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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.

A composite material of plasmonic nanoparticles embedded in a scaffold of nano-porous silicon offers unmatched capabilities for use as a SERS substrate. The marriage of these components presents an exclusive combination of tightly focused amplification of Localised Surface Plasmon (LSP) fields inside the material with an extremely high surface-to-volume ratio. This provides favourable conditions for a single molecule or extremely low concentration detection by SERS. In this work the advantage of the composite is demonstrated by SERS detection of Methylene Blue at a concentration as low as a few picomolars. We systematically investigate the plasmonic properties of the material by imaging its morphology, establishing its composition and the effect on the LSP resonance optical spectra.

Porous silicon layers on wafers are commonly converted into particles by mechanical milling or ultrasonic fragmentation. The former technique can rapidly generate large batches of microparticles. The latter technique is commonly used for making nanoparticles but processing times are very long and yields, where reported, are often very low. With both processing techniques, the porosity and surface area of the particles generated are often assumed to be similar to those of the parent film. We demonstrate that this is rarely the case, using air-dried high porosity and supercritically dried aerocrystals as examples. We show that whereas ball milling can more quickly generate much higher yields of particles, it is much more damaging to the nanostructures than ultrasonic fragmentation. The latter technique is particularly promising for silicon aerocrystals since processing times are reduced whilst yields are simultaneously raised with ultrahigh porosity structures. Not only that, but very high surface areas (> 500 m2/g) can be completely preserved with ultrasonic fragmentation.

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.

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.

Coupling between nanoplasmonics and semiconducting materials can enhance and complement the efficiency of almost all semiconductor technologies. It has been demonstrated that such composites enhance the light coupling to nanowires, increase photocurrent in detectors, enable sub-gap detection, allow DNA detection, and produce large non-linearity. Nevertheless, the tailored fabrication using the conventional methods to produce such composites remains a formidable challenge. This work attempts to resolve that deficiency by deploying the immersion-plating method to spontaneously grown gold clusters inside nano-porous silicon (np-Si). This method allows the fabrication of thin films of np-Si with embedded gold nanoparticles (Au) and creates nanoplasmonic���semiconductor composites, np-Si/Au, with fractional volume between 0.02 and 0.13 of the metallic component. Optical scattering measurements reveal a distinctive, 200 nm broad, localized surface plasmon (LSP) resonance, centered around 700 nm. Linear and non-linear properties, and their time evolution are investigated by optically pumping the LSP resonance and probing the optical response with short wavelength infra-red (2.5 ���� m) light. The ultrafast time-resolved study demonstrates unambiguously that the non-linear response is not only directly related to the LSP excitation, but strongly enhanced with respect to bare np-Si, while its strength can be tuned by varying the metallic component.

A variety of silicon-based nanostructures with dimensions in the 1–5 nm range now emit tunable photoluminescence (PL) spanning the visible range. Achievement of high photoluminescence quantum efficiency (PLQY) relies critically on their surface chemistry passivation and an impressive “tool box” of options have been developed. Two distinct PL bands are dominant. The “S-Band” (red–green emission with Slow microsecond decay) has PLQY that has steadily improved from ∼3% in 1990 to 65 ± 5% by 2017. The “F-Band” (blue–yellow with Fast nanosecond decay) has reported PLQY values that have improved from ∼0.1% in 1994 to as high as ∼90% by 2016. The vast literature on both bands is surveyed and for the S-band, size-structure-PL correlations and selective photo-excitation studies are highlighted. Resonant photoexcitation and single quantum dot studies have revealed the key role of quantum confinement and the excitonic phonon-assisted nature of the radiative transitions. For the F-band, in contrast, specific phenomenological studies are highlighted that demonstrate similar emission without the presence of silicon nanostructures. Low PLQY F-band emission from pure silicon–silica core shell systems is probably associated with oxide-related defects, but ultrahigh PLQY from many lower temperature synthesis routes is likely to be from carbon-based nanostructures or chromophores, not silicon nanostructures. Potential applications for both PL bands include sensing, medical imaging, theranostics, photovoltaics, LED colour converters and nano-thermometry. Emerging “green” synthesis routes are mentioned. If scalability and cost are significantly improved then a number of other proposed uses of ultra-efficient PL from “nano-Si” could become viable in cosmetics, catalysis, security and forensics.

Porous silicon has found increased attention as a drug delivery system due to its unique features such as high drug payloads, surface area and biodegradation. In this study supercritical fluid (SCF) assisted drying of ultrahigh porosity (>90%) silicon particles and flakes was shown to result in much higher mesopore volumes (~4.66 cm3/g) and surface areas (~680 m2/g) than with air-drying. The loading and physical state of the model drug (S)-(+)-Ibuprofen in SCF dried matrices was quantified and assessed using thermogravimetric analysis, differential scanning calorimetry, UV–Vis spectrophotometry, gravimetric analysis, gas adsorption and electron microscopy. Internal drug payloads of up to 72% were achieved which was substantially higher than values published for both conventionally dried porous silicon (17–51%) and other mesoporous materials (7–45%). In-vitro degradability kinetics of SCF-dried matrices in simulated media was also found to be faster than air-dried controls. The in-vitro release studies provided improved but sustained drug dissolution at both pH 2.0 and pH 7.4.

In this work, nanostructured porous silicon (pSi) prepared by a metal-assisted stain etched route is investigated for its ability to act as a carrier for sustained delivery of the antibacterial drug triclosan. The morphology, analyzed by transmission electron microscopy, reveals a rather different microstructure than traditional anodized porous silicon; as a consequence, such morphology manifests a different loaded drug crystallinity, triclosan release behavior, and associated antibacterial activity versus Staphococcus aureus relative to high porosity anodized porous silicon. In addition to electron microscopies and antibacterial disk diffusion assays, a combination of x-ray diffraction, thermogravimetric analyses, and UV/Vis spectrophotometric analysis of triclosan release are employed to carry out the above investigations.

© 2020 Wiley-VCH GmbH In influencing fundamental properties—and ultimately device performance—of lead halide perovskites, interfacial interactions play a major role, notably with regard to carrier diffusion and recombination. Here anodized porous Si (pSi) as well as porous silica particles are employed as templates for formation of methylammonium lead trihalide nanostructures. This allows synthesis of relatively small perovskite domains and comparison of associated interfacial chemistry between as-prepared hydrophobic hydrideterminated functionalities and hydrophilic oxide-terminated surfaces. While physical confinement of MAPbBr3 has a uniform effect on carrier lifetime, pore size (7–18 nm) of the silicon-containing template has a sensitive influence on perovskite photoluminescence (PL) wavelength maximum. Furthermore, identity of the surface functionality of the template significantly alters the PL quantum efficiency, with lowest PL intensity associated with the H-terminated pSi and the most intense PL affiliated with the oxideterminated pSi surface. These effects are explored for green-emitting MAPbBr3 as well as infrared-emitting MAPbI3. In addition, the role of silicon surface chemistry on the time-dependent stability of these perovskites packaged within a given mesoporous template is also evaluated, specifically, a lack of miscibility between MAPbI3 and the H-terminated pSi template results in a diffusion of this specific perovskite composition eluting from this porous matrix over time.

Recent developments in biodegradable nanostructured crystalline silicon and flexible silicon-based electronic devices raise the significant question of the stability of standard amorphous silicon transistor platforms in biologically relevant environments. In this work, we evaluate the biodegradation of hydrogenated amorphous silicon thin film transistors. Specifically, using a combination of gravimetric analysis, optical imaging, and X-ray fluorescence, we investigate the fundamental stability of a simple hydrogenated amorphous silicon thin film transistor structure immersed in phosphate-buffered saline at physiological temperature (37 °C). In addition to the possible galvanic influence of associated metal electrodes in the degradation of such devices, implications for future device platforms are also discussed. A study on the degradation of amorphous silicon-based devices may suggest ways to improve the performance of some biomedical implants. An international team of researchers led by Jeffery L. Coffer at Texas Christian University, USA, has immersed in a water-based salt solution kept at body temperature electronic devices based on amorphous silicon—a structurally disordered version of silicon that can be deposited as a thin film to realize flexible transistors. They observed how silicon and the metals used as electric contacts dissolve during 4 months, and how the local distribution of these materials affects dissolution. This investigation may prove useful to design devices that can be implanted in the body to record or stimulate the electrical activity of tissues and organs, and that biodegrade into non-toxic residues after a controlled amount of time.

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.

The chemical reactivity of as-anodized porous silicon is shown to have an adverse effect on a model drug (Lansoprazole) loaded into the pores. The silicon hydride surfaces can cause unwanted reactions with actives during storage or use. Techniques such as thermal oxidation or surface derivitization can lower the reactivity somewhat, by replacing the reactive silicon-hydride species with a more benign oxide or functional group. However, by using a trio of analytical techniques (fluorometric dye assay, HPLC assay, and chemography) we show that residual hydride is still likely to be present and only after combining thermal oxidation with surface derivitization can the residual reactivity be reduced to those values typically observed with sol-gel (porous) silica. Potential sources of residual reactivity are discussed, with reference to data obtained by trace metal analysis, residual solvents, and pH measurements.

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 (pSiO

The study of the stability enhancement of a peroxidase immobilized onto mesoporous silicon/silica microparticles is presented. Peroxidases tend to get inactivated in the presence of hydrogen peroxide, their essential co-substrate, following an auto-inactivation mechanism. In order to minimize this inactivation, a second protein was co-immobilized to act as an electron acceptor and thus increase the stability against self-oxidation of peroxidase. Two heme proteins were immobilized into the microparticles: a fungal commercial peroxidase and cytochrome c from equine heart. Two types of biocatalysts were prepared: one with only covalently immobilized peroxidase (one-protein system) and another based on covalent co-immobilization of peroxidase and cytochrome c (two-protein system), both immobilized by using carbodiimide chemistry. The amount of immobilized protein was estimated spectrophotometrically, and the characterization of the biocatalyst support matrix was performed using Brunauer–Emmett–Teller (BET), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), and Fourier transform infrared (FTIR) analyses. Stability studies show that co-immobilization with the two-protein system enhances the oxidative stability of peroxidase almost four times with respect to the one-protein system. Thermal stability analysis shows that the immobilization of peroxidase in derivatized porous silicon microparticles does not protect the protein from thermal denaturation, whereas biogenic silica microparticles confer significant thermal stabilization.

We demonstrate a process for realising mesoporous silicon from a range of land-based plants such as common grasses, bamboos, sugarcane and rice. Such plants act as “natural factories”, converting and concentrating vast quantities of soluble silicon in soil into nanostructured forms of silica in their roots, stems, branches or leaves. This porous biogenic silica is chemically extracted and then thermally reduced to porous silicon using magnesium vapor. Importantly, for larger batch size, an inexpensive thermal moderator such as salt, is added for control of the reaction exotherm and minimization of sintering. Mesoporous silicon of >350 m2/g with 8 nm wide pores has been obtained from a bamboo extract, for example. The same process is applicable to a wide range of “silicon accumulator plants”. The purity of this “naturally derived” porous silicon is likely to be raised to a level acceptable for a wide range of high volume applications outside of electronics and solar cell technology.

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.

Appendix A. Figures with essential colour discrimination: Certain figures in this article, particularly Figures 1, 3 and 4, are difficult to interpret in black and white. The full colour images can be found in the on-line version, at doi:10.1016/j.actbio.2010. 03.031. 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 24 h 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. Refereed/Peer-reviewed

The tunable pore size, biodegradability, and surface chemis- try of mesoporous silicon (BioSilicon ™ ) are important to a broad spectrum of uses for drug delivery. For the case of Boron Neutron Capture Therapy (BNCT), encapsulation of a given boron-containing drug molecule within a porous BioSilicon ™ microparticle provides a vehicle for a brachytherapy method that avoids the necessity of drug modification. In this work, the loading and characterization of three clinically approved BNCT drugs into mesoporous Si is demonstrated. Because of difficulties associated with light element detection, a method based on a Beer's Law analysis of selected FTIR vibrational bands has been developed to estimate boron-containing drug loading in these materials. As a complementary nanostructural platform, a cathodic deposition process for the surface enriched growth of selected drugs onto the surface of silicon nanowires is also described.

In this study, biodegradable mesoporous Si (BioSilicon ™ ) was loaded with cis-(2,2'-bipyridine) dichlororuthenium (II) (Ru complex) as a model hydrophobic compound. This ruthenium complex-loaded BioSilicon ™ was either partially embedded on the surface of electrospun polycaprolactone (PCL) fibers or fully encapsulated within the fibers. To study release profiles in the above model delivery systems, porous Si/PCL constructs were soaked in DI water at 37 °C and the UV―Vis absorption spectrum of the supernatant was measured as a function of time. These results show that the Ru complex was released in a sustained manner over 7-day period. In addition, it is shown that the controlled-release of this complex depends on both the spatial location of the complex in the PCL fibrous scaffolds as well as the amount of Ru compound loaded in the mesoporous Si.

In this study, the fabrication and characterization of highly porous composites composed of poly(epsilon-caprolactone) and bioactive mesoporous silicon (BioSilicon) prepared using salt-leaching and microemulsion/freeze-drying methods are described. The role of silicon, along with porosity, in the scaffolds on calcium phosphate deposition was assessed using acellular in vitro calcification analyses. The presence of bioactive silicon in these scaffolds is essential for the deposition of calcium phosphate while the samples are immersed in simulated body fluid (SBF). Silicon-containing scaffolds produced using salt-leaching methods are more likely to calcify as a consequence of SBF exposure than those produced using microemulsion methods. In vitro proliferation and cell viability assays of these porous composites using human embryonic kidney fibroblast cells indicate that no cytotoxic effects are present in the scaffolds under the conditions used. Preliminary analyses of bone sialoprotein and alkaline phosphatase expression using orthopedically relevant mesenchymal cells derived from bone marrow suggest that such scaffolds are capable of mediating osteoblast differentiation. Overall, the results show that these porous silicon-containing polymer scaffolds enhance calcification, can be considered nontoxic to cells, and support the proliferation, viability, attachment, and differentiation of bone precursor cells.

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.

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.

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.

We describe in this account a general, yet facile strategy for the directed assembly of bioactive composite materials comprised of an erodible organic polymer such as polycaprolactone and physiologically-resorbable inorganic mesoporous silicon. This method exploits a combination of capillary forces and selective interfacial coupling chemistry to produce isolable macroscale (mm sized) structures possessing a diverse range of geometries through simple mixing rather than intricate molding processes. Furthermore, we demonstrate the ability of such constructs to dissociate into their individual building blocks, with the concomitant release of embedded model compounds in a sustained manner.

This work describes the formation of porous composite materials based on a combination of bioactive mesoporous silicon and bioerodible polymers such as poly-caprolactone (PCL). The fabrication of a range of composites prepared by both salt leaching and microemulsion techniques are discussed. Particular attention to the influence of Si content in the composite on in vitro calcification assays are assessed. For each system, cytotoxicity and cellular proliferation are explicitly evaluated through fibroblast cell culture assays.

Here we describe a new algorithm of processing the optical spectra acquired from thin film samples, with emphasis to the infrared reflection-absorption spectra of porous silicon layers. A software package ProSpect-2 makes possible to remove oscillatory component of the spectra associated with interference fringes in fully automatic mode, thus essentially increasing the informative capability of IR spectra. It works in semiautomatic mode and fits experimental data with a batch of several sinusoidal signals slightly different in their frequencies (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon (Si) in the form of orthosilicic acid (Si(OH) 4 ) is vital for normal bone and connective tissue homeostasis. Porous Si films release Si(OH) 4 in aqueous solutions in the physiological pH range. This study investigates the release of Si(OH) 4 from porous Si films under physiological conditions with the aim of developing a bioavailable form of Si. Using a standardised technique, porous Si films released increasing Si with time. Dissolution was significant at pH 7 and above, and at a temperature of 37 °C. Higher porosity generally promoted dissolution, however multiple layer films did not show enhanced solubility over corresponding single layer controls. These properties will be used to optimise Si nanostructures that slowly deliver orthosilicic acid in the digestive tract.

Nanostructured mesoporous silicon powders are being developed for a diverse range of applications including drug delivery, therapeutics, biosensors, and explosives. Such powders are easily produced through mechanical milling of membranes taken from electrochemically anodized silicon wafers. Where applications require highvolume, low-cost production, however, wafer feedstock is not viable and alternative feedstocks such as metallurgical-grade silicon powders become more attractive. Porosification of powders can be achieved using electro-less processing such as stain etching. Although much of the stain-etch literature has been developed around silicon wafers, the process, involving immersion of the silicon in an aqueous solution of hydrofluoric and nitric acids, has been readily transferred to powders. 1,2 Scalable synthesis methodologies have been reported for powders, 3 albeit with limitations on achievable surface area (143 m 2 /g), pore volume (0.3 ml/g), and yield (5%). Encouraging results have been reported 4 on the use of alternative oxidants, containing transition metals, for porosification of wafers, thus avoiding issues such as quiescence periods, inhomogeneity and irreproducibility usually associated with hydrofluoric-nitric acid processing. Metal-assisted chemical etching of silicon wafers has been the subject of two reviews. 5,6 For the first time, to our knowledge, we have applied metal-assisted etching using the hydrofluoric acid (HF)-ferric chloride (FeCl3) process, 7,8 in modified form, to metallurgical-grade silicon powders. The metal-assisted etch reaction can be described as

To use porous silicon as an optical interferometric biosensor, the pores must be sufficiently large to allow easy ingress of reagents and the layer must also display Fabry-Perot optical cavity modes. Here the detection antibody is rabbit IgG and the analyte is α-rabbit IgG conjugated to horseradish peroxidase (HRP). For this model system, the pores should be >50 nm in diameter. Such diameters have been obtained in 0.05 Ω cm n-type silicon using anodisation followed by chemical etching in ethanolic KOH and also by anodising 0.005 Ω cm p-type material. The latter also displays optical cavity modes. The silicon surface is oxidised in ozone, silanised using aminopropylmethoxysilanes with one, two or three methoxy groups, and cross linked to IgG using glutaraldehyde. High specific binding is found for mono-, di- and tri-methoxy silanes, but the lowest non-specific binding is found for silanisation with the tri-methoxy silane.

The microstructure and chemical composition of silicon surfaces subjected to standard sterilization protocols in hospital autoclaves has been examined. Both metallic contamination and structural changes can accompany porous Si hydrolysis. The in-vitro incubation of bulk and porous Si discs with human lymphocytes reveals potential sources of cytoxicity such as silane emission, and the supersaturation of media with eluted Si.

The stability of derivatized mesoporous silicon mirrors in simulated human blood plasma has been assessed. The rate at which they are dissolved in-vivo is predicted to be tunable by surface chemistry over timescales of weeks to years, and high reflectivity can be maintained until the bottom of the multilayer stack starts to corrode. Such biodegradable optical components could be utilized to direct and define optical path lengths for therapeutic treatments and minimally-invasive diagnostics.

In this work, the incorporation and characterization of cis-platin (cis-diammine dichloroplatinum(II)), carbo-platin [cis-diammine(cyclobutane-1,1-dicarboxylato] platinum(II)), and Pt(en)Cl2 (ethylenediamminedichloro platinum(II)) within layers of calcium phosphate on porous Si/Si substrates are described. These materials have been characterized by scanning electron microscopy, secondary ion mass spectrometry, and X-ray energy dispersive spectroscopy. The diffusion of platinum species from the doped calcium phosphate layers has also been investigated by UV-visible absorption spectrometry and inductively-coupled plasma spectroscopy. The influence of initial platinum concentration, the impact of thermal annealing of the calcium phosphate/porous Si/Si matrix, as well as the effect of varying the ligand coordination sphere of the Pt complex on its ability to be delivered to the surroundings have also been analyzed. For the case of cis-platin, it is found that increasing the concentration of platinum complex in the electrolyte during cathodic growth of calcium phosphate results in a relatively greater concentration of Pt incorporated into the calcium phosphate layers and a larger amount of Pt which subsequently can be delivered to the surrounding medium upon exposure to solvent.

Lewis acid mediated hydrosilylation of alkynes and alkenes on non-oxidized hydride-terminated porous silicon derivatizes the surface with alkenyl and alkyl functionalities, respectively. A very broad range of chemical groups may be incorporated, allowing for tailoring of the interfacial characteristics of the material. The reaction is shown to protect and stabilize porous silicon surfaces from atmospheric or direct chemical attack without compromising its valuable material properties such as high porosity, surface area and visible room-temperature photoluminescence. The reaction is shown to act on alkenes and alkynes of all possible regiochemistries (terminal and internal alkynes; mono-, cis- and trans-, di-, tri-, and tetrasubstituted alkenes). FTIR as well as liquid- and solid-state NMR spectroscopies show anti-Markovnikov addition and cis stereochemistry in the case of hydrosilylated terminal alkynes. Material hydrosilylated with long-chain hydrophobic alkynes and alkenes shows a substantially slower surface oxidation and hydrolysis rate in air as monitored by long-term FTIR monitoring and chemography. BJH and BET measurements reveal that the surface area and average pore size of the material are reduced only slightly after hydrosilylation, indicating that the porous silicon skeleton remains intact. Elemental analysis and SIMS depth profiling show a consistent level of carbon incorporation throughout the porous silicon which demonstrates that the reaction occurs uniformly throughout the depth of the film. The effects of functionalization on photoluminescence were investigated and are shown to depend on the organic substituents.

Trends in the efficiency and small signal modulation behavior of porous silicon light emitting diodes (LEDs) are reported for devices formed by the anodization of bulk silicon p–n junctions. As the average size of the silicon skeleton is decreased, the external electroluminescence (EL) efficiency increases from 0.001% to 0.18% and there is a corresponding blue shift in the EL peak from 776 to 633 nm. An associated tenfold increase is observed in the photoluminescence efficiency while the diode resistance, at 2 V, increases from 3×103 to 1×106 Ω. Under small signal pulsed operation, the voltage dependence of the rising edge of the EL is well described by a carrier mobility of 3×10−4 cm2 s−1 V−1 which is independent of the average size of the luminescent regions of the silicon nanostructure. The falling edge of the EL transient is dominated by radiative recombination of quantum confined excitons. The modulation speed is found to be limited by a combination of carrier mobility in the silicon wires and radiat...

Large wavelength shifts have been measured in the reflectivity spectra of Bragg mirrors etched in porous silicon after exposure of the mirrors to vapor from organic solvents. The shift in the Bragg wavelength of the mirror arises from refractive index changes, induced by capillary condensation of the vapor in the mesoporous silicon, in the layers of the mirrors. Modeling of the reflectivity changes shows that the layer liquid volume fraction occurring in the measurements was 0.29 for acetone and 0.33 for chlorobenzene. Time-resolved measurements show that condensation occurs on the time scale of tens of seconds.

Experimental and theoretical techniques are used to examine the effects of microstructuring on the optical properties of multilayer, single and multiple microcavity structures fabricated from porous silicon. Measurements of the reflectivity and photoluminescence spectra of three multilayer samples are presented. The results are modelled using a transfer matrix technique including a negative absorption term to represent the effect of spontaneous emission which gives luminescence. The emitted light is strongly controlled by the optical modes of the structures and very good agreement is observed between theory and experiment.

In this work, the encapsulation of the transition metal complexes cis-Pt(NH3)2Cl2, Ru(bpy)32+, and Ru(phen)32+ within layers of hydroxyapatite on a porous Si/Si substrate are described. These specific complexes were selected as a consequence of their biological activity (cis-Pt(NH3)2Cl2) or polynucleotide binding ability (Ru(bpy)32+ and Ru(phen)32+). The complex-doped hydroxyapatite/porous Si/Si materials have been characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, secondary ion mass spectrometry, and photoluminescence spectroscopy. It is possible to exercise some control of the release of a given complex into the surrounding medium through thermal heating of the complex-doped hydroxyapatite/porous Si/Si structure.

It is desirable to know the location and type of the hydrogen within the nanostructure of porous silicon (PS). Hydrogen is, however, very difficult to detect and locate with conventional spectroscopic techniques. The presence of hydrogen in bulk crystalline and amorphous silicon is well-known, and it can exist in the form of intrinsic hydrogen, implanted hydrogen or as an impurity. Muon studies of amorphous and bulk crystalline silicon suggest that there are two important states for muonium and hence hydrogen in silicon, termed the `normal' and `anomalous' sites. Normal muonium is generally described as being located at the centre of the tetrahedral cage site. The second state is anisotropic and is believed to lie at the centre of a stretched Si–Si bond. Here we report muon implantation of PS and this is discussed in terms of the nanostructure dimensionality.

A range of bio-ceramics can induce calcium phosphate nucleation in simulated body fluids over timescales of hours to days. Such in-vitro behaviour is well correlated with in-vivo bone and tissue bonding ability. We demonstrate and discuss here how the “bio-activity” of porous silicon films might be tailored by manipulation of layer microstructure, chemical composition, and surface electrical charge. The kinetics of in-vitro calcification can be accelerated to occur within hours, suggesting that silicon bio-chips might be developed that could rapidly bond in-vivo with living tissue and bone. Crown Copyright © 1997 Published by Elsevier Science S.A.

We briefly review the performance to date of a variety of porous Si-based discrete optoelectronic devices. Light emitting diodes, waveguides and photodetectors have now been demonstrated, together with a number of non-linear optical effects that could be exploited in electro-optic or all optical modulators. In each case comparison is made with bulk Si devices to identify where exploitation of porosity has already resulted in significant performance gains, or might do in the future. Dramatic progress has recently been made with regard to visible light emitting diodes and efficiencies are now > 104 × higher than those of bulk Si devices. Preliminary studies suggest that porous Si should also be developed for both passive and active waveguiding. Key problems that are still to be addressed with regard to applications are also identified.

When heavily oxidized porous silicon is stored in ambient air for prolonged periods, a broad blue photoluminescence band with nanosecond decay times gradually appears. Not only PL intensity, but peak position, half-width and temperature dependence evolve with material aging. Very similar photoluminescence is shown to be characteristic of many high surface area materials stored in air for months. We stress here that hydrocarbon contamination can occur during air exposure, in addition to hydration by water vapour. Preliminary Fourier transform infrared spectroscopy and secondary ion mass spectrometry studies, together with published data on blue-emitting porous alumina and organics, suggest that the gradual build-up of low concentrations of carbonyl-related chromophores could give rise to the emergence of blue-green luminescence in aged oxidized porous Si structures.

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.

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 effect of supercritical drying (SCD) on the preparation of porous silicon (pSi) powders has been investigated in terms of photoluminescence (PL) efficiency. Since the pSi contains closely spaced and possibly interconnected Si nanocrystals (