Ligand Desorption and Surface Oxidation Drive Nanoparticle Coalescence in Diffusion-Limited Aggregation

We previously reported that peptides can drive diffusion-limited aggregation (DLA) of silver nanoparticles (AgNPs) into complex, fractal assemblies, but the mechanism behind this behavior remained unclear. In this study, we dissect how surface ligand chemistry governs this process: not merely by mediating interparticle interactions but by enabling structural transformation of the nanoparticle cores themselves. Using a panel of nine ligands spanning phosphine, thiolate, and polyphenol classes, we systematically examine how ligand identity dictates aggregation dynamics, redox behavior, and structural outcomes. Our findings reveal that aromatic phosphine ligands, especially bis(p-sulfonatophenyl)phenylphosphine (BSPP), uniquely promote DLA by facilitating partial ligand desorption and silver surface oxidation, which together enable nanoparticle coalescence and recrystallization into micron-scale fractal architectures. In contrast, thiol- and polyphenol-based ligands either bind too strongly to permit coalescence or lack the electronic features necessary to support oxidation-driven restructuring. We support this mechanism with multiple lines of evidence: zeta potential data demonstrate charge neutralization, X-ray photoelectron spectroscopy confirms the presence of oxidized silver species in BSPP-AgNP fractals, and enhanced light scattering suggests increased surface heterogeneity. Electron microscopy and elemental analyses validate the resulting architecture and ligand distribution. These findings establish a mechanistic framework for chemically directed DLA and introduce a tunable strategy for building structurally fused, hierarchical nanomaterials for biosensing, catalysis, and soft matter engineering.