Disentangling the Concentration of Au Nanoparticles Aggregates by Extinction, Photoacoustic Spectroscopy and Electrodynamics Modeling

Plasmonic nanoparticles are widely used as contrast agents in photoacoustic (PA) imaging owing to their efficient conversion of optical absorption into acoustic energy. In this work, we investigate the extinction and absorption properties of quasi-linear gold nanoparticle (AuNP) aggregates formed by self-assembly induced by the Ru(phen)32+ complex. Spherical citrate-stabilized AuNPs with an average diameter of 25 ± 5 nm were synthesized and characterized by transmission electron microscopy, UV–Vis spectroscopy, and photoacoustic spectroscopy. Upon addition of Ru(phen)32+, rapid aggregation occurs, giving rise to predominantly one-dimensional quasi-linear chains with controlled interparticle spacing and varying numbers of nanoparticles per aggregate. The aggregation process was monitored in real time by UV–Vis spectroscopy, while the absorption spectra of both isolated AuNPs and aggregates were independently measured using a calibrated photoacoustic setup. Rigorous electrodynamics simulations based on Mie theory and generalized multiparticle Mie theory, incorporating electronic confinement effects and an effective medium description of the nanoparticle environment, were employed to model the optical response. By combining experimental extinction and photoacoustic absorption spectra with theoretical simulations, we developed a quantitative methodology to determine the concentration and size distribution of AuNP aggregates in the colloidal dispersion. Despite their low abundance, larger aggregates were found to dominate the absorption response due to their significantly enhanced cross sections. The proposed approach provides a robust framework for quantitatively assessing the cluster-size composition of plasmonic nanoparticle assemblies and has important implications for the rational design and optimization of photoacoustic contrast agents for biomedical imaging applications.