Click-Ready Gold Nanoparticles from Aqueous Mechanochemistry: 2-Propynylamine as a Reducing Agent and Surface Ligand

We report a rapid aqueous method for synthesizing monodisperse gold nanoparticles (AuNPs), employing 2-propynylamine as both an intrinsic reducing agent and a surface-stabilizing ligand. This self-mediated process-achieved in a single step-yields spherical AuNPs with an average diameter of 4.0 ± 1.0 nm and a well-defined localized surface plasmon resonance band centered at 520 nm. Acting as a bifunctional molecule, 2-propynylamine simultaneously reduces HAuCl₄·3H₂O to elemental gold and passivates the nanoparticle surface through coordination via the amine group, while preserving a terminal alkyne (-C≡CH) functionality. This reactive moiety remains exposed and chemically accessible, enabling post-synthetic modification through Cu(I)-catalyzed azide-alkyne cycloaddition. Control experiments using alternate milling times and vial composition confirmed the essential role of 2-propynylamine in mediating both reduction and surface functionalization. The resulting alkyne-functionalized AuNPs serve as versatile "click-ready" platforms for bioconjugation, sensing, and advanced material assembly. Overall, this scalable, green approach eliminates the need for external reducing or capping agents and provides a modular route to chemically addressable nanomaterials with tunable surface reactivity.