Rapid biological synthesis of silver nanoparticles using plant leaf extracts
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- Song, J.Y. & Kim, B.S. Bioprocess Biosyst Eng (2009) 32: 79. doi:10.1007/s00449-008-0224-6
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Five plant leaf extracts (Pine, Persimmon, Ginkgo, Magnolia and Platanus) were used and compared for their extracellular synthesis of metallic silver nanoparticles. Stable silver nanoparticles were formed by treating aqueous solution of AgNO3 with the plant leaf extracts as reducing agent of Ag+ to Ag0. UV-visible spectroscopy was used to monitor the quantitative formation of silver nanoparticles. Magnolia leaf broth was the best reducing agent in terms of synthesis rate and conversion to silver nanoparticles. Only 11 min was required for more than 90% conversion at the reaction temperature of 95 °C using Magnolia leaf broth. The synthesized silver nanoparticles were characterized with inductively coupled plasma spectrometry (ICP), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and particle analyzer. The average particle size ranged from 15 to 500 nm. The particle size could be controlled by changing the reaction temperature, leaf broth concentration and AgNO3 concentration. This environmentally friendly method of biological silver nanoparticles production provides rates of synthesis faster or comparable to those of chemical methods and can potentially be used in various human contacting areas such as cosmetics, foods and medical applications.
KeywordsBiological synthesisNanoparticlesSilverPlant extractsSize control
Nanoparticles usually referred as particles with a size up to 100 nm [1, 2]. Nanoparticles exhibit completely new or improved properties based on specific characteristics such as size, distribution and morphology, if compared with larger particles of the bulk material they are made of. Nanoparticles present a higher surface to volume ratio with decreasing size of nanoparticles. Specific surface area is relevant for catalytic reactivity and other related properties such as antimicrobial activity in silver nanoparticles. As specific surface area of nanoparticles is increased, their biological effectiveness can increase due to the increase in surface energy .
Silver has long been recognized as having an inhibitory effect toward many bacterial strains and microorganisms commonly present in medical and industrial processes . The most widely used and known applications of silver and silver nanoparticles are in the medical industry. These include topical ointments and creams containing silver to prevent infection of burns and open wounds . Another widely used applications are medical devices and implants prepared with silver-impregnated polymers . In addition, silver-containing consumer products such as colloidal silver gel and silver-embedded fabrics are now used in sporting equipment.
Production of nanoparticles can be achieved through different methods. Chemical approaches are the most popular methods for the production of nanoparticles. However, some chemical methods cannot avoid the use of toxic chemicals in the synthesis protocol. Since noble metal nanoparticles such as gold, silver and platinum nanoparticles are widely applied to human contacting areas, there is a growing need to develop environmentally friendly processes of nanoparticles synthesis that do not use toxic chemicals. Biological methods of nanoparticles synthesis using microorganism [6–8], enzyme , and plant or plant extract  have been suggested as possible ecofriendly alternatives to chemical and physical methods. Using plant for nanoparticles synthesis can be advantageous over other biological processes by eliminating the elaborate process of maintaining cell cultures . It can also be suitably scaled up for large-scale synthesis of nanoparticles.
Shankar et al.  reported on the synthesis of pure metallic nanoparticles of silver and gold by the reduction of Ag+ and Au3+ ions using Neem (Azadirachta indica) leaf broth. However, little has been carried out about engineering approaches such as rapid nanoparticles synthesis using plant extracts and size control of the synthesized nanoparticles. The times required for more than 90% reduction of Ag+ and Au3+ ions using Neem leaf broth were about 4 and 2 h, respectively. If biological synthesis of nanoparticles can compete with chemical methods, there is a need to achieve faster synthesis rates. In this study, we screened several plant leaf extracts and compared their synthesis of silver nanoparticles by monitoring the conversion using UV-visible spectroscopy. We also investigated the effects of reaction conditions such as reaction temperature, leaf broth concentration and AgNO3 concentration on synthesis rate and particle size of the silver nanoparticles.
Materials and methods
Five plant leaves were collected and dried for 2 days at room temperature. They were Pine (Pinus desiflora), Persimmon (Diopyros kaki), Ginkgo (Ginko biloba), Magnolia (Magnolia kobus) and Platanus (Platanus orientalis). The plant leaf broth solution was prepared by taking 5 g of thoroughly washed and finely cut leaves in a 300 mL Erlenmeyer flask with 100 mL of sterile distilled water and then boiling the mixture for 5 min before finally decanting it. They were stored at 4 °C and used within a week.
Typically, 10 mL of leaf broth was added to 190 mL of 1 mM aqueous AgNO3 solution for reduction of Ag+ ions. The effects of temperature on synthesis rate and particle size of the prepared silver nanoparticles were studied by carrying out the reaction in water bath at 25–95 °C with reflux. The concentrations of AgNO3 solution and leaf broth were also varied at 0.1–2 mM and 5–50% by volume, respectively. The silver nanoparticle solution thus obtained was purified by repeated centrifugation at 15,000 rpm for 20 min followed by redispersion of the pellet in deionized water. UV-vis spectra were recorded as a function of reaction time on a UV-1650CP Shimadzu spectrophotometer operated at resolution of 1 nm. After freeze drying of the purified silver particles, the structure and composition were analyzed by scanning electron microscopy (SEM, Hitachi S-2500C), field emission transmission electron microscopy (FE-TEM, Tecnai F30 S-Twin, FEI), energy dispersive X-ray spectroscopy (EDS, Sigma), and X-ray photoelectron spectroscopy (XPS, ESCALAB 210). Silver concentrations and conversions were determined using inductively coupled plasma spectrometry (ICP, JY38Plus). Average particle size and distribution were measured using particle analyzer (NICOMP™ 380 ZLS).
Results and discussion
Synthesis and characterization of silver nanoparticles
Control of reaction rate and particle size
Currently, the mechanism of biological nanoparticles synthesis is not fully understood. For gold nanoparticles synthesized extracellularly by the fungus Fusarium oxysporum, it was reported that the reduction occurs due to NADH-dependent reductase released into the solution . With Neem leaf broth, it was reported that terpenoids are believed to be the surface active molecules stabilizing the nanoparticles and reaction of the metal ions is possibly facilitated by reducing sugars and/or terpenoids present in the Neem leaf broth . Recent results with Capsicum annuum L. extract indicated that the proteins which have amine groups played a reducing and controlling role during the formation of silver nanoparticles in the solutions, and that the secondary structure of the proteins changed after reaction with silver ions . More elaborate studies are required to elucidate the mechanism of biological nanoparticles synthesis. In conclusion, an environmentally friendly method using plant extracts was proposed to synthesize silver nanoparticles. Only 11 min was required for over 90% conversion by using Magnolia leaf broth and increasing the reaction temperature to 95 °C, which was faster or comparable to the synthesis rate of chemical methods. The average particle size could be controlled from 15 to 500 nm by changing the reaction temperature, leaf broth concentration and AgNO3 concentration.