Reflective interferometric fourier transform spectroscopy: a self-compensating label-free immunosensor using double-layers of porous SiO2

An interferometric biosensor comprised of two layers of porous Si, stacked one on top of the other, is described. A fast Fourier transform (FFT) of the reflectivity spectrum reveals three peaks that correspond to the optical thickness of the top layer, the bottom layer, and both layers together. Binding of immunoglobulin G to a protein A capture probe adsorbed to the surface of the top layer induces changes in reflectivity at the top layer/solution interface. The FFT method allows discrimination of target analyte binding from matrix effects due to nonspecific changes in the analyte solution. The sensor response is shown to be insensitive to the addition of 4000-fold excess sucrose or 80-fold excess bovine serum albumin interferents.     

A porous silicon-palladium composite film for optical interferometric sensing of hydrogen

Porous Si Fabry-Pérot films are coated with Pd via immersion plating. The materials are characterized by electron microscopy and infrared spectroscopy. The Pd-coated porous Si samples exhibit distinct Fabry-Pérot fringes in the optical reflection spectrum due to thin film optical interference in the porous Si layer, though the reflectivity spectrum loses fidelity upon Pd coating. The effect of H2 exposure on the interference spectrum is studied. Absorption of hydrogen into Pd induces a lattice expansion, which results in a shift of the optical fringes and a decrease in the reflected intensity. The detection limit measured at room temperature is approximately 0.2% (by volume) in an N2 carrier gas, with a response time of a few seconds.     

Local heating of discrete droplets using magnetic porous silicon-based photonic crystals

This paper describes a method for local heating of discrete microliter-scale liquid droplets. The droplets are covered with magnetic porous Si microparticles, and heating is achieved by application of an external alternating electromagnetic field. The magnetic porous Si microparticles consist of two layers. The top layer contains a photonic code and it is hydrophobic, with surface-grafted dodecyl moieties. The bottom layer consists of a hydrophilic silicon oxide host layer that is infused with Fe3O4 nanoparticles. The amphiphilic microparticles spontaneously align at the interface of a water droplet immersed in mineral oil, allowing manipulation of the droplets by application of a magnetic field. Application of an oscillating magnetic field (338 kHz, 18 A rms current in a coil surrounding the experiment) generates heat in the superparamagnetic particles that can raise the temperature of the enclosed water droplet to >80 degrees C within 5 min. A simple microfluidics application is demonstrated: combining complementary DNA strands contained in separate droplets and then thermally inducing dehybridization of the conjugate. The complementary oligonucleotides were conjugated with the cyanine dye fluorophores Cy3 and Cy5 to quantify the melting/rebinding reaction by fluorescence resonance energy transfer (FRET). The magnetic porous Si microparticles were prepared as photonic crystals, containing spectral codes that allowed the identification of the droplets by reflectivity spectroscopy. The technique demonstrates the feasibility of tagging, manipulating, and heating small volumes of liquids without the use of conventional microfluidic channel and heating systems.     

Quantum Characterization of Some Cluster-Based Materials

Three kinds of cluster-based materials are prepared by evaporation and inert gas condensation method. Their structures and properties are examined by transmission electron microscopy, Raman scattering, STM/STS, optical spectroscopy, etc. Some important results are obtained: (1) surface phonon modes of quasi-free Si clusters are observed when Si clusters softly land onto the mother skeleton of the porous silicon and/or through grazing angle collisions with the walls of the pores; (2) very sharp peaks of conductance resonances are obtained when the STM tip is right on the top of the Au cluster deposited on the H-terminated silicon crystal; and (3) large blue shifts and photoluminescence from violet to orange with main peaks in the blue range are observed from Ge cluster-based nanofilms at an excitation wavelength of 370 nm. Mechanisms are discussed including the quantum confinement effect of the Ge cluster cores, radiation transition from oxygen difficiency centers in the oxide surface layers, and exciton confinement in the interfacial layers between the crystalline cores and the oxide shells.     

Engineering the chemistry and nanostructure of porous silicon Fabry-Pérot films for loading and release of a steroid

A method for engineering the surface chemistry and pore dimensions in porous Si films for the purpose of controlling the loading and release of a hydrophobic drug is described. Loading of the steroid dexamethasone is confirmed by Fourier transform infrared spectroscopy, and the release rates are characterized by observation of the appearance of the drug in solution (UV-vis absorption spectroscopy) and by measurement of the Fabry-Perot fringes in the optical reflectivity spectrum of the porous Si film. Optical reflectivity changes provide a measure of the release rate of the drug that is amenable to in-vivo diagnostic applications. Fresh porous Si films are prepared by electrochemical etch and subsequently modified by hydrosilylation with 1-dodecene. The dodecene-modified samples are more robust in aqueous environments and exhibit slower release rates of the drug relative to freshly etched porous Si. Whereas the relatively large dexamethasone molecule is found to infiltrate the freshly etched samples, it does not enter the chemically modified films, because of steric crowding from the dodecyl species. To achieve a high degree of loading into these modified films, the pores are enlarged before hydrosilylation by treatment with an aqueous solution containing HF and dimethyl sulfoxide. The pore expanded, chemically modified samples admit approximately 70% of the dexamethasone that can be admitted into an unmodified (freshly etched) sample. Diffusion of the steroid from the modified, pore expanded films into phosphate-buffered saline solution is slower than from the unmodified sample by a factor of approximately 20, with 90% of the drug delivered in 3 days for the chemically modified films compared to 3 h for the unmodified films.     

硅基超薄多孔氧化铝膜的制备

将二次阳极氧化法应用于硅基铝膜的制备, 在草酸溶液中得到了厚度可控的硅基超薄多孔氧化铝膜(PAM), 厚度小于100 nm. 实验中记录了氧化电流随时间的实时变化曲线, 发现硅衬底的氧化电流在大幅下降前有一小幅波动. 对应于Al/Si界面的氧化过程中, 孔洞底部之间的残留铝岛被优先氧化, 可将此作为终止铝氧化的标志. 扫描电镜(SEM)观察表明, 二次氧化提高了孔洞分布的均匀性, 使得孔在一定的区域内呈现有序六角分布.这种模板可进一步用于硅基纳米器件和纳米结构的制备.     

Superhydrophobic and lipophobic properties of self-organized honeycomb and pincushion structures

This report describes the simple preparation of superhydrophobic and lipophobic surfaces by self-organization. Microporous polymer films of a fluorinated polymer with hexagonally arranged pores were prepared by casting from solution under humid conditions. Hexagonally packed water microdroplets were formed by evaporative cooling on the surface of the casting solution. After solvent evaporation, a honeycomb-patterned polymer film was formed with the water droplet array acting as a template; the water droplets themselves evaporated soon after the solvent. Two porous polymer layers were stacked vertically, separated by pillars at the hexagon vertexes. After peeling off the top layer using adhesive tape, a pincushion-like structure was obtained. Here, we show that superhydrophobic behavior was achieved, with the maximum contact angle, 170 degrees, observed using these pincushion structures. Theoretical calculations fit the experimental results well. The lipophobic properties of the films are also discussed.     

IR absorption and reflectometric interference spectroscopy (RIfS) combined to a new sensing approach for gas analytes absorbed into thin polymer films

Hydrophobic polymer layers (3 μm) were spin-coated on Si or Ge plates and placed in a flow through gas chamber. FTIR reflection spectra of the layers were recorded showing the characteristic IR absorption bands of the polymer and the interference pattern generated by layered structure of the polymer film. Upon exposure of the polymer layer to gaseous analytes enrichment in the polymer film occurred. This was evidenced by the appearance of analyte specific absorption particular in the mid-IR part of the spectrum, as well as by a shift in the interference pattern across the whole spectrum. Qualitative information concerning the analyte was accessible in the mid-IR part of the spectrum, whereas quantitative assessment was obtained from the interference pattern. Polyetherurethane, polydimethylsiloxane, Makrolon^® and polyisobutylene polymer layers were tested for such IR–RIfS measurements, whereas toluene, o-dichlorobenzene, m-xylene, ethyl acetate and cyclohexane were employed as analytes. There was no influence of water vapour neither on the IR absorptions nor the interference pattern as hydrophobic polymers were used.     

Size tuning of luminescent silicon nanoparticles with meso-porous silicon membranes

Size tuning of silicon (Si) nanoparticles (NPs) with the use of meso-porous silicon (meso-PS) free-standing layers is reported for the first time. Accumulation of Si NPs inside the membrane pores during the filtering process (NP transport through the meso-PS) leads to an auto-filtration effect (called Si-by-Si (SBS) filtration) allowing more efficient size selection of the NPs. General complex fractal shape and surface chemistry of the whole porous network, layer thickness as well as a given initial NP size dispersion determine final size of the NPs in the filtered solution. Moreover, quantum of step-like NP size increasing equal to 0.12 nm was found.     

Scalable Synthesis of Interconnected Porous Silicon/Carbon Composites by the Rochow Reaction as High‐Performance Anodes of Lithium Ion Batteries

Despite the promising application of porous Si‐based anodes in future Li ion batteries, the large‐scale synthesis of these materials is still a great challenge. A scalable synthesis of porous Si materials is presented by the Rochow reaction, which is commonly used to produce organosilane monomers for synthesizing organosilane products in chemical industry. Commercial Si microparticles reacted with gas CH₃Cl over various Cu‐based catalyst particles to substantially create macropores within the unreacted Si accompanying with carbon deposition to generate porous Si/C composites. Taking advantage of the interconnected porous structure and conductive carbon‐coated layer after simple post treatment, these composites as anodes exhibit high reversible capacity and long cycle life. It is expected that by integrating the organosilane synthesis process and controlling reaction conditions, the manufacture of porous Si‐based anodes on an industrial scale is highly possible.     

Hierarchically structured porous cadmium selenide polycrystals using polystyrene bilayer templates

In this study, a novel approach is demonstrated to fabricate hierarchically structured cadmium selenide (CdSe) layers with size-tunable nano/microporous morphologies achieved using polystyrene (PS) bilayered templates (top layer: colloidal template) via potentiostatic electrochemical deposition. The PS bilayer template is made in two steps. First, various PS patterns (stripes, ellipsoids, and circles) are prepared as the bottom layers through imprint lithography. In a second step, a top template is deposited that consists of a self-assembled layer of colloidal 2D packed PS particles. Electrochemical growth of CdSe crystals in the voids and selective removal of the PS bilayered templates give rise to hierarchically patterned 2D hexagonal porous CdSe structures. This simple and facile technique provides various unconventional porous CdSe films, arising from the effect of the PS bottom templates.     

In situ functionalization of porous silicon during the electrochemical formation process in ethanoic hydrofluoric acid solution

In this work, the results of a new method for the preparation of porous silicon (PS) layers with in situ simultaneous functionalization with organic molecules are reported. The molecules of interest are dissolved in the HF ethanoic solution used to prepare the PS layers by partial anodic dissolution of a Si electrode. The method has been proved to be effective with various molecules. In this Communication, the case of PS functionalization with heptyne molecules, studied by FTIR spectroscopy, is reported in detail. The results demonstrate that this new functionalization method, accompanied by a low-level oxidation, is simple, fast, and effective and that it can allow the confinement of the adsorbed molecules selectively in a single layer of a PS stack.     

Effects of substrate on the melting behavior of Cd arachidate Langmuir–Blodgett films

Nineteen monolayered Cd arachidate films were deposited on float glass substrate coated with Si and Ni over-layers. Two layers have been chosen with very different surface free energies. Melting behavior of films were studied using variable temperature X-ray specular reflectivity and Fourier transform infrared spectroscopy measurements. In conformity with earlier studies, melting of the multilayer precede by a transition from distorted hexagonal to hexaticlike phase. However, the transition temperature to hexaticlike phase as well as the melting temperature depend significantly on the type of layer. Both the transition temperatures are higher for the multilayer deposited on Ni layer as compared to those for the film deposited on Si layer. These results can be understood in terms of different surface free energies of Ni and Si layers. Further, in case of Ni layer, transition to hexaticlike phase is relatively sharper. Even in the molten state there is a significant difference in the structure of the Cd arachidate film on two layers; packing density of molecules in molten state is lower in case of substrate with higher surface free energy. These results suggest that the surface free energy of substrate plays a significant role in melting behavior of Langmuir–Blodgett films.     

Synthesis of Large‐Pore ECNU‐19 Material (12 × 8‐R) via Interlayer‐Expansion of HUS‐2 Lamellar Silicate

A large‐pore ECNU‐19 material with unique pore system consisting of 12‐ring (12R) pore channels intersected by 8R channels was post‐synthesized via interlayer‐expansion of HUS‐2 lamellar silicate with silylating agent of 1,3‐dimethyltetramethoxydisiloxane (DMTMDS). In consideration of the fact that the HUS‐2 precursor possessed a special structure with a malposition of the neighboring layers as well as silicon vacancies on layer surface, a “detemplating disassembly – intercalation reassembly – silylation” strategy was proposed to realize a successful interlayer‐expansion and structural amending. An acid treatment was firstly performed to remove a part of the structure‐directing agent molecules, which favored the following intercalation by bulk organic species. The intercalation not only rearranged the relative position of up‐down layers but also provided enough interlayer space for the insertion of dimeric silane molecules. Two –OH groups attached to one silicon atom of the silane molecule reacted with two close silanols on the up‐surface layer, while the other two –OH groups condensed with two silanols on the down‐surface layer, which then connected the two layers via ‐Si‐O‐Si‐ pillars and constructed new 12R pores along a axis and 8R pores along c axis, respectively.     

The effect of gas surface diffusion on the asymmetric permeability of two-layer porous membranes

The permeability is calculated for two-layer membranes composed of porous layers with nano- and microsized pores. It is found that, in nanosized pores, the gas transfer occurs in the free-molecular regime in the pore volume and via diffusion along the adsorption layer. The degree of adsorption layer filling is determined from the Langmuir isotherm. The dependence of the diffusion coefficient in the adsorption layer on the degree of surface coverage is taken into account. The transfer in the microsized pored is described in a hydrodynamic regime. The values of the membrane permeability are determined at different orientations with respect to the direction of a gas flow. It is shown that the difference between the permeability values may be as large as 60%.     

Planar monolithic porous polymer layers functionalized with gold nanoparticles as large-area substrates for sensitive surface-enhanced Raman scattering sensing of bacteria

For the first time, large-area surface-enhanced Raman scattering sensing active substrates using porous polymer monolithic layers have been successfully prepared. Our approach includes a simple photoinitiated polymerization process using glycidyl methacrylate and ethylene dimethacrylate in a glass mold, followed by a chemical reaction of the epoxy functionalities leading to thiols, and the attachment of preformed gold nanoparticles. We demonstrated that this very simple process produced uniform and reproducible large area surfaces that significantly enhance sensitivity of Raman spectroscopy. Experiments were also carried out that confirmed preferential adsorption of living bacteria Escherichia coli from a very dilute solution on the surface of the monolithic layer, and immediate detection of the captured microorganisms using the SERS spectrum.     

Mechanical properties of composite polymer microstructures fabricated by interference lithography

We demonstrate that organized, porous, polymer microstructures with continuous open nanoscale pores and a sub-micron spacing obtained via interference lithography can be successfully utilized in a non-traditional field of ordered polymer microcomposites. The examples presented here include porous matrices for the fabrication of binary, glassy-rubbery microcomposites with intriguing mechanical properties with large energy dissipation and lattice-controlled fracturing.     

Reduced protein adsorption by osmolytes

Osmolytes are substances that affect osmosis and are used by cells to adapt to environmental stress. Here, we report a neutron reflectivity study on the influence of some osmolytes on protein adsorption at solid-liquid interfaces. Bovine ribonuclease A (RNase) and bovine insulin were used as model proteins adsorbing at a hydrophilic silica and at a hydrophobic polystyrene surface. From the neutron reflectivity data, the adsorbed protein layers were characterized in terms of layer thickness, protein packing density, and adsorbed protein mass in the absence and presence of urea, trehalose, sucrose, and glycerol. All data point to the clear effect of these nonionic cosolvents on the degree of protein adsorption. For example, 1 M sucrose leads to a reduction of the adsorbed amount of RNase by 39% on a silica surface and by 71% on a polystyrene surface. Trehalose was found to exhibit activity similar to that of sucrose. The changes in adsorbed protein mass can be attributed to a decreased packing density of the proteins in the adsorbed layers. Moreover, we investigated insulin adsorption at a hydrophobic surface in the absence and presence of glycerol. The degree of insulin adsorption is decreased by even 80% in the presence of 4 M of glycerol. The results of this study demonstrate that nonionic cosolvents can be used to tune and control nonspecific protein adsorption at aqueous-solid interfaces, which might be relevant for biomedical applications.     

Time resolved alteration process of oxide glasses

Dissolution of oxide glasses by water has been studied by small angles X-ray scattering. It is shown that the altered residual surface layer due to dissolution and recondensation of Si atoms is a porous material with nanometer size pores. Based on five elements oxide glass (18NaO2-17B2O3-4CaO-yZrO2-(61-y)SiO2 with y=0, 1, 2, 4 and 8) the experiment highlights a strong influence of insoluble element on both the kinetic of alteration and the structure of the altered layer. It is shown that above 2% Zr content, the fraction of porous volume and the surface of exchange in the altered layer pass through a maximum value in the first hours of alteration corresponding to an overshoot of Si lixiviation without recondensation as the saturation limit is not reached. When the saturation limit is reached the porous volume fraction is just below the fraction of volume occupied primarily by the sodium and the boron.     

Origin of Porous Silicon Photoluminescence Peaks in the Wavelength Range 460–700 nm

The change in the photoluminescence peaks of porous silicon at λ = 640–670 and 540–560 nm at 300 and 77 K, as well as their behavior after low-temperature annealing of the samples at 500°С, has been studied. The change in these peaks correlated with that in the IR spectra. The peak at 640–670 nm has been explained by the existence Si–OH groups on the porous silicon layers and the peak at 540–560 nm, by the photoluminescence of the silicon matrix per se.