Understanding the Formation Dynamics and Physical Properties of Nanocapsules Using Charge Detection Mass Spectrometry

Characterizing the size, structure, and composition of nanoparticles is vital in predicting and understanding their macroscopic properties. In this work, charge detection mass spectrometry (CDMS) was used to analyze nanocapsules (∼10-200 MDa) consisting of a liquid oleic acid core surrounded by a dense silica outer shell. CDMS is an emerging method for nanoparticle analysis that can rapidly measure the mass and charge of thousands of individual nanoparticles. We find that increasing the feed volume of the tetraethylorthosilicate (TEOS) precursor added to form the silica shell of the nanocapsules yielded both higher and broader nanocapsule mass distributions with differentiable densities. A two-dimensional mass versus charge analysis also revealed the formation of two distinct populations of nanocapsules. These two nanocapsule morphologies were also present in transmission electron microscopy (TEM) images and exhibited low-density spherical cores and crescent-shaped cores where the remainder of the core volume was "filled in" by more dense silica. Nanocapsule shell growth kinetics over a ∼48-h synthesis period were also monitored by sampling the reaction mixture at various times, quenching the sampled aliquots, and then characterizing these time-resolved samples by CDMS. The CDMS data reveal three distinct growth phases in nanocapsule formation; rapid initial nucleation, an "inverted" distribution of silica growth, and a final slow growth phase where the rate of mass increase and final nanocapsule masses are dictated by the initial TEOS feed volumes. CDMS-enabled understanding of the diverse nanocapsule sizes, morphologies, and growth dynamics will allow us to better predict nanocapsule properties while reducing the experimental burden in optimizing nanocapsules for real-world applications.