Robust Calix[4]arene-Polyethyleneimine Coated Iron Oxide Nanoparticles for Enhanced Recovery of Gold and Platinum Chloride Complexes

Magnetic removal and recovery of precious metals from wastewater and complex biological media pose significant challenges mostly due to the need for efficient, selective, and stable materials. This work reports a methodology that allows to address these challenges by synthesizing iron oxide nanoparticles (IONPs) coated with a covalent layer of calix[4]arene-tetracarboxylate (X4C4) capable of binding polyethyleneimine (PEI) functionalities via electrostatic interactions. In contrast to citrate coating, which was previously utilized as an attachment layer for PEI, the reductive grafting of X4C4-tetra-diazonium salts onto IONPs results in a considerably more stable coating that proves to be an excellent substrate for the adsorption of PEI. This efficiently results in a synergistic interaction that significantly improves the durability of the PEI coating and maintains the particles in a dispersed state. The stability of the resulting IONPs@X4C4@PEI particles is demonstrated by their ability to withstand both acidic and alkaline conditions without significant particle aggregation or loss of magnetic properties. Moreover, these particles exhibit exceptional magnetic reusability, retaining their selectivity and recovery efficiency over multiple separation cycles. The selective affinity of IONPs@X4C4@PEI particles for Au and Pt stems from the specific binding interactions between the complexes formed by these metals in solution and the PEI coating, enabling efficient recovery of these precious metals. This work places these IONPs at the forefront in terms of stability, reusability, and selectivity, which will undoubtedly open new avenues for environmental remediation and purification applications.