Colloidal stability of polyelectrolyte-functionalized magnetic nanoparticles: experimental and theoretical studies

The commercially available magnetic nanoparticles (MNPs) in the solid powder form are typically colloidally unstable under the aqueous environment and can easily settle out without modification. Thus, surface functionalization with polyelectrolyte is required to improve the colloidal stability of MNP systems prior to their applications. In this study, the effect of various factors during the functionalization process, including type of polyelectrolyte, solution pH, ionic strength and particle-to-polyelectrolyte ratio, on the colloidal stability of the functionalized MNPs was evaluated experimentally and theoretically. It was revealed that PSS-70 k and PDDA(M) imparted better colloidal stability toward the functionalized MNPs as compared to PEG, PSS-1000 k and PDDA(L). The optimal pH to produce PSS-70 k- and PDDA(M)-functionalized MNPs is found to be pH 3 and 11, respectively. In addition, the optimum MNP/polyelectrolyte mass ratio was revealed to be 1:5 and 1:0.5 for the synthesis of PSS-70 k- and PDDA(M)-functionalized MNP systems, respectively. The colloidal stability of both PSS-70 k- and PDDA(M)-functionalized MNPs was severely impeded by the presence of free ionic species in the solution during the functionalization process. Lastly, the mathematical analysis based on DLVO theory was consistent and well aligned with the experimental findings.