Histidine-induced Synthesis Of Anisotropic Silver Nanoparticles And The Investigation Of Their Formation Mechanism

Anisotropic silver nanoparticles(Ag NPs)have presented essential application value and broad development prospects in areas,such as optical sensing,biochemical analysis,disease diagnosis and industrial catalysis,due to their unique optical,electrical and catalytic properties.In recent twenty years,anisotropic silver particles,such as rods,wires,polyhedrons,sheets,have been successfully prepared.However,the synthesis of these particles commonly requires an organic phase,which leads to environmental pollution and biotoxicity.For aqueous synthesis,the high reactivity of silver ions makes it harder to well regulate the synthesis of Ag NPs.It is still difficult to tune the component and reactivity of silver precursors efficiently to quantify the synthesis of various anisotropic silver particles.In this paper,we prepared silver particles with various morphologies,such as flower-like,octopod,hollow and dendritic Ag NPs,by using histidine as a ligand,silver nitrate as a silver source and hydroxylamine as a reductant under strong alkaline condition.We systematically investigated the effects of p H and histidine concentration on the reactivity of silver precursors and the formation mechanisms of silver particles with different morphologies.In addition,we explored the relationship between the size,morphology of silver particles and their surface-enhanced Raman scattering(SERS)and catalytic properties,preliminarily realizing the application of silver materials in the field of biological detection.The main contents of this paper are as follows:In Chapter 2,we synthesized nearly monodisperse silver nanoflowers(NFs)via one-pot method by using silver ion-histidine complex as a silver precursor.The experimental results showed that the introduction of histidine could regulate the branch lengths of silver nanoflowers by increasing the p H,because higher p H led to the higher reducing activity of hydroxylamine,which promoted the rapid reduction of silver ions.At the same time,a control experiment in lower histidine concentration proved that the reactivity of silver precursor was reduced after the complexation of silver ions with histidine,and the dominant growth of the crystal plane of silver particles(111)was promoted under the directing role of histidine.Silver particles with small size attached randomly onto the larger Ag particles,finally resulting in the formation of flower-like Ag NPs.The silver NFs products exhibited excellent peroxidase activity at near neutral p H,which was due to the stronger hydrogen bond and electrostatic interaction between the particles and the substrate under such a condition.Based on the reducing ability of ascorbic acid(AA)towards substrate 3,3,5,5-tetramethylbenzidine(TMB),we developed a simple colorimetric method for the detection of AA,which presented a linear range of 2.0～70.0μM.We also applied this method to the determination of AA concentration in bovine serum samples,which proved the potential applications of this strategy in clinical detection and biological research.In Chapter 3,we synthesized silver octopods by using silver oxide particles as precursors and templates after pseudomorphic transformation.By changing the concentration of histidine introduced into the reaction,the morphology of silver oxide evolved from cubes to eight-branched particles.After adding the reducing agent hydroxylamine,the silver oxide was transformed into silver particles through pseudomorphic transformation,which made the resulting silver particles perfectly retain the size and morphology of their silver oxide precursors.When the concentrations of histidine introduced were increased,the morphology of silver particles evolved from cubes to eight-branched structures,and the LSPR peak of these silver particles shifted from 499 nm to 1380nm.Under the optimized condition,the eight-branched silver particles showed highly reproducible and good SERS performance.Under 785 nm excitation,the enhancement factor(EF)of the particles towards p-mercaptobenzoic acid(p-MBA)was 7.6×10~5,which made the particles qualified as SERS active materials with significant competitive power in the NIR-I and II window.In chapter 4,we synthesized hollow silver particles by using Ag OH-coated silver oxide as a precursor.By adjusting p H and histidine concentration,we prepared hollow,semi-hollow,solid and branched solid particles,and their LSPR peak could be continuously tuned from 823 nm to 1165nm.By conducting control experiments,such as heating silver precursors and reducing them after stirring for a long time,which led to the formation of solid particles,we confirmed that the formation of hollow Ag structures was related to a small amount of silver hydroxide on the surface of silver oxide.The silver oxide inside the silver precursor particle diffused outwards,contributing to the hollow silver particles.In chapter 5,dendritic silver particles were synthesized with a mixture of silver oxide and silver hydroxide as precursors.By adjusting the p H value,the morphology of silver particles could be adjusted from spherical,sea urchin-like to cubic shapes.By characterizing silver precursors at different p H values and the investigation of the variation of the silver precursors with time,it is inferred that silver hydroxide in the silver precursors promoted the generation of dendritic silver particles.In addition,by controlling the concentration of histidine in the reaction,we obtained sea urchin-like,snowflake-like and cauliflower-lie silver particles,which further proved that histidine could induce the formation of silver dendritic structures.