Preparation of Colloidally Stable Positively Charged Hollow Silica Nanoparticles: Effect of Minimizing Hydrolysis on ζ Potentials.

Silica nanoparticles have received great attention as versatile nanomaterials in many fields such as drug delivery, sensing, and imaging due to their physical and chemical flexibility. Specifically, the silanol groups at the surface of silica nanoparticles have enabled various surface modifications and functionalization to tailor the nanoparticles for each application. Chemical tailoring to switch from negative to positive surface charge has been one important strategy to enhance cell internalization and biodistribution of the nanoparticles. However, efficient surface charge modification that is sustained upon dispersion is difficult to achieve and has not been well characterized though it can be a critical requirement for successful nanoparticle performance. In this study, solid spherical silica nanoparticles and hollow spherical silica nanoparticles around 45 nm in diameter were synthesized, both possessing tunable positive ζ potentials in aqueous colloidal suspension, to investigate the relationship between time-dependent ζ potential changes and their morphologies. A set of three different particles showing varied ζ potentials of approximately 5, 20, and >30 mV in both morphologies were prepared, and their colloidal surface electric potential fluctuations were measured. These studies reveal that the hollow morphologies are much more effectively able to maintain positive ζ potentials for 7 days of aqueous incubation, whereas the magnitude of the ζ potential of the solid silica spheres decreases uncontrollably, largely due to hydrolysis of the interior siloxane bonds, resulting in adsorption of the released silicic acid onto the nanoparticle surface.