Gold Nanoparticles Aggregation: Drastic Effect of Cooperative Functionalities in a Single Molecular Conjugate

Aggregation of gold nanoparticles (AuNPs) can be utilized in chemical and biomolecular sensing as a sensitive and easy-to-visualize process. However, interpretation of experimental results requires a clear understanding of physicochemical processes that take place upon multiple interactions between an analyte and AuNPs. In this article, interactions between citrate-stabilized AuNPs and organic compounds bearing various functional groups in an aqueous medium were experimentally and theoretically studied using spectrophotometry of the localized surface plasmon resonance (LSPR), transmission electron microscopy (TEM), conductometry, zeta potential measurements, and finite-difference time-domain (FDTD) modeling. As a result, it has been found that organic compounds containing both thiol and amine groups strongly promote the aggregation of AuNPs due to their cooperative functionalities. FDTD modeling has enabled consideration of the light extinction (i.e., LSPR response) properties of nanoparticle aggregates involving single, chain-like, and globular structures. Taking one billion distributions of differently structured aggregates into account, the theoretical light extinction was fitted to that of the experimental result with a root-mean-square deviation of 7%.