A predictive framework for the reproducible seed-mediated synthesis and functionalization of gold nanorods

Gold nanorods (AuNRs) are versatile plasmonic nanostructures due to their tunable geometry and optical properties. The localized surface plasmon resonance (LSPR) can be adjusted across visible and near-infrared spectral domains through controlled growth. Despite their broad utilization, reproducible and predictive synthesis of AuNRs via seed-mediated growth remains challenging, as minor variations in seed solution often led to large discrepancies in final nanorod physicochemical properties. In this work, we present a comprehensive experimental and predictive framework that links the actual concentration of gold nanoseeds (AuNSs) to AuNR properties and systematically dissects and quantifies critical seed synthesis parameters. By investigating the influence of NaBH4 concentration, aging time, alkaline treatment, and precursor variability, we demonstrate that the seed optical response at 478 nm provides a robust empirical marker of active seed concentration. Across five distinct protocols, a reproducible linear relationship is observed between the longitudinal plasmon resonance (LPR) and the inverse absorbance within a well-defined growth regimen. Geometrical modeling is combined with experimentally determined nucleation and gold deposition yields, enabling kinetic insight and establishing predictive relationships among nanorod length, width, volume, and concentration in the growth solution. Analysis of the volume intercept reveals that AuNR formation proceeds through cooperative growth involving multiple nanoseeds, rather than a one-seed-one-rod mechanism. The 3D (length, width, concentration) theoretical model predicts nanorod dimensions and plasmonic response in excellent agreement with experimental data. The integration of absorbance-based kinetic descriptors and a validated 3D growth model marks a shift from empirical optimization to rational synthesis. Moreover, we demonstrate effective post-synthetic functionalization through entire ligand exchange of cytotoxic cetyltrimethylammonium bromide with a multidentate thiol-based polymer, yielding colloidally stable and biocompatible AuNRs without altering their plasmonic properties.