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[en] Highly regulated γ-Fe2O3/(SiO2)x–SO3H/polypyrrole (FSxHP) composite core/shell/shell microspheres are successfully prepared through a SiO2 sol–gel process and an in situ polymerization of pyrrole monomers. γ-Fe2O3 functions as the “core,” whereas SiO2 and polypyrrole (PPy) act as the “shells” of the composite. In contrast to many previously reported FS2H-P and PPy-based composites, the FSxHP composites display substantially improved microwave absorption properties, which can reach nearly − 43.1 dB (15.1 GHz) with a broader effective absorption bandwidth (RL < − 10 dB) of 6.1 GHz. The excellent microwave-absorbing properties of the FSxHP composites are closely associated with the layer of SiO2 shells and can be attributed to the proper impedance matching and synergistic interaction of the special core/shell/shell structures, dielectric loss (SiO2 and PPy layers) and magnetic loss (γ-Fe2O3). These results show that the core/shell/shell structure may be an efficient and promising way to improve microwave absorption properties.
[en] To obtain outstanding electromagnetic microwave absorption (EMWA) properties, the rambutan-like dielectric–magnetic C@NiCo2O4 material was successfully prepared by a simple hydrothermal method, followed by a carbonization process. Benefiting from the unique rambutan-like structure, the dielectric–magnetic C@NiCo2O4 composites showed excellent microwave attenuation ability: minimum reflection loss (RLmin) value of − 39.0 dB at 17.4 GHz and wide effective absorption bandwidth (EAB, reflection loss exceeding − 10 dB) of 4.16 GHz (> 13.84 GHz) with a matching thickness of only 1.5 mm, which were much better than those of pure C and NiCo2O4. The superior properties might be due to multiple synergistic effects: magnetic loss (NiCo2O4), dielectric loss (C, NiCo2O4), the multi-reflections, scattering and interface relaxation resulting from mesoporous rambutan-like structures, and the dipole polarization to get good electromagnetic matching and high attenuation efficiency.
[en] Uniform γ-Fe2O3/microporous SiO2/polypyrrole (Fe/m-SiO2/PPy) microspheres (MSs) with “watermelon-like” structures were successfully fabricated using cetyltrimethylammonium bromide (CTAB) as a pore-directing agent. In the “watermelon-like” microspheres, γ-Fe2O3 nanoparticles represented the seeds, m-SiO2 the pulp, and PPy the rind. Through synergistic harnessing of the magnetic loss properties of γ-Fe2O3 and the dielectric loss properties of the m-SiO2/PPy core/shell structure, the Fe/m-SiO2/PPy MSs displayed outstanding excellent electromagnetic wave absorption (EMWA) properties. The maximum reflection loss (RLmax) of Fe/m-SiO2/PPy was − 51.24 dB (7.44 GHz) with a thickness of 4.0 mm, with an effective absorption bandwidth (EAB) (RL < − 10 dB) of 4.16 GHz at a loading of 14.2 wt% in a paraffin wax matrix. These results conclusively demonstrate that Fe/m-SiO2/PPy MSs-containing composites are very efficient EMWA materials and that porous core/shell/shell structures offer a promising approach for the rational design of lightweight high-performance EMW absorbers.
[en] Highlights: • Porous BDD/Ta sensor constructed using a Ni-assisted plasma etching method. • Sensor was applied to detection of dopamine and pyridoxine in human serum. • Electrochemical sensor has high sensitivity, selectivity, and long-term stability. • Selectivity of sensor is dependent on plane activity and surface chemistry. - Abstract: A porous boron-doped diamond (PBDD)/Ta sensing electrode was prepared for the fast, sensitive, stable, and discriminative detection of dopamine (DA) and pyridoxine (vitamin B6) in human serum. All the exposed surfaces of the diamond grains of the BDD layer were etched into a porous form, with pore sizes of less than 500 nm and an average depth of about 200 nm. The electrochemical performance characteristics of the PBDD layer and the reaction mechanisms enabling the detection of DA and vitamin B6 were studied. Large numbers of oxygen-containing groups on the PBDD surface, as well as the activity difference of the different planes, enabled us to successfully distinguish between DA and B6 by using the PBDD/Ta electrode. The low background current of PBDD, the large active area of the porous surface, and the high electron transfer properties led to the PBDD electrode having a high sensitivity. Therefore, this sensor can be used to stably detect DA and B6 in serum. Aberrant levels of DA and vitamin B6 in body fluids are key risk indicators for some diseases; thus, monitoring the levels of both and other species in serum is of great significance to clinical diagnoses.
[en] The authors describe a voltammetric sensor for simultaneous determination of dopamine (DA), uric acid (UA), L-tyrosine (Tyr), and the diuretic drug hydrochlorothiazide (HCTZ). The assay is based on the use of graphene nanowalls deposited on a tantalum substrate. The nanowalls are characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electrochemical impedance spectroscopy, and cyclic voltammetry. The nanowalls are vertically grown on the substrate by direct-current arc plasma jet chemical vapor deposition. The modified electrode is shown to enable simultaneous differential pulse voltammetric determination of DA, UA, Tyr, and HCTZ. The graphene nanowalls display a large specific surface, high conductivity, and a large number of catalytically active sites for oxidation of analytes. Simultaneous detection is performed best at a pH value of 7.0 and at peak potentials of 0.124 V (vs. SCE) for DA, 0.256 V for UA, 0.536 V for Tyr and 0.708 V for HCTZ. The respective detection limits are 0.04 μM, 0.1 μM, 0.6 μM and 0.4 μM. The results show that this graphene wall modified electrode is a promising tool for the design of sensitive, selective, and stable sensors. < Image>.
[en] Highlights: • MgO nanocrystals were prepared using DC arc plasma jet CVD method. • The growth time does not exceed 10 min in process of the synthesis. • The samples were found to consist of cubic MgO nanobelts and nanosheets. • Nanocrystals contain contacts, rough edges, vacancies, and doping defects. • The samples exhibited excellent electrochemical biosensing properties. - Abstract: MgO nanocrystals were prepared using a simple direct current arc plasma jet chemical vapor deposition method. Magnesium nitrate was used as source material and Mo film was used as a substrate and catalyst. The high-temperature plasma produced ensured rapid synthesis of the MgO nanocrystals. The as-prepared nanocrystals were characterized by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, energy-dispersive spectroscopy, Fourier transform infrared spectrometry, ultraviolet–visible spectrophotometry, and photoluminescence measurements. The as-synthesized samples were found to consist of cubic MgO nanobelts and nanosheets with large surface areas and low coordination oxide ions, and contained numerous contacts, rough edges, vacancies, and doping defects. The nanostructures exhibited excellent electrochemical sensing properties with high-sensing sensitivity toward ascorbic acid. Their high electrocatalytic activity was attributed to the effect of defects and the surface electron transfer ability of the one-dimensional MgO nanobelts
[en] Ti interlayers with different thicknesses were sputtered on Si substrates and then ultrasonically seeded in a diamond powder suspension. Nanocrystalline diamond (NCD) films were deposited using a dc arc plasma jet chemical vapor deposition system on the seeded Ti/Si substrates. Atomic force microscopy and scanning electron microscopy tests showed that the roughness of the prepared Ti interlayer increased with increasing thickness. The effects of Ti interlayers with various thicknesses on the properties of NCD films were investigated. The results show nucleation, growth, and microstructure of the NCD films are strongly influenced by the Ti interlayers. The addition of a Ti interlayer between the Si substrate and the NCD films can significantly enhance the nucleation rate and reduce the surface roughness of the NCD. The NCD film on a 120 nm Ti interlayer possesses the fastest nucleation rate and the smoothest surface. Raman spectra of the NCD films show trans-polyacetylene relevant peaks reduce with increasing Ti interlayer thickness, which can owe to the improvement of crystalline at grain boundaries. Furthermore, nanoindentation measurement results show that the NCD film on a 120 nm Ti interlayer displays a higher hardness and elastic modulus. High resolution transmission electron microscopy images of a cross-section show that C atoms diffuse into the Ti layer and Si substrate and form TiC and SiC hard phases, which can explain the enhancement of mechanical properties of NCD.
[en] Vertically stacked graphene nanosheet/titanium carbide nanorod array/titanium (graphene/TiC nanorod array) wires were fabricated using a direct current arc plasma jet chemical vapor deposition (DC arc plasma jet CVD) method. The graphene/TiC nanorod arrays were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction spectroscopy. The TiO2 nanotube array was reduced to the TiC nanorod array, and using those TiC nanorods as nucleation sites, the vertical graphene layer was formed on the TiC nanorod surface. The multi-target response mechanisms of the graphene/TiC nanorod array were investigated for ascorbic acid (AA), dopamine (DA), uric acid (UA), and hydrochlorothiazide (HCTZ). The vertically stacked graphene sheets facilitated the electron transfer and reactant transport with a unique porous surface, high surface area, and high electron transport network of CVD graphene sheets. The TiC nanorod array facilitated the electron transfer and firmly held the graphene layer. Thus, the graphene/TiC nanorod arrays could simultaneously respond to trace biomarkers and antihypertensive drugs. - Highlights: • Vertical graphene sheets were prepared with Ti as the catalyst via a CVD method. • TiO2 nanotubes were key transition layers in the formation of the TiC nanorods. • Vertical growth mechanism of graphene products was discussed. • Biomolecules were detected to be a chemical sensor. • Response mechanism for analytes at the graphene/TiC nanorod array was discussed.
[en] In this paper, we describe a method for fabricating dry electrodes for use in recording electroencephalograms (EEGs), which are based on the use of chitosan (Ch), gold (Au) particles, and titanium dioxide (TiO2) nanotube arrays deposited on titanium (Ti) thin sheets. The samples were characterized by scanning electron microscopy, X-ray diffraction, electrochemical impedance spectroscopy, and EEG signal collection. The TiO2 nanotube arrays were grown on the Ti thin sheet by an electrochemical anodic oxidation method. The Au particles were deposited on the bottom and surface layers of the TiO2 nanotube array using an electrochemistry-based multi-potential step technology. The fabricated dry Ch/Au-TiO2 electrodes have an efficient conversion interface for ion current/electron current, a high biocompatible contact surface, and a fast electron transfer channel. To confirm that the Ch/Au-TiO2 layer can be used in dry EEG electrodes, the impedance spectra of the electrodes in solution and skin were analyzed. The mean impedance values for skin were found to be approximately 169 ± 33.0 kΩ at 2.15 Hz and 67.4 ± 8.9 kΩ at 100 Hz. In addition, EEG signals from the forehead and sites with hair were collected using both the dry Ch/Au-TiO2 electrode and a wet Ag/AgCl electrode for comparison purposes. It was found that high quality EEG signal recordings could be obtained using the dry electrodes. The fact that electrolytes are not required means that the electrodes are suitable for use in long-term bio-potential testing. - Graphical abstract: We prepared dry electrodes based on chitosan, Au micro/nano-particles, and TiO2-nanotube-coated Ti films for use in the accurate monitoring of EEG bio-potentials on the forehead and sites with hair. - Highlights: • Chitosan (Ch)/Au-TiO2/Ti thin films have been successfully prepared. • The Ch/Au-TiO2/Ti thin films were used as dry electrodes and applied to EEG monitoring. • The EEG signal quality from the dry electrode is similar to that from wet electrode. • The Ch and Au-TiO2 nanotubes improved the interfacial stability and electron flow. • The dry electrode has applications in the recording of various bio-potentials.
[en] Graphical abstract: Display Omitted -- Highlights: •Graphene and MgO nanobelts are deposited on tantalum wires to form biosensors. •Ascorbic acid, dopamine and uric acid are determined with the biosensors. •The biosensors show high electrocatalytic activity for oxidation of these species. •The biosensors show high selectivity and good sensitivity. -- ABSTRACT: A promising electrochemical biosensor for simultaneous detection of ascorbic acid (AA), dopamine (DA) and uric acid (UA) was fabricated by electrochemical deposition of MgO nanobelts on a graphene-modified tantalum wire (denoted as MgO/Gr/Ta) electrode. The MgO nanobelts and graphene were verified by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Electrochemical performances of the electrodes were characterized by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The CV results show that AA, DA and UA could be detected simultaneously using MgO/Gr/Ta electrode with peak-to-peak separation of 300 mV, 147 mV and 447 mV for AA-DA, DA-UA and AA-UA, respectively. In the threefold co-existence system, the linear calibration plots for AA, DA and UA were obtained over the concentration range of 5.0–350 μM, 0.1–7 μM and 1–70 μM with detection limits of 0.03 μM, 0.15 μM and 0.12 μM, respectively. The modified electrode shows excellent selectivity, good sensitivity and good stability, making it attractive as a sensor for simultaneous detection of AA, DA and UA in biological fluids