Partial Aggregation of Silver Nanoparticles Induced by Capping and Reducing Agents Competition

It is well known that nanomaterials properties and applications are dependent on the size, shape, and morphology of these structures. Among nanomaterials, silver nanoparticles (AgNPs) have attracted attention since they have considerably versatile properties, such as a variable surface area to volume ratio, which is very useful for many biomedical and technological applications. Within this scenario, small nanoparticle aggregates can have their properties reduced due to the increased size and alterations in their shape/morphology. In this work, silver nanoparticles aggregation was studied through chemical reduction of silver nitrate in the presence of sodium borohydride (reducing agent) and sodium citrate (capping agent). By changing the amount of reducing agent along the reaction, unaggregated and partially aggregated samples were obtained and characterized by UV-vis, zeta potential, and SAXS techniques. pH was measured in every step of the reaction in order to correlate these results with those obtained from structural techniques. Addition of the reducing agent first causes the reduction of Ag+ to silver nanoparticles. For higher concentrations of sodium borohydrate, the average AgNPs size is increased and NPs aggregation is observed. It was found that zeta potential and pH values have a strong influence on AgNPs formation, since reducing agent addition can induce partial removal of citrate weakly associated on the AgNPs surface and increase the ionic strength of the solution, promoting partial aggregation of the particles. This aggregation state was duly identified by coupling SAXS, zeta potential and pH measurements. In addition, the SAXS technique showed that aggregates formed along the process are elongated-like particles due to the exponential decay evidenced through SAXS curves.