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Journal of Environmental Chemical Engineering
journal homepage: www.elsevier.com/locate/jece
Plant-mediated biosynthesis of metallic nanoparticles: A review ofliterature, factors affecting synthesis, characterization techniques andapplications
K. Vijayaraghavana,b,⁎, T. Ashokkumara,⁎⁎
a Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600036, Indiab Sino-Forest Applied Research Centre for Pearl River Delta Environment, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
A R T I C L E I N F O
Keywords:BiosynthesisNanotechnologyGreen chemistryBiosorptionPlant biotechnology
A B S T R A C T
Nanoparticles exhibit unique properties that enable them to find potential applications in various fields.Accordingly, significant research attention is being given to the development of novel strategies for the synthesisof nanoparticles. Among these, biological route of nanoparticle synthesis has been portrayed as an efficient, low-cost and environmental friendly technique. Biological materials such as bacteria, fungi, yeast, algae and planthave been reported to possess high bioreduction ability to synthesize various size and shape of metallic nano-particles. Of these biomaterials, this review focuses on plant-mediated biosynthesis of metallic nanoparticles.The biomolecules present in the plants such as terpenoids, flavones, ketones, aldehydes, proteins, amino acids,vitamins, alkaloids, tannins, phenolics, saponins, and polysaccharides play a vital role in reduction of metals. Asystematic comparison of literature, based on the bioreduction capacity of various plant biomass/extract towardsvarious metals under different experimental conditions, is also provided. Various instrumental techniques uti-lized to characterize nanoparticles are also discussed. Finally, this review also highlights the application ofbiosynthesized nanoparticles in different fields such as medicine, agriculture, catalytic, cosmetic and food. Thus,this article reviews the achievements and current status of plant-mediated biosynthesis, and hopes to provideinsights into this exciting research frontier.
1. Introduction
Nanoscience is a multidisciplinary field that involves the design andengineering of functional systems at the molecular scale. It is a field ofapplied science focused on the synthesis, characterization and appli-cation of materials and devices on the nanoscale. In general, na-noscience can be defined as the art and science of manipulating matterat the nanoscale to create new and unique materials. Nanoscaled ma-terials are usually categorized as materials having structured compo-nents with at least one dimension less than 100 nm. The emergence ofnanoscience has provided promising results in recent years by inter-secting with various other branches of science and forming impact onall forms of life. There are two basic approaches to attain nanos-tructures, viz. top-down and bottom-up methods [1]. Top-down ap-proach involves break down of bulk material into fine particles throughsize reduction using various lithographic techniques e.g. grinding,milling, sputtering and thermal/laser ablation [2]. In bottom-up ap-proach, the nanoparticles are fabricated from smaller entities, for
example by joining atoms, molecules and smaller particles [3]. Thebottom-up synthesis mostly relies on chemical and biological methodsof production. An important benefit of bottom-up approach is the in-creased possibility of preparing metallic nanoparticles with relativelylesser defects and more homogeneous chemical composition [4].
Nanoparticles are of great interest owing to their extremely smallsize and high surface to volume ratio, which alter their physical andchemical properties (such as mechanical properties, biological and sa-tirical properties, catalytic activity, thermal and electrical conductivity,optical absorption and melting point) compared to bulk of the samechemical composition [5]. Due to these beneficial properties, nano-materials have found potential applications in electronics, photonics,catalysis, information storage, chemical sensing and imaging, environ-mental remediation, drug delivery, biological labelling, cosmetics,biomedical, mechanics, optics, chemical industries, space industries,energy science, light emitters, single electron transistors and nonlinearoptical devices [6]. Functionalization facilitates targeted delivery ofthese nanoparticles to various cell types, bioimaging, gene delivery, and
http://dx.doi.org/10.1016/j.jece.2017.09.026Received 21 April 2017; Received in revised form 11 September 2017; Accepted 13 September 2017
⁎ Corresponding author.⁎⁎ Corresponding author.E-mail addresses: [email protected], [email protected], [email protected] (K. Vijayaraghavan), [email protected] (T. Ashokkumar).
Journal of Environmental Chemical Engineering 5 (2017) 4866–4883
Available online 14 September 20172213-3437/ © 2017 Elsevier Ltd. All rights reserved.
MARK
other therapeutic and diagnostic applications [7].Generally, nanoparticles are synthesized through three different
kinds of methods: physical, chemical, and biological [8,9]. Physicalmethods used for synthesis of nanoparticles include thermal decom-position, laser irradiation and electrolysis. For example, in thermaldecomposition method, the synthesizing process was carried out at veryhigh temperature [10]. The general disadvantages of physical methodsare that they usually are energy intensive and require costly vacuumsystems or equipment to prepare nanoparticles. The most often usedmethod for the chemical synthesis of nanoparticles is the chemical re-duction method using chemical like sodium borohydride or sodiumcitrate as reducing agents [5]. Even though chemical syntheses haveseveral advantages, the use of toxic chemicals on the surface of nano-particles and non-polar solvents in the synthesis procedure limits theirapplications in clinical fields [11]. Therefore, several researchers at-tempted to develop clean, biocompatible, non-toxic and eco-friendlymethods for nanoparticles synthesis [12]. This intensive research leadsto development of biological synthesis route for nanoparticle produc-tion. The biological synthesis of nanoparticles are being carried outusing different biomaterials such as bacteria [13], fungi [14], yeast[15], virus [16], microalgae [17], macroalgae [18] and plant biomass/extract [19]. The use of various biological organisms in this area israpidly developing due to their growing success and ease of formationof nanoparticles. Recently, the biosynthesis of metallic nanosized rods,wires, flowers, tubes, triangles, spherical, hexagonal, tetragonal andpentagonal were reported successfully [20]. These biological synthe-sized nanomaterials have potential applications in different areas suchas treatment, diagnosis, development surgical nanodevices and com-mercial product manufacturing [21]. Of various biological materials,plant biomass/extract possesses several inert advantages over othermicroscopic organisms in nanoparticle synthesis. Plant-mediated bio-synthesis of metallic nanoparticles occurs through biomolecules (suchas proteins, vitamins, amino acids, enzymes, polysaccharides and or-ganic acids such as citrates) present in the plant biomass [2]. Thus, theaim of present review paper is to highlight the recent trends and de-velopments in plant-mediated biosynthesis of metallic nanoparticles.The application of nanoparticles in medical and other sectors will alsobe highlighted.
2. Synthesis of nanoparticles
2.1. Physical methods
Physical methods for synthesis of metallic nanoparticles includeevaporation- condensation, laser ablation, electrolysis, diffusion,plasma arcing, sputter deposition, pyrolysis and high energy ball mil-ling [5]. Evaporation-condensation is generally carried out using a tubefurnace at atmospheric pressure. The source material within a boatcentered at the furnace is vaporized into a carrier gas. Nanoparticles ofvarious materials such as Ag, Au and indium tin oxide (In2O5Sn) havebeen synthesized using evaporation-condensation technique [22].However, synthesis of nanoparticles using a tube furnace at atmo-spheric pressure has some disadvantages; for example, tube furnaceoccupies a large space, consumes a great amount of energy whileraising the environmental temperature around the source material, andrequires a lot of time to achieve thermal stability [23]. The laser ab-lation method is typically designed to produce colloidal nanoparticlesin a variety of solvents. The pulse laser ablation process takes place inthe chamber under vacuum and in the presence of some inert gases[24]. One important advantage of laser ablation technique compared toother methods for production of metal colloids is the absence of che-mical reagents in solutions [5]. In plasma-arcing, the very high tem-peratures associated with the formation of an arc or plasma is used toeffectively separate the atomic species of feedstock, which quickly re-combine outside the plasma to form nanosized particles [25]. In recentyears, significant advances have been made in the field of spray
pyrolysis for the synthesis of nanoparticles and several investigatorsdemonstrated its versatility in producing particles of a variety of com-positions, shapes, and sizes [26]. In high energy ball milling, high im-pact collisions are used to reduce macroscale or microscale materialsdown into nano-crystalline structures without chemical change.
2.2. Chemical methods
Chemical methods for synthesis of nanoparticles include chemicalreduction, micro-emulsion/colloidal, electrochemical and thermal de-composition. Chemical reduction by inorganic and organic reducingagents is one of the most common methods to synthesize colloidal metalparticles because of ease in operation and equipment needed.Commonly used reducing agents are sodium borohydride (NaBH4) [27],potassium bitartrate (KC4H5O6) [28], methoxy polyethylene glycol(CH3O(CH2CH2O)nH) [29], trisodium citrate dihydrate (Na3C6H9O9)[30], ascorbate [5] and elemental hydrogen [31]. The aforementionedchemical agents reduce metallic ions and lead to the formation ofcorresponding metallic nanoparticles, which is followed by agglom-eration into oligomeric clusters. Turkevich et al. [32] described for thefirst time the reduction of Au(III) ions to Au(0) using citric acid andfurther suggested that the method can also be stabilized to formmonodispersed nanoparticles and could be exchanged to other ligand.Subsequently in 1994, Brust et al. [33] produced Au nanoparticles usingsodium borhydride as reducing agent to produce monodispersed na-noparticles which was easy to disperse in organic solvent and to re-isolate as pure powders. Micro-emulsion method is one of a versatileand reproducible method which enables control of particle propertiessuch as size, morphology, geometry, homogeneity and surface area[34]. As discussed in the review by Ganguli et al. [35], micro-emulsionprocess takes place in the aqueous cores of the reverse micelles whichare dispersed in an organic solvent and are stabilized by a surfactant.The dimensions of these aqueous cores are in the nanoregime and arethus being referred to as nanoreactors. The product obtained after thereaction is homogeneous. Several authors successfully attempted tocontrol the shape and size of the nanoparticles through micro-emulsiontechnique [36]. In the electrochemical method for the synthesis of na-noparticles, electricity is used as a controlling force. The method in-volves passing an electric current between two electrodes separated byan electrolyte and the nanoparticle synthesis occurs at the electrode/electrolyte interface [37]. Rodríguez-Sánchez et al. [38] used electro-chemical procedure, based on the dissolution of a metallic anode in anaprotic solvent, to prepare Ag nanoparticles ranging from 2 to 7 nm.The authors also demonstrated that it was possible to obtain differentAg particle sizes by changing the current density. Furthermore, Maet al. [39] synthesized spherical Ag nanoparticles (10–20 nm) withnarrow size distributions in an aqueous solution through electro-chemical method. Thermal decomposition is one of the most commonchemical techniques to produce stable monodisperse suspensions withthe ability of self-assembly [40]. Nucleation occurs when the precursormetal is added into a heated solution in the presence of surfactant,while the growth stage takes place at a higher reaction temperature[41]. The composition and the size of the formed particles depend onparameter such as the reaction time, the temperature and the surfactantmolecule length [40].
Even though chemical synthesis route has several benefits, the usageof excessive solvents, surfactants and other chemicals hinder the ap-plication aspects of synthesized nanoparticles.
2.3. Biological methods
In an attempt to develop low-cost and eco-friendly route, re-searchers have utilized the potential of biological materials for thesynthesis of metallic nanoparticles. Biological (green) synthesis in-volves the reduction of metal ions using biological mass/extract as asource of reductants either extra-cellularly or intra-cellularly. Apart
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from cost-effectiveness and eco-friendliness, the advantages of biolo-gical approach over traditional physical and chemical methods includeeffectiveness of the process to catalyze reactions in aqueous media atstandard temperature and pressure as well as flexibility of the processas it can be implemented in nearly any setting and at any scale [42].The constituents of biological materials are responsible for reductionand the process is often triggered by several compounds present in thecell such as phenolic, carbonyl, amine, amide groups, proteins, pig-ments, flavonones, terpenoids, alkaloids and other reducing agents[43]. More than one of these groups/agents may be responsible for theproduction of metallic nanoparticles. Considering the composition ofthese groups/agents vary with each type of biomaterial, elucidation ofexact mechanism associated with biosynthesis of nanoparticles may bedifficult and has not been completely understood as yet.
Bacteria, fungi, yeast, virus, microalgae, macroalgae and plantbiomass/extract are some of the important biological materials used insynthesis of metallic nanoparticles. Even though, the ability of biolo-gical materials to synthesize nanoparticles is a recent technique, theinteractions between metal ions and micro/macro-organism have beenknown for decades such as the ability of microorganisms to extract oraccumulate metals in processes of bioleaching and bioaccumulation[44].
Bacteria are a major group of unicellular living organisms belongingto the prokaryotes, which are ubiquitous in soil and water, and assymbionts of other organisms [45,46]. Bacteria are known to produceinorganic materials either intra-cellularly or extra-cellularly [47]. Someof the important bacteria studied for biosynthesis come under the fol-lowing genus: Bacillus [48], Pseudomonas [49]. Husseiny et al. [50]studied extra-cellular synthesis of Au nanoparticles using Pseudomonasaeruginosa. On the other hand, Kalimuthu et al. [51] synthesized Agnanoparticles of around 50 nm size using Bacillus licheniformis, isolatedfrom sewage collected from municipal wastes. Nair and Pradeep [52]synthesized nanocrystals of Au, Ag and their alloys by reaction of thecorresponding metal ions within cells of lactic acid bacteria present inbuttermilk. Platinum and palladium metal nanoparticles were producedby sulphate-reducing bacterium, Desulfovibrio desulfuricans [53]. Therewere reports on potential of Stenotrophomonas malophilia [54], E. coli[55], Geobacillus sp. [56], Pseudomonas proteolytica [57]. and Salmonellatyphirium [58] to extra-cellularly synthesize various metallic nano-particles. It is interesting to note that even dead/inactive bacterialbiomasses shown potential to reduce metal ions to nanoparticles owingto the presence of certain organic functional groups on cell wall [59].
Fungi are non-phototrophic and eukaryotic microorganisms com-prising of a rigid cell wall. The fungal cell wall comprises glycoproteinsand polysaccharides, which mainly include glucan and chitin [60].Fungi, when challenged with aqueous metal ions lead to the formationof nanoparticles both intra- and extra-cellularly [3]. Extra-cellularsynthesis is much rapid compared to the intra-cellular route; howeverthe synthesized nanoparticles are much bigger through extra-cellularroute [11]. Yadav et al. [60] explained that this difference in size couldbe possibly due to the nucleation of particles inside the fungus. Extra-cellular syntheses of nanoparticles by fungi such as Penicilium fellutanum[61], Fusarium solani [62], Phoma glomerata [9], Aspergillus oryzae [63],Aspergillus terreus [64] and Rhizopus nigricans [65] have been reported.On the other hand, only limited studies have been carried out on theintra-cellular synthesis of nanoparticles using fungal species [3,66].Few investigators also explored the possibility of nanoparticle forma-tion using macrofungi [67]. Yeast, which belongs to the class ascomy-cetes of fungi, has shown good potential for the synthesis of nano-particles. As a result, several research reports highlighted potential ofyeasts such as Yarrowia lipolytica [68] and Saccharomyces cerevisiae [69]in synthesis of nanoparticles.
Algae are simple plants; however they lack many distinct organs andstructures that characterize terrestrial plants. Some algae are micro-scopic and are able to float in surface waters (phytoplankton) due totheir lipid content, while others are macroscopic and attach to rocks or
other structures (seaweeds) [70]. Comparably, there is very little lit-erature supporting the use of algae in nanoparticle synthesis. Only fewreports highlighted the potential of microalgae in formation of nano-particles [71,72]. The brown marine algae (Sargassum wightii) showedpotential to synthesize Au nanoparticles extra-cellularly in the sizerange of 8–12 nm. Other brown seaweeds such as Turbinaria conoides[73] green seaweeds [74] and red seaweeds [75] also investigated fornanoparticle formation. Few researchers highlighted the involvement ofbiosorption followed by bioreduction in nanoparticle synthesis by sea-weeds [76,77]. During Au nanoparticle synthesis, Vijayaraghavan et al.[77] observed two phases: (1) rapid biosorption phase which lastedabout 1 h in which sorption of Au(III) ions onto the surface of seaweedoccurs and (2) slow bioreduction phase which involves reduction of Au(III) ions to Au(0).
Of the different biomaterials, the utilization of plant biomass/ex-tracts has been considered more reliable as well as eco-friendly methodfor the biosynthesis of nanoparticles. The advantages of plant-mediatedbiosynthesis over other biomaterials include [2,78],
• Easy availability
• Safe to handle
• Cost-effective
• One-step simple process
• Composed of various metabolites that may aid in reduction
• Elimination of elaborate maintenance of cell cultures
• Rapid rate of synthesis
• More environmental friendly
• More stable nanoparticles
• Better control over size and shape of nanoparticles
• Suitable for large-scale synthesis
Owing to these advantages, several researchers aimed to exploredifferent plant species for their potential to synthesize nanoparticles.Thus, the present review specifically focus on plant-mediated bio-synthesis of metal and metal oxide nanoparticles and their character-ization, influencing factors and applications in various fields in sub-sequent sections.
3. Plant-mediated biosynthesis of nanoparticles
For biosynthesis of nanoparticles, plants can be used in their liveform or dead/inactive form. Several plants are known for their metalaccumulation property and these accumulated metals later reduce tonanoparticles intra-cellularly [79]. Gold nanoparticles with a size rangeof 2–20 nm have been synthesized using the live alfalfa plants [80]. Baliand Harris [81] observed that metallophytes (Brassica juncea andMedicago sativa) initially accumulated Au from aqueous KAuCl4 solu-tion and later stored Au as nanoparticles throughout the epidermis,cortex and vascular tissue of both species. The authors also determinedthe particle sizes ranged between 2 nm–2 μm in B. juncea and2 nm–1 μm in M. sativa. However, recently much work has been doneusing inactive plant parts as reductant for synthesis of nanoparticles.The biomolecules present in the plants such as terpenoids, flavones,ketones, aldehydes, proteins, amino acids, vitamins, alkaloids, tannins,phenolics, saponins, and polysaccharides play a vital role in reductionof metals [47]. Plant biomass can either be used in powder form or asextracts [82]. In general, the plant biomass particle/extract is mixedwith a solution of the metal salt at room temperature and desired pHwith or without agitation. In a short span of time, synthesis of nano-particles will be completed. The experimental protocol for nanoparticlesynthesis using plant biomass is depicted in Fig. 1. Plant parts such asstem, leaf, flower, fruit, root, latex, seed and seed coat are being usedfor synthesis of metal nanoparticles. Bhati-Kushwaha and Malik [83]prepared leaf and stem extracts of Verbesina encelioides for biosynthesisof Ag. The authors confirmed that both extracts successfully synthesizedAg nanoparticles; however the rate of synthesis was faster in stem
K. Vijayaraghavan, T. Ashokkumar Journal of Environmental Chemical Engineering 5 (2017) 4866–4883
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extracts. Similarly, Paulkumar et al. [84] observed the differences insynthesis of Ag nanoparticles while using leaf and stem extracts of Pipernigrum. The authors noted that reaction started at 10 min and ended at2 h for leaf and 4 h for stem extracts. In addition, the size of the stem-derived Ag nanoparticles was 9–30 nm, whereas, small (4–14 nm) andlarge (20–50 nm) sized nanoparticles were observed in TEM images ofAg nanoparticles synthesized leaf (Piper nigrum) extracts. On the otherhand, Umadevi et al. [85] synthesized Ag nanoparticles using fruitextract of Solanum lycopersicums (tomato) plant. The authors indicatedthat citric acid present in S. lycopersicums fruit extract acted as reducingagent and malic acid was responsible for capping of the bioreduced Agnanoparticles. Sneha et al. [86] investigated Au nanoparticles usingcumin seed powder. The authors observed polydispersed Au nano-particles at pH 3 and 30 °C. Noruzi et al. [87] studied synthesis of Aunanoparticles using rose petals. The authors determined that the flowerextract medium contains abundant sugars and proteins and therebyresponsible for reduction of tetrachloroaurate salt into Au nano-particles. Some other important results obtained in biosynthesis of na-noparticles by different plant parts include, fenugreek seed extract forSe nanoparticles [88]; Hevea brasiliensis latex for Ag nanoparticles [89];mango peel extract for Au nanoparticle [90] and bark extracts of Ficusbenghalensis and Azadirachta indica for Ag nanoparticles [91].
Analysing the literature, we understand that much attention wasfocussed towards noble metals such as Ag, Au, Pt and Pd. These metalscome under “precious metal” category and nanoparticles of these me-tals are widely used in emerging interdisciplinary field of nanomedicineand nanotechnology. Literature on other metal/metal-oxide nano-particles is scare. In the subsequent sections, we have described the useof plant biomass or their extracts in the synthesis of various metal andmetal-oxide nanoparticles.
3.1. Silver nanoparticles
Of all the metals, Ag has been studied extensively for plant-medi-ated biosynthesis and it proved easier and more rapid method than thetedious and time-consuming microbial synthesis processes [92]. Fur-thermore, synthesis of Ag nanoparticles is of much interest to the sci-entific community because of their wide range of applications includingspectrally selective coating for solar energy absorption [93], surfaceenhanced Raman scattering for image [94] and many other biomedicalapplications [42].
Several plants and their respective portions had been utilized for thepreparation of Ag nanoparticles. Table 1 lists some of the important
results of plant-mediated biosynthesis of Ag nanoparticles. Plant bio-synthesis basically involves contacting silver nitrate salt (AgNO3) withextract/biomass of plants. The appearance of brownish yellow colorafter short span of contact confirms the formation of Ag nanoparticlesaccording to the following reaction:
Ag+NO3− + Plant molecule (OH, C]H, etc.) → Ag0 nanoparticles
(1)
Among earlier reports on Ag nanoparticle synthesis using plants,Shankar et al. [95] conducted Geranium (Pelargonium graveolens) leafassisted extracellular synthesis of Ag nanoparticles. Upon contactingleaf extract with AgNO3 solution, the authors observed rapid reductionof the Ag ions leading to the formation of highly stable, crystalline Agnanoparticles (16–40 nm) in solution. The authors also proved that therate of reduction of the Ag ions by the geranium leaf extract was fasterthan that observed for a fungal species, Fusarium oxysporum. Few yearslater, Huang et al. [6] showed that triangular or spherical shaped Agnanoparticles (55–80 nm) could be produced using sundried Cinna-momum camphora leaf extract. The authors identified polyol compo-nents and the water-soluble heterocyclic components in the leaf extractwere mainly responsible for the reduction of Ag ions. In 2008, Leela andVivekanandan [96] compared leaf extracts of different plant species(Basella alba, Helianthus annus, Oryza sativa, Sorghum bicolar, Saccharumofficinarum and Zea mays) for synthesis of Ag nanoparticles and con-cluded that H. annus exhibited highest potential and rapid reduction ofAg ions. Ahmad et al. [97] prepared broth of popular medicinal plant,basil (Ocimum sanctum) to synthesize Ag nanoparticles. The authorsobserved highly stable Ag nanoparticles in the size range of 10 ± 2and 5 ± 1.5 nm. Jeyaraj et al. [98] have reported synthesis of Agnanoparticles from leaf extract of Podophyllum hexandrum. The authorsobserved complete reduction of Ag ions within 2.5 h of contact at 60 °Cand pH 4.5, and process results in spherical shaped particles size rangedfrom 12 to 40 nm.
3.2. Gold nanoparticles
Gold nanoparticle also attracted several researchers in the field ofplant-mediated biosynthesis owing to their unique properties and ap-plications in nanoelectronics, biomedical, nonlinear optics, nanodevicesand catalysis [99]. In particular, Au nanoparticles provide promisingscaffolds for drug and gene delivery [100]. Their unique features suchas tunable core size, monodispersity, large surface to volume ratio, andeasy functionalization with virtually any molecule or biomolecule allow
Fig. 1. Experimental protocol for plant-mediated biosynth-esis of nanoparticles.
K. Vijayaraghavan, T. Ashokkumar Journal of Environmental Chemical Engineering 5 (2017) 4866–4883
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Table1
Green
synthe
sisof
variou
sno
blemetal
nano
particlesby
differen
tplan
tspecies.
Metals
Plan
tspecies
Expe
rimen
talco
nditions
Func
tion
almolecule
Shap
ean
dsize
App
lications
Referen
ce
pHCon
tact
time
Tempe
rature
Ag
Acalyph
aindica
Linn
Neu
tral
pH30
min
Roo
mtempe
rature
–Sp
herical;20
–30nm
Cytotox
icityactivity
[171
]Ag
Cheno
podium
albu
mleaf
2.0–
10.0
15min
Differen
ttempe
ratures
Carbo
nylan
dcarbox
ylategrou
psSp
herical;10
–30nm
–[154
]Ag
Hibiscusrosa
sinensisLe
af7.2–
8.5
––
Aminean
dcarbox
ylategrou
psSp
herical;13
nm–
[20]
Ag
Calendu
laoffi
cina
lisseed
3.0–
9.0
–30
and60
°CAmidegrou
pan
dam
inoacid
grou
psSp
herical;7.5nm
–[172
]Ag
Allo
phylus
cobbeleaf
8.0
6h
60°C
AmideII,a
minoan
dcarbox
ylategrou
psSp
herical;2–
10nm
Antibacterial
andAnti-biofi
lmactivities
[132
]Ag
Origanu
mvulgare
–10
min
60–9
0°C
Amide,
nitrile
andarom
atic
grou
ps–
Cytotox
ican
dan
timicrobial
activities
[173
]Ag
Ocimum
tenu
iflorum
leaf
extract
–10
min
Roo
mtempe
rature
Hyd
roxy
l,carbon
ylan
dpo
lysaccha
ride
sgrou
psSp
hericalan
dov
oid;
7–15
nmAntibacterial,a
nti-protease
and
mye
lope
roxida
seactivities
[174
]
Ag
Piperlongum
leaf
––
Roo
mtempe
rature
Alken
es,p
olyo
lsan
dalipha
ticam
ines
grou
pscytotoxicity
activity
[175
]Ag
Melia
azedarach
––
35–9
5°C
Carbo
nylgrou
pof
proteins,a
mines
and
polyph
enolicsgrou
psIn
vitroan
din
vivo
cytotoxicity
[176
]
Ag
Ann
onasqua
mosaleaf
––
Roo
mtempe
rature
Carbo
xylic
,phe
nols
andhy
drox
ylgrou
psSp
herical;20
–100
nmIn
vitrocytotoxicity
[177
]Ag
Sesban
iagran
difloraleaf
–24
hRoo
mtempe
rature
Carbo
xylic
,carbo
nylan
dam
inegrou
ps–
Invitroan
ticanc
eractivity
[178
]Ag
Aloeleaf
–20
min
Roo
mtempe
rature
Free
amino,
carbox
ylic
andam
idegrou
psSp
herical;20
nmAntim
icrobial
activity
[179
]Ag
Alternan
therasessilisLinn
.–
10min
–Po
lyph
enolsan
dalipha
ticam
ines
grou
ps–
Anti-microbial
andfree
radical
scav
enging
activities
[180
]
Ag
Artocarpu
sheteroph
yllusLa
m.
seed
–5min
Autoc
lave
(15psian
d12
1∘C)
Amides,c
arbo
xyl,am
inoan
dam
inoacid
grou
psIrregu
lar;
3–25
nmAntibacterial
activity
[181
]
Ag
Trigon
ella
foenum
-graecum
seed
–5min
–Ph
enolic
hydrox
yl,a
mides
andcarbox
ylgrou
psSp
herical;17
nmCatalytic
degrad
ationactivity
[169
]
Ag
And
rograp
hispa
niculata
––
30–9
5°C
Carbo
nyl,ke
tone
san
dhy
drox
ylgrou
psSp
herical;13
–27nm
Invitroan
tiox
idan
tan
dprotective
effect
onliv
erinjury
[182
]
Ag
Podo
phyllum
hexa
ndrum
leaf
4.5–
10.0
30–1
50min
20–6
0°C
Aromatic,am
ideIIIan
dII,p
olyp
heno
ls,
alka
nesan
dcarbon
ylgrou
psSp
herical;12
–40nm
Invitroan
ti-can
cerous
activity
[98]
Ag
Sina
pisarvensisseed
extract
––
25°C
Carbo
xyl,hy
drox
ylan
dam
inegrou
psSp
herical;1–
35nm
Antifun
galactivity
[163
]Ag
Buteamon
osperm
aleaf
––
–AmideIan
dIIgrou
psSp
herical;20
–30nm
Anti-alga
lprop
erty
[183
]Ag
Syzygium
cuminifruit
7.0–
9.0
2h
Roo
mtempe
rature
Aromatic
andalipha
ticam
ines,c
arbo
nyl
flav
onoids
andph
enolic
grou
psSp
herical;5–
20nm
Cytotox
ican
dan
tiox
idan
tactivity
[184
]
Au
Cassiafistulastem
bark
––
Roo
mtempe
rature
Hyd
roxy
l,am
idean
dalipha
ticam
ines
grou
psRectang
ular
andtriang
ular;
55.2–9
8.4nm
Diabe
tesmellitus
[165
]
Au
Trianthemadecand
raL.
–5min
–Hyd
roxy
l,carbon
ylan
dalde
hyde
grou
psSp
herical;37
.7nm
andhe
xago
nal;
79.9
nmAntim
icrobial
activity
[185
]
Au
Moringa
oleifera
flow
er–
60min
Roo
mtempe
rature
Hyd
roxy
lan
dcarbox
ylic
acid
grou
psTriang
ular,h
exag
onal
and
sphe
rical;5nm
Catalytic
andcytotoxicity
activities
[139
]
Au
Illicium
verum
2–11
15min
25–5
0°C
Polyph
enolsgrou
pTriang
ular
andhe
xago
nal;
20–5
0nm
Cytotox
icityactivity
[145
]
Au
Term
inalia
arjuna
leaf
–15
min
Roo
mtempe
rature
Amines
andcarbox
ylic
acid
grou
psSp
herical;20
–50nm
Polle
nge
rminationactivity
[186
]Au
Zingiber
officina
le7.4
20min
37an
d50
°C.
Alkan
ean
dcarbox
ylic
acid
grou
psSp
herical;5–
10nm
Bloo
dco
mpa
tibility
[166
]Au
Rosahy
bridape
tal
–5min
Roo
mtempe
rature
Amine,
hydrox
ylan
dcarbon
ylgrou
psSp
herical,triang
ular
and
hexa
gona
l;10
nm.
–[87]
Au
Term
inalia
chebulaseed
–20
sRoo
mtempe
rature
Hyd
roxy
l,alipha
tican
dcarbon
ylgrou
psTriang
ular,p
entago
nalan
dsphe
rical;6–
60nm
.Antim
icrobial
activity
[187
]
Au
Phoenixda
ctylife
raL.
leaf
––
Roo
mtempe
rature
Hyd
roxy
lan
dcarbon
ylgrou
psTriang
ular,tetrago
nalan
drod;
32–4
5nm
Catalytic
activity
[188
]
Au
Eucommia
ulmoidesba
rk5–
1330
min
30–6
0°C
Hyd
roxy
l,am
ines
andcarbox
ylic
acid
grou
psSp
herical
Catalytic
activity
[189
]Au
Oliv
eleaf
2.3–
9.6
30min
–Hyd
roxy
l,carbox
ylic
acid
andam
ines
grou
psTriang
ular,h
exag
onal,a
ndsphe
rical;50
to10
0nm
–[190
]
Au
Acoruscalamus
rhizom
e4–
9.2
–Atroom
tempe
rature
AmideIan
dam
inegrou
psSp
herical;10
nmAntibacterial
activity
[191
]Au
Pistacia
integerrim
a4.0–
13.0
20–8
0°C
Polyph
enols,
hydrox
ylan
dcarbox
ylic
acids
grou
ps–
Antibacterial
andan
tino
ciceptive
activities
[109
]
Au
Stevia
reba
diau
naleaf
6.0–
12.0
––
Amines,a
midean
dcarbon
ylgrou
psSp
herical;5–
20nm
–[192
]Au
Stachy
slavand
ulifo
liaVah
l7.4
20min
80°C
Flav
onoids
andmon
oterpe
nesgrou
psSp
hericalan
dtriang
ular;
–[193
](con
tinuedon
next
page)
K. Vijayaraghavan, T. Ashokkumar Journal of Environmental Chemical Engineering 5 (2017) 4866–4883
4870
Table1(con
tinued)
Metals
Plan
tspecies
Expe
rimen
talco
nditions
Func
tion
almolecule
Shap
ean
dsize
App
lications
Referen
ce
pHCon
tact
time
Tempe
rature
34–8
0nm
Au
Com
melinanu
diflora
5.0–
8.0
–30
–60°C
Amide,
phen
olic
acids,
suga
rmoietiesan
dalipha
ticam
inegrou
ps–
Antibacterial
andan
tiox
idan
tactivities
[21]
Au
Magno
liako
busan
dDiopyroska
kileav
es–
–25
–95°C
Amines,a
lcoh
ols,
ketone
s,alde
hyde
san
dcarbox
ylic
acidsgrou
psHexag
onal
andtriang
ular
–[105
]
Au
Curcumapseudo
mon
tana
root
–30
min
Roo
mtempe
rature
Carbo
nylan
dhy
drox
ylgrou
psSp
hericalshap
e;20
nmAntioxida
nt,an
tiba
cterialan
dan
ti-
inflam
matoryactivities
[194
]
Au
Citrus
limon
,Citrus
reticulataan
dCitrus
sinensis
–10
min
Roo
mtempe
rature
–Sp
hericalan
dtriang
ular;
15–8
0nm
–[195
]
Au
Dysosmapleian
tharhizom
e–
20min
30–6
0°C
Amide,
aminean
dcarbon
ylgrou
psSp
herical;12
7nm
Anti-metastaticActivity
[196
]Au
Zizyph
usmau
ritia
na–
10min
Roo
mtempe
rature
Hyd
roxy
lan
dcarbox
ylic
grou
psSp
herical;20
–40nm
Antim
icrobial
activity
[162
]Au
Hovenia
dulcisfruit
–30
min
Roo
mtempe
rature
Hyd
roxy
lan
dcarbon
ylgrou
psSp
hericalan
dhe
xago
nal;
10–2
0nm
Invitroan
tiox
idan
tactivity
[197
]
PdHippoph
aerham
noides
Linn
leaf
–25
min
80°C.
Polyph
enolsan
dcarbon
ylgrou
ps2.5–
14nm
Catalytic
activity
[198
]Pd
Cinna
mom
zeylan
icum
bark
extract
1.0–
11.0
72h
30°C
–Sp
herical;15
–20nm
–[137
]
PdBa
nana
peel
extract
2.0–
5.0
3min
40-10
0°C
Carbo
xyl,hy
drox
ylan
dam
idegrou
ps50
nm–
[199
]Pd
Cinna
mom
umcamph
oraleaf
–12
hRoo
mtempe
rature
Carbo
nyl,alde
hyde
,keton
es,c
arbo
xylic
acid
grou
psan
darom
atic
ring
sQua
si-sph
erical
andirregu
lar;
3.6–
9.9nm
–[116
]
PdCatha
ranthu
sroseus
leaf
extract
–2h
60°C
Carbo
xylan
dhy
drox
ylgrou
psSp
herical;38
nmDye
degrad
ation
[200
]Pd
Term
inalia
chebulafruit
–40
min
Roo
mtempe
rature
Polyph
enolsof
carbox
ylan
dhy
drox
ylgrou
ps–
–[201
]Pd
Ano
geissuslatifolia
–30
min
––
Sphe
rical;2.3–
7.5nm
Catalytic
activity
antiba
cterialactivities
[117
]Pt
Ana
cardium
occidentaleleaf
6.0–
8.0
–Roo
mtempe
rature
AmideIIan
dI,carbox
ylateions
ofam
ino
acid,h
ydroxy
lgrou
psIrregu
larrodshap
edCatalytic
activity
[202
]
PtCacum
enplatycladi
–25
h30
–90°C
Aldeh
ydean
dcarbox
ylic
acid
grou
psSp
herical;2–
2.9nm
–[111
]Pt
Asparagus
racemosus
root
extract
–5min
––
1.0–
6.0nm
–[203
]
K. Vijayaraghavan, T. Ashokkumar Journal of Environmental Chemical Engineering 5 (2017) 4866–4883
4871
targeting, transport, and tuning of delivery processes [101]. For most ofthe above applications, Au nanoparticles synthesized through eco-friendly as well as chemical-free route is preferred. Table 1 lists some ofthe important results achieved in green synthesis of Au nanoparticlesusing plant biomasses. In general, bioreduction of chloroauric acid(HAuCl4) to Au nanoparticles follows the below reaction,
H+Au3+4Cl−·4H2O + Plant molecule (OH, COOH, etc.) → Au0 nano-particles (2)
The above reaction generally turns the suspension to ruby red color,which confirms formation of Au nanoparticles in the solution. Shankaret al. [102] firstly reported the possibility of Au nanoparticle formationusing geranium leaves (Pelargonium graveolens). They noted that ter-penoids in the leaves acted as reducing and capping agents for rapidreduction of chloroaurate ions to stable Au nanoparticles of variablesize. In subsequent years, the same research group demonstratedsynthesis of Au nanoparticles using different plant species such aslemon grass plant extract [103] and Neem (Azadirachta indica) leafextract [78]. Begum et al. [104] produced sphere-, trapezoid-, prism-and rod-shaped Au nanoparticles using black tea leaf extract. The au-thors identified that extremely efficient reduction potential of leaf ex-tract was due to the presence of tea polyphenols, including flavonoids.Song et al. [105] reported that Magnolia kobus and Diospyros kaki leafbroths were capable of eco-friendly extracellular production of metallicAu nanoparticles (5–300 nm) with different triangular, pentagonal,hexagonal and spherical shapes within few minutes (up to 90% con-version at a reaction temperature of 95 °C). Due to the presence offlavonoids and phenol compounds, Babu et al. [106] noted that etha-nolic extract of Mentha arvensis leavesproduced hexagonal- and sphe-rical-shaped Au nanoparticles with an average size of 39 ± 15 nm.Pear fruit extract was used at room-temperature to biosynthesize tri-angular- and hexagonal-shaped Au nanoparticles (200–500 nm) [107].Due to the presence of phytochemicals such as organic acids, peptides,proteins, saccharides and amino acids, pear fruit extract when exposedto chloroaurate ions resulted in Au nanoparticles with plate-likemorphologies in a highly productive state. In 2014, the extract ofGarcinia combogia fruit was used for the production of Au nanoparticlesin spherical and anisotropic shapes [108]. The authors indicated thatshape of the Au nanoparticles depended on the extract quantity andreaction temperature. Islam et al. [109] reported the leaf galls extract ofPistacia integerrima to reduce ions to Au nanoparticles. Hydroxyl andcarboxylic acid groups of polyphenols were responsible for the reduc-tion. Polyphenols capping the nanoparticles makes them stable in dif-ferent pH solutions as well as at extreme of temperature and alsoshowed significant potential in enzyme inhibition, antibacterial, anti-fungal, antinociceptive, muscle relaxant and sedative activities.
3.3. Platinum nanoparticles
Platinum nanoparticles are widely used as catalysts as well as inmany biomedical applications [92]. Compared to Ag and Au, the re-ports on Pt nanoparticles are significantly limited. The interaction be-tween platinum solution and plant biomass usually leads to the fol-lowing reaction:
H2Pt+6Cl−·6H2O + Plant molecule (OH, COOH, etc.) → Pt0 nano-particles (3)
The first report on Pt nanoparticle synthesis using a plant extractwas by Song et al. [110], where the authors employed leaf extract ofDiopyros kaki as a reducing agent for extracellular synthesis of Pt na-noparticles (2–12 nm) from an aqueous chloroplatinic acid hexaydrate(H2PtCl6·6H2O) solution. They achieved 90% conversion of Pt ions intonanoparticles at reaction temperature of 95 °C and a leaf broth con-centration> 10%. With the aid of Cacumen platycladi extract, Zhenget al. [111] biologically synthesized Pt nanoparticles measuring
2.4 ± 0.8 nm obtained under the following conditions: reaction tem-perature, 90 °C; plant extract percentage, 70%; initial Pt(II) con-centration, 0.5 mM; and reaction time, 25 h. The authors also pointedout that reducing sugars and flavonoids played a vital role in reductionof Pt ions than proteins. Similarly, Soundarrajan et al. [112] used leafextract of Ocimum sanctum as a reducing agent for the synthesis of Ptnanoparticles (23 nm) from aqueous H2PtCl6·6H2O solution. Throughseveral instrumental techniques, the authors identified that plantcompounds such as ascorbic acid, gallic acid, terpenoids, certain pro-teins and amino acids acted as reducing agents for Pt reduction. Kumaret al. [113] demonstrated simple one step green synthesis of Pt nano-particles (< 4 nm) using naturally occurring plant polyphenols ob-tained from an aqueous extract of Terminalia chebula. They identifiedthat reduction of Pt(IV) to Pt(0) and subsequent stabilization was due tooxidised polyphenols in the plant extract. Other important results of Ptnanoparticle synthesis using plant materials are summarized in Table 1.
3.4. Palladium nanoparticles
Palladium nanoparticles are widely used as catalysts in various re-actions and plasmonic wave guiding as well in chemiresistor-typesensing devices [42].The reduction of palladium chloride (PdCl2) tonanoparticles by plant biomass follows the below equation:
Pd+Cl2− + Plant molecule (eC]C, eC]O, etc.)→ Pd0 nanoparticles(4)
Nadagouda and Varma [114] reported formation of Pd nanocrystalsusing coffee and tea extract at room temperature. The authors indicatedthat the obtained nanoparticles were in the size range of 20–60 nm andcrystallized in face centered cubic symmetry. Subsequently, Cinnamomzeylanicum bark extract mediated synthesis of Pd nanoparticles(15–20 nm) has been reported by Sathishkumar et al. [115]. Eventhough the authors were unclear on the exact mechanism, they believedthat terpenoids played an important role in biosynthesis of Pd nano-particle. Similarly, Yang et al. [116] used Cinnamomum camphora leafextract as bioreductant for the formation of Pd nanoparticles. The au-thors observed the mean size of Pd nanoparticles as 3.2–6.0 nm andthey believed that the polyols and the heterocyclic components wereresponsible for the reduction of Pd ions and the stabilization of Pdnanoparticles. Recently, Kora and Rastogi [117] synthesized Pd nano-particles from PdCl2 using plant polymer, gum ghatti (Anogeissus lati-folia). The authors confirmed the formation of Pd nanoparticles fromthe appearance of intense brown color in the solution and the producedPd nanoparticles were spherical in shape, polydisperse with averageparticle size of 4.8 ± 1.6 nm.
3.5. Other metals and metal-oxides
Other metal and metal-oxide nanoparticles through plant-mediatedbiosynthesis are represented by a considerably fewer number of reports(Table 2). Copper (Cu) nanoparticles were biosynthesized using Mag-nolia leaf extract [118]. When aqueous solution of copper (II) sulphatepentaydrate (CuSO4·5H2O) treated with the leaf extract, stable coppernanoparticles (37–110 nm) were formed. In another study, Cu nano-particles were synthesized extracellularly using stem latex of a medic-inally important plant, Euphorbia nivulia. The synthesized Cu nano-particles were stabilized and subsequently capped by peptides andterpenoids present within the stem latex. Copper oxide nanoparticleswere synthesized by Sivaraj et al. [119] using leaf extract of Ta-bernaemontana divaricate. Highly stable and spherical CuO nano-particles (48 ± 4 nm size) were synthesized using 50% concentrationof leaf extract.
Zinc oxide (ZnO) nanoparticle is an attractive semiconductor ma-terial for photonic and nano-electronic application. Highly stable andspherical ZnO nanoparticles (25–40 nm) were produced by Sangeetha
K. Vijayaraghavan, T. Ashokkumar Journal of Environmental Chemical Engineering 5 (2017) 4866–4883
4872
et al. [120] using Aloe barbadensis miller leaf extract. The authors re-ported more than 95% conversion to nanoparticles using aloe leaf brothconcentration greater than 25%. It was also shown that the ZnO na-noparticles were poly dispersed and particle size could be controlled byvarying the concentrations of leaf broth solution. Rajiv et al. [121]prepared highly stable, spherical and hexagonal ZnO nanoparticlesusing leaf extracts of Parthenium hysterophorus L. On the other hand,Vijayakumar et al. [122] synthesized ZnO nanoparticles using the leafextract of Plectranthus amboinicus.
Titanium dioxide (TiO2) is an important metal-oxide nanoparticleand is widely used in paints, printing ink, rubber, paper, cosmetics,sunscreens, car materials, cleaning air products, industrial photo-catalytic processes and decomposing organic matters in wastewater dueto their unique physical, chemical, and biological properties [123].Sundrarajan and Gowri [124] synthesized TiO2 nanoparticles(100–150 nm) from titanium isopropoxide solution using Nyctanthesarbor-tristis leaf extract. Alternatively, TiO2 nanoparticles (25–100 nm)have been synthesized using 0.3% aqueous extract prepared from latexof Jatropha curcas L. [125]. The authors have identified curcain (en-zyme) and cyclic peptides [such as curcacycline A (an octapeptide) andcurcacycline B (a nonapeptide)] as possible reducing and cappingagents present in the latex of J. curcas L. Other important metal andmetal-oxide nanoparticles synthesized using plants are summarized inTable 2.
4. Characterization of plant-mediated biosynthesis ofnanoparticles
Nanoparticles are typically characterized by their size, shape, sur-face area and dispersity nature. The common techniques of character-izing nanoparticles are as follows: UV–visible spectrophotometry,Fourier transform infrared spectroscopy (FTIR), powder X-ray diffrac-tion (XRD), scanning electron microscopy (SEM), transmission electronmicroscopy (TEM), energy dispersive X-ray spectroscopy (EDX) andatomic force microscopy (AFM).
4.1. UV–visible spectrophotometry
The formation of various metallic nanoparticles from their re-spective metallic salts gives characteristic peaks at different absorptionsthat can be monitored using UV–vis spectrophotometry. In particular,noble metallic nanoparticles such as Ag and Au possess strong absorp-tion in the visible region with the maximum in the range of400–450 nm and 500–550 nm, respectively, due to the surface plasmonresonance (SPR) phenomenon which occurs in metallic nanoparticles[126]. SPR in the UV–vis region of the spectrum originates from theresonant collective oscillations of the conduction electrons along thetransversal direction of the electromagnetic field [127]. SPR band in-tensity maximum and band width are influenced by the particle shape,dielectric constant of the medium and temperature [128]. Hence,UV–vis spectrophotometry analysis is usually the first technique used incharacterization of metallic nanoparticles and several authors used thistechnique to confirm the formation of nanoparticles [126].
On contacting the noble metal salt with plant biomass/extract, thecolor of the suspension changes and the color change was due to ex-citation of surface plasmon vibrations in the metal nanoparticles.Analysing the suspension using UV–vis spectrophotometer methodusually reveals a band from which adsorption peak can be determinedand the respective metal can be confirmed. A progressive increase inthe characteristic peak with increase in reaction time and concentrationof plant biomass/extract with metallic ion is a clear indicator of na-noparticle formation [129]. Narayanan and Sakthivel [130] observedthat addition of coriander leaf extract to aqueous HAuCl4 resulted in thecolor change to pink-ruby red after 12 h of reaction due to the pro-duction of Au nanoparticles. In addition, they determined Au SPR bandwas centered at about 536 nm and the intensity increases with increaseTa
ble2
Green
synthe
sisof
variou
smetal/m
etal-oxide
nano
particlesby
differen
tplan
tspecies.
Metals
Plan
ts/M
icrobe
sMetho
dology
impo
rtan
ceFu
nction
almolecule
Shap
ean
dsize
App
lications
Referen
ce
pH,
Con
tact
time
Tempe
rature
Cu
Citrus
medicaLinn
––
60–1
00°C
––
Antim
icrobial
activity
[204
]Cu
Lawsoniainermisleaf
11.0
––
Hyd
roxy
lan
dcarbox
ylic
grou
psSp
herical;83
nm–
[205
]Cu
Magno
liako
busleaf
–24
h25
–95°C
–Sp
herical;50
–250
nmAntibacterial
activity
[118
]Cu
Ocimum
sanctum
leaf
––
Roo
mtempe
rature
Terpen
oids,a
lcoh
ols,
ketone
s,alde
hyde
san
dcarbox
ylic
acid
grou
ps–
–[206
]CuO
Gum
karaya
–1h
75°C
Hyd
roxy
lan
dcarbox
ylategrou
psSp
herical;2–
10nm
Antim
icrobial
activity
[207
]CuO
Albizia
lebbeckleaf
–24
h37
°C–
Sphe
rical;10
0nm
–[208
]Fe
Dod
onaeaviscosa
––
–Carbo
nyl,po
lyols,
hydrox
yl,a
ndam
ideIgrou
psSp
herical;50
–60nm
.Antibacterial
effect
[209
]Fe
Eucalyptus
globules
leaf
extract
––
–Hyd
roxy
lgrou
psof
phen
ol–
–[210
]
FeEu
calyptus
leaf
extract
––
Roo
mtempe
rature
Phen
ols,
amines,carbox
ylan
dcarbon
ylgrou
ps–
Treatm
ento
fswinewastewater
[211
]Fe
3O4
Plan
tain
peel
extract
–2h
70°C
Polyph
enolic,c
arbo
hydrates,c
arbo
xylic
acidsan
dam
inegrou
psSp
herical;
>50
nm–
[212
]TiO2
Calotropisgigantean
–2h
90°C
Prim
aryam
ines,a
mides,c
arbo
nylan
dhy
drox
ylgrou
ps–
Acaricida
lactivity
[213
]TiO2
Ann
onasqua
mosafruitpe
el–
6h
Roo
mtempe
rature
–Sp
herical;23
±2nm
–[214
]TiO2
Jatropha
curcas
L.latex
–12
h50
°CAmideII,a
mideIII,carbox
ylic
acid
andam
ines
grou
psSp
herical;25
–50nm
–[125
]TiO2
Solanu
mtrilo
batum
L.leaf
–6h
Roo
mtempe
rature
Hyd
roxy
l,alka
nes,
mon
osub
stituted
alky
nes,
viny
lethe
rs,a
ldeh
ydes,
beta
lacton
es,a
ndalipha
ticam
ines
grou
ps–
Pedicu
locida
lan
dlarvicidal
Activities
[123
]
ZnO
Calotropisproceralatex
122h
Roo
mtempe
rature
–Sp
herical;5–
40nm
–[215
]Zn
OAloeba
rbad
ensisMiller
leaf
extract
–5–
6h
150°C
Phen
olic
compo
unds,terpe
noids,
proteins,freeam
inoan
dcarbox
ylic
grou
psSp
hericalan
dhe
xago
nal;25
–55nm
–[120
]
K. Vijayaraghavan, T. Ashokkumar Journal of Environmental Chemical Engineering 5 (2017) 4866–4883
4873
in contact time. Similarly, Ashokkumar and Vijayaraghavan [131]confirmed the formation of Au nanoparticles by observing the colorchange of reaction mixture from brown to ruby red and surface plasmonresonance centered at 525 nm (Fig. 2). During synthesis of Ag nano-particles, Gurunathan et al. [132] visualized color change from palegreen to deep brown on contacting Allophylus cobbe leaf extract andAgNO3. Using UV–vis spectrophotometer, the authors determinedstrong and broad peak at about 420 nm confirming the formation of Agnanoparticles. On the other hand, Yang et al. [116] reported that thecolor of the solution gradually turned from brownish yellow into darkbrown in 12 h and the absence of the absorption peaks above 300 nm inthe sample indicated the generation of Pd nanoparticles. There was alsoa report by Venu et al. [133], which indicates the color of the Pt re-duced solutions gradually turned from yellow to black and absorbancedominates near 200 nm wavelength confirms the generation of Pt na-nostructures.
4.2. Fourier transform infrared (FTIR) spectroscopy
Fourier transform infrared (FTIR) spectroscopy is a surface chemicalanalytical technique, which measures the infrared intensity versuswavenumber (wavelength) of light. The nature of the functional groupsand their involvement during bioreduction can be approximatelyevaluated using the FTIR spectroscopy. The identification of these plantcomponents is vital to develop new routes in synthesis of nanoparticles.In general, FTIR spectrum of virgin plant biomass/extract and that ofsynthesized nanoparticles will be compared to gather information aboutfunctional groups responsible for bioreduction. Several researchersused FTIR technique to elucidate various plant biomolecules re-sponsible for metal bioreduction as depicted in Table 3. As discussedpreviously, the important plant biomolecules that acts as reducing andcapping agents are polysaccharides, proteins, terpenoids and flavo-noids. Analysing Table 3, it can be inferred that significant absorptionbands have observed in the region of 1000–1800 cm−1, which confirmthe role of above biomolecules in bioreduction. It should also be notedthat stretching vibrations of NeH and OeH groups appear in the rangeof 3200–3500 cm−1. A typical FTIR spectrum of T. ornata extract andsynthesized gold nanoparticles using T. ornata extract is shown inFig. 3.
4.3. X-ray diffraction (XRD)
The X-ray diffraction (XRD) technique is used to study structuralinformation about crystalline metallic nanoparticle. The energetic X-rays can penetrate deep into the materials and provide informationabout the bulk structure [6]. If the nanoparticles are produced in anamorphous structure, no diffraction peak is observed and this techniquecannot help to identify the sample [126]. The broadening of the peaksin XRD confirms the formation of particles in nano size. The size of
nanoparticle can be calculated by means of the Debye–Scherrer equa-tion [134]. The Debye–Scherrer equation was used to derive the par-ticle size from the XRD data by determining the width of the (1 1 1)Bragg reflection according to the following equation:
=d Kλβ θcos (5)
where d is the particle size (nm), K is the Scherrer constant, βis the fullwidth half maximum, θ is half of Bragg angle and λ is the wavelength ofX-ray [135]. Gao et al. [136] observed XRD pattern of newly synthe-sized Au nanoparticles showed intense peaks of (111), (200), (220),(311) and (222), which confirmed the monophasic nature of pure Auwith face centered cubic symmetry. In addition, the peaks are broadwith fair intensity indicating the nanocrystalline nature of Au powder.Khazaei et al. [137] reported that the XRD pattern showed threecharacteristic peaks at (1 1 1), (2 0 0) and (3 1 1). These characteristicpeaks indicated crystallographic planes of the Pd (0) nanoparticles.From the XRD pattern, the size and structure of ZnO nanoparticlessynthesized from leaf extract of Tamarindus indica was determined byElumalai et al. [138]. The authors identified that synthesized ZnO na-noparticles were 16–37 nm and crystalline in nature owing to thestrong and narrow diffraction peaks. For illustration purpose, the XRDpattern of gold nanoparticles synthesized using leaf extract of Portulacagrandiflora is presented in Fig. 4.
4.4. Electron microscope
Scanning electron microscopy (SEM) provides information aboutthe topography and morphology of the nanoparticles. In addition,electron microscopy techniques can also be used to measure the averagesize of nanoparticles using statistical software. Through SEM images,Anand et al. [139] confirmed that Au nanoparticles synthesized byMoringa oleifera flower extracts were in spherical shape with the size of100 nm. Similarly, Rajiv et al. [121] distinguished spherical and hex-agonal-shaped ZnO nanoparticles synthesized using Parthenium hyster-ophorus extract through SEM images. Fig. 5 shows SEM images ofsynthesized nanoparticles using the green seaweed extract of star likeAg-Au bimetallic NPs (data not published). Few researchers also usedatomic force microscopy (AFM) to analyse morphology of nano-particles. Das et al. [140] studied the surface morphology of the Ses-bania grandiflora-synthesized Ag nanoparticles using AFM. Throughtwo- and three- dimensional topography of the Ag nanoparticles pro-vided by AFM, the authors determined Ag nanoparticles were sphericalin shape and 10–25 nm in size. Through AFM images, Bhat et al. [141]demonstrated that Au nanoparticles were irregular in shape with or-ganic shell over it. The average size of the Au nanoparticles from theAFM data was found to be 12–15 nm.
To study surface morphology and shape of nanoparticles, manyresearchers preferred transmission electron microscopy (TEM). TEMtechnique has greater magnification and resolution than SEM and theimages provide more accurate information regarding size, shape andcrystallography of the nanoparticles [142]. Philip [20] observed tri-angular, hexagonal, dodecahedral and spherical shaped Ag and Aunanoparticles with average size 13–14 nm in TEM images after bior-eduction of Ag and Au ions by Hibiscus rosa-sinensis leaf extract. Simi-larly, Dhayananthaprabhu et al. [143] conducted Cassia auriculataflower extract-mediated biosynthesis of Au nanoparticles and noticedspherical, hexagonal and triangular shaped Au nanoparticles with sizeranging from 10 to 55 nm through TEM images. Another advantage ofTEM analysis over SEM images is that TEM images can be used todistinguish amorphous structures from crystalline structures using theselected area electron diffraction technique [126]. Fig. 6 shows TEMimages of gold nanoparticles synthesized using leaf extract of Portulacagrandiflora.
Elemental composition of metal nanoparticles can be establishedusing energy dispersive X-ray spectroscopy (EDX) [144]. Each element
Fig. 2. UV–visible spectra of synthesized gold nanoparticles using seaweed extract of T.ornata Reprinted from Ashokkumar and Vijayaraghavan [131] with permission fromVinanie Publishers copyright (2016).
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Table 3Exemplary stretching frequencies observed in FTIR spectra during plant-mediated biosynthesis of metal nanoparticles in some important studies.
Metal Type of plant biomass Wavenumber (cm−1) Surface functional groups identified References
Native biomass Biomass after metal reduction
Ag Boswellia serrata extract of gumolibanum
3395 3435 -OH groups [216]2963,2926 2963,2925 Methylene groups2858 28552145 2014 CeC groups1651,1520 1630,1537 Amide I and II1605,1420,1383 1454,1398 C]O groups13831261,1093 1261,1093 CeO groups1022 1026
Ag Nigella sativa leaf extract 3397 3394 -NH groups [217]2939 – –1605 1608 eC]C groups1413 1389 -CeH groups
Ag Sesbania grandiflora leaf extract 3397 3394 NeH groups [140]1646 1646 C]C groups1397 1406 CeO groups1070 1070 CeO-C groups
Ag Tribulus terrestris L. fruit 3488 3419 -OH groups [218]2811,2727 2726 CeH groups2164 2171 C]C groups– 1613 CeC groups1348,1124 1348,1125 CeN groups
Au Cajanus cajan seed coat 3433 3434 -OH groups [219]2924 2924 CeH groups2161 2142 C]C groups1614 1629 C]O groups1415 1419 CeH groups1089 – CeO groups1031,947,903,819 1031 Ester groups
Au Couroupita guianensis flower extract 3414 3615,3547 -OH groups [220]– 2924,2855 CeH groups2133 2402,2129 C]C groups1648 – C]C groups– 1744 C]O groups1383 1378 CeN groups1288 – CeO groups1073 1071 CeN groups
Au Moringa oleifera flower extract 3390 3378 Amide groups [139]2848,2916 – -OH groups1641 1635 C]O groups1108 – CeO groups
Au Phoenix dactylifera 3415 3398 eOH groups [188]1632 1651,1613 eC]O groups
Au Terminalia chebula – 3145 eOH groups [187]2922 CeH groups1720 C]O groups1613 C]C groups1445 CeN groups1384 CeO groups1205 CeO-H groups1020 CeO-C groups
Au Zingiber officinale extract – 1638, 515 C]C groups [166]1275,1151,1033 C]O groups1603,1190,935,850, 813,723 heterocyclic compounds such as alkanoids, flavoinds and
alkaloids
Pd Hippophae rhamnoides Linn leaf extract 3375 3387 eOH groups [198]1722 1714 eC]O groups1674 – –1522 1462 C]C groups1396 1381 CeOH groups1122 1074 –
Pd Terminalia chebula fruit 3400 3430 OH groups [201]1610 1620 eC]C groups1040 1040 CeOeC Groups
Pt Anacardium occidentale dried leafpowder
3400 3200 eOH groups [202]1720,1641 1720,1641 C]O groups
(continued on next page)
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has a unique atomic structure making a unique set of peaks on its X-rayspectrum which, in turn, leads to the characterization of the elements[126]. Fig. 7 shows EDAX spectrum of synthesized gold nanoparticlesusing T. ornata extract. Sathishkumar et al. [145] synthesized Au na-noparticles using Illicium verum and the EDX spectrum confirmed thepresence of Au in the measured sample. The authors assigned the peakat 2 keV to Au nanoparticles. With the aid of EDX spectrum, Dipankarand Murugan [146] confirmed synthesis of Ag nanoparticles using Ir-esine herbstii leaf extract. The authors observed a sharp signal peak ofAg, which in turn indicated the reduction of AgNO3 to Ag nanoparticles.
5. Factors influencing biosynthesis of nanoparticles
Several factors affect the synthesis, characterization and applicationof nanoparticles during plant-mediated biosynthesis. Some of the
important factors are summarized below.
5.1. Solution pH
The solution pH plays an important role in plant-mediated bio-synthesis of nanoparticles. Several reports indicated that pH of the so-lution medium influences the size, shape and rate of the synthesizednanoparticles [147]. This phenomenon is due to the formation of nu-cleation centres, which increases with increase in pH. As the nucleationcentre increases, the reduction of metallic ion to metal nanoparticlesalso increases. At the same instance, the solution pH also influences theactivity of the functional groups in the plant extract/biomass and alsoinfluences the rate of reduction of a metal salt [81]. Armendariz et al.[147] identified that the size of Au nanoparticle produced by Avena
Table 3 (continued)
Metal Type of plant biomass Wavenumber (cm−1) Surface functional groups identified References
Native biomass Biomass after metal reduction
1519 – Amide II groups1448 1448 CeOH groups1222 1222 Polyols
Fig. 3. FTIR spectra of T. ornata extract and synthesized gold nanoparticles using T. or-nata extract Reprinted from Ashokkumar and Vijayaraghavan [131] with permission fromVinanie Publishers copyright (2016).
Fig. 4. XRD pattern of crystalline gold nanoparticles synthesizedusing leaf extract of Portulaca grandiflora. Reprinted from Ashokkumaret al. [221] with permission from American Institute of ChemicalEngineers copyright (2016).
Fig. 5. SEM images of synthesized nanoparticles using the green seaweed extract of starlike Ag-Au bimetallic NPs. (data not published).
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sativa was strongly dependent on the solution pH. They determined thatlarge size Au nanoparticles (25–85 nm) were formed at pH 2, in smallquantities. On the other hand, smaller sized nanoparticles were formedat pH 3 and 4 in large quantities. They speculated that at low pH (pH 2),the Au nanoparticles prefer to aggregate to form larger nanoparticlesrather than to nucleate and form new nanoparticles. In contrast, at pH 3and 4, more functional groups (carbonyl and hydroxyl) were availablefor Au binding; thus a higher number of Au(III) complexes bounded tothe plant biomass at the same time. Zhan et al. [148] studied influenceof pH on biosynthesis of Au nanoparticles by leaf extracts of Cacumenplatycladi. The authors observed that the size of the Au nanoparticlesdecreased whereas the intensity of absorbance peaks increased withincrease in pH. They also noted that high pH leads to rapid reductionrate of the chloroaurate ions, boosts the homogenous nucleation, anddecreases the anisotropy growth. In contrast, slow reduction rate oc-curred under acidic condition resulted in heterogeneous nucleation andsecondary nucleation of small Au seeds. Moreover, Jacob et al. [149]studied the reduction rates of Au and Ag ions at pH 3.3 and 10.8. Their
work revealed that at pH 3.3, Ag ions reduced very slowly compared toAu ions and the resulting nanoparticles precipitated in 2 days. Never-theless, at pH 10.8, simultaneous reductions occurred for both metalions and a single SPR band was observed. The TEM image showedhomogenous electron density, suggesting that alloyed nanoparticleswere formed. They further stated that high pH value also contributed tothe formation of AgO− and AgO which strongly interact with hydroxylgroups in plant extracts hence effectively covered the surface of nano-particles.
5.2. Temperature
Temperature is another important factor that influences the size,shape and rate of nanoparticles. Similar to pH, formation of nucleationcentres increase with increase in temperature, which in turn increasethe rate of biosynthesis. Sneha et al. [59] studied the effect of tem-perature on biosynthesis of Au nanoparticles by Piper betle leaf extract.Through TEM images, the authors visualized mainly nanotriangles at200 °C, whereas nanoplatelets and nanoparticles ranging in size from 5to 500 nm were observed at 30–40 °C. Further at 50 and 60 °C, theauthors noticed the size and morphology of the formed nanoparticleswere reasonably consistent since the density of the spherical nano-particles dominated the nanotriangles and octahedral platelets. Shenyet al. [150] compared the reduction processes of Au and Ag ions by leafextract of Anacardium occidentale at different temperature values todetermine the optimum condition for bimetallic Au-Ag synthesis. Theauthors determined that a greater amount of leaf extract was requiredto synthesize stable nanoparticles at low reaction temperature than athigh temperature. To be precise, only 0.6 mL of leaf extract was re-quired for biosynthesis at 100 °C while as much as 2.5 mL of leaf extractwas required for synthesis at 27 °C. Also, the authors observed thatnanoparticles formed at high temperature were not only more stablebut also larger in size. In an attempt to investigate the effect of tem-perature, Iravani and Zolfaghari [151] heated the reaction mixture(Pinus eldarica bark extract and AgNO3) to different temperatures (25,50, 100 and 150 °C). By increasing reaction temperature, the authorsobserved an increase in absorbance and decrease in size of Ag nano-particles.
5.3. Reaction time
The size, shape and extent of nanoparticle synthesis using plant-based biomaterials also greatly influenced by the length of reactiontime the suspension medium is incubated. Nazeruddin et al. [152] ob-served rapid synthesis of Ag nanoparticles by seed extract of Corian-drum sativum within 1–2 h as compared to 2–4 days required by mi-croorganisms. Similarly, Noruzi et al. [87] reported that the Rosahybrid petal mediated synthesis of Au nanoparticles reaction was rapidand completed within 5 min. Even though plant-mediated biosynthesisof nanoparticles is usually rapid compared to microorganism, severalauthors observed efficient production rate at high reaction times [153].Dwivedi and Gopal [154] observed that Chenopodium album leafmediated synthesis of Ag and Au nanoparticles started within 15 min ofthe reaction. In addition, they found that an increase in contact timestrongly influenced the sharpening of the peaks in both Ag and Aunanoparticles. Few reports also indicated that size and shape of thenanoparticles alter with the extent of reaction time [155,59]. During Agnanoparticles synthesis using Capsicum annuum L. extract, Li et al. [155]identified five hours reaction time led to spherical and polycrystallineshaped nanoparticles (10 ± 2 nm). With increase in reaction time to9 h and 13 h, the size of the nanoparticles was increased to 25 ± 3 nmand 40 ± 5 nm, respectively.
5.4. Plant extract/biomass dosage
The concentration of plant biomass/extract often decides the
Fig. 6. TEM images of gold nanoparticles synthesized using leaf extract of Portulacagrandiflora. Reprinted from Ashokkumar et al. [221], with permission from AmericanInstitute of Chemical Engineers copyright (2016).
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efficiency of nanoparticle synthesis. Several investigators identified thatincrease in biomass dosage enhances the production of nanoparticles aswell as alter the shape of nanoparticles [111,156]. Hence, it is oftenessential to determine optimum biomass dosage for the process.Chandran et al. [157] used the Aloe vera leaf extract to modulate theshape and the size of the synthesized Au nanoparticles. Most of the Aunanoparticles were triangular in the size range of 50–350 nm, whichdepended on the extract quantity. The addition of low amounts of theextract to HAuCl4 solution resulted in the formation of nanogold tri-angles in larger sizes. Also, when the extract quantity was increased, theratio of nanotriangles to spherical nanoparticles decreased.
6. Applications of nanoparticles
Due to their unique properties, nanoparticles are widely used indifferent fields such as medicine, agriculture, cosmetic industry, drugdelivery, catalysis and wastewater treatment. Fig. 8 illustrate applica-tions of nanopartilces in various fields. In particular, biosynthesizedmetallic nanoparticles are much preferred especially in medical-basedapplications. The nanoparticles exhibit high differential uptake effi-ciency in the target cells over normal cells through preventing themfrom prematurely interacting with the biological environment,
enhanced permeation and retention effect in disease tissues and im-proving their cellular uptake, resulting in decreased toxicity [158]. Theuse of nanoparticles for cancer therapy has been gaining popularity inrecent years. Several researchers explored the cytotoxic effects of var-ious metallic nanoparticles on different cell lines synthesized usingplant biomass/extracts [139,145,159]. Raghunandan et al. [160] syn-thesized Au and Ag nanoparticles using guava and clove plant extractsand subsequently examined their anti-cancer efficacy against differentcancer cell lines including human colorectal adenocarcinoma, humankidney, human chronic myelogenous, leukemia, bone marrow, andhuman cervix. The authors proved that these nanoparticles as potentialanti-cancer agents. Multi-functional Au nanoparticles have been de-monstrated to be highly stable and versatile scaffolds for drug deliverydue to their unique size, coupled with their chemical and physicalproperties [101]. The ability to tune the surface of the particle providesaccess to cell-specific targeting and controlled drug release. Yallappaet al. [161] synthesized Au nanoparticles using Mappia foetida leavesextract and subsequently conjugated Au nanoparticles with activatedfolic acid and doxorubicin complex to deliver the drug to human cancercells. The authors determined that the maximum amount of drug re-leased was at pH 5.3, which was toxic to human cancer cells viz., MDA-MB-231, HeLa, SiHa and Hep-G2. In addition, Au and Ag nanoparticles
Fig. 7. EDAX spectrum of synthesized goldnanoparticles using T. ornata extractReprinted from Ashokkumar andVijayaraghavan [131] with permission fromVinanie Publishers copyright (2016).
Fig. 8. Various applications of nanoparticles.
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have been commonly found to have wide spectrum of antimicrobialactivity against pathogens [109,162,163]. The antimicrobial activity ofmetallic nanoparticles recommend its possible application in the foodpreservation field and also as a potent sanitizing agent for disinfectingand sterilizing food industry equipment and containers against the at-tack and contamination with foodborne pathogenic bacteria [164]. Aunanoparticles synthesized from Cassia fistula (stem bark) extract havealso exhibited anti-diabetic activity [165]. Praveenkumar et al. [166]described the blood compatibility of Au nanoparticles synthesized withginger (Zingiber officionale) extract. Zargar et al. [167] reported thatAg/V. negundo films have a biocompatible and rough surface used forspecial biological applications, such as cell immobilization.
The application of nanoparticles as catalysts is also receiving in-creased research attention. Compared to bulk materials, nanoparticleshave high surface-area-to volume ratio and thus found to exhibit higherturnover frequencies. The catalytic activity of Au, Ag Pd and Pt in thedecomposition of H2O2 to oxygen is well known [47]. Ghosh et al.[168] reported that Au nanoparticles synthesized using Gnidia glaucaflower extract exhibited remarkable catalytic properties in a reductionreaction of 4-nitrophenol to 4-aminophenol by NaBH4 in aqueousphase. In addition, Trigonella foenum-graecum bioreduced Ag nano-particles exhibited remarkable size dependent catalytic properties in areduction reaction of organic dyes such as methyl orange, methyleneblue and eosin Y [169].
The nanosized metal nanoparticles such as Au, Ag and Pt arebroadly being applied for various commercial personal care productssuch as shampoo, soap, detergent, anti-ageing creams and perfumes[129]. Applications of nanoparticles are emerging in crop protectionand agriculture. Several types of nanoparticles have also been shown toreduce the microbial loads in treated wastewater effluent as well [170].
7. Conclusions and future directions
In recent years, the biological synthesis of metal nanoparticlesgarnered significant attention and has emerged as an important scien-tific field. A wide number of biological materials, including bacteria,fungi, yeast, micro-algae, seaweeds and plant parts have shown po-tential to synthesize various metal and metal oxide nanoparticles suchas Ag, Au, Pd, Pt, Cu, ZnO, CuO and TiO2. Of these biomaterials, theplant extract/biomass has gained great importance in nanoparticlessynthesis due to the fact that plant-mediated biosynthesis is generallyinexpensive, more environmental friendly, simple one-step process andsafe to handle. It was understood that the reaction rate strongly de-pends on pH, temperature, reaction time and plant dosage.Biosynthesized nanoparticles have been characterized using FTIR, SEM,TEM, EDX, AFM, XRD and UV-spectrophotometry. Several studies alsopointed out that plant synthesized metallic nanoparticles found usage inmany fields in particular medicine and catalysis.
Although several research reports highlighted potential of variousplant species in nanoparticle synthesis, the exact mechanism is yet to beunderstood. Some researchers hypothesized involvement of certainchemical agents/functional groups in plant biomass during synthesis ofnanoparticles. However considering the diversity of plants and theirvaried chemical composition, more efforts should be made to elucidatemechanism through experimental techniques. Also additional stepsshould be taken to determine potential application of biosynthesizednanoparticles in fields other than medicine and catalysis. Hence, moreresearch investment and an interdisciplinary team work are thereforerequired to elevate the plant-biosynthesis method to successfully com-pete with chemical and physical syntheses of nanoparticles.
Acknowledgement
This work was financially supported by Ramalingaswami Re-entryFellowship (Department of Biotechnology, Ministry of India, India).
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