Self-assembly Of Silver Nanoclusters Directed By Amphiphilic Block Copolymer

The ultra-small size(2-3 nm)of metal nanoclusters(MNCs)enables them many molecular-like characteristics,such as discrete energy level structure,enhanced fluorescence,magnetism,catalytic properties and so on.In recent years,self-assembly of metal nanoclusters mediated by small molecules has become a hot topic.On the one hand,the fluorescence can be enhanced,on the other hand,the fluorescence mechanism of metal nanoclusters will be enriched.However,the relatively poor stability and environment related driving forces limit the further development in this field.The key issue is how to improve the stability of metal nanoclusters during self-assembly process.Amphiphilic block copolymers can self-assemble into desirable and controllable morphologies under certain conditions because of the unique chemical structure.Based on above discussions,silver nanoclusters(Ag NCs)were synthesized by the ionic amphiphilic block copolymer,polystyrene block polymethacrylic acid(PS-b-PMAA).Fluorescent metal nanoclusters may self-assemble into sphere or vesicles morphologies driven by amphiphilic block copolymer,which is benefit for their applications in bio-imaging and theranostics.The main contents are shown as follows:1.The different block copolymer PS₄₁-b-PMMA₆₂ and PS₄₁-b-PMMA₁₈ were prepared successfully by Atom Transfer Radical Polymerization(ATRP)method(all the subscripts refer to the number of repeating units of polymer).Their structures were determined by FT-IR and ¹H NMR.PS₄₁-b-PMAA₆₂ and PS₄₁-b-PMAA₁₈ were obtained by hydrolysis and acidification of PS₄₁-b-PMMA₆₂ and PS₄₁-b-PMMA₁₈ under alkaline conditions.Dissolved in good solvent DMF followed by adding water as a selective solvent,polymer micelles were prepared successfully by dialysis.FESEM and FETEM indicated that both of the amphiphilic block copolymers can self-assemble into spherical micelles.Their sizes are dependent on the composition and concentration of the block copolymers.When the concentrations of PS₄₁-b-PMAA₆₂ and PS₄₁-b-PMAA₁₈ were fixed at 2.5 mg/m L,the average sizes of micelles were about 90 nm and 40 nm,respectively.When the concentration of PS₄₁-b-PMAA₆₂ increased to 10 mg/m L,the size increased to 120 nm.In addition,PS₄₁-b-PMAA₁₈ micelles can further assemble into nanofibers with width of 40-100 nm and length of 40?m,due to hydrogen bonding and electrostatic interaction between the hydrophilic blocks.2.The spherical micelles were used both as a template and directing agent for the synthesis of Ag NCs.The Ag NCs with red fluorescence were successfully prepared by photoreduction.The structure of the silver nanoclusters was determined by UV-vis absorption spectra,FT-IR and XPS.It also proved that the polymer structure remained during this process.Further explored the influence of different reaction conditions on the fluorescence properties of silver nanoclusters,such as the molar ratio of Ag⁺to MAA,UV irradiation time and pH.When the molar ratio of Ag⁺to MAA was 1:2,UV irradiation time was 15 h and pH was 7,the absolute fluorescence quantum yield of Ag NCs was 2.7%,and the average fluorescence lifetime was 0.98 ns.The FETEM images indicated that Ag NCs with size about 3 nm were successfully prepared templated by micelles.These nanoclusters exhibited good stability and the fluorescence intensity didn't show obvious change after 2months storage in dark.This work successfully synthesized Ag NCs mediated by polymer micelles.The effects of micelles size,composition and different reaction conditions on the fluorescence properties of Ag NCs are studied.It provides an avenue for the synthesis of metal nanoclusters using polymer both as self-assembly directing agent and template,and is expected to broad the applications of metal nanoclusters in intelligent response and nano medicine field.