Aging effect on the enhancement of thermal stability, mechanical stiffness and fluorescence property of a peptide-appended naphthalenediimide based two component hydrogel

A dipeptide attached naphthalenediimide (NDI)-containing amphiphilic molecule (NDIP) self-assembles into nanotubes in aqueous solution at pH 6.6 as revealed by high-resolution transmission electron microscopy. This peptide-appended NDI forms a two component hydrogel in the presence of tartaric acid in molar ratio 1:2. The morphological transformation was observed from nanotube structure in the non-gel aggregated state of peptide appended NDI to interconnected cross-linked nanofibers of the two component hydrogel in the presence of tartaric acid. Interestingly, the gel exhibits an unusual behavior upon aging compared to the fresh gel. It is found that the thermal stability and gel stiffness increase very significantly upon aging. Another important feature noted is that very weakly-fluorescent nature of the fresh gel is transformed to a bright greenish fluorescence upon aging. This result suggests that intermolecular interactions among the gelator molecules and tartaric acid in the gel phase slowly increase with time to form a mechanically very stiff and thermally robust gel.