Tunable glycyrrhizic acid supramolecular hydrogels via metal ion complexation

Glycyrrhizic acid (GA) is a natural edible chiral triterpene saponin that can undergo hierarchical self-assembly in water to form supramolecular hydrogels. Customizing the properties of GA supramolecular hydrogels is crucial for their wide food and biomedical applications, and the incorporation of specific metal ions can be an attractive way for this purpose. Herein, this study investigates the effects of metal ions with different valence states and concentrations on the linear and nonlinear rheological properties and network structures of GA supramolecular hydrogels. Monovalent metal ions (Na+ and K+), exhibiting weak binding affinity to GA, rely on the electrostatic screening effects at high concentrations (e.g., 50 mM) to enhance the GA interfibrillar interactions and thus the formation of a more compact and ordered gel network, which displays a pronounced nonlinear rheological behavior with a typical transition from elastic to viscous response. Polyvalent metal ions (Ca2+, Zn2+, and Al3+), owing to their greater charge density and stronger binding with GA, significantly enhance the network strength of the hydrogels at low concentrations (e.g., 5 mM). Nonetheless, at high ion concentrations (e.g., 50 mM), the GA-Mn+ forms discrete aggregated network structures due to excessively strong cation-carboxylate complexation, leading to the irregular nonlinear rheological responses and the lower resistance to large deformations. These findings can deepen our understanding of the highly tunable rheological behaviors and network structures of GA hydrogels, which demonstrates a new possibility for the design and development of responsive natural supramolecular hydrogels with controlled properties through the strategy of metal ion complexation.