The influence of Fe2+ on the self-assembly of a bipyridine containing homopolymer: From bowl-shaped nanoparticles to vesicles

Abstract

Metal ion coordination has critical influence on the self-assembly behavior of amphiphilic polymers and the morphology of the obtained assemblies. Herein, an amphiphilic homopolymer with 2,2′-bipyridine (BPy) as side chain is synthesized (noted as PBPyAA), which can self-assemble into bowl-shaped nanoparticles (BNPs) in tetrahydrofuran (THF)/water. Taking advantage of the coordination interaction between BPy and metal ions, Fe²⁺ is chosen to regulate the self-assembly behavior and the morphology of the assemblies of PBPyAA in two pathways. (Ⅰ) The aqueous solution of Fe²⁺ with various concentrations is added to the THF solution of PBPyAA during self-assembly. (Ⅱ) Fe²⁺ is added into the THF solution of PBPyAA before self-assembly, followed by the addition of deionized water to promote the self-assembly. The results show that the pathway Ⅰ facilitates the coordination of BPy and Fe²⁺. With the increase of the concentration of Fe²⁺ aqueous solution, the coordination efficiency of BPy increases from 0.419 % to 7.789 %, leading to the transformation of BNPs to vesicles. Though the coordination efficiency of BPy also increases with the concentration of Fe²⁺ in pathway Ⅱ, which is still quite low of 0.274 % to 0.366 %, and the morphology of the BNPs barely changes. In addition, ethylene diamine tetraacetic acid (EDTA), a strong chelating agent, is also added to promote the competitive complexation with BPy, resulting in the dissociation of BPy and Fe²⁺ and the reversible transformation from vesicles to BNPs. Overall, the effect of Fe²⁺ coordination on the self-assembly behavior of PBPyAA in two pathways is investigated and the reversible transformation of BNPs to vesicles is also achieved.