A green chemical approach for the synthesis of gold nanoparticles: characterization and mechanistic aspect

This IRCSET-EMPOWER(Irish Research Council for Science, Engineering and Technology Postdoctoral Research Grant) project aims to improve current methodology for the synthesis of metal nanoparticles (NPs). The development of efficient methodology for
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  PROJECT UPDATE A green chemical approach for the synthesis of goldnanoparticles: characterization and mechanistic aspect Sujoy K. Das • Enrico Marsili Ó Springer Science+Business Media B.V. 2010 Abstract ThisIRCSET-EMPOWER(IrishResearchCouncil for Science, Engineering and TechnologyPostdoctoral Research Grant) project aims to improvecurrent methodology for the synthesis of metalnanoparticles (NPs). The development of efficientmethodology for metal nanomaterials synthesis is aneconomical and environmental challenge. While thecurrent methods for NPs synthesis are often energy-intensive and involve toxic chemicals, NPs biosynthe-sis can be carried on at circumneutral pH and mildtemperature, resulting in low cost and environmentalimpact. Nanomaterial biosynthesis has been alreadyobserved in magnetotactic bacteria, diatoms, andS-layerbacteria,however,controlledNPsbiosynthesisis a relatively new area of research with considerablepotential for development. A thorough understandingof the biochemical mechanism involved in NPsbiosynthesis is needed, before biosynthetic methodscan be economically competitive. The analysis andidentification of active species in the nucleation andgrowth of metal NPs is a daunting task, due to thecomplexity of the microbial system. This project work focuses on the controlled biosynthesis of gold NPs byfungal microorganisms and aims to determine thebiochemical mechanism involved in nucleation andgrowth of the particles. Keywords Nanobiotechnology Á Gold nanoparticles Á Microbial synthesis Á Living nanofactory Á Green chemical approach 1 Introduction The synthesis of metal nanoparticles (NPs) is agrowing area of research in materials science becausethey exhibit unique properties (Gratzel2001; Xiaet al.2003), different from those of bulk metals dueto their unique size and shape dependent character-istics. Because of stability, oxidation resistance, andbiocompatibility, gold NPs find wide applications inelectronics and photonics, catalysis, informationstorage, chemical sensing and imaging, drug deliveryand biological labeling (Elghanian et al.1997; Caoet al.2002). For each application, NPs of differentsize and shape are needed (Jin et al.2001; Sun andXia2002), thereby synthetic protocols for theproduction of size and shape controlled monodisperseNPs are required. Since Faraday’s pioneering work in1857 on the synthesis of colloidal gold by reducingNaAuCl 4 with a solution of phosphorus in carbondisulphide (Fig.1) (The Royal Institution of GreatBritain2008; Thomson2007), several physical and chemical methods have been developed to producegold NPs. Synthetic techniques based on the reduc-tion of metal ions with sodium citrate or sodiumborohydride, followed by surface modification of the S. K. Das Á E. Marsili ( & )School of Biotechnology, Dublin City University,Collins Avenue, Dublin 9, Irelande-mail:  123 Rev Environ Sci BiotechnolDOI 10.1007/s11157-010-9188-5  produced particles with suitable capping ligands andorganic solvents (Niemeyer et al.1998; Le´vy et al.2004), raised environmental concerns, because of thetoxic compounds used in the process. Also, it isdifficult to obtain NPs of defined size and shapes(e.g., spheres, rod, cubes, hexagons, etc.) in highyield. Current synthetic methods result in mixed-shape NPs that require expensive and low-yieldpurification procedures, such as differential centrifu-gation (Murphy2002). These limitations invite neweco-friendly (‘‘green chemistry’’) methodology forproduction of nanocrystals with desired shape.Natural processes such as biomineralization maybe mimicked to design efficient NP synthesis tech-niques. Biomineralization processes exploit biomo-lecular templates that interact with the inorganicmaterial throughout its formation resulting in thesynthesis of particles with defined shape and size(Mann1993). Bones, teeth and shells are typicalexamples of structural materials produced by naturalbiomineralization processes. A general scheme forNP synthesis in microorganisms is illustrated inFig.2.The adoption of biomineralization methods in thesynthesis of nanostructured materials is expected toyield novel and more complex structural entitiescomparedtothoseobtainedwithconventionalmethods(Brown et al.2000; Klaus et al.1999; Mukherjee et al. 2001; Xie et al.2007a,2007b). Both uni and multicel- lularmicroorganismsarereportedtoproduceinorganicnanomaterials either intra- or extracellularly. Somewell-known examples (Mann et al.1990; Oliver et al.1995; Sleytr et al.1999) of microbial mediated synthesis of inorganic materials include magnetotacticbacteria Magnetospirillum magnetotacticum (whichsynthesize magnetite NPs), S-layer bacteria Synecho-coccus sp., Bacillus stearothermophilus (whichproduce gypsum and calcium carbonate layers), anddiatoms Coscinodiscus sp., Cylindrotheca fusiformis (which synthesize siliceous materials). However, theuse of microorganisms in the deliberate and controlledNP synthesis is a new area of research.1.1 Bacteria in nanoparticle synthesisAmong the microorganisms, prokaryotes havereceived the most attention in the area of NPbiosynthesis. Beveridge and Murray (1980) havedemonstrated that gold NPs readily precipitate inbacterial cells following incubation of the cells withAu ? 3 ions under ambient temperature and pressure.Organic phosphate compounds play a role in thein vitro development of octahedral Au, possibly asbacteria–Au complexing agents. Fe ? 3 reducing bacte-ria Shewanella algae can reduce Au ? 3 ions in anaer-obic environments. In the presence of  S. algae andhydrogen gas, the Au ions are completely reducedand 10–20 nm gold NPs are formed (Konishi et al.2004). Klaus–Joerger and co-workers (Klaus et al.1999) have demonstrated that Pseudomonas stutzeri AG259 isolated from a silver mine reduces Ag ? ionsand forms silver NPs of well-defined size andmorphology, ranging from a few to 200 nm or more,within the periplasmic space. Fig. 1 Faraday’s colloidalsuspension of gold ( a ) (TheRoyal Institution of GreatBritain2008); Highresolution transmissionelectron microscopic image( b ) of individual colloidalgold particles (at amagnification of 10 7 9 ),prepared according toFaraday’s recipe (Edwards,Thomas2007)Rev Environ Sci Biotechnol  123  1.2 Fungi in nanoparticle synthesisThe use of fungi in the synthesis of NPs is a relativelyrecent addition and holds promise for large scale NPproduction. In fact, fungi secrete large amounts of theenzymes involved in NP synthesis and are simplerto grow both in the laboratory and at industrialscale. Different fungal and actinomycete species, i.e., Fusarium oxysporum , Verticillium sp., Thermomo-nospora sp., Rhodococcus sp . have been reported(Ahmad et al.2003; Mandal et al.2006) to synthesize NPs intra- or extracellularly. Shankar et al. (2004)synthesized gold nanoplates by fungal extracts.A brief over view on microbial synthesis of metalNPs is given in Table1.1.3 Mechanism of nanoparticle synthesisWhile a large number of microbial species are capableof producing metal NPs, the mechanism of nanopar-ticle biosynthesis has not been established. Themetabolic complexity of viable microorganisms com-plicatestheanalysisandidentificationofactivespeciesin the nucleation and growth of metal NPs. Recentworks by Xie et al. (2007a) demonstrated that proteinsare the principal biomolecules involved in the algal Table 1 Synthesis of nanoparticles by different microorganismsMicroorganisms Metal nanoparticle ReferencesBacteria  Bacillus subtilis Gold Beveridge and Murray (1980) Shewanella algae Gold Konishi et al. (2004) Pseudomonas stutzeri Silver Klaus et al. (1999)  Lactobacillus Gold, silver, Au–Ag alloy Nair and Pradeep (2002) Escherichia coli Gold Brown et al. (2000)  Rhodococcus Gold He et al. (2007)Fungi Verticillium Gold, silver Mukherjee et al. (2001) Fusarium oxysporum Gold, silver, Au–Ag alloy Ahmad et al. (2003); Mandal et al. (2006); and there in Colletotrichum sp. Gold Shankar et al. (2003) Fig. 2 Schematicof biomineralizationprocess fornanoparticlesynthesisRev Environ Sci Biotechnol  123  synthesis of gold NPs. Other researchers (Ahmad et al.2003; He et al.2007) have postulated that microor- ganisms secrete enzymes, which may be responsiblefor the reduction of metal ions, which result in the NPsnucleation and growth. Ahmad et al. (2003) postulatedthat a NADH-dependent reductase is involved in AgNPs synthesis by Fusarium oxysporum . However, thebiochemical mechanism of metal ion reduction andsubsequent NP formation remains unexplored.The elucidation of the biochemical pathways lead-ing to gold biomineralization is necessary to develop arational approach to NP biosynthesis. A number of issues need to be addressed from the nanotechnologyand microbiological points of view before suchbiosynthetic procedures can compete with the conven-tional protocols.Preliminary experiments (Fig.3) carried out in ourlaboratory demonstrated the synthesis of single crystalgold NPs from HAuCl 4 when incubated with myceliaorcell-freeextractof   Rhizopusoryzae (Dasetal.2008,2009).‘Green chemistry’ aims to employ environmentallybenign solvents and nontoxic chemicals in syntheticmethods, thereby reducing their environmental impact(AnastasandWarner1998).ThisIRCSET-EMPOWERproject will adopt such ‘green chemistry’ approach tosynthesize gold NPs using microorganisms as a‘living nano-factory’ by avoiding any chemicalagents, to determine the biochemical mechanismsinvolved in the biomineralization process, and toisolate and purify the enzymes involved in the NPformation. 2 Methodological approaches The present project work aims to (1) synthesize goldNPs through room temperature reduction of gold ionsby R. oryzae ; and (2) determine of the biochemicalmechanism involved in the nucleation and growth of gold NPs. R. oryzae will be grown in the laboratory aspreviouslydescribed(Dasetal.2009)andthecell-free Fig. 3 Atomic forcemicroscopic images ( a – b )of gold nanoparticlessynthesized on the surfaceof the fungal mycelia ( a lowresolution; b highresolution); high resolutiontransmission electronmicroscopic image ( c )shows that the averageparticles size is 20 nm;selected area electrondiffraction (SAED) pattern( d ) indicates that thesynthesized goldnanoparticles are singlecrystalRev Environ Sci Biotechnol  123  extract will be prepared from R. oryzae myceliafollowing harvesting from the growth medium. Thecell-free extract will then be used for the synthesis of gold NPs.The NP will be characterized through TransmissionElectron Microscopy (TEM), Atomic Force Microscopy(AFM), X-ray photoelectron spectroscopy (XPS),UV–visible and Fourier Transform Infrared spectros-copy (FTIR). The crystal structure of the NPs will bedetermined with the selected area electron diffraction(SAED) pattern obtained from TEM image.The biochemical pathway involved in the NPbiosynthesis will be assessed using heat inactivatedor metabolically inhibited microorganisms (e.g.,through sodium cyanide, formaldehyde, and 2,4-dini-trophenol) as control experiment. The enzyme(s)responsible for reduction of Au ? 3 to Au 0 will beobtained from the cell-free extract through severalpurification steps: ammonium sulphate fractionation,anion-exchange chromatography, chromatofocusing,and gel filtration. The purified enzyme(s) will then beused for the synthesis of gold NPs as described above.Kinetic measurements of enzyme activity for gold NPsynthesiswillthenbeundertakenattheoptimalpHandtemperature, as determine in separate experiments.The shape controlled synthesis of gold NPs by purifiedenzyme(s) will be also performed using varyingreaction conditions, such as the concentration of goldions, the pH of the solution, and the incubation period.Alltheseconditions controlthecrystalgrowth kineticsand final NP morphology. Since the enzyme(s)secreted by R. oryzae act both as reducing and cappingagent,theadsorptionofsuchenzyme(s)onthegrowingcrystals and their reducing activity changes with theabove mentioned conditions, thereby resulting indifferent crystal shapes. 3 Impact of the proposed research Metal NPs are relevant to numerous emerging tech-nologies. The development of high yield and low costmethods for NP production is therefore an importantchallenge. Current methods for metal NP productionrequire harsh chemicals and energy-intensive pro-cesses. It is consequently important to develop an eco-friendlysustainable(‘‘greenchemistry’’)alternativetothe existing chemical methods. The microbial-medi-ated synthesis of metal NPs may replace some of thecurrent physical and chemical methods in use for NPproduction.However,severalissuesneedtobeaddressedbefore such biosynthetic procedures can competewith established protocols. Comprehension of bio-chemical mechanism involved in gold nanoparticleformation is crucial to the development of innovativeand low-energy NPs production processes. This IRC-SET-EMPOWER project deals with pioneer researchon the synthesis of gold nanostructures employingmicroorganism simultaneously as the reducing andcapping agent. This environmental-friendly method-ology may be applied in various pharmaceutical andbiomedical formulations, as well as in cellular imag-ing, biosensing, and drug delivery. Acknowledgment Funding source: IRCSET-EMPOWER2009 Postdoctoral Research Grant. References Ahmad A, Mukherjee P, Senapati S, Mandal D, Khan MI,Kumar R, Sastry M (2003) Extracellular biosynthesis of silver nanoparticles using the fungus Fusarium oxyspo-rum . Colloids Surf B Biointerfaces 28:313–318Anastas PT, Warner JC (1998) Green chemistry: theory andpractice. Oxford University Press, New York, p 30Beveridge TJ, Murray RGE (1980) Site of metal deposition inthe cell wall of  Bacillus subtilis . J Bacteriol 141:876–887Brown S, Sarikaya M, Johnson EA (2000) Genetic analysis of crystal growth. J Mol Biol 299:725–735Cao YC, Jin R, Mirkin CA (2002) Nanoparticles with Ramanspectroscopic fingerprints for DNA and RNA detection.Science 297:1536–1540Das SK, Das AR, Guha AK (2008) Synthesis of gold nano-particles: a green chemical approach. International con-ference on soft system ICSS-2008, Kolkata, India, 3–15February 2008Das SK, Das AR, Guha AK (2009) Gold nanoparticles:microbial synthesis and application in water hygienemanagement. Langmuir 25:8192–8199Edwards PP, Thomas JM (2007) Gold in a metallic dividedstate—from faraday to present-day nanoscience. AngewChem Int Ed 46:5480–5486Elghanian R, Storhoff JJ, Mucic RC, Letsinger RL, Mirkin CA(1997) Selective colorimetric detection of polynucleotidesbased on the distance-dependent optical properties of goldnanoparticles. Science 277:1078–1081Gratzel M (2001) Photoelectrochemical cells. Nature 414:338–344He S, Guo Z, Zhang Y, Zhang S, Wang J, Gu N (2007) Bio-synthesis of gold nanoparticles using the bacteria Rho-dopseudomonas capsulate . Mater Lett 61:3984–3987Jin R, Cao Y, Mirkin CA, Kelly KL, Schatz GC, Zheng JG(2001) Photoinduced conversion of silver nanospheres tonanoprisms. Science 294:1901–1903Rev Environ Sci Biotechnol  123
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