A sensitive immunosensor using colloidal gold as electrochemical label

A sensitive immunosensor using colloidal gold as electrochemical label is described. In this method, the capture protein was first immobilized on a carbon paste electrode surface through passive adsorption to bind quantitatively with corresponding antigen and colloidal gold labeled antibody to perform a sandwich assay. To detect the amount of the colloidal gold captured on the electrode surface, the colloid was first oxidized electrochemically to produce AuCl₄⁻ ions which were adsorbed strongly on the electrode surface. Adsorptive voltammetry was then employed for the determination of the adsorbed AuCl₄⁻ ions. A linear relationship between reduction wave peak current and the antigen concentration (human IgG) from 10 to 500 ng/ml is obtained with a detection limit of 4.0 ng/ml.     

Electrochemical etching of silicon carbide

Both n- and p-type SiC of different doping levels were electrochemically etched by HF. The etch rate (up to 1.5 μm/min) and the surface morphology of p-type 6H-SiC were sensitive to the applied voltage and the HF concentration. The electrochemical valence of 6.3 ± 0.5 elementary charge per SiC molecule was determined. At p-n junctions (p-type layer on a n-type 6H-SiC substrate) a selective etching of the p-type epilayer could be achieved. For a planar 6H-4H polytype junction (n-type, both polytypes with equal doping concentrations) the 4H region was selectively etched under UV illumination. Thus polytype junctions could be marked by electrochemical etching. With HCl instead of HF no etching of SiC occurs, but a SiO₂ layer (thickness up to 8 μm) is formed by anodic oxidation. Received: 29 October 1998 / Accepted: 27 January 1999     

Wettability-gradient surface fabricated by combining electrochemical etching and lithography

This paper proposes a simple, precise, and controllable method to fabricate wettability-gradient surfaces. Combining electrochemical etching and lithography, different micro/nanostructures can be obtained by adjusting the etching time. After being modified by low energy substances, low adhesive superhydrophobic and sticky hydrophobic regions can be obtained on one surface. Based on the obtained adhesion gradient, droplets of different volumes can be controlled to roll off at dissimilar tilted angles via designing sticky hydrophobic tracks with different widths. Directional transportation of water droplets on curve tracks is also realized based on the anisotropic sliding angles parallel and perpendicular to the tracks.     

Mechanisms of surface processes in silicon etching

A unified view on the mechanism allowing one to explain the experimental features governing spontaneous silicon etching by atomic fluorine is presented. Analysis of the phenomenological equation of adsorption shows a significant difference between etching mechanisms at high and low heat of adsorption on the surface being etched. As follows from the parameter estimates, one or another case can be realized under different experimental conditions. At steady-state the etching is argued to be determined only by the processes taking place on the SiF. film surface. To describe the process, it is necessary to understand the mechanism of overcoming the surface barrier for fluorine penetration into the film. At low heat of fluorine adsorption the barrier is overcome by thermal activation. In the opposite case the etching mechanism includes electron tunneling from silicon to adatoms and creation of a surface electric field. The field lowers the high energetic barrier for fluorine penetration. Based on the kinetic equations describing the electronic and atomic processes on the surface, the equation of the field strength is obtained. This equation is analyzed in different limit cases. The observed features are shown to be reproduced at some conditions on the parameters. Definite predictions on the temperature dependence of the etch rate are made.     

Size-controllable synthesis of monodispersed colloidal silica nanoparticles via hydrolysis of elemental silicon

A new method is presented for preparing monodisperse and uniform-size silica nanoparticles using a two-stage hydrolysis of silicon powder in aqueous medium. The influence of synthesis conditions including solution composition and temperature on the formation of silica nanoparticles were systematically investigated. The structure and morphology of the silica particles were characterized via transmission electron microscopy (TEM) and dynamic light scattering (DLS). Various-sized particles in the range 10–100 nm were synthesized. The size of the nanoparticles can be precisely controlled by using a facile regrowth procedure in the same reaction media.     

Morphological and compositional evolution of polymeric colloidal monolayer during UV irradiation

In this study, we investigated the morphological and compositional evolution polymeric colloidal monolayer during UV irradiation. A PS colloidal monolayer with interparticle bridges was prepared and exposed to the UV light. As a consequence of photochemical reactions containing chain-scission, UV irradiation induced morphological changes in the monolayer surface including changes in the size, shape, and packing structure of PS particles. By manipulating the UV irradiation time, fine tuning of size and shape of the interstice in the monolayer was achieved. In these procedures, the interparticle bridges play an important role. The UV irradiation induced the formation of polar groups in the PS particle surface and thus the particle surface became highly hydrophilic.     

pH and ionic strength responsive photonic polymers fabricated by using colloidal crystal templating

In this paper, monodispersed silica particles were synthesized using tetraethoxysiliane hydrolyzing in ethanol by a Stöber–Fink–Bobn method and then self-assembled on cleaning glass slides to form silica colloidal crystals. After photopolymerization of methacrylic acid mixing with ethylene glycol dimethylacrylate and hydrofluoric acid etching, the pH-responsive polymers were obtained with highly 3D-ordered macroporous structures templated by silica colloidal crystals. These polymers films can swell or deswell in response to external stimuli, causing a change of Bragg diffraction to read pH or ionic strength of various solutions by optical signals or electrochemical signals. As an application, they can be used as chemical sensors to detect pH or ionic strength variation of environment.     

The study of electrochemical palladium behavior using the quartz crystal microbalance

The electrochemical quartz crystal microbalance (EQCMB) method has been used to evaluate the processes which occur in/on the palladium electrode in basic solutions. Hydrogen electrosorption in palladium is accompanied by an additional frequency shift that can be attributed to the stresses generated inside the Pd metal. A non-linear dependence between the mass change and the charge consumed during hydrogen oxidation in the Pd electrode has been found for hydrogen absorbed in the α- and β-phases. This effect precludes the objective estimation of the amount of hydrogen absorbed inside the Pd electrode. The EQCMB method has been used, however, for studying the surface electrode processes on the Pd electrode, i.e. specific anion adsorption, surface oxidation and dissolution. Also, the structure of the palladium oxide formed on the Pd surface during electrochemical oxidation is discussed in this paper and the effect of the anodic limiting potential on the oxide structure is reported. Received: 10 August 1999 / Accepted: 24 September 1999     

Electron-impact-induced anisotropic etching of silicon by hydrogen

The etch rate of silicon in a hydrogen low-pressure discharge plasma can be strongly enhanced by electron bombardment, reaching presently up to 1000 Å/min. The etch rate increases linearly with increasing electron current density and hydrogen pressure (range 0.05–0.7 mbar) and decreases with increasing temperature, yielding an activation energy of –4.2 kcal/mole in a temperature range of 80 to 300°C. The etching remains anisotropic within the whole pressure range studied.     

Thin film processing using S-layer proteins: biotemplated assembly of colloidal gold etch masks for fabrication of silicon nanopillar arrays

We explored the bionanofabrication of silicon nanopillar structures using ordered gold nanoparticle arrays generated from microbial surface layer (S-layer) protein templates. The S-layer template used for these thin film processing experiments was isolated from the Gram-positive bacterium Deinococcus radiodurans. In this preliminary work, S-layers preimmobilized onto chemically modified silicon substrates were initially used to template the fabrication of a nanolithographic hard mask pattern comprised of a hexagonally ordered array of 5-nm gold nanoparticles (lattice constant = 18 nm). Significantly, the use of the biotemplated gold nanoparticle mask patterns in an inductively coupled plasma (ICP) etching process successfully yielded silicon nanopillar structures. However, it was found that the resultant nanopillars (8–13 nm wide at the tip, 15–20 nm wide at half-height, 20–30 nm wide at the base, and 60–90 nm tall) appeared to lack any significant degree of translational ordering. The results suggest that further studies are needed in order to elucidate the optimal plasma processing parameters that will lead to the generation of long-range ordered arrays of silicon-based nanostructures using S-layer protein templates.     

Enhancement and stabilization of the photoluminescence from porous silicon prepared by Ag‐assisted electrochemical etching

In this paper, we present the results of studies on the photoluminescence (PL) of porous silicon (PSi) samples obtained by etching with the assistance of silver metal in different ways. If the Si sample, after being coated with a layer of silver nanoparticles, is electrochemically etched, its PL intensity becomes hundreds of times stronger than the PL intensity when it is chemically etched in the similar conditions. The difference in the PL intensities is explained partly by the anodic oxidation of silicon which occurs during the electrochemical etching process. The most obvious evidence that silicon had been oxidized anodically in the electrochemical etching process is the disappearance of the PSi layer and the appearance of the silicon oxide layer with mosaic structure when the anodization current density is large enough. The anodic oxidation has the effect of PSi surface passivation. Because of that, the PL of obtained PSi becomes stronger and more stable with time. Copyright © 2012 John Wiley & Sons, Ltd.     

Preparation of non-contact ordered array of polystyrene colloidal particles by using a metallic thin film of fused hemispheres

A thin metallic bilayer consists of fused hollow hemispheres of 930 nm in diameter is fabricated by sputter deposition of Ti and Fe at 50 and 5 nm of thickness, respectively, onto an array of spherical polystyrene colloidal particles. The fused metal hemispheres are utilized to assemble polystyrene colloidal particles of smaller diameters (800 nm) into non-contact two-dimensional periodic array by trapping them in the metallic wells.     

Fabrication of Cs2.5H0.5PW12O40 three-dimensional ordered film by colloidal crystal template

A three-dimensional ordered polyoxometalate periodic film was synthesized using dodecatungstophosphoric acid (H₃PW₁₂O₄₀) and Cs₂CO₃ as precursors and colloidal crystals as templates by an inverse opal method. The samples were characterized by elemental analysis, XRD, IR spectra, UV-Vis diffuse reflectance spectra (DR-UV-Vis) and SEM techniques. This arrayed film constructed by pure cesium salt of dodecatungstophosphoric acid Cs_2.5H_0.5PW₁₂O₄₀ nanoparticles shows well-defined lamellar array with inverse opal structure, which exhibits a well-defined photonic band gap.     

Study of fibrinogen adsorption on hydroxyapatite and TiO2 surfaces by electrochemical piezoelectric quartz crystal impedance and FTIR-ATR spectroscopy

The electrochemical piezoelectric quartz crystal impedance (EQCI), a combined technique of piezoelectric quartz crystal impedance (PQCI), electrochemical impedance (EI), and Fourier transform infrared spectroscopy-attenuated total internal reflectance spectroscopy (FTIR-ATR) were used to in situ study the adsorption process of fibrinogen onto the surface of biomaterials—TiO₂ and hydroxyapatite (Ca₅(PO₄)₃OH, HAP). The equivalent circuit parameters, the resonance frequencies and the half peak width of the conductance spectrum of the two biomaterial-modified piezoelectric quartz crystal (PQC) resonances as well as the FTIR-ATR spectra of fibrinogen during fibrinogen adsorption on TiO₂ and HAP particles modified electrode surface were obtained. The adsorption kinetics and mechanism of fibrinogen were investigated and discussed as well. The results suggested that two consecutive steps occurred during the adsorption of fibrinogen onto TiO₂ and hydroxyapatite (HAP) surface. The fibrinogen molecules were firstly adsorbed onto the surface, and then the rearrangement of adsorbed fibrinogen or multi-layered adsorption occurred. The FTIR-ATR spectroscopy investigations showed that the secondary structure of fibrinogen molecules was altered during the adsorption and the adsorption kinetics of fibrinogen related with the variety of biomaterials. These experimental results suggest a way for enriching biological analytical science and developing new applications of analytical techniques, such as PQCI, EI, and FTIR-ATR.     

Rigidity of colloidal alloys as studied by reflection spectroscopy in sedimentation equilibrium

Rigidities of colloidal alloys of binary mixtures of colloidal silica spheres (CS82; 103 nm in diameter) with larger silica spheres (CS91; 110 nm, CS121; 136 nm and CS161; 184 nm) have been measured by reflection spectroscopy in sedimentation equilibrium. Substitutional-solid-solution-type alloy structures are formed for mixtures of CS82 and CS91 and for CS82 and CS121. A superlattice, probably MgCu₂ type, is formed for CS82 and CS161 mixtures. The rigidities of the colloidal crystals of the single component of the spheres increase as the sphere size increases at the same number density of spheres. The rigidities of the colloidal alloys decrease when a comparatively small number of the larger spheres are mixed with the small spheres at the same total sphere number density. Received: 14 June 2000 Accepted: 3 November 2000     

Physical adsorption of protamine for heparin assay using a quartz crystal microbalance and electrochemical impedance spectroscopy

This study attempted to determine absolute heparin concentration in phosphate buffer solution (PBS, pH 7.4) by using quartz crystal microbalance (QCM) as an affinity biosensor. Electrochemical impedance spectroscopy (EIS) was also used to investigate immobilization of protamine and heparin assay. In addition, the effectiveness of physical adsorption in immobilizing protamine was confirmed by examining the preparation conditions, including the incubation time and protamine concentration. It induced maximum decrease (ca. −100 Hz) in oscillating frequency of QCM by applying 20 mg/ml protamine and 20 min for incubation in PBS. Heparin adsorption onto protamine-modified electrode in PBS revealed an exponential-like binding curve and long duration for reaching the steady state in frequency response of QCM. Moreover, two linear calibration curves were obtained judging from the initial slope (df/dt) and the frequency change (Δf) of QCM obtained after a binding interval (600 s) for heparin concentrations from 0 to 3.0 and 7.0 U/ml, respectively. In EIS analysis, calibration curves with linear concentration range of 0-3.0 U/ml were obtained for heparin in PBS when ferrocyanide was used as an electroactive marker.     

Highly conformal growth of microstructured polypyrrole films by electrosynthesis on micromachined silicon substrates

A novel technology for fabricating microstructured polypyrrole (PPy) films is presented based on PPy electrosynthesis on micromachined silicon substrates. PPy light-activated electropolymerization is performed on n-type microstructured silicon featuring lattices of square-like pores with pitch of 8 μm, size (s) of 5 μm, and depth (d) from 5 μm up to 50 μm. Scanning electron microscopy (SEM) highlights as light-activation allows a highly conformal polymer growth yielding a three-dimensional PPy structure perfectly replicating the silicon microstructure to be achieved up to high aspect-ratio (HR = d/s). Arrays of highly ordered PPy hollow microtubes with depth up to 50 μm and thickness up to 1 μm are obtained. Chemical analysis of microstructured PPy films is performed by X-ray photoelectron spectroscopy (XPS) and their electrochemical activity is verified by cyclic voltammetry (CV).     

Surface bonding on silicon surfaces as probed by tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been used to obtain the Raman signal of surface species on silicon single crystal surfaces without the necessity for surface enhancement by addition of Ag nanoparticles. By illuminating the hydrogen terminated silicon surface covered with a droplet of 4-vinylpyridine with UV light, a 4-ethylpyridine modified silicon surface can be easily obtained. By bringing a scanning tunneling microscope (STM) Au tip with a nanoscale tip apex to a distance of ca. 1 nm from the m...     

Photon trapping by the internal Bragg reflection of colloidal crystals

Fluorescence light emitted from photoexcited rhodamine 6G (R6G) doped in colloidal crystals of exhaustively deionized colloidal silica suspension is partially trapped within a crystal cage. This photon trapping is caused by Bragg reflection in crystal lattices. The photon trapping efficiencies were quantitatively examined as a function of the thickness of measurement cell. The efficiency increased from about 40 to 60% as the cell thickness increased from 1 to 10 mm for an R6G concentration of 5×10⁻⁶ mol/L. This result is attributed to an increase in the number of crystal layers perpendicular to the observation direction; these are formed in the cell with a large optical path length. On the other hand, the trapping efficiencies were constant irrespective of the angle between the incident and observed light of the cylindrical cells. The constant efficiencies are attributed to the fact that the heterogeneous crystal layers around the inner cell wall have the same thickness.     

Parasitic ion-implantation produced by a Kaufman-type ion source used for planar etching of surfaces

Total reflection X-ray fluorescence spectrometry (TXRF) and photoelectron spectroscopy (XPS) were used to characterize ion-etched surfaces of Si wafers, a quartz glass and vacuum-metallized multilayer on a Si wafer. These samples were first treated with low kinetic energy ions of a broad beam Kaufman-type ion source. By repeating this method of sputtering, layers can be removed stepwise and in combination with surface analysis by TXRF a multielement depth profiling can be carried out. The depth resolution is of the order of 3 nm. Unfortunately, ion-etching leads to uncontrollable ion implantation and thus to a contamination of the samples with some parasitic elements which may disturb the multielement characterization of the original surfaces. XPS with Ar sputtering was used to confirm the implantation of interfering elements in the freshly ion-etched samples.Dedicated to the memory of Wilhelm Fresenius