Iron nanoparticles-based supramolecular hydrogels to originate anisotropic hybrid materials with enhanced mechanical strength

Here, we report the synthesis and structural characterization of novel iron nanoparticles (FeNPs)-based short-peptide supramolecular hydrogels. These hybrid hydrogels composed of Fmoc-diphenylalanine (Fmoc-FF) peptide and FeNPs were prepared through the self-assembly of Fmoc-FF in a suspension containing FeNPs in the presence or absence of an external magnetic field. Optical images of these hydrogels revealed the formation of column-like aggregates of FeNPs when the gels were formed in the presence of a magnetic field. Moreover, the intricate structure derived from the interwoven nature of the fiber peptides with these FeNP column-like aggregates resulted in anisotropic materials, more rigid under shear forces applied perpendicularly to the direction of the aggregates, presenting under these conditions values of G' (storage modulus) about 7 times those of the native hydrogel. To the best of our knowledge, this is the first example in which the mechanical properties of peptide hydrogels were strongly enhanced due to the presence of FeNPs. A theoretical model trying to explain this phenomenon is presented. Quite interesting CD, FTIR and synchrotron X-ray diffraction analyses indicated that the anti-parallel beta-sheet arrangement of Fmoc-FF peptide was highly conserved in the hydrogels containing FeNPs. Moreover, FLCS measurements showed that the diffusion of a small solute through the hydrogel network was improved in hydrogels containing FeNPs, probably caused by the formation of preferential channels for diffusion. Taken together, our results provide a new method for the synthesis of novel hybrid Fmoc-FF-FeNPs anisotropic hydrogels with enhanced mechanical strength and water-like diffusion behavior, thus easing their application in drug delivery and tissue engineering.