Taming non-radiative recombination in Si nanocrystals interlinked in a porous network.

A range of the distinctive physical properties, comprising high surface-to-volume ratio, possibility to achieve mechanical and chemical stability after a tailored treatment, controlled quantum confinement and the room-temperature photoluminescence, combined with mass production capabilities offer porous silicon unmatched capabilities required for the development of electro-optical devices. Yet, the mechanism of the charge carrier dynamics remains poorly controlled and understood. In particular, non-radiative recombination, often the main process of the excited carrier's decay, has not been adequately comprehended to this day. Here we show, that the recombination mechanism critically depends on the composition of surface passivation. That is, hydrogen passivated material exhibits Shockley-Read-Hall type of decay, while for oxidised surfaces, it proceeds by two orders of magnitude faster and exclusively through the Auger process. Moreover, it is possible to control the source of recombination in the same sample by applying a cyclic sequence of hydrogenation-oxidation-hydrogenation processes, and, consequently switching on-demand between Shockley-Read-Hall and Auger recombinations. Remarkably, irregardless of the recombination mechanism, the rate constant scales inversely with the average volume of individual silicon nanocrystals contained in the material. Thus, the type of the non-radiative recombination is established by the composition of the passivation, while its rate depends on the degree of the charge carriers' quantum confinement.

The effects of drying technique and surface pre-treatment on the cytotoxicity and dissolution rate of luminescent porous silicon quantum dots in model fluids and living cells.

Tailoring of the biodegradation of photoluminescent silicon quantum dots (Si QDs) is important for their future applications in diagnostics and therapy. Here, the effect of drying and surface pretreatment on the dissolution rate of Si QDs in model liquids and living cells was studied in vitro using a combination of photoluminescence and Raman micro-spectroscopy. Porous silicon particles were obtained by mechanical milling of electrochemically etched mesoporous silicon films, and consist of interlinked silicon nanocrystals (QDs) and pores. The samples were subjected to super-critical drying with CO2 solvent (SCD) or air drying (AD) and then annealed at 600 °C for 16 hours in 1% oxygen to obtain nano-sized Si QDs. The obtained samples were characterized by a core-shell structure with a crystalline silicon core and a SiO2 layer on the surface. The sizes of the crystalline silicon cores, calculated from Raman scattering spectra, were about 4.5 nm for the initial AD-SiQDs, and about 2 nm for the initial SCD-SiQDs. Both the AD-Si QDs and the SCD-Si QDs exhibited visible photoluminescence (PL) properties due to quantum confinement effects. The dissolution of the nanocrystals was evaluated through their PL quenching, as well as by the presence of a low-frequency shift, broadening, and a decrease in the intensity of the Raman signal. The stability of the AD-Si QDs and the complete dissolution of the SCD-Si QDs during 24 hours of incubation with cells have been demonstrated. This might explain the apparent lower cytotoxicity observed for SCD-Si QDs.

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.

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.

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.

Green synthesis of mesoporous and biodegradable iron silicide nanoparticles for photothermal cancer therapy.

Photothermal nanomaterials have shown great potential for photothermal therapy. In this study, we developed a simple green method of magnesiothermic co-reduction for the synthesis of mesoporous, magnetic and biodegradable iron silicide nanoparticles (FeSi NPs) as applied to photothermal therapy (PTT). Starting from biogenic tabasheer extracted from bamboo and Fe2O3, the resultant FeSi NPs with a much lower band gap exhibited excellent optical absorption with a photothermal conversion efficiency of 76.2%, indicating a good photothermal performance. The weight extinction coefficient was measured to be 13.3 L g-1 cm-1 at 1064 nm (second near-infrared window, NIR-II), which surpassed the performance of other competitive Si-based and Fe-based photothermal agents. Results of the cell viability assay showed that cells could be killed by NIR-II laser irradiation with the synthesized FeSi NPs. In vivo results on mice showed clearly an efficient suppression of tumour growth by photothermal treatment with FeSi NPs. FeSi NPs were found to be biodegradable in simulated body fluids. The results from our work indicate that FeSi NPs are a new class of promising photothermal agents (PTAs) for application in cancer therapy.

Nanoporous Silicon as a Green, High-Tech Educational Tool.

Pedagogical tools are needed that link multidisciplinary nanoscience and technology (NST) to multiple state-of-the-art applications, including those requiring new fabrication routes relying on green synthesis. These can both educate and motivate the next generation of entrepreneurial NST scientists to create innovative products whilst protecting the environment and resources. Nanoporous silicon shows promise as such a tool as it can be fabricated from plants and waste materials, but also embodies many key educational concepts and key industrial uses identified for NST. Specific mechanical, thermal, and optical properties become highly tunable through nanoporosity. We also describe exceptional properties for nanostructured silicon like medical biodegradability and efficient light emission that open up new functionality for this semiconductor. Examples of prior lecture courses and potential laboratory projects are provided, based on the author's experiences in academic chemistry and physics departments in the USA and UK, together with industrial R&D in the medical, food, and consumer-care sectors. Nanoporous silicon-based lessons that engage students in the basics of entrepreneurship can also readily be identified, including idea generation, intellectual property, and clinical translation of nanomaterial products.

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.

Plant-Derived Tandem Drug/Mesoporous Silicon Microcarrier Structures for Anti-Inflammatory Therapy.

The properties of nanostructured plant-derived porous silicon (pSi) microparticles as potential candidates to increase the bioavailability of plant extracts possessing anti-inflammatory activity are described in this work. pSi drug carriers were fabricated using an eco-friendly route from the silicon accumulator plant bamboo (tabasheer) powder by magnesiothermic reduction of plant-derived silica and loaded with ethanolic extracts of Equisetum arvense, another silicon accumulator plant rich in polyphenolic compounds. The anti-inflammatory properties of the active therapeutics present in this extract were measured by sensitive luciferase reporter assays; this active extract was subsequently loaded and released from the pSi matrix, with a clear inhibition of the activity of the inflammatory signaling protein NF-κB over a period of hours in a sustained manner. Our results showed that after loading the extracts of E. arvense into pSi microparticles derived from tabasheer, enhanced anti-inflammatory activity was observed owing to enhanced solubility of the extract.

Hierarchical Nanostructuring of Porous Silicon with Electrochemical and Regenerative Electroless Etching.

Hierarchically nanostructured silicon was produced by regenerative electroless etching (ReEtching) of Si powder made from pulverized anodized porous silicon. This material is characterized by ∼15 nm mesopores, into the walls of which tortuous 2-4 nm pores have been introduced. The walls are sufficiently narrow that they support quantum-confined crystallites that are photoluminescent. With suitable parameters, the ReEtching process also provides control over the emission color of the photoluminescence. Ball milling and hydrosilylation of this powder with undecylenic acid produces nanoparticles with hydrodynamic diameter of ∼220 nm that exhibit robust and bright luminescence that can be excited with either one ultraviolet/visible photon or two near-infrared photons. The long-lived, robust visible photoluminescence of these chemically passivated porous silicon nanoparticles is well-suited for bioimaging and theranostic applications.

Reconstructing charge-carrier dynamics in porous silicon membranes from time-resolved interferometric measurements.

We performed interferometric time-resolved simultaneous reflectance and transmittance measurements to investigate the carrier dynamics in pump-probe experiments on thin porous silicon membranes. The experimental data was analysed by using a method built on the Wentzel-Kramers-Brillouin approximation and the Drude model, allowing us to reconstruct the excited carriers' non-uniform distribution in space and its evolution in time. The analysis revealed that the carrier dynamics in porous silicon, with ~50% porosity and native oxide chemistry, is governed by the Shockley-Read-Hall recombination process with a characteristic time constant of 375 picoseconds, whereas diffusion makes an insignificant contribution as it is suppressed by the high rate of scattering.

Biogenic porous silica and silicon sourced from Mexican Giant Horsetail (Equisetum myriochaetum) and their application as supports for enzyme immobilization.

Porous silica-based materials are attractive for biomedical applications due to their biocompatibility and biodegradable character. In addition, inorganic supports such as porous silicon are being developed due to integrated circuit chip compatibility and tunable properties leading to a wide range of multidisciplinary applications. In this contribution, biosilica extracted from a rarely studied plant material (Equisetum Myriochaetum), its conversion to silicon and the potential for both materials to be used as supports for enzyme immobilization are investigated. E. myriochaetum was subject to conventional acid digestion to extract biogenic silica with a% yield remarkably higher (up to 3 times) than for other Equisetum sp. (i.e. E. Arvense). The surface area of the isolated silica was ∼400 m2/g, suitable for biotechnological applications. Biogenic silicon was obtained by magnesiothermic reduction. The materials were characterized by SEM-EDX, XRD, FT-IR, ICP-OES, TGA and BET analysis and did not contain significant levels of class 1 heavy elements (such as Pb, Cd, Hg and As). Two commercial peroxidases, horseradish peroxidase (HRP) and Coprinus cinereus peroxidase (CiP) were immobilized onto the biogenic materials using three different functionalization routes: (A) carbodiimide, (B) amine + glutaraldehyde and (C) amine + carbodiimide. Although both biogenic silica and porous silicon could be used as supports differences in behaviour were observed for the two enzymes. For HRP, loading onto biogenic silica via the glutaraldehyde immobilization technique (route B) was most effective. The loading of CiP showed a much higher peroxidase activity onto porous silicon than silica functionalized by the carbodiimide method (route A). From the properties of the extracted materials obtained from Equisetum Myriochaetum and the immobilization results observed, these materials appear to be promising for industrial and biomedical applications.

Accelerated calcification in electrically conductive polymer composites comprised of poly(epsilon-caprolactone), polyaniline, and bioactive mesoporous silicon.

In this study the fabrication and characterization of an electrically conductive composite material comprised of poly(epsilon-caprolactone) (PCL), polyaniline (PANi), and bioactive mesoporous silicon (BioSilicon) is discussed. The influence of PANi and silicon on calcium phosphate induction was assessed via ex vitro calcification analyses (by acellular simulated body fluid (SBF) exposure) both with and without electrical bias. Acceleration of calcium phosphate formation is one possible desirable feature of "smart" synthetic scaffolds for selected orthopedic-relevant applications. In addition, electrical stability assays were performed in growth medium (DMEM) to determine the stability of such structures to bias in an authentic electrolyte during a typical cell experiment. The cytocompatibility of the composites was evaluated in vitro using human kidney fibroblasts (HEK 293) cell proliferation assays, along with more orthopedically relevant mesenchymal stem cells from mouse stroma. Importantly, these composites demonstrate accelerated calcification in SBF when electrical bias is applied cathodically to the scaffold. Furthermore, these scaffolds exhibit noncytotoxic behavior in the presence of fibroblasts over an 8-day culture period, and attachment of stromal cells to the semiconducting scaffold was directly imaged via scanning electron microscopy. Overall, these results suggest that materials of this type of composition have potential merit as a biomaterial.

Evaluation of mammalian cell adhesion on surface-modified porous silicon.

Porous silicon is a promising biomaterial that is non-toxic and biodegradable. Surface modification can offer control over the degradation rate and can also impart properties that promote cell adhesion. In this study, we modified the surface of porous silicon surface by ozone oxidation, silanisation or coating with collagen or serum. For each surface, topography was characterised using atomic force microscopy, wettability by water contact angle measurements, degradation in aqueous buffer by interferometric reflectance spectroscopy and surface chemistry by Fourier-transform infrared spectroscopy. The adhesion of rat pheochromocytoma (PC12) and human lens epithelial cells to these surfaces was investigated. Cells were incubated on the surfaces for 4 and 24 h, and adhesion characteristics were determined by using a fluorescent vital stain and cell counts. Collagen coated and amino silanised porous silicon promoted cell attachment for both cell lines whereas cells attached poorly to ozone oxidised and polyethylene glycol silanised surfaces. We showed that the two cell lines had different adhesion characteristics on the various surfaces at different time points. The use of the vitality assays Alamar Blue (redox based assay) and neutral red (active cellular uptake assay) with porous silicon was also investigated. We reveal incompatibilities between certain resazurin (Alamar Blue), lysosomal incorporation assays (neutral red) and porous silicon.

All-optical modulation in Mid-Wavelength Infrared using porous Si membranes.

We demonstrate for the first time the possibility of all-optical modulation of self-standing porous Silicon (pSi) membrane in the Mid-Wavelength Infrared (MWIR) range using femtosecond pump-probe techniques. To study optical modulation, we used pulses of an 800 nm, 60 femtosecond for pump and a MWIR tunable probe in the spectral range between 3.5 and 4.4 µm. We show that pSi possesses a natural transparency window centred around 4 µm. Yet, about 55% of modulation contrast can be achieved by means of optical excitation at the pump power of 60 mW (4.8 mJ/cm(2)). Our analysis shows that the main mechanism of the modulation is interaction of the MWIR signal with the free charge carrier excited by the pump. The time-resolved measurements showed a sub-picosecond rise time and a recovery time of about 66 ps, which suggests a modulation speed performance of ~15 GHz. This optical modulation of pSi membrane in MWIR can be applied to a variety of applications such as thermal imaging and free space communications.

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.

The role of nanostructured mesoporous silicon in discriminating in vitro calcification for electrospun composite tissue engineering scaffolds.

The impact of mesoporous silicon (PSi) particles-embedded either on the surface, or totally encapsulated within electrospun poly (ε-caprolactone) (PCL) fibers-on its properties as a tissue engineering scaffold is assessed. Our findings suggest that the resorbable porous silicon component can sensitively accelerate the necessary calcification process in such composites. Calcium phosphate deposition on the scaffolds was measured via in vitro calcification assays both at acellular and cellular levels. Extensive attachment of fibroblasts, human adult mesenchymal stem cells, and mouse stromal cells to the scaffold were observed. Complementary cell differentiation assays and ultrastructural measurements were also carried out; the levels of alkaline phosphatase expression, a specific biomarker for mesenchymal stem cell differentiation, show that the scaffolds have the ability to mediate such processes, and that the location of the Si plays a key role in levels of expression.

Generation of reactive oxygen species from porous silicon microparticles in cell culture medium.

Nanostructured (porous) silicon is a promising biodegradable biomaterial, which is being intensively researched as a tissue engineering scaffold and drug-delivery vehicle. Here, we tested the biocompatibility of non-treated and thermally-oxidized porous silicon particles using an indirect cell viability assay. Initial direct cell culture on porous silicon determined that human lens epithelial cells only poorly adhered to non-treated porous silicon. Using an indirect cell culture assay, we found that non-treated microparticles caused complete cell death, indicating that these particles generated a toxic product in cell culture medium. In contrast, thermally-oxidized microparticles did not reduce cell viability significantly. We found evidence for the generation of reactive oxygen species (ROS) by means of the fluorescent probe 2',7'-dichlorofluorescin. Our results suggest that non-treated porous silicon microparticles produced ROS, which interacted with the components of the cell culture medium, leading to the formation of cytotoxic species. Oxidation of porous silicon microparticles not only mitigated, but also abolished the toxic effects.

The biocompatibility of porous silicon in tissues of the eye.

In this report, we explore the biocompatibility of thermally-oxidised, aminosilanised porous silicon membranes and their potential to support human ocular cells in vitro and in vivo, in the rat eye. A colorimetric assay for silicic acid showed that membranes with pore sizes of 40-60nm slowly dissolved, but the material could be maintained in tissue culture medium in vitro for at least two weeks without visible degradation. When implanted under the rat conjunctiva, the material did not erode the underlying or overlying tissue. The implant underwent slow dissolution, but remained visible at the operating microscope for over 8 weeks. End-stage histology indicated the presence of a thin fibrous capsule surrounding the implant, but little evidence of any local accumulation of acute inflammatory cells or vascularization. Human lens epithelial cells and primary human corneal explants adhered to the porous silicon membranes, where they remained viable and underwent division. Primary corneal epithelial cells supported on membranes were labelled with a cell tracker dye and implanted under the rat conjunctiva. Seven days later, labelled cells had moved from the membrane into the ocular tissue spaces. A porous silicon membrane may have value as a biomaterial that can support the delivery of cells to the ocular surface and improve existing therapeutic options in patients with corneal epithelial stem cell dysfunction and ocular surface disease.