Orientation of N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-4- methylbenzenesulfonamide on silver nanoparticles: SERS studies

Article abstract of DOI:10.1016/j.saa.2014.04.125

In the present study, the silver nanoparticles were synthesized using a solution combustion method with urea as fuel. The prepared silver nanoparticles show an FCC crystalline structure with particle size of 59 nm. FESEM image shows the prepared silver is a rod like structure. The surface-enhanced Raman scattering (SERS) spectrum indicates that the N-(1-(2-chlorophenyl)-2-(2- nitrophenyl)ethyl)-4-methylbenzenesulfonamide (CS) molecule adsorbed on the silver nanoparticles. The spectral analysis reveals that the sulfonamide is adsorbed by tilted orientation on the silver surface. The Hatree Fock calculations were also performed to predict the vibrational motions of CS. This present investigation has been a model system to deduce the interaction of drugs with DNA.

Full text of DOI:10.1016/j.saa.2014.04.125

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 261–267
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Orientation of N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-4-
methylbenzenesulfonamide on silver nanoparticles: SERS studies
M. Anuratha a, A. Jawahar b, M. Umadevi c, V.G. Sathe d, P. Vanelle e, T. Terme e, V. Meenakumari f,
A. Milton Franklin Benial f,
⇑
a Department of Chemistry, Thamirabharani Engineering College, Tirunelveli, Tamil Nadu, India
b Department of Chemistry, N.M.S.S.V.N. College, Madurai 625 019, Tamil Nadu, India
c Department of Physics, Mother Teresa Women’s University, Kodaikanal 624 101, Tamil Nadu, India
d UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India
e Laboratory of Radical Pharmaco-Chemistry, UMR CNRS 6264, University of the Méditerranée, Faculty of Pharmacy, 27 Bd Jean Moulin, 13385 Marseille Cedex 5, France
f Department of Physics, N.M.S.S.V.N. College, Madurai 625 019, Tamil Nadu, 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
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Silver nanoparticles were synthesized
using a solution combustion method.
It is an fcc crystalline structure with
particle size of 59 mm.
The prepared silver nanoparticle is a
rod like structure.
CS is adsorbed by tilted orientation on
the silver surface.
HF calculations are also performed to
study the vibrational features of CS.
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study, the silver nanoparticles were synthesized using a solution combustion method with
urea as fuel. The prepared silver nanoparticles show an FCC crystalline structure with particle size of
59 nm. FESEM image shows the prepared silver is a rod like structure. The surface-enhanced Raman scat-
tering (SERS) spectrum indicates that the N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-4-methylben-
zenesulfonamide (CS) molecule adsorbed on the silver nanoparticles. The spectral analysis reveals that
the sulfonamide is adsorbed by tilted orientation on the silver surface. The Hatree Fock calculations were
also performed to predict the vibrational motions of CS. This present investigation has been a model sys-
tem to deduce the interaction of drugs with DNA.
Received 13 March 2014
Received in revised form 10 April 2014
Accepted 22 April 2014
Available online 30 April 2014
Keywords:
N-(1-(2-chlorophenyl)-2-(2-
nitrophenyl)ethyl)-4-
methylbenzenesulfonamide
Surface enhanced Raman scattering
Silver nanoparticles
Combustion method
XRD
Ó 2014 Elsevier B.V. All rights reserved.
Hartree Fock
Introduction
SERS is a surface-sensitive technique, which enhances the
Raman scattering by molecules adsorbed on rough metal surfaces.
⇑
Corresponding author. Tel.: +91 4522458152.
E-mail address: miltonfranklin@yahoo.com (A. Milton Franklin Benial).
http://dx.doi.org/10.1016/j.saa.2014.04.125
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

262
M. Anuratha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 261–267
The enhancement factor of a molecule that is adsorbed on a rough
metal surface is in the order of 104–1015, than the normal Raman
signal. This means that the technique may detect single molecules
[1,2]. The enhancements of 5–6 orders of magnitude over regular
Raman scattering can be observed dependent on the surface. The
enhancement comes from the increase of the local optical field
intensity in the proximity of sharp points of textured metals such
as Au, Ag, Cu, or in the nano-scale gaps between colloidal particles.
Another main advantage of SERS over normal Raman is that fluo-
rescence, which interferes with the recording of Raman scattering,
if the molecule is adsorbed on the surface of the metal. The key
point of generating SERS is to make the ideal metal surface. It is
found that SERS activity critically depends on the nature of metal
and surface roughness; therefore substrate preparation is always
of central concern [3]. It is highly desirable to apply in situ SERS
to obtain molecular level information about the breaking and for-
mation of bonds in the reaction, the reaction intermediate on the
surface and finally, to distinguish the reaction products. This tech-
nique can be coupled with a wide range of other surface techniques
to study corrosion, catalysis, and sensors [4]. Due to the reproduc-
ibility and level of enhancement, SERS is becoming a powerful tool
to detect and study trace levels of molecular species. SERS com-
bined with TEM and SEM techniques, which helps to study the
relationship between the morphology of the particles and SERS
effect [1]. SERS has several biophysical applications. SERS-active
substrate in fiber optics probe will enhance Raman signals
collected by the fiber [5,6]. SERS is recently used as a powerful
whole-organism finger printing technique. It is used in microbial
systematics for the rapid identification of bacteria and fungi [7].
SERS is used to obtain quantitative in vivo glucose measurements
from an animal model [8].
In this present work, silver nanoparticles were prepared by a
combustion method using urea as fuel. The orientation of CS on
the prepared silver nanoparticles was studied by Raman analysis
of CS and CS on silver nanoparticles. The present analysis is a
model system to deduce the interaction of drugs with DNA.
Experimental
Reagents
Silver nitrate, urea, DMF, 2-nitrobenzyl chloride, N-tosylimine,
tetrakis(dimethylamino)ethylene were obtained from Sigma
Aldrich chemicals and were used without further purifications.
All glass wares were properly washed with distilled water and
dried in hot air oven before use.
Preparation of silver nanoparticles
For the synthesis of Ag nanoparticles, the stoichiometric com-
position of the solution of urea was calculated according to the
principle of propellant chemistry, keeping the oxidizer (silver
nitrate) to fuel (urea) ratio as unity. The stoichiometric amount
of silver nitrate was dissolved in a minimum quantity of deionized
water and then urea was added into it. The solution was kept in the
hot plate at 300 °C. Initially, the solution boils and goes through
dehydration followed by disintegration with the evolution of the
large amount of gasses. After the solution reached the point of
spontaneous combustion, it start on to burn and released a lot heat,
vaporizing the complete solution directly and the combustion
reaction was ended in 20 min. Finally loose grayish black color
powder was formed which was compressed and grounded
thoroughly.
The sulfonamide derivatives possess many pharmacological
activities, antibacterial, insulin-release stimulation, carbonic
anhydrase inhibition, diuretic and antithyroid properties. Benzene
sulfonamide derivatives were found to have antiproliferative
activities toward L1210 murine leukemia cells [9]. Sulfonamide
Synthesis of N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-4-
methylbenzenesulfonamide
compounds were used in the treatment of a disease called otitis
media [10]. The sulfonamide antibiotics occupy the important posi-
tion of being the first synthetic compounds to be used in human
therapy [11]. The sulfonamide ASO2NHA group occurs in various
biologically active compounds like antimicrobial drugs, saluretics,
carbonic anhydrase inhibitors, insulin-releasing sulfonamides,
antithyroid agents and antitumour drugs. Aliphatic sulfonamide
derivatives are acting as antimicrobial agents [12]. Sulfonamide anti-
biotics are extracted from agricultural soils using pressurized liquid
extraction, followed by solid-phase extraction and liquid chroma-
tography [13]. The sulfonamide compounds are used for decortica-
tions in the treatment of empyema in children [14]. The
sulfonamides are an important class of drugs, with several types
of pharmacological agents possessing antibacterial, anti-carbonic
anhydrase, diuretic, hypoglycemic and antithyroid activity among
others. A large number of structurally novel sulfonamide derivatives
were reported to show antitumor activity in vitro and in vivo [15].
In a two-necked flask equipped with a drying tube (silica gel)
and a nitrogen inlet was added 15 mL of an anhydrous DMF solu-
tion of 2-nitrobenzyl chloride (0.5 g, 2.92 mmol) and N-tosylimine
(3.21 mmol). The solution was cooled to À20 °C, maintained at
this temperature for 30 min and then tetrakis(dimethylamino)
ethylene (TDAE) (1 equiv.) was added dropwise (via a syringe).
The solution was vigorously stirred at À20 °C for 1 h and then
maintained at room temperature for 2 h. After that, TLC analysis
(CH2Cl2) clearly showed the total consumption of starting product.
The solution was filtered (to remove the octamethyl-oxamidinium
dichloride) and hydrolyzed with H2O (100 mL). The aqueous
solution was extracted with chloroform (3 Â 50 mL), the
combined organic layers washed with H2O and dried over MgSO4.
After evaporation of the solvent, the white solid was recovered
by diethyl ether and filtered yielding the chlorosulfonamide (CS)
(N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-4-methylbenzene-
sulfonamide) (Fig. 1).

M. Anuratha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 261–267
263
CH3
O
S
HN
Cl
O
NO2
Fig. 1. Structure of N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-4-methylben-
zenesulfonamide (CS).
Characterization
The crystal structure and crystallite size of the sample were
characterized by X-ray diffraction. The XRD patterns were col-
lected from PANalytical X-ray diffractometer using Cu Ka radiation
(k = 1.5406 Å) and the crystallite size was calculated at particular
peaks with low and high intensity in the range 2h (20° 6 2h
6 80°) as shown in Fig. 2. The field emission scanning electron
microscopy analyzer (Advanced Micro Analysis solution AMETEK)
was also used.
Fig. 3. Scanning electron microscopic image of CS.
shape. The size and the shape of the silver nanorod strongly
depend on the preparation technique [16]. Generally, homoge-
neous, small and uniform nanoparticles were prepared using com-
bustion synthesis. But the local sintering due to high temperature
formed during the reaction force gives rise to the observed rod like
structure.
Results and discussion
XRD analysis of silver nanoparticles
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 = 38.2°, 44.3°, 64.5° and 77.5° (Fig. 2), which corre-
spond to (111), (200), (220) and (311) Bragg’s reflections of face
centre cubic structure of silver respectively (ICDD 04-0783). 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
cosh, where D is the particle 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 particle size was
found around 59 nm.
Computational studies
In computational studies, the Hartree Fock (HF) method is an
approximate method for the determination of the ground-state
wave function and ground-state energy of quantum many-body
systems. The HF method allows us to calculate the structure, elec-
trostatic properties and Raman scattering intensities of molecules
and also assign their Raman bands [17].
Calculations of the structure and vibrational spectra of the
investigated compound were performed using the Gaussian 03
program package. All calculations were carried out by the
restricted Hartree Fock method with a 6-31G basis set. The dipole
moment of CS is 6.82D.
The imaginary wavenumbers were absent in the calculated
vibrational spectrum confirms that the structure deduced corre-
sponds to the minimum energy. The assignments of the calculated
wavenumbers were based on the animation option of the program,
which gives a visual presentation of the vibrational modes. The
optimized (before and after optimization) geometry of CS and
Mullikan atomic charge distribution of CS are shown in Fig. 4.
SEM analysis of silver nanoparticles
SEM image of the microstructure of silver nanoparticles is
shown in Fig. 3, which reveals that the particles exhibit a rod like
HOMO–LUMO
The HOMO and LUMO describes the understanding of the tran-
sition from HOMO to LUMO. The HOMO and LUMO represents the
ability to donate an electron and accept electron respectively. Fig. 5
shows the LUMO (a) and HOMO (b) structure of CS with surface
contour. In the present case, the HOMO is mainly localized on chlo-
rophenyl and LUMO is localized on nitrophenyl.
Vibrational assignments
The CS molecule has S@O, NH, NO2 and Cl as functional group.
The calculated vibrational frequencies were scaled by 0.8929. The
orientation of chlorosulfonamide (CS) on the silver surface can be
predicted from the ring stretching/breathing vibrations, in-plane
and out-of-plane vibrations. Fig. 6 shows the normal Raman and
Fig. 2. XRD pattern of silver nanoparticles.

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M. Anuratha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 261–267
Fig. 5. (a) Lowest unoccupied molecular orbital of CS with surface contour and (b)
highest occupied molecular orbital of CS with surface contour.
Fig. 6. Normal Raman and SERS spectra of CS.
vibrational mode occurring at 1647 cmÀ1 in nRs, which is assigned
to the NAH deformation mode. The vibrational modes fall in the
region 1565–1475 cmÀ1 are also corresponding to NH deformation
in secondary amide [18]. In the present case, the vibrations from
1558 to 1490 cmÀ1 are assigned to NH deformation.
Fig. 4. Optimized geometry (before (a) and after (b) optimization) and Mullikan
atomic charge distribution (c) of N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-4-
methylbenzenesulfonamide (CS).
SERS spectra of CS. Table 1 shows the vibrational modes and its
corresponding assignments.
The benzene ring in aromatic compounds shows a sharp peak
for ring stretching mode in the region 1615–1590 cmÀ1 [18]. Nor-
mally, the CAC stretching mode is observed at 1602 cmÀ1 [19].
Normally in halogen substituted aromatic compounds, C@C
The NAH bond in primary amide undergoes NH deformation
in the region 1650–1620 cmÀ1 [18]. In the present case, the

M. Anuratha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 261–267
265
Table 1
Vibrational modes observed for CS and its corresponding assignments.
nRs (cmÀ1
)
Calculated (cmÀ1
)
SERS (cmÀ1
)
Assignments
1647
1610
1601
1574
1558
1524
1490
1450
Ring stretching; NH deformation
Ring stretching
Ring stretching
1592
1569
1595
1550
Ring stretching
NH deformation; Ring stretching; NO2 antisymmetric stretching
NH deformation; Ring stretching; NO2 antisymmetric stretching
NH deformation; Ring stretching; CAN stretching; NO2 antisymmetric stretching
Ring stretching; CH2 Scissoring
Ring stretching
NO2 symmetric stretching; SO2 antisymmetric stretching
CAN stretching
CAN stretching
CAN stretching; CH in-plane bending; SO2 symmetric stretching
SO2 symmetric stretching
SO2 symmetric stretching
SO2 symmetric stretching
SO2 symmetric stretching; Ring Breathing
Ring Breathing
1470
1333
1194
1439
1349
1349
1299
1288
1201
1204
1170
1151
1087
1047
1016
905
1144
1047
905
1059
1024
Ring Breathing
858
838
855
826
NO2 bending; CACl stretching
CACl stretching
818
815
CACl stretching
804
CACl stretching
754
733
769
738
CACl stretching
CACl stretching; CH2 rocking
CACl stretching; CH2 rocking; CAH out-of-plane bending
Symmetric PhASAPh stretching; CACl stretching
In-plane ring deformation; CACl stretching
NO2 rocking; out-of-plane ring deformation
Out-of-plane ring deformation; CACl deformation
CACl deformation
CACl deformation
CACl deformation
CACl deformation
CACl deformation; AgAN stretching
710
663
630
669
630
482
425
387
300
283
266
237
327
247
224
stretching vibrations occur in the region 1430–1650 cmÀ1 [20]. In
pyridine, CAC stretching mode appears at 1603–1430 cmÀ1 [21].
In phenothiazine, ring stretching mode is observed in the region
1570–1605 cmÀ1 [22]. In the present case, the vibrational mode
occurring in the region 1610–1439 cmÀ1 both nRs and SERS are
assigned to ring stretching mode.
The NO2 in aromatic nitro compounds show NO2 antisymmetric
stretching at 1550–1490 cmÀ1 [18]. In the present case, the vibra-
tional modes in the region 1588 to 1490 cmÀ1, which are assigned
to NO2 antisymmetric stretching mode.
the region 1047–1204 cmÀ1 in nRs and SERS, which are assigned
to symmetric SO2 stretching mode.
The carbon ring in cyclic compound shows strong ring breath-
ing mode in Raman spectroscopy from 1030 cmÀ1 to 950 cmÀ1
[18]. In the present case, the ring breathing mode appears in the
region 1047 to 905 cmÀ1 in nRs and SERS spectra of CS. The NO2
bending vibration occurs at 865 cmÀ1 [26]. In the present case,
the vibrational mode occurs at 858 cmÀ1 in nRs, which is assigned
to NO2 bending vibration.
Normally, the CACl stretching vibration occurs in the region
850–550 cmÀ1 in chloro compounds [18]. The CACl stretching
band occurs in the region 750–580 cmÀ1 in dihalogen substituted
benzene derivatives [27]. In the present case, the vibrational
modes appeared in the region 858–630 cmÀ1, which are assigned
to the CACl stretching vibration.
The CH2 rocking mode in methylene chain of hydrocarbons
occurs at 740–720 cmÀ1 and the intensity depends on chain length
[18]. In the present case, the vibrations at 733 cmÀ1 in nRs and
710 cmÀ1 in SERS, which are assigned to CH2 rocking in methylene.
The vibrational mode appeared at 710 cmÀ1 in SERS, which is also
assigned to CAH out-of-plane bending [28,29].
The vibrational mode occurs at 1450 cmÀ1 in nRs, which is
assigned to CH2 scissoring vibration in aliphatic compounds [18].
The NO2 in aromatic nitro compounds show NO2 symmetric
stretching at 1360–1320 cmÀ1 [18]. In the present case, the vibra-
tional mode occurring at 1349 cmÀ1 in both nRs and SERS are
assigned to NO2 symmetric stretching.
In general, SO2 in sulfonamide shows a very strong band for SO2
antisymmetric stretching in the region 1360–1335 cmÀ1 [18]. The
asymmetric SO2 stretching mode of thiazine dioxide is observed
at 1350 and at 1368 cmÀ1 [23]. In this case, the vibrational modes
occurr at 1349 cmÀ1 in nRs and SERS, which are also assigned to
SO2 asymmetric stretching.
Aromatic amines show CAN stretching in the region
1280–1180 cmÀ1. In the present case, CAN stretching mode is
observed at 1299 cmÀ1 in nRs and 1288 and 1201 cmÀ1 in SERS.
CH in-plane bending vibrations are appeared in the region
1200–1184 cmÀ1 [24].
Sulfonamide usually shows SO2 symmetric stretching around
1170–1145 cmÀ1 [18]. The Sulfoxide S@O stretching vibration
appears in the region 1050–1200 cmÀ1, which depends upon the
presence of substituents [25]. In the present case, the peaks in
The symmetric PhASAPh stretching is observed at 663 cmÀ1.In
naphthalenes, the ring deformation mode appears in the region
645–615 cmÀ1 [18]. In the present case, it is observed at
630 cmÀ1. The NO2 rocking in nitro compound falls in the region
530–470 cmÀ1[18]. It is observed at 482 cmÀ1in SERS. The out-
of-plane ring deformation for pyridine derivatives are seen in
405, 400 and 512 cmÀ1 [25]. In the present case, the vibrational fre-
quency at 425 cmÀ1 in nRs and 482 cmÀ1 in SERS, which are
assigned to the out-of-plane ring deformation mode. The CACl
deformation vibration appeared in the region 460–175 cmÀ1 [30].

266
M. Anuratha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 261–267
In the present case, the vibrational modes observed in the region
425–237 cmÀ1, which are assigned to CACl deformation.
ring of CS to silver surface or
surface to benzene ring of CS.
According to SERS investigations of benzene derivatives, the
wavenumber of ring stretching modes have to red shift or decrease
by around 10 cmÀ1 and also the band width should increase, when
p electron back donation from silver
Moskovits et al. have observed a band at 257 cmÀ1 in the SER
spectrum of urea adsorbed on colloidal silver and assigned to the
AgAN stretching vibration [31]. Similarly a strong band observed
at 247 cmÀ1 in SERS, which is assigned to AgAN stretching vibra-
tional mode due to the formation of a metal–nitrogen bond.
the molecules adsorb on the metal surface via their
p system [37].
It shows that the red shift can be due to the bond weakening
caused by the electron back donation from the metal to the anti-
bonding p⁄ orbital of the benzene moiety [37].
Orientation studies
In the present case, the intense peak observed at 1595 cmÀ1 of
SERS for ring stretching is downshifted by 6 cmÀ1. Similarly the
ring stretching mode observed at 1588 cmÀ1 for nRs is downshifted
by 8 cmÀ1 in case of SERS. In SERS, this mode is found in 1595 cmÀ1
which is very much enhanced in intensity. This is an evidence of
flat-on orientation of CS on the silver surface. In addition to down-
shift of ring stretching vibration the high enhanced and increased
SERS effect exists, when a molecule adsorbs on a metal sub-
strate and interacts with it strongly. Based on the relative intensi-
ties of SERS signals, one can investigate the adsorption orientation
of
a molecule. For this purpose, surface selection rules by
Moskovits et al. for SERS have been used [32,33]. The purpose of
the present study is to find the adsorption orientation of a mole-
cule on a metal substrate using SERS effect.
The great enhancement due to the SERS effect is generally inter-
preted from two mechanisms: an electromagnetic effect and a
chemical effect [34,35]. The Chemical theory only applies for com-
pounds which form a chemical bond with the surface, so it cannot
explain the observed signal enhancement in all cases, but the elec-
tromagnetic theory can be applied even in molecules, which are
physisorbed to the surface.
In electromagnetic mechanism, increase in intensity occurs
because the metals (NPs) on which SERS worked, support optically
excited surface plasmon and exhibit a surface Plasmon resonance.
Hence, enhancement in an electric field is provided by the surface.
For molecules with a lone pair of electrons, in which the mole-
cules can bond to the surface, chemical mechanism which involves
charge transfer between chemisorbed species and the metal sur-
face is proposed. When HOMO and LUMO of the adsorbate fall
symmetrically about the Fermi level of the metal surface, the min-
imum energy is used to make the transition. When a molecule is
chemisorbed on the surface, those atoms or groups involved in
the adsorption will have vibrational mode at different wavenum-
bers than from the corresponding modes in the free state. The ori-
entation of the adsorbed molecule on the metal surface can be
investigated by analyzing the change in band width, intensity var-
iation and shift in vibrational wavenumbers of the modes in SERS
compared to that in nRs.
bandwidth shows that CS adsorb on the metal surface via ring
system.
p
Generally, SERS bands of adsorbate undergo band broadening.
The homogeneous and inhomogeneous contributions are the rea-
son for the observed broadening of peaks in SERS. When the mol-
ecule approaches a metal surface, the electrons present in its levels
undergo tunneling between the molecule and the metal. As a
result, the metal gives it a finite lifetime and hence larger FWHM
[38]. The larger FWHM values noticed in the ring stretching mode
shows inhomogeneous band broadening along with strong interac-
tions between the benzene ring and silver surface.
The selection rule suggests that if the out-of-plane bending
modes for a molecule is more enhanced than its in-plane bending
modes, then the molecule is adsorbed flat-on the silver surface and
vice versa when it is adsorbed stand-on to the surface [39,40]. In
CS, the CH in-plane bending mode occurs at 1204 cmÀ1 in nRs
and at 1201 cmÀ1 in SERS. The CH out-of-plane bending modes
are observed in 733 cmÀ1 for nRs of CS and at 710 cmÀ1 for SERS.
The out-of-plane bending mode of the CS is enhanced in intensity
than the in-plane vibrational modes in SERS with respect to nRs.
The observed higher intensity of out-of-plane vibrational modes
suggests that CS is adsorbed flat-on the silver surface.
The presence or absence of the benzene ring CH stretching
vibration is a tool for the perpendicular or parallel orientation
respectively, of the benzene ring with respect to the surface
[41,42]. In the present case, the ring CH stretching band is absent
(so it is not shown in Fig. 6). This means that the dye is oriented
flat-on the silver surface.
The surface enhancement also depends on the orientation of the
chemical compound on the metal nanoparticles i.e. flat-on or
stand-on. Another effect involves the presence of electron with-
drawing or donating groups (polar substitute) in the chemical
compound [36].
In the present case, NO2 symmetric stretching and SO2 asym-
metric stretching modes occur at 1349 cmÀ1 in nRs and SERS of
In the present study, CS has different binding sites such as lone
pair of electron on Cl atom, O atom of S@O, and also the electron
withdrawing NO2 group in the nitrobenzene ring and benzene ring
p
system. The binding sites may lead to the adsorption of the mole-
cule on the silver surface. The orientation of the dye CS on the silver
surface can be obtained from ring stretching/ breathing vibrations,
changes observed in functional groups and surface selection rules.
For benzene and its derivatives, a significant downshift in SERS
wavenumber and band broadening of ring breathing modes upon
adsorption, which indicates the ring-surface
p orbital overlap. It
is a strong evidence for a flat-on orientation of adsorbate molecules
on a metal surface [37]. The downshifted peaks in SERS usually
occur as a result of the weakening of the bonds in the ring system
caused by either ring-surface
electron back donation.
p electron donation or surface-ring p
In the present case, the ring breathing mode is observed around
1047 cmÀ1 in nRs of CS. The intensity of this peak is greatly
enhanced in SERS with respect to nRs. Eventhough there is no
change in wavenumber, the observed high intensity and band
width indicate that there is
p
electron donation from the benzene
Fig. 7. Orientation of CS on silver nanoparticle.

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The vibrational mode at 1047 cmÀ1 in SERS is enhanced in
intensity with respect to nRs. This peak is also due to SO2 symmet-
ric stretching. Hence, this also paves additional support for the pos-
sibility of orientation of the CS through a lone pair of electron in
S@O group. The observed high intensity AgAN stretching mode
in SERS and its absence in nRs indicates that the CS molecule
adsorbs through a lone pair of electrons of nitrogen.
The observed spectral analysis evidenced that CS would adsorb
tilted orientation on the silver surface through the binding sites
such as lone pair of electron on Cl atom, N atom of NH, O atom
of S@O, and also through benzene ring
p system. Fig. 7 represents
the orientation of CS (molecular orbital) on silver nanoparticle sur-
face. The molecular orbital diagram is taken from HF theoretical
calculations. It gives an idea of the interaction of molecular orbitals
of CS with silver surface.
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Conclusion
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Orientation of N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-
4-methylbenzenesulfonamide (CS) adsorbed on silver nanoparticle
was studied using SERS. Silver nanoparticle was synthesized using
a solution combustion method with urea as fuel. The prepared sil-
ver nanoparticle has an fcc crystalline structure with the particle
size of ꢁ59 nm. The nRs and SERS spectral analysis reveals that
the CS adsorbed tilted orientation on the silver surface. The mor-
phology of the silver nanoparticles was also studied by SEM. HF
calculations were performed to study the vibrational features of CS.
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Acknowledgements
The authors M. Anuratha, A. Milton Franklin Benial, A. Jawahar
and V. Meenakumari thank their College management for encour-
agement and permission to carry out this work. One of the authors
M. Umadevi is thankful to UGC-DAE-CSR, Indore, DST-CURIE and
DST-SERB, New Delhi for financial assistance.
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