Preparation of water-dispersible silver-decorated polymer vesicles and micelles with excellent antibacterial efficacy

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Abstract
It is well known that silver nanoparticles have excellent antibacterial activities. However, to prepare well-defined, water-dispersible and long-term stable silver nanoparticles still remains insufficient. Presented in this paper are the design and preparation of new water-dispersible silver-decorated polymer vesicles and micelles based on a new kind of amphiphilic block-statistical copolymer, PEO-b-P(DMA-stat-tBA) (polymer 1) and its partially hydrolyzed derivative, PEO-b-P(DMA-stat-tBA-stat-AA) (polymer 2). Here PEO stands for poly(ethylene oxide), DMA for 2-(dimethylamino)ethyl methacrylate, tBA for t-butyl acrylate and AA for acrylic acid. PEO and poly(tBA) are designed to provide permanent hydrophilicity and hydrophobicity for macromolecular self-assembly in water, respectively. Poly(DMA) is introduced for the coordination of Ag+ ions to form silver nanoparticles in situ upon reduction. Poly(AA) is designed to serve the scaffold for the silver nanoparticle formation in the micelle core by electrostatic interactions with Ag+ ions. The silver-decorated polymer vesicles and micelles have been prepared in five steps: Firstly, polymer 1 was synthesized by Atom Transfer Radical Polymerization (ATRP) in the presence of CuBr/PMDETA in methanol at 60 °C. Secondly, a fraction of hydrophobic poly(tBA) in polymer 1 was hydrolyzed into hydrophilic and functional poly(acrylic acid) (PAA) to form polymer 2 in the presence of trifluoroacetic acid (TFA). Thirdly, polymer 1 was self-assembled into polymer vesicles with tuneable sizes ranged from 100–700 nm in basic water–DMF and polymer micelles with ∼50 nm diameter in neutral water–DMF, respectively. Polymer 2 was self-assembled into small micelles with ∼20 nm diameter by direct dissolution in water. Fourthly, the silver precursor AgNO3 was introduced into the polymer micelle or polymer vesicle solution where the Ag+ ion was absorbed in the core of the micelles or the membrane of the vesicles. Finally, the Ag nanoparticles were formed in the core of the micelles or the membrane of the vesicles when the reducing agent sodium borohydride was introduced. 1H NMR, GPC, DLS, UV and TEM were employed to characterize the structure and composition of the block copolymers, polymer micelles and vesicles, and silver-decorated polymer micelles and vesicles. Those water-dispersible silver-decorated polymer micelles and vesicles showed excellent antibacterial efficacy against Escherichia coli (E. coli) with quite low minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC).

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