Tryptophan-stabilized gold-manganese oxide nanocomposites for enhanced superoxide dismutase activity

Syntheses of gold–manganese oxide nanocomposites were attempted by a redox-mediated growth method under varying mild reaction conditions with amino acid as a stabilization agent; finally, the nanocomposites were applied for superoxide dismutase (SOD)-mimic activity. Growth reaction was performed by the reduction of Au(III) with Mn(0) powder on the surface of citrate-stabilized gold nanoparticles as seeds. Variable reaction conditions were attempted to monitor the effect of the pH and, finally, optimized based on the critical properties of the nanocomposites including their long-term stability. In a neutral medium, tryptophan-stabilized Au–Mn3O4 nanocomposites were obtained. Stable Au–Mn2O3 nanocomposites were formed at basic pH in the presence of hydrophobic amino acids. The present work elucidates the role of amino acids, especially tryptophan, in stabilizing gold–manganese oxide nanocomposites. The effect of crystalline vs. the amorphous nature of Mn3O4 sheets in the tryptophan-stabilized nanocomposites was evaluated in SOD-mimetic applications. The IC50 values for the newly synthesized Au–Mn3O4 nanocomposites with crystalline or amorphous Mn3O4 sheets at room temperature were found to be 125 times and 25 times better with respect to the reported Mn3O4 nanoparticles synthesized after calcination at 600 °C. These results provide useful insights into the synthesis of gold–manganese oxide nanocomposites with tunable properties and their potential applications in the growing field of nanozymes.