Green Synthesis of Hydroxylatopillar[5]arene-Modified Gold Nanoparticles and Their Self-Assembly, Sensing, and Catalysis Applications

A novel, green, one-pot synthesis of gold nanoparticles (AuNPs) was obtained by the redox reaction between AuCl₄^– and hydroxylatopillar[5]­arene (HP5) in aqueous solution with the aid of OH^– at room temperature without the need of a traditional harsh reducing agent such as NaBH₄, N₂H₄, etc. Monodisperse AuNPs with a uniform diameter of ∼5.0 nm are fabricated via the proposed one-step colloidal synthesis route by using HP5 as both reducing agent and stabilizer, while AuNPs cannot be effectively protected by noncyclic monomers of HP5. The FTIR, ¹³C NMR, and XPS studies demonstrated that the hydroxy groups in HP5 reduce Au³⁺ into Au⁰, which leads to nucleation, growth, and formation of AuNPs, and the hydroxy groups themselves are oxidized to carboxyl groups. It is surprising that the HP5 functionalized AuNPs can self-assemble and form multiple well-defined architectures, including vesicles, like nanotubes, and one-/two-dimensional (1D/2D) nanostructures without the need of a guest mediator. The self-assembly mechanism was also studied. Moreover, the prepared HP5@AuNPs could be employed as not only scaffolds but energy acceptors for turn-on fluorescence sensing based on a competitive host–guest interaction. In addition, the AuNPs exhibited very excellent catalytic activity for the reduction of 4-nitrophenol (4-NP). We believe that the versatile HP5@AuNPs could be potentially used in the field of self-assembly, sensing, and catalysis