Phytomediated selenium doped Au/ZnO/Fe₃O₄ nanocomposites for synergistic photocatalytic dye degradation and Mercury(II) detection

The sustainable fabrication of multifunctional nanocomposites remains a major challenge in environmental nanotechnology. In this study, a selenium-doped Au/ZnO/Fe₃O₄ nanocomposite (GW@Se-AZF) was successfully synthesized through a green-mediated route using Geranium wallichianum extract, which acted simultaneously as a reducing and stabilising agent. Structural and functional characterization confirmed the multiphase composition and nanoscale features of the product. UV-Vis analysis revealed a surface Plasmon resonance peak corresponding to Au incorporation, while FTIR spectra indicated phytochemical capping groups and metal-oxygen vibrations. XRD confirmed crystalline phases of ZnO, Fe₃O₄, Au, and Se with an average crystallite size of ~ 22 nm. SEM images demonstrated quasi-spherical, uniformly distributed nanoparticles (NPs), and EDX confirmed the elemental presence of Zn, Fe, Au, Se, and O. Functionally, the GW@Se-AZF nanocomposite exhibited dual performance. It showed highly selective and sensitive colorimetric detection of Hg²⁺ ions with a detection limit of 92.7 nM, attributable to synergistic interactions between Se doping and Au NPs. GW@Se-AZF demonstrated efficient photocatalytic activity under sunlight, achieving rapid degradation of organic dyes including methylene blue (≈ 91%), methyl orange (≈ 89%), and methyl violet (≈ 90%) within 60 min. This work demonstrates a sustainable strategy for developing multifunctional nanocomposites with integrated sensing, photocatalytic, and magnetic properties, offering significant potential for water purification and heavy metal monitoring in real environmental systems.