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

evidences that the SERS spectrum obtained using 514.4 nm laser line falls within the resonant Raman conditions, where both electromagnetic effects due to surface plasmon resonance from[r]

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Original Article

C.R Rekha, V.U Nayar, K.G Gopchandran*

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 February 2018 Received in revised form 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

2468-2179/© 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/)

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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 The solution was kept

undisturbed for 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 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 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

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were large enough which facilitated the investigations on shape dependent plasmonic properties

Figs 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

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 Agỵions 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), (2 0), (2 0) and (3 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 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 Absorption spectra of (a) supernatant solution and (b) precipitated solution

Fig FE-SEM images of (a) supernatant solution and (b) precipitated solution

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assigned as (1 1), (2 0), (2 0) and (3 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

SERS spectra of CV, MG and NBC molecules under 514.4 nm and 784.8 nm excitation laser sources are shown in Figs 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 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 Histograms showing the aspect ratio distribution of nanorods: (a) R1, (b) R2, and (c) R3

Fig (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 [29e31] The enhancement factors corre-sponding to the prominent vibrational frequencies present in the

Fig 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

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 1170 In-plane vibrations of ring C-H 1292 Ring C-C stretching

1362 N-phenyl stretching 1615 Ring C-C stretching Nile blue

chloride

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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

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

Analytical enhancement factors calculated for nanorods R1, R2 and R3

Dye molecule Raman peaks R1 R2 R3

514.4 nm 784.8 nm 514.4 nm 784.8 nm 514.4 nm 784.8 nm

CV 206 2.22 105 1.89 103 9.07 104 3.56 105 1.37 104 4.61 106

916 5.85 105 1.83 104 3.09 105 2.31 105 7.15 104 3.68 106

1169 4.95 105 2.01 104 1.14 105 3.16 105 6.71 104 3.78 106

1620 8.60 106 3.90 104 3.27 105 4.42 105 7.85 104 4.74 106

MG 230 7.77 104 2.20 104 4.24 103 2.57 105 3.04 102 6.91 105

1170 9.24 105 4.60 105 5.97 104 6.63 105 4.44 103 7.36 105

1367 4.22 105 1.40 105 1.14 104 3.90 105 2.76 103 4.22 105

1615 7.50 105 4.02 105 1.25 104 5.10 105 5.00 103 5.87 105

NBC 592 1.27 105 9.33 105 3.25 104 6.67 107 8.78 103 1.44 108

662 3.13 105 1.17 105 1.29 104 3.49 107 3.57 103 6.86 107

1491 3.33 105 4.18 105 2.19 104 9.28 106 5.95 103 4.90 107

1641 4.04 105 3.31 105 2.28 104 6.63 106 6.75 103 3.07 107

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

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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 [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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