Feasibility of Electrostatic-Mediated Post-Functionalization to Induce Long Term Colloidal Stability and Stability After Freeze Drying of Amphoteric Nanoparticles

A commercialized iron oxide nanoparticles (of amphoteric nature) were surface modified with poly(sodium 4-styrenesulfonate) (PSS) via a simple electrostatic-mediated post-functionalization technique. The modification is successful if the iron oxide nanoparticles experience a complete alteration in surface properties. Such observation was obtained when the functionalization was conducted at pH 3.5, in which the iron oxide nanoparticles and the PSS molecules were of opposite charge. Here, the PSS can be attached onto the iron oxide via electrostatic attraction. In contrast, poor surface modification was resulted when the functionalization was conducted at pH > 9, in which iron oxide and PSS repulse each other upon having similar surface charge. Later on, the ability of the electrostatic-mediated post-functionalization to continually impose good colloidal stability on the iron oxide nanoparticles for a long period (6 months) and its ability to sustain from lyophilization process were evaluated by tracking the evolution of particle hydrodynamic size.