One-Step Green Synthesis of Gold Nanoparticles Using Black Cardamom and Effect of pH on Its Synthesis
© Singh and Srivastava. 2015
Received: 1 July 2015
Accepted: 19 August 2015
Published: 4 September 2015
In the present article, an effective, one-step, and environmentally benign protocol for the synthesis of gold nanoparticles has been discussed. The black cardamom extract is used as a reducing agent for HAuCl4.3H2O. In order to synthesize gold nanoparticles, an aqueous solution of HAuCl4.3H2O was mixed with an optimized concentration of black cardamom extract where 1,8-cineole is the dominant component. Choosing black cardamom extract as a reducing agent can be justified under the light of the fact that it has a very fast reducing ability. Gold nanoparticles with different shapes and sizes were synthesized by varying the ratio of AuCl4 ions to black cardamom extract. Kinetics of reactions has been evaluated through monitoring of surface plasmon behavior of gold nanoparticles as a function of time. Based on Fourier transform infrared spectroscopy (FTIR) studies, a tentative mechanism of reduction of Au nanoparticles has also been proposed which includes oxidation of 1,8-cineole to 2-oxo-1,8-cineole. Further, a comprehensive study to investigate the effect of pH on the synthesis of Au nanoparticles has been carried out.
Attaining of unique properties by tailoring the materials at atomic level can be achieved by the process of nanotechnology . There have been impressive developments in the field of nanotechnology in the recent past years, with numerous methodologies formulated to synthesize nanoparticles of particular size and also of shape depending on specific requirement.
Recently, a resurgence of interest in metal nanoparticles has resulted due to their potential applications in the emerging field of plasmonics [2–4]. Plasmonics uses the unique optical properties of metallic nanomaterials to manipulate the transfer of light on the nanoscale and is a promising technology for integrating the large data-carrying capacity of optical interconnects with nanoscale electronic devices.
Metal nanoparticles are known to display tremendous potential for biological and chemical sensing [5–10] and cancer therapy . They can serve as a model system to experimentally probe the effects of quantum confinement on electronic, magnetic, and other fated properties [12–14]. They have also been widely exploited for use in photography , catalysis [16, 17], photonics , optoelectronics , information storage , surface-enhanced Raman scattering (SERS) [21–23], and formulation of magnetic ferrofluids .
Among various methods available for the synthesis of metal nanoparticles, laser evaporation and chemical reduction are the major ones . However, each method has certain limitations. For example, the use of costly and sophisticated instruments and the problems associated with their handling in case of laser evaporation and also the yields of nanoparticles are quite low. Similarly, chemical reduction method may end up with the adsorption of some toxic chemical species on the surface even though gold nanoparticles are considered biocompatible which needs an additional step of removal of these toxic species. This may have some adverse effects in medical applications. Therefore, there is an urgent need to develop an environmentally benign nanoparticle synthesis protocol. This tempts the researchers in the field of nanoparticle synthesis and assembly to utilize some eco-compatible natural compounds for the reduction of Ag- and Au-containing salts for the synthesis of Ag and Au nanoparticles [26, 27]. Recently, microorganisms mediated nanoparticle synthesis and gained much importance because of biocompatibility and facile assembly of nanoparticles [28–33]. Sastri et al. opened an avenue to the synthesis of metal nanoparticles by eukaryotic organisms [34, 35]. Later, they carried out extracellular synthesis of Ag and Au nanoparticles, using fungi [36–38]. They demonstrated that the shift from bacteria to fungi as a means of developing ‘natural nanofactories’ has the added advantage for the processing and handling of the biomass.
Further, plant extracts have received considerable attention as an effective reducing agent for Ag- and Au-containing salts to synthesize the Ag and Au nanoparticles. Jose-Yacaman et al. have reported the first living plant-mediated synthesis of silver and gold nanoparticles . Similar biosynthesis of nanoparticles was achieved by Sastri et al. by using plant leaf extracts, and they explored further potential applications . They studied bio reduction of silver ions and chloroaurate ions by the broth of geranium leaf  or neem leaf . They also demonstrated the synthesis of gold nanotriangles from tamarind leaf extract and studied their potential application in vapor sensing . Recently, some scientist synthesized the gold nanotriangles and silver nanoparticles, using aloe vera plant extract .
With this literature background, we herein report a novel, eco-compatible, and green synthesis of gold nanoparticles from AuIII salts by using extract of black cardamom as a natural reducing agent. Black cardamom is widely used extensively in India, in foods, beverages, mouth fresheners, and native medicine. Black cardamom has been used as reducing agent in our synthesis protocols. Synthesis of gold nanoparticles by employing Black cardamom as a reducing agent and the effect of pH on synthesis have been discussed and described in this article.
Materials and Methods
HAuCl4.3H2O, procured from Sigma Aldrich and dried black cardamom easily available in commercial market, have been adopted as staring materials. These materials have been used without any further purification.
Preparation of Black Cardamom Extract
The effect of varying amount of reducing agent
The role of pH on the synthesis of Au nanoparticles
For the first investigation, the synthesis steps involve the mixing of a 50-mL (0.001 M) aqueous solution of HAuCl4.3H2O to different amounts of black cardamom extract under constant stirring of 250 rpm at room temp. Three different concentration ratio, 1:1, 1:0.5, and 1:0.1 have been selected for this purpose. Subsequent changes in color (which occurs within minutes) indicate the successful synthesis of Au nanoparticles.
Synthesis of Gold Nanoparticles
To study the effect of varying the amount of black cardamom extract on the synthesis of nanoparticles, three samples with different ratios of HAuCl4 and black cardamom extract (1:1, 1:0.5, 1:0.1) have been prepared. Subsequent changes in color within minutes, depending on the shape and size of the particles, clearly indicate the formation of gold nanoparticles. Solutions were further centrifuged at 5000 rpm for nearly 10 min. Gold nanoparticles thus obtained were collected and resuspended in double-distilled water, and in order to remove impurities, this process was repeated for three times.
To investigate the phase formation and crystal structure, X-ray diffraction (XRD) analysis has been carried out by using X-ray diffractometer (PAN—analyst BV, the Netherlands with a built in graphite monochromator) using Cu Kα radiation with Ni filter in a wide range of Bragg angle (20 < 2θ < 80). For this purpose, the Au nanoparticles obtained through the process described above were placed on a glass disk (~5 mm diameter), allowed to dry, and then mounted in the specimen port of diffractometer.
TEM Analysis of Gold Nanoparticles
After the completion of a successful synthesis process, suspended centrifuged particles have been sampled for TEM analysis. In this process, the samples of gold nanoparticles were prepared by placing a drop of obtained suspension after centrifugation on the Formvar-coated copper grids. The grids were further dried and used for TEM analysis. For shape, size, and microstructural details of these as-synthesized gold nanoparticles, TECNAI 20 G2 electron microscope, operated at an accelerating voltage of 200 kV, has been used.
UV-Visible Spectroscopic Studies
The evolution of nanoparticles from AuCl4 − ions has been observed through monitoring the UV-visible spectra of synthesized Au nanoparticles. The samples were analyzed by employing Perkin Elmer Lambda 750S UV-Visible spectrometer with a resolution of 1 nm.
FTIR Spectroscopic Study
Fourier transform infrared spectroscopy (FTIR) spectra of black cardamom extract, before and after bio reduction of Au nanoparticles, have been taken by employing Perkin Elmer Spectrum 100 instrument for unrevealing the mechanism of formation of Au nanoparticles through the reduction of AuCl4 − ions.
Results and Discussions
UV-Visible Spectroscopy of Synthesized Gold Nanoparticles
Instant change in color of aqueous solution of HAuCl4.3H2O after the addition of black cardamom extract clearly indicates the formation of gold nanoparticles. One milliliter of this reaction mixture, diluted with 3.0 mL of double-distilled water, has instantaneously been taken for the UV-visible spectroscopic investigations. Three different samples of gold nanoparticles have been prepared with varying ratio of HAuCl4.3H2O to black cardamom extract (i.e., 1:1, 1:0.2, and 1: 0.1) resulting in distinct colors arising due to the difference in shape and size of nanoparticles
This leads to the conclusion that reaction took place within first 10 to 20 min. After that, the rate of the reaction gets slowed down with no significant change in total absorbance of reaction mixture. A similar conclusion which can be drawn from Fig. 2c is the absorption spectra of gold nanoparticles, synthesized with HAuCl4 to black cardamom extract ratio of 1:0.1. In this case, a broad peak at 565 nm appears. This peak is further red shifted, and initial four spectra are more distinctly separated with respect to the spectra of Fig. 2a, b. Therefore, it can be concluded that the reaction has further been slowed down, and the reaction took nearly 30 to 40 min to get completed. All these spectra shown in Fig. 2 provide an indication to follow a regular pattern with the size of the particles, which allowed us to conclude that as the amount of black cardamom extract in the reaction (i.e., 10 to 1 mL) have been reduced, the size of the particles increased. Another distinct feature of these UV-vis spectra of Au nanoparticles is that it becomes broader around the prime peak as we decrease the concentration of black cardamom extract. This broadening occurs because as we decrease the concentration of black cardamom extract, particles with different shapes and sized begin to form. We know that different sizes of nanoparticles contribute to different positions of SPR maxima. Since UV-vis spectra depict the collective oscillation of surface plasmons of all nanoparticles, therefore, broadening occurs as a result of polydispersity. It has been confirmed in the TEM investigation of synthesized gold nanoparticles.
TEM Analysis of Gold Nanoparticles
FTIR Analysis of Black Cardamom Extract
There are few reports available on pharmaceutical composition of black cardamom which indicate that out of 40 identified compounds of black cardamom, 1,8-cineole (65 %), β-Pinene (0.85 %), and α-Terpineol (7.92 %) are the main constituents .
This spectrum provides the information of changes in functional groups of chemicals, basically found in black cardamom extract which was further utilized to deduce a plausible mechanism for the bio reduction of gold nanoparticles. Sample for FTIR of black cardamom extract after the reduction of gold nanoparticles has been obtained by the centrifugation of reaction mixture at 15000 rpm for nearly 15 min. In this process, the particles get settled at the bottom and supernatant have been collected for further analysis. One milliliter of black cardamom extract before and after the bio reduction of HAuCl4 has further been employed for recording the FTIR spectra. FTIR spectra of black cardamom extract before reduction clearly show a large number of peaks from 4000 to 3200 cm−1 . Around 3200 cm−1, there is a broad peak which attributes –OH.
However, this radical disappears in the spectra recorded after the bio reduction, which indicates the consumption of –OH in the reduction of HAuCl4. Rest other bands up to 1800 cm−1 appear in both the spectra, i.e., spectra obtained before and after the reduction, are common. Below 1800 cm−1, a new band at 1770 cm−1 appears after the reduction. This is the fingerprint signature of –C=O functional group in stretching mode. After 1770 to 500 cm−1, positions of bonds are again almost similar as those before reduction, with varying intensity.
After the formation of gold nanoparticles, –OH (visible at 3200 cm−1) group disappears.
–C=O group (1770 cm−1) appears after the formation of gold nanoparticles.
Rest other groups like (R–CH) are common in both the spectra.
Based on the above FTIR results, attempts have been made to propose a viable growth mechanism of gold nanoparticles, as discussed in the next section.
Reduction Mechanism of Gold Nanoparticles
To understand the change in –OH group that whether this change is due to the chemical-containing –OH group, which is present in the aqueous black cardamom extract medium or due to the aqueous medium itself. In order to check this, the experiment has been repeated in organic solvent with no –OH group like hexane (though HAuCl4 has low solubility in organic solvent). The results have not shown any indication of formation of gold nanoparticles which leads to conclude that the presence of water molecule is an essential part for the reduction of energy.
When the reaction starts, coordinately bonded Cl reacts with α-H of 1,8-cineole followed by the removal of HCl and forming a bond between Cl and Au3+. Here, Au is again in +3 oxidation state, therefore, one Cl atom will get coordinately bonded with Au.
Since water was used as medium in this experiment, in which oxygen molecule have two pairs of electron, therefore, one of these two lone pairs attacks on Au to form Au-O bond. This bond formation leads to the removal of HCl where Cl− and H+ atoms came from Au and water, respectively.
Now, the lone pair of –OH attacks on C, therefore, carbon gives its bond pair to Au to maintain its oxidation number zero. Since oxygen has given its lone pair to carbon, it will acquire a positive charge, which will be neutralized by elimination of H+ after the removal of Cl− from AuCl to form HCl. In this step, Au3+ gets reduce to Au+
In this step, C–H Bond pair migrates to form C=O and H+ gets removed by forming HCl. Further, the bond pair between O and Au moves to Au+ which leads to the reduction of Au+ to Au
Effect of pH on the Synthesis of Gold Nanoparticles
Variation in absorbance maxima with change in pH synthesis of gold nanoparticle by oxidation of 1,8-cineole
Amount of NaOH Added (μL)
pH of Solution Before Reduction
pH of Solution After Reduction
Absorbance Maxima (nm)
UV-Visible Spectroscopic Study
In conclusion, a simple, one-step, and fast green route for the synthesis of gold nanoparticles of different shapes and sizes has been demonstrated. It has been found that these physical parameters (shape and size) can be tuned easily by varying the ratio of the HAuCl4 to black cardamom extract. The UV-visible spectroscopic study provides some clue about the evolutions of nanoparticles with time and variation in sizes at the same time. These observations can further be confirmed by histogram plot of particle size distribution obtained from TEM analysis. These analyses lead to the conclusion that the size of particles increased when the concentration of black cardamom extract is decreased. The XRD analysis explicitly shows the formation of gold nanoparticles as the diffraction peaks match well with the standard value of gold lattice structure. Reduction of HAuCl4 takes place because of 1,8-cineole. A plausible mechanism of reduction has been put forward, based on analysis of FTIR spectra of black cardamom extract before and after the bio reduction of HAuCl4.
Authors acknowledge with gratitude the CSIR, New Delhi, for financial assistance in the form of Junior and Senior Research Fellowships. DST-UNANST is also gratefully acknowledged for further financial support.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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