Controlled Oxidation and Halide Coordination for Fractal Gold Nanoparticles with Applications in Photoacoustic Imaging.

Hierarchical nanoparticle assemblies formed through diffusion-limited aggregation (DLA) exhibit broad plasmonic coupling, amplified electromagnetic hotspots, and enhanced optical properties. We recently demonstrated that peptide-induced oxidative coalescence drives DLA in bis(p-sulfonatophenyl) phenylphosphine (BSPP)-stabilized silver nanoparticles, where silver's low reduction potential (Ag+/Ag0 = +0.80 V) permits the surface dissolution and redeposition that fuse particles into fractal architectures. Extending this approach to gold has remained elusive: gold's nearly 2-fold higher reduction potential (Au3+ /Au0 = +1.50 V) renders surface atoms inert to the mild oxidants that readily mobilize silver. Here, we overcome this redox barrier by treating BSPP-stabilized gold nanoparticles with hydrogen peroxide (H2O2) in the presence of halide ions and the cationic peptide RRK. A systematic screen of halide identity (F-, Cl-, Br-, I-) reveals that bromide uniquely balances oxidative etching against structural preservation: Bromide coordination lowers the effective gold oxidation potential, enabling H2O2-driven surface atom mobilization without complete dissolution. The order of reagent addition is critical─halide must interact with the gold surface before peptide-mediated clustering, or assembly is suppressed. X-ray photoelectron spectroscopy confirms partial surface oxidation, and electron microscopy reveals branched, hierarchical assembly spanning hundreds of nanometers. The resulting gold fractals exhibit broad near-infrared extinction (680-950 nm) and produce ∼2-fold photoacoustic signal enhancement over dispersed or aggregated AuNPs at 700 nm, establishing a route to DLA-derived plasmonic contrast agents that circumvent the cytotoxicity and structural instability of their silver counterparts.