Engineering the morphology of Mn(ii)-chelated hybrid polyion nanocomplexes via tryptophan-directed assembly of double-hydrophilic block-copolymers for enhanced magnetic relaxation

Abstract

Self-assembled nanostructures from double-hydrophilic block copolymers (DHBCs) offer versatile platforms for engineering nanomaterials with tunable morphologies. Herein, we report a rational design strategy employing a tryptophan (Trp)-functionalized poly(acrylic acid)-block-poly(ethylene glycol) (PAA-b-PEG), where Trp end groups enable hydrophobic interactions and acid-triggered Mannich-type dimerization, driving vesicle formation in aqueous media. The resulting Trp-PAA-b-PEG vesicles display surface-exposed, metal-chelating PAA residues, providing a reactive interface for post-assembly functionalization. Two distinct Mn(II) chelation pathways were explored to modulate morphology. In the first, pre-formed Trp-PAA-b-PEG vesicles were exposed to aqueous Mn(II), yielding Mn(II)-coated vesicular nanostructures (v-Mn) via selective chelation to the PAA exterior. Second, Mn(II) ions were introduced during the hydration of dry-state polymer, enabling concurrent chelation and self-assembly. This co-assembly pathway favoured the formation of toroid-like micellar nanostructures (m-Mn), as confirmed by TEM, in which Trp groups cluster within the core while Mn-chelating PAA segments are localized internally and stabilized by PEG coronas. These two chelation modes—post-assembly surface coordination and in situ co-assembly—enabled controlled transformation between vesicular and toroidal morphologies. This Trp-guided, coordination-assisted assembly overcomes the intrinsic limitations of Mn(II)-driven DHBC self-assembly, enabling the fabrication of Mn(II)-chelating nanostructures with programmable morphology and tailored relaxometric profiles. This dual-pathway strategy demonstrates that combining Trp-mediated dimerization with metal coordination provides a facile route to morphology-controlled, Mn(II)-chelating DHBC nanostructures, offering tunable structural and functional properties for potential bioimaging applications.