Roles Of Active Components From Plant Extract In The Biosynthesis Of Au Nanoparticles

Currently,the plant-inspired synthesis of gold nanoparticles(GNPs)has become a hot research issue due to mild reaction conditions,environmental friendly process and low toxicity of products.Dozens of plant wre reported to prepare GNPs with various size and morphologies.However,the study on the plant-inspired synthesis of GNPs still remained in the preliminary stage because of plant species diversity and complexity in components of extract.In the previous studies,the plant extract was identified through the analysis of infrared spectra or determination of the active composition before and after the reduction,which was difficult to avoid interference and influence between the different effective components.In this dissertation,Cacumen platycladi leaves extract(CPLE)was chosen to synthesize GNPs and the formation mechanism of GNPs,the reducing and stabliizing capacity of different biomolecules including sugars,protein,polyphenols and flavonoids,and the morphology and size regulation of nanoparticles were investigated.Firstly,the roles of representative sugars and proteins according to the group difference in the synthesis of GNPs were identified.The results showed the reduction ability of sugars was the weakest and the interaction between the sugars compounds/functional groups and GNPs was poor.For the functional groups in sugars,the sequence of reduction ability from strong to weak followed the order of aldehyde group,ketone group and hydroxyl group.Compared with sugars,the reaction rate and reduction capacity of amino acid are different due to the variety of functional groups.The sequence of reaction rate from high to low was acidic,neutral and alkaline amino acid.For the reduction capacity of functional groups,phenolic hydroxyl and imidazole were the strongest,less with benzene ring and hydroxyl,and the least with amino,carboxyl and guanidine.In addition,the protective ability of amino acids was higher than that of sugars,but it was also relatively weak.The amino and carboxyl group tend to be coated directly on the GNPs.Phenol hydroxyl,benzene ring,guanidine enhanced the isotropic growth of GNPs,while imidazole was beneficial to the anisotropic growth of GNPs.In addition,the reduction ability of macromolecular protein was weaker than that of amino acids and sugars,however,its stabilizing capacity was strong,leading to formation of monodisperse spherical GNPs with small and uniform size.Secondly,the reducing and stabliizing capacity of typical polyphenols and flavonoids/functional groups in the biosynthesis of GNPs were investigated.It was found that the reduced ability of polyphenols was the strongest.The UV-Vis characterization and density functional theory(DFT)calculations showed that the more ortho phenolic hydroxyl groups in the polyphenols,the stronger the reducing ability,and the presence of carboxyl groups can enhance the electron donating capability to improve reducing power.Meanwhile,the phenolic hydroxyl groups in polyphenols had the strongest protective ability for GNPs,which was favorable for the isotropic growth of nanoparticles to form spherical nanoparticles.Kinetic studies indicated that,when the concentration of polyphenolic compounds increased,the reaction rate constant k became greater,the nucleation rate turned faster,and the Avrami index n decreased.Thus,the growth dimension of nanoparticles decreased,and the isotropic growth of GNPs increased.In addition,the reducing and stabliizing capacity of flavonoids were stronger than those of sugars and protein,but weaker than polyphenols.Finally,according to the differential polarity in CPLE,the main biological components were separated by flash column chromatography.The interaction between plant active compounds and GNPs,the formation mechanism of GNPs with different morphologies,and the morphology and size control of GNPs were also studied.The results manifested that polyphenols showed the strongest reduction ability among the different active components in CPLE,followed by flavonoids,and sugars exhibited the weakest reduction ability.The reduction ability of the main functional groups,in turn decreased on adjacent phenolic hydroxyl,phenolic hydroxyl,aldehyde,ketone and hydroxyl group.Different active components in CPLE exhibited varied adsorption capacity and protective performance for GNPs.The adsorption capacity of polyphenols was stronger than that of flavonoids and sugars.In terms of the protective performance of different active components,sugars were weak,polyphenols were easy to provide isotropic protection,and flavonoids tend to coat GNPs directionally.The formation mechanism of different morphology nanoparticles was deduced according to the particle evolution of TEM images during the synthesis.Phenolic hydroxyl group was easy to isotropically adsorbed on the surface of GNPs to form spherical particles.Under the slow nucleation rate,the functional groups such as the carbonyl group,which was the oxidation product of phenolic hydroxyl,could be directed to form {111} plane of GNPs and lead to flaky nanoparticles.When the nucleation rate was accelerated,there was not enough time for carbonyl group to directionally cover on the surface of GNPs,leading to the anisotropic growth of GNPs and the formation of flower-like particles.