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Surface Enhanced Raman Spectroscopic investigations of 2-bromo-3-methylamino-1,4-naphthoquinone on silver nanoparticles
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1967–1973
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Surface Enhanced Raman Spectroscopic investigations of 2-bromo-3methylamino-1,4-naphthoquinone on silver nanoparticles K. Geetha a, M. Umadevi a,⇑, G.V. Sathe b, P. Vanelle c, T. Terme c, O. Khoumeri c a
Department of Physics, Mother Teresa Women’s University, Kodaikanal 624101, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India c Laboratoire de Pharmaco-Chimie Radicalaire, Faculté de Pharmacie, Aix-Marseille Univ, CNRS, Institut de Chimie Radicalaire ICR, UMR 7273, 27 Boulevard Jean Moulin, 13385 Marseille Cedex 05, France b
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
Silver nanoparticles were synthesized
by solution combustion method using citric acid as fuel. Prepared silver nanoparticles are fcc structure. nRs and SERS studies were performed for BMANQ molecule. Higher enhancement observed for C@O and CABr stretching modes. Orientation of BMANQ molecule on silver nanoparticles is ‘stand-on’.
a r t i c l e
i n f o
Article history: Received 21 June 2014 Received in revised form 4 August 2014 Accepted 27 October 2014 Available online 1 November 2014 Keywords: Silver nanoparticles Solution combustion method 2-Bromo-3-methylamino-1,4naphthoquinone (BMANQ) Surface Enhanced Raman scattering
a b s t r a c t Surface Enhanced Raman Spectroscopic technique has been employed to investigate the orientation of 2bromo-3-methylamino-1,4-naphthoquinone (BMANQ) on silver nanoparticles. Silver nanoparticles have been prepared by solution combustion method with citric acid as fuel. Silver nanoparticles were characterized by X-ray Diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Electron Microscopy (SEM). XRD and morphological results confirmed the nanocrystalline nature of the prepared silver nanoparticles. The observed intense C@O stretching, CABr stretching and NH2 vibration suggests that the BMANQ molecule may be adsorbed in a ‘stand-on’ orientation to the silver surface. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy show that charge transfer occurs within the molecule. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Surface Enhanced Raman Spectroscopic (SERS) has been of interest in the fields of Physics, Chemistry, Surface Science, Nanoscience, and Biomedical Science. SERS is a high sensitivity spectrum without damage to samples. It has high potential in ⇑ Corresponding author. Tel.: +91 4542 245685. E-mail address: [email protected] (M. Umadevi). http://dx.doi.org/10.1016/j.saa.2014.10.119 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
providing some useful information about the nature, orientation of adsorbed molecular species and the adsorbate–metal interaction mechanism [1]. SERS technology has been well established for obtaining detailed information of molecules adsorbed on the surfaces of silver, gold or other noble metals, such as the adsorption configuration of molecules and the interaction mechanism of the molecules with the surfaces of substrates [2–7]. SERS is widely used to explain in sequence, the performance of biomolecules adsorbed at the metal surfaces, orientation of adsorbed species
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K. Geetha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1967–1973
and the changes in the orientation brought by peripheral features. In addition, the adsorption of molecules on metal particles reduces the fluorescence environment and hence the technique is useful in the study of biological samples [8–10]. The enhancement of selective vibrational modes and band shifts observed in SERS have usually been described in terms of the charge-transfer model and are established to be sensitive to the orientation of the molecules with respect to the surface [11–13]. The technique is therefore expected to provide interesting information on the sites through which the interaction takes place and also the molecular orientation with respect to the metal surfaces. Naphthoquinone (NQ) is the most important derivatives of naphthalene. They are of significant interest in chemical and biochemical fields. Naphthoquinone has been widely used in various fields such as dye for cosmetics, textiles, foods, and for curative purposes, including antiphthisic, antitumor, anti-inflammatory, antimicrobial agents, and antimalarial action and can be used for pharmacological treatment of different types of respiratory diseases, especially cancer [14,15]. Recently, our group has done SERS works for 2,3-dibromo-1,4-naphthoquinone and 1,4-dibromonaphthalene [16,17]. In this present investigation, SERS spectral analysis of 2-bromo3-methylamino-1,4-naphthoquinone (BMANQ) on silver surface and the analysis of vibrational modes observed both in experimental and computational method were studied. HOMO–LUMO analysis of BMANQ molecule were also investigated.
Experimental Materials Silver nitrate (AgNO3) and citric acid (C6H8O7) were purchased from MERCK. 2-bromo-3-methylamino-1,4-naphthoquinone (BMANQ) was synthesized according to the literature [19]. All the chemicals were of analytical grade and were used as obtained without further purification. Double-distilled water was used throughout the experiment. Preparation of silver nanoparticles (Ag NPs) using solution combustion method For the preparation of Ag nanoparticles, the stoichiometric composition of the solution components (fuels and oxidizer) was calculated according to the principle of propellant chemistry, keeping the oxidizer (metal nitrate) to fuel (citric acid) ratio as unity [18]. Stoichiometric amount of silver nitrate was dissolved in minimum quantity of deionised water and then citric acid was added into it. The solution was kept on the hotplate at 300 °C. Initially, the solution boiled and went through dehydration followed by disintegration with the evolution of large amount of gasses. After the solution reached the point of spontaneous combustion, it started to burn and releases a lot of heat, vaporizing the complete solution directly and the combustion reaction ended in 20 min. Finally loose greyish black colour powder was formed which was compressed and ground thoroughly.
washed with H2O and dried over MgSO4 and concentrated under vacuum. Recrystallization from ethanol gave the BMANQ [19].
Instrumentations and characterization The X-ray Diffraction (XRD) patterns were recorded using PANalytical X-ray diffractometer using Cu Ka radiation (k = 1.5406 Å) operated at 50 kV and 100 mA. The experiments were performed in the diffraction angle range of 2h = 20–80°. The size, composition and atomic structure of the nanoparticles were analyzed by Transmission Electron Microscopy (TEM) analysis and it was done using a JEOL JEM 2100 High Resolution Transmission Electron Microscope, operating at 200 kV. Field emission scanning electron microscopy (FESEM) analysis was done using an advanced micro analysis solution AMETEK. The Raman spectra were obtained by micro Raman system from Jobin Horibra LABRAM-HR visible spectrometer at room temperature with He–Ne laser. The excitation wavelength was 632 nm.
Result and discussion Structural studies of AgNPs The structure of prepared silver (Ag) nanoparticles has been studied by X-ray Diffraction (XRD) analysis. Fig. 1. shows a typical XRD pattern of the prepared silver nanoparticles using citric acid as fuel in stoichiometric. The XRD peak positions were consistent with the silver and the sharp peaks of XRD indicate the crystalline nature. The peaks were observed at 2h = 38.1°, 44.3°, 64.4° and 77.4° which correspond to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) Bragg’s reflections of face centre cubic structure of silver respectively (PCPDF# 893722). The lattice constant values are also calculated and are very close to the standard data. The calculated lattice constants of the unit cells are a = 4.084. No peak of other impurities can be detected from this pattern. This indicates that pure silver metal was obtained under the present synthesis conditions. The particle size (D) of the Ag nanoparticles was calculated using the Debye–Scherrer formula as D = 0.9k/b cos h where D is the particles size (nm), k is the wavelength of the X-ray (k = 1.5406 Å), b is the full width at half-maximum intensity of the diffraction line (in radians) and h is the Bragg angle (degree) of the reflection peak. The calculated average particles size was found around 46 nm.
Preparation of 2-bromo-3-methylamino-1,4-naphthoquinone (BMANQ) To a solution of 2,3-dibromonaphthalene-1,4-dione (10 g, 31.65 mmol), dioxane (100 mL) and methylamine (13 mL, 316.5 mmol) was added. The reaction mixture was stirred at room temperature for 2 h, the mixture was poured in water (100 mL) and extracted with CHCl3 (2 50 mL). The organic phase was
Fig. 1. X-ray Diffraction pattern of prepared by silver nanoparticles.
K. Geetha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1967–1973
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Morphological studies of AgNPs The morphology, size and microstructure of the products were investigated in detail through HRTEM and SEM. A representative HR-TEM and SEM images of the silver nanoparticles are shown in Figs. 2 and 3 from which the silver nanoparticles are demonstrated to be dispersed almost as spherical particles. The twining of silver nanoparticles in the TEM image is due to the planar defects. It is clearly seen that average particle size of the silver nanoparticles was about 15–44 nm from both HR-TEM and SEM. The SEM image of silver nanoparticles shows that the little negated and spaces are observed in the image because of the escaping gases during combustion reaction. This may be due to escaping of gas with high pressure. The morphology of the powders reveals the inherent spirit of the combustion process. This agreed very well with the result of XRD measurement. HOMO–LUMO analysis
Fig. 3. SEM image of prepared silver nanoparticles.
The most important orbitals in a molecule are frontier molecular orbitals: highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), which is the result of a significant degree of intermolecular charge transfer (ICT) from the end-capping electron-donor groups through p-conjugated path. The strong charge transfer interaction through p-conjugated bridge results in significant ground state donor–acceptor mixing and the appearance of a charge transfer band in the electronic absorption spectrum. These molecules interact with other species. The HOMO–LUMO molecular orbital gap helps to characterize the chemical reactivity; optical polarizability and chemical hardness– softness of a molecule [20]. Surfaces for the HOMO–LUMO molecular orbitals were drawn to understand the bonding scheme of the title compound. Fig. 4 shows the calculated HOMO–LUMO molecular orbitals for BMANQ at HF/6-311G. The calculated energy gap of BMANQ is 1.3778 eV at HF/6-311G. The chemical stiffness and faintness of a molecule is a superior suggestion of the chemical stability of the molecule. The molecules that contain small energy gap are more polarizable because they require small energy for excitation. The HOMO and LUMO energy gap explains the fact that final charge transfer interaction is taking place within the molecule. Vibrational assignments The structure of BMANQ molecule is shown in Fig. 5. The normal Raman (nRs) and SERS spectrum of BMANQ are shown in Figs. 6 and 7. The wavenumber of the observed Raman and SERS Fig. 4. HOMO and LUMO structure of BMANQ molecule.
Fig. 2. TEM image of prepared silver nanoparticles.
bands, their relative intensities and the assignments are given in Table 1. The CAC stretching (ring stretching) vibrations are very important in the spectrum of naphthalene derivatives and are highly characteristic of the aromatic ring itself. Naphthalene ring CAC stretching vibrations are expected in the region 1600–1250 cm 1 and these vibrations are found to make a major contribution in the Raman and SERS Spectra [21]. In this view, the CAC aromatic stretches are observed at 1630–1336 cm 1 in the Raman spectrum and the strong peaks are observed at 1649–1330 cm 1 in the SERS spectrum. The CAC stretching vibration, were assigned to coupled vibration of NAH bending and CH3 deformation vibrations. The CAH in-plane bending vibration usually occurs in the region 1300–1000 cm 1 and is very useful for characterization purposes [22]. In the present case, CAH in-plane bending vibrational
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K. Geetha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1967–1973 Table 1 Vibrational assignment of BMANQ and BMANQ adsorbed on the silver nanoparticles. Wavenumber (cm
Fig. 5. Structure of 2-bromo-3-methylamino-1,4-naphthoquinone (BMANQ).
1
)
Vibrational assignments
nRs
SERS
1741 1711 1680 1630
1745 1721 1686 1649 1627
1599 1553 1516
1602 1559 1522
1479
1488
1451 1429 1414 1395 1380
1457 – 1417
1336 1284 1256 1200 1169 1126 1104 1080 1030 1005
1383 1358 1330 1293 1259 1219 1188 1163 1120 1092 1043 1005
975 907
987 913 876
854 808
857
758 724 696 665
764 727
616 – 492
671 653 619 585 492
Fig. 6. Raman spectrum (nRs) of BMANQ molecule. 455 430 399 350 266 211 186 152 115
Fig. 7. SERS spectrum of BMANQ molecule on silver nanoparticles.
452 393 344 276 220 161 117
C@O stretching C@O stretching C@O stretching CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching CAC stretching CAC stretching CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending, CH3 rocking CAH in-plane bending, CH3 rocking CAH in-plane bending, CH3 rocking CAH in-plane bending CAH in-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending Ring breathing CAH out-of-plane bending Skeletal deformation Skeletal deformation Skeletal deformation Skeletal deformation Skeletal deformation Skeletal deformation Skeletal deformation CABr stretching CABr stretching AgAN stretching CH3 torsion CABr in-plane bending CH3 torsion
modes are observed in the region 1284–975 cm 1 in nRs and SERS spectra in the region 1293–987 cm 1. The aromaticity of the compound was clearly proved by the presence of strong peak below 900 cm 1 and the substitution patterns on the ring can be evaluated from the out of plane bending of the ring CAH bond in the region 900–667 cm 1 which are more informative [23]. In the present study, the CAH out-of-plane bending modes of BMANQ molecule occur in the region 907–665 cm 1 in nRs and the weak and strong bands of SERS at 913–727 cm 1 confirm the CAH out-of plane bending vibrations which agrees well with the above said literature values [24,25]. The changes in the frequencies of these deformations from the values in naphthalene are almost determined exclusively by the relative position of the substituent and are almost independent of their nature.
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BMANQ under consideration possesses a CH3 group in the sidesubstituted chain. For the assignments of CH3 group frequencies, one can expect that nine fundamentals can be associated with each CH3 group, namely the symmetrical stretching in CH3 (CH3 symmetric stretch) and asymmetrical stretching (CH3 asymmetric stretch) in-plane stretching modes (i.e., in-plane hydrogen stretching mode); the symmetrical (CH3 symmetric deform) and asymmetrical (CH3 asymmetric deform) deformation modes; the in-plane rocking (CH3 ip), out-of-plane rocking (CH3 op), and twisting (tCH3) modes. Methyl groups are generally referred as electron-donating substituent in the aromatic ring system. The methyl hydrogen atoms in BMANQ are subjected simultaneously to hyper conjugation and back donation, which causes the decrease in stretching wavenumber and infrared intensities, as reported in literature [26]. The CH3 deformation mode occurs in the region 1650–1515 cm 1. These assignments are also supported by the literature [27]. In the present case, nRs vibrational mode observed in the region 1451–1380 cm 1 and 1457–1358 cm 1 SERS are assigned to CH3 deformation modes which have appeared as coupled vibrations with CAC stretching modes. The methyl rocking modes are active in the region 1070–1010 cm 1 [28] with a band intensity which is sometimes weak, although the band can also be of medium intensity or sometimes even be strong. In the present work, the medium intensity peak at 1080–1005 cm 1 in nRs and 1092–1005 cm 1 SERS spectrum are assigned to CH3 rocking modes which are appeared as coupled vibrations with CAH inplane bending modes. The CH3 torsion vibrational modes are observed at 186 and 115 cm 1 in Raman spectrum and SERS vibrational mode is observed in the region 117 cm 1. The methyl group assignments proposed in this study is also in agreement with the literature values [29–31]. Strong characteristic adsorption due to the CABr stretching vibration is observed with the position of the bend, being influenced by neighbouring atom or group, smaller the halide atom, greater the influence of the neighbour. Bands with weak to medium intensity are also observed for the CABr stretching vibrations. Most aromatic CABr stretching vibrational modes are observed strongly in the region 377 and 267 cm 1 [22]. In the present case, a band is observed at 350–266 cm 1 in both nRs and SERS are assigned to CABr stretching vibration for BMANQ molecule. The CABr in-plane bending mode is observed at 152–162 cm 1 in both nRs and SERS spectrum. The skeletal deformation mode of NQ molecule occurs in the region 600–250 cm 1 [31]. The skeletal deformation mode was observed at 616–399 cm 1 in nRs and SERS band observed in the region 619–393 cm 1 are assigned to skeletal deformation of NQ. In NQ, ring breathing vibrational mode is observed at 690 cm 1 [32]. The ring breathing mode was observed at 665 cm 1 in nRs and 671 cm 1 in SERS spectra. The C@O stretching vibration gives rise to characteristic bands in the Raman and SERS spectra and the intensity of these bands can increase due to conjugation or the formation of hydrogen bonds [22]. Generally, the carbonyl group stretching vibration modes has been observed around 1800–1700 cm 1 [33]. If a carbonyl group is part of a conjugated system, then the wavenumber of the carbonyl stretching vibration decreases, the reason being that the double bond character of the C@O group is less due to the p-electron conjugation being localized. For the title compound, the stretching C@O stretching mode is seen as a strong band at 1741–1680 cm 1 in the nRs and 1745–1686 cm 1 in SERS spectra. N-H bending is usually observed in the region 1650–1515 cm 1, which is called the amide band [34]. The N-H bending mode was observed in the region 1630–1516 cm 1in nRs and SERS band are observed in the region 1649–1522 cm 1 are assigned to NAH bending of NQ.
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The band at 220 cm 1 is due to the stretching mode between metal and adsorbate. In most of the study on nitrogen heterocycles adsorbed on silver electrodes, this line is recognized to the weak AgAN bond [35]. In the present case, the nRs band at 211 cm 1 and SERS band observed at 220 cm 1 was assigned as AgAN stretching vibrational mode. SERS studies of BMANQ molecule on silver surface The surface enhancement depends on the following factors. The first effect is the kind of adsorption between chemical compound and metal nanoparticles, i.e. chemisorption or physisorption. Chemisorptions shows high surface enhancement when contrasted to physisorption. The second effect depends on the orientation of the chemical compound on the metal nanoparticles i.e. flat-on or stand-on. The third effect involves the polar substitute of the chemical compound i.e. withdrawal of electron or donation of electron [36]. Molecules chemisorbed on metal surface show a larger enhancement than the physisorbed molecules, signifying some chemical effect between the molecule and the surface. It is known that the absorption coefficients of chemisorbed molecules are larger than those of strong over-layers [37]. Some theories expect that charge oscillations between molecular orbitals and the metal surface enlarge the adsorption coefficient of adsorbates by intensity borrowed from the charge oscillations [38,17]. Two enhancement mechanisms commonly explain SERS effect, the electromagnetic mechanism and chemical mechanism. In the electromagnetic mechanism, local electric fields in the surroundings of the metal nanoparticles were enhanced due to the surface plasmon excitation, leading to more intense electronic transition in molecules located near the nanoparticles, and enhanced Raman scattering. The electromagnetic mechanism depends on tunable optical properties of the metal nanoparticles that can be optimized in order to achieve higher SERS enhancements. The enhancement in chemical mechanism consists of increasing the molecular polarizability of the adsorbate due to the charge transfer interaction of the absorbent with metal nanoparticle surface. In the present case, the SERS enhancement in the silver surface was explained through electromagnetic and chemical enhancement mechanism. There are two possible orientations that the molecule that may adsorb through chemisorption process, i.e. ‘face-on’ and ‘stand-on’. The BMANQ molecule has three binding sites, i.e. aromatic ring, long pair of electron of the nitrogen and carbonyl groups. These features may lead to the adsorption of the molecule on the silver surface [39]. The orientation of the molecule on the silver surface can be inferred from aromatic CAC stretching, ring breathing, inplane bending, out-of-plane bending, and the SERS surface selection rule. The possible ring breathing mode represents sufficient information of the orientation in NQ. The ring breathing mode of SERS that occurs at 671 cm 1 was upshifted by about 6 cm 1 and the bandwidth was decreased, compared to 665 cm 1 for nRs. In the SERS spectra, the metal–molecule interactions increase the frequency of the ring breathing mode when compared to the ‘free’ molecule in the solid state. It was clearly proposed that BMANQ molecule adsorbed on the silver surface in a ‘stand-on’ orientation. Generally, quinone derivatives are adsorbed on the metal surface through the C@O binding site. One more promising way in which the BMANQ molecule ‘stand-on’ adsorption occurs on the silver surface is through the C@O. Carboxylate group can interact strongly with metal through the formation of charge transfer compound between adsorbed and metal nanoparticles, where adsorbate operates like a donor and the metal as acceptor. The intensity of the peak increases and the wavenumber is downshifted with respect to the corresponding nRs band [40]. In the present case, bands are observed in region 1741, 1711 and 1680 cm 1 in nRs and SERS bands are observed in the region 1745, 1721 and
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1686 cm 1 of BMANQ molecule on silver surface due to C@O stretching. The observed peak intensity and upshifted wavenumber of C@O stretching mode indicates that the molecule is adsorbed on the metal through C@O stretching. In addition, the vibrations relating to atoms that are close to the silver surface will be enhanced. When the wavenumber difference between nRs and SERS spectra is not more than 10 cm 1, the molecular plane will be perpendicular to the silver surface [40]. In the present case the difference between nRs and SERS not more than 10 cm 1 wavenumber for C@O stretching which also evidence that the BMANQ has ‘stand-on’ orientations. The strength of out-of-plane vibrational modes decreases substantially relative to the in-plane vibrational modes, when the adsorbed orientation is distorted from parallel to stand-on orientation. The high intensity of in-plane vibrational modes occurs in the region 1293–987 cm 1 in SERS and nRs vibrational modes occur in the region 1284–975 cm 1. The CAH out-of-plane vibrational modes were observed in the region 913–653 cm 1 in both nRs and SERS. In our case, the intensity of in-plane vibrational modes has increased with respect to the out-of-plane bending mode. This once again confirms the ‘stand-on’ orientation of BMANQ adsorbed silver nanoparticles [41]. When the molecules adsorbed ‘stand-on’ the silver surface, the polarizability tensor corresponding to out-ofplane vibration was parallel to the metal surface. Some of the outof-plane vibrational modes are not observed in the SERS spectrum. The intensity of the peak at 854, 758, 724 and 665 cm 1in nRs is less than that of the peak at 857, 764, 727 and 671 cm 1 in SERS also indicates that the greater possibility of the ‘stand-on’ orientation of BMANQ molecule adsorbed on silver surface. The CAC stretching vibrations are generally most important in the spectrum of NQ and its derivatives are characteristic of aromatic ring. CAC ring stretching vibration modes usually occur in the region 1600–1250 cm 1 [21]. The nRs wavenumber of CAC stretching vibrational modes have decreased by greater than 10 cm 1 and their bandwidth increases considerably when the molecule adsorbs on the metal surface via their p system. The nRs and SERS shows that sixteen ring stretching vibration mode occur in the region 1649–1330 cm 1. The bands at 1627– 1330 cm 1 in SERS are upshifted in the nRs spectrum and bandwidths are affected. But in the case of CAC stretching vibration modes the SERS modes are shifted with respected to nRs modes. In our case, the overall vibrational of CAC stretching modes increases with respect to nRs. It clearly recommends that the BMANQ molecule adsorbed on the silver surface in a ‘stand-on’ orientation. From the observed up-shifted peaks and its wavenumber, it was seen that it is difficult to make orientation of the BMANQ molecules on the silver surface using CAC stretching vibrations. Another possible way in which the BMANQ molecule in a ‘stand-on’ adsorption occurs on the silver surface is through the CABr stretching. Generally, in quinone the CABr stretching vibrational band is very strong in intensity [41]. In the present case, bands are observed in region 344 and 267 cm 1 in nRs and SERS bands are observed in the region 348 and 276 cm 1 of BMANQ molecule on silver surface due to CABr stretching. The CABr inplane bending vibration modes also occur in the region 152 and 161 cm 1 in nRs and SERS. In both case these modes are enhanced in SERS and shifted by 9 cm 1. It was evidently proposed that BMANQ molecule was adsorbed on the silver surface in a ‘standon’ orientation through Br. In the present case, a new band appears at 220 cm 1 in SERS was due to Ag-N stretching vibration mode [35]. This is indicating that the BMANQ molecule was adsorbed on the silver surface through the nitrogen atom in ‘stand-on’ orientation. The observed high SERS signal indicates that the prepared silver nanoparticles are good source for physical, chemical and biomedical application as SERS substrate.
From the above discussion, it is concluded that BMANQ molecules adsorb as ‘stand-on’ orientation on the silver surface through the coordinating site C@O, CABr and NH2. Conclusion Silver nanoparticles were synthesized by solution combustion method using citric acid as fuel. Its structural and morphological properties were investigated and confirm that the particles have a cubic crystalline structure. The SERS spectral analysis indicates that the silver nanoparticles reveal high SERS activity. The vibrational features of C@O, CABr and NH2 bending modes suggest that the BMANQ molecule was adsorbed through a ‘stand-on’ orientation on the silver surface. The HOMO–LUMO analysis confirms that the energy gap value has significant influence on the intermolecular charge transfer and that the BMANQ molecule has quite established configuration. Acknowledgements The author M. Umadevi is thankful to DST-CURIE, New Delhi, DST-SERB, New Delhi and UGC-DAE-CSR, Indore for financial assistance. References [1] Y.J. Kwon, S.B. Lee, K. Kim, M.S. Kim, J. Mol. Struct. 318 (1994) 25–35. [2] M. Umadevi, V. Ramakrishnan, J. Raman Spectrosc. 34 (2003) 13–20. [3] D.A. Stern, L. Laguren-Davidson, K.G. Frank, J.Y. Gui, C.H. Lin, F. Lu, G.N. Salaita, N. Walton, D.C. Zapien, A.T. Hubbard, J. Am. Chem. Soc. 111 (1989) 877–891. [4] K. Geetha, M. Umadevi, G.V. Sathe, P. Vanelle, T. Terme, O. Khoumeri, J. Mol. Struct. 1059 (2014) 87–93. [5] K. Zawada, J. Bukowska, Surf. Sci. 34 (2002) 507–510. [6] W.B. Cai, B. Ren, X.Q. Li, C.X. She, F.M. Liu, X.W. Cai, Z.Q. Tian, Surf. Sci. 406 (1998) 9–22. [7] A. Kudelski, M. Janik-Czachor, M. Pisarek, J. Bukowska, P. Mack, M. Dolata, A. Szummer, Surf. Sci. 441 (2002) 507–510. [8] Y.Q. Wu, B. Zhao, W.Q. Xu, G.W. Li, B. Li, Langmuir 15 (1999) 1247–1251. [9] M.M. Miranda, Vib. Spectrosc. 29 (2002) 229–234. [10] M.M. Miranda, N. Neto, G. Sbrana, J. Mol. Struct. 205 (1997) 410–411. [11] E. Hesse, J.A. Creighton, Chem. Phys. Lett. 303 (1999) 101–110. [12] J. Sallack, A.K. Maiti, R. Aroca, J.R. Menendez, J. Mol. Struct. 217 (1997) 410– 411. [13] Y.H. Chen, C.Y. Yeh, Colloids Surf. A 197 (2002) 133–139. [14] P. Rajeshwar Verma, Anti Cancer Agents Med. Chem. Med. Chem. 6 (2006) 489–499. [15] O. Zakharova, L. Goryunov, N. Troshkova, Eur. J. Med. Chem. 45 (2010) 270– 274. [16] M. Anuratha, A. Jawahar, M. Umadevi, V.G. Sathe, P. Vanelle, T. Terme, V. Meenakumari, A. Milton Franklin Benial, Spectrochim. Acta Part A 105 (2013) 218–222. [17] K. Geetha, M. Umadevi, G.V. Sathe, R. Erenler, Spectrochim. Acta Part A 116 (2013) 236–241. [18] O. Khoumeri, M. Montana, T. Terme, P. Vanelle, P. First, Tetrahedron 64 (2008) 11237–11242. [19] S.T. Aruna, A.S. Mukasyan, Curr. Opin. Solid State Mater. Sci. 12 (2008) 44–50. [20] V. Balachandran, G. Santhi, Vib. Spectrosc. 52 (2012) 11425–11433. [21] N.R. Sheela, S. Muthu, S. Sampath Krishnan, Der. Pharma Chemica 4 (2012) 169–183. [22] H. Kirk, M. Michaelian, M.S. Ziegler, Appl. Phys. 27 (1972) 1. [23] N.B. Colthup, L.H. Daly, S.E. Wilberley, Introduction to infrared and Raman spectroscopy, Academic Press, New York, 1990. [24] M. Karabacak, M. Cinar, Z. Unal, M. Kurt, J. Mol. Struct. 982 (2010) 22–27. [25] A. Altun, K. Golcuk, M. Kumru, J. Mol. Struct. (Theochem.) 637 (2003) 155–162. [26] G. Raja, K. Saravanan, K. Sivakumar, Int. J. Appl. Phys. Math. 1 (2011) 1–4. [27] Dasari Jagadeeswara Rao, Y. Ramakrishna1, V. Padmarao, B. Venkateswara Rao, Int. J. Adv. Res. Sci. Technol. 1 (2012) 135–159. [28] J.P. Kalyani, N. Kalaiselvi, N. Muniyandi, J. Powder Sources 111 (2002) 232– 238. [29] M. Karabacak, M. Cinar, S. Ermec, M. Kurt, J. Raman Spectrosc. 41 (2010) 98– 105. [30] A.U. Rani, N. Sundaraganesan, M. Kurt, M. Cinar, M. Karabacak, Spectrochim. Acta A 75 (2010) 1523–1532. [31] C. Sengamalai, M. Arivazhagan, K. Sampathkumar, Elixir Comp. Chem. 56 (2013) 13291–13295. [32] M. Moskovits, J. Phys. Chem. 92 (1988) 6327–6329. [33] N. Sundaraganesan, B. Dominic Joshua, T. Radjakoumar, Ind. J. Pure Appl. Phys. 47 (2009) 248–258.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Surface Enhanced Raman Spectroscopic investigations of 2-bromo-3methylamino-1,4-naphthoquinone on silver nanoparticles K. Geetha a, M. Umadevi a,⇑, G.V. Sathe b, P. Vanelle c, T. Terme c, O. Khoumeri c a
Department of Physics, Mother Teresa Women’s University, Kodaikanal 624101, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India c Laboratoire de Pharmaco-Chimie Radicalaire, Faculté de Pharmacie, Aix-Marseille Univ, CNRS, Institut de Chimie Radicalaire ICR, UMR 7273, 27 Boulevard Jean Moulin, 13385 Marseille Cedex 05, France b
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
Silver nanoparticles were synthesized
by solution combustion method using citric acid as fuel. Prepared silver nanoparticles are fcc structure. nRs and SERS studies were performed for BMANQ molecule. Higher enhancement observed for C@O and CABr stretching modes. Orientation of BMANQ molecule on silver nanoparticles is ‘stand-on’.
a r t i c l e
i n f o
Article history: Received 21 June 2014 Received in revised form 4 August 2014 Accepted 27 October 2014 Available online 1 November 2014 Keywords: Silver nanoparticles Solution combustion method 2-Bromo-3-methylamino-1,4naphthoquinone (BMANQ) Surface Enhanced Raman scattering
a b s t r a c t Surface Enhanced Raman Spectroscopic technique has been employed to investigate the orientation of 2bromo-3-methylamino-1,4-naphthoquinone (BMANQ) on silver nanoparticles. Silver nanoparticles have been prepared by solution combustion method with citric acid as fuel. Silver nanoparticles were characterized by X-ray Diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Electron Microscopy (SEM). XRD and morphological results confirmed the nanocrystalline nature of the prepared silver nanoparticles. The observed intense C@O stretching, CABr stretching and NH2 vibration suggests that the BMANQ molecule may be adsorbed in a ‘stand-on’ orientation to the silver surface. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy show that charge transfer occurs within the molecule. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Surface Enhanced Raman Spectroscopic (SERS) has been of interest in the fields of Physics, Chemistry, Surface Science, Nanoscience, and Biomedical Science. SERS is a high sensitivity spectrum without damage to samples. It has high potential in ⇑ Corresponding author. Tel.: +91 4542 245685. E-mail address: [email protected] (M. Umadevi). http://dx.doi.org/10.1016/j.saa.2014.10.119 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
providing some useful information about the nature, orientation of adsorbed molecular species and the adsorbate–metal interaction mechanism [1]. SERS technology has been well established for obtaining detailed information of molecules adsorbed on the surfaces of silver, gold or other noble metals, such as the adsorption configuration of molecules and the interaction mechanism of the molecules with the surfaces of substrates [2–7]. SERS is widely used to explain in sequence, the performance of biomolecules adsorbed at the metal surfaces, orientation of adsorbed species
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and the changes in the orientation brought by peripheral features. In addition, the adsorption of molecules on metal particles reduces the fluorescence environment and hence the technique is useful in the study of biological samples [8–10]. The enhancement of selective vibrational modes and band shifts observed in SERS have usually been described in terms of the charge-transfer model and are established to be sensitive to the orientation of the molecules with respect to the surface [11–13]. The technique is therefore expected to provide interesting information on the sites through which the interaction takes place and also the molecular orientation with respect to the metal surfaces. Naphthoquinone (NQ) is the most important derivatives of naphthalene. They are of significant interest in chemical and biochemical fields. Naphthoquinone has been widely used in various fields such as dye for cosmetics, textiles, foods, and for curative purposes, including antiphthisic, antitumor, anti-inflammatory, antimicrobial agents, and antimalarial action and can be used for pharmacological treatment of different types of respiratory diseases, especially cancer [14,15]. Recently, our group has done SERS works for 2,3-dibromo-1,4-naphthoquinone and 1,4-dibromonaphthalene [16,17]. In this present investigation, SERS spectral analysis of 2-bromo3-methylamino-1,4-naphthoquinone (BMANQ) on silver surface and the analysis of vibrational modes observed both in experimental and computational method were studied. HOMO–LUMO analysis of BMANQ molecule were also investigated.
Experimental Materials Silver nitrate (AgNO3) and citric acid (C6H8O7) were purchased from MERCK. 2-bromo-3-methylamino-1,4-naphthoquinone (BMANQ) was synthesized according to the literature [19]. All the chemicals were of analytical grade and were used as obtained without further purification. Double-distilled water was used throughout the experiment. Preparation of silver nanoparticles (Ag NPs) using solution combustion method For the preparation of Ag nanoparticles, the stoichiometric composition of the solution components (fuels and oxidizer) was calculated according to the principle of propellant chemistry, keeping the oxidizer (metal nitrate) to fuel (citric acid) ratio as unity [18]. Stoichiometric amount of silver nitrate was dissolved in minimum quantity of deionised water and then citric acid was added into it. The solution was kept on the hotplate at 300 °C. Initially, the solution boiled and went through dehydration followed by disintegration with the evolution of large amount of gasses. After the solution reached the point of spontaneous combustion, it started to burn and releases a lot of heat, vaporizing the complete solution directly and the combustion reaction ended in 20 min. Finally loose greyish black colour powder was formed which was compressed and ground thoroughly.
washed with H2O and dried over MgSO4 and concentrated under vacuum. Recrystallization from ethanol gave the BMANQ [19].
Instrumentations and characterization The X-ray Diffraction (XRD) patterns were recorded using PANalytical X-ray diffractometer using Cu Ka radiation (k = 1.5406 Å) operated at 50 kV and 100 mA. The experiments were performed in the diffraction angle range of 2h = 20–80°. The size, composition and atomic structure of the nanoparticles were analyzed by Transmission Electron Microscopy (TEM) analysis and it was done using a JEOL JEM 2100 High Resolution Transmission Electron Microscope, operating at 200 kV. Field emission scanning electron microscopy (FESEM) analysis was done using an advanced micro analysis solution AMETEK. The Raman spectra were obtained by micro Raman system from Jobin Horibra LABRAM-HR visible spectrometer at room temperature with He–Ne laser. The excitation wavelength was 632 nm.
Result and discussion Structural studies of AgNPs The structure of prepared silver (Ag) nanoparticles has been studied by X-ray Diffraction (XRD) analysis. Fig. 1. shows a typical XRD pattern of the prepared silver nanoparticles using citric acid as fuel in stoichiometric. The XRD peak positions were consistent with the silver and the sharp peaks of XRD indicate the crystalline nature. The peaks were observed at 2h = 38.1°, 44.3°, 64.4° and 77.4° which correspond to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) Bragg’s reflections of face centre cubic structure of silver respectively (PCPDF# 893722). The lattice constant values are also calculated and are very close to the standard data. The calculated lattice constants of the unit cells are a = 4.084. No peak of other impurities can be detected from this pattern. This indicates that pure silver metal was obtained under the present synthesis conditions. The particle size (D) of the Ag nanoparticles was calculated using the Debye–Scherrer formula as D = 0.9k/b cos h where D is the particles size (nm), k is the wavelength of the X-ray (k = 1.5406 Å), b is the full width at half-maximum intensity of the diffraction line (in radians) and h is the Bragg angle (degree) of the reflection peak. The calculated average particles size was found around 46 nm.
Preparation of 2-bromo-3-methylamino-1,4-naphthoquinone (BMANQ) To a solution of 2,3-dibromonaphthalene-1,4-dione (10 g, 31.65 mmol), dioxane (100 mL) and methylamine (13 mL, 316.5 mmol) was added. The reaction mixture was stirred at room temperature for 2 h, the mixture was poured in water (100 mL) and extracted with CHCl3 (2 50 mL). The organic phase was
Fig. 1. X-ray Diffraction pattern of prepared by silver nanoparticles.
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Morphological studies of AgNPs The morphology, size and microstructure of the products were investigated in detail through HRTEM and SEM. A representative HR-TEM and SEM images of the silver nanoparticles are shown in Figs. 2 and 3 from which the silver nanoparticles are demonstrated to be dispersed almost as spherical particles. The twining of silver nanoparticles in the TEM image is due to the planar defects. It is clearly seen that average particle size of the silver nanoparticles was about 15–44 nm from both HR-TEM and SEM. The SEM image of silver nanoparticles shows that the little negated and spaces are observed in the image because of the escaping gases during combustion reaction. This may be due to escaping of gas with high pressure. The morphology of the powders reveals the inherent spirit of the combustion process. This agreed very well with the result of XRD measurement. HOMO–LUMO analysis
Fig. 3. SEM image of prepared silver nanoparticles.
The most important orbitals in a molecule are frontier molecular orbitals: highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), which is the result of a significant degree of intermolecular charge transfer (ICT) from the end-capping electron-donor groups through p-conjugated path. The strong charge transfer interaction through p-conjugated bridge results in significant ground state donor–acceptor mixing and the appearance of a charge transfer band in the electronic absorption spectrum. These molecules interact with other species. The HOMO–LUMO molecular orbital gap helps to characterize the chemical reactivity; optical polarizability and chemical hardness– softness of a molecule [20]. Surfaces for the HOMO–LUMO molecular orbitals were drawn to understand the bonding scheme of the title compound. Fig. 4 shows the calculated HOMO–LUMO molecular orbitals for BMANQ at HF/6-311G. The calculated energy gap of BMANQ is 1.3778 eV at HF/6-311G. The chemical stiffness and faintness of a molecule is a superior suggestion of the chemical stability of the molecule. The molecules that contain small energy gap are more polarizable because they require small energy for excitation. The HOMO and LUMO energy gap explains the fact that final charge transfer interaction is taking place within the molecule. Vibrational assignments The structure of BMANQ molecule is shown in Fig. 5. The normal Raman (nRs) and SERS spectrum of BMANQ are shown in Figs. 6 and 7. The wavenumber of the observed Raman and SERS Fig. 4. HOMO and LUMO structure of BMANQ molecule.
Fig. 2. TEM image of prepared silver nanoparticles.
bands, their relative intensities and the assignments are given in Table 1. The CAC stretching (ring stretching) vibrations are very important in the spectrum of naphthalene derivatives and are highly characteristic of the aromatic ring itself. Naphthalene ring CAC stretching vibrations are expected in the region 1600–1250 cm 1 and these vibrations are found to make a major contribution in the Raman and SERS Spectra [21]. In this view, the CAC aromatic stretches are observed at 1630–1336 cm 1 in the Raman spectrum and the strong peaks are observed at 1649–1330 cm 1 in the SERS spectrum. The CAC stretching vibration, were assigned to coupled vibration of NAH bending and CH3 deformation vibrations. The CAH in-plane bending vibration usually occurs in the region 1300–1000 cm 1 and is very useful for characterization purposes [22]. In the present case, CAH in-plane bending vibrational
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K. Geetha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1967–1973 Table 1 Vibrational assignment of BMANQ and BMANQ adsorbed on the silver nanoparticles. Wavenumber (cm
Fig. 5. Structure of 2-bromo-3-methylamino-1,4-naphthoquinone (BMANQ).
1
)
Vibrational assignments
nRs
SERS
1741 1711 1680 1630
1745 1721 1686 1649 1627
1599 1553 1516
1602 1559 1522
1479
1488
1451 1429 1414 1395 1380
1457 – 1417
1336 1284 1256 1200 1169 1126 1104 1080 1030 1005
1383 1358 1330 1293 1259 1219 1188 1163 1120 1092 1043 1005
975 907
987 913 876
854 808
857
758 724 696 665
764 727
616 – 492
671 653 619 585 492
Fig. 6. Raman spectrum (nRs) of BMANQ molecule. 455 430 399 350 266 211 186 152 115
Fig. 7. SERS spectrum of BMANQ molecule on silver nanoparticles.
452 393 344 276 220 161 117
C@O stretching C@O stretching C@O stretching CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching, NAH bending CAC stretching CAC stretching CAC stretching CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching, CH3 deformation CAC stretching CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending CAH in-plane bending, CH3 rocking CAH in-plane bending, CH3 rocking CAH in-plane bending, CH3 rocking CAH in-plane bending CAH in-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending CAH out-of-plane bending Ring breathing CAH out-of-plane bending Skeletal deformation Skeletal deformation Skeletal deformation Skeletal deformation Skeletal deformation Skeletal deformation Skeletal deformation CABr stretching CABr stretching AgAN stretching CH3 torsion CABr in-plane bending CH3 torsion
modes are observed in the region 1284–975 cm 1 in nRs and SERS spectra in the region 1293–987 cm 1. The aromaticity of the compound was clearly proved by the presence of strong peak below 900 cm 1 and the substitution patterns on the ring can be evaluated from the out of plane bending of the ring CAH bond in the region 900–667 cm 1 which are more informative [23]. In the present study, the CAH out-of-plane bending modes of BMANQ molecule occur in the region 907–665 cm 1 in nRs and the weak and strong bands of SERS at 913–727 cm 1 confirm the CAH out-of plane bending vibrations which agrees well with the above said literature values [24,25]. The changes in the frequencies of these deformations from the values in naphthalene are almost determined exclusively by the relative position of the substituent and are almost independent of their nature.
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BMANQ under consideration possesses a CH3 group in the sidesubstituted chain. For the assignments of CH3 group frequencies, one can expect that nine fundamentals can be associated with each CH3 group, namely the symmetrical stretching in CH3 (CH3 symmetric stretch) and asymmetrical stretching (CH3 asymmetric stretch) in-plane stretching modes (i.e., in-plane hydrogen stretching mode); the symmetrical (CH3 symmetric deform) and asymmetrical (CH3 asymmetric deform) deformation modes; the in-plane rocking (CH3 ip), out-of-plane rocking (CH3 op), and twisting (tCH3) modes. Methyl groups are generally referred as electron-donating substituent in the aromatic ring system. The methyl hydrogen atoms in BMANQ are subjected simultaneously to hyper conjugation and back donation, which causes the decrease in stretching wavenumber and infrared intensities, as reported in literature [26]. The CH3 deformation mode occurs in the region 1650–1515 cm 1. These assignments are also supported by the literature [27]. In the present case, nRs vibrational mode observed in the region 1451–1380 cm 1 and 1457–1358 cm 1 SERS are assigned to CH3 deformation modes which have appeared as coupled vibrations with CAC stretching modes. The methyl rocking modes are active in the region 1070–1010 cm 1 [28] with a band intensity which is sometimes weak, although the band can also be of medium intensity or sometimes even be strong. In the present work, the medium intensity peak at 1080–1005 cm 1 in nRs and 1092–1005 cm 1 SERS spectrum are assigned to CH3 rocking modes which are appeared as coupled vibrations with CAH inplane bending modes. The CH3 torsion vibrational modes are observed at 186 and 115 cm 1 in Raman spectrum and SERS vibrational mode is observed in the region 117 cm 1. The methyl group assignments proposed in this study is also in agreement with the literature values [29–31]. Strong characteristic adsorption due to the CABr stretching vibration is observed with the position of the bend, being influenced by neighbouring atom or group, smaller the halide atom, greater the influence of the neighbour. Bands with weak to medium intensity are also observed for the CABr stretching vibrations. Most aromatic CABr stretching vibrational modes are observed strongly in the region 377 and 267 cm 1 [22]. In the present case, a band is observed at 350–266 cm 1 in both nRs and SERS are assigned to CABr stretching vibration for BMANQ molecule. The CABr in-plane bending mode is observed at 152–162 cm 1 in both nRs and SERS spectrum. The skeletal deformation mode of NQ molecule occurs in the region 600–250 cm 1 [31]. The skeletal deformation mode was observed at 616–399 cm 1 in nRs and SERS band observed in the region 619–393 cm 1 are assigned to skeletal deformation of NQ. In NQ, ring breathing vibrational mode is observed at 690 cm 1 [32]. The ring breathing mode was observed at 665 cm 1 in nRs and 671 cm 1 in SERS spectra. The C@O stretching vibration gives rise to characteristic bands in the Raman and SERS spectra and the intensity of these bands can increase due to conjugation or the formation of hydrogen bonds [22]. Generally, the carbonyl group stretching vibration modes has been observed around 1800–1700 cm 1 [33]. If a carbonyl group is part of a conjugated system, then the wavenumber of the carbonyl stretching vibration decreases, the reason being that the double bond character of the C@O group is less due to the p-electron conjugation being localized. For the title compound, the stretching C@O stretching mode is seen as a strong band at 1741–1680 cm 1 in the nRs and 1745–1686 cm 1 in SERS spectra. N-H bending is usually observed in the region 1650–1515 cm 1, which is called the amide band [34]. The N-H bending mode was observed in the region 1630–1516 cm 1in nRs and SERS band are observed in the region 1649–1522 cm 1 are assigned to NAH bending of NQ.
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The band at 220 cm 1 is due to the stretching mode between metal and adsorbate. In most of the study on nitrogen heterocycles adsorbed on silver electrodes, this line is recognized to the weak AgAN bond [35]. In the present case, the nRs band at 211 cm 1 and SERS band observed at 220 cm 1 was assigned as AgAN stretching vibrational mode. SERS studies of BMANQ molecule on silver surface The surface enhancement depends on the following factors. The first effect is the kind of adsorption between chemical compound and metal nanoparticles, i.e. chemisorption or physisorption. Chemisorptions shows high surface enhancement when contrasted to physisorption. The second effect depends on the orientation of the chemical compound on the metal nanoparticles i.e. flat-on or stand-on. The third effect involves the polar substitute of the chemical compound i.e. withdrawal of electron or donation of electron [36]. Molecules chemisorbed on metal surface show a larger enhancement than the physisorbed molecules, signifying some chemical effect between the molecule and the surface. It is known that the absorption coefficients of chemisorbed molecules are larger than those of strong over-layers [37]. Some theories expect that charge oscillations between molecular orbitals and the metal surface enlarge the adsorption coefficient of adsorbates by intensity borrowed from the charge oscillations [38,17]. Two enhancement mechanisms commonly explain SERS effect, the electromagnetic mechanism and chemical mechanism. In the electromagnetic mechanism, local electric fields in the surroundings of the metal nanoparticles were enhanced due to the surface plasmon excitation, leading to more intense electronic transition in molecules located near the nanoparticles, and enhanced Raman scattering. The electromagnetic mechanism depends on tunable optical properties of the metal nanoparticles that can be optimized in order to achieve higher SERS enhancements. The enhancement in chemical mechanism consists of increasing the molecular polarizability of the adsorbate due to the charge transfer interaction of the absorbent with metal nanoparticle surface. In the present case, the SERS enhancement in the silver surface was explained through electromagnetic and chemical enhancement mechanism. There are two possible orientations that the molecule that may adsorb through chemisorption process, i.e. ‘face-on’ and ‘stand-on’. The BMANQ molecule has three binding sites, i.e. aromatic ring, long pair of electron of the nitrogen and carbonyl groups. These features may lead to the adsorption of the molecule on the silver surface [39]. The orientation of the molecule on the silver surface can be inferred from aromatic CAC stretching, ring breathing, inplane bending, out-of-plane bending, and the SERS surface selection rule. The possible ring breathing mode represents sufficient information of the orientation in NQ. The ring breathing mode of SERS that occurs at 671 cm 1 was upshifted by about 6 cm 1 and the bandwidth was decreased, compared to 665 cm 1 for nRs. In the SERS spectra, the metal–molecule interactions increase the frequency of the ring breathing mode when compared to the ‘free’ molecule in the solid state. It was clearly proposed that BMANQ molecule adsorbed on the silver surface in a ‘stand-on’ orientation. Generally, quinone derivatives are adsorbed on the metal surface through the C@O binding site. One more promising way in which the BMANQ molecule ‘stand-on’ adsorption occurs on the silver surface is through the C@O. Carboxylate group can interact strongly with metal through the formation of charge transfer compound between adsorbed and metal nanoparticles, where adsorbate operates like a donor and the metal as acceptor. The intensity of the peak increases and the wavenumber is downshifted with respect to the corresponding nRs band [40]. In the present case, bands are observed in region 1741, 1711 and 1680 cm 1 in nRs and SERS bands are observed in the region 1745, 1721 and
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1686 cm 1 of BMANQ molecule on silver surface due to C@O stretching. The observed peak intensity and upshifted wavenumber of C@O stretching mode indicates that the molecule is adsorbed on the metal through C@O stretching. In addition, the vibrations relating to atoms that are close to the silver surface will be enhanced. When the wavenumber difference between nRs and SERS spectra is not more than 10 cm 1, the molecular plane will be perpendicular to the silver surface [40]. In the present case the difference between nRs and SERS not more than 10 cm 1 wavenumber for C@O stretching which also evidence that the BMANQ has ‘stand-on’ orientations. The strength of out-of-plane vibrational modes decreases substantially relative to the in-plane vibrational modes, when the adsorbed orientation is distorted from parallel to stand-on orientation. The high intensity of in-plane vibrational modes occurs in the region 1293–987 cm 1 in SERS and nRs vibrational modes occur in the region 1284–975 cm 1. The CAH out-of-plane vibrational modes were observed in the region 913–653 cm 1 in both nRs and SERS. In our case, the intensity of in-plane vibrational modes has increased with respect to the out-of-plane bending mode. This once again confirms the ‘stand-on’ orientation of BMANQ adsorbed silver nanoparticles [41]. When the molecules adsorbed ‘stand-on’ the silver surface, the polarizability tensor corresponding to out-ofplane vibration was parallel to the metal surface. Some of the outof-plane vibrational modes are not observed in the SERS spectrum. The intensity of the peak at 854, 758, 724 and 665 cm 1in nRs is less than that of the peak at 857, 764, 727 and 671 cm 1 in SERS also indicates that the greater possibility of the ‘stand-on’ orientation of BMANQ molecule adsorbed on silver surface. The CAC stretching vibrations are generally most important in the spectrum of NQ and its derivatives are characteristic of aromatic ring. CAC ring stretching vibration modes usually occur in the region 1600–1250 cm 1 [21]. The nRs wavenumber of CAC stretching vibrational modes have decreased by greater than 10 cm 1 and their bandwidth increases considerably when the molecule adsorbs on the metal surface via their p system. The nRs and SERS shows that sixteen ring stretching vibration mode occur in the region 1649–1330 cm 1. The bands at 1627– 1330 cm 1 in SERS are upshifted in the nRs spectrum and bandwidths are affected. But in the case of CAC stretching vibration modes the SERS modes are shifted with respected to nRs modes. In our case, the overall vibrational of CAC stretching modes increases with respect to nRs. It clearly recommends that the BMANQ molecule adsorbed on the silver surface in a ‘stand-on’ orientation. From the observed up-shifted peaks and its wavenumber, it was seen that it is difficult to make orientation of the BMANQ molecules on the silver surface using CAC stretching vibrations. Another possible way in which the BMANQ molecule in a ‘stand-on’ adsorption occurs on the silver surface is through the CABr stretching. Generally, in quinone the CABr stretching vibrational band is very strong in intensity [41]. In the present case, bands are observed in region 344 and 267 cm 1 in nRs and SERS bands are observed in the region 348 and 276 cm 1 of BMANQ molecule on silver surface due to CABr stretching. The CABr inplane bending vibration modes also occur in the region 152 and 161 cm 1 in nRs and SERS. In both case these modes are enhanced in SERS and shifted by 9 cm 1. It was evidently proposed that BMANQ molecule was adsorbed on the silver surface in a ‘standon’ orientation through Br. In the present case, a new band appears at 220 cm 1 in SERS was due to Ag-N stretching vibration mode [35]. This is indicating that the BMANQ molecule was adsorbed on the silver surface through the nitrogen atom in ‘stand-on’ orientation. The observed high SERS signal indicates that the prepared silver nanoparticles are good source for physical, chemical and biomedical application as SERS substrate.
From the above discussion, it is concluded that BMANQ molecules adsorb as ‘stand-on’ orientation on the silver surface through the coordinating site C@O, CABr and NH2. Conclusion Silver nanoparticles were synthesized by solution combustion method using citric acid as fuel. Its structural and morphological properties were investigated and confirm that the particles have a cubic crystalline structure. The SERS spectral analysis indicates that the silver nanoparticles reveal high SERS activity. The vibrational features of C@O, CABr and NH2 bending modes suggest that the BMANQ molecule was adsorbed through a ‘stand-on’ orientation on the silver surface. The HOMO–LUMO analysis confirms that the energy gap value has significant influence on the intermolecular charge transfer and that the BMANQ molecule has quite established configuration. Acknowledgements The author M. Umadevi is thankful to DST-CURIE, New Delhi, DST-SERB, New Delhi and UGC-DAE-CSR, Indore for financial assistance. References [1] Y.J. Kwon, S.B. Lee, K. Kim, M.S. Kim, J. Mol. Struct. 318 (1994) 25–35. [2] M. Umadevi, V. Ramakrishnan, J. Raman Spectrosc. 34 (2003) 13–20. [3] D.A. Stern, L. Laguren-Davidson, K.G. Frank, J.Y. Gui, C.H. Lin, F. Lu, G.N. Salaita, N. Walton, D.C. Zapien, A.T. Hubbard, J. Am. Chem. Soc. 111 (1989) 877–891. [4] K. Geetha, M. Umadevi, G.V. Sathe, P. Vanelle, T. Terme, O. Khoumeri, J. Mol. Struct. 1059 (2014) 87–93. [5] K. Zawada, J. Bukowska, Surf. Sci. 34 (2002) 507–510. [6] W.B. Cai, B. Ren, X.Q. Li, C.X. She, F.M. Liu, X.W. Cai, Z.Q. Tian, Surf. Sci. 406 (1998) 9–22. [7] A. Kudelski, M. Janik-Czachor, M. Pisarek, J. Bukowska, P. Mack, M. Dolata, A. Szummer, Surf. Sci. 441 (2002) 507–510. [8] Y.Q. Wu, B. Zhao, W.Q. Xu, G.W. Li, B. Li, Langmuir 15 (1999) 1247–1251. [9] M.M. Miranda, Vib. Spectrosc. 29 (2002) 229–234. [10] M.M. Miranda, N. Neto, G. Sbrana, J. Mol. Struct. 205 (1997) 410–411. [11] E. Hesse, J.A. Creighton, Chem. Phys. Lett. 303 (1999) 101–110. [12] J. Sallack, A.K. Maiti, R. Aroca, J.R. Menendez, J. Mol. Struct. 217 (1997) 410– 411. [13] Y.H. Chen, C.Y. Yeh, Colloids Surf. A 197 (2002) 133–139. [14] P. Rajeshwar Verma, Anti Cancer Agents Med. Chem. Med. Chem. 6 (2006) 489–499. [15] O. Zakharova, L. Goryunov, N. Troshkova, Eur. J. Med. Chem. 45 (2010) 270– 274. [16] M. Anuratha, A. Jawahar, M. Umadevi, V.G. Sathe, P. Vanelle, T. Terme, V. Meenakumari, A. Milton Franklin Benial, Spectrochim. Acta Part A 105 (2013) 218–222. [17] K. Geetha, M. Umadevi, G.V. Sathe, R. Erenler, Spectrochim. Acta Part A 116 (2013) 236–241. [18] O. Khoumeri, M. Montana, T. Terme, P. Vanelle, P. First, Tetrahedron 64 (2008) 11237–11242. [19] S.T. Aruna, A.S. Mukasyan, Curr. Opin. Solid State Mater. Sci. 12 (2008) 44–50. [20] V. Balachandran, G. Santhi, Vib. Spectrosc. 52 (2012) 11425–11433. [21] N.R. Sheela, S. Muthu, S. Sampath Krishnan, Der. Pharma Chemica 4 (2012) 169–183. [22] H. Kirk, M. Michaelian, M.S. Ziegler, Appl. Phys. 27 (1972) 1. [23] N.B. Colthup, L.H. Daly, S.E. Wilberley, Introduction to infrared and Raman spectroscopy, Academic Press, New York, 1990. [24] M. Karabacak, M. Cinar, Z. Unal, M. Kurt, J. Mol. Struct. 982 (2010) 22–27. [25] A. Altun, K. Golcuk, M. Kumru, J. Mol. Struct. (Theochem.) 637 (2003) 155–162. [26] G. Raja, K. Saravanan, K. Sivakumar, Int. J. Appl. Phys. Math. 1 (2011) 1–4. [27] Dasari Jagadeeswara Rao, Y. Ramakrishna1, V. Padmarao, B. Venkateswara Rao, Int. J. Adv. Res. Sci. Technol. 1 (2012) 135–159. [28] J.P. Kalyani, N. Kalaiselvi, N. Muniyandi, J. Powder Sources 111 (2002) 232– 238. [29] M. Karabacak, M. Cinar, S. Ermec, M. Kurt, J. Raman Spectrosc. 41 (2010) 98– 105. [30] A.U. Rani, N. Sundaraganesan, M. Kurt, M. Cinar, M. Karabacak, Spectrochim. Acta A 75 (2010) 1523–1532. [31] C. Sengamalai, M. Arivazhagan, K. Sampathkumar, Elixir Comp. Chem. 56 (2013) 13291–13295. [32] M. Moskovits, J. Phys. Chem. 92 (1988) 6327–6329. [33] N. Sundaraganesan, B. Dominic Joshua, T. Radjakoumar, Ind. J. Pure Appl. Phys. 47 (2009) 248–258.
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