Water-Phase Synthesis of Cationic Silica/Polyamine Nanoparticles

Abstract

Silica particles are commonly functionalized with amine groups on their surface through the hydrolytic condensation of aminotrialkoxysilanes for use in bioimaging, enzyme immobilization, and other applications. Eliminating this aminotrialkoxysilane condensation step could simplify and improve the efficiency of the synthesis of amine-functionalized silica. Here, we describe a one-pot, ambient-condition, water-phase method to synthesize silica-based nanoparticles (NPs) that present surface amine groups. The formation mechanism involves the electrostatic cross-linking of cationic polyallylamine hydrochloride by citrate anions and the infusion of the resulting polymer/salt aggregates by silicic acid. The particles were unimodal with average diameters in the range of 40 to 100 nm, as determined by the size of the templating polymer-salt aggregates. Colorimetric analysis using Coomassie brilliant blue and zeta potential measurements confirmed the presence of surface amine groups on the hybrid silica/polymer NPs. The point of zero-charge value for these NPs was ∼5, between the corresponding values of unfunctionalized and aminopropyltriethoxysilane-functionalized silica particles (∼2 and ∼10, respectively). Surface charge calculations indicated the hybrid NPs had a lower amine surface density than aminopropyltriethoxysilane-functionalized silica (0.057 #/nm2 vs 0.169 #/nm2 at pH 7). The polymer-salt aggregate synthesis chemistry is a new approach toward controlling the amine surface density and point of zero-charge of hybrid silica/polymer NPs.