Development of novel carbon thin film electrodes for electrochemical analysis of trace heavy metals in aqueous solutions

Contamination and mismanagement of water resources have released toxic metals such as mercury (Hg), lead (Pb), cadmium (Cd) and copper (Cu), etc. into the environment. The presence of these toxic metals in aquatic ecosystems affects directly or indirectly biota and human being. Hence, fast detection and determination of trace toxic heavy metals in aqueous solutions are necessary to reduce fatal cases due to misconsumption of polluted water. Anodic stripping voltammetry (ASV) has been widely used for detection of heavy metals in solutions due to its remarkably low detection limit (ng/L), capability of simultaneous determination of multi-elements, low operating power and relatively low cost. The stripping step of ASV can be pulse, squarewave, linear or staircase. Square-wave anodic stripping voltammetry (SWASV) has been recognized as a powerful technique for detection of trace heavy metals in various aqueous solutions, because of its unique accumulation/preconcentration of analyte species contained in the solutions. In the past, glassy carbon electrode (GCE) has been widely used in electroanalytical applications because of its robust and smooth surface nature, as well as a large potential window. However, its electroanalytical performance frequently suffers from gradual loss of surface activity. In order to improve reproducibility, stability and sensitivity, a bismuth (Bi) thin film was coated on a GC substrate whose surface was modified with a porous thin layer of polyaniline (PANI) via multipulse potentiostatic electropolymerization to form a novel type of Bi/PANI/GCE in this study. The new electrodes were successfully used to simultaneously detect Cd2+ and Pb2+ ions with reference to SWASV signals. The experimental results depicted that the environmentally-friendly Bi/PANI/GCEs had the ability to rapidly monitor trace heavy metals even in the presence of surface-active species in the solutions. The electroanalytical performance of GCEs coated with PANI-multiwalled carbon nanotube (MWCNT) nanocomposite coatings (PANI-MWCNT/GCE) was investigated by detecting the Pb2+ ions in a 0.1 M acetate buffer solution using SWASV. It was found that the PANI-MWCNT/GCEs had a better performance than the bare GCEs. Different solvents were attempted for better dispersion of MWCNTs in the PANI matrices for more sensitive stripping signals. The surface morphology and structure of the PANI-MWCNT/GCEs were examined using field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy, showing that the conductive PANI matrices worked as both a conductor to electrically connect the individual MWCNTs, and a binder to mechanically join the MWCNTs. Recently, graphene-based electrochemical sensors have also been developed to trace toxic heavy metals in aqueous solutions. Graphene possesses various unique properties with its atomic carbon layers of nanometer thicknesses, high electrical conductivity, fast transfer of electrons and alleviation of the fouling effect of surfactants. Graphene-based electrochemical sensors can be modified with nafion to improve their sensitivity in tracing heavy metals, thus greatly enhancing stripping current signals. There are several viable deposition techniques for fabrication of doped-graphene based electrode materials, such as chemical vapour deposition (CVD), physical vapour deposition (PVD) and spin coating, which are usually followed by high temperature treatment. In this work, few-layer graphene ultrathin films were synthesized via a novel solid-state carbon diffusion method by rapid thermal processing (RTP) of nickel/amorphous carbon (Ni/a-C) bilayers or Ni-C mixed layers, which were all sputtering-coated on silicon (Si) substrates with or without a silicon dioxide (SiO2) layer. For the Ni/a-C bilayer coated samples, the samples were heated at 1000 °C for 3 min to allow the C atoms from the a-C layers to diffuse into the top Ni layers to form C rich surface layers. Upon rapid cooling, the saturated C atoms in the C rich surfaces of the Ni layers precipitated and formed the ultrathin graphene films on the top of the remaining Ni/a-C layers. The formation of the ultrathin graphene films was confirmed by Raman spectroscopy, HR-TEM, electron diffraction, FE-SEM, X-ray photoelectron spectroscopy (XPS), and electrical impedance measurement by a 4-point probe. The formation mechanism of the graphene films was investigated with respect to Ni/a-C bilayer thickness and substrate surface condition (with or without a SiO2 layer). It was found that SiO2 nanowires arose on the thermally treated Ni/a-C bilayer coated Si substrates without a SiO2 layer, which may be due to the reactions between the thermally diffused Si atoms from the Si substrates and the residual oxygen in the RTP chamber, with the Ni layers as a catalyst. The key factors that prevent the formation of the SiO2 nanowires were discussed. The synthesized ultrathin graphene films were used as the working electrodes for simultaneous detection of trace Pb2+ and Cd2+ ions (as low as 7 nM) in acetate buffer solutions (pH 5.3) using SWASV. The effects of substrate surface condition, Ni layer thickness, and preconcentration potential and time on the structure and electrochemical properties of the graphene electrodes were systematically investigated. Compared to conventional diamond-like carbon (DLC) electrodes, the graphene electrodes developed in this study had better repeatability, higher sensitivity and higher resistance to passivation caused by surface active species in the solutions. The interference between the Cd2+ and Pb2+ stripping peaks was also investigated. With further modifications by using PANI porous layer and/or Bi nanoparticles, the graphene electrodes showed good repeatability, ultrahigh sensitivity (as low as 0.33 nM) and good resistance to passivation during the simultaneous detection of trace Pb2+ and Cd2+ ions. For the Ni-C mixed layer coated samples, the graphene thin films were synthesized using the same thermal processing method. During heating, the C atoms dissolved into the Ni lattices. However, during rapid cooling, the solubility of C atoms in Ni was sharply reduced, leading to the precipitation of excess C atoms and the formation of graphene thin films on the outer surfaces of the Ni-C layers. Raman spectroscopy and XPS were used to characterize the structure and composition of both the as-deposited and the thermally treated Ni-C coated samples with respect to the C content of the Ni-C thin films. The graphene thin film electrodes were used as the working electrodes in the simultaneous detection of trace Pb2+, Cd2+ and Cu2+ ions in acetate buffer solutions modified with bismuth (Bi). The Bi-modified graphene electrodes showed the significantly enhanced electroanalytical performance. The electroanalytical performance of the graphene electrodes was also investigated with respect to the Si substrate surface conditions (with or without a SiO2 layer).DOCTOR OF PHILOSOPHY (MAE

Graphene ultrathin film electrode for detection of lead ions in acetate buffer solution

Few-layer graphene ultrathin films were synthesized via solid-state carbon diffusion from amorphous carbon (a-C) thin layers sputtering coated on Si substrates with or without a SiO2 layer, which an a-C layer was covered by a nickel (Ni) layer as a catalyst. When the Ni/a-C bilayer coated samples were heated at 1000 °C the carbon (C) atoms from the a-C layers diffused into the top Ni layers to form a C rich surface. Upon rapid cooling, the C atoms accumulated on the surface of the Ni layers and formed graphene ultrathin films through nucleation and growth processes. The formation of graphene ultrathin films was confirmed by Raman spectroscopy, high resolution transmission electron microscopy (HR-TEM), electron diffraction, field-emission scanning electron microscopy (FE-SEM) and 4-point probe. The synthesized graphene ultrathin films were used as working electrodes for detection of trace heavy metal ions (Pb2+, as low as 7 nM) in acetate buffer solutions (pH 5.3) using square wave anodic stripping voltammetry (SWASV). The effects of substrate surface condition and Ni layer thickness on the structure and electrochemical properties of graphene ultrathin film electrodes were investigated in detail. Compared to conventional diamond-like carbon (DLC) electrodes, the graphene electrodes developed in this study had better repeatability, higher sensitivity and higher resistance to passivation caused by surface active species

Design and Modeling of a Test Bench for Dual-Motor Electric Drive Tracked Vehicles Based on a Dynamic Load Emulation Method

Dual-motor Electric Drive Tracked Vehicles (DDTVs) have attracted increasing attention due to their high transmission efficiency and economical fuel consumption. A test bench for the development and validation of new DDTV technologies is necessary and urgent. How to load the vehicle on a DDTV test bench exactly the same as on a real road is a crucial issue when designing the bench. This paper proposes a novel dynamic load emulation method to address this problem. The method adopts dual dynamometers to simulate both the road load and the inertia load that are imposed on the dual independent drive systems. The vehicle’s total inertia equivalent to the drive wheels is calculated with separate consideration of vehicle body, tracks and road wheels to obtain a more accurate inertia load. A speed tracking control strategy with feedforward compensation is implemented to control the dual dynamometers, so as to make the real-time dynamic load emulation possible. Additionally, a MATLAB/Simulink model of the test bench is built based on a dynamics analysis of the platform. Experiments are finally carried out on this test bench under different test conditions. The outcomes show that the proposed load emulation method is effective, and has good robustness and adaptability to complex driving conditions. Besides, the accuracy of the established test bench model is also demonstrated by comparing the results obtained from the simulation model and experiments

Tuning magnetofluidic spreading in microchannels

Magnetofluidic spreading (MFS) is a phenomenon in which a uniform magnetic field is used to induce spreading of a ferrofluid core cladded by diamagnetic fluidic streams in a three-stream channel. Applications of MFS include micromixing, cell sorting and novel microfluidic lab-on-a-chip design. However, the relative importance of the parameters which govern MFS is still unclear, leading to non-optimal control of MFS. Hence, in this work, the effect of various key parameters on MFS was experimentally and numerically studied. Our multi-physics model, which combines magnetic and fluidic analysis, showed excellent agreement between theory and experiment. It was found that spreading was mainly due to cross-sectional convection induced by magnetic forces, and can be enhanced by tuning various parameters. Smaller flow rate ratio, higher magnetic field, higher core stream or lower cladding stream dynamic viscosity, and larger magnetic particle size can increase MFS. These results can be used to tune magnetofluidic spreading in microchannels.ASTAR (Agency for Sci., Tech. and Research, S’pore

Diagenesis and its influence on reservoir quality and oil-water relative permeability: A case study in the Yanchang Formation Chang 8 tight sandstone oil reservoir, Ordos Basin, China

Different from conventional reservoirs, unconventional tight sand oil reservoirs are characterized by low or ultra-low porosity and permeability, small pore-throat size, complex pore structure and strong heterogeneity. For the continuous exploration and enhancement of oil recovery from tight oil, further analysis of the origins of the different reservoir qualities is required. The Upper Triassic Chang 8 sandstone of the Yanchang Formation from the Maling Oilfield is one of the major tight oil bearing reservoirs in the Ordos Basin. Practical exploration demonstrates that this formation is a typical tight sandstone reservoir. Samples taken from the oil layer were divided into 6 diagenetic facies based on porosity, permeability and the diagenesis characteristics identified through thin section and scanning electron microscopy. To compare pore structure and their seepage property, a high pressure mercury intrusion experiments (HPMI), nuclear magnetic resonance (NMR), andwater-oil relative permeability test were performed on the three main facies developed in reservoir. The reservoir quality and seepage property are largely controlled by diagenesis. Intense compaction leads to a dominant loss of porosity in all sandstones, while different degrees of intensity of carbonate cementation and dissolution promote the differentiation of reservoir quality. The complex pore structure formed after diagenesis determines the seepage characteristics, while cementation of chlorite and illite reduce the effective pore radius, limit fluid mobility, and lead to a serious reduction of reservoir permeability

Label-free alignment of nonmagnetic particles in a small uniform magnetic field

Label-free manipulation of biological entities can minimize damage, increase viability and improve efficiency of subsequent analysis. Understanding the mechanism of interaction between magnetic and nonmagnetic particles in an inverse ferrofluid can provide a mechanism of label-free manipulation of such entities in a uniform magnetic field. The magnetic force, induced by relative magnetic susceptibility difference between nonmagnetic particles and surrounding magnetic particles as well as particle–particle interaction were studied. Label-free alignment of nonmagnetic particles can be achieved by higher magnetic field strength (Ba ), smaller particle spacing (R), larger particle size (rp1 ), and higher relative magnetic permeability difference between particle and the surrounding fluid (Rμr ). Rμr can be used to predict the direction of the magnetic force between both magnetic and nonmagnetic particles. A sandwich structure, containing alternate layers of magnetic and nonmagnetic particle chains, was studied. This work can be used for manipulation of nonmagnetic particles in lab-on-a-chip applications

An Experimental Study on Hysteresis Characteristics of a Pneumatic Braking System for a Multi-Axle Heavy Vehicle in Emergency Braking Situations

This study aims to investigate the hysteresis characteristics of a pneumatic braking system for multi-axle heavy vehicles (MHVs). Hysteresis affects emergency braking performance severely. The fact that MHVs have a large size and complex structure leads to more nonlinear coupling property of the pneumatic braking system compared to normal two-axle vehicles. Thus, theoretical analysis and simulation are not enough when studying hysteresis. In this article, the hysteresis of a pneumatic brake system for an eight-axle vehicle in an emergency braking situation is studied based on a novel test bench. A servo drive device is applied to simulate the driver’s braking intensions normally expressed by opening or moving speed of the brake pedal. With a reasonable arrangement of sensors and the NI LabVIEW platform, both the delay time of eight loops and the response time of each subassembly in a single loop are detected in real time. The outcomes of the experiment show that the delay time of each loop gets longer with the increase of pedal opening, and a quadratic relationship exists between them. Based on this, the pressure transient in the system is fitted to a first-order plus time delay model. Besides, the response time of treadle valve and controlling pipeline accounts for more than 80% of the loop’s total delay time, indicating that these two subassemblies are the main contributors to the hysteresis effect

Simulations of self-propelled anguilliform swimming using the immersed boundary method in OpenFOAM

This study extends the existing immersed boundary method (IBM) in the open source toolbox OpenFOAM for solving fluid-structure interactions involving the immersed structure with changeable shapes. To handle a changeable-shape problem, the existing discrete-forcing direct-imposition approach of IBM in OpenFOAM with a pressure-implicit split-operator (PISO) solver is used to solve the fluid domain with interpolated immersed boundary conditions. In the solved fluid domain, the interactions between the fluid domain and the immersed structure are calculated. Making use of these interactions, a user-defined solid solver is applied to compute and update the boundary conditions on the immersed structure. To validate this methodology, models of anguilliform swimming, a typical FSI problem with moving boundaries which has been well studied, is simulated. The accuracy of the present work is examined by comparing the numerical results with the published data. In simulations, anguilliform swimmers are modeled as deformable immersed boundaries. The immersed swimmers propagate in a path determined by the forces from the surrounding fluid acting upon their bodies, as generated by their prescribed changing shapes. The results including the velocity, the thrust, the wake morphology, and the force distribution along the immersed boundaries are presented and discussed. The good agreement of the computational results with reported works validates the extension of IBM in OpenFOAM

In vitro corrosion behaviors of Mg67Zn28Ca5 alloy : from amorphous to crystalline

Mg-based metallic glasses show attractive properties making it as potential materials for implants in biomedical applications, especially compared to traditional crystalline Mg alloys. In this study, the corrosion behavior of melt-spun glassy Mg67Zn28Ca5 ribbons before and after heat treatment at different temperatures was systematically investigated in simulated body fluid. Electrochemical tests and the rate of hydrogen evolution indicated that the corrosion behavior strongly depended on the structure of ribbons. The slowest corrosion rate (strongest corrosion resistance) was achieved for the ribbon with a partially crystallized structure (metastable crystalline Mg102.08Zn39.6 and amorphous matrix). Surface morphology analysis revealed that amorphous ribbons were more susceptible to pitting corrosion than the corresponding partially and fully crystallized ribbons. A Zn-rich passivation layer was detected on the surface of ribbons after immersion test, indicating the corrosion was mainly caused by the loss of Mg- and Ca-containing components, resulting in the enrichment of metallic Zn with improved corrosion resistance as the outcome

PDIA3 gene induces visceral hypersensitivity in rats with irritable bowel syndrome through the dendritic cell-mediated activation of T cells

This study investigated the mechanism of protein disulfide-isomerase A3 (PDIA3)-induced visceral hypersensitivity in irritable bowel syndrome (IBS). Rats were treated with saline (control), acetic acid and restraint stress (IBS model), empty vector (RNAi control) and PDIA3-RNAi vector (PDIA3-RNAi). Mesenteric lymph node DCs (MLNDCs) and splenic CD4+/CD8+ T cells were isolated for co-cultivation. Compared with control, MLNDCs co-cultured with CD4+ or CD8+ T cells showed an increased ability to promote T cell proliferation and produced more IL-4 or IL-9 secretion. Compared with the RNAi control, MLNDCs from the PDIA3 knockdown models were less effective in promoting the proliferation of CD4+/CD8+ T cells. It is concluded that PDIA3 plays an important role in the development of IBS through the DC-mediated activation of T cells, resulting in degranulation of MCs and visceral hypersensitivity