Lignin-Functionalized Fe3O4 Nanoparticles Facilitate the Extraction and Reduction of Metal Ions to Form Binary and Quinary Composite Nanoparticles

Lignin functionalized magnetic nanoparticles (NPs) were used for the interfacial biomineralization of Fe3O4-Au, Fe3O4-Ag, Fe3O4-Pd, Fe3O4-Pt binary, and Fe3O4-Au-Ag-Pd-Pt quinary nanocomposites in the aqueous phase at room temperature. The weak reducing ability of lignin adsorbed on the solid-liquid interface of magnetic NPs facilitated the interfacial reduction and extraction of metal ions. It was controlled by the availability of surface-adsorbed lignols as well as steric hindrances faced by the molecular geometry of the complex ions. Interfacial biomineralization synthesized pure Au, Ag, and Pd NPs in conjunction with Fe3O4 NPs to produce binary nanocomposites, whereas lignin preferred to reduce PtCl62- ions into PtO NPs. Quinary nanocomposites were mainly constituted by a combination of Au-Ag-Pd-Pt alloy and Fe3O4 NPs, where alloy NPs were synthesized by autocatalytic reduction in a stepwise manner. Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses determined the elemental composition of nanocomposites which placed the amounts of different metal NPs in the order Au > Ag > Pd > Pt. The maximum and minimum amounts of Au and Pt, respectively, were controlled by a delicate balance between the reduction potential and steric hindrances. Both binary and quinary nanocomposites strongly responded to the external magnetic field, thus facilitating their extraction and applicability in the aqueous phase.