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The greener synthesis of nanoparticlesOxana V. Kharissova1, H.V. Rasika Dias2, Boris I. Kharisov3, Betsabee Olvera Perez3,and Victor M. Jimenez Perez3

1 Universidad Auto noma de Nuevo Leo n, Facultad de Ciencias Fısico-Matema ticas, CIIDIT, Ciudad Universitaria, Monterrey,

NL 66450, Mexico2 Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, TX 76019, USA3 Universidad Auto noma de Nuevo Leo n, Facultad de Ciencias Quımicas, CIIDIT, Ciudad Universitaria, Monterrey, NL 66450,

Mexico

In this review, we examine ‘greener’ routes to nanopar-ticles of zerovalent metals, metal oxides, and salts withan emphasis on recent developments. Products fromnature or those derived from natural products, such asextracts of various plants or parts of plants, tea, coffee,banana, simple amino acids, as well as wine, table sugarand glucose, have been used as reductants and as cap-ping agents during synthesis. Polyphenols found in plantmaterial often play a key role in these processes. Thetechniques involved are simple, environmentally friend-ly, and generally one-pot processes. Tea extracts withhigh polyphenol content act as both chelating/reducingand capping agents for nanoparticles. We discuss thekey materials used in the field: silver, gold, iron, metalalloys, oxides, and salts.

Green chemistry and nanotechnologyThe ‘greener’ environmentally friendly processes in chem-istry and chemical technology are becoming increasinglypopular and are much needed as a result of worldwideproblems associated with environmental contamination.Within the last 2 years, several ‘green chemistry’ bookshave been published describing green processes in general[1–4] and their specialized aspects, including ultrasound,microwaves, and other methods in synthesis [5,6], greenanalytical chemistry [7], green tribology [8], polymers andgreen polymerization [9,10], green engineering andmanufacturing [11,12], food [13], textiles [14], hydrogenand syngas production and uses [15,16], wastewater treat-ment [17], particle technology [18], biofuels, biomass andbiocomposites [19–21], and other ‘green chemistry’ areas[22,23]. The 12 principles of green chemistry [24] have nowbecome a classic guide for chemists and chemical technol-ogists worldwide in developing less hazardous chemicalsyntheses.

Although UV irradiation, aerosol technologies, lithog-raphy, laser ablation, ultrasonic fields, and photochemicalreduction techniques have been used successfully to pro-duce nanoparticles, they remain expensive and involve theuse of hazardous chemicals. Therefore, there is a signifi-cant interest in the development of environmentallyfriendly and sustainable methods [25]. At the same time,despite intensive developments of the nanotechnology, theadverse effects of nanomaterials are still relativelyunknown. The synthesis of such materials using

environmentally friendly and biocompatible reagentscould lower the toxicity of the resulting materials andthe environmental impact of the byproducts [26,27]. Toreach this goal, non-toxic solvents (preferably water),closed reactors, ‘green’ techniques without contacting re-action media and air (ultrasound, microwave, hydrother-mal, magnetic, biological methods, among others), and lowtemperatures, can be used. In this short review whichfocuses on the most recent developments, we describethe methods which we consider as relatively greener routesto nanoparticles and nanomaterials involving extracts ofplants [28] and other natural products.

Green production of nanoparticlesThe techniques for obtaining nanoparticles using natu-rally occurring reagents such as vitamins, sugars, plantextracts, biodegradable polymers, and microorganismsas reductants and capping agents could be consideredattractive for nanotechnology. These syntheses have ledto the fabrication of limited number of inorganicnanoparticles (mainly metal nanoparticles, althoughseveral metal oxides and salts are also reported). Amongthe reagents mentioned above, plant based materialsseem to be the best candidates and they are suitablefor large-scale ‘biosynthesis’ of nanoparticles [29]. Plantparts such as leaf, root, latex, seed, and stem are beingused for metal nanoparticle synthesis. The key activeagent in some of these syntheses are believed to bepolyphenols, present for example, in tea, wine and win-ery waste, red grape pomace. Greener synthesis of nano-particles provides advancement over other methods as itis simple, cost-effective, and relatively reproducible andoften results in more stable materials [30]. Microorgan-isms can also be utilized to produce nanoparticles but therate of synthesis is slow and only limited number of sizesand shapes are amenable compared to routes involvingplant based materials. At present, fungi are gainingworldwide popularity as nano-factories for the greensynthesis of nanoparticles [31]. Overall, biological mate-rials provide an environmentally friendly or greenerchemical method to produce invaluable materials be-cause the biomaterial based routes eliminate the needto use harsh or toxic chemicals [32]. Mechanisms of thesebioreductive transformations [33–35], as well as catalyticproperties of materials obtained via these routes [36–39]have been discussed in a series of reports.

Review

Keywords: ‘green’ synthesis; plant extracts; tea extract; nanoparticles

240 0167-7799/$ – see front matter � 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tibtech.2013.01.003 Trends in Biotechnology, April 2013, Vol. 31, No. 4

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Silver nanoparticles are the classic and the most pre-ferred target of the above mentioned ‘green’ methods (Ta-ble 1) [40–43]. This is related to the antibacterialproperties of zerovalent silver and easy reduction of sil-ver(I) salts to form zerovalent silver. Gold nanoparticlesare also of a considerable interest, although to a muchlesser degree than Ag. Other metal and other inorganicnanoparticles are represented by small number of reports.We discuss various greener routes for the synthesis ofthese nanoparticles below.

In general, as it will be seen below, ‘green’ methods arenot so widespread (although rapidly being developed) innanoscience and nanotechnology as classical wet-chemistryroutes for nanoparticles fabrication [44,45]. This relativelyhot topic possesses such important advantage as lesser oralmost zero contaminations for the environment. Wasteproducts are usually compatible with environmentalrequirements (except microbial synthesis of nanoparticles,where the waste material could be dangerous depending onthe type of microbe applied) because they are derived fromnatural plant extracts. Precursor costs may vary, but gen-erally they are low. On reviewing a series of reports ofauthors from different countries, we observed that in gen-eral commonly available and economical products for thoseregions are used, for example, nut putamen (India) or cactusextracts (Mexico). Obviously, the ‘greener’ methods do notpretend to substitute all conventional techniques; however,at least a small decrease in damage to nature can be reached.

Silver (Ag)A large number of reports are dedicated to greener synthe-sis of silver nanoparticles, therefore here we will examineonly the most representative examples. Silver nanoparti-cles ranging from 5 to 10 nm in size have been synthesizedunder microwave irradiation conditions using glutathione(Figure 1a), an absolutely benign antioxidant that servesas the reducing as well as the capping agent in aqueous

medium [46]. The process takes only 30–60 min and isadaptable for the synthesis of palladium, platinum, andgold nanoparticles. The formation of dendritic nanostruc-tures on the transmission electron microscopy (TEM) gridwas observed when the reaction mixtures without completeconversion of AgNO3 were loaded on the TEM grid prior tothe washing procedure. The copper and carbon present onthe TEM grid are responsible for catalyzing the reaction.Synthesis of silver nanoparticles (1–10 nm) has also beeninvestigated using geraniol (Figure 1b) [47]. A concentrationof 1 mg/ml of these silver nanoparticles could inhibit cancercell line (Fibrosarcoma-Wehi 164) growth by less than 30%.

Sugars have been used with silver salts as a part of thewell-known Tollen’s test to produce silver mirrors. Sugarshave also been used in Ag nanoparticle fabrication. Synthe-sis of biocompatible Ag nanoparticles (10–20 nm) employingtable sugar as a capping and a reducing agent is a relativelyeconomical, easily adaptable, and greener route [48]. Aremarkable antitumor potency of the resulting Ag nanopar-ticles was seen against HT144 (malignant skin melanoma)and H157 (squamous cell lung carcinoma) cells as comparedto the clinically used reference compounds vincristine andmethotrexate. Ag nanoparticles have also been synthesizedin a natural polymeric matrix, using silver nitrate, gelatin,glucose, and sodium hydroxide as silver precursor, stabiliz-er, reducing agent, and accelerator reagent, respectively[49]. The resultant nanoparticles had faced centered cubic(fcc) structures smaller than 20 nm in diameter. Gelatin wasused as the stabilizer and matrix during this synthesis. Thepossible chemical equations (Equations 1 and 2) for prepar-ing the Ag nanoparticles are as follows:

AgþðaqÞþgel ! ½AgðgelÞ�þðaqÞ [1]

2½AgðgelÞ�þðaqÞþ2OH�þC5H11O5-CHO ! 2Ag þ 2gelþH2O þ C5H11O5-COOH [2]

Table 1. Synthesis of silver nanoparticles by greener methods (precursor AgNO3 in all cases)

Reducing agent Conditions, observations Properties of formed Ag nanoparticles Refs

Aqueous extracts of the manna of

hedysarum plant and the soap-root

(Acanthe phylum bracteatum) plant

13–20 min at 25–95 8C Average diameter of the prepared

nanoparticles in solution was about 29–68 nm

[96]

Extract of Argemone mexicana leaf 4 h at room temperature Average size of the particles synthesized was

20 nm, size range was 10–50 nm, with cubic

and hexagonal shape

[97]

Culture supernatants of Aspergil

lusterreus

24 h at 28 8C; reduced nicotinamide

adenine dinucleotide (NADH) was found

to be an important reducing agent for the

biosynthesis, and the formation of

AgNPs; this could be an enzyme-

mediated extracellular reaction process

Polydispersed spherical particles ranging in

size from 1 to 20 nm

[98]

Leaf extract of Euphorbia hirta L. 50–95 8C Relatively spherical shape with diameter range

40–50 nm

[99]

Euphorbia milii Irradiation of a silver nitrate and solution

by light from a xenon lamp followed by

ultra-short laser pulses; the irradiation

times were 1, 3, and 5 min

Nanoparticles are present after the xenon lamp

illumination, and after the laser irradiation the

silver nanoparticles sizes are reduced to the

range of 10–50 nm

[100]

F. vulgare leaves Room temperature 127 nm [101]

Basic amino acids, such as L-lysine

(Figure 1d) or L-arginine, as reducing

agents and soluble starch as a protecting

agent

The mixture was heated to 150 8C by

microwave under magnetic stirring

and was maintained at this temperature

for 10 s

26 nm (average size); this synthetic process can

be readily applied to large-scale production, for

example, a reaction yielding 0.1 g of nearly

monodisperse silver nanoparticles can be

performed in a 80 ml microwave sealed vessel

[40]

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Plant extracts have also been used for Ag nanoparticlesynthesis. The reactions have generally been carried out atroom temperature, although in some processes a heatingwas applied, mainly below 100 8C. For example, silvernanoparticles have been synthesized (at room temperatureand 60 8C) using Polyalthia longifolia leaf extract as areducing and capping agent along with D-sorbitol usedto increase the stability of the nanoparticles [50]. Thesynthesized silver nanoparticles were more highly toxicto Gram-positive bacteria than Gram-negative bacteria. Inaddition, silver nanoparticles have been synthesized usingsulfated polysaccharide isolated from marine red algae(Porphyra vietnamensis) [51]. The resulting silver nano-particles showed surface plasmon resonance centered at404 nm with average particle size measured to be 13 �3 nm. The involvement of the sulfate moiety of the poly-saccharide for reduction of silver nitrate was revealedspectroscopically.

Silver nanoparticles have been synthesized from aque-ous silver nitrate (1 mM) using leaf broth of Ocimumsanctum as reductant and stabilizer [52]. The resultingsilver nanoparticles ranged in size from 3 to 20 nm. Leafextract of Morinda citrifolia L. was assessed for the syn-thesis of silver nanoscale particles at different tempera-tures and reaction times [53]. High amounts of phenoliccompounds in the plant extract are likely to influence thereduction process and stabilize nanoparticles preventing

agglomeration. Among many other plant extracts appliedfor reduction of Ag+, we note aqueous extracts of Moringaoleifera [54] and Vitex negundo L. [55].

Extracts from well-known commercial products likecoffee and tea were used in silver chemistry as well. Thus,by reducing silver nitrate in solutions of tea extract orepicatechin (Figure 1c) of varying concentrations, sphericalsilver nanoparticles were formed that had controllable sizedistributions depending on the concentration of tea extractor epicatechin in the samples [56]. The nanoparticles werein fact interacting with the keratinocytes. These nanopar-ticles are potentially biocompatible and warrant furtherevaluation in other biological systems. In another example,the use of soap nuts [57] resulted in Ag nanoparticles thatshowed more inhibitory activity on pathogenic Gram-posi-tive than Gram-negative bacteria.

Among the engineering achievements in the productionof nanoparticles of silver and other metals, we especiallynote the reactor designed for Ag nanoparticles obtained bylixivium of sundried Cinnamomum camphora [58]. Here,synthesis was carried out in a continuous-flow tubularmicroreactor (Figure 2). According to the temperatureprofiles, at the inlet of the microreactors at 90 8C, the highfluid temperature induced the burst of silver nuclei byhomogeneous nucleation. Subsequently, the nuclei grewgradually along the reactors into silver nanoparticlesfrom 5 to 40 nm. Polydisperse particles were formed by

Glutathione

(a) (b)

(c) (d)

Geraniol

Epicatechin

HO

HN

HN

OH

OH

HO

OH

OH

OH

H2N

H3C CH3

CH3

OH

HS

O

O

O

O

O

L-lysine

H2N

NH2

OH

H

O

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Figure 1. Chemical structures of some components of plant extracts.

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a combination of heterogeneous nucleation and Ostwaldripening along the tubes at 60 8C.

Gold (Au) and its alloysGold nanoparticles can also be obtained by greener meth-ods [59,60], but the number of reports is significantly fewercompared to that of silver. The size of gold particles mayvary in a considerable range (from 20 to 300 nm in distinctreports). For example, a variety of gold nanoparticles(average size 300 nm; strictly speaking, some of theseare microparticles, according to a 100-nm accepted sizerestriction of the nano-area) have been synthesized bybanana peel extract (BPE) as a simple, non-toxic,eco-friendly green material [61]. The boiled, crushed,acetone-precipitated, air-dried banana peel powder wasused to reduce chloroauric acid. The aggregation of thenanoparticles into microcubes and microwire networkstowards the periphery of the air-dried samples wasobserved. The involvement of carboxyl, amine, and hydrox-yl groups during the synthetic process was indicated.Nanocomposites of Au nanoparticles can also be obtainedusing tea extracts. Tea leaf extracts in 1-methyl-2-pyrro-lidinone (NMP) solution were used for fabrication of 20 nmAu nanoparticles [62]. These nanoparticles were incorpo-rated in NMP solution of polyaniline emeraldine base(PANIEB) to cast the nanocomposite films. TEM analysisconfirmed the presence of gold nanoparticles in polyanilinematrix. In addition, a facile and completely greener routeto Au nanoparticles by mixing the Au(III) ion-dissolved ricewine and soda at pH 6.5 at a slightly elevated temperature(25–55 8C) in the absence of extra protective agents wasreported [63]. Rice wine was used as a solvent and areducing agent; soda not only functioned as a protectiveagent but also played the role of base catalyst. The resul-tant solution was quite stable: no precipitation occurredeven after several months.

The effects of synthetic conditions on the morphology ofbiologically-formed Au nanoparticles were examined.

Thus, the effect of halide ions on the formation of biogeni-cally prepared gold nanotriangles using the leaf extract oflemongrass (Cymbopogon flexuosus) plant was studied[64]. Iodide ions have a greater propensity to transformflat gold nanotriangles into circular disk-like structures ascompared to other halide ions. Also, the presence of Cl�

ions during the synthesis promotes the growth of nano-triangles, whereas the presence of I� ions distorts thenanotriangle morphology and induces the formation ofaggregated spherical nanoparticles (Figure 3). By varyingthe temperature of reaction condition, the shape, size, andoptical properties of anisotropic nanoparticles can be fine-tuned. The ability to tailor the size/morphology of goldnanoparticles and thus their optical properties by simplevariation in experimental conditions by a fully green chem-istry approach could be important in applications such ashyperthermia and architectural optical coatings.

Some gold alloys were also produced by greener routes.Dextrose and the ethanol extract of black tea (Camelliasinensis) were used as natural reducing agents for prepar-ing Au–Ag alloy nanoparticles (molar ratio 3:1, althoughthe Au3+/Ag+ mixture was selected with a molar ratio of1:1) under microwave irradiation [65]. The resulting alloynanoparticles were further purified with the agarose gel

Silver hydrosol

Silicone tubeKey:

Lixivium ofC. camphora leaf

Aqueoussilver nitrate

Peristal�cpump

Glycerin bath

Rota�ng direc�on ofelectric sha�

Heater

Electric sha�Rota�ng and moving cylinder Tubular reactor

‘T’ junc�on

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Figure 2. Schematic of the experimental setup for continuous-flow biosynthesis of silver nanoparticles. Reproduced/adapted, with permission, from [58].

Cl- l-

100 nm 100 nm

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Figure 3. Influence of halide ions on gold nanoparticle morphology. Reproduced/

adapted, with permission, from [64].

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electrophoresis technique. Purified Ag–Au alloy nanopar-ticles were spherical in the size range of �200 nm, andpossessed an average size of 92 nm. The final concentra-tions of glucose and plant extract should be considered asthe main parameters affecting the Ag/Au ratio in the Ag–Au alloy nanoparticles. In addition, a facile and eco-friend-ly method for the preparation of Au–Pd bimetallic nano-particles (�7 nm) was developed based on simultaneousbioreduction of Au(III) and Pd(II) precursors with Cacu-men Platycladi leaf extract in aqueous environment [66]. Itwas shown that the C=O and C–O groups in the plantextract play a critical role in capping the nanoparticles.

Iron (Fe), its alloys, and oxidesNanoscale iron particles, FexOy and FeOOH (in total 16polymorphic forms) are of significant interest because oftheir rapidly developing applications for disinfection ofwater and remediation of heavy metals from soils[67,68]. Therefore, methods using mainly tea extracts onthe polyphenol basis, as well other plant extracts, havebeen utilized for obtaining Fe-containing nanoparticles[69] and their further uses for the purposes above. Thus,nanoscale zerovalent iron (NZVI) particles (Figure 4), pre-pared using tea polyphenols [70], were assessed withmethyl tetrazolium [MTS, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazoli-um] and lactate dehydrogenase (LDH) toxicological assays.Some of them were found to be nontoxic compared to thecontrol samples prepared using the conventional borohy-dride reduction protocols.

In addition to ‘pure’ iron nanoparticles produced usingextracts of green tea leaves, the samples may containmainly iron oxide and iron oxohydroxide [71]. The nano-particles were then utilized as a Fenton-like catalyst fordecolorization of aqueous solutions containing methyleneblue (MB) and methyl orange (MO) dyes. The resultsindicated fast removal of the dyes with the kinetic data

of MB following a second order removal rate, whereas thoseof MO were closer to a first order removal rate. Almostcomplete removal of both dyes from water takes place overa wide range of concentrations (10–200 mg/l). Superpara-magnetic Fe3O4 nanoparticles [blocking temperature (TB)of 150 K and saturation magnetization of 37.1 emu/g] weresynthesized via soya bean sprout (SBS) templates at roomtemperature and normal atmospheric pressure [72].Spherical Fe3O4 nanoparticles with an average diameterof 8 nm simultaneously formed on the epidermal surfaceand the interior stem wall of SBS, which were responsiblefor size and morphologic control during the whole forma-tion of nanoparticles. In addition, NixCu0.25Zn0.75–xFe2O4

(x = 0.25, 0.35, 0.5) polycrystalline nanopowders (15–40 nm) with spinel structure were prepared by a simplesolution method using metal nitrates and Aloe vera plantextract solution [73]. The samples exhibited ferromagneticbehavior. Finally, iron alloys are also available by thesemethods. Thus, membranes containing reactive nanopar-ticles (Fe and Fe/Pd) immobilized in a polymer film [poly-acrylic acid (PAA)-coated polyvinylidene fluoride (PVDF)membrane] were prepared with the use of tea extract [74].The membrane-supported nanoparticles were used for thedegradation of a common and highly important pollutant,trichloroethylene (TCE).

Other metalsCopper (Cu)

Other metals and their compounds are represented by aconsiderably fewer number of reports. Highly stabledispersions of nanosized copper particles with an aver-age particle size less than 2 nm have been synthesizedusing a straightforward and cost-effective method,utilizing nontoxic L-ascorbic acid as both a reducing agentand capping agent precursor in aqueous medium [75].Also, greener fabrication of nanoparticles of copperand other metals using various fruit extracts and otherproducts of natural origin (such as red wine) waspatented [76].

Palladium (Pd)

Palladium nanoparticles (PdNps), which are of interestbecause of their catalytic properties and affinity for hydro-gen, have been phyto-synthesized under moderate pH androom temperature using Solanum trilobatum [77] andconjugated with lipoic acid (S-PdNp-LA) and vitamins(S-PdNp-Vitamin-LA). S-PdNp are polydisperse and ofdifferent morphologies ranging from 60 to 70 nm (S-PdNp),65 to 80 nm (S-PdNp-LA), and 75 to 100 nm (S-PdNp-Vitamin-LA) in size. Bulk quantities of nanocrystals(20–60 nm) of Pd, as well as other noble metals such asAg, have been obtained using coffee and tea extracts atroom temperature [78]. The reported method is generaland may be extended to other noble metals such as Au andPt. One-pot biogenic fabrication of palladium nanoparticles(small size from 3.2 to 6.0 nm) was also carried out by asimple procedure using broth of Cinnamomum camphoraleaf without extra surfactant, capping agent, and/or tem-plate [79]. The size of PdNPs could be easily controlled byvariation of the initial concentration of Pd(II) ions. Like-wise, the reaction between broth of C. camphora leaf and

3000

1

2

3

400 500 600 700 800

AKey:

CB

Wavelength (nm)

Abso

rban

ce (a

.u.)

TRENDS in Biotechnology

Figure 4. UV spectra of (a) Fe(NO3)3 control, (b) control tea extract, and (c) the

reaction product (Fe nanoparticles) obtained from Fe(NO3)3 and tea extract. The

inset shows the photographic image of the tea extract, the control Fe(NO3)3solution, and after mixing them (from left to right vials). Reproduced/adapted, with

permission, from [70].

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the Pd(II) species might occur according to the followingequation (Equation 3):

nPdðIIÞ þ 2R-ðOHÞn! nPdð0Þ þ 2nR ¼ O þ 2nHþ [3]

Finally, palladium nanoparticles (15 nm) were synthesizedusing a Glycine max (soybean) leaf extract-mediated bio-synthesis process [80]. Protein-rich soybean leaf extractacts as an effective reducing agent for palladium ions. Also,these biosynthesized palladium nanoparticles can be usedin catalysis, specifically for the degradation of azo dyes.

Platinum (Pt)

The leaf extract of Diopyros kaki has been used as areducing agent in the eco-friendly extracellular synthesisof platinum nanoparticles (2–12 nm, depending on thereaction temperature and concentrations of the leaf brothand PtCl6

2�) from an aqueous H2PtCl6.6H2O solution [81].

A greater than 90% conversion of platinum ions to nano-particles was achieved with a reaction temperature of95 8C and a leaf broth concentration of >10%. Platinumnanoparticle synthesis using Diopyros kaki may not be anenzyme-mediated process. Other related methods for plat-inum nanoparticle formation are reviewed in [82].

Metal oxides and metal saltsTitanium(IV) oxide (TiO2)

In addition to the above-mentioned iron oxide nanoparti-cles, other metal oxide nanoparticles like TiO2 (one of themost important compounds in nanotechnology) have alsobeen obtained by greener methods. Titanium(IV) oxidenanoparticles (100–150 nm) have been prepared from tita-nium isopropoxide solution using nyctanthes leaf extract[83]. Alternatively, TiO2 nanoparticles (25–100 nm) havebeen prepared by using 0.3% aqueous extract preparedfrom latex of Jatropha curcas L. [84]. Curcain (enzyme) andcyclic peptides [namely curcacycline A (an octapeptide) andcurcacycline B (a nonapeptide)] have been identified aspossible reducing and capping agents present in the latexof J. curcas L. There are two broad categories of nanopar-ticles – the first have diameters from 25 to 50 nm and aremostly spherical in shape; the second have larger anduneven shapes.

Zinc(II) oxide (ZnO)

Highly stable and spherical zinc oxide nanoparticles (25–40 nm) are produced by using zinc nitrate and Aloe barba-densis Miller leaf extract [85]. As a result, greater than 95%conversion to nanoparticles was achieved using aloe leafbroth concentration greater than 25%. It was shown thatthe zinc oxide nanoparticles were poly dispersed. Theparticle size could be controlled by varying the concentra-tions of leaf broth solution. Alternatively, ZnO nanoparti-cles (30–60 nm) were synthesized via the sol–gel method ingelatin media [86], where long-chain gelatin compoundswere utilized to terminate the growth of ZnO nanoparticlesand to stabilize them. It was concluded by authors thatgelatin is an interesting material that can be used as astabilizer in the sol–gel technique for preparing small ZnO-NPs. In addition, formation of ZnO (and also gold, silver,copper, and gold–silver–copper alloy) in living plants isdiscussed in an extremely interesting recent review [87].

Copper oxides (Cu2O and CuO)

Cu2O nanoparticles were prepared by reduction of Barfoed’ssolution, using agriculture wastes of Arachis hypogaea L.(Fabaceae) leaf extracts containing reducing sugars, whichact as reducing agent at room temperature [88]. The alde-hyde group present in reducing sugar plays an excellent rolein the formation of cuprous oxide nanoparticles in the solu-tion. The cuprous oxide nanoparticles had antibacterialeffects on Gram-negative Escherichia coli. The morphologyof divalent copper oxide in 15–30 nm nanoparticles can becontrolled by tuning the amount of Aloe vera extract [89].This eco-friendly approach for synthesis is suitable for large-scale commercial production and health-related applica-tions of CuO nanoparticles.

Indium(III) oxide (In2O3)

In addition, cubic In2O3 nanoparticles (5–50 nm) weresynthesized by a simple, cost-effective and environmental-ly friendly route using indium acetylacetonate and Aloevera plant extract solution followed by a calcinations stepin air at 400–600 8C for 2 h [90]. Alternatively, In2O3

hollow spheres were ‘one-pot’ synthesized by hydrothermalreaction of glucose and InCl3 mixtures at 180 8C for 12 h,followed by calcination in air at 600 8C for 1 h [91]. Thesespheres were sensitive to six flammable organic vaporsincluding alcohol, acetone, 93# gasoline, formaldehyde,chloroform, and acetonitrile.

Lead(II) sulfide (PbS)

Finally, the intracellular synthesis of stable cubic leadsulfide nanoparticles by a marine yeast, Rhodosporidiumdiobovatum, has been reported [92]. The particles werefound to be composed of lead and sulfur in a 1:2 ratioindicating that PbS nanoparticles were capped by a sul-fur-rich peptide. It was revealed that 55% of lead in themedium was accumulated in the exponential phase, where-as a further 35% was accumulated in the stationary phase,thus providing 90% overall recovery of PbS nanoparticles.

Concluding remarks and future perspectivesFabrication of metal, metal oxides, and salt nanoparticlesusing natural substances is a very promising area in nano-technology, which came to prominence only a few years ago.Useful materials can be produced easily even in reasonablescale because the biomaterial-based routes eliminate theneed to use harsh or toxic chemicals. Waste products arerelatively innocuous and easier to dispose of because theyare mostly composed of leftover natural plant extracts. Themajority of greener synthetic efforts reported thus far arededicated to obtaining silver nanoparticles, perhaps due totheir ease of preparation, importance in disinfection science,and utility in several applications. Several other metals (Au,Fe, Pt, Pd, Cu), their alloys, oxides, and salts have also beenobtained by ‘greener’ methods.

In these greener routes, products from nature or derivedfrom plant materials such as extracts of various plants, tea,coffee, banana, simple amino acids, as well as wine, tablesugar, and glucose, have been utilized as reductants andcapping agents. A considerable number of efforts are de-voted to tea extracts, which, owing to a number of poly-phenols present, can act as both chelating/reducing and

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capping agents for the nanoparticles. Therefore, the result-ing particles are protected from further reactions andaggregation, which increases their stability and longevity.Greener methods involving plant materials that have beenused in nanoparticle synthesis are generally single-potreactions, without the use of additional surfactants, cap-ping agents, and/or templates. Frequently, the techniquedeveloped for a given metal and metal oxide nanoparticlecould also be applied for other metals.

We note that the bulk of these investigations have beencarried out at research laboratories in small scale (which isnormal) whereas there are no reports on pilot plant orindustrial-scale fabrication of nanomaterials using naturalproducts. We believe that there are good opportunities fordeveloping large-scale greener processes, where nanopar-ticles have important applications. For example, iron nano-particles derived from cheap widespread naturalprecursors for remediation of heavy metals or disinfectionof water could be an attractive option in nanotechnologicalenvironmental applications. Systematic studies are alsoneeded for uncovering some of the reaction mechanismsand to reach more firm conclusions. For example, there areseveral theories on Ag+ reduction to Ag0 and the disinfec-tion action of silver nanoparticles [93–95]. Overall, the useof natural products for greener synthesis of nanoparticles/nanomaterials is an emerging and exciting area of nano-technology and may have a significant direct impact onfurther advances in biotechnology.

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