(583bm) Catalysis By Hybrid Mesoporous Nanoparticles: Experiments, Mechanism and Modeling

We utilize the block co-polymer based self-assembly route for the synthesis of mesoporous silica nanoparticles (SBA-15) of different morphology, namely, spherical, cubic, rod-like, fiber-like etc. This is followed by functionalization by a second component (another nanoparticle or an enzyme) for achieving catalytic applications in this platform.

To this end, in the first step, hexagonal phase of self-assembled micelles were used for SBA-15 nanoparticle synthesis, having either 7 nm or 12 nm internal pore diameter. In the second step, pores of these SBA-15 particles were further impregnated with different functional constituents, either by a 3.5-4 nm diameter SnO₂ /TiO₂ nanoparticle, or by glucose oxidase (GOD) enzyme molecules of 5.5 nm size. The resulting hybrid mesoporous particles were thus tailored for selective access of a guest species (dissolved in a solvent) to the functionalized component, enhancing catalysis.

This general principle is illustrated in two catalytic systems - firstly, in photo-degradation of rhodamine-B dye by inorganic SnO₂ /TiO₂ nanoparticle for removal of the dye; and secondly, in biocatalysis of glucose, by the GOD enzyme, for designing a glucose sensor. In both systems, we find that spherical or cubic SBA-15 nanoparticles allow much better diffusion, adsorption and reaction rates of all reactant and product species, compared to rod- or fiber-shaped SBA-15 particles, since the latter have increasingly higher length to diameter aspect ratio.

Finally, using our experimental data for validation, we developed a mathematical model by combining mass transport and relevant chemical reactions in each case, accounting for the particle morphology and pore dimensions. Such a general model for catalysis by a hybrid mesoporous system is helpful in delineating the relative importance of pore-diffusivity, adsorption isotherms and reaction kinetics on catalytic conversion. Thus, we could optimize the SBA-15 nanoparticle morphology, external particle dimensions and internal pore diameter, in order to achieve the best dye degradation rate or enzyme activity, respectively.

In summary, porous nanoparticles synthesized by us have two important dimensions – external morphology and size of the nanoparticles, as well as diameter of the internal pores. While the external particle dimension (from 150 nm diameter to 50 micrometer length) and morphology is tunable across spherical, cubic to cylindrical shapes; the internal pore diameter can be precisely varied from 2.5 nm to 12 nm, by using different surfactants and block co-polymers as templates. Furthermore, impregnation or in-situ generation of a second reactive component, as shown here – e. g. a glucose oxidase (GOD) enzyme molecule or solid SnO₂ /TiO₂ nanoparticle  – imparts catalytic functionality to resulting hybrids. It is hence desirable to have the materials possess a uniform dispersion of the functional additive only inside the pores, with good interfacial contact and selective access of reactants, like glucose or dye molecules etc.

It is for the above reason, that we synthesize these nanomaterials in multiphase (immiscible liquid-liquid or liquid-solid) colloidal systems, which are naturally dominated by the presence of large interfacial area between the constituent phases. Eventually, our aim is to build-up a complete description from templates to material synthesis, structure and finally functionality, as illustrated by the current examples of inorganic and biocatalytic applications.