21
2µm
Figure 2. (A) SOS particles formed with the co-condensation of TMOS and (B) N2 adsorption isotherm profile generated after CTAB templating and hydrothermal treatment.
ratios. For the SOS particles, both the nanospheres and microspheres are nearly free of mesopores, but does exhibit some microporosity generated from the organic part of the silane. The interstitial porosity generated from the packing of nanospheres provides surface macroporosity, which is the platform for liquid phase separation.
This was the initial principles of this mechanism, but it was further utilised in order to introduce mesoporosity into the shell of the particles. Although MPTMS has been used for the preparation, MPTMS was required to co-condense with TEOS or TMOS to form surfactant-templated mesopores or covalently attached to preformed mesoporous silica with interesting surface morphology [12] (Figure 2). The resulting particles exhibited higher surface area reaching up to 680 m2
/g. Other precursors were co-condensed with MPTMS and
this has resulted in a change in shell morphology and porosity. This method was further explored to generate more complex structures that are analogous to the structure of a fractal [13], where the porosity of the spheres is removed, resulting in the surface area being attributable to the morphology of the particle rather than the pore diameter. The type of pores within the particles is critically important for certain applications, especially bio-separations, as it can greatly affect mass transfer effects between the bulk flow and pore regions.
Use of Spheres-on-Sphere silica microspheres in bio-separation
Figure 3. (A) SEM images of SOS particles modified with ZIF-8 MOF. (B) The diol-functionalized SOS particles show improved efficiency for separation of toluene, 2,4-di-tert-butylphenol, o-nitroaniline, and cinnamyl alcohol mixture. Mobile phase: heptane:dioxane (90:10, v/v), 10 µL injection, flow rate 1 cm3/min, back pressure 14 bars, column dimension 4.6 mm I.D. X 50 mm L.
2µm
The SOS particles were functionalised with different chemistries and used for the separation of small molecules such as nitroaniline, acetophenone, biphenyl and sugars [10,12,14]. It was interesting to observe that these particles were capable of separating such small molecules. Other new chemistries were also explored such as the entrapment of a crystalline phase by forming a layer of metal–organic framework (MOF) nanocrystals within the porous shell [15,16]. MOFs are a type of crystalline porous materials via the linkage of metal
ions and organic ligands. Most MOFs exhibit microporosity of varied morphologies although great effort (e.g., by ligand extension, combining the synthesis with surfactant templating) has been made to prepare mesoporous MOFs. The pore size, pore shape, and pore surface functionality are well defined in MOFs, which are suitable for highly selective separation of gas molecules or small molecule liquids [17]. MOFs can be difficult to pack into columns, which presents a major issue [18]. However, columns that are packed with MOFs demonstrate low separation efficiency and column stability. This is because MOFs are normally prepared as irregularly shaped micro- particles. The porous nature of the SOS silica particles combined with surface functionality enabled a unique entrapment of MOFs, such as HKUST-1 and ZIF-8, and exhibited some control over the crystal shape growth with enhanced separation efficiency. It was demonstrated that the SOS particles can be combined with other materials to produce unique separations. However, the major focus of this material is the use of standard SOS silica particle as a stationary phase for biomolecules separation.
The morphology of the porous structure within SOS silica is inherently stochastic, but it has been determined that it has a fractional
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