Thin films of polyelectrolyte-encapsulated catalase microcrystals for biosensing

Polyelectrolyte (PE)-encapsulated catalase microcrystals were assembled onto gold electrodes by their sequential deposition with oppositely charged PEs, utilizing electrostatic interactions to form enzyme thin films for biosensing. The PE coating around the microcrystals provided a regular surface charge, thus facilitating the stepwise film growth, and it effectively prevented catalase leakage from the assembled films. The encapsulated catalase was shown to retain both its biological and its electrochemical activity. Direct electron transfer between catalase molecules and the gold electrode was achieved without the aid of any electron mediator. In pH 5.0 phosphate buffer solution, the apparent formal potential (E(o)') of catalase was -0.131 V (vs Ag/AgCl). As a H2O2 biosensor, films consisting of one layer of the encapsulated catalase displayed considerably higher (approximately 5-fold) and more stable electrocatalytic responses to the reduction of H2O2 than did corresponding films made of one layer of nonencapsulated catalase or solubilized catalase. An increase in either the number of "precursor" PE layers between the gold electrodes and the catalase microcrystal layers in the film or the number of PE layers encapsulating the catalase microcrystals was found to decrease the electrocatalytic activity of the electrode. At low precursor PE layer numbers (approximately 2) and PE encapsulating layers (approximately 4), the current response was proportional to the H2O2 concentration in the range 3.0 x 10(-6) to 1.0 x 10(-2) M. The overall electroactivity of the multilayer film increased for the first two layers of encapsulated catalase, after which a plateau was observed. This was attributed to the increasing difficulty of electron transfer and substrate diffusion limitations. The current approach of using immobilized PE-encapsulated enzyme microcrystals for biosensing provides a versatile method to prepare high enzyme content films with high and tailored enzyme activities.     

Electromigration in gold and copper thin film conductors

The electromigration behavior of gold thin film conductors with molybdenum, niobium, hafnium and zirconium adhesion layers and of copper thin film conductors with chromium and niobium adhesion layers was investigated under various fabrication and testing conditions. Niobium increases the electromigration lifetime of gold conductors by more than an order of magnitude, while hafnium and zirconium also improve it but by lesser amounts. Sputter-deposited gold conductors appear to be superior to vapor-deposited gold conductors. Pulse powering also increases the electromigration lifetime. In copper conductors chromium adhesion layers do not affect the electromigration lifetime, while niobium adhesion layers appear to reduce it.     

Optical properties of self assembled oriented island evolution of ultra-thin gold layers

Gold layers with a thickness of only 8 to 21 nm were sputtered on soda-lime-silica glasses. Subsequent annealing at 300 and 400 °C for 1 and 24 h resulted in the formation of separated round gold particles with diameters from 8 to 200 nm. Crystal orientations were described using X-ray diffraction and electron backscatter diffraction. The gold particles are oriented with their (111) planes perpendicular to the surface. Most gold nano particles are single crystalline, some particles are twinned. Thermal annealing of sputtered gold layers resulted in purple samples with a coloration comparable to that of gold ruby glasses. The color can be controlled by the thickness of the sputtered gold layer and the annealing conditions. The simple method of gold film preparation and the annealing temperature dependent properties of the layers make them appropriate for practical applications.     

复合纳米金膜的制备及其光学性质

本文利用化学还原法制备了不同尺寸的金纳米颗粒,并利用离子自组装多层技术在玻璃基底上沉积了基于金纳米颗粒的复合纳米金膜,研究了颗粒尺寸和成膜厚度对复合金膜光学性质的影响。不同比例的柠檬酸钠与氯金酸产生的金纳米颗粒溶液的紫外-可见光谱随着金颗粒直径增大而红移展宽。适量比例的柠檬酸钠与氯金酸能够产生平均直径为14±1.2nm且尺寸分布均匀的金纳米球;其溶液在518nm处有一特征吸收峰。不同大小的金纳米颗粒形成的薄膜的紫外-可见光谱形状不同,局域表面等离子体共振峰的位置随着颗粒直径的减小而向短波方向迁移。薄膜的沉积层数越多,薄膜表面的颗粒分布越均匀,局域表面等离子体峰的峰值变化也将减小。本工作证实了利用离子自组装多层技术能够快速、简易、低成本地在玻璃基底上沉积具有局域表面等离子体共振的复合纳米金膜。     

Optical studies of Au@SnO<Subscript>2</Subscript>/poly-(vinyl) alcohol core-shell nanoparticle thin films

Au@SnO₂ core-shell nanoparticles dispersed in poly-(vinyl) alcohol films were fabricated on glass substrate by employing dip-coating technique. The structure and morphology of the films were investigated by X-ray diffraction technique and field-emission transmission electron microscopy, respectively. Optical properties of the films were studied by UV–visible spectroscopy. Surface plasmon spectrum of the core-shell nanoparticles film remained unaltered with increase in the number of layers. However, gold nanoparticles films have shown peak broadening and shifting with increase in the number of layers. Our investigations showed that the surface plasmon band of the gold nanoparticles could be tailored and preserved by controlled deposition of tin dioxide shell.     

Thin Gold Covered Titanium Transition Edge Sensor for Optical Measurement

A thin-gold-film-covered titanium transition edge sensor is newly developed for highly reliable optical photon detection. The aim of the gold film is to prevent a formation of a surface oxidation layer (typically 2.8?nm) on titanium that causes severe degradation of the titanium superconductivity. Optical properties for the gold-covered titanium TES embedded in an optical cavity are calculated, and we find that the maximum absorptance and absorption bandwidth will be reduced with increasing a thickness of the gold film. However, more than 99% absorptance can be possible for the gold (10?nm in thickness) and titanium (30?nm) if 11 dielectric layers are used in an anti-reflection coating. A depth profile of a chemical state for the fabricated device was analysed by an X-ray photoelectron spectroscopy. The profile shows no evidence of TiO₂ existence in photoelectron spectrum. Superconducting critical temperature covered with the 10?nm gold were in the range of 200?mK to 320?mK depending on the titanium thickness of 18?nm to 26?nm.     

Electrochemical deposition of polypyrrole nanolayers on discontinuous ultrathin gold films

Ultrathin layers of polypyrrole (PPy) were electrochemically grown between microelectrodes on a Si/SiO(2) substrate. Conducting nanolayers of PPy are directly grown onto ultrathin discontinuous gold (Au) film between the microelectrodes, with thicknesses in the range 10-100?nm. The system therefore forms a novel (PPy/Au) nanocomposite conductor. Atomic force microscopy (AFM) imaging and conductivity measurements indicate that at all thicknesses a relatively uniform film is formed but with significant roughness that reflects the roughness of the metallic island layer. In PPy/Au films with thickness ～10?nm, the small barriers around the gold islands dominate the conduction, and as the film thickness increases to 100?nm the intrinsic conductivity of highly doped PPy dominates the charge transport.     

Epitaxial growth of the Au/C/Ag system in ultrahigh vacuum

Carbon layers were evaporated onto silver single crystals (that had been previously exposed to air) at room temperature in an ultrahigh vacuum (UHV), and a gold film was subsequently evaporated onto the specimens without change of temperature. The gold films were examined by transmission electron microscopy to determine whether or not epitaxy had occurred. Three cases were observed: complete epitaxy when the average carbon film thickness was 1 Å or less; completely disordered structures for amorphous films 3 Å thick or more; and a partially oriented structure in the intermediate range. The fact that epitaxy is destroyed before the coalescence stage of the carbon film is reached suggests that the carbon islands block the mobility of the nuclei that are growing epitaxially; there is no need for carbon to cover the silver crystal completely for epitaxy to be destroyed. The experimental results obtained also indicate that not only is direct contact between substrate and evaporated film necessary for epitaxy but that the substrate surface must be “suitably” contaminated.     

Effects of diffusion on interfacial fracture of gold-chromium hybrid microcircuit films

In this study, the effects of diffusion on gold-chromium film durability was determined from interfacial fracture energy measurements on laboratory samples aged to simulate long term service. The samples were prepared by sputter deposition of gold films and chromium adhesive layers on sapphire substrates. Some films were left in the as-deposited condition while others were given an accelerated age to drive the chromium off the interface and into the gold film. Stressed overlayers and nanoindentation were then used to induce interfacial delamination and blister formation from which interfacial fracture energies were determined using mechanics-based models. These tests showed that the fracture energies for interfacial failure of the as-deposited and annealed films occurred near 1.3 J m^–2 even when diffusion had driven all chromium into solution. These results clearly demonstrate that chromium in solution is as effective in promoting adhesion as continuous chromium adhesive layers.     

Effect of angle of incidence during deposition on Ti-Pd-Au conductor film adhesion

The influence of a number of the preparation conditions on the adhesive properties of the Ti-Pd-Au metallization on Ta₂N film resistors was investigated. Test samples consisted of a filament-evaporated conductor film on an unpatterned Ta₂N film on glass, sapphire and Al₂O₃ ceramic substrates. The adhesive properties of the Ti-Pd-Au films were examined as a function of the angle of incidence at which the Ti and Pd atoms were deposited. In addition the adhesive properties were examined after heat treatment in air and after electroplating gold. Transmission electron microscopy and density measurements established that at higher angles of incidence (&#62;60°) Ti and Pd films became increasingly porous and that adhesion at the Ti-Pd interface degraded rapidly upon exposure to air. Both evaporated and plated Au layers appear to offer little protection for films evaporated under these conditions. Good adhesion was found to be independent of the substrate material and of the method used for gold deposition when the Ti-Pd films were evaporated at angles of incidence of 48° or less.     

High-energy radiation generated during electric explosion of a conductor in a longitudinal magnetic field

Our recent experiments [1] with the electric explosion of conductors in vacuum with an applied strong longitudinal magnetic field showed that this process was accompanied by radiation capable of penetrating through metal layers of considerable thickness. The high intensity of this radiation allowed it to be reliably detected outside the vacuum chamber using a photographic film with an amplifying luminescent (ZnS) screen situated in a paper envelope at a distance above 3 m from the region of explosion. In the same series of experiments, it was established that elements of the setup situated in the vicinity of the explosion exhibited residual radioactivity. This radioactivity was manifested, in particular, by flare spots on a photographic film in contact with an amplifying phosphor screen contained in a paper cassette that was placed inside the solenoid gap upon the discharge. No such explosion-induced radioactivity was observed in analogous experiments performed without an applied magnetic field. This paper presents the first results of investigations of the induced radioactivity, in which the activation detector was a gold ring situated at a distance of about 0.2 m from the axis of the exploding conductor.     

Optical thin film filters of colloidal gold and silica nanoparticles prepared by a layer-by-layer self-assembly method

A novel fabrication method for optical thin film filters based on the self-organization of alternating layers of colloidal gold and silica nanoparticles (NP) is reported. The filter is designed to work in the deep-UV to visible spectral range. The spectral absorption peaks are tuned by three parameters: the organic capping ligand of the gold NPs (citrate, chitosan, poly (diallyl-dimethylammonium)-chloride or PDDA); the capping environment (bare, chitosan, or PDDA) of the silica NPs and the thickness of the film. Precise control of the transmission color (less than 1% color distance per layer), is achieved by changing the film thickness. Exploitation of the self-assembly process should lead to the facile production of highly reliable large area thin film optical filters at considerably lower costs.     

Corona Generation and Deposition of Metal Nanoparticles on Conductive Surfaces and Their Effects on the Substrate Surface Texture and Chemistry

A corona discharge ion bombardment technique was used successfully to generate gold particles of submicron diameters. In a negative corona discharge, the glow region contains electrons, negative ions, and positive ions. Positive ions collided with the negative corona tip electrode, causing it to sputter and emit fine particles of the electrode material. These nanoparticles were deposited on grounded metal substrates or thin mica sheets supported by grounded metal substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the size distribution and deposition pattern of the metal nanoparticles. The diameter of these nanoparticles was dependent upon the material of the electrode and ranged from 20 to 450 nm for gold and from 15 to 240 nm for tungsten. The nanoparticles were deposited on aluminum, mica, and carbon steel test panels for different amounts of time. The electrochemical response of the carbon steel panels exposed to aerated salt solution was measured by direct current (DC) polarization technique before and after the gold nanoparticles were deposited. This technique was employed to determine the changes in the surface chemistry because of the presence of gold nanoparticles, and it proved to be a sensible method for detecting the presence of fine layers of nanoparticles on the metallic substrate. The presence of the gold nanoparticles increased the electrochemical potential E_corr from -0.640 V to -0.211 V, compared with the value for a noncorrosive surface, like that of pure gold, which is 0 V.     

A reusable surface-enhanced Raman scattering (SERS) substrate prepared by atomic layer deposition of alumina on a multi-layer gold and silver film

A thermally stable, reusable surface-enhanced Raman scattering (SERS) substrate consisting of a gold/silver bi-layer film with a protective alumina coating is reported. The film is synthesized by thermally evaporating sequential layers of gold and silver followed by coating an ultra-thin alumina layer using atomic layer deposition. The use of gold as the foundational layer improves the thermal stability of the metal bi-layer film while providing the additional ability to tune the SERS response. Deposition of the thin alumina overlayer on the bi-layer film creates a SERS substrate capable of enduring multiple high-temperature exposures to 400 °C with minimal loss of enhancement capabilities. We demonstrate the multi-use capability of the substrate by measuring the SERS spectrum of rhodamine 6G followed by a thermal treatment at 400 °C to remove the analyte. A representative substrate was used to acquire SERS spectra of rhodamine 6G up to five repeat measurements, thus establishing the reusability of this relatively simple, inexpensive, and stable substrate.     

Diffusional processes in thin films of Au/CeO2 on NiCr superalloy

Gold films deposited onto CeO₂ intermediate layers on Inconel substrates by means of thermal decomposition of organometallic solutions lose their coating identity, i.e. reflectivity, as a result of heating at temperatures between 500 and 850 °C in air. The main cause is the out-diffusion of chromium from the Inconel substrate through the CeO₂ intermediate layer to the gold film surface and the subsequent oxidation of the chromium. The predominant diffusion mechanism was found to be through grain boundaries in the gold film, with an activation energy of 1.00±0.05 eV. This relatively low value of the activation energy was attributed to in situ grain growth in the gold films that take place during the diffusion. In this temperature regime it could be assumed that the CeO₂ intermediate layer is a semidiffusion barrier, since no concentration of gold could be observed in the Inconel substrate. However, at temperatures higher than 750 °C, slight decomposition of the CeO₂ was detected.     

Wavelength-scanning surface plasmon resonance imaging

A new surface plasmon resonance (SPR) imaging technique was proposed. After measurements were conducted at varying wavelengths, the wavelength affording the minimum brightness (SPR wavelength) was determined at each pixel of the image. A two-dimensional map of the SPR wavelength could be converted to a thickness profile by use of a nonlinear calibration curve, which was obtained by Fresnel calculation. An array of protein thin layers on a gold film was evaluated in air to present the layers' surface structure in nanometer scale.     

Attenuated total reflection surface-enhanced infrared absorption microspectroscopy for identification of small thin layers on material surfaces

Attenuated total reflection surface-enhanced infrared absorption microspectroscopy (micro-ATR-SEIRA) was developed for the identification of sub-mm size and nm-thick layers on material surfaces by using gold island films deposited on the surface of micro-ATR crystals. A thin layer of triphenyl phosphate (TPP) on a poly(tetrafluoroethylene) (PTFE) membrane filter was used to evaluate the enhancement of the absorption bands. Three types of crystals: diamond, silicon, and germanium, were evaluated. Diamond gave the greatest enhancement with a 12 nm thick gold island film. The enhancement factor was 200 times compared to bare diamond crystal, whereas it was 10 times for germanium crystal. This variation of enhancement factor according to crystal types was presumed to be due to the morphology of the gold films on the crystals. We also obtained an enhanced ATR map over an area of approximately 2 x 6 mm for a thin layer (approximately 1 nm thick) of di-2-ethylhexylphthalate on PTFE using gold-coated hexagonal silicon micro-ATR crystals. This micro-ATR-SEIRA technique has major potential for analyzing small and thin substances on material surfaces.     

Layer-by-layer self-assembled mutilayer films of gold nanoparticles for surface-assisted laser desorption/ionization mass spectrometry

Layer-by-layer (LBL) self-assembled multilayer films of gold nanoparticles (AuNPs) on a silicon wafer were demonstrated to be promising substrates for surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) of peptides and environmental pollutants for the first time. LBL multilayer films, (AuNPs/PAHC)n, consisting of alternating layers of ammonium citrate capped AuNPs and poly(allylamine hydrochloride) (PAHC) were prepared on a silicon surface. Silicon plates with aggregated AuNPs were more suitable than those with dispersed AuNPs for the SALDI-MS of peptides. The number of particle layers had a significant effect on the laser desorption/ionization of angiotensin I; the peak intensity of the peptide (molecular ion amount) increased with an increase in the number of layers of AuNPs. As a result, the (AuNPs/PAHC)5 multilayer films increased the sensitivity of the angiotensin I to subfemtomoles and raised the useful analyte mass range, thus making it possible to detect small proteins (a 12 kDa cytochrome c). The signal enhancement when using (AuNPs/PAHC)5 may be due to (i) the high absorption of the UV laser light at 337 nm by the AuNP layers, (ii) the low thermal conductivity due to the AuNPs being covered with a thin monolayer of PAHC, and (iii) the increase in the surface roughness (approximately 100 nm) with the number of AuNP layers. Thus, laser-induced rapid high heating of AuNPs for effective desorption/ionization of peptides is possible. In addition, it was found that (AuNPs/PAHC)5 could be used to extract environmental pollutants (pyrene and dimethyldistearylammonium chloride) from very dilute aqueous solutions with concentrations less than 10(-10) mg/mL, and the analytes trapped in the LBL film could be identified by introducing the film directly into the SALDI mass spectrometer without needing to elute the analytes out of the film.     

Layer-by-layer assembled charge-trap memory devices with adjustable electronic properties

We describe a versatile approach for preparing flash memory devices composed of polyelectrolyte/gold nanoparticle multilayer films. Anionic gold nanoparticles were used as the charge storage elements, and poly(allylamine)/poly(styrenesulfonate) multilayers deposited onto hafnium oxide (HfO2)-coated silicon substrates formed the insulating layers. The top contact was formed by depositing HfO2 and platinum. In this study, we investigated the effect of increasing the number of polyelectrolyte and gold nanoparticle layers on memory performance, including the size of the memory window (the critical voltage difference between the 'programmed' and 'erased' states of the devices) and programming speed. We observed a maximum memory window of about 1.8 V, with a stored electron density of 4.2 x 1012 cm-2 in the gold nanoparticle layers, when the devices consist of three polyelectrolyte/gold nanoparticle layers. The reported approach offers new opportunities to prepare nanostructured polyelectrolyte/gold nanoparticle-based memory devices with tailored performance.     

Thin plasma-polymerized layers of hexamethyldisiloxane for humidity sensor development

The response of resistive-type sensors based on thin hexamethyldisiloxane layers to relative humidity (RH) was evaluated. Humidity sensitive layers were plasma polymerized at low frequency glow discharge using a capacitively coupled parallel plate reactor. The sensor design comprises the absorbing layer deposited on clean glass substrate with comb-shape aluminum electrodes (interdigitated structure). The change in electrical impedance of the sensing film was monitored as the device was exposed to humidity. The variation of the plasma-polymerization parameters resulted in different humidity sensing properties which could be correlated to the results of Fourier transform infrared spectroscopy (FTIR). The deposited films exhibited a detectable response to RH ranging from 30 to 95% with low hysteresis, good reproducibility and stability in long-term use. Films with a greater thickness showed a significant decrease in the humidity sensing capability. FTIR analysis revealed the presence of SiH bonding groups, which are frequently linked to the film density. The increase in the plasma discharge power induced also a significant decrease in the diffusion process of water vapor inside the sensitive layer bulk.