Histidinehydroxamic acid as new biomolecule to produce molecular-like fluorescent gold nanoclusters: Possible mechanisms for metal ion sensing

[Display omitted] •Hydroxamic acid-stabilized gold nanoclusters (Au NCs) were produced for the first time.•Few-atomic nanostructures possess intense blue emission with quantum yield of 4%•Chelating hydroxamate moiety on NCs’surface is effective to design metal ion sensors.•Copper ions can exchange the ligand from the surface causing fluorescence quenching.•Fluorescence enhancement is detected for coordinating zinc ions on metallic surface. We firstly demonstrate a new biomolecule, Histidinehydroxamic acid (HisHA), to produce few-atomic fluorescent gold nanoclusters (Au NCs) in aqueous medium. The preparation protocol has been optimized by studying the effect of metal ion/biomolecule molar ratio, metal ion concentration, pH, temperature, reaction time as well as the role of citrate as mild reducing agent on appearance of blue-emitting (λem = 440 nm, λex = 365 nm) molecular-like NCs. Structural studies confirmed that imidazole-N and amino-N of the HisHA stabilize the formed metallic cores. The quantum yield of ∼ 4% and fluorescence lifetime of 4.2 ns were determined. Moreover, these NCs show suitable stability under high inert salt (cNaCl = 2.5 M) concentration as well. Verifying its ability to detect metal ions, dual strategies were discovered. We confirmed that the copper ions cause fluorescence quenching (LOD = 2.49 µM) by pushing the higher amount of soft HisHA ligand from the metallic surface and forming complexes with dominantly hydroxamate-[O,O] coordination mode in the aqueous medium. For Zn2+-ions, a “turn-on” sensing mechanism was observed; the smallest detectable amount of Zn2+ is 7.5 µM. Linear increase of the quantum yield (from ∼ 4% to ∼ 11.5%) was identified above 75 μM of Zn2+ due to the binding of the Zn2+-ions on the cluster surface via hydroxamate-[O,O] donors.