Biological Buffer Tris as a New Capping Agent for the Synthesis of Zero-Valent Nickel Nanoparticles

We present a straightforward and effective approach for synthesizing highly stable, pure, and well-dispersed zero-valent nickel nanoparticles (Ni NPs) with a distinct bipyramid morphology in an aqueous medium. The synthesis was carried out at room temperature through the sequential addition of tris(hydroxymethyl)aminomethane (Tris), sodium hydroxide (NaOH), and ascorbic acid. In this method, NaOH serves to adjust the pH, while ascorbic acid functions as the reducing agent. The amine groups of Tris coordinate with Nickel(II) nitrate hexahydrate (Ni(NO3)2.6H2O) via lone pair electrons, promoting the formation of a more stable Ni(II)-amine complex. At an alkaline pH of 9, ascorbic acid is deprotonated to ascorbate ions, which reduce Ni2+ to elemental Ni0. Simultaneously, the amine ligands remain anchored to the surface of the nanoparticles, serving as stabilizing agents. Furthermore, the hydroxyl groups of Tris contribute to stabilization through non-covalent hydrogen bonding with the Ni0 surface. Together, the amine and hydroxyl functionalities provide steric protection, effectively shielding the Ni0 particles from oxidation by repelling dissolved oxygen and hydroxide ions. The formation of zero-valent Ni NPs was confirmed by UV-Visible spectroscopy (UV-Visible), while Fourier-transform infrared spectroscopy (FTIR) verified the presence of surface-bound amine and hydroxyl groups. Zeta potential analysis confirmed that the zero-valent Ni NPs possess a surface charge attributed to Tris capping. X-ray diffraction (XRD) analysis confirmed the face-centered cubic (fcc) structure of Ni NPs. Scanning electron microscopy (SEM) revealed a bipyramidal nanoparticle morphology, and transmission electron microscopy (TEM) indicated a moderate size distribution. These findings demonstrate that Tris, a common biological buffer, serves as an excellent chelating and capping agent for the sustainable synthesis of zero-valent Ni NPs. The resulting zero-valent Ni NPs may thus have potential applications in spectroscopy, catalysis, electrochemical sensing, and biological systems.