Bio-fabrication of gold nanoparticles using aqueous extract of red tomato and its use as a colorimetric sensor
© Barman et al.; licensee Springer. 2013
Received: 20 December 2012
Accepted: 12 April 2013
Published: 19 April 2013
In this work, we report a green method for the synthesis of gold nanoparticles (GNP) using the aqueous extract of red tomato (Lycopersicon esculentum). We believe that citric acid and ascorbic acid present in tomato juice are responsible for the reduction of gold ions. This biosynthesized GNP in the presence of sodium dodecyl sulfate has been used as a colorimetric sensor to detect and estimate the pesticide, methyl parathion. The GNP in the presence of methyl parathion shows a new peak at 400 nm due to the formation of 4-nitrophenolate ion by catalytic hydrolysis of methyl parathion in alkaline medium. A calibration curve between the absorption coefficients of the 400-nm peak versus the concentration of the pesticide allows the quantitative estimation of the 4-nitrophenolate ion, thereby enabling indirect estimation of methyl parathion present in the system.
KeywordsGold nanoparticles Green synthesis Sodium dodecyl sulfate Colorimetric sensor Detection of pesticide Estimation of pesticide Calibration curve
The last 2 decades have witnessed rapid advancement in various technologies for the fabrication of nanoparticles. Among the various classes of nanoparticles, metal nanoparticles are receiving much attention due to their application in various fields of science and technology. A number of approaches are available for the synthesis of silver and gold nanoparticles, for example, reduction of solution [1–3]; thermal , electrochemical , and sonochemical decomposition ; microwave-assisted synthesis ; and recently, using of green chemistry [8–11]. Using plants in the biosynthesis of metal nanoparticles, especially gold and silver nanoparticles, has received more attention as suitable alternative to chemical procedures and physical methods. Bioreduction of metal nanoparticles using a combination of biomolecules found in plant extract, e.g., enzymes, proteins, amino acids, vitamins, polysaccharides, and organic acids such as citrates is environmentally benign yet chemically complex. Extracts from plants may act as both reducing and capping agents in nanoparticle synthesis. Gardea-Torresdey et al. [12, 13] were the first to report the formation of gold and silver nanoparticles inside living plants. A rapid reduction of the silver ions was observed when the silver nitrate solution comes to contact with geranium leaf extract . A competition reduction of Au3+ and Ag+ ions was observed when presented simultaneously in neem (Azadirachta indica) leaf extract . A simple biosynthesis procedure of applying green tea extract has been used for gold and silver nanoparticle synthesis by Vilchis-Nestor et al. .
In this work, we report a green method for the synthesis of gold nanoparticles (GNP) using the aqueous extract of red tomato (Lycopersicon esculentum). The tomato is a member of the Solanaceae family. Nutritionally, the tomato is a good source of vitamins A and C . Composition data vary due to the wide range of species, stage of ripeness, year of growth, climatic conditions, light, temperature, soil, fertilization, irrigation, and other conditions of cultivation, handling, and storage . Average dry matter content of the ripe fresh food is between 5.0% and 7.5% .
The pectins, arabinogalactans, xylans, arabinoxylans, and cellulose are the major polysaccharides present in tomato. Glutamic acid comprises up to 45% of the total weight of free amino acids in fresh tomato juice with the next highest in concentration being aspartic acid. Citric acid is the most abundant organic acid with some malic acid also present . Thus, the water extract of the tomato juice mostly contains proteins and water-soluble organic acids like citric acid, malic acid, amino acids, and vitamins. We believe that the presence of citric acid and ascorbic acid in the aqueous extract of tomato juice is responsible for the reduction of gold ions while the soluble proteins and amino acids are responsible for the stabilization of GNP.
This biosynthesized GNP in the presence of sodium dodecyl sulfate (SDS) has been used as a colorimetric sensor for the detection and estimation of the pesticide present in water and in alkaline medium. The pesticide methyl parathion is chosen because it is a highly neurotoxic agricultural chemical that is used extensively worldwide to control a wide range of insect pests. Its residue in the soil causes pollution in the environment and poses a serious risk to human health. The sensor properties were studied by examining the UV-visible spectral change due to the addition of methyl parathion at parts per million (ppm) levels.
Chloroauric acid and SDS, both of AR grade, were purchased from Sigma-Aldrich Chemical Ltd. (Powai, Mumbai, India). Sodium hydroxide and methyl parathion were purchased from Merck (Whitehouse Station, NJ, USA). Double-distilled deionized water was used in all experiments.
The red tomato (Lycopersicon esculentum) was collected from the local market and washed with double-distilled deionized water. The skin was removed from the tomato, and the whole mass was squeezed to get the juice. This was filtered using a Whatman filter paper (Maidstone, UK) and diluted with twice its volume of water. This solution was used as the tomato extract. From this extract, we prepared diluted extract having different compositions like 5:5 (5 ml extract and 5 ml water), 6:4 (6 ml extract and 4 ml water), 7:3 (7 ml extract and 3 ml water), 8:2 (8 ml extract and 2 ml water), 10:0 (10 ml extract ), and so on.
The absorbance spectra of the GNP were analyzed using a Shimadzu UV-1800 spectrophotometer (Chestnut Ridge, NY, USA), and transmission electron microscopy (TEM) images were taken using a JEOL JEM-2100 high-resolution transmission electron microscope (HR-TEM, Akishima-shi,Japan). Samples for the TEM studies were prepared by placing a drop of the aqueous suspension of GNP on a carbon-coated copper grid followed by solvent evaporation under a vacuum. The crystalline nature of the GNP was examined using an X’Pert Pro X-ray diffractometer operated at a voltage of 40 kV and a current of 30 mA with CuKα radiation.
3Ten milliliters of the as-prepared GNP was added to an equal volume of 3×10-3 (M) concentration of alkaline SDS. The pH of the solution was maintained at 9 to 9.5 by varying the amount of NaOH solution (0.15 (M)) added. The mixture was heated at 80°C for 30 min during which the color of the mixture deepened. This solution was used to detect the presence of methyl parathion. The concentration of methyl parathion in the alkaline GNP solution was varied from 0 to 200 ppm. Five hundred microliters of a solution containing different concentrations of methyl parathion was added to 5 ml of alkaline GNP solution, and the mixture was heated for 5 min with stirring. The deep reddish-violet color changed into brownish red. The intensity of the brownish red gradually increased with the increase of methyl parathion.
Results and discussion
Synthesis of nanoparticles is an important activity in modern nanotechnology, and the biosynthesis of nanoparticles using plant extracts is presently getting much attention. The development of biological processes for the synthesis of nanoparticles is evolving as an important branch of nanotechnology. The present study deals with the synthesis of gold nanoparticles (GNP) using aqueous tomato extract.
Shifting of gold plasmon band to the higher value may be explained as follows: tomato extract is a strong reducing agent but not a good capping agent. So, it induces rapid nucleation but cannot restrict the growth of gold nanoparticles. Hence, polydispersed gold nanoparticles are observed. When we use tomato extract (100%), the band shifts to 540 nm and the extinction coefficient decreases appreciably. This might be due to colloidal instability. The polydispersity and the colloidal instability (agglomeration tendency of gold nanoparticle) may be the reason for a broad spectrum of gold sol along with a shift in the peak position. The shifting of the peak position may be related to the increase of the size of gold nanoparticles.
The newly found peak might be due to the 4-nitrophenolate ions which are produced by the hydrolysis of methyl parathion in alkaline medium . It is known that due to the catalytic hydrolysis of methyl parathion, two hydrolyzed products, 4-nitrophenolate ions and sodium di-O-methyl thiophosphonate are produced (Figure 1). The literature confirms that 4-nitrophenolate ion shows a characteristic absorption peak at 400 nm . The increase in the concentration of methyl parathion in the mixture quantitatively increases the amount of the 4-nitrophenolate ions in the medium which are reflected in the absorption spectra (Figure 3A).
A calibration curve between the absorption coefficient of the 400-nm peak and the concentration of the pesticide allows quantitative estimation of the methyl parathion present in a sample at ppm levels (Figure 3B). This calibration curve enables the estimation of methyl parathion indirectly by estimating the 4-nitrophenolate ions present in the medium. The corresponding decrease in the absorption peak of GNP at 532 nm may be due to the agglomeration of GNP which is facilitated by the presence of the other hydrolyzed product sodium di-O-methyl thiophosphonate containing sulfur. The formation of agglomeration of GNP is indicated by the broadening of the 532-nm peak in the presence of methyl parathion. A control experiment (Figure 3C) was carried out by taking methyl parathion only, and no peak was found at 400 nm. The peak at 400 nm emerges only when the hydrolysis of methyl parathion occurs in the presence of GNP in water.
The TEM image of Figure 5C is due to GNP with methyl parathion in alkaline medium in the presence of SDS. It appears that the restructuring of GNP occurs after the addition of methyl parathion and agglomeration of particles is observed. It is likely that the surface of the GNP forms an Au-S coordination bond as the sol is being heated after addition of methyl parathion and some hydrolyzed product sodium di-O-methyl thiophosphonate get adsorbed on the Au surface by replacing SDS. As it is anionic in alkaline medium, its adsorption on the GNP surface lowers the surface charge, and thus, they agglomerate and particle clustering is observed (Figure 1).
A green method has been used for the synthesis of gold nanoparticles using the aqueous extract of red tomato. Citric acid and ascorbic acid present in tomato juice are believed to be responsible for the reduction of gold ions. This biosynthesized GNP has been used as colorimetric sensor for detection and estimation of methyl parathion present in water in the presence of SDS. A new peak generated at 400 nm due to the formation of 4-nitrophenolate ion when methyl parathion added in the alkaline medium of the GNP. The variations of the absorbance of this peak have been used for estimation of methyl parathion present in the solution. To quantitatively estimate methyl parathion present in water, a calibration curve between the absorbance of 400-nm peak versus concentration of methyl parathion has been drawn.
JKL is Associate Professor and Head of the Department of Chemistry, Midnapore College, West Bengal, India. GB and SM are research scholars of this department.
We gratefully acknowledge the financial support received from UGC (Ref. no. F. PSW-096 / 10–11.). We are also thankful to the Central Research Facility at IIT Kharagpur, India for the HR-TEM and XRD measurements.
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