Metal nanoparticles harbour numerous exceptional physiochemical properties absolutely different from those of bulk metal as afunction of their extremely small size and large superficial area to volume. Naked metal nanoparticles are synthesized by variousphysical and chemical methods. Chemical methods involving metal salt reduction in solution enjoy an extra edge over otherprotocols owing to their relative facileness and capability of controlling particle size along with the attribute of surface tailoring.Although chemical methods are the easiest, they aremarred by the use of hazardous chemicals such as borohydrides.This has led toinclination of scientific community towards eco-friendly agents for the reduction of metal salts to form nanoparticles. Tannic acid,a plant derived polyphenolic compound, is one such agent which embodies characteristics of being harmless and environmentallyfriendly combined with being a good reducing and stabilizing agent. In this review, first various methods used to prepare metalnanoparticles are highlighted and further tannic acid mediated synthesis of metal nanoparticles is emphasized. This review bringsforth the most recent findings on this issue.
1. Introduction
Owing to the nanoscale dimension (in the range of 1–1000 nm), nanoparticles enjoy a leading edge in the fieldsof nanoscience and nanotechnology. Recent years havewitnessed increased interests of scientific community innanomaterials particularly metal nanoparticles in variousareas ranging from material science to nanotechnology [1–3]. Although nanomaterials have begun to be sought afteronly recently, the notion dates back to the early 20th century[3]. Humans have developed and used nanomaterials since avery long time as evidenced by the ruby red colour of someglass which is due to the entrapment of gold nanoparticlesin the glass matrix. Appearance of ruby red colour of thecolloidal gold solution results as a formation of small goldnanoparticles [4]. Medieval potteries embellished with glazeor luster display different characteristic colours that resultfrom special optical properties of the glaze which themselvesarise due to the random dispersion of metallic sphericalnanoparticles within the glaze [3].The properties of the glazehave been well enumerated in 1857 by Michael Faraday in
his revolutionary study “Experimental relations of gold (andother metals) to light” [3, 5].
Thenanoscale dimension and high surface area to volumeratio of nanoparticles makes their physicochemical proper-ties quite different from those of the bulkmaterials [3, 5].Thismakes nanomaterials capable of being potentially applied indiverse fields including photonics and electronics, sensing,imaging, information storage, environmental remediation,drug delivery, and biolabelling [3]. It has been well docu-mented that the optical, electronic, and catalytic propertiesof metal nanoparticles are functions of nanoparticle size,shape, and crystal structure. For instance, differently shapednanostructures of silver and gold embody unique opticalscattering properties [3, 6]. While a single scattering peak isshown by highly symmetric spherical particles; multiple scat-tering peaks in the UV-vis range are exhibited by anisotropicnanoparticles like rods, triangular prisms, and cubes exhibitas a result of highly localized charge polarizations at cornersand edges [3]. This developed increased interest of scientificcommunity in the synthesis of metal nanoparticles of definedmorphology. Various procedures have been developed for the
Hindawi Publishing CorporationJournal of NanotechnologyVolume 2014, Article ID 954206, 11 pageshttp://dx.doi.org/10.1155/2014/954206
synthesis of metal nanoparticles and nanomaterials includ-ing physical, chemical, and biological methods. Numerousreducing agents have been used to reduce metal salts to formmetal nanoparticles; for example, tri-sodium citrate [7–9]and sodiumborohydride [10] are being used for the reductionof gold chloride and silver nitrate solutions to form gold andsilver nanoparticles, respectively, for decades. Tannic acid, aplant derived polyphenolic compound [11] (Figure 1), has alsobeen exploited as a reductant of metal salt solutions. Ostwaldin as early as 1917 reported that chloroauric acid solutionscan be reduced to gold nanoparticles employing tannin evenwhen tap water is used to prepare aqueous solutions [12].Ostwald’s protocol was replicated by Turkevich et al. [13] in1951 who reported generation of gold particles with size tobe 12.0 ± 3.6 nm. Mulpfordt in 1982 [14] prepared colloidalgold nanoparticles employing tannic acid as an additionalreductant. Moreover, tannic acid has also been an essentialcomponent of the extensively used Slot and Geuze protocolproposed in 1985 [15] for synthesizing gold nanoparticles inthe size range of 3–17 nm.This gives the historical significanceof tannic acid being used as a reductant formetal nanoparticlesynthesis. Despite that the first exploitation of tannic acidmediated nanoparticle synthesis possibly dates back to early20th century [11], its use remained subdued for a finiteperiod of time after that owing to the extra edge enjoyedby citrate as a reducing agent for metal salts. However,scientific community has recently revisited interest in thiscompound owing to its properties of being a reducing as wellas stabilizing agent [16, 17].
Tannic acid has been well studied for its antioxidant,antimutagenic, and anticancarcinogenic properties [18, 19].Tannic acid has been reported to harbour inhibitoryaction against skin, lung, and forestomach tumors causedby polycyclic aromatic hydrocarbon carcinogens and
N-methyl-N-nitrosourea in mice [18, 20, 21]. Glucoseoccupies the central core position in the tannic acid structurewhose hydroxyl groups are attached to one or more galloylresidues [16]. At its natural acidic pH, tannic acid behaves asa weak reducing agent which can induce growth of only seedsto nanoparticles at room temperature [16, 22]. Tannic acidowing to the pKa value between seven and eight, as a functionof the degree of dissociation, partially gets hydrolysed intoglucose and gallic acid moieties under mild acidic/basicconditions [16, 23, 24]. At alkaline pH, gallic acid inducesformation of silver nanoparticles from silver nitrate rapidlyat room temperature [16, 25], but the poor stabilizationpotential of gallic acid leads to aggregation of particles insolution. However, glucose harbours the property of beinga good stabilizing agent at alkaline pH but concomitantlyis a weak reducing agent at room temperature [26]. Thesefacts enumerate the attribute of tannic acid of being an idealreducing and stabilizing agent under alkaline conditionsat room temperature. The aggressive reducing propertiesof tannic acid owe to the numerous phenolic groups inits structure. These phenols take part in redox reactionsby forming quinones and donating electrons. The donatedelectrons reduce the oxidisedmetal ions inmetal salts to formcorresponding metal nanoparticles.The reaction mechanismof phenol (present in tannic acid) based reduction of metalions is outlined in Figure 2. In this review first we willdiscuss the various methods used for the synthesis of metalnanoparticles in general. Further, we will specifically reviewthe synthesis procedures for silver, gold, and palladiumnanoparticles with special emphasis on tannic acid mediatedsynthesis of these particles since tannic acid has begun to beused as a universal reductant of gold, silver, and palladiumsalts for the production of respective nanoparticles. We willdiscuss the most recent findings on this issue.
Journal of Nanotechnology 3
Phenolic form Quinone form
(from metal salt)
Tannic acid
OH
O
O
+ 2H++ 2e−
M+
M0
HO
HO
HOHO
HO
HO
HO
HO HO
HO
HO
HO
OH OHOH
OH
OH
OH
OH
OH
OH
OHOH
OH
OH
OO
O
O
O
OO
OO O
O
O
O
OO
O
OO
O
OO
Figure 2: Reaction mechanism of tannic acid based reduction of metal salts. The phenolic groups in the tannic acid get oxidised to quinoneswith subsequent release of electrons which reduce the metal ions.
2. Synthesis of Metal Nanoparticles
Numerous physical and chemical methods have been usedto carry out the synthesis of metal nanoparticles that includelaser ablation, ion sputtering, solvothermal synthesis, chem-ical reduction, and sol-gel method [3, 27–31]. Moreover,biological method is also being used to fabricate metalnanoparticles [3, 32, 33]. Nanoparticle synthesis approachescan be basically divided into two: the top-down approachand the bottom-up approach [3]. The concept of top-downapproaches is to create nanoscale objects by using bulky,externally controlled microscopic devices for directing theirassembly, while bottom-up approaches adoptmolecular com-ponents that are built up into more complex assemblies[3]. Microfabrication techniques are often used for top-down approaches, wherein externally controlled equipmentare exploited for shaping materials into the required shapeand size by cutting or milling. Photolithography and inkjetprinting which fall under micropatterning techniques arewell-known examples of top-down approach. On the othercontrary, bottom-up approaches use the self-assembled prop-erties of single molecules into some useful conformation [3].Various physical and chemical methods for metal nanoparti-cle synthesis are described in the following section.
2.1. Laser Ablation. Laser ablation can be used to obtaincolloidal nanoparticles solutions in a number of solvents[3, 28]. Plasma plume generated in response to laser ablationof bulk metal plate dipped in liquid solution on condensationforms nanoparticles. This method being conceived as “green
technique” is used in place of the chemical reductionmethodfor synthesizing noble metal nanoparticles. Nevertheless, thismethod suffers a setback owing to high energy required perunit of metal nanoparticles generated and the feeble controlover the growth rate of the metal nanoparticles.
2.2. Inert Gas Condensation. The most widely appliedmethod at laboratory scale for metal nanoparticle synthesisis inert gas condensation (IGC) which was introduced byGleiter in 1984 for iron nanoparticle synthesis [3]. In IGC,the small particles synthesized as a result of condensation ofmetals evaporated in ultra high vacuum chamber grow byBrownian coagulation and coalescence and ultimately formnanoparticles. This technique has recently been applied forsize-controlled synthesis of gold or palladium nanoparticlesand varied-sized gold nanoclusters for nonvolatile memorycell applications [34, 35].
2.3. Sol-Gel Method. The sol-gel process is a recently devel-oped wet-chemical technique for the synthesis of nanomate-rials. Sol-gel process is used for the generation of inorganicnanostructures by first forming a colloidal suspension (sol)and then gelation of the sol to integrated network in con-tinuous liquid phase (gel). Size and stability of metal andmetal oxide nanoparticles have been controlled by invertedmicelles, polymer blends, block copolymers, porous glasses,and ex situ particle-capping techniques [3, 36–39]. Despitethese features, the aqueous sol-gel chemistry is marred bythe complexity of process and amorphous nature of the as-synthesized particles.
4 Journal of Nanotechnology
2.4. Hydrothermal and Solvothermal Synthesis. Synthesis ofinorganic materials by hydrothermal and solvothermal tech-nique is amongst the important methodologies for nano-material synthesis [3]. For hydrothermal synthesis, the syn-thetic process is required to occur in aqueous solution attemperature higher than the boiling point of water, whilein solvothermal method the reaction takes place in organicsolvents above their boiling points (at temperatures 200–300∘C). TiO
2photocatalysts have been reported to be pro-
duced by hydrothermal process [40]. Low cost and lowerenergy consumption make hydrothermal process suitable forindustrial production. Solvothermal process on the otherhand increases the synthetic diversity as it enables to chooseamong numerous solvents or mixture thereof. For instance,highly reactive TiO
2nanocrystals were produced in hydrogen
fluoride and 2-propanol solvent mixture [41].
2.5. Colloidal Methods. Colloidal methods have been usedfor long to control the nucleation and growth of metalnanoparticles [3, 12, 42–44]. In colloidal method, metal saltsare reduced by chemical reducing agents such as borohydride,citrate and hydrazine to produce metal nanoparticles whichare further stabilized by capping agents to avoid coalescenceof the particles. Varying the concentration of these chemicalreductants influences the size, shape, and dispersity of pro-duced nanocrystals. Moreover, concentration of metal saltsalso affects the size and dispersity of generated nanoparticleswhen such chemical reducing agents are used to reducemetalsalts. Recently, reduction of metal salts by tannic acid hasattracted attention of scientific community since it fulfils therequirement of a chemical reductant, but itself is a plantderived compound. Hence, it renders the characteristics ofgreen synthesis. In the succeeding section, we will be dis-cussing synthesis of silver, gold, and palladium nanoparticlesemploying tannic acid.
The unique size and shape dependent optical, electrical, andchemical properties of silver nanoparticles open avenues fortheir application in diverse fields. They possess antimicrobialproperties and can be incorporated in biosensor materials,composite fibres, cryogenic superconducting materials, cos-metic products, and electronic components [10]. Numer-ous physical and chemical methods have been exploitedfor generating and stabilizing silver nanoparticles [45, 46].Silver nanoparticles have been synthesized using chemicalreduction method, electrochemical procedure, physiochem-ical reduction, or radiolysis. For chemical reduction, variousorganic and inorganic reducing agents are in use [10]. Thesechemicals are usually hazardous and sometimes requireenergy inputs. Therefore, there is increasing interest in pro-ducing nanoparticles using environment friendly methods,that is, green methods. In this context, use of environmen-tally safe “green” reducing agents is entailed. Several recentreports have made significant progress towards this goal byusing amino acids, vitamins, polysaccharides, and extracts
of bioorganisms. However, very recently, tannic acid being apolyphenolic plant extract has been given attention since it isan aggressive reducing agent and behaves as stabilizer as well.
The study conducted by Cataldo et al. [47] reveals that sil-ver nanoparticles synthesized employing tannin make morestable colloidal solutions than those prepared by reductionof silver salt by NaBH
4. Sivaraman et al. have demonstrated
that silver nanoparticle size can be controlled by molar ratiovariation of tannic acid to silver nitrate [16]. They foundthat tannic acid can be used as a reducing and stabilizingagent for silver nanoparticle synthesis within a few minutes.An increase in particle size with increasing molar ratio oftannic acid/silver nitrate indicated that tannic acid acts as anorganizer for facilitating nucleation. Moreover, the synthesiswas found to occur at room temperature itself. The silvernanoparticles were synthesized over a wide range of valuesof the initial molar ratio of tannic acid to silver nitrate. Stablecolloidal dispersions were formed in all instances. Therefore,they infer that each tannic acid molecule acts as a five-armedchelator and that atomic reorganization occurs within suchcomplexes facilitating nucleation.
Although tannic acid has been previously utilized as areducing agent in the presence of additional stabilizers [48]or as both the reducing and stabilizing agent [49, 50] butthe reaction times reported lie between half to two and ahalf hours at 80∘C, whereas the particle sizes reported are>15 nm. The crucial difference between the study conductedby Sivaraman et al. [16] and the earlier reports is that the pHofthe tannic acid solution was adjusted prior to the addition ofmetal salt.The alkaline pH environment enhanced the reduc-ing and stabilizing capability of tannic acid allowing roomtemperature synthesis of silver nanoparticles in seconds, andalso enabling variation of the mean size from 3 to 22 nm.The increase in particle size with increasing molar ratio oftannic acid to silver nitrate indicates a third role for tannicacid as an organizer for facilitating nucleation. This conceptof using tannic acid at alkaline pH as a reducing, organizing,and stabilizing agent is easily extendable to other elementsthat are known to chelate with tannic acid [16].
Moreover, Calinescu et al. [51] have demonstrated thesynthesis of silver nanoparticles in the presence of tannicacid along with poly-vinyl alcohol. Their study enumeratesthat by varying the molar ration between silver nitrate/poly-vinyl alcohol and silver nitrate/tannic acid at different NaOHconcentration reaction time, temperature and microwavepower level renders to develop a fine control over thenanoparticle size and distribution. Gupta et al. [52] havefound that silver nanoparticles synthesized employing tannicacid bear antibacterial activity against multidrug resistanthuman pathogens. Silver nanoparticles synthesized usingtannic acid have also been evaluated for release of silverin natural waters [53]. It has been found that tannic acidfunctionalized silver nanoparticles are more prone to silverrelease in comparison to citrate coated silver particles inwaterreservoirs.
Nanoplates as well as nanoshells of silver have beensynthesized using tannic acid. Tannic acid has been reportedto induce formation of silver nanoplates at room temperature.The synthesis has been found to be a seedless process where
Journal of Nanotechnology 5
tannic acid plays the role of a reducing as well as a cappingagent [54]. Very recently, tannic acid (along with sodiumcitrate) mediated controlled synthesis of low polydispersityAg@SiO
2core-shell nanoparticles has been reported. These
nanoparticles have been found to have applications in plas-monics [55].
The unique optical, electronic, and molecular-recognitionproperties of gold nanoparticles make them the subjectof substantial research embodying applications in a widevariety of areas, including electron microscopy, electronics,nanotechnology, and materials science [18]. Specifically goldnanoparticles in the 1–10 nm size range have size-tunablephysicochemical properties that are useful in a wide vari-ety of applications such as cancer diagnostics, catalysis,Raman and fluorescence spectroscopy, selective ionisation ofbiomolecule, water purification, drug delivery, photothermaltherapy, and optoelectronics [18, 56–59]. Colloidal syntheticprocedure being versatile and relatively easy is better thanother chemical methods in vogue for the synthesis of goldnanoparticles [60].This method allows control over seed sizeand seed size distribution since the experimental parameterslike reactant concentration, addition of stabilizers, mixingrate, and so forth, can be tuned and manipulated as per therequirement. A variety of chemical reagents being able toreduce gold ions have been exploited for gold nanoparticlesynthesis. Preparation of thiol-functionalized gold nanopar-ticles by chemical reduction of soluble Au(1)-thiolates hasbeen reported byCorbierre and Lennox [61]. Long et al.[62] prepared gold nanoparticles exploiting trisodium citrateas reductant and further studied their optical properties.Gold nanoparticles have also been generated by ethanolassisted reduction of tetrachloroaurate ion in the presence ofsodium linoleate [63]. Moon et al. [64] have demonstratedthe cetyltrimethylammonium bromide (CTAB) mediatedsynthesis of gold nanoparticles in sodium dodecyl sulfonate(SDS) aqueous solution.
Synthesis of gold nanoparticles using tannic acid hasbegun to be investigated recently owing to its attribute ofbeing a “green technique.” Influence of tannic acid alone andin combination with citrate has been observed on the sizeof synthesized gold nanoparticles [65]. It has been reportedthat when the mixture of tannic acid and citrate is usedto reduce gold chloride solution, the gold nanoparticlesformed are significantly smaller than those observed whenonly tannic acid is used as a reductant [65]. Mahl et al.[66] have reported synthesis of gold, silver, and gold-silverbimetallic nanoparticles using a modified citrate reductionmethod where tannic acid has been used. The use of mixtureof citrate and tannic acid led to reduction in size of formednanoparticles by a factor of 3. Sivaraman et al. [11] studiedhow pH affects the size distribution of gold nanoparticlesproduced in response to reduction of gold chloride solutionby tannic acid.They report that the pHof precursor solutions,mode of contacting, and the dynamics of stabiliser adsorption
vis-a-vis Brownian collision frequency play critical rolesin tuning nanoparticle formation, growth, and coalescence.Their study also demonstrates that gold nanoparticles withreduced polydispersity are produced when chloroauric acidsolution is added into tannic acid slowly as compared tothose produced by faster addition protocols. They reportedthat dropwise addition protocol leads to synthesis of size-controlled gold nanoparticles (in the size range of 2–10 nm)in a few minutes at room temperature. They stated that“the optimal process is shown to be similar to a one-shot nucleation-seeded growth technique and the growthmechanism is identified to be surface-reaction controlledunder these conditions. These insights on independentlymanipulating reactivity and stabilisation can be extended toother redox reactions for rapidly synthesising size-controlledmetal nanoparticles, as most systems exhibit pH dependentreactivity and stabilization.” Aswathy Aromal and Philip [18]have reported facile one-pot-synthesis of varied sized goldnanoparticles at 373K employing tannic acid as reducing andstabilizing agent.They have investigated tannic acidmediatedsynthesis of stable and spherical gold nanoparticles. Theycome up with a simple, economically viable, and efficientprotocol for preparing gold nanoparticles. Nanoparticle sizewas found to be a function of the amount of tannic acid andpH. They state that phenyl hydroxyl and carboxylate groupspresent in the tannic acid might play a putative role in theformation of gold nanoparticles. Their study has enumeratedthat adjusting one or more experimental parameters isrequired to harness gold nanoparticles in a particular sizerange and with particular morphology. Aswathy Aromal andPhilip [18] also report that gold nanoparticles formed usingtannic acid exhibit good catalytic activity in the reduction of4-nitrophenol to 4-aminophenol by excess NaBH
4. Ahmad
and Khan [67] very recently investigated the effect of higherchloroauric acid concentrations on the size evolution of goldnanoparticles synthesized using tannic acid. They observedtwo different patterns of size evolution of gold particles inresponse to higher chloroauric acid concentrations.The sizesobtained below 1mM chloroauric acid solution exhibited ageneral decrease with increase in molarity of chloroauricacid solution. In contrast, the sizes of particles formed atchloroauric acid concentrations greater than or equal to 1mMwere found to increase with increase in chloroauric acidconcentration. Very recently, tannic acid functionalized goldnanoparticles have also been found to assist in SNP detectionin the NanoBioArray chip at room temperature [68].
In addition to forming gold nanoparticles (nanospheres),tannic acid has also been reported in synthesizing parti-cles of other morphologies like nanoplates and nanorods.Nanostructures of various morphologies are obtained bycontrolling aspect ratios of particles in seeded growth. Tannicacid is popularly known to form metal seeds which underthe influence of variation in crucial growth factors like pH,temperature andmolar ratio result in nanoparticles of diverseshapes. Zhang et al. [69] have demonstrated tannic acidassisted synthesis of gold nanoplates. They have reportedthat mixing aqueous tannic acid and HAuCl
4solutions at
room temperature may be an effective route towards greenpreparation of gold nanoplates. Catalytic property of gold
6 Journal of Nanotechnology
nanoplates towards the oxidation and reduction of H2O2has
been enumerated in the study. Moreover, they successfullydemonstrate that such gold nanoplates can be used asbuilding blocks for creating biosensor capable of detectingglucose in buffer as well as human serum.Moreover, Unteneret al. [70] have demonstrated a distinctive uptake of tannicacid coated gold nanorods by endosomes and their uniquedistribution within the cell. Their study reveals that tannicacid functionalized gold nanorods maintain their integrityafter being endocytosed by the keratinocyte endosomeswhich indicates their potential candidature as nanodeliveryagents, nanobioimaging tools, and nanotherapeutics.
Although in the last few decades, the coinage metals like goldand silver had been the focus of the nanoscientific communityleaving the transition metals at the backdrop of the researchfield.However, recently transitionmetals have begun to provetheir mettle when their properties as metal nanoparticles arebeing harnessed [71, 72]. Transition metal nanoparticles aremajorly being exploited in the area of catalysis as the high sur-face area to volume ratio of nanomaterialsmakes them highlyefficient as potential catalysts. Palladium nanoparticles areone of the transitionmetal nanoparticles beingwidely appliedin homogenous and heterogenous catalysis [72–75] althoughthey find applications in sensing, chemooptical transducers,and plasmonic wave guiding as well [72, 76–78]. Palladiumnanoparticles being capable of adsorbing hydrogen are exten-sively applied in hydrogen storage [72, 79, 80]. Palladiumnanoparticles have been prepared by various methodologiesincluding chemical and electrochemical reduction [81, 82],ion exchange [83], vapor deposition, thermal decomposition[84, 85], and polyol method [86]. As discussed earlier,biological and eco-friendly green technologies are at theleading edge of nanoscience and nanotechnology in thecurrent era [87, 88]. Moreover, wide and unique applicationsof nanoparticles demand economically viable approaches fortheir synthesis. Metal nanoparticle synthesis entails the use ofa reducing agent and a stabilizer for the reduction of a metalsalt and further capping of metal nanoparticles, respectively,to avoid nanoparticle collapse and coalescence [89]. Tannichas been used as a reducing and protective agent in goldnanoparticle synthesis since time immemorial and recentlyit has begun to be widely applied for synthesis of other metalnanoparticles which include silver and palladium.
Very recently, Kumari et al. [72] demonstrated an eco-nomically viable and efficient procedure for the prepa-ration of palladium nanoparticles exploiting tannic acid.They enumerate a green method for the reduction of Pd2+ions to nanometer size using tannic acid. The proceduredemonstrated by them neither necessitates the use of anysuperfluous stabilizing or capping agent nor involves anyextreme operating conditions such as high pressure. Theysynthesized palladium nanoparticles both at room tempera-ture and boiling conditions and found that the nanoparticlesformed at the former environmental cue were more stable
than those formed at latter condition. Hence, they concludethat stable palladium nanoparticles can be prepared at roomtemperature by sheer use of tannic acid as reducing agent forpalladium salt and capping agent of formed nanoparticles.FTIR analysis performed in the study demonstrates the roleof poly-phenolic groups in reducing Pd2+ ions.They state thedevelopment of that simple method for the synthesis of metalnanoparticles is desirable over other methods because of itsfacile, environment friendly, quick, and one-step approach.
Moreover,Devarajan et al. [90] have reported that simul-taneous addition of trisodium citrate and tannin to the goldand palladium salt aqueous solutions leads to formation of4–7 nm sized bimetallic gold-palladium nanoparticles.
6. Other Metal Nanoparticles SynthesizedUsing Tannic Acid
There are also other metals which have been reduced tonanoparticles employing tannic acid wholly or as a coreduc-tant. Tannins isolated from the biomass of Medicago sativa(alfalfa) have been found to be capable of reducing goldchloride to gold nanorods and zinc salts to zinc nanoparticles[91]. Moreover, they were also found capable of formingbimetallic nanoparticles, lanthanide clusters, iron oxide, andmagnetite nanoclusters [91]. Herrera-Becerra et al. [92]have also demonstrated tannin mediated biosynthesis of ironoxide nanoparticles. In a recent study, magnetic carbon-ironoxide nanoparticles have been synthesized using tannin incombination with a microwave-based thermolytic process[93]. The iron oxide nanoparticles generated employing thisprocess have been found to be embedded within a carbonmatrix in small nanoclusters (lees than or equal to 100 nm).Recently, Chang et al. [94] have reported preparation of 1,5-diaminoanthraquinone nanofibers (DAAQNFs) decoratedwith small platinum nanoparticles (PtNPs) synthesized usingtannic acid as a reducing agent. They found the resultantPtNP/DAAQNF composites to exhibit a good catalytic activ-ity toward reduction of 4-nitrophenol to 4-aminophenol byNaBH4. In an earlier study, Huang et al. [95] exploitedbayberry tannin as a stabilizer to prepare supported plat-inum nanocatalysts. Preparation of gold-platinum bimetallicnanoparticles has also been reported employing tannic acid[96]. To a mixture of gold chloride and platinum chloride,addition of tannic acid and citrate led to the formation ofbimetallic gold-platinum nanoparticles [96].
Ni/graphene nanocomposites have also been preparedusing tannic acid. Very recently, Lu et al. [97] prepared novelNi(II)-based metal-organic coordination polymer nanopar-ticle/reduced graphene oxide (NiCPNP/rGO) nanocompos-ites employing hydrothermal treatment of the mixture oftannic acid functioned graphene oxide and NiCl(2) aque-ous solution in N,N-dimethylformamide. They found thatthe NiCPNP/rGO nanocomposite-modified electrode couldexhibit high electrocatalytic activity for glucose oxidation inalkaline medium and hence could be used as a glucose sensorin human blood serum.
Journal of Nanotechnology 7
Reduction
Electrostericstabilization
: COO−
: polymer moiety
:
M : metal
Tannic acid (C76H52O46)
M0
M+
M0
HO
HO
HOHO
HO
HO
HO
HO HO
HO
HO
HO
OH OHOH
OH
OH
OH
OH
OH
OH
OHOH
OH
OH
OO
O
O
O
OO
OO O
O
O
O
OO
O
OO
O
OO
Figure 3: Putative mechanism of metal nanoparticle synthesis and stabilization by tannic acid.
Tannic acid
Reduced metal seeds
Variation in pH, temperature, molar ratio
e
Nanospheres
Nanoplates
Nanorods
M+
Figure 4: Schematic representation of various morphologies ofmetal nanoparticles obtained upon tannic acid mediated reductionof metal ions in response to variation in pH, temperature and molarratio.
7. Reduction Mechanism of Metal Salts byTannic Acid
Tannic acid mediated reduction of metal salts and synthesisof metal nanoparticles is outlined in Figure 3. Gallic acidmolecules and glucose polymerise to form tannic acid. Tannicacid being an antioxidant is rich in electrons and embodiesthe capability of liberating freely reactive hydrogen atom[18]. Presence of a hydrophobic “core,” a hydrophilic “shell,”and above all the polyphenolic nature of tannic acid makeit an effective antioxidant [18] combined with an aggressivereducing agent. Numerous polyphenols bear antioxidant
nature owing to the relative facileness of donating hydroxylgroup to a free radical and the potency of the aromatic ringto carry an unpaired electron. Hence, the hydroxyls of thephenolic groups present in the tannic acid may be respon-sible for reducing chloroauric acid. Carboxylic acid groups(COOH) present in the tannic acid lose their hydrogen atomto become carboxylate ion (COO−) during the reductionprocess. The COO− so formed attaches to the surface ofmetal nanoparticles along with the remaining part of thepolymer to act as surfactant and stabilize metal nanoparticlesby electrosteric stabilization [18]. Various factors like pH,temperature, and molar ratio of metal salts to tannic acidhave been found to play a decisive role in the generation ofnanoparticles of various shapes (Figure 4).
8. Conclusion and Future Perspective
Herein, we first reviewed variousmethods formetal nanopar-ticle synthesis and further shifted our focus to tannic acidmediated synthesis of metal nanoparticles since tannic acidbeing a good reductant and eco-friendly compound is cur-rently in vogue for the reduction of metal salts to form metalnanoparticles. From the findings enumerated in the review,it becomes apparent that tannic acid can act as a universal
8 Journal of Nanotechnology
reducing agent for silver, gold, palladium, platinum, nickel,zinc, iron, and other metallic salts to form their respectivemetal nanoparticles. Exploitation of tannic acid provides anopportunity for rapid, facile, economically viable, and eco-friendly synthesis of metal nanoparticles in general withoutrequiring specific chemical reductant for each metal salt. pH,temperature, and molar ratio of the metal salt and tannicacid have been found to influence the synthesis of metalnanoparticles from tannic acid. However, a defined patternof nanoparticle synthesis for various metals in response tovariation in these parameters cannot be worked out fromstate-of-the-art findings. To obtain a complete picture inthis regard, further studies on tannic acid mediated metalnanoparticle synthesis have to be performed. Since thenanoparticles formed employing tannic acid have smallerdiameters, this can open avenues in future for them to beexploited in therapeutics, drug delivery, and bioimaging assmaller particles cannot be uptaken by the phagocytic systemof the body, remain in circulation for a long time, andtherefore can have enhanced targeting potential.
Conflict of Interests
The authors declare that there is no conflict of interests.
References
[1] R. Guo, Y. Song, G. Wang, and R. W. Murray, “Does coresize matter in the kinetics of ligand exchanges of monolayer-protected Au clusters?” Journal of the American ChemicalSociety, vol. 127, no. 8, pp. 2752–2757, 2005.
[2] M.-C. Daniel and D. Astruc, “Gold nanoparticles: assembly,supramolecular chemistry, quantum-size-related properties,and applications toward biology, catalysis, and nanotechnol-ogy,” Chemical Reviews, vol. 104, no. 1, pp. 293–346, 2004.
[3] S. K. Das and E. Marcili, “Bioinspired metal nanoparticle:synthesis, properties and application,” in Nanotechnology andNanomaterials, chapter 11, pp. 253–278, InTech, 2011.
[4] P. S. Prasoon, “Synthesis and characterization of colloidal goldnanoparticles suspension using liquid soaps,” Chemistry andMaterials Research, vol. 2, no. 1, pp. 82–87, 2012.
[5] M. Faraday, “Experimental relations of gold (and other metals)to light,” Philosophical Transactions of the Royal Society ofLondon, vol. 147, pp. 145–181, 1857.
[6] E. Roduner, “Size matters: why nanomaterials are different,”Chemical Society Reviews, vol. 35, no. 7, pp. 583–592, 2006.
[7] K. Zabetakis, W. E. Ghann, S. Kumar, andM.-C. Daniel, “Effectof high gold salt concentrations on the size and polydispersityof gold nanoparticles prepared by an extended Turkevich-Frensmethod,” Gold Bulletin, vol. 45, no. 4, pp. 203–211, 2012.
[8] X. Ji, X. Song, J. Li, Y. Bai, W. Yang, and X. Peng, “Size control ofgold nanocrystals in citrate reduction: the third role of citrate,”Journal of the American Chemical Society, vol. 129, no. 45, pp.13939–13948, 2007.
[9] J. Kimling, M. Maier, B. Okenve, V. Kotaidis, H. Ballot, andA. J. Plech, “Turkevich method for gold nanoparticle synthesisrevisited,” Journal of Physical Chemistry B, vol. 110, no. 32, pp.15700–15707, 2006.
[10] H. Korbekandi and S. Iravani, “Silver nanoparticles,” inThe Delivery of Nanoparticles, A. A. Hashim, Ed., InTech,
[11] S. K. Sivaraman, S. Kumar, and V. Santhanam, “Room-temperature synthesis of gold nanoparticles—size-control byslow addition,” Gold Bulletin, vol. 43, no. 4, pp. 275–286, 2010.
[12] W. Ostwald, An Introduction toTheoretical and Applied Colloid,John Wiley & Sons, New York, NY, USA, 1917.
[13] J. Turkevich, P. C. Stevenson, and J. Hillier, “A study of thenucleation and growth processes in the synthesis of colloidalgold,” Discussions of the Faraday Society, vol. 11, pp. 55–75, 1951.
[14] H.Mulpfordt, “The preparation of colloidal gold particles usingtannic acid as an additional reducing agent,” Experientia, vol.38, no. 9, pp. 1127–1128, 1982.
[15] J. W. Slot and H. J. Geuze, “A new method of preparing goldprobes for multiple-labeling cytochemistry,” European Journalof Cell Biology, vol. 38, no. 1, pp. 87–93, 1985.
[16] S. K. Sivaraman, I. Elango, S. Kumar, and V. Santhanam,“A green protocol for room temperature synthesis of silvernanoparticles in seconds,” Current Science, vol. 97, no. 7, pp.1055–1059, 2009.
[17] X. Tian,W.Wang, andG. Cao, “A facile aqueous-phase route forthe synthesis of silver nanoplates,”Materials Letters, vol. 61, no.1, pp. 130–133, 2007.
[18] S. Aswathy Aromal and D. Philip, “Facile one-pot synthesisof gold nanoparticles using tannic acid and its application incatalysis,” Physica E, vol. 44, no. 7-8, pp. 1692–1696, 2012.
[19] I. Gulcin, Z. Huyut, M. Elmastas, and H. Y. Aboul-Enein,“Radical scavenging and antioxidant activity of tannic acid,”Arabian Journal of Chemistry, vol. 3, no. 1, pp. 43–53, 2010.
[20] R. E. Vance and R. W. Teel, “Effect of tannic acid on rat liverS9 mediated mutagenesis, metabolism and DNA binding ofbenzo[a]pyrene,” Cancer Letters, vol. 47, no. 1-2, pp. 37–44, 1989.
[21] W. A. Khan, Z. Y. Wang, M. Athar, D. R. Bickers, and H.Mukhtar, “Inhibition of the skin tumorigenicity of (±)-7𝛽,8𝛼-dihydroxy-9𝛼,10𝛼-epoxy-7,8,9,10- tetrahydrobenzo[a]pyreneby tannic acid, green tea polyphenols and quercetin in Sencarmice,” Cancer Letters, vol. 42, no. 1-2, pp. 7–12, 1988.
[22] X. Tian,W.Wang, andG. Cao, “A facile aqueous-phase route forthe synthesis of silver nanoplates,”Materials Letters, vol. 61, no.1, pp. 130–133, 2007.
[23] B. H. Cruz, J. M. Diaz-Cruz, C. Arino, and M. Esteban,“Heavy metal binding by tannic acid: a voltammetric study,”Electroanalysis, vol. 12, no. 14, pp. 1130–1137, 2000.
[24] W. Bors, L. Y. Foo, N. Hertkorn, C. Michel, and K. Stettmaier,“Chemical studies of proanthocyanidins and hydrolyzable tan-nins,” Antioxidants and Redox Signaling, vol. 3, no. 6, pp. 995–1008, 2001.
[25] G. A. Martinez-Castanon, N. Nino-Martınez, F. Martınez-Gutierrez, J. R. Martınez-Mendoza, and F. Ruiz, “Synthesis andantibacterial activity of silver nanoparticles with different sizes,”Journal of Nanoparticle Research, vol. 10, no. 8, pp. 1343–1348,2008.
[26] J. Liu, G. Qin, P. Raveendran, and Y. Ikushima, “Facile “green”synthesis, characterization, and catalytic function of 𝛽-D-glucose-stabilized Au nanocrystals,” Chemistry A, vol. 12, no. 8,pp. 2131–2138, 2006.
[27] F. Mafune, J.-Y. Kohno, Y. Takeda, T. Kondow, and H. Sawabe,“Formation of gold nanoparticles by laser ablation in aqueoussolution of surfactant,” Journal of Physical Chemistry B, vol. 105,no. 22, pp. 5114–5120, 2001.
Journal of Nanotechnology 9
[28] M. Raffi, A. K. Rumaiz, M. M. Hasan, and S. I. Shah, “Studies ofthe growth parameters for silver nanoparticle synthesis by inertgas condensation,” Journal of Materials Research, vol. 22, no. 12,pp. 3378–3384, 2007.
[29] M. J. Rosemary and T. Pradeep, “Solvothermal synthesis ofsilver nanoparticles from thiolates,” Journal of Colloid andInterface Science, vol. 268, no. 1, pp. 81–84, 2003.
[30] N. K. Chaki, S. G. Sudrik, H. R. Sonawane, and K. Vijayamo-hanan, “Single phase preparation of monodispersed silver nan-oclusters using a unique electron transfer and cluster stabilisingagent, triethylamine,” Chemical Communications, no. 1, pp. 76–77, 2002.
[31] S. Shukla and S. Seal, “Cluster size effect observed for goldnanoparticles synthesized by sol-gel technique as studied by X-ray photoelectron spectroscopy,”Nanostructured Materials, vol.11, no. 8, pp. 1181–1193, 1999.
[32] P. T. Anstas and J. C. Warner, Green Chemistry: Theory andPractice, Oxford University Press, New York, NY, USA, 1998.
[33] J. A.Dahl, B. L. S.Maddux, and J. E.Hutchison, “Toward greenernanosynthesis,”Chemical Reviews, vol. 107, no. 6, pp. 2228–2269,2007.
[34] E. Perez-Tijerina, M. Gracia Pinilla, S. Mejıa-Rosales, U.Ortiz-Mendez, A. Torres, and M. Jose-Yacaman, “Highly size-controlled synthesis of Au/Pd nanoparticles by inert-gas con-densation,” Faraday Discussions, vol. 138, pp. 353–362, 2008.
[35] I.-S. Kang, M.-H. Kang, E. Lee, H.-S. Seo, and C. W. Ahn,“Facile, hetero-sized nanocluster array fabrication for investi-gating the nanostructure-dependence of nonvolatile memorycharacteristics,” Nanotechnology, vol. 22, no. 25, Article ID254018, 5 pages, 2011.
[36] T. Gacoin, “Sol-gel transition in CdS colloids,” Journal ofMaterials Chemistry, vol. 7, no. 6, pp. 859–860, 1997.
[37] Y. Yuan, J. H. Fendler, and I. Cabasso, “Photoelectron transfermediated by size-quantized CdS particles in polymer-blendmembranes,” Chemistry of Materials, vol. 4, no. 2, pp. 312–318,1992.
[38] B. L. Justus, R. J. Tonucci, and A. D. Berry, “Nonlinear opticalproperties of quantum-confined GaAs nanocrystals in Vycorglass,”Applied Physics Letters, vol. 61, no. 26, pp. 3151–3153, 1992.
[39] V. Sankaran, J. Yue, R. E. Cohen, R. R. Schrock, and R. J.Silbey, “Synthesis of zinc sulfide clusters and zinc particleswithin microphase-separated domains of organometallic blockcopolymers,” Chemistry of Materials, vol. 5, no. 8, pp. 1133–1142,1993.
[40] W. Ren, Z. Ai, F. Jia, L. Zhang, X. Fan, and Z. Zou, “Low tem-perature preparation and visible light photocatalytic activity ofmesoporous carbon-doped crystalline TiO2,” Applied CatalysisB, vol. 69, no. 3-4, pp. 138–144, 2007.
[41] H. G. Yang, C. H. Sun, S. Z. Qiao et al., “Anatase TiO2 singlecrystals with a large percentage of reactive facets,” Nature, vol.453, no. 7195, pp. 638–641, 2008.
[42] A. R. Tao, S. Habas, and P. Yang, “Shape control of colloidalmetal nanocrystals,” Small, vol. 4, no. 3, pp. 310–325, 2008.
[43] G. Frens, “Particle size and sol stability in metal colloids,”Colloid and Polymer Science, vol. 250, no. 7, pp. 736–741, 1972.
[44] M. Brust,M.Walker, D. Bethell, D. J. Schiffrin, and R.Whyman,“Synthesis of thiol-derivatised gold nanoparticles in a two-phase liquid-liquid system,” Journal of the Chemical Society,Chemical Communications, no. 7, pp. 801–802, 1994.
[45] S. Senapati, Biosynthesis and Immobilization of NanoparticlesandTheir Applications, University of Pune, Maharashtra, India,2005.
[46] T. Klaus-Joerger, R. Joerger, E. Olsson, and C.-G. Granqvist,“Bacteria as workers in the living factory: metal-accumulatingbacteria and their potential for materials science,” Trends inBiotechnology, vol. 19, no. 1, pp. 15–20, 2001.
[47] F. Cataldo, O. Ursini, and G. Angelini, “A green synthesis ofcolloidal silver nanoparticles and their reaction with ozone,”European Chemical Bulletin, vol. 2, no. 10, pp. 700–705, 2013.
[48] L. Sun, Z. J. Zhang, Z. S. Wu, and H. X. Dang, “Synthesis andcharacterization of DDP coated Ag nanoparticles,” MaterialsScience and Engineering A, vol. 379, no. 1-2, pp. 378–383, 2004.
[49] S.-Y. Zhao, S.-H. Chen, D.-G. Li, X.-G. Yang, and H.-Y. Ma, “Aconvenient phase transfer route for Ag nanoparticles,” PhysicaE, vol. 23, no. 1-2, pp. 92–96, 2004.
[50] E. Bulut andM. Ozacar, “Rapid, facile synthesis of silver nanos-tructure using hydrolyzable tannin,” Industrial and EngineeringChemistry Research, vol. 48, no. 12, pp. 5686–5690, 2009.
[51] I. Calinescu, M. Patrascu, A. I. Gavrila, A. Trifan, and C.Boscornea, “Synthesis and characterisation of silver nanopar-ticles in the presence of PVA and tannic acid,” U.P.B. ScientificBulletin B, vol. 73, no. 4, pp. 3–10, 2011.
[52] N. Gupta, A. Panwar, R. Kumar, S. K. Sharma, R. K. Sharma,and V. Agrawal, “Green synthesis of silver nanoparticles andtheir antibacterial activity against multi-drug resistant humanpathogens,” Advanced Science, Engineering and Medicine, vol. 5,no. 4, pp. 355–361, 2013.
[53] J. Dobias and R. Bernier-Latmani, “Silver release from silvernanoparticles in natural waters,” Environmental Science &Technology, vol. 47, pp. 4140–4146, 2013.
[54] M. Bayat and M. Khatibzadeh, “A review on green methods forsynthesis of silver nano particles,” in Proceedings of the Inter-national Conference Nanomaterials: Applications and Properties,vol. 2, no. 2, p. 6, 2013.
[55] L. Rainville, M.-C. Dorais, and D. Boudreau, “Controlled syn-thesis of low polydispersity Ag@SiO2 core-shell nanoparticlesfor use in plasmonic applications,” RSC Advances, vol. 3, pp.13953–13960, 2013.
[56] S. Trudel, “Unexpected magnetism in gold nanostructures:making gold even more attractive,” Gold Bulletin, vol. 44, no.1, pp. 3–13, 2011.
[57] T. Pradeep andA.Anshup, “Noblemetal nanoparticles forwaterpurification: a critical review,” Thin Solid Films, vol. 517, no. 24,pp. 6441–6478, 2009.
[58] W. Cai, T. Gao, H. Hong, and J. Sun, “Applications of goldnanoparticles in cancer nanotechnology,” Nanotechnology Sci-ence and Application, vol. 1, pp. 17–32, 2008.
[59] D. Philip, K. G. Gopchandran, C. Unni, and K. M. Nis-samudeen, “Synthesis, characterization and SERS activity ofAu-Ag nanorods,” SpectrochimicaActa A, vol. 70, no. 4, pp. 780–784,2008.
[60] G. C. Hadjipanayis and R. W. Seigel, Nanophase Materials: Syn-thesis, Properties andApplications, KluwerAcademic,Dodrecht,The Netherlands, 1994.
[61] M. K. Corbierre and R. B. Lennox, “Preparation of thiol-cappedgold nanoparticles by chemical reduction of soluble Au(I)-thiolates,” Chemistry of Materials, vol. 17, no. 23, pp. 5691–5696,2005.
[62] N. N. Long, L. V. Vu, C. D. Kiem et al., “Synthesis and opticalproperties of colloidal gold nanoparticles,” Journal of Physics,vol. 187, Article ID 012026, 2009.
[63] R. Das, S. S. Nath, and R. Bhattacharjee, “Optical properties oflinoleic acid protected gold nanoparticles,” Journal of Nanoma-terials, vol. 2011, Article ID 630834, 4 pages, 2011.
10 Journal of Nanotechnology
[64] S. Y. Moon, T. Kusunose, and T. Sekino, “CTAB-assistedsynthesis of size- and shape-controlled gold nanoparticles inSDS aqueous solution,” Materials Letters, vol. 63, no. 23, pp.2038–2040, 2009.
[65] A. Alshammari, A. Kockritz, V. N. Kalevaru, A. Bagabas, and A.Martin, “Influence of single use and combination of reductantson the size, morphology and growth steps of gold nanoparticlesin colloidal mixture,”Open Journal of Physical Chemistry, vol. 2,no. 4, pp. 252–261, 2012.
[66] D. Mahl, J. Diendorf, S. Ristig et al., “Silver, gold, and alloyedsilver-gold nanoparticles: characterization and comparativecell-biologic action,” Journal of Nanoparticle Research, vol. 14,no. 1153, pp. 1–13, 2012.
[67] T. Ahmad and W. Khan, “Size variation of gold nanoparticlessynthesized using tannic acid in response to higher chloroauricacid concentrations,” World Journal of Nano Science and Engi-neering, vol. 3, no. 3, pp. 62–68, 2013.
[68] A. Sedighi and P. C. H. Li, “Gold nanoparticle assists SNPdetection at room temperature in the nanoBioArray chip,”International Journal of Materials Science and Engineering, vol.1, no. 1, pp. 45–49, 2013.
[69] Y. Zhang, G. Chang, S. Liu, W. Lu, J. Tian, and X. Sun, “A newpreparation of Au nanoplates and their application for glucosesensing,” Biosensors and Bioelectronics, vol. 28, no. 1, pp. 344–348, 2011.
[70] E. A. Untener, K. K. Comfort, E. I. Maurer, C. M. Grabinski,D. A. Comfort, and S. M. Hussain, “Tannic acid coated goldnanorods demonstrate a distinctive form of endosomal uptakeand unique distribution within cells,” ACS Applied Materials &Interfaces, vol. 5, no. 17, pp. 8366–8373, 2013.
[71] J. Cookson, “The preparation of palladium nanoparticles,”Platinum Metals Review, vol. 56, no. 2, pp. 83–98, 2012.
[72] M. M. Kumari, S. A. Aromal, and D. Philip, “Synthesis ofmonodispersed palladium nanoparticles using tannic acid andits optical non-linearity,” Spectrochimica Acta A, vol. 103, pp.130–133, 2013.
[73] S. Cheong, J. D. Watt, and R. D. Tilley, “Shape control ofplatinum and palladium nanoparticles for catalysis,”Nanoscale,vol. 2, no. 10, pp. 2045–2053, 2010.
[74] H. Chen, G. Wei, A. Ispas, S. G. Hickey, and A. Eychmuller,“Synthesis of palladium nanoparticles and their applicationsfor surface-enhanced Raman scattering and electrocatalysis,”Journal of Physical Chemistry C, vol. 114, no. 50, pp. 21976–21981,2010.
[75] K. R. Gopidas, J. K. Whitesell, and M. A. Fox, “Synthesis,characterization, and catalytic applications of a palladium-nanoparticle-cored dendrimer,” Nano Letters, vol. 3, no. 12, pp.1757–1760, 2003.
[76] S. Cherevko, N. Kulyk, J. Fu, and C.-H. Chung, “Hydrogensensing performance of electrodeposited conoidal palladiumnanowire and nanotube arrays,” Sensors and Actuators B, vol.136, no. 2, pp. 388–391, 2009.
[77] P. Tobiska, O. Hugon, A. Trouillet, and H. Gagnaire, “Anintegrated optic hydrogen sensor based on SPR on palladium,”Sensors and Actuators B, vol. 74, no. 1–3, pp. 168–172, 2001.
[78] P. R. West, S. Ishii, G. V. Naik, N. K. Emani, V. M. Shalaev, andA. Boltasseva, “Searching for better plasmonic materials,” Laserand Photonics Reviews, vol. 4, no. 6, pp. 795–808, 2010.
[79] M. Yamauchi, R. Ikeda, H. Kitagawa, and M. Takata, “Nanosizeeffects on hydrogen storage in palladium,” Journal of PhysicalChemistry C, vol. 112, no. 9, pp. 3294–3299, 2008.
[80] S.-U. Rather, R. Zacharia, S. W. Hwang, M.-U. Naik, and K. S.Nahm, “Hydrogen uptake of palladium-embedded MWCNTsproduced by impregnation and condensed phase reductionmethod,”Chemical Physics Letters, vol. 441, no. 4–6, pp. 261–267,2007.
[81] M. Tristany, J. Courmarcel, P. Dieudonne et al., “Palladiumnanoparticles entrapped in heavily fluorinated compounds,”Chemistry of Materials, vol. 18, no. 3, pp. 716–722, 2006.
[82] Y. Xiong, J. Chen, B. Wiley, Y. Xia, Y. Yin, and Z.-Y. Li, “Size-dependence of surface plasmon resonance and oxidation for Pdnanocubes synthesized via a seed etching process,”Nano Letters,vol. 5, no. 7, pp. 1237–1242, 2005.
[83] A. Sartre, M. Phaner, L. Porte, and G. N. Sauvion, “STM andESCA studies of palladium particles deposited on a HOPGsurface,” Applied Surface Science, vol. 70-71, no. 1, pp. 402–406,1993.
[84] S. U. Son, Y. Jang, K. Y. Yoon, E. Kang, and T. Hyeon, “Facilesynthesis of various phosphine-stabilized monodisperse palla-dium nanoparticles through the understanding of coordinationchemistry of the nanoparticles,” Nano Letters, vol. 4, no. 6, pp.1147–1151, 2004.
[85] S.-W. Kim, J. Park, Y. Jang et al., “Synthesis of monodispersepalladium nanoparticles,” Nano Letters, vol. 3, no. 9, pp. 1289–1291, 2003.
[86] Y. Xiong, J. Chen, B. Wiley, Y. Xia, S. Aloni, and Y. Yin,“Understanding the role of oxidative etching in the polyolsynthesis of Pd nanoparticles with uniform shape and size,”Journal of the American Chemical Society, vol. 127, no. 20, pp.7332–7333, 2005.
[87] P. Mohanpuria, N. K. Rana, and S. K. Yadav, “Biosynthesis ofnanoparticles: technological concepts and future applications,”Journal of Nanoparticle Research, vol. 10, no. 3, pp. 507–517, 2008.
[88] M. Gericke and A. Pinches, “Biological synthesis of metalnanoparticles,” Hydrometallurgy, vol. 83, no. 1–4, pp. 132–140,2006.
[89] N. Karousis, G.-E. Tsotsou, F. Evangelista, P. Rudolf, N. Ragous-sis, and N. Tagmatarchis, “Carbon nanotubes decorated withpalladium nanoparticles: synthesis, characterization, and cat-alytic activity,” Journal of Physical Chemistry C, vol. 112, no. 35,pp. 13463–13469, 2008.
[90] S.Devarajan, P. Bera, and S. Sampath, “Bimetallic nanoparticles:a single step synthesis, stabilization, and characterization of Au-Ag, Au-Pd, and Au-Pt in sol-gel derived silicates,” Journal ofColloid and Interface Science, vol. 290, no. 1, pp. 117–129, 2005.
[91] V. Sanchez-Mendieta andA. R.Vilchis-Nestor, “Green synthesisof noble metal (Au, Ag, Pt) nanoparticles, assisted by plant-extracts,” inNobleMetals, Y.-H. Su, Ed., chapter 18, pp. 392–408,Intech, 2012.
[92] R. Herrera-Becerra, J. L. Rius, and C. Zorrilla, “Tannin biosyn-thesis of iron oxide nanoparticles,” Applied Physics A, vol. 100,no. 2, pp. 453–459, 2010.
[93] C. Finlay, G. Gunawan, A. S. Biris et al., “Novel microwave-assisted synthesis of renewable-resource based carbon-magnetite nanocomposites,” Journal of Wood Chemistry andTechnology, vol. 32, no. 3, pp. 268–278, 2012.
[94] G. Chang, Y. Luo, X. Qin et al., “Synthesis of Pt nanopar-ticles decorated 1, 5-diaminoanthraquinone nanofibers andtheir application toward catalytic reduction of 4-nitrophenol,”Journal of Nanoscience and Nanotechnology, vol. 12, no. 9, pp.7075–7080, 2012.
[95] X. Huang, L. Li, X. Liao, and B. Shi, “Preparation of platinumnanoparticles supported on bayberry tannin grafted silica
Journal of Nanotechnology 11
bead and its catalytic properties in hydrogenation,” Journal ofMolecular Catalysis A, vol. 320, no. 1-2, pp. 40–46, 2010.
[96] A. Shah, L.-U. Latif-Ur-Rahman, R. Qureshi, and Z.-U. Zia-Ur-Rehman, “Synthesis, characterization and applications ofbimetallic (Au-Ag, Au-Pt, Au-Ru) alloy nanoparticles,” Reviewson Advanced Materials Science, vol. 30, no. 2, pp. 133–149, 2012.
[97] W. Lu, X. Qin, A. M. Asiri, A. O. Al-Youbi, and X. Sun, “Facilesynthesis of novel Ni(II)-based metal-organic coordinationpolymer nanoparticle/reduced graphene oxide nanocompositesand their application for highly sensitive and selective nonen-zymatic glucose sensing,” Analyst, vol. 138, no. 2, pp. 429–433,2013.
