Self-Organization of Magnetic Nanoparticles and Inclusion of Hydrogen by Borohydride Reduction

 Different structures of the interglobular space or voids between self-organized nanoparticles lead to differences in the measurable magnetic properties of single-domain particle chains of similar composition, grain size, and amorphous structure of the single globules. The volumes and radii of nanoparticles obtained by application of a magnetic field (3 to 15 nm) are larger than those determined without application of a magnetic field during the borohydride reduction process. Two types of hydrogen containing nanotubes with diameters of up to 2 (small-size containers) and 5 nm (large-size containers) are produced using as a driving force the domain wall formation energy between ferromagnetic nanoparticles with quantum size effected dimensions prepared by this reduction method at room temperature and ambient atmosphere. Nanoscale hydrogen containers can be used instead of MeH nanoparticle electrodes as perfect energy charge transfer media of high efficiency (close to 100%) using Li ion electrolytes. No influence on the electrode temperature and no participation of OH⁻ and H₂O in the main charge/discharge transfer reactions were observed.     

Magnetic field effects on microstructural variation of electrodeposited cobalt films

The electrodeposition process of Co films in a sulfuric acid solution was examined in a magnetic field (0–5 T). The surface morphology of Co films electrodeposited without a magnetic field was drastically modified with the variation of hydrogen gas evolution rate. Crystalline α-Co was formed in the range of pH = 1.5–6.0, while β-Co was not observed. When the magnetic field was superimposed perpendicular to the electric field in the acidic solution (pH = 1.5), the hydrogen evolution rate was promoted by MHD convection, which enhanced the ionic mass transfer (H⁺ and Co²⁺) near the electrode surface. Moreover, crystalline β-Co was formed simultaneously with the appearance of the elongated ridge-shape precipitates under a higher magnetic field (≥3 T). Contribution to special issue “Magnetic field effects in Electrochemistry”     

Sensitive fluorescent probes for determination of hydrogen peroxide and glucose based on enzyme-immobilized magnetite/silica nanoparticles

Sensitive fluorescent probes for the determination of hydrogen peroxide and glucose were developed by immobilizing enzyme horseradish peroxidase (HRP) on Fe₃O₄/SiO₂ magnetic core–shell nanoparticles in the presence of glutaraldehyde. Besides its excellent catalytic activity, the immobilized enzyme could be easily and completely recovered by a magnetic separation, and the recovered HRP-immobilized Fe₃O₄/SiO₂ nanoparticles were able to be used repeatedly as catalysts without deactivation. The HRP-immobilized nanoparticles were able to activate hydrogen peroxide (H₂O₂), which oxidized non-fluorescent 3-(4-hydroxyphenyl)propionic acid to a fluorescent product with an emission maximum at 409 nm. Under optimized conditions, a linear calibration curve was obtained over the H₂O₂ concentrations ranging from 5.0 × 10⁻⁹ to 1.0 × 10⁻⁵ mol L⁻¹, with a detection limit of 2.1 × 10⁻⁹ mol L⁻¹. By simultaneously using glucose oxidase and HRP-immobilized Fe₃O₄/SiO₂ nanoparticles, a sensitive and selective analytical method for the glucose detection was established. The fluorescence intensity of the product responded well linearly to glucose concentration in the range from 5.0 × 10⁻⁸ to 5.0 × 10⁻⁵ mol L⁻¹ with a detection limit of 1.8 × 10⁻⁸ mol L⁻¹. The proposed method was successfully applied for the determination of glucose in human serum sample.     

Influence of Al2O3 Nanoparticles on the Thermal Stability of Ultra-Fine Grained Copper Prepared by High Pressure Torsion

Summary.  Ultra-fine grained (UFG) Cu (grain size 80 nm) containing 0.5 wt.% Al₂O₃ nanoparticles (size 20 nm) was prepared by high pressure torsion (HPT). Positron lifetime spectroscopy was employed to characterize the microstructure of this material, especially with respect to types and concentration of lattice defects. The evolution of microstructure with increasing temperature was studied by positron lifetime spectroscopy and X-ray diffraction measurements. The thermal stability of the Cu + 0.5 wt.% Al₂O₃ nanocomposite was compared with that of pure UFG Cu prepared by the same technique. The processes taking place during thermal recovery of the initial nanoscale structure in both studied materials are described. Received October 5, 2001. Accepted (revised) December 20, 2001     

Cathodoluminescence Depth Profiling in SiO2:Ge Layers

 For investigation of the luminescent center profile cathodoluminescence measurements are used under variation of the primary electron energy E ₀ = 2…30 keV. Applying a constant incident power regime (E ₀·I ₀ = const), the depth profiles of luminescent centers are deduced from the range of the electron energy transfer profiles dE/dx. Thermally grown SiO₂ layers of thickness d = 500 nm have been implanted by Ge⁺-ions of energy 350 keV and doses (0.5–5)10¹⁶ ions/cm². Thus Ge profiles with a concentration maximum of (0.4 – 4) at% at the depth of d_m≅240 nm are expected. Afterwards the layers have been partially annealed up to T _a = 1100 °C for one hour in dry nitrogen. After thermal annealing, not only the typical violet luminescence (λ = 400 nm) of the Ge centers is strongly increased but also the luminescent center profiles are shifted from about 250 nm to 170 nm depth towards the surface. This process should be described by Ge diffusion processes, precipitation and finally Ge nanocluster formation. Additionally, a Ge surface layer is piled-up extending to a depth of roughly 25 nm.     

Electrodeposition of Platinum Nanoparticles on Multi-Walled Carbon Nanotubes for Electrocatalytic Oxidation of Methanol

Platinum (Pt) nanoparticles were deposited at the surface of well-aligned multi-walled carbon nanotubes (MWNTs) by potential cycling between +0.50 and −0.70 V at a scanning rate of 50 mV · s⁻¹ in 5 mM Na₂PtCl₆ solution containing 0.1 M NaCl. The electrocatalytic oxidation of methanol at the nanocomposites of Pt nanoparticles/nanotubes (Pt_nano/MWNTs) has been investigated using 0.2 M H₂SO₄ as supporting electrolyte. The effects of various parameters, such as Pt loading, concentration of methanol, medium temperature as well as the stability of Pt_nano/MWNTs electrode, have been studied. Compared to glassy carbon electrode, carbon nanotube electrode significantly enhances the catalytic efficiency of Pt nanoparticles for methanol oxidation. This improvement in performance is due not only to the high surface area and the fast electron transfer rate of nanotubes but also to the highly dispersed Pt nanoparticles as electrocatalysts at the tips and the sidewalls of nanotubes.     

Determination of rutin using a CeO2 nanoparticle-modified electrode

CeO₂ nanoparticles approximately 12 nm in size were synthesized and subsequently characterized by XRD, TEM and UV-vis spectroscopy. Then, a gold electrode modified with CeO₂ nanoparticles was constructed and characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The modified electrode demonstrated strong catalytic effects with high stability towards electrochemical oxidation of rutin. The anodic peak currents (measured by differential pulse voltammetry) increased linearly with the concentration of rutin in the range of 5.0 × 10⁻⁷–5.0 × 10⁻⁴ mol · L⁻¹. The detection limit (S/N = 3) was 2.0 × 10⁻⁷ mol · L⁻¹. The relative standard deviation (RSD) of 8 successive scans was 3.7% for 5.0 × 10⁻⁶ mol · L⁻¹ rutin. The method showed excellent sensitivity and stability, and the determination of rutin in tablets was satisfactory.     

Observation of nanoparticles and film formation by scanning tunneling microscopy: methyl methacrylate/butyl methacrylate nanoparticles prepared by microemulsion polymerization

 Latexes as dispersions of poly(methyl methacrylate-co-butyl methacrylate) copolymeric nanoparticles within water were produced by microemulsion polymerization of the respective comonomers. Polymer yield, number-average and weight-average molecular weights, polydispersity index, and the glass-transition temperature of the copolymer produced were 50%, 8.8 × 10⁴, 2.54 × 10⁵, 2.87, and 45 °C. Scanning tunneling microscopy (STM) images of the latex nanoparticles and film formation on highly oriented pyrolitic graphite (HOPG) were obtained with a 2 V sample bias and a tunneling current of 20 pA. The STM pictures revealed that the particle size was 18 ± 3 nm. There was no film formation in the case of dehydration at room temperature. There was some coalesence of particles when the HOPG surface was preheated at 55 °C, while complete film formation was achieved when the latexes were annealed at 55 °C in an oven for about 10 min. Received: 23 August 1999 Accepted: 17 January 2000     

Direct Synthesis of Nanocrystalline Hydroxyapatite by Hydrothermal Hydrolysis of Alkylphosphates

Summary.  The influence of reaction conditions (temperature, type of catalyst, time) on the base-catalyzed reaction of mono-, di-, and trialkylphosphates (alkyl = methyl, ethyl, i-propyl, n-butyl) with Ca²⁺ ions and on the structure and composition of the reaction products was studied. The composition of the calcium phosphates depends mainly on the reaction temperature. At temperatures below 100°C, a nanocrystalline solid product transforming into dicalcium phosphate by heating (calcination) was found. Pure nanocrystalline hydroxyapatite was prepared by hydrothermal synthesis at 160°C from mono- and dialkylphosphates. The size of hydroxyapatite crystallites was about 1 nm, the particle size about 150 nm. Agglomerated particles of hydroxyapatite larger than 2 μm were prepared at 200°C. Hydrothermal reaction of trialkylphosphates with Ca²⁺ ions at 200°C produced CaHPO₄. The experimental results were used to propose a reaction mechanism for base-catalyzed hydrothermal reactions of alkylphosphates with Ca²⁺ ions. Received October 4, 2001. Accepted (revised) November 19, 2001     

Electrocatalytic reduction of oxygen on the surface of glassy carbon electrodes modified with plumbagin

The preparation and electrochemical characterization of glassy carbon electrodes modified with plumbagin were investigated by employing cyclic voltammetry, chronoamperometry and rotating disc electrode techniques. The cyclic voltammograms of the electroreduction of oxygen showed an enhanced current peak at approximately −0.289 V in air-saturated phosphate buffer pH = 7 and scan rate 10 mV s⁻¹. The thermodynamic and kinetic parameters of the reduction of oxygen at glassy carbon have been evaluated using cyclic voltammetry. The experimental parameters were optimized and the mechanism of the catalytic process was discussed. The obtained values of E°′ (V vs. Ag/AgCl), the apparent electron transfer rate constant k_s (s⁻¹), heterogeneous rate constant for the reduction of O₂ at the surface of the modified electrode k_h (M⁻¹ s⁻¹) and α (charge transfer coefficient of oxygen) were as follows: −0.146, 23.4, 9.9 × 10³ and 0.57, respectively. In addition, plumbagin exhibited strong catalytic activity toward the reduction of H₂O₂.     

Solid State Analysis with the New Leipzig High-Energy Ion Nanoprobe

 The high-energy ion nanoprobe LIPSION at the University of Leipzig has been operational since October 1998. The ultrastable single ended 3.5 MV SINLETRON^TM accelerator supplies the H⁺ or He⁺ ion beam. A magnetic scanning system moves the focused beam across the sample. At present, a resolution of 150 nm in the low current mode and 300 nm at 5 pA could be achieved. The UHV grade experimental chamber is equipped with electron-, energy dispersive X-ray-, and particle detectors. They can be used simultaneously to analyse the sample by means of PIXE (particle induced X-ray emission), RBS (Rutherford backscattering) and in the case of thin samples STIM (scanning transmission ion microscopy). A goniometer allows the application of channeling measurements in single crystals in combination with these methods. The detection limits depend on the elements to be analysed and range from (1000⋯1) μg/g relative and (1⋯0.01) pg absolute. The analysis is nondestructive, but the sample has to be vacuum resistant. Applications of the nanoprobe in the field of semiconductor research, biomedicine, and archaeology will be described.     

Size characterization and quantification of silver nanoparticles by asymmetric flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry

A method for determining the size of silver nanoparticles and their quantification by asymmetric flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry (ICP-MS) is proposed and was tested in consumer products. Experimental conditions were studied in detail to avoid aggregation processes or alteration of the original size distributions. Additionally, losses from sorption processes onto the channel membrane were minimized for correct quantification of the nanoparticles. Mobile phase composition, injection/focusing, and fractionation conditions were evaluated in terms of their influence on both separation resolution and recovery. The ionic strength, pH, and the presence of ionic and nonionic surfactants had a strong influence on both separation and recovery of the nanoparticles. In general, better results were obtained under those conditions that favored charge repulsions with the membrane. Recovery values of 83 ± 8% and 93 ± 4% with respect to the content of silver nanoparticles were achieved for the consumer products studied. Silver nanoparticle standards were used for size calibration of the channel. The results were compared with those obtained by photon correlation spectroscopy and images taken by transmission electron microscopy. The quantification of silver nanoparticles was performed by direct injection of ionic silver standard solutions into the ICP-MS system, integration of the corresponding peaks, and interpolation of the fractogram area. A limit of detection of 5.6 μg L^-1 silver, which corresponds to a number concentration of 1×10¹² L^-1 for nanoparticles of 10 nm, was achieved for an injection volume of 20 μL.     

Oleic acid-assisted preparation of LiMnPO<Subscript>4</Subscript> and its improved electrochemical performance by Co doping

LiMnPO₄, with a particle size of 50–150 nm, was prepared by oleic acid-assisted solid-state reaction. The materials were characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The electrochemical properties of the materials were investigated by galvanostatic cycling. It was found that the introduction of oleic acid in the precursor led to smaller particle size and more homogeneous size distribution in the final products, resulting in improved electrochemical performance. The electrochemical performance of the sample could be further enhanced by Co doping. The mechanism for the improvement of the electrochemical performance was investigated by Li-ion chemical diffusion coefficient ( [(D)\tilde]_\textLi ) \left( {{{\tilde{D}}_{\text{Li}}}} \right) and electrochemical impedance spectroscopy measurements. The results revealed that the [(D)\tilde]_\textLi {\tilde{D}_{\text{Li}}} values of LiMnPO₄ measured by cyclic voltammetry method increase from 9.2 × 10⁻¹⁸ to 3.0 × 10⁻¹⁷ cm² s⁻¹ after Co doping, while the charge transfer resistance (R _ct) can be decreased by Co doping.     

Kinetic Studies on the Complexation of Chromium(III) with some Amino Acids in Aqueous Acidic Medium

Summary.  The kinetics of the formation of the 1:3 complex of chromium(III) with L-glutamic acid and DL-lysine were studied spectrophotometrically at and 550 nm. The reaction was found to be first order in both reactants. Increasing the hydrogen ion concentration from 3.2×10⁻⁵ to 1.0×10⁻³ molċdm⁻³ retarded the reaction rate which is of the form . Values of 28.8 and 63.6 kJċmol⁻¹ were obtained for the energy of activation and −184 and −116 Jċ K⁻¹ċmol⁻¹ for the entropy of activation for L-glutamic acid and DL-lysine. The logarithms of the formation constants of the two complexes were found to be 5.9 and 5.1. Received January 7, 2000. Accepted (revised) March 8, 2000     

A Highly Sensitive Resonance Rayleigh Scattering Method for the Determination of Vitamin B1 with Gold Nanoparticles Probe

In weakly acidic buffer medium, vitamin B₁ (VB₁) interacts with gold nanoparticles to form a binding product, which resulted in a significant enhancement of resonance Rayleigh scattering (RRS) intensity and the appearance of a new RRS spectrum. The maximum RRS peak was at 368 nm, and there are three smaller scattering peaks that were at 284 nm, 440 nm and 495 nm, respectively. The enhanced RRS intensity (ΔI) was directly proportional to the concentration of VB₁ in the range of 0–2.8 × 10⁻⁷ mol L⁻¹. The method had high sensitivity and its detection limit (3σ) was 0.9 ng mL⁻¹. The optimum conditions and the influencing factors have been investigated. The method had good selectivity, which could be observed from the influence of coexisting substances. A sensitive, simple and fast RRS method for the determination of VB₁ with gold nanoparticle probe has been developed. In addition, the reasons for RRS enhancement were discussed.     

Electrochemical reduction and kinetics of hydrogen peroxide on the rotating disk palladium-plated aluminum electrode modified by Prussian blue film as an improved electrocatalyst

This paper describes the use of an aluminum electrode plated by metallic palladium and modified by Prussian blue (PB/Pd-Al) in the electrocatalytic reduction of hydrogen peroxide (H₂O₂). The effect of pH on the electroreduction of H₂O₂ on the modified electrode is investigated and a simple irreversible reduction pathway is suggested. The electroreduction kinetics including transfer coefficient α, potential-dependent charge transfer rate constants k _f, and diffusion coefficient D are estimated by means of forced hydrodynamic voltammetry using a rotating disk PB/Pd-Al electrode. The mean values obtained for kinetics are 0.38, 10⁻² cm⁻¹, and 7.6 × 10⁻⁶ cm² s⁻¹, respectively. The long-term stability of the modifying layers on the Al substrate was studied.     

Exploring the Limits of Emulsion Polymerization of Styrene for the Synthesis of Polymer Nanoparticles

Summary. Suspensions of polymer nanoparticles in water (latices) with average particle diameters between 20 and 80 nm were synthesized by batch emulsion polymerization of styrene using sodium dodecyl sulphate (SDS) as surfactant and potassium persulphate (KPS) as initiator. The influence of surfactant concentration, initiator concentration, monomer concentration, and reaction temperature on the final average particle diameters and size distributions of the latices were studied. The number of particles generated was proportional to the 0.56 power of the emulsifier concentration and to the 0.37 power of the initiator concentration in the whole concentration range which was observed. Furthermore, the final number of particles was dependant on the reaction temperature to the 2.06 power. With these correlations the average particle number as well as the average particle size could be estimated, and the results were in good agreement (±6%) with the experimental values. A reduction of the monomer/water ratio from 1:5 to 1:20 yielded smaller particle diameters, while leaving the particle number unaffected. The lower particle size limits for monomer ratios of 1:10 and 1:15 were estimated with diameters of about 18 and 16 nm.     

Electrochemical study of brucine on an electrode modified with magnetic carbon-coated nickel nanoparticles

A novel type of glassy carbon electrode modified with magnetic carbon-coated nickel nanoparticles (C-Ni/GCE) was fabricated and the electrochemical properties of brucine were studied using it. The carbon-coated nickel nanoparticles showed excellent electrocatalytic activity for the redox of brucine and an enhanced electron transfer rate. The electrochemical behavior of brucine on the C-Ni/GCE was explored by cyclic voltammetry (CV), and a redox mechanism for brucine was proposed. A series of electrochemical parameters were calculated for brucine by CV and controlled-potential electrolysis. The C-Ni/GCE showed good sensitivity, selectivity and stability, and was applied to determine the concentration of brucine. The differential pulse voltammetry (DPV) response of the C-Ni/GCE showed that the catalytic current was linear with the concentration of brucine in the range of 4.7 × 10⁻⁸ to 2.4 × 10⁻⁴ mol l⁻¹, with a correlation coefficient of 0.998. The detection limit was 1.4 × 10⁻⁸ mol l⁻¹.     

Fabrication of gold nanoparticles/polypyrrole composite-modified electrode for sensitive hydroxylamine sensor design

A highly sensitive hydroxylamine (HA) electrochemical sensor is developed based on electrodeposition of gold nanoparticles with diameter of 8 nm on the pre-synthesized polypyrrole matrix and formed gold nanoparticles/polypyrrole (GNPs/PPy) composite on glassy carbon electrode. The electrochemical behavior and electrocatalytic activity of the composite-modified electrode are investigated. The GNPs/PPy composite exhibits a distinctly higher electrocatalytic activity for the oxidation of HA than GNPs with twofold enhancement of peak current. The enhanced electrocatalytic activity is attributed to the synergic effect of the highly dispersed gold metal particles and PPy matrix. The overall numbers of electrons involved in HA oxidation, the electron transfer coefficient, catalytic rate constant, and diffusion coefficient are investigated by chronoamperometry. The sensor presents two wide linear ranges of 4.5 × 10⁻⁷–1.2 × 10⁻³ M and 1.2 × 10⁻³–19 × 10⁻³ M with the detection limit of 4.5 × 10⁻⁸ M (s/n = 3). In addition, the proposed electrode shows excellent sensitivity, selectivity, reproducibility, and stability properties.