Photochromic and photothermal hydrogels derived from natural amino acids and heteropoly acids†
A new class of supramolecular hydrogels have been designed and synthesized via the co-assembly of basic amino acids (AAs) and heteropoly acids (HPAs) under acidic conditions. The formation of gel-like samples is identified using an inverted tube method, rheology, and scanning and transmission electron microscopy. Fourier transform infrared spectroscopy reveals that the structural integrity of the HPAs is maintained during the gelation process. X-ray photoelectron spectroscopy (XPS) and proton nuclear magnetic resonance spectroscopy demonstrate that the anionic HPAs interact with both the protonated α-NH2 and the protonated side groups of the basic amino acids, initiating the preferential growth of one-dimensional nanofibers. These nanofibers bundle and entangle with each other to form extended three-dimensional network structures. The resulting AA/HPA supramolecular hydrogels show clear stereoselectivity of the basic amino acids. With the decreasing enantiomeric excess of the basic amino acids, the gelation propensity of the AA/HPA complexes is found to be depressed. The co-assembled hydrogels show the UV-responsive photochromic behaviour because of the presence of HPAs. The corresponding XPS data confirm that the photochromism of the hydrogels is attributed to the intervalence charge-transfer transition resulting from the reduction of HPAs. Interestingly, the reduced HPAs within the hydrogel matrix can absorb the near-infrared (NIR) light and exhibit photo-thermal conversion properties, which elevates the bulk temperature of the AA/HPA hydrogels and induces the gel-to-sol transition. This study unveils that HPAs have unique capacity to promote the gelation of basic amino acids for the construction of supramolecular soft materials with functional features.