Biogenic Nanostructured Porous Silicon as a Carrier for Stabilization and Delivery of Natural Therapeutic Species.

Nanostructured mesoporous silicon (pSi) derived from the silicon-accumulator plant Tabasheer (Bambuseae) is demonstrated to serve as a potential carrier matrix for carrying and stabilizing naturally active, but otherwise metastable, therapeutic agents. Particularly, in this study, garlic oil containing phytochemicals (namely, allicin) that are capable of inhibiting Staphylococcus aureus (S. aureus) bacterial growth were incorporated into Tabasheer-derived porous silicon. Thermogravimetric analysis (TGA) indicated that relatively high amounts of the extract (53.1 ± 2.2 wt %) loaded into pSi are possible by simple infiltration. Furthermore, by assessing the antibacterial activity of the samples using a combination technique of agar disk diffusion and turbidity assays against S. aureus, we report that biogenic porous silicon can be utilized to stabilize and enhance the therapeutic effects of garlic oil for up to 4 weeks when the samples were stored under refrigerated conditions (4 °C) and 1 week at room temperature (25 °C). Critically, under ultraviolet (UV) light (365 nm) irradiation for 24 h intervals, plant-derived pSi is shown to have superior performance in protecting garlic extracts over porous silica (pSiO2) derived from the same plant feedstock or extract-only controls. The mechanism for this effect has also been investigated.

In Vitro Gene Delivery with Large Porous Silicon Nanoparticles Fabricated Using Cost-Effective, Metal-Assisted Chemical Etching.

The cytocompatibility, cell membrane affinity, and plasmid DNA delivery from surface oxidized, metal-assisted stain-etched mesoporous silicon nanoscale particles (pSiNPs) to human embryonic kidney (HEK293) cells is demonstrated, suggesting the possibility of using such material for targeted transfection and drug delivery.

Single Plant Derived Nanotechnology for Synergistic Antibacterial Therapies.

Multiple new approaches to tackle multidrug resistant infections are urgently needed and under evaluation. One nanotechnology-based approach to delivering new relevant therapeutics involves silicon accumulator plants serving as a viable silicon source in green routes for the fabrication of the nanoscale drug delivery carrier porous silicon (pSi). If the selected plant leaf components contain medicinally-active species as well, then a single substance can provide not only the nanoscale high surface area drug delivery carrier, but the drug itself. With this idea in mind, porous silicon was fabricated from joints of the silicon accumulator plant Bambuseae (Tabasheer) and loaded with an antibacterial extract originating from leaves of the same type of plant (Bambuseae arundinacea). Preparation of porous silicon from Tabasheer includes extraction of biogenic silica from the ground plant by calcination, followed by reduction with magnesium in the presence of sodium chloride, thereby acting as a thermal moderator that helps to retain the mesoporous structure of the feedstock. The purified product was characterized by a combination of scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and low temperature nitrogen gas adsorption measurements. Antimicrobial activity and minimum inhibitory concentration of a leaf extract of Bambuseae arundinacea was tested against the bacteria Escherichia Coli (E. Coli) and Staphylococcus aureus (S. Aureus), along with the fungus Candida albicans (C. Albicans). A S. aureus active ethanolic leaf extract was loaded into the above Tabasheer-derived porous silicon. Initial studies indicate sustained in vitro antibacterial activity of the extract-loaded plant derived pSi (25 wt %, TGA), as measured by disk diffusion inhibitory zone assays. Subsequent chromatographic separation of this extract revealed that the active antimicrobial species present include stigmasterol and 2,6-dimethoxy-p-benzoquinone.

Influence of Surface Chemistry on the Release of an Antibacterial Drug from Nanostructured Porous Silicon.

Nanostructured mesoporous silicon possesses important properties advantageous to drug loading and delivery. For controlled release of the antibacterial drug triclosan, and its associated activity versus Staphylococcus aureus, previous studies investigated the influence of porosity of the silicon matrix. In this work, we focus on the complementary issue of the influence of surface chemistry on such properties, with particular regard to drug loading and release kinetics that can be ideally adjusted by surface modification. Comparison between drug release from as-anodized, hydride-terminated hydrophobic porous silicon and the oxidized hydrophilic counterpart is complicated due to the rapid bioresorption of the former; hence, a hydrophobic interface with long-term biostability is desired, such as can be provided by a relatively long chain octyl moiety. To minimize possible thermal degradation of the surfaces or drug activity during loading of molten drug species, a solution loading method has been investigated. Such studies demonstrate that the ability of porous silicon to act as an effective carrier for sustained delivery of antibacterial agents can be sensitively altered by surface functionalization.

Determination of excitation profile and dielectric function spatial nonuniformity in porous silicon by using WKB approach.

We develop an analytical model based on the WKB approach to evaluate the experimental results of the femtosecond pump-probe measurements of the transmittance and reflectance obtained on thin membranes of porous silicon. The model allows us to retrieve a pump-induced nonuniform complex dielectric function change along the membrane depth. We show that the model fitting to the experimental data requires a minimal number of fitting parameters while still complying with the restriction imposed by the Kramers-Kronig relation. The developed model has a broad range of applications for experimental data analysis and practical implementation in the design of devices involving a spatially nonuniform dielectric function, such as in biosensing, wave-guiding, solar energy harvesting, photonics and electro-optical devices.

Taste and mouthfeel assessment of porous and non-porous silicon microparticles.

Unlike the trace minerals iron, copper and zinc, the semiconductor silicon has not had its organoleptic properties assessed. Nanostructured silicon provides the nutrient orthosilicic acid through hydrolysis in the gastrointestinal tract and is a candidate for oral silicon supplements. Mesoporous silicon, a nanostructured material, is being assessed for both oral drug and nutrient delivery. Here we use taste panels to determine the taste threshold and taste descriptors of both solid and mesoporous silicon in water and chewing gum base.Comparisons are made with a metal salt (copper sulphate) and porous silica. We believe such data will provide useful benchmarks for likely consumer acceptability of silicon supplemented foodstuffs and beverages.

Luminescence of mesoporous silicon powders treated by high-pressure water vapor annealing.

We have studied the photoluminescence of nanocrystalline silicon microparticle powders fabricated by fragmentation of PSi membranes. Several porosities were studied. Some powders have been subjected to further chemical etching in HF in order to reduce the size of the silicon skeleton and reach quantum sizes. High-pressure water vapor annealing was then used to enhance both the luminescence efficiency and stability. Two visible emission bands were observed. A red band characteristic of the emission of Si nanocrystals and a blue band related to localized centers in oxidized powders. The blue band included a long-lived component, with a lifetime exceeding 1 sec. Both emission bands depended strongly on the PSi initial porosity. The colors of the processed powders were tunable from brown to off-white, depending on the level of oxidation. The surface area and pore volume of some powders were also measured and discussed. The targeted applications are in cosmetics and medicine.

Sustained antibacterial activity from triclosan-loaded nanostructured mesoporous silicon.

In this work, nanostructured particles of porous silicon are demonstrated to act as an effective carrier for the sustained delivery of antibacterial agents with an enhanced inhibitory activity. Methods are described for the incorporation of significant amounts of the established antibacterial compound triclosan (Irgasan) into mesoporous silicon of varying porosities. Such materials were characterized by a combination of scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), and antimicrobial assays. Assessment of antibacterial activity was carried out versus the bacterium Staphylococcus aureus as a function of time with concomitant assessment of triclosan release; significant, sustained inhibition of bacterial growth is demonstrated in the triclosan-containing porous Si for time intervals greater than 100 days. Significantly, enhanced dissolution (relative to room temperature equilibrium solubility) of the triclosan was observed for the initial 15 days of drug release, inferring some amorphization or nanostructuring by the porous Si matrix.

Evaluation of mesoporous silicon/polycaprolactone composites as ophthalmic implants.

The suitability of porous silicon (pSi) encapsulated in microfibers of the biodegradable polymer polycaprolactone (PCL) for ophthalmic applications was evaluated, using both a cell attachment assay with epithelial cells and an in vivo assessment of biocompatibility in rats. Microfibers of PCL containing encapsulated pSi particles at two different concentrations (6 and 20 wt.%) were fabricated as non-woven fabrics. Given the dependence of Si particle dissolution kinetics on pSi surface chemistry, two different types of pSi particles (hydride-terminated and surface-oxidized) were evaluated for each of the two particle concentrations. Significant attachment of a human lens epithelial cell line (SRA 01/04) to all four types of scaffolds within a 24h period was observed. Implantation of Si fabric samples beneath the conjunctiva of rat eyes for 8 weeks demonstrated that the composite materials did not cause visible infection or inflammation, and did not erode the ocular surface. We suggest that these novel composite materials hold considerable promise as scaffolds in tissue engineering with controlled release applications.

Mesoporous silicon: a platform for the delivery of therapeutics.

Nanostructuring materials can radically change their properties. Two interesting examples highlighted here are nanoscale porosity inducing biodegradability, and nanoscale confinement affecting the physical form of an entrapped drug. Mesoporous silicon is under increasing study for drug-delivery applications, and is the topic of this review. The authors focus on those properties of most relevance to this application, as well as those recent studies published on small molecule and peptide/protein delivery.