Synthesis of ultrabright nanoporous fluorescent silica discoids using an inorganic silica precursor

The templated sol-gel synthesis of ultrabright fluorescent nanoporous silica particles based on the use of organic silica sources has previously been reported. The use of organosilanes as the main silica precursors has a number of issues, in particular, the low robustness of the synthesis due to instability of the organic silica source. Here we report on a novel synthesis of ultrabright fluorescent nanoporous silica discoids (a specific shape in-between the sphere and disk) of 3.1 ± 0.7 microns in size, which were prepared using a stable inorganic sodium silicate silica source. Organic fluorescent dye Rhodamine 6G (R6G) was physically (non-covalently) entrapped inside cylindrical nanochannels of ∼4-5 nm in diameter. In contrast to the synthesis with organic silica precursors, the obtained particles showed an excessive leakage of dye. To prevent this leakage, we modified the synthesis by adding a small amount of a secondary silica source. The synthesized particles show virtually no leakage, high photostability, and a brightness equivalent to the fluorescence of up to 7 × 10⁷ free R6G molecules. This is about 7 times higher than the fluorescent brightness of particles of the same size made of CdSe/ZnS quantum dots, and 420 times higher than the brightness of the same volume of aqueous solution of free R6G dye.