TiO2-graphene nanocomposite for electrochemical sensing of adenine and guanine

TiO₂-graphene nanocomposite was prepared by hydrolysis of titanium isopropoxide in colloidal suspension of graphene oxide and in situ hydrothermal treatment. It provides an efficient and facile approach to yield nanocomposite with TiO₂ nanoparticles uniformly embedded on graphene substrate. The electrochemical behavior of adenine and guanine at the TiO₂-graphene nanocomposite modified glassy carbon electrode was investigated. The results show that the incorporation of TiO₂ nanoparticles with graphene significantly improved the electrocatalytic activity and voltammetric response towards these species comparing with that at the graphene film. The TiO₂-graphene based electrochemical sensor exhibits wide linear range of 0.5–200 μM with detection limit of 0.10 and 0.15 μM for adenine and guanine detection, respectively. The excellent performance of this electrochemical sensor can be attributed to the high adsorptivity and conductivity of TiO₂-graphene nanocomposite, which provides an efficient microenvironment for electrochemical reaction of these purine bases.     

Synthesis and electrochemical properties of CeO2 nanoparticle modified TiO2 nanotube arrays

In this paper, a cerium dioxide (CeO₂) modified titanium dioxide (TiO₂) nanotube array film was fabricated by electrodeposition of CeO₂ nanoparticles onto an anodized TiO₂ nanotube array. The structural investigation by X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicated that the CeO₂ nanoparticles grew uniformly on the walls of the TiO₂ nanotubes. The composite was composed of cubic-phase CeO₂ crystallites and anatase-phase TiO₂ after annealing at 450 °C. The cyclic voltammetry and chronoamperometric charge/discharge measurement results indicated that the CeO₂ modification obviously increased the charge storage capacity of the TiO₂ nanotubes. The charge transfer process at the surface, that is, the pseudocapacitance, was the dominate mechanism of the charge storage in CeO₂-modified TiO₂ nanotubes. The greater number of surface active sites resulting from uniform application of the CeO₂ nanoparticles to the well-aligned TiO₂ nanotubes contributed to the enhancement of the charge storage density.     

A novel non-enzymatic hydrogen peroxide sensor based on single walled carbon nanotubes–manganese complex modified glassy carbon electrode

A simple procedure was developed to prepare a glassy carbon (GC) electrode modified with single wall carbon nanotubes (SWCNTs) and phenazine derivative of Mn-complex. With immersing the GC/CNTs modified electrode into Mn-complex solution for a short period of time 20–100 s, a stable thin layer of the complex was immobilized onto electrode surface. Modified electrode showed a well defined redox couples at wide pH range (1–12). The surface coverages and heterogeneous electron transfer rate constants (k_s) of immobilized Mn-complex were approximately 1.58 × 10⁻¹⁰ mole cm⁻² and 48.84 s⁻¹. The modified electrode showed excellent electrocatalytic activity toward H₂O₂ reduction. Detection limit, sensitivity, linear concentration range and k_cat for H₂O₂ were, 0.2 μM and 692 nA μM⁻¹ cm⁻², 1 μM to 1.5 mM and 7.96(±0.2) × 10³ M⁻¹ s⁻¹, respectively. Compared to other modified electrodes, this electrode has many advantageous such as remarkable catalytic activity, good reproducibility, simple preparation procedure and long term stability.     

Synthesis and electrochemiluminescence of the CeO2/TiO2 composite

CeO₂/TiO₂ composite with kernel–shell structure was synthesized by a sol–gel process. The characterization results show that the composite is made up of anatase phase TiO₂ and cubic system CeO₂. The electrochemiluminescence (ECL) behavior of the CeO₂/TiO₂ composite was studied by a cyclic voltammetry in the presence of persulfate, and the effect factors on ECL emission were discussed. Based on a series of experiments, it is proposed that the strong dual ECL emission produced by the CeO₂/TiO₂ composite resulted from the benefit ECL effect of interface heterojunction in composite.     

Morphology-Controllable Synthesis of CeO2 on a Pt Electrode

Nanoscale cerium dioxides with shape of nanoparticles, nanorods, and nanotubes were electrochemically synthesized. The morphology of CeO₂ was modulated by changing electrode potential and potential direction. CeO₂ nanorods and CeO₂ nanotubes were synthesized via the potentiostatic and cyclic voltammeteric methods, respectively. The morphology and structure of the obtained CeO₂ were characterized by field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). A possible formation mechanism has been suggested to illuminate the relationship between the preparation condition and the morphology of CeO₂.     

Hydrogen production by oxidative steam reforming of methanol using ceria promoted copper–alumina catalysts

The performance of different Cu/CeO₂/Al₂O₃ catalysts of varying compositions is investigated for the oxidative steam reforming of methanol (OSRM) in order to produce the hydrogen selectively for polymer electrolyte membrane (PEM) fuel cell applications. All the catalysts were prepared by co-precipitation method and characterized for their surface area, pore volume and oxidation–reduction behavior. The effect of various operating parameters studied are as follows: reaction temperature (200–300 °C), contact-time (W/F = 3–15 kg_cat s mol^− 1) and oxygen to methanol (O/M) molar ratio (0–0.5). The steam to methanol (S/M) molar ratio = 1.5 and pressure = 1 atm were kept constant. Among all the catalysts studied, catalyst Cu–Ce–Al:30–20–50 exhibited 100% methanol conversion and 179 mmol s^− 1 kg_cat^− 1 hydrogen production rate at 280 °C with carbon monoxide formation as low as 0.19%. The high catalytic activity and hydrogen selectivity shown by ceria promoted Cu/Al₂O₃ catalysts is attributed to the improved specific surface area, dispersion and reducibility of copper which were confirmed by characterizing the catalysts through temperature programmed reduction (TPR), CO chemisorption, X-ray diffraction (XRD) and N₂ adsorption–desorption studies. Reaction parameters were optimized in order to produce hydrogen with carbon monoxide formation as low as possible. The time-on-stream stability test showed that the Cu/CeO₂/Al₂O₃ catalysts were quite stable.     

A novel kojic acid amperometric sensor based on hollow CuO/Fe2O3 hybrid microspheres immobilized in chitosan matrix

Hollow CuO/Fe₂O₃ hybrid microspheres with small uniform holes were synthesized using a convenient hydrothermal method and were applied to fabricated an amperometric sensor for kojic acid. The resulting materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) and then were immobilized into the chitosan (Chi) matrix onto a glassy carbon electrode to obtain CuO/Fe₂O₃–Chi/GCE. The potential utility of the constructed electrodes were demonstrated by applying them to the analytical determination of kojic acid concentration. The electrochemical behavior of kojic acid on CuO/Fe₂O₃–Chi/GCE was explored. The modified electrode displayed excellent amperometric response for kojic acid with a linear range from 0.2 μM to 674 μM with a detection limit of 0.08 μM at a signal-to-noise ratio of 3. In order to validate feasibility, the CuO/Fe₂O₃–Chi/GCE has been used for quantitative detecting kojic acid in real samples.     

Co2SnO4–multiwalled carbon nanotubes composite as a highly reversible anode material for lithium-ion batteries

We report a facile strategy to synthesize the composite of Co₂SnO₄ nanoparticles and multiwalled carbon nanotubes (MWCNTs) as a highly reversible anode material for high-performance lithium-ion batteries. Galvanostatic charge/discharge, cyclic voltammograms(CVs) and electrochemical impedance spectra (EIS) testing results indicate that the Co₂SnO₄–MWCNTs composite display large reversible capacity, excellent cyclic performance and good rate performance, highlighting the importance of the added MWCNTs for maximum utilization of electrochemically active Co₂SnO₄ nanoparticles for energy storage applications in high-performance lithium-ion batteries.     

Study of the biosensor based on platinum nanoparticles supported on carbon nanotubes and sugar–lectin biospecific interactions for the determination of glucose

Highly sensitive electrochemical platform based on Pt nanoparticles supported on carbon nanotubes (Pt_nano-CNTs) and sugar–lectin biospecific interactions is developed for the direct electrochemistry of glucose oxidase (GOD). Firstly, Pt_nano-CNTs nanocomposites were prepared in the presence of carbon nanotubes (CNTs), and then the mixture was cast on a glassy carbon electrode (GCE) using chitosan as a binder. Thereafter, concanavalin A (Con A) was adsorbed onto the precursor film by the electrostatic force between positively charged chitosan and the negatively charged Con A. Finally, the multilayers of Con A/GOD films were prepared based on biospecific affinity of Con A and GOD via layer-by-layer (LBL) self-assembly technique. The electrochemical behavior of the sensor was studied using cyclic voltammetry and chronoamperometry. The electrochemical parameters of GOD in the film were calculated with the results of the electron transfer coefficient (α) and the apparent heterogeneous electron transfer rate constant (k_s) as 0.5 and 5.093 s⁻¹, respectively. Experimental results show that the biosensor responded linearly to glucose in the range from 1.2 × 10⁻⁶ to 2.0 × 10⁻³ M, with a detection limit of 4.0 × 10⁻⁷ M under optimized conditions.     

A hydrogen peroxide sensor based on a horseradish peroxidase/polyaniline/carboxy-functionalized multiwalled carbon nanotube modified gold electrode

We have developed a polyaniline/carboxy-functionalized multiwalled carbon nanotube (PAn/MWCNTCOOH) nanocomposite by blending the emeraldine base form of polyaniline (PAn) and carboxy-functionalized multiwalled carbon nanotubes (MWCNT) in dried dimethyl sulfoxide (DMSO) at room temperature. The conductivity of the resulting PAn/MWCNTCOOH was 3.6 × 10⁻³ S cm⁻¹, mainly as a result of the protonation of the PAn with the carboxyl group and the radical cations of the MWCNT fragments. Horseradish peroxidase (HRP) was immobilized within the PAn/MWCNTCOOH nanocomposite modified Au (PAn/MWCNTCOOH/Au) electrode to form HRP/PAn/MWCNTCOOH/Au for use as a hydrogen peroxide (H₂O₂) sensor. The adsorption between the negatively charged PAn/MWCNTCOOH nanocomposite and the positively charged HRP resulted in a very good sensitivity to H₂O₂ and an increased electrochemically catalytical current during cyclic voltammetry. The HRP/PAn/MWCNTCOOH/Au electrode exhibited a broad linear response range for H₂O₂ concentrations (86 μM–10 mM). This sensor exhibited good sensitivity (194.9 μA mM⁻¹ cm⁻²), a fast response time (2.9 s), and good reproducibility and stability at an applied potential of −0.35 V. The construction of the enzymatic sensor demonstrated the potential application of PAn/MWCNTCOOH nanocomposites for the detection of H₂O₂ with high performance and excellent stability.     

Visible light photocatalytic degradation of thiophene using Ag–TiO2/multi-walled carbon nanotubes nanocomposite

Ag–TiO₂ nanocatalyst, supported on multi-walled carbon nanotubes, was synthesized successfully via a modified sol–gel method, and the prepared photocatalyst was used to remediate aqueous thiophene environmentally by photocatalytic oxidation under visible light. The prepared Ag–TiO₂/multi-walled carbon nanotubes nanocomposite photocatalyst was characterized through X-ray diffraction, Brunauer–Emmett–Teller (BET), transmission electron microscopy, and UV–vis spectra (UV–vis). The results showed that both Ag and TiO₂ nanoparticles were well-dispersed over the MWCNTs and formed a uniform nanocomposite. Ag doping can eliminate the recombination of electron–hole pairs in the catalyst, and the presence of MWCNTs in the TiO₂ composite can change surface properties to achieve sensitivity to visible light. The optimum mass ratio of MWCNT:TiO₂:Ag was 0.02:1.0:0.05, which resulted in the photocatalyst's experimental performance in oxidizing about 100% of the thiophene in a 600 mg/L solution within 30 min and with 1.4 g L⁻¹ amount of catalyst used.     

Simultaneous determination of acetaminophen and dopamine using SWCNT modified carbon–ceramic electrode by differential pulse voltammetry

A highly sensitive method was investigated for the simultaneous determination of acetaminophen (AC) and dopamine (DA) using single wall carbon nanotubes modified carbon–ceramic electrode (SWCNT/CCE). The SWCNT/CCE displayed excellent electrochemical catalytic activities towards the oxidation of AC and DA. Under optimized experimental conditions in differential pulse voltammetry technique, AC and DA gave linear response over the ranges 0.2–100.0 μM (R² = 0.996) and 0.4–150.0 μM (R² = 0.999), respectively. The detection limits (S/N = 3) were found to be 0.12 μM for AC and 0.22 μM for DA. The present method was applied to the determination of AC and DA in some commercial pharmaceutical samples.     

Nanostructured CeO2 microspheres synthesized by a novel surfactant-free emulsion

Nanostructured CeO₂ microspheres with an average diameter of 11 μm were prepared by a novel surfactant-free emulsion for the first time. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Brunauer-Emmet-Teller (BET) measurements were used to characterize the products. The images of SEM showed that the CeO₂ microspheres consisted of petal-like nanostructures with petal thickness in a range of 60 nm to 100 nm. The BET measurements showed the specific surface area of the CeO₂ microspheres exceeded 43 m²/g. The XRD analysis indicated the nanostructured CeO₂ microspheres were of cubic lattice. A possible mechanism of an interfacial precipitation reaction with the droplets of solid-stabilized emulsion as templates was proposed.     

Sonochemical synthesis of magnetic core–shell Fe3O4@ZrO2 nanoparticles and their application to the highly effective immobilization of myoglobin for direct electrochemistry

In this study, bifunctional Fe₃O₄@ZrO₂ magnetic core–shell nanoparticles (NPs), synthesized by a simple and effective sonochemical approach, were attached to the surface of a magnetic glassy carbon electrode (MGCE) and successfully applied to the immobilization/adsorption of myoglobin (Mb) for constructing a novel biosensor platform. With the advantages of the magnetism and the excellent biocompatibility of the Fe₃O₄@ZrO₂ NPs, Mb could be easily immobilized on the surface of the electrode in the present of external magnetic field and well retained its bioactivity, hence dramatically facilitated direct electron transfer of Mb was demonstrated. The proposed Mb/Fe₃O₄@ZrO₂ biofilm electrode exhibited excellent electrocatalytic behaviors towards the reduction of H₂O₂ with a linear range from 0.64 μM to 148 μM. This presented system avoids the complex synthesis for protecting Fe₃O₄ NPs, supplies a simple, effective and inexpensive way to immobilize protein, and is promising for construction of third-generation biosensors and other bio-magnetic induction devices.     

Preparation of Pt/CeO2/HCSs anode electrocatalysts for direct methanol fuel cells

Hollow carbon spheres (HCSs) were prepared through a simple hydrothermal method using silica particles and glucose as the template and carbon precursor, respectively. HCSs used as supports for platinum catalysts deposited with cerium oxide (CeO₂) were prepared for application as anode catalysts in direct methanol fuel cells. The composition and structure of the samples were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The electrocatalytic properties of the as-prepared catalysts for methanol oxidation were investigated by cyclic voltammetry (CV). The Pt/CeO₂/HCSs catalyst heated at 550 °C for 1 h exhibited the best catalytic activity for methanol oxidation.     

The role of CeO2–ZrO2 as support in the ZnO–ZnCr2O4 catalysts for autothermal reforming of methanol

Autothermal reforming of methanol for hydrogen production was investigated over ZnO–ZnCr₂O₄ supported on a series of metal oxides (Al₂O₃, CeO₂, ZrO₂ and CeO₂–ZrO₂). CeO₂–ZrO₂ mixed oxides with Ce /Zr molar ratio of 4/1 was found to be the optimal support which showed significant effect on the catalytic activity and selectivity. The ZnO–ZnCr₂O₄/CeO₂–ZrO₂ and ZnO–ZnCr₂O₄ catalysts were characterized by XRD, TEM, H₂-TPR and XPS. The results show that CeO₂–ZrO₂ mixed oxides have significant effect on the catalytic performance and the supported catalyst shows more uniform temperature distribution in the catalyst bed which was mainly due to its reasonable redox properties.     

Fabrication of CeO2 nanoparticles modified glassy carbon electrode and its application for electrochemical determination of UA and AA simultaneously

A glassy carbon electrode modified with CeO₂ nanoparticles was constructed and was characterized by electrochemical impedance spectrum (EIS) and cyclic voltammetry (CV). The resulting CeO₂ nanoparticles modified glassy carbon electrode (CeO₂ NP/GC electrode) was used to detect uric acid (UA) and ascorbic acid (AA) simultaneously in mixture. This modified electrode exhibits potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards UA and AA with activation overpotential. For UA and AA in mixture, one can well separate from the other with a potential difference of 273 mV, which was large enough to allow the determination of one in presence of the other. The DPV peak currents obtained in mixture increased linearly on the UA and AA in the range of 5.0 × 10⁻⁶ to 1.0 × 10⁻³ mol/L and 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ mol/L, with the detection limit (signal-to-noise ratio was 3) for UA and AA were 2.0 × 10⁻⁷ and 5.0 × 10⁻⁶ mol/L, respectively. The proposed method showed excellent selectivity and stability, and the determination of UA and AA simultaneously in serum was satisfactory.     

Synthesis of dendritic silver nanostructures and their application in hydrogen peroxide electroreduction

Well-defined silver (Ag) dendritic nanostructures were successfully synthesized by electrodeposition without the use of any template or surfactant. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to investigate the as-prepared Ag nanomaterials. These dendrites are aggregates of Ag nanoparticles, which are highly crystalline in nature. The concentration of AgNO₃ affects the shape of the nanoparticles. In addition, the electrochemical properties of the Ag dendrite-modified glassy carbon electrode (Ag/GC) were characterized by cyclic voltammetry and chronoamperometry. Results indicated that the as-obtained Ag dendrites exhibited favorable electroreduction activity towards oxygen (O₂) and hydrogen peroxide (H₂O₂). When used as a sensor, the Ag/GC electrode exhibited a wide linear range of 0.005–12 mM H₂O₂, with a remarkable sensitivity of 7.39 μA/mM, a detection limit of 0.5 μM, estimated at a signal-to-noise ratio of 3, and a rapid response time (within 5 s). Moreover, the electrode showed good reproducibility, anti-interferant ability and long-term stability.     

A highly reactive catalyst for CO oxidation: CeO2 nanotubes synthesized using carbon nanotubes as removable templates

In this work, the nanocomposites, carbon nanotubes (CNTs) coated with nanosized ceria, were prepared by a facile solvothermal method. The obtained nanocomposites have a dense overlayer which is made of nanoparticles with the diameter of ～10 nm. Ceria nanotubes with a porous and hollow structure were fabricated by the removal of CNTs, which possess high surface area and remarkable thermal stability. The products were characterized by X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. The parameters affecting the formation of CeO₂ nanotubes were discussed in details. The key steps involved in the formation of the CeO₂ nanotubes are solvothermal modifications of CNTs and controlled calcinations. CeO₂ nanotubes have an excellent catalytic performance for the CO oxidation. The remains of the templates in the CeO₂ nanotubes are considered to play an important part in the enhanced catalytic activity for the formation of CeO_2?xC_x.     

Formation and thermoelectric property of TiO2 nanotubes covered by Te–Bi–Pb nanoparticles

A variety of strategies have been attempted to improve the performance of thermoelectric materials. The primary approach is to employ low-dimensional materials to reduce the lattice thermal conductivity as described by the Wiedemann–Franz law. That is, to decrease the thermal conductivity, rattling structures, point defects, vacancies and nanocomposites have been used to efficiently scatter phonons within or between the unit cell crystals. Complex crystalline structures have been used to decouple the electrical conductivity and thermal conductivity to achieve this goal. Based on such considerations, we have prepared TiO₂ nanotubes from titanium foils. These nanotubes are low-dimensional, thus, preferable to achieve low lattice thermal conductivity to generate favorable thermoelectric properties. Moreover, scattered Te–Bi–Pb nanoparticles have been deposited on the surface of the TiO₂ nanotubes via electrochemical method. The purpose of the nanoparticles is to further enhance the performance of the thermoelectricity, specifically in our case, to increase the Seebeck coefficient. From the results obtained, the best Seebeck coefficient for pure TiO₂ nanotubes is about 90 μV/K; while the best Seebeck coefficient for TiO₂ nanotubes covered with scattered Te–Bi–Pb nanoparticles is about 155 μV/K. This significant improvement could be explained by the quantum confinement in such a peculiar nanostructure.