Enhancing Stability of Metallic Magnesium Nanoparticles toward Oxidation in Water via PEG-Phosphonate Passivation

Metallic magnesium nanoparticles (MgNPs) offer unique opportunities for nanoplasmonics due to their optical properties, sustainability, and low cost. Yet their rapid oxidation in water severely limits practical use. In this work, we report the structural stabilization of colloidally synthesized MgNPs toward oxidation through surface functionalization with α-methoxy-ω-phosphonic acid poly(ethylene glycol) (PPEG1000). Three functionalization strategies were evaluated: post-synthesis grafting, one-pot addition after nucleation, and pre-addition of PPEG prior to MgNP formation. The role of functionalization timing on nanoparticle morphology and structural stability was assessed. TEM analysis shows that bare MgNPs form well-defined hexagonal platelets, whereas early PPEG addition disrupts Mg(II) reduction, yielding polymer-embedded aggregates. In contrast, one-pot introduction after initial nucleation arrests further growth, producing a star-like morphology, while post-functionalization retains the anisotropic platelet morphology. FTIR and TGA confirm phosphonate binding and PPEG surface coverage, with polymer loadings of 18 wt% (one-pot) and 4 wt% (post-functionalized). Water-dispersion assays reveal dramatically improved stability, extending from minutes for bare MgNPs to 40 min (one-pot) and 3 h (post-functionalized). These findings establish phosphonic-acid PEG ligands as effective passivating agents for MgNPs as a key parameter for morphology and stability control in aqueous environments.