AbstractTunable pore size and outer particle diameter of spherical mesoporous silica particles in the nanometer range were successfully synthesized using a novel water/oil-phase synthesis technique. This method involves (i) simultaneous hydrolytic condensation of tetraorthosilicate to form silica and polymerization of styrene into polystyrene (PS), (ii) self-assembly nanocluster silica and nanocluster PS to form silica/PSL nanoparticle, and (iii) calcination process to remove organic components and to produce mesoporous silica particle. In this study, an amino acid (e.g. lysine) was utilized to catalyze and to maintain the silica formation due to its ability in covering prepared silica after reaction. Further, another advantage of this catalyst is more harmless than other catalysts (e.g. ammonia, N2H4). The result showed spherical particles with controllable pore size (from 4 to 15 nm) and outer diameter (from 20 to 80 nm) was produced. The ability to control pore size and outer diameter was drastically altered by adjusting the concentration of styrene and hydrophobic molecules, respectively. After the preparation of particle was understood clearly, the large-molecule-adsorption performance of the prepared porous particles was conducted. As expected, relatively large organic molecules (i.e. Rhodamine B) were well-absorbed in the prepared sample but not by the commercial non-porous particles. With this reason, the prepared mesoporous silica particles may be used efficiently in various applications, including electronic devices, sensors, pharmaceuticals, and environmentally sensitive pursuits, due to its harmless process, compatibility for bio-application, and excellent adsorption properties.