Synthesis of highly stable silver nanorods and their application as sers substrates

Original ArticleSynthesis of highly stable silver nanorods and their application as SERS substrates Department of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram,

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Original Article

Synthesis of highly stable silver nanorods and their application as

SERS substrates

Department of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram, Kerala 695581, India

a r t i c l e i n f o

Article history:

Received 5 February 2018

Received in revised form

7 March 2018

Accepted 21 March 2018

Available online 29 March 2018

Keywords:

Silver nanorods

Seed mediated synthesis

Surface enhanced Raman scattering

Crystal violet

Malachite green

Nile blue chloride

Rhodamine 6G

a b s t r a c t

We report on the improved stability and yield of silver nanorods with well controlled aspect ratios synthesized using a modiﬁed seed mediated approach conducted at room temperature It is found that the longitudinal surface plasmon resonance of these nanoparticles can be tuned in the spectral range 400e

700 nm by varying the concentration of seed particles The surface enhanced Raman scattering (SERS) activity of these nanorods with varying aspect ratios was tested with four dye molecules viz., crystal violet, malachite green, nile blue chloride and rhodamine-6G, using visible and near-infrared laser excitation sources viz., 514.4 and 784.8 nm, respectively The mechanism of enhancement for the dye molecules adsorbed on these nanorods was investigated in detail A maximum enhancement factor in the order of 108 was obtained when factors such as the peak wavelength corresponding to the plasmon of the nanorods, the absorption of dye and the excitation line were in close approximation The linearity obtained in the cali-bration curves drawn for intense Raman peaks in the SERS spectra of different dye molecules indicated that these substrates are suitable for applications such as biosensing

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Demanding investigations have been committed to the

syn-thesis and characterization of metal nanostructures in recent years

due to their outstanding optical, electrical and mechanical

prop-erties, which drastically vary from their bulk counterparts Surface

plasmon property of nanostructures has become the subject of

extensive study in variousﬁelds in the recent years and it deals

with the collective oscillation of free electrons in the conduction

band of noble metals at the nanoscale Surface Enhanced Raman

Scattering (SERS) is one of the signiﬁcant applications of metallic

nanoparticles, which utilises their surface plasmon properties to

enhance the Raman scattering signals from the molecules attached

to the surface of these nanostructures The SERS enhancement

varies with the excitation wavelength used and it is highest when

the plasmon resonance of the metal matches with the laser

exci-tation wavelength The analyte molecule under observation should

adsorb onto the substrate for getting the scattered Raman signal

enhanced[1e4] Recently SERS has been widely used in many areas

like biosensing [5], monitoring water pollutants, environmental

monitoring[6], paint analyses and food safety[7] The advantage of using SERS is that the quantity of probe molecules required for the analysis is very less compared to other conventional methods and the signal to noise ratio that can be obtained is very high, quenching theﬂuorescence, if any By a careful selection of suitable excitation laser sources, the order of Raman enhancement can still

be made higher if the molecular resonance from the dye molecule also contributes to the overall enhancement factor, which is known

as Surface Enhanced Resonance Raman Scattering (SERRS)[8] The choice of suitable substrates is crucial in all the cases where the SERS technique is employed and the quality of the signals obtained

is highly dependent on the stability and reproducibility of the substrates Silver, gold and copper metallic nanoparticles are the most popular substrates used for SERS, in which silver nano-particles have the largest SERS enhancement capabilities compared

to the others, due to their relatively large sensitivity and sharpness

of scattered signals[9,10]

In the recent years, silver nanorods have attracted extensive research interest due to their superior optical properties The advantage of these nanorods is that their optical absorption prop-erties can beﬁne-tuned by varying their aspect ratio[11e13] Silver nanorods can be synthesized using a variety of techniques including the seed mediated growth, photochemical and electrochemical routes[14e18] The seed mediated synthesis is a solution based

* Corresponding author.

E-mail address: gopchandran@yahoo.com (K.G Gopchandran).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.03.003

Journal of Science: Advanced Materials and Devices 3 (2018) 196e205

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silver seeds were stabilized in citrate [14] This work ignited a

number of researchers to work on solution based methods for the

growth of metallic anisotropic nanostructures such as rods, wires,

cubes, spheroids, pyramids etc[10,22e24] Difﬁculty in controlling

the exposed silver crystal facets after the nucleation of the Ag seeds

and the abundant presence of self-nucleating spherical and

aniso-tropic by-products prevented the reproducible synthesis of silver

nanorods

In this work, a modiﬁed seed mediated synthesis followed by a

static precipitation method is described, which improved the yield

and stability of the silver nanorods signiﬁcantly It was found that

the aspect ratios of silver nanorods could beﬁne tuned by varying

the amount of seed added to the growth solution The SERS

ac-tivity of these nanorods with varying aspect ratios was studied

with four different dye molecules crystal violet (CV), malachite

green (MG), nile blue chloride (NBC) and rhodamine-6G (R6G),

having absorption at different wavelengths 593, 612, 644 and

527 nm, using two different (514.4 and 784.8 nm) excitation laser

sources

2 Experimental

2.1 Materials and synthesis methods

Silver nitrate (AgNO3, 99.95%), Cetyltrimethylammonium

bro-mide (CTAB, 99%), Sodium borohydride (NaBH4, 99%), L-ascorbic

acid (99%) and sodium hydroxide (99%) were purchased from

Sigma-Aldrich Double distilled water was used for the synthesis

All glasswares were cleaned with aqua regia, thoroughly rinsed

with double distilled water, and dried prior to usage

Silver nanorods were prepared according to a modiﬁed seed

mediated synthesis[14] The seed solution was prepared by mixing

0.01M AgNO3and 80mL of 0.1M CTAB stock solution This was made

up to 20 mL with double distilled water and to this 0.6 mL of 0.01 M

NaBH4 was added and stirred for 2 min The solution was kept

undisturbed for 1 h, before adding to the growth solution To

pro-duce silver nanorods with varying aspect ratios, 10 mL of a growth

solution consisting of 0.01M CTAB, 0.25 mL 0.02 M AgNO3 and

0.5 mL 0.1 M ascorbic acid was taken To this, a varied amount of

seed solution (0.1, 0.125 and 0.25 mL) was added Finally, 0.10 mL of

1 M NaOH was added to each set and shaken gently The colour of

the solution was seen to change gradually to green, violet or pink

depending upon the concentration of the seed added The

as-prepared colloidal solution contained many nanospheres and

nanoplates mixed with nanorods and from this colloid, the silver

nanorods were puriﬁed by static precipitation, which is an effective

method for the isolation of nanorods from a mixture of spheres,

short rods and plates

2.2 SERS sample preparation

Surface Enhanced Raman Scattering (SERS) measurements were

conducted using four dye analytes CV, MG, NBC and R6G In order to

study the variation of SERS enhancement factor with varying aspect

range 200e900 nm Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX) analysis

of the samples were done using the electron microscope-NOVA NANOSEM450 employed with xT microscope control software X-ray diffraction (XRD) was carried out with Bruker D8 ADVANCE with DAVINCI design diffractometer with Niﬁltered CuKa1 radia-tion (l¼ 0.15405 nm) High resolution TEM (HRTEM) analysis was performed with Joel JEM 2100 Raman measurements were done with a LabRAM HR 800 (HORIBA Scientiﬁc-Jobin Yvon Tech-nologtery) spectrometer, equipped with an Argon-Ion laser exci-tation source emitting a wavelength of 514.4 nm and a semiconductor laser emitting a wavelength of 784.8 nm A holo-graphic grating with 1800 groves/mm for 514.4 nm and 600 groves/

mm for 784.8 nm enabled the spectral resolution Samples were placed in a 1 cm path length cuvette mounted in an L-shaped adapter

3 Results and discussion

Atﬁrst, in this work silver seeds were prepared by reducing silver precursor (AgNO3) with NaBH4 using CTAB as a stabilizer, instead of trisodium citrate used in previous reports [14] The crystalline phase of the initial seed particles is one of the critical factors which determines the growth of nanorods Some reports have shown that the stability of metallic nanorods formed from CTAB-capped seeds is better than the citrate-capped seeds[25,26]

In this synthesis, it is found that the formation and yield of silver nanorods are highly sensitive to the ratio of metal salt to seed concentration used in the growth solution AgNO3precursor having different concentrations was reduced with ascorbic acid in the presence of CTAB and it was found that by keeping the molarity of the seed solution constant, the yield was high for the sample syn-thesized using 0.02 M AgNO3, maintaining a high degree of ho-mogeneity The reduction of silver ion by a weak reducing agent, ascorbic acid can produce silver nanorods in a very limited manner,

in the absence of seeds CTAB in the growth solution acts as a structure-directing agent and it forms a tightly bound cationic bilayer on silver nanoparticles, thereby making constrains and restricting the growth in one dimension NaOH is added in theﬁnal stage of the synthesis in order to adjust the pH of the solution, thereby enhancing the growth and formation of nanorods[14] The as-prepared colloidal solution contained a mixture of spheres, plates and rods and in order to isolate silver nanorods from this mixture, static precipitation process was conducted by keeping the solution aside for 3 h without any disturbance During the static process, silver nanorods were precipitated naturally to the bottom

of the glass beaker, due to electrostatic aggregation and were then separated [20] Hence, silver nanorods could be easily puriﬁed without centrifugation orﬁltration by means of static precipitation The precipitation of nanorods to the bottom and their separation prevented further growth and formation of nanorods with smaller aspect ratios, maintaining the monodispersity obtained for these nanorods The quantities of nanorods obtained in this protocol

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were large enough which facilitated the investigations on shape

dependent plasmonic properties

Figs 1 and 2show the absorption spectra and the corresponding

morphologies of supernatant solution and precipitated typical

nanorod suspension respectively, prepared using a 0.125 mL seed

solution The optical absorption spectrum of silver nanorods shows

two plasmon peak positions corresponding to the transverse

sur-face plasmon resonance (TSPR) and longitudinal sursur-face plasmon

resonance (LSPR) centred at 428 and 575 nm, respectively The

FE-SEM images (Fig 2) show that the precipitated solution consists of

silver nanorods with high yield and homogeneity The supernatant

solution consists of elongated nanostructures which look like short

rods in between It is evident that the strategy used is simple,

conducted at room temperature (300 K), and the quality,

homo-geneity and yield of silver nanorods obtained are comparable or

better than those reported elsewhere[14,21] The stability of the

colloidal suspensions of nanorods was then tested in different time

intervals using absorption measurements and no signiﬁcant change

was observed even after three months, indicating that these

sus-pensions are highly stable (Fig 3)

Fig 4shows the absorption spectra of silver nanorods prepared

with different seed concentrations along with that of the seed

particles The optical absorption spectrum of the seed solution

shows a band centred at 403 nm The aspect ratios of nanorods

were calculated from FE-SEM images (Fig 5) and it comes to

6± 0.86 (R1), 11 ± 0.43 (R2) and 15 ± 2.87 (R3) corresponding to

seed concentrations of 0.25, 0.125 and 0.1 mL, respectively The

aspect ratio distributions of the nanorods produced are shown in

Fig 6

The surface plasmon properties of these nanorods are found to

be highly sensitive to the seed concentration It is found that the

LSPR peak shifts towards the red region with decrease in seed

concentration, which in turn indicates the increase in the aspect ratio of nanorods In the extreme case obtained in this work, the aspect ratio was as large as 15± 2.87, with LSPR at 633 nm With increase in the amount of seed concentration from 0.1 to 0.25 mL, the LSPR is found to shift from 633 to 527 nm The concentrations of seed and base relative to that of silver ion concentration are vital in the seed mediated synthesis in determining the aspect ratio of anisotropic nanostructures The number of nucleation sites for sil-ver atoms for the growth and formation of nanorods decreases with decrease in seed concentration This causes the frequency of attachment of silver atoms to a particular nucleation site to in-crease, resulting in the growth along the longitudinal direction, which in turn is found to increase the length of the nanorods in this case When the seed concentration increases, the probability of getting Agþions attached to a particular nucleation site diminishes, making the formation of nanostructures with high aspect ratio become difﬁcult

Fig 7(a) shows the typical X-ray diffraction (XRD) pattern ob-tained for silver nanorods with an aspect ratio of 11± 0.43 nm The pattern clearly shows main peaks at (2q) 38.2, 44.3, 64.50 and 77.5 which correspond to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, respectively The XRD pattern is indexed according to the JCPDSﬁle no: 04-0783 of cubic silver From the X-ray diffraction pattern, it can

be seen that the prominent diffraction peak is at 38.2which in-dicates that the preferential growth is along the (111) crystal plane

Fig 7(b) shows the energy dispersive spectrum of the synthesised nanorods and it suggests the presence of silver as an ingredient element The synthesised silver nanorods show strong absorption

in the region 2.5e4 KeV[27]

Fig 8 shows a typical high resolution TEM image and the selected area electron diffraction pattern (SAED) of silver nanorods synthesised using 0.125 mL seed solution The ring patterns are

Fig 1 Absorption spectra of (a) supernatant solution and (b) precipitated solution Fig 3 UVevis absorption spectra of nanorod suspension at various time intervals.

C.R Rekha et al / Journal of Science: Advanced Materials and Devices 3 (2018) 196e205

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assigned as (1 1 1), (2 0 0), (2 2 0) and (3 1 1) orientations of the

crystal planes of cubic silver nanoparticles [JCPDSﬁle no: 04-0783]

The HRTEM shows the crystalline structure with a d-spacing of

0.229 nm, which closely matches with the (111) plane of cubic

silver and this conﬁrms that the longitudinal growth is the

pref-erential one and is along the (111) plane

where Cnrmis the concentration of the analyte, which produces a Raman signal Inrm under non-SERS conditions and Csers is the concentration of the same analyte solution on a SERS substrate with different concentrations and gives a SERS signal Isers, under iden-tical experimental conditions Before analysing the SERS spectrum, the aqueous SERS solutions were equilibrated for 15 min

SERS spectra of CV, MG and NBC molecules under 514.4 nm and 784.8 nm excitation laser sources are shown in Figs 9 and 10

respectively Analysing the SERS spectra obtained, the highest Raman enhancement for all dyes under 514.4 nm laser excitation was shown by substrate of R1 type whereas it was with substrates

Fig 4 Absorption spectra of silver nanorods prepared with different concentrations of

seed particles: (a) 0.25 mL, (b) 0.125 mL, and (c) 0.1 mL.

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of R3 type when laser excitation line used was 784.8 nm This is

because SERS mainly arises due to an electromagnetic effect from

the surface plasmon resonance of the nanoparticles, which is

characteristically wavelength dependent and the chemical effect If

the laser excitation wavelength matches with the plasmon reso-nance of the metallic nanosubstrates used, Raman scattered signal intensity will be highly enhanced The highest enhancement factor for substrate R1 can be attributed to the fact that Raman signal is

Fig 6 Histograms showing the aspect ratio distribution of nanorods: (a) R1, (b) R2, and (c) R3.

Fig 7 (a) XRD pattern obtained from R2; (b) Energy dispersive spectrum of R2.

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highly enhanced when the plasmon resonance wavelength of silver

nanorods (527 nm) closely matches the laser excitation line

(514.4 nm) It can be seen that the enhancement factor decreases

with the shift of LSPR away from the laser excitation wavelength

Similar trend was observed in the Raman spectra for all the three

dye molecules when 784.8 nm excitation laser source was used

Enhancement can also occur even when the difference of

wave-length between the plasmon and the laser excitation is small and is

known as a pre-resonant condition The enhancement factor for

substrate R3 was highest under near infrared laser excitation and in

this case the plasmon band is relatively nearer to the laser

excita-tion wavelength

The Raman spectra for CV and MG looked similar as they have

similar chemical structures The Raman peak enhancement for the

same vibrational frequencies under different excitation lines was

different for all the analytes Relative intensity for the same Raman

band can vary according to the magnitude of the localﬁeld at the

substrate surface and the orientation of the polarizability

de-rivatives This is because for a well-oriented chemisorbed species,

the perpendicular and tangential components of the localﬁeld on

the silver colloids could be quite different in magnitude for laser

lines on different sides of the plasmon resonance[28]

Raman peak frequencies and their corresponding assignments

are given in Table 1 [29e31] The enhancement factors

corre-sponding to the prominent vibrational frequencies present in the

Fig 9 SERS spectra of CV, MG and NBC using silver nanorods R1 (b, f, j), R2 (c, g, k) and R3 (d, h, l) using 514.4 nm excitation laser source and (a, e, i) represent the corresponding normal Raman spectra of the dyes.

Fig 10 SERS spectra of CV, MG and NBC using silver nanorods R1 (b, f, j), R2 (c, g, k) and R3 (d, h, l) using 784.8 nm excitation lasers source and (a, e, i) represent the corresponding normal Raman spectra of the dyes.

Table 1 Peak frequencies and assignments for analytes adsorbed on silver nanorods Dye molecules Raman shift

(cm1)

Band assignment

Crystal violet 206 Breathing of central bonds

336 In-plane vibration of phenyl-C-phenyl

423 Out-of-plane vibrations of

phenyl-C-phenyl

524, 561 Ring skeletal vibration of radical

orientation

724, 913 Out-of-plane vibrations of ring C-H

1169 In-plane vibrations of ring C-H

1297 Ring C-C stretching

1389 N-phenyl stretching

1538, 1620 Ring C-C stretching Malachite

green

230 In-plane vibration of phenyl-C-phenyl

422, 447 Out-of-plane vibrations of

phenyl-C-phenyl

798, 915 Out-of-plane vibrations of ring C-H

1615 Ring C-C stretching Nile blue

chloride

592 C-C-C and C-N-C vibrations

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Raman spectra were calculated for nanorods with different aspect

ratios, which were used as substrates

The calculated enhancement factors for CV, MG and NBC for

nanorods with different aspect ratios under 514.4 nm and 784.8 nm

excitation lines are given inTable 2

The molecular resonance from the dye analyte can also

contribute to the overall enhancement along with the surface

plasmon resonance of the substrate viz., the SERRS The model

an-alyte used for SERRS detection should contain a chromophore,

which is the part of a molecule responsible for its colour The laser

excitation frequency should be close to or coincident with the

electronic transition of the chromophore[32,33] On analysing the enhancement factors obtained for all the three dye molecules, the highest enhancement factors obtained for NBC and CV can be attributed to SERRS.Fig 11shows the absorption spectra of the three dyes CV, MG and NBC and are centred around 593, 612 and 640 nm respectively; insets show their corresponding chemical structures For nanorod substrate R1, using crystal violet as a model analyte and 514.4 nm as a laser excitation line, surface as well as molecular resonance enhancement can be considered to contribute to the overall enhancement as the plasmon resonance of the nanorods (527 nm) as well as the absorption wavelength of CV (590 nm) is

Table 2

Analytical enhancement factors calculated for nanorods R1, R2 and R3.

Fig 11 Absorption spectra of (a) Crystal Violet, (b) Malachite Green, and (c) Nile blue Chloride.

Fig 12 SERS spectra of Rhodamine 6G using silver nanorods with aspect ratios R1 and R3 under a) 514.4 nm excitation, b) 784.8 nm excitation, and c) optical absorption spectrum

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overall enhancement along with the plasmon resonance of the

substrate R3 under 784.8 nm excitation, which may be the reason

for the highest signal to noise ratio for NBC molecules

In order to validate the effect of molecular resonance on the

enhancement factor, another model analyte R6G with well

recog-nized vibrational characteristics was chosen, whose absorption

wavelength lies very closely to one of the laser excitation sources,

i.e 514.4 nm.Fig 12shows the SERS spectra obtained for R6G with

tion is made on the basis that the plasmon absorption wavelength

of R1 lies very closely to the absorption maximum of R6G whereas that of R3 lies away from it Here, the enhancement factor was calculated for vibrational frequencies; 1362 cm1(in-plane C-H bend), 1511 cm1(C-N stretch), and 1649 cm1(in-plane C-H bend) and is given inTable 3 [34] The table shows that enhancement factor for R6G is highest for substrates having plasmon in the close approximation of the excitation line used

Fig 13 Dependence of SERS on the concentration of probe molecules; (a) CV under 514.5 nm and (b) NBC under 784.4 nm excitations.

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Comparing the enhancement factors, it is highest for silver

nanorods R1, under 514.4 nm laser excitation (1.81 107) This

evidences that the SERS spectrum obtained using 514.4 nm laser

line falls within the resonant Raman conditions, where both

elec-tromagnetic effects due to surface plasmon resonance from the

substrates and the molecular resonance from R6G contribute to a

strong SERRS signal and this is not observable when the 784.8 nm

excitation line was used

The sensitivity performance of these SERS substrates towards

variation of analyte concentrations was also studied, in order to

identify their limits of detection (LOD) for a particular analyte

Silver nanorods R1 and R3, which showed highest enhancement

factors, were chosen to study the sensitivity

Fig 13(a) shows the SERS spectrum of CV with varying

molar-ities in the range (104e107M), under 514.4 nm laser excitation

using R1 and Fig 13(b) shows the SERS spectrum of NBC with

varying molarities in the range (105e108 M), under 784.8 nm

laser excitation using R3 type substrates The LOD for the Raman

band at 1620 cm1, which showed the highest enhancement for CV,

was 107M and in addition the characteristic peaks such as 206,

913 and 1169 cm1are also visible at this concentration The Raman

band at 593 cm1, which showed the highest enhancement for NBC,

can be recognized even at a very low analyte concentration of

108M, along with discernible recognition of other Raman peaks

such as 664, 1359 and 1640 cm1

The linear ﬁt calibration curves for intense Raman peaks are

plotted for different molarity concentrations of the dyes and are

shown inFig 14 The coefﬁcient of determination (R2) of the linear

ﬁt for the peaks 206, 913 and 1169 and 1620 cm1of CV reached to

97.74, 97.66, 96.53 and 94.17 respectively The R2 value for the

Raman peaks 593, 664, 1359 and 1640 cm1, of NBC reached to

95.85, 96.67, 97.87 and 96.45 respectively The measure of linearity

responses obtained here indicates that these silver nanorods can

ﬁnd potential applications in SERS based sensors

4 Conclusion

Silver nanorods with different aspect ratios were prepared using

a modiﬁed seed mediated synthesis It is found that ﬁne-tuning of

the aspect ratios of these nanorods is possible by controlling the

amount of seed solution used in the synthesis Nanorods classiﬁed

into three types based on aspect ratios were then tested for SERS

activity Four different dye molecules viz., crystal violet, malachite

green, nile blue chloride and rhodamine-6G were used as analytes

and two excitation lines 514.4 nm and 784.8 nm were used for

Raman measurements A maximum enhancement of 1.44 108

was observed for nanorods having the high aspect ratio of 15± 2.87,

when tested with NBC probe molecule under a laser excitation

wavelength of 784.8 nm We have validated the SERRS effect by

using nanorods having aspect ratio 6± 0.86 and Rh-6G analyte with

514.4 nm excitation line The minimum concentration of analyte

used in the SERS measurement is vital in determining the

sensi-tivity of these substrates Raman measurements conducted with

varying molar concentrations of the analyte molecules have shown

promising detection limits up to 107for CV and 108for NBC An

attempt was also made to plot the linearﬁt calibration curves for

intense Raman peaks and the observed values of the coefﬁcient of

determination are found to be good for a variety of applications

Acknowledgements

Rekha C.R acknowledges the ﬁnancial support from

Depart-ment of Science and Technology, India through PURSE programme

of University of Kerala

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