Organic Base-Free Synthesis of Tunable Mesoporous Silica Nanoparticles Triggered by CO₂‑Derived Cyclic Carbonates

Mesoporous silica nanoparticles (MSNs) with controlled size (d ∼ 30-70 nm) and monodisperse size distribution are attracting significant interest for multiple applications, especially drug delivery. Particles with such structural features could be obtained in some cases through aqueous protocols employing TEOS (tetraethyl orthosilicate) as the silicon source and CTAB (cetyltrimethylammonium bromide) as the surfactant, but in the presence of toxic, volatile, and flammable triethylamine (TEA). There is a lack of safer protocols able to produce MSNs with the above-mentioned desirable characteristics by using nonvolatile inorganic bases. In this work, we explore the formation of MSNs using CTAB, TEOS, and NaOH as the base in the presence of cosolvents of different polarities in well-defined molar ratios. When using highly polar, CO₂-based cyclic carbonates as cosolvents above a specific CTAB/cyclic carbonate ratio, formation of monodisperse MSNs with a tunable size below 100 nm (and specifically in the 40-60 nm size range for propylene carbonate) and a well-defined starburst morphology was observed. Based on experimental evidence, a growth mechanism for carbonate-based MSNs is derived where strong coordination of micelle and silica condensation nuclei by the cyclic carbonates drives particle growth, yield, and morphology.