Green synthesis, characterization of Au-Ag core-shell nanoparticles using gripe water and their applications in nonlinear optics and surface enhanced Raman studies

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l O P Publishing | Vietnam Academy of Science and Technology Advances in Natural Sciences: Nanoscience and Nanotectinology Adv Nat SCI : Nanosci. Nanolechnol. 5 (2014) 045006 (6pp) doiil 0.1088/2043^62/5/4/045006

Green synthesis, characterization of Au-Ag core-shell nanoparticles using gripe water and their applications in nonlinear optics and surface enhanced Raman studies

E Kirubha' and P K Palanisamy^

' Department of Physics, Anna University, Chennai, Tamil Nadu 600025, India

^Department of Medical Physics, Anna University, Chennai, Tamil Nadu 600025, India E-mail' [email protected]

Received 20 June 2014

Accepted for publication 9 September 2014 Published 17 October 2014 Abstract

In recent years there has been excessive progress in the 'green' chemistry approach for the synthesis of gold and silver nanoparticles. Bimetallic nanoparticles have gained special significance due to their unique tunable optical properties. Herein we report a facile one-pot, eco- friendly synthesis of A u - A g bimetallic core-shell nanoparticles using gripe water as reducing as well as stabilizing agent. The as-synthesized Au-Ag nanoparticles are charactenzed using U V - Vis spectroscopy to determine the surface plasmon resonance, and using ti^ansmission electron microscopy to study die morphology and the particle size. The optical nonlinearity of the bimetallic nanoparticles investigated by z-scan technique using femtosecond Ti:sapphhe is in the order of 10^. The nonlinear optical parameters such as the nonhnear refractive index «2i nonlinear absorption coefficient /? and the third order nonlinear susceptibihty x' are measured for various wavelengths from 700 nm to 950 nm. The Au-Ag nanoparticles are also used in surface enhanced Raman spectroscopic studies to enhance the Raman signals of rhodamine 6G, Keywords; A u - A g nanoparticles, z-scan, SERS, gripe water

Madiematics Subject Classification: 2,04, 2.09, 4,02, 5.04

1. Introduction Cassia angusiifolia [12], Daucus carota (Z>. carota) extract [13], honey [14], edible mushroom extract [15], hibiscus rosa Metal nanoparticles are of great interest because of their sinensis [16], banana peel extract [17] and mulberry leaf distinct optical properties and theu advances in various extract [18].

applications such as catalysis, biosensing, drug dehvery and In our previous work [19], we have reported the other physiochemical, optoelectronic properties and surface synthesis of gold nanoparticles with size and dispersity enhanced Raman scattering and detection (1-10]. The bime- control using gnpe water. Adopting a similar method, in this tallic nanoparticles, with core-shell structures are commonly work we present the complete 'green' synthesis of A u - A g synthesized using chemical reduction technique and by seed core-shell nanoparticles reduced and stabilized at room growth methods. Usually the synthesis procedure involves the temperature using gripe water. This is a facile one-pot reduction of metal ions using a reducing agent and capped by method for the synthesis of Au-Ag nanoparticles. Gripe another stabilizing agent. The use of such reactive chemicals water is a well-known antacid, which is prescribed to treat poses potential risk to the environment. Hence, there is a gripe, indigestion and acidity for infants [20]. It is easily growing need to develop clean, nontoxic and environmental available in all medical stores all over the worid. The as- friendly 'green chemistry' approaches to synthesis of nano- synthesized nanoparticles are characterized using UV-Vis particles [11]. Recent green chemistry methods for the spectroscopy and transmission electron microscopy (TEM).

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Adv Nat Rri - Nanosci, Nanotechnol 5 |2014) 045006

E Kinjbtia and P K Palatfean 2.8-r

2,6-

^ 2.4-

Absorbance (a Normalised

1.2

(b) 1 Au-Ag = 463nm

Wavelength (nm) Wavelength (nm)

Figure 1. (a) Time depend™, surface ptemon band of Au-Ag nanoparticles reduced using gripe wain, (b) surface plasmon band of Au-Ag nanopamcles reduced using gnpe water

analyzed using smgle beam z-scan technique. Nonlinear optical parameters «2. /? and / of die Au-Ag nanoparticles are analyzed and calculated using closed and open aperture z-scan. The as-syndiesized nanoparticles are apphed in surface enhanced Raman spectroscopic (SERS) analysis. The enhancement of rhodamine 6 G dye is analyzed by adduig the bimetallic nanoparticles.

2. Experimental 2.1. Materials

an 80 cm focal length spectrometer and a He-Ne 633 nm wifli 11 mW power on the sample. The specti-a are acquired over a wavenumber region of 1800 to 600 cm"' with an acquisition time of 10 sec.

2.3. Synthesis of Au-Ag nanoparticles

The core-shell nanoparticles were prepared by taking the source solutions of HAuCU and AgNGs in 1:1 molar ratio. To the stirring source solution of 10 mL of gripe water and 0,3 mM of AgNOj was added to 0.3 mM of HAuCL, and kepi under continuous stirring.

Chloroauric acid HAuCU and silver nitrate AgNOj are pur-

chased from Sigma-Aldrich, Woodward's gripe water is 3 , R e s u l t s a n d d i s c u s s i o n purchased from TTK Healtii Care Ltd, INDIA. Millipore

water (18 Mi3) is used as solvent m our expenments. 3 i_ Surface plasmon resonance 2.2. Characterization

The surface plasmon bands of die particles are recorded using UV-Vis spectrometer. The absorption spectra for all the particles are recorded using Shimadzu spectrometer, widi a resolution of 0.5 nm taking die nanoparticles in a 10 mm optical length quartz cuvette. The particle size and shape have been monitored using a high resolution transmission electron microscope (HR-TEM, JEOL JEM 3010) operated at 200 kV, Samples for TEM images are prepared by placing a drop of colloidal silver on carbon coated copper gnd 300 mesh. The IR spectra are recorded on JASCO Fr/IR-4100 Fourier transform infrared spectrometer in the range 400-4000 cm"' with a resolution of 4cm~'. The z-scan studies were per- formed using a tunable, mode locked Ti:sapphire at a repe- tition rale of 80 MHz witii a pulse widdi of 160 fs laser beam (Mai Tai—deep SEE) focused by a lens of 32 mm focal length. A photodetector (COHERENT) connected to the digital power meter Field Master Gs-COHERENT measured the laser power. The Raman spectra for die SERS studies are recorded using a LabRAM HR800, Jobin Yvon, France, with

During the synthesis procedure, gripe water and AgNOs ate added together to HAuCU solution to avoid precipitation of AgCl. Although die reducing agent is added to both the source solutions at the same lime, Au nanoparticles are formed first as the growth kinetics of Au is faster Uian diat of the Ag nanoparticles. Therefore A u - A g nanoparticles aie grown by successive reduction of HAuCU and AgNOs by gripe water. The nanoparticles attained saturauon only after 22 h of reaction. The characteristic surface plasmon band (SPR) of the as-prepared nanoparticles is observed only after one hour of reaction at 463 nm. Figure 1 (a) illustrates the time dependent SPR band of the nanoparticles and figure 1(b) shows die SPR band of die A u - A g core-shell nanoparticles.

From time dependent UV-Vis specfrum, it is seen that the SPR band of the nanoparticles increases in intensity and peak broadening is not observed. This imphes that the number of nanoparticles increases and the size does not vary. The SPR band of the observed nanoparticles is at 463 nm for the Au- Ag core-sheU nanoparticles. Every 5 ml of used gripe watCT contains (as per its label) sodium bicarbonate (Sarjikakshara) of 0.05 g, dill oil (Anethum Graveolens) of 0.005 ml, sugar of

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Adv Nat, Sci - Nanosci, Nanolectinol 5(2014)045006 E Kirubha and P K Palanisamy

Figure 2. TEM images of Au-Ag core-shell bimettalic nanoparticles synthesized using gripe water given in different scales, (a) 20 nm and (b) 10 nm.

Figure 3. Schematic set up of z-scan using Ti:sapphire femtosecond laser.

1.1 g with the preservatives bronopol, sodium benzoate, sodium methylparaben, and sodium propylparaben. Sodium benzoate, sodium methyl paraben, sodium propylparaben, and bronopol (also called 2-bromo 2-nitropropanel,3-diol) present in gripe water stabilize the nanoparticles, while sodium bicarbonate and sucrose reduce the A u - A g nanoparticles.

3.2. Transmission electmn microscopy

A transmission electron microscope (TEM) is used to deci- pher the surface morphology and the particle size of the synthesized nanoparticles and the images are shown in figure 2, The TEM images reveal that the bimetallic nanoparticles are core-shells structures, with Au as core and Ag as shells. It is difficult to distinguish Au and Ag from the TEM images as both they have the same lattice parameters, yet with the high electron density of Au atoms, the Au core is dif- ferentiated with die Ag sheU. The average particle size of die Au-Ag nanoparticles attained fixim TEM images is found to be lOnm.

Table 1. Measured nonlinear parameters of Au-Ag nanoparticles.

A (nm) 700 750 800 850 900 950

dTp.^

0 71 0.80 0.87 0.94 1.00 1.09

d*0 2.13 2.40 2.61 2.82 3.00 3.27

H^xlO"**

(cm^ W"') 1.18 1.42 1.65 1.89 2.13 2.45

^ x 10""' (cm W"') 1.69 1.56 1.45 1.34 1.12 1.23

^ x l O - ' » (esu)

5.30 6 39 7.42 8.51 9.57 11.03

3.3. Z-scan: nonlinear optical properties

Using the single beam z-scan techmque, the nonlinear optical behavior of the Au-Ag nanoparticles is studied [21]. The closed aperture z-scan technique is used to measure the sign and magnitude of the nonlinear refraction «2 of the nanoparticles, while the open aperture z-scan quantifies the nonhnear absorption coefficient p. The experimental set up of z- scan is schematically represented in figure 3, The A u - A g nanoparticles whose nonlineanty is to be studied is taken in

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J Nat, SCI Nanosci, Nanolectinol 5(2014)045006

E Kirubha and P K P

Z(mm) Z (mm) Figure 4. Z-scan plot of Au-Ag nanoparticles. (a) Closed aperture and (b) open aperture.

11

s'°

1'

3 B

near

* e 5

(c)

. -

•

1

T

^ 3 .

;

Wavelength (nm)

Figure 5. Vanation of Ihe nonhnear parameters (a) /J2, (b) jJ and (c) %i of Au-Ag nanoparticles with wavelength.

colloidal form in quartz cuvette with a width of I mm and mounted on a translation stage. Along the direction of pro- pagation of the laser beam, the stage is moved across the z direction from - 1 0 to -nlO mm {-z to 4-z).

The intensity of the laser beam transmitted from the sample is collected and measured through an aperture by a photo-detector attached to the digital power meter for the closed aperture z-scan, while in the case of open aperture the

aperture is removed and the entire laser beam is focused into the detector through a lens. When the sample is moved towards the focus of the lens ^ = 0, the beam irradiance increases leading to pre-focal peak and when moved away from the focus, the beam irradiance decreases suddenly leading to post-focal valley. This shows the negative nonlinearity of the nanoparticles. The nonlinear refractive vaAe/.

/i2 is deduced from the closed aperture and the nonlineai

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Adv. Nat, Sci.: Nanosd. Nanolechnol 5 (2014) 045006 1400

1300 1200 1100 3 1000 s soo

• f 800 9 700 -^ 600 1 SOO

^ 400

100 0

•

. .

.

.J.

S

N,

XSsra

^

{:;

lisaa^

s Y

' ,^

• ^

s

1

n

• ^

j R6G 1 Au-Aq

~

fe

U AS

Al ""

r/l y

uWV./

1000 1200 1400 Raman Shift (cm'^)

1600 1800

Figure 6. Surface enhanced Raman specttum of rfiodamine 6G usmg Au-Ag core-shell nanoparticles.

E Kinjbha and P K Palanisamy core-shell nanoparticles are tabulated in table 1, Closed and open aperture z-scan plots of the nanoparticles for different wavelengths are illustrated in figure 4,

The as-synthesized Au-Ag nanoparticles indicate negative nonlinearity as their closed aperture z-scan traces show peak followed by valley. The nonhnear refractive mdex n2 of the nanoparticles is in the order of 10~^cm W ~ \ The 02 value increases from 1.18 to 2,45x IO"^cm^W"' with the increase in the incident wavelength for the bimetallic nanoparticles. The nonlinear absorption coefficient ^ obtained from the open aperture z-scan decreases from 1.69 to 1.23x 10"^cm W~' with the increase in wavelength. The third order nonhnear susceptibihty /^ increases with the increase in the incident wavelength. Variation of the nonlinear opucal parameters /12, ^ and / ^ with wavelength is shown in figure 5. The concentration of the as synthesized Au-Ag nanoparticles is obtained from inductive coupled plasma (ICP). The concentration of Au is 0.250 moi L~' and Ag is 0 . 3 1 0 m o l L - ' .

absorption coefficient fi is inferred from the open aperture z- scan, The optical nonlinear parameters are obtained from equations

«2 = , , , . (1)

A0o = - kh Left

AT^.y 0.406(1 - 5 ) " -

_ 2^AT

(3) where 02 is the nonlinear refractive index, k is the wave number (k=2]i/X) and A0o is the on-axis phase shift at the focus, /Q is the peak intensity within the sample at the focus.

Left is the effective thickness of the sample (1 mm—quartz cuvette), S is the linear aperture transmittance (0.5) and ATp.^, is the normalized peak valley differences obtained from the closed aperture z-scan trace, AT=i-Tp, where Tp is the normalized peak value from the open aperture plot [22-25].

Third order nonlinear susceptibihty x^ of the nanoparticles were obtained usmg the relations as in equations

= ,/Re(z'f+Im(7'y

Re / ' : 10-'

(4)

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I ^ ( , 3 ) = , 0 - 1 ^ A _2 gpC " 0 (6) 47t'

where, £0 = 8.854 X 1 0 " ' ' ' F m " ' is the vacuum permittivity, c is the speed of light in vacuum ( 3 x 1 0 cm sec" ) and TIQ is the linear refractive index (1.33). The real part R e ( ^ ) is proportional to the nonhnear refractive index of the nanoparticles, while the imaginary part imij^y is proportional to the nonhnear absorption coefficient ^. Z-scan is carried out for different incident wavelengtiis from 700 nm to 950 nm.

The measured nonhnear parameters of the prepared Au-Ag

3.4. SERS analysis

Surface enhanced Raman scattering (SERS) of rhodamine 6G (R6G) dye molecules is studied using the bimetallic Au-Ag nanoparticles. In general the SERS studies are carried out by coating the probe molecules to be smdied to the metal nanoparticles on a substrate. This leads to hot-spots, with large enhancement occturing at the junctions between the nanoparticles. The hot-spots are the regions where the enhancement is substantially increased due to a number of factors [26, 27]. However in this study there are no such constraints as the whole experiment is carried out as such in liquid state. The Au-Ag nanoparticles and the rhodamine 6G dye solution are taken in hquid form in a quartz cuvette. This leads to the enhancement of al] the predominant peaks of the dye. The concentiation of the dye R6G was very low as high concentrations led to blinding of the Raman signals. Equal volume ratio (1:1) of R6G (1 x 10"'^ M) and nanoparticles is taken for the analysis as this ratio resulted with maximum enhancement of Raman signal, SERS enhancement factor (EF) IS defined as the ratio of the enhancement obtained to what would be obtained for the same molecule in non-SERS conditions [28]. The average SERS EF is calculated accord- ing to the formula

EF^

^A'SERS (7)

where IQ and /SERS are the peak intensities of the Raman measurement under normal and SERS conditions, respectively [29], A'o and A'SERS are the number of R6G molecules in die scattering volume for the normal Raman measurement and SERS measurement, respectively. The SERS spectmm of rhodamine 6G enhanced using Au-Ag nanoparticles is shown in figure 6.

The vibrational bands of the R6G dye are from 1800 to 600 cm"'. The vibrational bands observed in the R6G spectra from 1314—1651cm"' are due the aromatic C-C stretching.

The other wavenumber 1124 cm"' is a weak band due to the

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Adv. I^al Sci: Nanosci. Nanotechnol 5(2014)045006 e Kinjbha and P K P a l a n i ^ C-H (ip) bend, while 774 cm"' arises from die C-H (oop)

bend and 614 cm"' is due to C-C-C (ip) bend [30]. After tiie addition of nanoparticles to die dye all die nine distinctively seen peaks are enhanced and their enhancement factor is obtauied. The enhancement factor is calculated by averaging the enhancement of all the peaks. The average EF attained is 2.72 X 10^. Our results also show diat die core-shell nanoparticles act as good analytes for surface Raman scattering of rhodamine 6G dye.

4. Conclusion

In essence, gripe water is used to develop a facde and a complete green method to synthesize Au-Ag bimetallic nanoparticles at room temperamre with great ease The synthesized nanoparticles are core-shell structures with Au being the core and Ag as the shell. The as-synthesized nanoparticles are subjected to nonlinear optical studies by z-scan using Ti:

sapphire femtosecond pulses and tuning the wavelengdi from 700 nm to 950 nm. The nonlinear optical parameters such as the nonlinear refractive index W2 are found to be in the order of 10"^cm^W"', nonhnear absorption coefficient ji is obtained in tiie order of 10"^cmW"' and the diird order nonhnear susceptibihty ^ is in the order of 10"'" (esu). The Au-Ag nanoparticles are highly compatible with die dye rhodamine 6G and the surface Raman scattering enhancement due to the nanoparticles is observed and the average enhancement factor is found to be in the range of 10^. Hence, diis study proves to be an excellent tool for the enhancement of Raman signals and can be extended for biosensing.

Acknowledgment

Author P K Palanisamy acknowledges the Emeriuis fellow- ship award of University Grants Commission (UGC), New Delhi.

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