Bimodal Morphology Transition Pathway in the Synthesis of Ultrasmall Fluorescent Mesoporous Silica Nanoparticles

Morphological transitions during the surfactant-directed synthesis of mesoporous silica nanoparticles (MSNs) are of great interest, as these materials are highly desirable for applications in catalysis, separation, and drug delivery. We investigate the transition pathway in the formation of ultrasmall fluorescent MSNs of two different morphologies synthesized through micelle templating. Increasing the concentration of pore expander, trimethylbenzene [TMB], drives a transition from single-pore MSNs to silica rings. We show that in the transition region, although their relative composition varies, both particle structures maintain constant pore and particle sizes as a function of [TMB]. Beyond the transition region, an increase in the size of the silica rings is observed. The bimodal nature of this transition is corroborated by a combination of gel permeation chromatography, fluorescence correlation spectroscopy, dynamic light scattering, and transmission electron microscopy investigations and can be influenced by solution stirring rate. We expect that insights from the study of such transition pathways will be essential for the ability to synthesize advanced-generation nanomaterials for applications including nanomedicine.