Gelation of graphene oxides induced by different types of amino acids

We have investigated gelation of graphene oxide (GO) sheets induced by amino acids. For gelation of single layer GO sheets, six different types of amino acids were added to GO suspension as gelators. To understand gelation mechanism, we varied the concentration and type of amino acids as well as the pH of suspension, and monitored the morphology and rheological properties of reaction mixtures. Gelation was observed in acidic pH only with three types of amino acids (arginine, tryptophan, and cysteine) whereas no gel was formed at other pH values (neutral and basic). As the type of amino acid was varied, both the binding strength between amino acid and GO and the moduli (G′ and G′′) of reaction mixtures followed the same order, arginine > cysteine > tryptophan > asparagine > aspartic acid > glycine. To rationalize these results, we considered interactions between amino acid side chains and GO sheets (i.e., electrostatic interaction, π–π stacking, and hydrogen bonding), and found that the hydrogel formed by electrostatic attraction with arginine exhibited shorter gel time and larger moduli than the other samples. Finally, synthesis of GO hydrogel at physiological pH was demonstrated by increasing the concentration of GO sheets.