Plant-mediated Synthesis Of Silver And Gold Nanomaterials By Biomass-based Reduction And Their Potential Applications

Noble metal nanomaterials have attracted extensive attention owing to their uniquecatalytic and optical properties.Therefore,their preparation and application have become oneof research highlights in the field of nanotechnology.Compared with the traditional physicaland chemical methods of synthesizing metal nanomaterials,the bioreduction methods basedon microorganisms or plants have emerged as cost-effective and environmentally benignapproaches to highly stable metal nanomaterials in recent years.Therefore,we envision thatthey will be used to produce commercial metal nanomaterials with promising market prospect.In contrast to microorganisms,the use of readily available plants and their extracts cancircumvent laborious biological screening and cultivation.Hence,plants and their extractswill be better options for synthesis of metal nanomaterials than microorganisms.However,atpresent,the researches on plant-based bioreduction have been focused on simple synthesisand characterization of nanomaterials.Many fundamental problems are far from clear.Forinstance,shape and size control of noble metal nanomaterials have met very limited success.Moreover,process design of plant-based biosynthesis and optimization of operation conditionhave not been reported yet.Sloving such problems are of great significance for establishing anew protocol on shape and size control of noble metal nanomaterials,and practicaltechnology.Silver and gold nanomaterials were fabricated via reduction of Ag⁺ and[AuCl₄]^- by driedplant biomass and their extract in this work.Concerning the above status of plant-basedbiosynthesis,through screening of plant biomass,process design and optimization ofplant-based biosynthesis,and design of reactors,this work aimed at establishing a newprotocol on shape and size control of Ag and Au nanomaterials.Furthermore,application ofthe Ag nanoparticles (AgNPs) as antimicrobials and Au nanoparticles (AuNPs) as catalystswas preliminarily explored,respectively.Firstly,AgNPs and AuNPs were synthesized by sundried C.camphora leaf.Ag or Auseeds were added to the mixture of the precursors and C.camphora leaf extract to tune theshape and size of AgNPs or AuNPs.Not only could silver nanoparticles ranging from 55 to 80nm in size be fabricated,but also gold nanotriangles with edge length range of 25-150 nm orspherical gold nanoparticles with size range of 10-35 nm could be easily modulated byadjusting the amount of C.camphora leaf biomass.The presence of Ag seeds promotedanisotropic growth of AgNPs while that of Au seeds improved the monodispersity of AuNPs.The polyol components such as polysaccharide,reducing sugar,flavones,etc were mainlyresponsible for the reduction of silver ions or chloroaurate ions.Secondly,richful and readily available plant biomass of traditional Chinese medicines(TCMs) were used to tune the shape of Ag and Au nanomaterials and particle size of AgNPsand AuNPs at 30℃.And general knowledge based on the biosynthesis by TCMs was proposed.AgNPs and AuNPs could be synthesized by dried powder of F.Carthami,F.Chrysanthemi,H.Epimedii Brevicornus,S.Prunellae Vulgaris and H.Lysimachiae.AgNWswith mean diameter of 45 nm and length of at least 10μm could be fabricated by driedpowder of F.Gardeniae and F.Lonicerae.The nanowires grew along the direction of[110]through chain connection of spheriodal AgNPs.Furthermore,Ag nanoplates could beprepared by dried powder of F.Lonicerae and F.Magnoliae Officinalis.The overlappinggrowth of Ag nanoplates led to dentritic Ag crystal.Anisotropic growth index (S) of plantbiomass based on their FTIR analyses could be used as one of referred indexes of screeningplant biomass.Anisotropic Ag nanostructures were promoted when S was more than 1.0,while isotropic Ag nanostructures were promoted when S was less than 1.0.This workexemplified the universal application of plant bioresources for the synthesis of AuNPs.Thesize,shape and associated optical properties of the AuNPs could be tuned by different TCMs.The nascent gold nanoplates were formed by aggregation of small AuNPs less than 10 nm.Thirdly,continuous-flow biosynthesis of Ag nanoparticles (AgNPs) by C.camphora leafextract in tubular microreactors was investigated.At the proper glycerin bath temperature 90℃,nucleation of silver nuclei and growth of AgNPs could be separated and secondarynucleation could be avoided to attain AgNPs with narrow size distribution.The proper angleof Y junction is 60°or 90°.And the proper volumetric flow rate is 0.5 or 0.8 mL.min^-1.At thesame volumetric flow rate,the nucleating rate of silver nuclei could be increased bydecreasing the inner diameter to produce AgNPs with narrow size distribution.The properinner diameters of the tubes were 2 or 3 mm.Lastly,on one hand,the antibacterial properties of AgNPs by the methods of liquidculture medium and solid culture medium were evaluated and the representative E.coli(gram-positive bacteria) and S.aureus (gram-negative bacteria) were used as tested strains.On the other hand,supported Au/TiO₂ catalysts were fabricated by in-situ bioreduction withthe extract of C.Platycladi and their catalytic activities were tested by catalytic reduction of4-nitrophenol (4-NP) into 4-acetaminophenol (4-AP) as model reaction.The results showedthat AgNPs have a good antibacterial property against these two strains;the minimuminhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of AgNPstowards E.coli is 1.4 ppm and 27 ppm,respectively,whilst the MIC of AgNPs towards S.aureus is 5.4 ppm.The AuNPs produced by dried powder of C.Platycladi were highly stableand exhibited excellent catalytic activity towards reduction of 4-NP to 4-AP.Furthermore,theAu/TiO₂ catalysts also showed good catalytic activity towards reduction of 4-NP to 4-AP.Generally,the Au/TiO₂ catalysts fabricated at 30℃possessed better catalytic activity thanthose at 60 or 90℃.And their catalytic performance could be enhanced by calcination at 300℃.