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Green synthesis of metal nanoparticles using plants Article in Green Chemistry · October 2011 DOI: 10.1039/C1GC15386B
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Green synthesis of metal nanoparticles using plants Siavash Iravani* Received 10th April 2011, Accepted 20th July 2011 DOI: 10.1039/c1gc15386b In recent years, the development of efficient green chemistry methods for synthesis of metal nanoparticles has become a major focus of researchers. They have investigated in order to find an eco-friendly technique for production of well-characterized nanoparticles. One of the most considered methods is production of metal nanoparticles using organisms. Among these organisms plants seem to be the best candidates and they are suitable for large-scale biosynthesis of nanoparticles. Nanoparticles produced by plants are more stable and the rate of synthesis is faster than in the case of microorganisms. Moreover, the nanoparticles are more various in shape and size in comparison with those produced by other organisms. The advantages of using plant and plant-derived materials for biosynthesis of metal nanoparticles have interested researchers to investigate mechanisms of metal ions uptake and bioreduction by plants, and to understand the possible mechanism of metal nanoparticle formation in plants. In this review, most of the plants used in metal nanoparticle synthesis are shown.
1.
Introduction
“Nanotechnology is the application of science to control matter at the molecular level”.1 Tremendous growth in nanotechnology has opened up novel fundamental and applied frontiers in materials science and engineering, such as nanobiotechnology,2 bionanotechnology,3 quantum dots,4 surface-enhanced Raman scattering (SERS),5 and applied microbiology. Developments in the organization of nanoscale structures into predefined superstructures ensure that nanotechnology will play a critical role in many key technologies. It is gaining importance in areas such as mechanics, optics, biomedical sciences, chemical industry, electronics, space industries, drug-gene delivery, energy science, catalysis,6,7 optoelectronic devices,8,9 photoelectrochemical applications,10 and nonlinear optical devices.11,12 For instance, nanometre-scale geranium quantum dots (less than 10 nm) could be controllably formed for novel optoelectronic device applications such as single electron transistors (SETs) and light emitters.13 The ability to tune the optical absorption/emission properties of quantum dots (semiconductor nanoparticles) by simple variation in nanoparticle size is particularly attractive in the facile band-gap engineering of materials14 and the growth of quantum dot lasers.15 Moreover, advances in nanotechnology are creating a novel class of magnetic resonance image contrast-enhancing agents such as small particles of iron oxide, fullerenes encapsulating Gd3+ ions (gadofullerenes),
Biotechnology Department, Faculty of Pharmacy and Pharmaceutical Sciences, Isfahan University of Medical Sciences, Isfahan, Iran. E-mail:
[email protected]; Fax: +98 311 6251011; Tel: +98 9132651091
2638 | Green Chem., 2011, 13, 2638–2650
and single-walled carbon nanotube nanocapsules encapsulating Gd3+ ion clusters (gadonanotubes).16 Nanoparticles are of great interest due to their extremely small size and large surface to volume ratio, which lead to both chemical and physical differences in their properties (e.g. mechanical properties, biological and sterical properties, catalytic activity, thermal and electrical conductivity, optical absorption and melting point) compared to bulk of the same chemical composition.17–19 Therefore, design and production of materials with novel applications can be achieved by controlling shape and size at nanometre scale. Nanoparticles exhibit size and shape-dependent properties which are of interest for applications ranging from biosensing and catalysts to optics, antimicrobial activity, computer transistors, electrometers, chemical sensors, and wireless electronic logic and memory schemes. These particles also have many applications in different fields such as medical imaging, nanocomposites, filters, drug delivery, and hyperthermia of tumors.20–23 There are many important applications for metal nanoparticles in medicine and pharmacy. Gold and silver nanoparticles are the most common ones used for biomedical applications and in emerging interdisciplinary field of nanobiotechnology. For instance, oligonucleotidecapped gold nanoparticles have been used for polynucleotide or protein detection using various detection/characterization methods, including atomic force microscopy, gel electrophoresis, scanometric assay, surface plasmon resonance imaging, amplified voltammetric detection, chronocoulometry, and Raman spectroscopy.24,25 Furthermore, gold nanoparticles have been employed in immunoassay,26 protein assay,27 cancer nanotechnology (especially detection of cancer cells),28 and capillary electrophoresis.29 In the field of medicine, gold nanoparticles This journal is © The Royal Society of Chemistry 2011
are used for different proposes. They can be used as markers for biological screening test. After cellular uptake, they can act as precise and powerful heaters (thermal scalpels) to kill cancer.30,31 Moreover, gold nanoparticles are capable of inducing apoptosis in B cell-chronic lymphocytic leukemia (chronic lymphoid leukemia).32 Silver nanoparticles have drawn the attention of researchers because of their extensive applications in areas such as integrated circuits,33 sensors,34 biolabelling,34 filters,34 antimicrobial deodorant fibres,35 cell electrodes,36 and antimicrobials.103,104 Antimicrobial properties of silver nanoparticles caused the use of these nanometals in different fields of medicine, various industries, animal husbandry, packaging, accessories, cosmetics, health and military. Silver nanoparticles show potential antimicrobial effects against infectious organisms such as Escherichia coli, Bacillus subtilis, Vibria cholera, Pseudomonas aeruginosa, Syphillis typhus, and Staphylococcus aureus.37,38 Nanoparticles have been produced physically and chemically for a long time, but recent developments show the critical role of microorganisms and biological systems in production of metal nanoparticles (Fig. 1 and 2). The use of organisms in this area is rapidly developing due to their growing success and ease of formation of nanoparticles. Moreover, biosynthesis of metal nanoparticles is an environmentally friendly method (green chemistry) without use of harsh, toxic and expensive chemicals.39–42 For instance, production of silver nanoparticles by chemical reduction (e.g., hydrazine hydrate, sodium borohydride, DMF, and ethylene glycol) may lead to absorption of harsh chemicals on the surfaces of nanoparticles raising the toxicity issue. The organisms used in nanoparticle synthesis vary from simple prokaryotic bacterial cells to complex eukaryotes.43 Actually, the ability of organisms in production of metal nanoparticles has opened a new exciting approach toward the development of these natural nano-factories. The important aspects which
might be considered in the process of producing highly stable and well-characterized nanoparticles are as follows: 1. Selection of the best organisms: In order to choose the best candidates for metal nanoparticle production, researchers have focused on the important intrinsic properties of the organisms such as enzyme activities and biochemical pathways. For example, plants which have great potential in heavy metal accumulation and detoxification are the best candidates for nanoparticle synthesis. 2. Optimal conditions for cell growth and enzyme activity: Optimization of the growth conditions is very important. The nutrients, inoculum size, light, temperature, pH, mixing speed, and buffer strength should be optimized. The presence of the substrates or related compounds in subtoxic levels from the beginning of the growth would increase the activity of the enzymes. 3. Optimal reaction conditions: In order to use the organisms for synthesis of metal nanoparticle in industrial scale, the yield and the production rate are important issues to be considered. Therefore, we need to optimize the bioreduction conditions in the reaction mixture.39,43 The substrate concentration, the biocatalyst concentration, the electron donor and its concentration, pH, exposure time, temperature, buffer strength, mixing speed, and light need to be controlled and optimized. For example, it was demonstrated that reaction time and dodecanethiol (as the capping ligand) were important parameters in controlling size and morphology of the gold nanoparticles produced by reaction of the AuCl4 - ions with Bacillus megatherium biomass.44 Moreover, researchers have used some complementary factors such as visible light or microwave irradiation, and boiling which could affect the size, morphology, and rate of reaction. Optimization of these crucial factors could control morphologies and other properties of nanoparticles. This exciting improvement toward the use of eco-friendly methods for production of nanoparticles with desired morphological characteristics and
Fig. 1 Some important manufacturing methods used in nanoparticle synthesis.
This journal is © The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 2638–2650 | 2639
Fig. 2 Some important synthetic methods for synthesis of gold, silver, palladium, zinc oxide, copper, magnetite, and indium oxide nanoparticles.
sizes might facilitate researcher’s abilities to overcome many limitations of this field. Biosynthesis of metal nanoparticles by plants is currently under exploitation. The biological synthesis of metal nanoparticles (especially gold and silver nanoparticles) using plants (inactivated plant tissue, plant extracts and living plant) has received more attention as a suitable alternative to chemical procedures and physical methods. Synthesis of metal nanoparticles using plant extracts is very cost effective, and therefore can be used as an economic and valuable alternative for the large-scale production of metal nanoparticles. Extracts from plants may act both as reducing and capping agents in nanoparticle synthesis. The bioreduction of metal nanoparticles by combinations of biomolecules found in plant extracts (e.g. enzymes, proteins, amino acids, vitamins, polysaccharides, and organic acids such as citrates) is environmentally benign, yet chemically complex. Because of the important and critical roles of plants in bio-based protocols for metal nanoparticle production, the green synthesis of metal nanoparticles using plants has been discussed in this review.
2.
Plant biomass and/or living plant
Plants have shown great potential in heavy metal accumulation and detoxification.45 Several studies widely reported detoxification and hyper-accumulation of toxic metals by plants, such as Arabidopsis halleri and Thlaspi caerulescens.46,47 Different kinds of plants such as Acanthopanax sciadophylloides, Maytenus 2640 | Green Chem., 2011, 13, 2638–2650
founieri, Brassica juncea, Ilex crenata, Sesbania drummondii, and Clethra barbinervis have potential capacity for phytoremediation of heavy metals.45–48 Trace elements (heavy metals and metalloids) are important environmental pollutants, and are toxic even at very low concentrations. The use of plant biomass for metal removal from aqueous solutions (known as biosorption) gained attention because it has shown to be very promising for the removal of contaminants from effluents in an eco-friendly approach. The natural phenomenon of heavy metal tolerance of plants has interested researchers to investigate the related biological mechanisms as well as physiology and genetics of metal tolerance in hyperaccumulator plants (Fig. 3). Researchers pay attention to use of plants with potential in phytomining and phytoremediation of heavy metals in order to phytosynthesise metallic nanoparticles. Gardea-Torresdey et al. have reported for the first time the formation of gold and silver nanoparticles inside living plants.49,50 They have demonstrated the synthesis of gold and silver nanoparticles within live Medicago sativa (alfalfa) plants by gold and silver ion uptake, respectively, from solid media. The alfalfa plants were grown in an AuCl4 - -rich environment. The information about uptake and formation of these nanoparticles was confirmed by X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM). The gold nanoparticles were in crystalline state. Additionally, icosahedral particles (~4 nm) and face centered cubic (fcc) twinned particles (6–10 nm) were observed. In a related report, agricultural biomass was used to This journal is © The Royal Society of Chemistry 2011
Fig. 3 Important mechanisms of heavy metal tolerance in plants.
reduce Cr(VI) to Cr(III) ions, indicating that biological methods could be very efficient in decontaminating polluted waters and soil polluted with heavy metal ions.51 Morphologies of gold particles obtained by reacting gold(III) with alfalfa biomass include irregular shaped, fcc tetrahedral and hexagonal platelet particles which were larger than decahedral and icosahedral multiple twinned particles.52 Magnetite nanoparticles attracted great attention for many important technological and biomedical applications such as drug delivery, cancer hyperthermia, optical and nanoelectronic devices, magnetic separation, and magnetic resonance imaging enhancement.53 Various chemical and physical methods have been reported in order to synthesise magnetite nanoparticles, but some of the chemical procedures involved in the synthesis of these nanoparticles use toxic solvents which could potentially generate unsafe and hazardous byproducts, and often involve high energy consumption.54–56 New methods for growing nanoparticles are exploring the use of biological systems. It was reported that iron oxide nanoparticles could be synthesized by using alfalfa biomass.57 pH 10 yielded smaller particles with greater proportion of the Fe2 O3 , and the size could be controlled in the range of 1–4 nm. When pH decreased (pH 5), larger nanoparticles were produced. In another study, AuFe3 O4 composite nanoparticles were prepared with a combined chemical and biological reducing process (semi-biosynthesis method). Magnetic cores were primarily produced using a fabrication method consisting of coprecipitation of Fe2+ and Fe3+ . An ethanol extract of Eucalyptus camaldulensis was used for the reduction of Au+3 on the surface of the magnetite nanoparticles and for the functionalization of the Au-Fe3 O4 nano-composite particles.58 Armendariz et al.59 reported for the first time the formation of rod-shaped nanoparticles by biomaterials. They characterized the gold nanoparticles formed by wheat biomass exposed to a 0.3 mM potassium tetracholoaurate solution at pH values of 2–6 at room temperature. It was concluded that wheat biomass was able to reduce Au(III) to Au (0) forming fcc tetrahedral, hexagonal, decahedral, icosahedral multitwinned, irregular, and rod-shaped nanoparticles. In another study, pH This journal is © The Royal Society of Chemistry 2011
dependent synthesis of rod-shaped Au nanoparticles using Avena sativa (oat) has shown that biomass might carry more positive functional groups such as positive amino groups, sulfhydryl groups and carboxylic groups which allowed the Au(III) ions to get more closure to binding sites and approved the reduction of Au(III) to Au(0).60,61 A 0.1 mM solution of Au(III) was reacted with powdered oat biomass at pH values of 2–6 for one hour. As in the case of wheat, oat biomass produced fcc tetrahedral, hexagonal, decahedral, icosahedral multitwinned, irregular, and rod-shaped nanoparticles. It was reported that most of the nanoparticles synthesized by using alfalfa, wheat, and oat at pH 2 had an irregular shape. However, it seems that pH has a major impact on the size of the produced nanoparticles rather than on the shape of them. Sterilized geranium leaves (P. graveolens) when exposed to chloroaurate ions separately resulted in rapid reduction of the metal ions and formation of stable gold nanoparticles of variable size. The reduction of the AuCl4 - ions was nearly complete after 60 min of reaction and the particles (20–40 nm) were predominantly decahedral and icosahedral in shape.62 Not only silver nanoparticles (55 to 80 nm) could be produced, but also triangular or spherical gold nanoparticles could be easily formed by reaction of the novel sundried biomass of Cinnamomum camphora leaf with aqueous silver or gold precursors at ambient temperature.42 Size dispersity of quasispherical silver nanoparticles as well as triangular or spherical shapes of gold nanoparticles could be facilely controlled by simple variation of the amount of biomass reacting with aqueous solution of AgNO3 or HAuCl4 . When Huang et al. switched the amount of the dried biomass from 0.1 to 0.5 g subjected to the same chloroauric acid, the particles shifted from nanotriangles to spherical particles. The utilization of sundried C. camphora leaf for biosynthesis of the nanoparticles has some defects. For example, the drying process of the leaf in the sun was time-consuming. The polyol components and the water soluble heterocyclic components were mainly found to be responsible for the reduction of silver or chloroaurate ions and the stabilization of the nanoparticles, respectively. Huang et al.63 investigated biological production of silver nanoparticles by lixivium of sundried C. camphora leaf in continuous-flow tubular microreactors. They introduced polyols in the lixivium as possible reducing agents. Harris et al.64 have investigated the limits (substrate metal concentration and time exposure) of uptake of metallic silver by two common metallophytes, Brassica juncea and Medicago sativa. They demonstrated that B. juncea and M. sativa could be used in phytosynthesis (a broad application of phytoextraction) of metallic silver nanoparticles. B. juncea, when exposed to an aqueous substrate containing 1000 ppm silver nitrate for 72 h, accumulated up to 12.4 wt% silver. M. sativa accumulated up to 13.6 wt% silver when exposed to an aqueous substrate containing 10 000 ppm silver nitrate for 24 h. In the case of M. sativa, an increase in metal uptake was observed with a corresponding increase in the exposure time and substrate concentration. In both cases, TEM analysis showed the presence of roughly spherical silver nanoparticles, with a mean size of 50 nm. In another study, after a 9-week growth in gold, silver, and copper-enriched soil, seeds of B. juncea grow into a plant containing Au–Ag–Cu alloy nanoparticles.65 Green Chem., 2011, 13, 2638–2650 | 2641
Sesbania drummondii is a medium-sized perennial shrub in the legume family Fabaceae. S. drummondii (leguminous shrub) seedlings could uptake high amounts of gold(III) ions, resulting in intracellular formation of monodispersed spherical gold nanoparticles (6–20 nm) inside plant cells or tissues.66 The catalytic function of the nanoparticle-rich biomass was substantiated by the reduction of aqueous 4-nitrophenol (hazardous and toxic pollutant). Attractively, nanoparticle-bearing biomatrix of S. drummondii reduced aqueous 4-nitrophenol (4hydroxynitrobenzene).
3.
Plant extracts
Current research in biosynthesis of nanometals using plant extracts has opened a new era in fast and nontoxic methods for production of nanoparticles. Many researchers have reported the biosynthesis of metal nanoparticles by plant leaf extracts and their potential applications.40,41,62,67–72 Sastry et al. have studied bioreduction of gold and silver ions by leaf broth of Pelargonium graveolens62,67 and Azadirachta indica.71 Moreover, they have explored the formation mechanism of triangular gold nanoprisms by Cymbopogon flexuosus (lemongrass) extracts, the nano-triangles seemed to grow by a process involving rapid bioreduction, assembly and room-temperature sintering of ‘liquid-like’ spherical gold nanoparticles.40 Also rapid synthesis of stable gold nanotriangles using Tamarindus indica (tamarind) leaf extract as reducing agent could be achieved.72 The shape of metal nanoparticles considerably changed their optical and electronic properties.73 They have demonstrated synthesis of gold and silver nanoparticles with variety of shapes (spherical and triangular) and sizes using Aloe vera plant extracts, as well.68 It was explained that only biomolecules of molecular weights less than 3 kDa caused reduction of chloroaurate ions, leading to the formation of gold nanotriangles. Nevertheless, the bioreduction of silver ions proceeded merely in the presence of ammonia. The aqueous solution of gold ions when exposed to Coriandrum sativum leaf extract was reduced and resulted in the extracellular biosynthesis of gold nanoparticles with spherical, triangle, truncated triangle and decahedral morphologies ranging from 6.75 nm to 57.91 nm. These nanoparticles were stable in solution over a period of one month at room temperature.74 Shankar et al.71 have reported the biosynthesis of pure metallic nanoparticles of silver and gold by the reduction of aqueous Ag+ and AuCl4 - ions and also the synthesis of bimetallic core-shell nanoparticles of gold and silver by simultaneous reduction of aqueous Ag+ and AuCl4 - ions with the broth of neem leaves (A. indica). They observed that the metal particles were stable in solution even 4 weeks after their synthesis. Moreover, stabilizing the nanoparticles was possibly facilitated by reducing sugars and/or terpenoids present in the neem leaf broth. The silver nanoparticles formed were predominantly spherical and polydisperse with diameters in the range 5 to 35 nm. Gold nanoparticles synthesized using neem leaf broth appeared to have a propensity to form thin, planar structures rather than just spherical particles. The planar particles formed were predominantly triangular with a very small percentage of hexagonal shaped particles.71 Geranium leaf (P. graveolens) broth, when exposed to aqueous silver nitrate solution, resulted in extracellular enzymatic synthe2642 | Green Chem., 2011, 13, 2638–2650
sis of stable crystalline silver nanoparticles.67 The bioreduction of the metal ions was fairly rapid, occurred readily in solution, and resulted in a high density of stable silver nanoparticles in the size range 16–40 nm. The produced nanoparticles appeared to be assembled into open, quasilinear superstructures and were predominantly spherical in shape. It was believed that proteins, terpenoids and other bio-organic compounds in the geranium leaf broth participated in the bioreduction of silver ions and in the stabilization of the nanoparticles thus formed by surface capping. Shankar et al.62 reported the possibility of terpenoids from geranium leaf in the silver nanoparticle synthesis. Polyols such as terpenoids, polysaccharides, and flavones in the C. camphora leaf were believed to be the main cause of the reduction of silver and chloroaurate ions.42 Moreover, green synthesis of nanosilver particles using methanol extract of Eucalyptus hybrida (safeda) leaf was reported. Flavanoid and terpenoid constituents which present in E. hybrid leaf extract are responsible for stabilization of produced silver nanoparticles (50–150 nm).75 Cinnamon zeylanicum bark extract could be used in biosynthesis of cubic and hexagonal silver nanocrystals (31– 40 nm).76 The particle size distribution varied with variation in the dosage of C. zeylanicum bark extract. The number of particles increased with increasing dosage due to the variation in the amount of reductive biomolecules. Small nanoparticles were formed at high pH. The shape of silver nanoparticles at high pH was more spherical in nature rather than ellipsoidal. Moreover, bactericidal effect of produced nano-crystalline silver particles was tested against E. coli strain. As a result, the various tested concentrations of 2, 5, 10, 25, and 50 mg L-1 produced inhibition of 10.9, 32.4, 55.8, 82, and 98.8%, respectively. The minimum inhibitory concentration was found to be 50 mg L-1 . C. zeylanicum bark is rich in terpenoids (linalool, methyl chavicol, and eugenol) and in chemicals (such as cinnamaldehyde, ethyl cinnamate, and b-caryophyllene) which contribute to its special aroma. Furthermore, proteins are also present in the bark. It was believed that water-soluble organics present in C. zeylanicum bark were the reasons of the bioreduction of silver ions to nanosized Ag particles. Moreover, proteins from C. zeylanicum bark capped the produced nanoparticles either through free amine groups or cysteine residues, and thus stabilized them.76 Complimentary investigations have explained that Capsicum annuum L. extract contains proteins with amine groups, which played a reducing and controlling role during the formation of silver nanoparticles in the solutions.77 Li et al.78 have shown rapid precipitation of a-Se/protein in room temperature using proteins extracted from C. annuum L. They also demonstrated that proteins and vitamin C presented in C. annuum L. extract were responsible for the synthesis of a-Se nanoparticles. The proteins also stabilized nanoparticles via precipitation on their surfaces and formation of a-Se/protein composites. The size and shell thickness of the a-Se/protein composites were increased in high concentrations of C. annuum L. extract, but decreased at low pH values. The reaction of aqueous silver ions with Desmodium trifolium extract resulted in extracellular production of silver nanoparticles at room temperature.79 The authors of this article believed that H+ ions produced along with NAD during glycolysis were responsible for formation of nanosilver particles as well as water soluble antioxidative agents (e.g. ascorbic acids). These This journal is © The Royal Society of Chemistry 2011
antioxidative agents were especially participating in reduction of silver ions. Jha et al.80 used biotechnological method for production of silver nanoparticles. They demonstrated that Cycas leaf extract could be used in order to produce stable silver nanoparticles. X-Ray data indicated that silver nanoparticles had fcc unit cell structure. Phytochemicals such as polyphenols, glutathiones, metallothioneins (a family of cysteine-rich, low molecular weight proteins), and ascorbates probably were responsible for formation of the nanoparticles. Glutathione was implicated to play an important role in plants exposed to metal stress, and metallothioneins have the capacity to bind both xenobiotic (e.g. cadmium, silver, mercury, and arsenic) and physiological (e.g. zinc, selenium, and copper) heavy metals through the thiol group of their cysteine residues. Antioxidant action of phenolic compounds (these compounds consist of catechin, taxifolin, procyanidins of various chain lengths formed by catechin and epicatechin units, and phenolic acids) is due to their high tendency to chelate metals. Phenolic compounds possess hydroxyl and carboxyl groups, may inactivate iron ions by chelating and additionally suppressing the superoxidedriven Fenton reaction, which is believed to be the most important source of reactive oxygen species (ROS). Therefore, plants with high content of phenolic compounds (e.g. Pinus species) are one of the best candidates for nanoparticle synthesis. High density of extremely stable silver nanoparticles (16–40 nm) were rapidly synthesized by challenging silver ions with Datura metel (a plant of family Solanaceae) leaf extract.81 Leaf extracts of this plant contain biomolecules such as alkaloids, proteins/enzymes, amino acid, alcoholic compound, and polysaccharides which could be used as reluctant to react with silver ions and scaffolds to direct the formation of silver nanoparticles in solution. Quinol (alcoholic compound) and chlorophyll pigment were responsible for reduction of silver ions and stabilization of produced nanoparticles. The use of pure natural constituents to reduce and stabilize the metal nanoparticles is under investigation. Kasthuri et al.82 have shown the ability of apiin compound extracted from henna leaves to produce anisotropic gold and quasi-spherical silver nanoparticles. Secondary hydroxyl and carbonyl groups of apiin compound were responsible for the bioreduction of metal salts. In order to control the size and shape of nanoparticles, they used different amounts of apiin compound (as the reducing agent). The nanoparticles were stable in water for 3 months, which could be attributed to surface binding of apiin compound with the reduced materials. In another study, geraniol as a volatile compound obtained from P. graveolens was used for biosynthesis of silver nanoparticles (1–10 nm). The cytotoxicity of the produced silver nanoparticles was evaluated in vitro against Fibrosarcoma Wehi 164 at different concentration (1–5 g ml-1 ).83 As a result, the presence of 5 g ml-1 of silver nanoparticles significantly inhibited the cell line’s growth (up to 60%). Therefore, it seems that silver nanoparticles have inhibitory effects against the proliferation of cancer cells. Bayberry tannin (a natural plant polyphenol) was used for the one-step synthesis of Au@Pd core– shell nanoparticles in aqueous solution at room temperature. Bayberry tannin is able to preferentially reduce Au3+ ions to Au nanoparticles when placed in contact with an Au3+ /Pd2+ mixture, and subsequently the formed gold nanoparticles served This journal is © The Royal Society of Chemistry 2011
as in situ seeds for the growth of a Pd shell, resulting in the formation of Au@Pd nanoparticles.84 Song et al.85 have elucidated that Pinus desiflora, Diospyros kaki, Ginko biloba, Magnolia kobus and Platanus orientalis leaves broth synthesized stable silver nanoparticles with average particle size ranging from 15 to 500 nm, extracellularly. In the case of M. kobus and D. kaki (persimmon) leaf broth, synthesis rate and final conversion to silver nanoparticles became faster when reaction temperature increased. But the average particle sizes produced by D. kaki leaf broth decreased from 50 nm to 16 nm when temperature increased from 25 ◦ C to 95 ◦ C. They also illustrated that only 11 min was required for more than 90% conversion at the reaction temperature of 95 ◦ C using M. kobus leaf broth.85 Bimetallic Au/Ag nanoparticles were formed with some cubic structures, by the interactions between the bound bio-organic capping molecules and gold and silver nanoparticles.86 In another study, it was reported that M. kobus and D. kaki were capable of ecofriendly extracellular production of metallic gold nanoparticles (5–300 nm) with different triangular, pentagonal, hexagonal, and spherical shapes within a few minutes (for up to 90% conversion at a reaction temperature of 95 ◦ C).87 It was suggested that the rate of synthesis of the nanoparticles was related to the reaction and incubation temperature, and increased temperature levels allowed nanoparticle growth at a faster rate. Moreover, by increasing the temperatures and leaf broth concentrations, size of nanoparticles became smaller. Fourier transformed infrared spectroscopy (FTIR) analysis has shown that gold nanoparticles produced by M. kobus extract were surrounded by proteins and metabolites (such as terpenoids having functional groups of amines, aldehydes, carboxylic acid, and alcohols). It was also reported that the use of low concentration of phyllanthin extract reacting with HAuCl4 led to production of hexagonal or triangular gold nanoparticles, but spherical nanoparticles could be formed by addition of higher concentration of the extract.87 Vilchis-Nestor et al.88 have used green tea (Camellia sinensis) extract to produce gold nanoparticles and silver nanostructures in aqueous solution at ambient conditions. They also investigated control of size, morphology, and optical properties of the nanostructures and reported initial concentrations of metal ions and tea extract as controlling factors. It was investigated that when the amount of C. sinensis extract was increased, the resulted nanoparticles were slightly bigger and more spherical. The authors of this study believed that phenolic acid-type biomolecules present in C. sinensis extract were responsible for production and stabilization of silver and gold nanoparticles. Caffeine and theophylline present in tea extracts might be responsible for catalysis and synthesis of nanoparticles. In another study, black tea leaf extracts were used in production of Au and Ag nanoparticles.89 The nanoparticles are stable and have different shapes, such as spheres, trapezoids, prisms, and rods. Findings of this study have demonstrated that polyphenols and flavonoids were responsible for synthesis of nanoparticles. Mode et al.90 reported production of spherical silver nanoparticles (60–80 nm) using callus extract of Carica papaya. Proteins and other ligands seemed to be responsible for the synthesis and stabilization of silver nanoparticles. Furthermore, fcc silver nanoparticles (10–20 nm) were synthesized by using the latex of Jatropha curcas as reducing and capping agent.91 It was Green Chem., 2011, 13, 2638–2650 | 2643
demonstrated that leaf extracts from the aquatic medicinal plant, Nelumbo nucifera (Nymphaeaceae), could be able to reduce silver ions and produce silver nanoparticles (with an average size of 45 nm) in different shapes.92 Biosynthesized silver nanoparticles showed larvicidal activity against malaria (Anopheles subpictus) and filariasis (Culex quinquefasciatus) vectors. Silver and gold nanoparticles were synthesized biologically by using Sorbus aucuparia leaf extract within 15 min. Produced nanoparticles were found to be stable for more than 3 months. The authors of this study believed that sorbate ion in the leave extract of S. aucuparia encapsulated the nanoparticles and this matter was responsible for maintenance of the stability.93 The effect of leaf extract quantity, substrate concentration, temperature, and contact time were also evaluated to optimize the process of producing nanoparticles. With increase in the concentration of metal ions from 10-4 to 10-2 M, increase in particle size was also found. Biosynthesis of silver and gold nanoparticles using fruit extracts has been investigated by some authors. For example, Emblica officinalis fruit extract produced highly stable Ag and Au nanoparticles extracellularly.41 Pear fruit extract could be used in order to room-temperature biosynthesis of triangular and hexagonal gold nanoparticles (200–500 nm).94 Pear extract contains essential phytochemicals consisting of organic acids, peptides, proteins, and amino acids. In addition, it contains saccharides which provide synergetic reduction power for the bioreduction of chloroaurate ions into gold nanoparticles. The pear fruit extract when exposed to chloroaurate ions in an alkaline condition resulted in gold nanoparticles with plate-like morphologies in a highly productive state. The gold nanoparticles formed under normal conditions also exhibited plate-like morphologies with a low productivity. In addition, green synthesis of silver and gold nanoparticles with spherical and triangular shapes by fruit extract of Tanacetum vulgare has been reported.95 Ag and Au nanoparticles were 16 and 11 nm in size. Carbonyl group was involved in synthesis of these nanoparticles. The effect of pH on zeta potential of the produced nanoparticles has been investigated. Zeta potential values reveal details about the surface charge and stability of the synthesized metal nanoparticles.95 Actually, stability of produced metal nanoparticles are evaluated with zeta potentiometer and corresponding surface plasmon spectra.96,97 Ag nanoparticles demonstrated lower zeta potential value at strongly acidic pH, but when the pH increased higher zeta potential values were obtained. Furthermore, larger particle size could be achieved by decreasing the pH. It has been indicated that larger quantities of T. vulgare extract leads to an increase in peak absorbance in UV/Vis spectrum. Moreover, decrease in particle size of Ag and Au nanoparticles has been reported due to an increase in extract amount.95 The possibility of controlling the properties of nanoparticles by changing the composition of the reaction mixture has resulted in the use of different amount of biomass or cell extract and substrate concentration in order to formation of nanoparticles with desired shape and size. It appears that the particle size decreased with an increase in the leaf broth concentration or decrease in the concentration of metal ions. Moreover, the rate of formation of metal nanoparticles was found to be slower at lowest concentration.93,96,98,99 Bankar et al.99 reported banana peel (Musa paradisiaca) extract as a new source 2644 | Green Chem., 2011, 13, 2638–2650
for synthesis of silver nanoparticles. The banana peels were washed and boiled in distilled water for 30 min at 90 ◦ C. It is well known that the temperature at which the reaction mixtures are incubated affects the process of metal reduction. When the temperature is increased, the reactants are consumed rapidly eventually leading to the production of smaller nanoparticles. The produced silver nanoparticles displayed antifungal activity against C. albicans and C. lipolytica, and antibacterial activity against E. coli, Shigella sp., Klebsiella sp. and E. aerogenes. The antimicrobial activity of the banana peel extract-mediated silver nanoparticles was comparable to that synthesized by the trisodium citrate method. Nanoparticles of silver were prepared by using Bryophyllum sp., Cyperus sp., and Hydrilla sp. plant extracts.100 X-Ray analysis demonstrated that silver nanoparticles (2–5 nm) have fcc unit cell structure. The reduction of silver ions was due to water-soluble phytochemicals such as flavones, quinones, and organic acid (e.g., oxalic, malic, tartaric, and protocatecheuic acid) present in plant tissues. It was demonstrated that silver nanoparticles might have resulted due to different aforementioned metabolites or fluxes and other oxido-reductively labile metabolites such as ascorbates or catechol/protocatecheuic acid.100 In the case of Chenopodium album (an obnoxious weed), organic acids were also responsible for nanoparticle biosynthesis. The plant leaf has a high level of oxalic acid as well as lignin which can act as reducing agents. C. album leaf extract was used for the single-pot bio-inspired synthesis of spherical silver and gold nanoparticles.96 Quasi-spherical shapes were observed for produced nanoparticles within range of 10–30 nm. Deshpande et al.101 have investigated the rapid and extracellular formation of gold nanocrystals by using dried clove buds (Syzygium aromaticum) solution within a few minutes. The XRD (X-ray diffraction) and EDAX (energy dispersive X-ray analysis) demonstrated that the particles were crystalline in nature. The probable biochemical pathway of biosynthesis was studied using FTIR. Water-soluble flavonoids of clove buds were responsible for bioreduction and stabilization of nanoparticles (colloidal solution was stable more than 6 weeks). Kumar et al.102 reported that only high polar soluble constituents of Syzygium cumini were responsible for synthesis of silver nanoparticles. It was also demonstrated that the amount of polyphenols and biochemical constituents could be one of the important parameters affecting the size and distribution of silver nanoparticles. The various synthetic and natural polymers such as poly(ethylene glycol), poly-(N-vinyl-2-pyrrolidone), starch, heparin, poly-cationic chitosan (1-4-linked 2-amino-2-deoxy-b-Dglucose), sodium alginate (a polysaccharide gum derived from the cell walls of brown algae), and gum acacia have been reported as reducing and stabilizing agents for the biosynthesis of gold and silver nanoparticles. Kora et al.98 reported that monodispersed and spherical silver nanoparticles (3 nm) were biosynthesized by using gum kondagogu (non-toxic polysaccharide derived as an exudate from the bark of Cochlospermum gossypium). It was suggested that carboxylate and hydroxyl groups were involved in complexation and bioreduction of silver ions into nanoparticles. This method is compatible with green chemistry principles as the gum serves a matrix for both bioreduction and stabilization of the synthesized nanoparticles. Due to the availability of low cost plant derived biopolymer, this This journal is © The Royal Society of Chemistry 2011
method could be implemented for the large scale production of highly stable monodispersed nanoparticles. The formed silver nanoparticles with an average size of 4.5 nm were used for checking the antibacterial activity. As a result, the nanoparticles had significant antibacterial effects against gram negative (E. coli and P. aeruginosa) and positive (S. aureus) bacteria. Acalypha indica (Euphorbiaceae) leaf extracts have produced silver nanoparticles (20–30 nm) within 30 min.103 These nanoparticles had excellent antimicrobial activity against water borne pathogens E. coli and V. cholera (minimum inhibitory concentration (MIC) = 10 mg ml-1 ). Furthermore, spherical silver nanoparticles (40–50 nm) were produced using leaf extract of Euphorbia hirta.104 These nanoparticles had potential and effective antibacterial property against Bacillus cereus and S. aureus. In another study, silver nanoparticles (with an average size of 57 nm) were produced when 10 ml of Moringa oleifera leaf extract was mixed to 90 ml of 1 mM aqueous of AgNO3 and was heated at 60–80 ◦ C for 20 min. The formed nanoparticles had considerable antimicrobial activity against pathogenic microorganisms, including S. aureus, Candida tropicalis, K. pneumoniae, and C. krusei.105 It has been reported that cotton fibers loaded with biosynthesized silver nanoparticles (~20 nm) using natural extracts of Eucalyptus citriodora (neelagiri) and Ficus bengalensis (marri) had excellent antibacterial activity against gram-negative E. coli bacteria. These fibers have potential for utilization in burn/wound dressings as well as in the fabrication of antibacterial textiles and finishings.106 Garcinia mangostana (mangosteen) leaf extract could be used as reducing agent in order to synthesize silver nanoparticles. The aqueous silver ions when exposed to leaf extract were reduced and resulted in silver nanoparticles with an average size of 35 nm. These nanoparticles had high effective antimicrobial activity against E. coli and S. aureus.107 It was reported that Ocimum sanctum (tulsi) leaf extract could reduce silver ions into crystalline silver nanoparticles (4–30 nm) within 8 min of reaction time. These nanoparticles were stable due to the presence of proteins which may act as capping agent. O. sanctum leaves contain ascorbic acid which may play a role in bioreduction of silver ions into metallic nanoparticles. Biosynthesized silver nanoparticles have shown strong antimicrobial activity against both gram-negative (E. coli) and gram-positive (S. aureus) microorganisms.108 Furthermore, biosynthesis of silver nanoparticles by Cacumen platycladi extract was investigated. Reducing sugars and flavonoids in the extract were mainly responsible for the bioreduction of the silver ions, and their reductive capability promoted at 90 ◦ C, leading to the formation of silver nanoparticles (18.4 ± 4.6 nm) with narrow size distribution. The nanoparticles had significant antibacterial activity against E. coli and S. aureus.109 Ultrafine palladium (Pd) nanoparticles have application both in heterogeneous and homogeneous catalysis, due to their high surface-to-volume ratio and high surface energy.110 The common synthetic methods for synthesis of palladium nanoparticles involve chemical reduction of Pd(II) by alcohol,111 polyethylene glycol,112 NaBH4 /ascorbic acid113 or CNCH2 COOK,114 thermally induced reduction of Pd(Fod)2 in o-xylene,115 reduction of Pd(OAc)2 by dimethylamine-borane in supercritical carbon dioxide (CO2 ),116 and sonochemical reduction of Pd(NO3 )2 .117 Sathishkumar et al.118 have investigated phyto-crystallization of palladium through a reduction process using C. zeylanicum bark This journal is © The Royal Society of Chemistry 2011
extract. As a result, nano-crystalline palladium particles (15– 20 nm) were successfully synthesized. It was demonstrated that reaction conditions such as pH, temperature, and biomaterial dosage had no major effects on the shape and size of produced nanoparticles. In another study, nano-crystalline palladium particles (10–15 nm) have been synthesized using Curcuma longa tuber extract as biomaterial. pH and temperature had no major effect on size and shape of the nanoparticles. It was found that the zeta potential of formed palladium nanoparticles was negative and it increased with increase in pH. It has been also reported that palladium nanoparticles could be synthesized by using coffee and tea extract. The produced nanoparticles were in the size range of 20–60 nm and crystallized in face centered cubic symmetry.119 It was reported that crude water extract of Gardenia jasminoides Ellis could be used for bioreduction of palladium chloride. Identified antioxidants, including geniposide, chlorogenic acid, crocins and crocetin were reducing and stabilizing agents for synthesizing palladium nanoparticles (3– 5 nm) in water crude extract. The particle size and disparity of nanoparticles were temperature dependent, and the best dispersity was revealed at 70 ◦ C.120 Indium oxide (In2 O3 ) is an important n-type semiconductor which has interesting properties such as high transparency to visible light, strong interaction between certain poisonous gas molecules and its surface, and high electrical conductance.121,122 Indium oxide is an intersecting material for a wide range of applications such as solar cells, organic light emitting diodes, architectural glasses, panel displays, and field emission.123–125 Indium oxide nanoparticles have been intensively studied to be used as promising materials for gas sensor applications.126 In addition to chemical synthetic methods, these nanoparticles could be biologically synthesized using A. vera (Aloe barbadensis Miller) plant extracted solution. In2 O3 nanoparticles (5–50 nm) have been produced by using indium acetylacetonate and A. vera plant extracted solution.126 The nanoparticles are formed after calcination of the dried precursor of indium oxide in air at 400–600 ◦ C. The morphology and size of indium oxide materials were affected by the temperature of calcination. Copper (Cu) nanoparticles were biosynthesized using magnolia leaf extract. When aqueous solutions of CuSO4 ·5H2 O treated with the leaf extract, stable copper nanoparticles (40–100 nm) were formed. Foams coated with biologically synthesized copper nanoparticles showed higher antibacterial activity against E. coli cells.127 In another study, extracellular production of copper nanoparticles was carried out using stem latex of a medicinally important plant, Euphorbia nivulia. The produced nanoparticles were stabilized and subsequently capped by peptides and terpenoids present within the latex. The copper nanoparticles are toxic to adenocarcinomic human alveolar basal epithelial cells (A549 cells) in a dose-dependent manner. It was concluded that the non-toxic aqueous formulation of latex capped copper nanoparticles could be directly used for administration/in vivo delivery of nanoparticles for cancer therapy.128 Zinc oxide (ZnO) is an attractive semiconductor material for photonic and nano-electronic applications.129 In addition to various chemical and physical synthetic approaches (e.g., sol-gel, vapor phase oxidation, micro-emulsion, thermal vapor transport and condensation, sono-chemical, precipitation, hydrothermal, and polyol methods), ZnO nanoparticles could be Green Chem., 2011, 13, 2638–2650 | 2645
Table 1 Important examples of nanoparticle biosynthesis using plants Plant origin
Nanoparticle
Size (nm)
Morphology
References
Aloe vera Aloe vera (Aloe barbadensis Miller) Acalypha indica Apiin extracted from henna leaves Apiin extracted from henna leaves Avena sativa (oat)
gold & silver indium oxide silver silver gold gold
spherical, triangular spherical spherical spherical, triangular quasi-spherical rod-shaped
68
Azadirachta indica (neem)
spherical, triangular, hexagonal
71
Black tea leaf extracts Brassica juncea (mustard) Bryophyllum sp. Camellia sinensis (green tea) Carica papaya Chenopodiumalbum Cinnamomum camphora
gold, silver & silver-gold alloys gold & silver silver silver gold silver gold & silver gold & silver
— 5–50 20–30 39 7.5–65 5–20 (pH 3 & 4), 25–85 (pH 2) 5–35 & 50–100
89
Cinnamomum camphora Cinnamon zeylanicum (cinnamon) Cinnamon zeylanicum (cinnamon) Citrus limon (lemon) Cochlospermum gossypium Coriandrum sativum (coriander)
palladium silver palladium silver silver gold
3.2–6 31–40 15–20
