In this work, we investigated the kinetic balance between ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) and subsequent condensation over the growth of silica particles in the Stöber method. Our results reveal that, at the initial stage, the reaction is dictated by TEOS hydrolysis to form silanol monomers, which is denoted as pathway I and is responsible for nucleation and growth of small silica particles via condensation of neighboring silanol monomers and siloxane network clusters derived thereafter. Afterward, the reaction is dictated by condensation of newly formed silanol monomers onto the earlier formed silica particles, which is denoted as pathway II and is responsible for the enlargement in size of silica particles. When TEOS hydrolysis is significantly promoted, either at high ammonia concentration (≥0.95 M) or at low ammonia concentration in the presence of LiOH as secondary catalyst, temporal separation of pathways I and II makes the Stöber method reminiscent of in situ seeded growth. This knowledge advance enables us not only to reconcile the most prevailing aggregation-only and monomer-addition models in literature into one consistent framework to interpret the Stöber process but also to grow monodisperse silica particles with sizes in the range 15-230 nm simply but precisely regulated by the ammonia concentration with the aid of LiOH.

A series of tris(1,10-phenanthroline)ruthenium ion (Ru(phen)(3)(2+)) doped silica nanoparticles were prepared by introducing the dye at different stages of the Stöber process. The emission properties of the doped silica particles were found to be dependent on the time (0-8 h) of the dye introduced into the reaction system. A turnover of the emission properties was identified for the doped silica particles by introducing the dye before and after 3 h of the reaction. Compared to the particles prepared by adding the dye at the beginning of the reaction (0 h doping), the particles prepared by introducing the dye before 3 h of the reaction (3 h doping) showed enhanced emission intensity and blue-shifted emission with the delayed addition time. The particles prepared by introducing the dye during the period of 3-8 h of the reaction showed decreased emission intensity and red-shifted emission with the delayed addition time compared to those prepared by introducing the dye at 3 h of the reaction. The emission intensity of the 3 h doping silica particles was about 3.3 times that of the 0 h doping particles, and the emission maximum shifted from 592 to 575 nm correspondingly. The 8 h doping particles showed emission maximum at 581 nm, and their emission intensity was only 15% of that of the 3 h doping particles. However, both the emission intensity and maximum of the 8 h doping particles would be similar to those of the 3 h doping particles after further deposition of silica protection layer. The switching of the emission properties of the doped silica particles prepared by introducing the dye before and after 3 h is attributed to the suppressed aggregation of the dye molecules and decreased thickness of the silica protection layer, respectively.

Nearly monodispersed sphere-like Fe 3 O 4 /SiO 2 composite particles were prepared via a direct silica coating using Tween-80 of modified aggregates of Fe 3 O 4 nanoparticles obtained by the emulsion droplet solvent evaporation method. The size and magnetite proportion of the composite particles were tunable by controlling the size of the aggregates and/or the thickness of the silica coating layer. Preparation of Tween-80-modified aggregates of Fe 3 O 4 nanoparticlesAggregates of Fe 3 O 4 nanoparticles were prepared by the emulsion droplet solvent evaporation method. In a typical procedure, 0.3 g oleic acid-modified Fe 3 O 4 nanoparticles was dispersed into 1.5 mL cyclohexane with the aid of ultrasound to form an oil

The performance of fluorescein isothiocyanate (FITC) and tris(1, 10-phenanathroline) ruthenium ion (Ru(phen)32+) co-doped silica particles as pH indicator was evaluated. The emission intensity ratios of the pH sensitive dye (FITC) and the reference dye (Ru(phen)32+) in the particles were dependent on pH of the environment. The changes in emission intensity ratios of the two dyes under different pH could be measured under single excitation wavelength and readily visualized by naked eye under a 365-nm UV lamp. In particular, such FITC and Ru(phen)32+ co-doped silica particles were identified to show high sensitivity to pH around the pKa of FITC (6.4), making them be potential useful as visualized pH indicator for detection of intracellular pH micro-circumstance.

A negatively charged dye, 8-hydroxypyene-1,3,6-sulfonic acid trisodium salt (HPTS), was successfully incorporated into a silica matrix by using a positively charged polyelectrolyte, poly (diallyldimethylammoniumchloride) (PDADMAC) as a bridge. The positive charges of PDADMAC were partially neutralized by HPTS upon the formation of PDADMAC-HPTS complexes. After being introduced into a St€ ober system and pre-hydrolyzed for a finite time, the surface charge of the complexes was rapidly reversed from positive to negative due to the absorption of the negatively charged silica species on their surface, which was important to avoid flocculation of the complexes and allow the growth of the silica particles. Stability of the complexes and thus the size of resulting particles were tunable by changing either the positive charges of the complexes or the pre-hydrolyzation time of the St€ ober system. The optical performance of the silica particles was improved and the leakage of the dye from the particles was greatly suppressed due to the electrostatic interaction between the dye and polyelectrolyte.

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Multicolor particles were prepared by incorporating two dyes, one fluorescent (fluorescein isothiocyanate) and one phosphorescent (tris(1,10-phenanathroline) ruthenium ion), into the silica matrix. Colors of the particles can be easily tuned by either varying the doping ratios of the two dyes or changing the excitation wavelength while fixing the ratios. The multicolor character of the particles is less sensitive to the location of the two dyes in the silica, since the luminescence of the particles is independent of Förster resonance energy transfer (FRET).

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