Synthesis of spherical mesoporous silica nanoparticles with nanometer-size controllable pores and outer diameters
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.Introduction
The synthesis of mesoporous materials has been of great interest due to an ever-expanding list of uses, ranging from chemical sensors to drug delivery [1]. In the past decade, many mesoporous have been developed. One of the well-developed mesoporous materials is silica. Silica is attractive because it is chemically inert, thermally stable, harmless, and inexpensive [2].
Many preparation methods of mesoporous silica materials with various morphologies from thin film, sphere, fiber, as well as bulkform, such as the MCM series [3] and the SBA series [4], have been reported. In these synthetic reactions, the organic template-driven synthesis process is commonly used [5]. Although the previously reported methods are feasible for industry, they have several disadvantages: (i) use of harmful chemicals (e.g. ammonia [6,7] or N2H4 [8] as a catalyst) that can be a problem for bio-applications; and (ii) the difficult preparation of spherical particles with controllable size less than 100 nm in diameter.
Methods for circumventing the above problems have been suggested by several research groups [9], such as utilizing functional amines as ‘‘bio”-catalysts, which reduce the diameter to approximately 50 nm. However, even with the current method, synthesis
of spherical particles smaller than 50 nm remains difficult. Furthermore, the pore size of the prepared particle has been limited to less than 5 nm. In this range of pore size, difficulties associated with mass transfer, diffusivity, and penetration of molecules into or out of the pore system are well-known, limiting several applications [10]. In addition, while current methods have been used to create porous particles, to the best of our knowledge, reports of materials comprised of spherical particles with controllable particle size in the nanometer range and controllable pore size greater than 5 nm are virtually non-existent. Meanwhile, with a pore size of the greater than 5 nm, many applications (such as adsorbing large molecules) could be possible. Given a size in the nanometer range, a higher surface area could be gained. Further, we also can control the pore size with the prospect of controlling the optical properties, such as the refractive index.
In our previous studies, we prepared several meso- and macroporous inorganic particles with diameters in the submicron range using a spray method and polymer particles as the template. The morphology of the resulting prepared particles is controllable and the pore size is greater than 30 nm [11–14]. The purpose of the present study was to develop a method for the preparation of spherical nano-sized silica particles (Hiroshima Mesoporous Mate-