From "nano-bags" to "micro-pouches". Understanding and tweaking flocculation-based processes for the preparation of new nanoparticle-composites.

Abstract

The control of simple parameters involved in the process of classic bridging flocculation allow the preparation and fine-tuning a new class of hybrid nanomaterials with respect to size, composition, and morphology. The resulting nanoparticle-filled "nano-bags" are obtained in aqueous suspension by mixing three basic components, a polyelectrolyte, a multivalent ion, and nanoparticles in different ratios. The size range in which nano- and micropouches can be prepared seems to start at about 25 nm; these are oligo-nanoparticle aggregates whose size are clearly related to the size of the nanoparticles themselves and seem to extend up to about 5 microm. By controlling the stoichiometric balance between the global number of positive and negative charges on the polycation and on the multivalent anion and by controlling the absolute concentrations and the ratios, namely of the polyelectrolyte and the nanoparticles, one has access to a wide range of different nanopouch morphologies and compositions. Interestingly, the process of nanopouch formation seems not to be restricted to a single type of nanoparticles since, at least, citrate-stabilized gold and iron oxide nanoparticles showed indistinguishable results in transmission electron microscopy. The outer surface of the nanoparticle-filled nanopouches is easily functionalized further through layer-by-layer assembly. As a first example, we have enhanced the stability of nanopouch suspensions at increased ionic strength by electrostatic adsorption of a polyanion on the nanobag exterior.