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Two-Step Synthesis of Nanosize Hollow SiO2 Particles for Multiplexing

Gerson R. Aguirre1, Alexander Couzis2, Charles Maldarelli3, and M. Lane Gilchrist2. (1) Chemistry Department, City College and Graduate Center of the City University of New York, 140th Street at Convent Avenue, NY, NY 10031, (2) Chemical Engineering, City College and Graduate Center of the City University of New York, 140th Street at Convent Avenue, NY, NY 10031, (3) The Levich Institute and the Chemical Engineering Department, City College and Graduate Center of the City University of New York, 140th Street at Convent Avenue, NY, NY 10031

Entrapment and retention of materials inside synthesized silica particles is an active research field due to its many industrial, biological and chemical activities including as molecular sieves due to their porosity. Many methods include layer-by-layer templating techniques, emulsion droplets, and others use template-assisted synthesis such as latex, colloidal particles to vesicles, even gold nanoparticles. Templating techniques are more effective given the ease with which hollow particles can be obtained. These methods require the removal of the templating agent by acid burning or calcination of the organic material that can be detrimental if a sensitive material needs to be preserved and more importantly to retain its intended function if absorption of such materials is restricted by pore size or chemistry. What follows is a fabrication scheme that will allow for a more benign particle synthesis and allows for retention of introduced material or molecules inside the silica particle that can be beneficial in biological applications.

In this study we report on the formation of silica particles that have entrapped fluorescent quantum dots. We have used an oil-in-water suspension approach by the use of high CMC value of cationic surfactant (CTAB) in a highly alkaline solution. The TBOS (tretrabutoxysilane) oil droplets hydrolyze at the interface, TBOS has a very slow rate of hydrolysis due to its longer alkyl chain, facilitated by the high concentration of charge density provided by the ionized surfactant, according to Huo, et al. At this pH, condensation is enhanced but limited by the rate-limiting hydrolysis step. When the appropriate extent of hydrolysis is achieved at this alkaline regime rapid gelation at the oil/water interface occurs by the driving force of condensation.

In addition to synthesizing these silica micronsize particles we have also encapsulated in the core fluorescing semiconductor nanoparticles, CdSe/ZnS quantum dots (QDs). The distributed oil phase provides the vestibule for the QDs. As the silica particle is formed at the oil-water interface the QDs are trapped in the core. The hydrophobic nature of the QD surface stabilizes their dispersion in the oil phase. The hydrophobic termination of the QDs is provided by a TriOctyl-PhosphineOxide (TOPO) and octadecylamine adsorbed cap. This cap is chemisorbed against the ZnS shell of the QDs and extends its hydrophobic tails into the oil phase, resulting in the stabilization of the dispersion. Confocal microscopy images confirm the quantitative capture of the QDs within the formed silica particle.