Chirality control of multi-stimuli responsive and self-healing supramolecular metallo-hydrogels

Abstract

A metallo-hydrogel based on histidine and Zn²⁺ ions is designed using coordination driven self-assembly. The hydrogel formation is instantaneous and it exhibits stimuli-responsive behaviour with respect to pH, heat and external chemicals. The gelation ability depends on the optical purity and enantiomeric excess of the amino acids. The supramolecular chirality mediated by the Zn–histidine complex and subsequent fiber formation showed sign inversion of the CD signal with respect to that of native amino acids (L- or D-) and can be tuned by changing the ratio of the two chiral species, L-, D-amino acids. The Job's plot from the circular dichroism probing supramolecular chirality confirms the 1 : 1 complex of Zn and histidine. The viscoelastic properties and kinetics of the hydrogel can conveniently be tuned by changing its enantiomeric excess (ee%), concentration and the ratio of Zn and histidine. Moreover, the resulting metallo-hydrogel is thixotropic and exhibits instantaneous, intrinsic self-healing behaviour.