A disposable electrochemical immunofiltration test strip for rapid detection of α-fetoprotein

A disposable electrochemical immunofiltration test strip for the rapid detection of α-fetoprotein (AFP) was developed. The test strip was constructed by assembly of screen-printed carbon electrodes, absorption-water pad, nitrocellulose membrane modified by anti-AFP antibody and glass fiber membrane conjugated with ferrocene monocarboxylic acid (FC) labeling AFP. The analytical system utilizes flow-through immunofiltration and competitive immunoassay techniques in combination with an amperometric sensor. The parameters affecting the immunoassay such as selection of filter membrane, membrane pore-size, and antibody binding capacity were investigated and optimized. The immunofiltration system allows us to specifically and directly detect AFP in serum with a low detection limit of 6 ng/mL. The working range is from 6 to 500 ng/mL with an overall analysis time of 5 min for one sample. This electrochemical immunoassay system enabled us to construct a novel point-of-care testing device for the monitoring of biomarker including AFP.     

Controlled synthesis and gas-sensing properties of hollow sea urchin-like α-Fe2O3 nanostructures and α-Fe2O3 nanocubes

Hollow sea urchin-like α-Fe₂O₃ nanostructures were successfully synthesized by a hydrothermal approach using FeCl₃ and Na₂SO₄ as raw materials, and subsequent annealing in air at 600 °C for 2 h. The hollow sea urchin-like α-Fe₂O₃ nanostructures with the diameters of 2–4.5 μm consist of well-aligned α-Fe₂O₃ nanorods with an average length of about 1 μm growing radially from the centers of the nanostructures, have a hollow interior with a diameter of about 2 μm. α-Fe₂O₃ nanocubes with a diameter of 700–900 nm were directly obtained by a hydrothermal reaction of FeCl₃ at 140 °C for 12 h. The response S_r (S_r = R_a/R_g) of the hollow sea urchin-like α-Fe₂O₃ nanostructures reached 2.4, 7.5, 5.9, 14.0 and 7.5 to 56 ppm ammonia, 32 ppm formaldehyde, 18 ppm triethylamine, 34 ppm acetone, and 42 ppm ethanol, respectively, which was excess twice that of the α-Fe₂O₃ nanocubes and the nanoparticle aggregations. Our results demonstrated that the hollow sea urchin-like α-Fe₂O₃ nanostructures were very promising for gas sensors for the detection of flammable and/or toxic gases with good-sensing characteristics.     

Molecular sieve 4A–TiO2–K2Cr2O7 coexisted system as sensor for chemical oxygen demand

The nanocomposite TiO₂/molecular sieve 4A as photocatalyst was fabricated and employed to develop an effective, rapid, simple and environmental friendly method for chemical oxygen demand (COD) detection. This new approach overcomes the problems with direct use of TiO₂ for COD detection techniques such as photo-decay and difficulties in recycling. Here, the COD value was calculated from the changed absorbance of Cr(VI) and the mechanism of the photocatalytic oxidation was discussed. Under the optimal condition, the COD sensor gave a detection limit of 0.24 mg l⁻¹ and a linear range from 3.0 to 15 mg l⁻¹. With the recoveries from 97 to 103% and without any pretreatments for complicated samples, the developed sensor was successfully applied to the determination of COD in real samples.     

Critical thermodynamic evaluation and optimization of the MnO–SiO2–“ TiO2 ”–“ Ti2O3 ” system

A complete review, critical evaluation, and thermodynamic optimization of phase equilibrium and thermodynamic properties of the MnO–SiO₂–“ TiO₂”–“ Ti₂O₃” systems at 1 bar pressure are presented. The molten oxide phase was described by the Modified Quasichemical Model. The Gibbs energies of the manganosite, spinel, pyrophanite and pseudobrookite and rutile solid solutions were taken from the previous study. A set of optimized model parameters for the molten oxide phase was obtained which reproduces all available reliable thermodynamic and phase equilibrium data within experimental error limits from 25 ^°C to above the liquidus temperatures over the entire range of compositions and oxygen partial pressure in the range of pO₂ from 10⁻²⁰ bar to 10⁻⁷ bar. Complex phase relationships in these systems have been elucidated, and discrepancies among the data have been resolved. The database of model parameters can be used along with software for Gibbs energy minimization in order to calculate any phase diagram section or thermodynamic properties.     

Critical thermodynamic evaluation and optimization of the MnO–“ TiO2 ”–“ Ti2O3 ” system

A complete review, critical evaluation, and thermodynamic optimization of the phase equilibrium and thermodynamic properties of the MnO–“ TiO₂”–“ Ti₂O₃” systems at 1 bar pressure are presented. The molten oxide phase was described by the Modified Quasichemical Model. The Gibbs energy of spinel, pyrophanite and pseudobrookite solid solutions were modeled using the Compound Energy Formalism, and rutile solid solution was treated as a simple Henrian solution. Manganosite solid solution was assumed to dissolve both Ti⁴⁺ and Ti³⁺. A set of optimized model parameters for all phases was obtained which reproduces all available reliable thermodynamic and phase equilibrium data within experimental error limits from 25 ^°C to above the liquidus temperatures over the entire composition ranges and in the range of pO₂ from 10⁻²⁰ to 10⁻⁷ bar. Complex phase relationships in these systems have been elucidated, and discrepancies among the data have been resolved. The database of model parameters can be used along with software for Gibbs energy minimization in order to calculate any phase diagram section or thermodynamic properties.     

Synthesis and enhanced gas sensing properties of crystalline CeO2/TiO2 core/shell nanorods

Crystalline CeO₂/TiO₂ core/shell nanorods were fabricated by a hydrothermal method and a subsequent annealing process under the hydrogen and air atmosphere. The thickness of the outer shell composed of crystal TiO₂ nanoparticles can be tuned in the range of 5-11 nm. The crystal core/shell nanorods exhibited enhanced gas-sensing properties to ethanol vapor in terms of sensor response and selectivity. The calculated sensor response based on the change of the heterojunction barrier formed at the interface between CeO₂ and TiO₂ is agreed with the experimental results, and thus the change of the heterojunction barrier at different gas atmosphere can be used to explain the enhanced ethanol sensing properties.     

Solid-state potentiometric SO2 sensor combining NASICON with V2O5-doped TiO2 electrode

A compact tubular sensor based on NASICON (sodium super ionic conductor) and V₂O₅-doped TiO₂ sensing electrode was designed for the detection of SO₂. In order to reduce the size of the sensor, a thick-film of NASICON was formed on the outer surface of a small Al₂O₃ tube; furthermore, a thin layer of V₂O₅-doped TiO₂ with nanometer size was attached on the NASICON as a sensing electrode. This paper investigated the influence of V₂O₅ doping and sintering temperature on the characteristics of the sensor. The sensor attached with 5 wt% V₂O₅-doped TiO₂ sintered at 600 °C exhibited excellent sensing properties to 1–50 ppm SO₂ in air at 200–400 °C. The EMF value of the sensor was almost proportional to the logarithm of SO₂ concentration and the sensitivity (slope) was −78 mV/decade at 300 °C. It was also seen that the sensor showed a good selectivity to SO₂ against NO, NO₂, CH₄, CO, NH₃ and CO₂. Moreover, the sensor had speedy response kinetics to SO₂ too, the 90% response time to 50 ppm SO₂ was 10 s, and the recovery time was 35 s. On the basis of XPS analysis for the SO₂-adsorbed sensing electrode, a sensing mechanism involving the mixed potential at the sensing electrode was proposed.     

An ultraviolet selective photodetector based on a nanocrystalline TiO2 photoelectrochemical cell

The UV component of solar light is responsible for skin cancer and a number of other skin disorders. An inexpensive and simple ultraviolet (UV) selective photodetector would be convenient to measure the UV exposure. A sealed two-electrode photoelectrochemical cell (PEC) based on a novel double-layer of nanocrystalline TiO₂ has been designed and constructed as a UV-selective-sensor. The properties of the sensor, including spectral response, current-voltage characteristics, sensitivity, linearity and response time are reported. The results demonstrate the potential for the use of this low cost UV-photodetector - which does not require UV selective filters - to provide a warning of harmful solar UV-radiation levels.     

Microstructure control of TiO2 nanotubular films for improved VOC sensing

Porous gas sensing films composed of TiO₂ nanotubes were fabricated for the detection of volatile organic compounds (VOCs), such as alcohol and toluene. In order to control the microstructure of TiO₂ nanotubular films, ball-milling treatments were used to shorten the length of TiO₂ nanotubes and to improve the particle packing density of the films without destroying their tubular morphology and crystal structure. The ball-milling treatment successfully modified the porosity of the gas sensing films by inducing more intimate contacts between nanotubes, as confirmed by scanning electron microscopy (SEM) and mercury porosimetry. The sensor using nanotubes after the ball-milling treatment for 3 h exhibited an improved sensor response and selectivity to toluene (50 ppm) at the operating temperature of 500 °C. However, an extensive ball-milling treatment did not enhance the original sensor response, probably owing to a decrease in the porosity of the film. The results obtained indicated the importance of the microstructure control of sensing layers in terms of particle packing density and porosity for detecting large sized organic gas molecules.     

Structural and properties of nanocrystalline WO3/TiO2-based humidity sensors elements prepared by high energy activation

Nanocrystalline WO₃/TiO₂-based powders have been prepared by the high energy activation method with WO₃ concentration ranging from 1 to 10 mol%. The samples were thermal treated in a microwave oven at 600 °C for 20 min and their structural and micro-structural characteristics were evaluated by X-ray diffraction, Raman spectroscopy, EXAFS measurements at the Ti K-edge, and transmission electron microscopy. Nitrogen adsorption isotherms and H₂ Temperature Programmed Reduction were also carried out for physical characterization. The crystallite and particle mean sizes ranged from 30 to 40 nm and from 100 to 190 nm, respectively. Good sensor response was obtained for samples with at least 5 mol% WO₃ activated for at least 80 min. Ceramics heat-treated in microwave oven for 20 min have shown similar sensor response as those prepared in conventional oven for 120 min, which is highly cost effective. These results indicate that WO₃/TiO₂ ceramics can be used as a humidity sensor element.     

Photoelectrocatalytic regeneration of NADH at poly(4,4′-diaminodiphenyl sulfone)/nano TiO2 composite film modified indium tin oxide electrode

Herein we report the photoelectrocatalytic regeneration of NADH at poly(4,4′-diaminodiphenyl sulfone)/nano TiO₂ (PDDS/TiO₂) composite modified indium tin oxide (ITO) electrode. The PDDS film growth was confirmed through in situ electrochemical quartz crystal microbalance (EQCM) studies. The prepared PDDS/TiO₂ composite was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) studies. SEM and AFM results confirmed that TiO₂ nanoparticles size is between 130 and 180 nm. XRD results showed that TiO₂ nanoparticles are crystalline and belong to anatase phase. Electrochemical impedance spectroscopy (EIS) and light induced EIS results substantiate a rapid electron transfer process at PDDS/TiO₂ composite surface. Cyclic voltammetry (CV) results demonstrated that composite film showed excellent response to the photoelectrocatalytic regeneration of NADH. The photoelectrocatalytic oxidation of NADH at composite film surface irradiated for 5 min (optimized irradiation time) produced a notable enhancement in anodic peak current and it was 18-fold higher than that of PDDS film and several folds higher than that of TiO₂ and bare ITO electrodes. Further, composite film showed higher sensitivity of 124.1 μA μM⁻¹ for NADH. From Square wave voltammetry (SWV) results, sensitivity of the irradiated composite film was obtained as 0.252 μA nM⁻¹ of NADH. The linear concentration range was between 23 and 39 nM NADH respectively. Further, the composite film exhibits good selectivity towards NADH and no significant interference effect was observed even when 200-fold excess of ascorbic acid (AA), dopamine (DA) and uric acid (UA) coexist in the same supporting electrolyte solution.     

Synthesis of Pt nanoparticles functionalized WO3 nanorods and their gas sensing properties

A novel sensor material of Pt nanoparticles (NPs) functionalized WO₃ hybrid nanorods was fabricated via a one-pot method. The obtained Pt NPs decorated WO₃ nanorods (Pt-WO₃) were analyzed by means of X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). A comparative gas sensing study was carried out on both the Pt NPs decorated and undecorated WO₃ nanorods in order to investigate the influence of Pt NPs on the gas sensing performances. Obtained results showed that the Pt-WO₃ sensor exhibited fast response and recovery as well as high sensitivity compared with the undecorated sensor. The improved sensing properties were attributed to the spillover effect of Pt NPs and the electronic metal-support interaction.     

Preparation and characterization of nanocrystalline α-Fe2O3 by a sol-gel process

α-Fe₂O₃ ultra-fine powder with an average particle size of 6–26nm has been prepared by a sol-gel process. Thermal analysis, X-ray diffraction and transmission electron microscope were used to study its formation process and micro-structure. The temperature dependence of the electric conductance of the elements made of nanocrystalline α-Fe₂O₃ shows that the gas-sensing properties are strongly related to its surface. The elements exhibited good sensitivity and selectivity to ethyl alcohol, indicating it is a promising alcohol-sensing material.     

Nanoplates of α-SnWO4 and SnW3O9 prepared via a facile hydrothermal method and their gas-sensing property

Nanoplates of α-SnWO₄ and SnW₃O₉ were selectively synthesized in large scale via a facile hydrothermal reaction method. The final products obtained were dependent on the reaction pH and the molar ratio of W⁶⁺ to Sn²⁺ in the precursors. The as-prepared nanoplates of α-SnWO₄ and SnW₃O₉ were characterized by X-ray powder diffraction (XRD), N₂-sorption BET surface area, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The XPS results showed that Sn exists in divalent form (Sn²⁺) in SnW₃O₉ as well as in α-SnWO₄. The gas-sensing performances of the as-prepared α-SnWO₄ and SnW₃O₉ toward H₂S and H₂ were investigated. The hydrothermal prepared α-SnWO₄ showed higher response toward H₂ than that prepared via a solid-state reaction due to the high specific surface area. The gas-sensing property toward H₂S as well as H₂ over SnW₃O₉ was for the first time reported. As compared to α-SnWO₄, SnW₃O₉ exhibits higher response toward H₂S and its higher response can be well explained by the existence of the multivalent W (W⁶⁺/W⁴⁺) in SnW₃O₉.     

Ag nanoparticles modified TiO2 spherical heterostructures with enhanced gas-sensing performance

Monodispersed TiO₂ spherical colloids with diameters of about 250 nm were prepared by a sol-gel method. Heterostructural Ag-TiO₂ spheres were manipulated by surface engineering, in which the Ag nanoparticles with an average size of 10 nm were uniformly distributed on the surface of the TiO₂ nanospheres by in situ reduction and growth. The gas-sensing properties of the TiO₂ nanospheres and heterostructural Ag-TiO₂ nanospheres to ethanol and acetone were measured at 350 °C. The results indicated that Ag nanoparticles greatly enhanced the response, stability and response characteristic of TiO₂ nanospheres to the tested gases. Response times of Ag-TiO₂ sensor to 30 ppm acetone and 50 ppm ethanol were <5 s.     

Experimental determination of the liquidus in the high basicity region in the Al2O3(30 mass%) - CaO - MgO - SiO2 system

The liquidus in the high basicity region in the Al₂O₃(30 mass%)-CaO-MgO-SiO₂ system were determined experimentally at 1773 and 1873 K using the quench technique followed by EPMA analysis. Based on the experimental data, a phase diagram of the Al₂O₃(30 mass%)-CaO-MgO-SiO₂(<20 mass%) section was constructed for 1773 and 1873 K. The solubilities of 2CaO.SiO₂ and 3CaO.SiO₂ at 1773 K were found to be considerably higher in comparison with the existing phase diagram. Even the solubility of MgO at 1873 K was found to be somewhat higher. In addition, the activities of MgO, CaO and Al₂O₃ at 1773 K were estimated using the phase diagram information.     

Exploration of artificial neural network to predict morphology of TiO2 nanotube

Artificial neural network (ANN) was developed to predict the morphology of TiO₂ nanotube prepared by anodization. The collected experimental data was simplified in an innovative approach and used as training and validation data, and the morphology of TiO₂ nanotube was considered as three parameters including the degree of order, diameter and length. Applying radial basis function neural network to predict TiO₂ nanotube degree of order and back propagation artificial neural network to predict the nanotube diameter and length were emphasized in this paper. Some important problems such as the selection of training data, the structure and parameters of the networks were discussed in detail. It was proved in this paper that ANN technique was effective in the prediction work of TiO₂nanotube fabrication process.     

Effect of surface defects on biosensing properties of TiO2 nanotube arrays

In this paper, highly ordered titania nanotube (TNT) arrays fabricated by anodization were annealed at different temperatures in CO to create different concentrations of surface defects. The samples were characterized by SEM, XRD and XPS. The results showed different concentrations of Ti³⁺ defects were doped in TNT arrays successfully. Furthermore, after co-immobilized with horseradish peroxidase (HRP) and thionine chloride (Th), TNT arrays was employed as a biosensor to detect hydrogen peroxide (H₂O₂) using an amperometric method. Cyclic voltammetry results and UV-Vis absorption spectra presented that with an increase of Ti³⁺ defects concentration, the electron transfer rate and enzyme adsorption amount of TNT arrays were improved largely, which could be ascribed to the creation of hydroxyl groups on TNT surface due to dissociative adsorption of water by Ti³⁺ defects. Annealing in CO at 500 °C appeared to be the most favorable condition to achieve desirable nanotube array structure and surface defects density (0.27%), thus the TNT arrays showed the largest adsorption amount of enzyme (9.16 μg/cm²), faster electron transfer rate (1.34 × 10⁻³ cm/s) and the best response sensitivity (88.5 μA/mM l⁻¹).     

Polyaniline–TiO2 nano-composite-based trimethylamine QCM sensor and its thermal behavior studies

Inorganic/organic composites are very attractive due to synergetic behavior and a wide range of potential use. A polyaniline–TiO₂ nano-composite, obtained by combination of chemical polymerization and a sol–gel method, was deposited on the electrode of quartz crystal to implement a quartz crystal microbalance (QCM) chemical sensor. The morphology of the composite film was studied by scanning electron microscopy (SEM) measurements. The coated quartz crystal and a non-coated quartz crystal were mounted in a sealed chamber, and their frequency difference was monitored. When analyte vapor was injected into the chamber, gas absorption decreased the frequency of the coated quartz crystal and thereby caused an increase of the frequency difference between the two crystals. The frequency difference change response towards trimethylamine was evident and could be recovered by N₂ purgation easily. The calibration curve towards trimethylamine, its long-term stability and selectivity were investigated. The thermal behavior of the sensing characteristics was compared with that of a polyaniline QCM sensor. Fourier transform infrared (FTIR) spectra of polyaniline and polyaniline–TiO₂ nano-composite and QCM data under various conditions were used to study the effect of thermal treatment.     

Nano-composite ZrO2/Au film electrode for voltammetric detection of parathion

This work describes the fabrication of ZrO₂/Au nano-composite films and its application for voltammetric detection of organophosphate pesticides. The nano-composite ZrO₂/Au film was prepared through a combination of sol–gel procedure and electroless plating that can be carried out in a general chemistry lab with no need for special facilities and reagents. The sensing performance of the ZrO₂/Au nano-composite film electrode toward parathion was studied with square wave voltammetry. The nano-ZrO₂ showed a strong affinity toward the phosphate group on parathion molecules, which provides sensitivity and selectivity of the sensing film. A linear relationship was obtained between the peak currents and the concentration of parathion, with a detection limit for standard samples of 3 ng/ml. In addition, interference studies showed that structurally similar compounds without phosphate groups would not interfere with the response toward parathion of the film electrode.