Effect of Large Ammonium Cations on the Aggregation Kinetics of Citrate Capped Gold Nanoparticles

It is known that citrate capped gold nanoparticles (Ct-AuNP) aggregate when an electrolyte is added to the medium due to the electrostatic screening produced by the cations on the nanoparticle surface. Small cations are mostly employed when studying this effect. Instead, here we use the large monovalent cations coming from tetrabutylammonium chloride (Bu4N+Cl-), tetrapropylammonium chloride (Pr4N+Cl-) and tetraethylammonium chloride (Et4N+Cl-) in order to induce the aggregation of Ct-AuNP of 20 nm of hydrodynamic diameter. Then, we have followed the aggregation kinetics by means of dynamic light scattering and ultraviolet-visible spectroscopy. Also, we have measured the zeta potential and taken transmission electron microscopy images. We show that these large cations produce a quick aggregation which is stronger as larger is the cation. Thus, the results indicate that larger cations screen more efficiently the Ct-AuNP surface which can be ascribed to the weaker hydration of larger cations. On the other hand, the greater destabilising effect produced by larger cations is in agreement with the direct Hofmeister series.