. Chem. Soc., Faraday Trans. II (75) (1979) 790–798. [15] A.P. Roy, S.K. Deb, M.A. Rekha, A.K. Sinha, Ind. J. Pure Appl. Phys. 30 (1992) 724–728. [16] A.D. Becke, J. Chem. Phys. 98 (1993) 1372–1377.

32

S. Thomas et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 27–32

[17] M.J. Frisch, et al. Gaussian 98, Revision A.11.2, Gaussian, Inc., Pittsburgh, PA, 2001. [18] F. Sue Legge, G.L. Nyberg, J. Barrie Peel, J. Phys. Chem. A105 (2001) 7905–7916. [19] S. Naumov, S. Kapoor, S. Thomas, S. Venkateswaran, T. Mukherjee, J. Mol. Str. (Theochem) 685 (2004) 127–131. [20] S. Thomas, N. Biswas, S. Venkateswaran, S. Kapoor, R. D’Cunha, T. Mukherjee, Chem. Phys. Lett. 402 (2005) 361–366. [21] S. Thomas, N. Biswas, S. Venkateswaran, S. Kapoor, S. Naumov, T. Mukherjee, J. Phys. Chem. A 109 (2005) 9928–9934. [22] N. Biswas, S. Thomas, S. Kapoor, A. Misra, S. Wategaonkar, S. Venkateswaran, T. Mukherjee, J. Phys. Chem. A 110 (2006) 1805–1811. [23] N. Biswas, S. Kapoor, H.S. Mahal, T. Mukherjee, Chem. Phys. Lett. 444 (2007) 338–345. [24] N. Biswas, S. Thomas, A. Sarkar, T. Mukherjee, S. Kapoor, J. Chem. Phys. 129 (2008). 184702-1–184702-15. [25] N. Biswas, S. Thomas, A. Sarkar, T. Mukherjee, S. Kapoor, J. Phys. Chem. C 113 (2009) 7091–7100. [26] N. Biswas, S. Thomas, A. Sarkar, T. Mukherjee, S. Kapoor, Chem. Phys. Lett. 479 (2009) 248–254.

[27] S. Thomas, N. Biswas, V.V. Malkar, T. Mukherjee, S. Kapoor Chem, Phys. Lett. 491 (2010) 59–64. [28] J.A. Creighton, in: R.K. Chang, T.E. Furtak (Eds.), Surface Enhanced Raman Scattering, Plenum Press, New York, 1982, pp. 315–337. [29] E.J. Liang, C. Engert, W. Kiefer, J. Raman Spectrosc. 24 (1993) 775–779. [30] S. Sanchez-Cortes, J.V. Garcia-Ramos, G. Morcillo, A. Tinti, J. Colloid Interface Sci. 175 (1995) 358–368. [31] K.M. Mukherjee, T.N. Misra, Bull. Chem. Soc. Jpn. 70 (1997) 301–305. [32] J. Chowdhury, M. Ghosh, J. Colloid Interface Sci. 277 (2004) 121–127. [33] P.N. Sanda, J.M. Warlaumont, J.E. Demuth, J.C. Tsang, K. Christmann, J.A. Bradley, Phys. Rev. Lett. 45 (1980) 1519–1523. [34] S.K. Kim, T.H. Joo, S.W. Suh, M.S. Kim, J. Raman Spectrosc. 17 (1986) 381–386. [35] T.H. Joo, K. Kim, M.S. Kim, J. Phys. Chem. 90 (1986) 5816–5819. [36] J. Chowdhury, M. Ghosh, P. Pal, T.N. Misra, J. Colloid. Interface Sci. 263 (2003) 318–326. [37] S.Y. Feng, J. Chen, Q. Chen, J. Pan, R. Chen, S. Lin, C. Li, Proc. SPIE 7519 (2009). 75191R-1–75191R-8.