Influence of polycation/cation competition on the aggregation threshold of magnetic nanoparticles

Abstract

Precise control of the aggregation of magnetic nanoparticles in aqueous environment is a key to their successful application in the biological and medical fields. We investigate here the critical aggregation threshold of PAA-coated maghemite nanoparticles in aqueous solution, in the presence of ionenes, polycations with a regular, tunable and pH-insensitive chain charge density, and a variety of ammonium-based salts. Aggregation of the nanoparticles is followed by a combination of dynamic light scattering and light absorbance measurements, along a dilution pathway from initially dispersed systems at high salt concentration (3 M). The position of the aggregation threshold, or critical salt concentration, is found to depend on two parameters: volume of the salt cation (Vcation) and volume of the charged monomer unit of the polycation (Vmono+, reflecting the polycation charge density). Combining these two parameters, a master curve emerges, showing that the critical salt concentration follows in a linear fashion the ratio of the above two volumes, Vcation/Vmono+. Interpreted within the theory of Collins and coworkers, the master curve is a clear demonstration of the importance of local ion-pairing in governing the stability of the system, which remains unaccounted for in the continuum electrostatic theories attempting to describe polyelectrolyte complex formation. Moreover, such a master curve provides a very simple rule for the control of interactions between charged polyelectrolytes moderated by salts.