(Invited) Green Synthesis of Porous Silicon Derived from Accumulator Plants: Associated Morphologies and Stabilization of a Natural Bioactive Extract

Elemental silicon, in nanostructured mesoporous form, is attracting extensive attention in biomedically-relevant applications such as biosensing, drug delivery, and tissue engineering. The use of electronic grade silicon as a feedstock material for production of this material has economic obstacles to high volume applications. Alternatives, such as open circuit stain etching processes in conjunction with lower cost metallurgical grade Si, for example, are appealing, yet face common issues like high volume use of hydrofluoric acid and other challenges such as generation of uniform high porosity frameworks.

   In contrast to the above, a more eco-friendly option is the use of established silicon accumulator plants as sustainable starting materials for high surface area porous silicon structures. We have recently reported¹ a facile route that employs well-known plants such as horsetail (equisetum arvense, equisetum telmatia) and bamboo (bambuseae, tabasheer) as feedstocks. This process first entails the calcination of a given plant material to porous silica, and its subsequent magnesiothermic reduction to porous silicon in the presence of a thermal moderator/porogen.

    In this presentation we focus on two distinct, yet complementary, issues. The first is the influence of plant component on the morphology of the resultant porous silicon. This is investigated for the case of equisetum telmatia, where we have evaluated the morphology and properties of porous silicon derived from the stem component of the plant separately from that from the fronds.  A combination of electron microscopies (FESEM, TEM) along with low temperature nitrogen adsorption measurements (and corresponding BET surface area analyses) are used to characterize differences in nanostructure morphology that is produced from each type of plant material.

    The second issue is the ability of this plant-derived material to act as a carrier for the stabilization and delivery of otherwise metastable natural product-derived therapeutic species. As an example, we have evaluated the antibacterial activity of extracts of garlic loaded into porous silicon derived from tabasheer; such extracts have been reported in the literature to possess possible antimicrobial, antifungal, and cholesterol–lowering functions.² Garlic consists of a complex mixture of sulfur containing components, dominated by the organosulfur compound allicin, that rapidly degrade (seconds) under ambient conditions.  For these experiments, after determining the extent of extract loading of tabasheer-derived pSi with thermogravimetric analysis (TGA), a combination of agar disc diffusion as well as minimum inhibition concentration (MIC) methods are used to evaluate the antibacterial activity of drug-loaded pSi against Staphylococcus aureus. The ability of this particular plant-derived pSi is contrasted with the free extract as a function of time, thereby providing an assessment of this biogenic porous silicon carrier to extend the shelf life of an otherwise unstable active biogenic extract.

    Taken in concert, such experiments explore the utility and appeal of this low cost, naturally derived nanostructured porous matrix in higher volume controlled release applications.  

¹ L. Batchelor, A. Loni, L.T. Canham, M. Hasan, and J.L. Coffer, Silicon, 2012, 4, 259-266.

² L.D. Lawson and Z.J. Wang, J. Agric. Food Chem. 2005, 53, 1974−1983.