Biomimetic synthesis of silver nanoparticles by Citrus limon (lemon) aqueous extract and theoretical prediction of particle size
Introduction
Noble metal nanoparticles are being widely used nowadays in the fields of medicine, biology, material science, physics and chemistry [1], [2]. Metal nanoparticles have been shown to possess enormous application potential in the areas such as photography, catalysis, biological labeling, photonics, optoelectronics and surface-enhanced Raman scattering (SERS) [3], [4]. Traditionally nanoparticles have been prepared and stabilized by a number of physical and chemical methods; of them, chemical reduction, electrochemical techniques and photochemical reduction are being most widely used [5], [6]. Of the noble metal nanoparticles, silver is of particular interest because of distinctive properties, such as good electrical conductivity, chemical stability, catalytic and antibacterial activity.
Most of the traditionally used chemical and physical methods of synthesis of silver nanoparticles are expensive and often involve the use of toxic, hazardous chemicals which may pose environmental risks [7]. Also most of the current synthetic methods mostly rely on the use of organic solvents due to the hydrophobicity of the capping agents used [8]. Lately the quest for cleaner methods of synthesis has led to the development of bio-inspired approaches. Bio-inspired methods have been put forward to be advantageous over other synthetic methods as they are cost effective and do not involve the use of toxic chemicals, high pressure, energy and temperatures [9].
Recently a number of organisms have been used for the synthesis of nanoparticles, noble metals in particular. Synthesis using bio-organisms is compatible with the green chemistry principles: the bio-organism is eco-friendly as are the reducing agent employed and the capping agent of the reaction [10]. The nanoparticles may either be synthesized intracellularly or extracellularly using bacteria, yeast, fungi and plant materials which have been used for various applications.
Synthesis of nanoparticles using microorganisms started with the use of Pseudomonas stutzeri AG259, isolated from silver mines, which has been shown to produce silver nanoparticles [11], [12]. Recently Bacillus licheniformis has been studied to produce silver nanocrystals extracellularly [13]. Eukaryotic organisms like fungi have been used for the synthesis of different types of nanoparticles such as Verticillium sp. [14], and Fusarium oxysporum [15].
Microbe mediated synthesis of nanoparticles are not industrially feasible as they require the maintenance of highly aseptic conditions [16]. Compared to microbe assisted synthesis, plant mediated synthesis of nanoparticles is a relatively under exploited field and is recently gaining wide attention. Some of the plants/plant parts that have been used for synthesis are the Capsicum annum L. extract [10], leaves of Azadirachta indica [17] and lemongrass plant extract [18] to name a few.
Silver nanoparticles have been synthesized using the salt of citrate as the reducing agent [19] and gold nanoparticles have been synthesized using ascorbic acid as the reducing agent [20]. Since lemons are a rich source of citric acid and ascorbic acid [21], [22], in the present study, fruits of Citrus limon (lemon) were used as a bioreductant for the synthesis of silver nanoparticles. Also the other reason behind selecting this particular plant is (i) lemon is a commonly available fruit and (ii) antimicrobial properties of the synthesized nanoparticles might be enhanced.
The principal objective of this work was to study in detail the influence of various process variables like reductant concentrations, mixing ratio of the reactants and interaction time to develop a bio-based synthesis process of silver nanoparticles by using fruits of C. limon (lemon) and to compare the theoretical and experimental results with regard to particle size by statistical analyses for this particular process.
Section snippets
Materials
Fresh lemons (C. limon) were procured from a local market in Vellore, India. Silver nitrate, l-ascorbic acid, citric acid (anhydrous), potassium iodide and potassium iodate were purchased from SD Fine Chemicals Ltd., Mumbai, India.
Preparation of extract
Lemons (C. limon) were squeezed to extract the juice which was later strained through a fine pore nylon mesh. The juice obtained was centrifuged at 10,000 rpm for 10 min to remove any undesired impurities. This juice was used for further experiments.
Synthesis of silver nanoparticles
In order to
Silver nanoparticles: biosynthesis and characterization
Commercially available sodium citrate has been used extensively for the synthesis of silver nanoparticles at higher temperatures [19]. Also ascorbic acid though a weak reducing agent has been used for the synthesis of gold nanoparticles [20]. Lemon juice is a rich source of citric acid and ascorbic acid [21], [22] and also known for water softening and food additive properties. Lemon juice was thus used in this study for the one point green synthesis of silver nanoparticles at room temperature.
Summary
An interesting room temperature green synthesis method has been developed for the preparation of silver nanoparticles. This preparatory method is a novel and cost effective method that excludes the use of external stabilizing/capping agents. The effects of various process variables like the reductant concentration, mixing ratio of silver nitrate solution to lemon juice and interaction time on the formation of silver nanoparticles was studied. With an interaction time of 4 h, nanoparticles below
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