Self-Templated HPEI Nanoreactors: Cross-Linking-Directed In Situ Growth of Gold Nanoparticles in Reusable Polymer–Metal Catalytic Films

Hyperbranched polyethylenimine (HPEI)-derived cross-linked polymers ( P 25 – P 100 ) were investigated as multifunctional platforms for the in situ synthesis and stabilization of gold nanoparticles (Au NPs). The intrinsic redox activity of the polymer enabled Au(III) reduction without external reducing agents, yielding localized surface plasmon resonance (LSPR) bands in the 500–540 nm region. Comparative studies using hydrazine and citrate revealed that external reductants accelerated nucleation but did not improve long-term colloidal stability. Systematic variation of cross-linking density demonstrated that nanoparticle formation, dispersion, and optical response are strongly governed by the structural flexibility of the polymer network, with the intermediate cross-linked system P 50 providing the optimal balance between reduction efficiency and stabilization. Transmission electron microscopy (TEM) analysis confirmed the formation of quasi-spherical Au NPs confined within the polymer matrix, while Fourier transform infrared spectroscopy (FTIR) indicated coordination of nitrogen-containing groups to the nanoparticle surface. The resulting P 50 /Au NPs film exhibited efficient catalytic activity in the reduction of nitroaromatic compounds (NACs) under mild conditions, maintaining activity over multiple cycles without detectable metal leaching. These findings establish cross-linked HPEI matrices as multifunctional reducing, stabilizing, and supporting platforms, positioning them as promising materials for the design of reusable heterogeneous polymer–metal hybrid catalysts that combine facile recovery with sustained operational stability.