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

Chloroauric acid HAuCl₄ and silver nitrate AgNO₃ are purchased from Sigma-Aldrich. Woodward's gripe water is purchased from TTK Health Care Ltd, INDIA. Millipore water (18 MΩ) is used as solvent in our experiments.

The surface plasmon bands of the particles are recorded using UV–Vis spectrometer. The absorption spectra for all the particles are recorded using Shimadzu spectrometer, with a resolution of 0.5 nm taking the 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 grid 300 mesh. The IR spectra are recorded on JASCO FT/IR-4100 Fourier transform infrared spectrometer in the range 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹. The z-scan studies were performed using a tunable, mode locked Ti:sapphire at a repetition rate of 80 MHz with a pulse width 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 the SERS studies are recorded using a LabRAM HR800, Jobin Yvon, France, with an 80 cm focal length spectrometer and a He-Ne 633 nm with 11 mW power on the sample. The spectra are acquired over a wavenumber region of 1800 to 600 cm⁻¹ with an acquisition time of 10 sec.

The core–shell nanoparticles were prepared by taking the source solutions of HAuCl₄ and AgNO₃ in 1:1 molar ratio. To the stirring source solution of 10 mL of gripe water and 0.3 mM of AgNO₃ was added to 0.3 mM of HAuCl₄ and kept under continuous stirring.

During the synthesis procedure, gripe water and AgNO₃ are added together to HAuCl₄ solution to avoid precipitation of AgCl. Although the reducing agent is added to both the source solutions at the same time, Au nanoparticles are formed first as the growth kinetics of Au is faster than that of the Ag nanoparticles. Therefore Au–Ag nanoparticles are grown by successive reduction of HAuCl₄ and AgNO₃ by gripe water. The nanoparticles attained saturation 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 the SPR band of the Au–Ag core–shell nanoparticles. From time dependent UV–Vis spectrum, it is seen that the SPR band of the nanoparticles increases in intensity and peak broadening is not observed. This implies 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–shell nanoparticles. Every 5 ml of used gripe water contains (as per its label) sodium bicarbonate (Sarjikakshara) of 0.05 g, dill oil (Anethum Graveolens) of 0.005 ml, sugar of 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-nitropropane1,3-diol) present in gripe water stabilize the nanoparticles, while sodium bicarbonate and sucrose reduce the Au–Ag nanoparticles.

Zoom In Zoom Out Reset image size

A transmission electron microscope (TEM) is used to decipher 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 differentiated with the Ag shell. The average particle size of the Au–Ag nanoparticles attained from TEM images is found to be 10 nm.

Zoom In Zoom Out Reset image size

Using the single beam z-scan technique, 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 n₂ of the nanoparticles, while the open aperture z-scan quantifies the nonlinear absorption coefficient β. The experimental set up of z-scan is schematically represented in figure 3. The Au–Ag nanoparticles whose nonlinearity is to be studied is taken in colloidal form in quartz cuvette with a width of 1 mm and mounted on a translation stage. Along the direction of propagation of the laser beam, the stage is moved across the z direction from −10 to +10 mm (−z to +z).

Zoom In Zoom Out Reset image size

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 z = 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 index n₂ is deduced from the closed aperture and the nonlinear absorption coefficient β is inferred from the open aperture z-scan. The optical nonlinear parameters are obtained from equations

$$\begin{eqnarray}&&{{n}_{2}}=\frac{\Delta {{\Phi }_{0}}}{k{{I}_{0}}\ \;{{L}_{{\rm eff}}}},\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&\Delta {{\Phi }_{0}}=\frac{\Delta {{T}_{p-v}}}{0.406{{(1-S)}^{0.25}}},\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&\beta =\frac{2\sqrt{2}\Delta T}{{{I}_{0}}\ \;{{L}_{{\rm eff}}}},\end{eqnarray} \tag{ 3 }$$

where n₂ is the nonlinear refractive index, k is the wave number (k = 2π/λ) and ΔΦ₀ is the on-axis phase shift at the focus, I₀ is the peak intensity within the sample at the focus, L_eff is the effective thickness of the sample (1 mm—quartz cuvette), S is the linear aperture transmittance (0.5) and ΔT_p-v is the normalized peak valley differences obtained from the closed aperture z-scan trace, ΔT = 1−T_p, where T_p is the normalized peak value from the open aperture plot [22–25]. Third order nonlinear susceptibility χ³ of the nanoparticles were obtained using the relations as in equations

$$\begin{eqnarray}&&{{\chi }^{3}}=\sqrt{\operatorname{Re}{{({{\chi }^{3}})}^{2}}+Im{{({{\chi }^{3}})}^{2}}},\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&\operatorname{Re}({{\chi }^{3}})={{10}^{-4}}\frac{{{\varepsilon }_{0}}{{c}^{2}}n_{0}^{2}}{\pi }{{n}_{2}},\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{\rm Im}({{\chi }^{3}})={{10}^{-2}}\frac{{{\varepsilon }_{0}}{{c}^{2}}n_{0}^{2}}{4{{\pi }^{2}}}\beta ,\end{eqnarray} \tag{ 6 }$$

where, ε₀ = 8.854 × 10⁻¹⁴ F m⁻¹ is the vacuum permittivity, c is the speed of light in vacuum (3 × 10¹⁰ cm sec⁻¹) and n₀ is the linear refractive index (1.33). The real part Re(χ³) is proportional to the nonlinear refractive index of the nanoparticles, while the imaginary part Im(χ³)' is proportional to the nonlinear absorption coefficient β. Z-scan is carried out for different incident wavelengths from 700 nm to 950 nm. The measured nonlinear parameters of the prepared Au–Ag 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.

Zoom In Zoom Out Reset image size

The as-synthesized Au–Ag nanoparticles indicate negative nonlinearity as their closed aperture z-scan traces show peak followed by valley. The nonlinear refractive index n₂ of the nanoparticles is in the order of 10⁻⁸ cm² W⁻¹. The n₂ value increases from 1.18 to 2.45 × 10⁻⁸ cm² W⁻¹ with the increase in the incident wavelength for the bimetallic nanoparticles. The nonlinear absorption coefficient β obtained from the open aperture z-scan decreases from 1.69 to 1.23 × 10⁻³ cm W⁻¹ with the increase in wavelength. The third order nonlinear susceptibility χ³ increases with the increase in the incident wavelength. Variation of the nonlinear optical parameters n₂, β and χ³ 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 mol L⁻¹ and Ag is 0.310 mol L⁻¹.

Zoom In Zoom Out Reset image size

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 studied to the metal nanoparticles on a substrate. This leads to hot-spots, with large enhancement occurring 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 liquid form in a quartz cuvette. This leads to the enhancement of all the predominant peaks of the dye. The concentration 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 × 10⁻¹² 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 according to the formula

$$\begin{eqnarray}&&EF=\frac{{{I}_{{\rm SERS}}}{{N}_{0}}}{{{I}_{0}}{{N}_{{\rm SERS}}}},\end{eqnarray} \tag{ 7 }$$

where I₀ and I_SERS are the peak intensities of the Raman measurement under normal and SERS conditions, respectively [29], N₀ and N_SERS are the number of R6G molecules in the scattering volume for the normal Raman measurement and SERS measurement, respectively. The SERS spectrum of rhodamine 6G enhanced using Au–Ag nanoparticles is shown in figure 6.

Zoom In Zoom Out Reset image size

The vibrational bands of the R6G dye are from 1800 to 600 cm⁻¹. The vibrational bands observed in the R6G spectra from 1314–1651 cm⁻¹ are due the aromatic C-C stretching. The other wavenumber 1124 cm⁻¹ is a weak band due to the C-H (ip) bend, while 774 cm⁻¹ arises from the C-H (oop) bend and 614 cm⁻¹ is due to C-C-C (ip) bend [30]. After the addition of nanoparticles to the dye all the nine distinctively seen peaks are enhanced and their enhancement factor is obtained. The enhancement factor is calculated by averaging the enhancement of all the peaks. The average EF attained is 2.72 × 10⁹. Our results also show that the core–shell nanoparticles act as good analytes for surface Raman scattering of rhodamine 6G dye.