Synthesis and characterization of novel blue-emitting nicotinamide-gold nanoclusters with “chain-breaker” antioxidant property

Abstract

• Synthesis of nicotinamide-reduced gold nanoclusters was optimized. • Molar ratio and pH have dominant role on the formation of blue-emitting nanoclusters. • Fluorescence emission and lifetimes confirm uniform size with few-atomic composition. • These novel clusters possess intense blue fluorescence and antioxidant effect as well. In this paper, we firstly demonstrate a simple and reproducible synthesis protocol to produce nicotinamide-stabilized gold nanoclusters (NAM-Au NCs) under mild conditions. The dominant role of the pH and the ligand to metal ion molar ratio on the formation of these few atomic fluorescent NCs were proven. The prepared NCs show intensive blue-emission at 380 nm with two main lifetime components (4.7 and 6.7 ns), which refers to an ultra-small Au NCs structure and nearly uniform composition. The metallic oxidation state of the metal content was identified by X-ray photoelectron spectroscopy, while infrared studies strongly confirm the binding of aromatic-N and the amide-N to the metal core. The oxygen radical capacity (ORAC) tests were carried out in the case of the stabilizing nicotinamide and the prepared NAM-Au NCs using Trolox molecule as reference between 1 and 100 µM concentration range. Based on the fluorometric measurements, we demonstrated that our blue-emitting Au NCs also possess antioxidant activity with 0.48 ± 0.03 Trolox equivalent value. We clearly proved that our sub-nanometer-sized NAM-Au NCs show novel molecular-like optical features against the pure nicotinamide, but the antioxidant effect was also preserved. These coupled properties make our NCs as potential agents for biocatalytic reactions.