The Synthesis And Chiroptical Studies Of Ag Nanoparticles Modified With Small Chiral Organic Molecules

Ag nanomaterials have a wide range of applications in different fields due to their unique optical,electronic and catalytic properties.In this paper,chiral organic molecules such as mercapto-cholesterol,sodium cholate and cysteine were used as chiral inducing agents.Silver nanoparticles and Ag@mSiO₂ nanoparticles were prepared by photochemical reduction,liquid-phase chemical reduction and St?ber method,respectively.By varying the experimental conditions,the factors that influence the CD signal of silver nanoparticles were explored.By the liquid-phase chemical reduction method,sodium citrate-stabilized silver nanoparticles were prepared by reducing of AgNO₃ with NaBH₄ in the presence of sodium citrate as a stabilizer.Mercapto-cholesterol modified silver nanoparticles were obtained by adding organic chiral small molecule mercapto-cholesterol to sodium citrate-stabilized silver sol.The addition of mercapto-cholesterol promoted the aggregation of silver nanoparticles,resulting in the appearance of new absorption peaks at 520 nm for the silver nanoparticles.The circular dichroism signal appeared as a split Cotton effect.The addition of mercapto-cholesterol has a significant effect on the CD signal.With the increase of the amount of mercapto-cholesterol,the CD signal of the system increases.In order to improve the stability of silver nanoparticles,PVP-stabilized silver nanoparticles were prepared by ligand exchange reaction.The mercapto-cholesterol modified silver nanoparticles obtained from this system showed a positive Cotton effect,indicating that the degree of aggregation of silver nanoparticles affects its circular dichroism response.Using photochemical reduction method,sodium cholate modified silver nanoparticles were prepared by using sodium cholate as a reducing agent to reduce AgNO₃ under ultraviolet light.In this system,sodium cholate also acts as a chiral inducing agent and stabilizer in addition to acting as a reducing agent.As the illumination time increases,the amounts of silver nanoparticles gradually increase and the CD signals gradually enhance.The concentration of sodium cholate has an effluence on the CD signal of silver nanoparticles.As the concentration of sodium cholate increases,the obtained silver nanoparticles are more stable,which results in the decrease of the degree of aggregation of Ag nanoparticle and the weakening of the CD signal.It was found that adding KCl to the system helps to shorten the time of photochemical reaction,but too much KCl will decrease the stability of the silver nanoparticles and cause the aggregation of the nanoparticles.Cysteine-modified silver nanoparticles were prepared by in-situ reduction of AgNO₃ in the presence of L-Cys and D-Cys as chiral agents.The concentration of cysteine affects the circular dichroism signal.When the cysteine concentration is relatively low,the prepared silver nanoparticles with bimodal size distribution exhibit weaker CD signals.When the concentration of cysteine is relatively high,the prepared silver nanoparticles with average sizes below 3 nm show a strong CD signal,and the CD signals come from the absorption of cysteine molecules adsorbed on the surface of the silver nanoparticles.Ag@mSiO₂ was synthesized by the St?ber method.By adding cysteine to the surface of the silver core,Ag@mSiO₂ composite nanoparticles with circular dichroism response were prepared.The concentration of L-Cys has effect on the circular dichroism signal.By increasing the concentration of cysteine in the solution,the circular dichroism signal of the prepared Ag@mSiO₂ composite nanoparticles gradually enhances.