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Preparation and characterization of thin silicate-coated fluorescent silica nanoparticles with enhanced stability.

Silica nanoparticles (SNPs) were functionalized by aminosilanes of different chain lengths, such as (3-aminopropyl) trimethoxysilane (APTMS), [3-(2-aminoethylamino)propyl] trimethoxysilane (AEAPTMS), and N-(3-trimethoxysilylpropyl)diethylenetriamine (TMSP), followed by the electrostatic conjugation with carboxyl quantum dots (QDs). The resulting QD-anchored SNPs (Q-SNPs) were treated with a silicate solution to afford thin silica-coated fluorescent nanoparticles. The Q-SNPs prepared using AEAPTMS exhibited the highest photoluminescence (PL) intensity compared to those prepared using APTMS and TMSP. Moreover, the conjugation of amine-terminated SNPs with carboxyl QDs was found to be very strong under acidic pH conditions. The silicate-coated Q-SNPs exhibited a long-lasting PL intensity compared to the pristine Q-SNPs because the silica coating prevented the oxidative degradation and/or detachment of the anchored QDs from the SNPs. Such a protective coating strategy would be a useful guideline to prepare stable nanostructured materials for prolonged applications. The composite particles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), and photoluminescence (PL) spectroscopy.