A Generalized Mechanism for Ligand-Induced Dipolar Assembly of Plasmonic Gold Nanoparticle Chain Networks

The mechanism of gold nanoparticle chain assembly associated with the induction of electric dipole-dipole interactions arising from the partial ligand exchange of surface-adsorbed citrate ions by mercaptoethanol is investigated. UV-vis spectrophotometry and electron microscopy are used, respectively, to determine the kinetics and time-dependent structural changes associated with formation of the 1D nanoparticle superstructures between 5 and 50 degrees C. The results indicate that assembly of the plasmonic nanoparticle networks is extremely sensitive to changes in temperature. Formation of the nanoparticle chains is optimized at 25-30 degrees C and follows first order kinetics with increasing reaction rates attained for higher initial nanoparticle concentrations. Below 25 degrees C, plasmonic nanoparticle networks are produced but at a considerably reduced rate. In contrast, above 30 degrees C, short-chain networks form rapidly but the process is superseded by a secondary mechanism that limits chain growth and produces small fragments and isolated Au nanoparticles. The changes in assembly behavior are attributed to the temperature-dependent ordering and disordering of mercaptoethanol molecules associated with the gold nanoparticle surface. The results provide a general mechanistic model for the self-assembly of metallic nanoparticles based on ligand-induced electric dipolar interactions, which are globally under thermodynamic control but sensitive to kinetic aspects. It is also shown that the dipolar mechanism can be further exploited to introduce larger nanoparticles as topological dopants that reside specifically at branching points or termini in the self-assembled 1D nanoparticle networks.