Microwave-radiated synthesis of gold nanoparticles/carbon nanotubes composites and its application to voltammetric detection of trace mercury(II)

Gold nanoparticles/carbon nanotubes (Au-NPs/CNTs) composites were rapidly synthesized by microwave radiation, and firstly applied for the determination of trace mercury(II) by anodic stripping voltammetry (ASV). The structure and composition of the synthesized Au-NPs/CNTs nanocomposites were characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), UV–vis absorption spectroscopy and cyclic voltammetry. Au-NPs/CNTs nanocomposites modified glassy carbon electrode (Au-NPs/CNTs/GCE) exhibited excellent performance for Hg(II) analysis. A wide linear range (5 × 10⁻¹⁰–1.25 × 10⁻⁶ mol/L) and good repeatability (relative standard deviation of 1.84%) were obtained for Hg(II) detection. The limit of detection was found to be 3 × 10⁻¹⁰ mol/L (0.06 μg/L) at 2 min accumulation, while the World Health Organization’s guideline value of mercury for drinking water is 1 μg/L, suggesting the proposed method may have practical utility.     

Using flowerlike polymer–copper nanostructure composite and novel organic–inorganic hybrid material to construct an amperometric biosensor for hydrogen peroxide

A new type of amperometric hydrogen peroxide biosensor was fabricated by entrapping horseradish peroxidase (HRP) in the organic–inorganic hybrid material composed of zirconia–chitosan sol–gel and Au nanoparticles (ZrO₂–CS–AuNPs). The sensitivity of the biosensor was enhanced by a flowerlike polymer–copper nanostructure composite (pPA–FCu) which was prepared from co-electrodeposition of CuSO₄ solution and 2,6-pyridinediamine solution. Several techniques, including UV–vis absorption spectroscopy, scanning electron microscopy, cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy were employed to characterize the assembly process and performance of the biosensor. The results showed that this pPA–FCu nanostructure not only had excellent redox electrochemical activity, but also had good catalytic efficiency for hydrogen peroxide. Also the ZrO₂–CS–AuNPs had good film forming ability, high stability and good retention of bioactivity of the immobilized enzyme. The resulting biosensors showed a linear range from 7.80 × 10^?7 to 3.7 × 10^?3 mol L^?1, with a detection limit of 3.2 × 10^?7 mol L^?1 (S/N = 3) under optimized experimental conditions. The apparent Michaelis–Menten constant was determined to be 0.32 mM, showing good affinity. In addition, the biosensor which exhibits good analytical performance, acceptable stability and good selectivity, has potential for practical applications.     

Voltammetric sensor for buzepide methiodide determination based on TiO2 nanoparticle-modified carbon paste electrode

In this work, we have prepared nano-material modified carbon paste electrode (CPE) for the sensing of an antidepressant, buzepide methiodide (BZP) by incorporating TiO₂ nanoparticles in carbon paste matrix. Electrochemical studies indicated that the TiO₂ nanoparticles efficiently increased the electron transfer kinetics between drug and the electrode. Compared with the nonmodified CPE, the TiO₂-modified CPE greatly enhances the oxidation signal of BZP with negative shift in peak potential. Based on this, we have proposed a sensitive, rapid and convenient electrochemical method for the determination of BZP. Under the optimized conditions, the oxidation peak current of BZP is found to be proportional to its concentration in the range of 5 × 10⁻⁸ to 5 × 10⁻⁵ M with a detection limit of 8.2 × 10⁻⁹ M. Finally, this sensing method was successfully applied for the determination of BZP in human blood serum and urine samples with good recoveries.     

One-step fabrication of chitosan–hematite nanotubes composite film and its biosensing for hydrogen peroxide

This work reports on a novel chitosan–hematite nanotubes composite film on a gold foil by a simple one-step electrodeposition method. The hybrid chitosan–hematite nanotubes (Chi–HeNTs) film exhibits strong electrocatalytic reduction activity for H₂O₂. Interestingly, two electrocatalytic reduction peaks are observed at −0.24 and −0.56 V (vs SCE), respectively, one controlled by surface wave and the other controlled by diffusion process. The Chi–HeNTs/Au electrode shows a linear response to H₂O₂ concentration ranging from 1 × 10⁻⁶ to 1.6 × 10⁻⁵ mol L⁻¹ with a detection limit of 5 × 10⁻⁸ mol L⁻¹ and a sensitivity as high as 1859 μA μM⁻¹ cm⁻².     

Simultaneous determination of acetaminophen,dopamine and ascorbic acid using a PbS nanoparticles Schiff base-modified carbon paste electrode

A highly sensitive method was investigated for the simultaneous determination of acetaminophen (AC), dopamine (DA), and ascorbic acid (AA) using a PbS nanoparticles Schiff base-modified carbon paste electrode (PSNSB/CPE). Differential pulse voltammetry peak currents of AC, DA and AA increased linearly with their concentrations within the ranges of 3.30 × 10⁻⁸–1.58 × 10⁻⁴ M, 5.0 × 10⁻⁸–1.2 × 10⁻⁴ M and 2.50 × 10⁻⁶–1.05 × 10⁻³ M, respectively, and the detection limits for AC, DA and AA were 5.36 × 10⁻⁹, 2.45 × 10⁻⁹ and 1.86 × 10⁻⁸ M, respectively. The peak potentials recorded in a phosphate buffer solution (PBS) of pH 4.6 were 0.672, 0.390, and 0.168 V (vs Ag/AgCl) for AC, DA and AA, respectively. The modified electrode was used for the determination of AC, DA, and AA simultaneously in real and synthetic samples.     

Silica–zirconia membranes for nanofiltration

Inorganic nanofiltration membranes were fabricated from silica–zirconia composite colloidal sol (molar ratio Si/Zr=9/1) using a sol–gel process. Molecular weight cut-off (MWCO) was successfully controlled between 200 and 1000 Da by regulating the colloidal diameters of sol solutions in the final coating stage. The pure water permeabilities ranged from 0.15×10⁻¹¹ to 1.5×10⁻¹¹ m³ m⁻² s⁻¹ Pa⁻¹. Pore size and pore size distribution were estimated based on the dynamic method of humid air permeation, and found to be from 1.0 to 2.9 nm. The MWCO obtained from NF experiments using neutral organic solutes corresponds well with the pore diameters estimated from the dynamic permeation method. Silica–zirconia membranes were found to be stable in aqueous solution for periods in excess of four months.     

Choline biosensors based on a bi-electrocatalytic property of MnO2 nanoparticles modified electrodes to H2O2

An interesting mode of reactivity of MnO₂ nanoparticles modified electrode in the presence of H₂O₂ is reported. The MnO₂ nanoparticles modified electrodes show a bi-direction electrocatalytic ability toward the reduction/oxidation of H₂O₂. Based on this property, a choline biosensor was fabricated via a direct and facile electrochemical deposition of a biocomposite that was made of chitosan hydrogel, choline oxidase (ChOx) and MnO₂ nanoparticles onto a glassy carbon (GC) electrode. The biocomposite is homogeneous and easily prepared and provides a shelter for the enzyme to retain its bioactivity. The results of square wave voltammetry showed that the electrocatalytic reduction currents increased linearly with the increase of choline chloride concentration in the range of 1.0 × 10⁻⁵ –2.1 × 10⁻³ M and no obvious interference from ascorbic acid and uric acid was observed. Good reproducibility and stability were obtained. A possible reaction mechanism was proposed.     

Kinetics of the diazotization and azo coupling reactions of procaine in the micellar media

The kinetics of the diazotization reaction of procaine in the presence of anionic micelles of sodium dodecyl sulfate (SDS) and cationic micelles of cetyltrimethyl ammonium bromide (CTAB), dodecyltrimethyl ammonium bromide (DDTAB) and tetradecyltrimethyl ammonium bromide (TDTAB) were carried out spectrophotometrically at λ_max = 289 nm. The values of the pseudo first order rate constant were found to be linearly dependent upon the [NaNO₂] in the concentration range of 1.0 × 10⁻³ mol dm⁻³ to 12.0 × 10⁻³ mol dm⁻³ in the presence of 2.0 × 10⁻² mol dm⁻³ acetic acid. The concentration of procaine was kept constant at 6.50 × 10⁻⁵ mol dm⁻³. The addition of the cationic surfactants increased the reaction rate and gave plateau like curve. The addition of SDS micelles to the reactants initially increased the rate of reaction and gave maximum like curve. The maximum value of the rate constant was found to be 9.44 × 10⁻³ s⁻¹ at 2.00 × 10⁻³ mol dm⁻³ SDS concentration. The azo coupling of diazonium ion with β-naphthol (at λ_max = 488) nm was found to linearly dependent upon [ProcN₂⁺] in the presence of both the cationic micelles (CTAB, DDTAB and TDTAB) and anionic micelles (SDS). Both the cationic and anionic micelles inhibited the rate of reactions. The kinetic results in the presence of micelles are explained using the Berezin pseudophase model. This model was also used to determine the kinetic parameters e.g. k_m, K_s from the observed results of the variation of rate constant at different [surfactants].     

Enhanced electrochemiluminescence based on Ru(bpy)32+-doped silica nanoparticles and graphene composite for analysis of melamine in milk

A sensitive electrochemiluminescence (ECL) sensor for melamine analysis was fabricated based on Ru(bpy)₃²⁺-doped silica (Ru(bpy)₃²⁺@SiO₂) nanoparticles and graphene composite. Spherical Ru(bpy)₃²⁺@SiO₂ nanoparticles with uniform size about 55 nm were prepared by the reverse microemulsion method. Since per Ru(bpy)₃²⁺@SiO₂ nanoparticle encapsulated a great deal of Ru(bpy)₃²⁺, the ECL intensity has been greatly enhanced, which resulted in high sensitivity. Due to its extraordinary electric conductivity, graphene improved the conductivity and accelerated the electron transfer rate. In addition, graphene could work as electronic channel improving the efficient luminophor amount participating in the ECL reaction, which further enhanced the ECL signal. This proposed sensor was used to melamine analysis and the ECL intensity was proportional to logarithmic melamine concentration range from 1 × 10⁻¹³ M to 1 × 10⁻⁸ M with the detect limit as low as 1 × 10⁻¹³ M. In application to detect melamine in milk, satisfactory recoveries could be obtained, which indicated this sensor having potential application in melamine analysis in real samples.     

Stripping voltammetric analysis of organophosphate pesticides based on solid-phase extraction at zirconia nanoparticles modified electrode

A sensitive electrochemical stripping voltammetric method for analyzing organophosphate (OP) compounds was developed based on solid-phase extraction (SPE) at zirconia (ZrO₂) nanoparticles modified electrode. ZrO₂ nanoparticles were proved as a new sorbent for SPE of OP pesticides. Because of the strong affinity of ZrO₂ for the phosphoric group, nitroaromatic OPs can strongly bind to the ZrO₂ nanoparticle surface. The combination of SPE with square-wave voltammetry (SWV) provided a fast, sensitive, and selective electrochemical method for nitroaromatic OP compounds using methyl parathion (MP) as a model. The stripping response was highly linear over the MP range of 0.003–2.0 μg/mL, with a detection limit of 0.001 μg/mL. The fast extraction ability of ZrO₂ nanoparticles makes it promising sorbent for various solid-phase extractions.     

A highly sensitive and selective resonance Rayleigh scattering method for Hg2+ based on the nanocatalytic amplification

In 0.2 mol/L HCl–0.22 mol/L HNO₃ medium, trace Hg²⁺ catalyzed NaH₂PO₂ reduction of HAuCl₄ to form gold nanoparticles (AuNPs), which exhibited a strong resonance Rayleigh scattering (RRS) effect at 370 nm. With increasing of [Hg²⁺], the RRS effect enhanced due to more AuNP generated from the catalytic reaction. Under the chosen conditions, the enhanced RRS intensity at 370 nm is linear to Hg²⁺ concentration in the range of 5.0–450 × 10⁻⁹ mol/L, with a detection limit of 0.1 nmol/L. This RRS method was applied for the determination of Hg in water samples, with high sensitivity and good selectivity, and its results were agreement with that of atomic fluorescence spectrometry.     

Proton-conducting membranes based on poly(2,5-benzimidazole) (ABPBI) and phosphoric acid prepared by direct acid casting

We report the preparation of phosphoric acid doped poly(2,5-benzimidazole) (ABPBI) membranes for PEMFC by simultaneously doping and casting from a poly(2,5-benzimidazole)/phosphoric acid/methanesulfonic acid (MSA) solution. The evaporation of MSA yields a very homogeneous membrane having a better controlled composition, avoiding the use of solvent-intensive procedures. Membranes have been prepared with contents of up to 3.0H₃PO₄ molecules per ABPBI repeating unit. These membranes achieve a maximum conductivity of 1.5 × 10⁻² S cm⁻¹ at temperatures as high as 180 °C in dry conditions. These ABPBI membranes are more conveniently prepared than those conventionally formed and doped in separate steps while featuring comparable conductivities (ABPBI × 2.7H₃PO₄ prepared by the soaking method showed a conductivity of 2.5 × 10⁻² S cm⁻¹ at 180 °C in dry conditions).     

Highly selective thiocyanate electrode based on bis-bebzoin-semitriethylenetetraamine binuclear copper(II) complex as neutral carrier

A novel selective thiocyanate PVC membrane electrode based on bis-bebzoin-semitriethylenetetraamine binuclear copper(II) [Cu(II)₂–BBSTA] as neutral carrier is reported, which displays an anti-Hofmeister selectivity sequence in following order: SCN⁻ > ClO₄⁻ > I⁻ >Sal⁻ >SO₃²⁻ >NO₃⁻ > H₂PO₄⁻ > Cl⁻ >NO₂⁻ > SO₄²⁻. The electrode exhibits Nernstian potential linear range to thiocyanate from 1.0 × 10⁻¹ to 9.0 × 10⁻⁷ mol/l with a detection limit 7.0 × 10⁻⁷ mol/l and a slope of −57.0 mV/decade in pH 5.0 of phosphorate buffer solution at 25 °C. The response mechanism is discussed in view of the AC impedance technique and the UV spectroscopy technique. From comparison of potentiometric response characteristics between the binuclear metallic complex copper(II) [Cu(II)₂–BBSTA] and mononuclear copper(II) metallic complex [Cu(II)–BBSDA], an enhanced response towards thiocyanate from the electrode based on binuclear metallic complex copper (II) [Cu(II)₂–BBSTA] was observed. The electrode based on binuclear copper(II) compound was used to determine the thiocyanate content in waste water with satisfactory results.     

Reduction reactions of bovine serum albumin and lysozyme by CO- .2 radical in polyvinyl alcohol solution: a pulse radiolysis study

Pulse radiolytic reduction of bovine serum albumin (BSA) and lysozyme by CO^⨪₂ radical in presence of polyvinyl alcohol (PVA) has been studied. At pH 6.8 in presence of 2% (w/v) PVA, reduction of BSA and lysozyme (both at 1×10⁻⁴ mol dm⁻³) give an additional transient peak at 390 nm along with the usual 420 nm peak. The bimolecular rate constants for the reaction of CO^⨪₂ radical at pH 6.8±0.2 with BSA are 2.7×10⁸ and 7.13×10⁸ dm³ mol⁻¹ s⁻¹ at 420 nm and 390 nm respectively. The same for lysozyme are 3.2×10⁸ and 5.6×10⁸ dm³ mol⁻¹ s⁻¹ at 420 nm and 390 nm, respectively. Dimethyl disulfide also gives 390 nm and 420 nm peaks in this system upon reduction with CO^⨪₂ radical. The 390 nm peak is ascribed to the sulfenium radical (RSS(H)R). Studies on the variation of pH suggests the protonation of RSSR^⨪ radical (420 nm) to form RSS(H)R radical (390 nm) in this viscous media. The decay of RSS(H)R radical occurs via formation of RS radical and RS(H)S(H)R, the final product being RSH in both cases.     

Activity and stability of horseradish peroxidase in hydrophilic room temperature ionic liquid and its application in non-aqueous biosensing

The activity and stability of horseradish peroxidase (HRP) were investigated in a hydrophilic room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluroborate ([bmim][BF₄]) by electrochemical methods. Although no detectable activity exhibited in anhydrous [bmim][BF₄], HRP was active in the presence of a small amount of water (4.53%, v/v). And its activity can be improved by immobilization in agarose hydrogel. The immobilized HRP possesses excellent activity at 65 °C. It remained 80.2% of its initial activity after being immersed for 10.5 h in an aqueous mixture of [bmim][BF₄] with some hydrogen peroxide (H₂O₂) under room temperature, implying extremely high stability. Moreover, the immobilized HRP was found to be very sensitive and stable in H₂O-containing [bmim][BF₄] for the detection of H₂O₂, with a wide linear range of 6.10 × 10⁻⁷ to 1.32 × 10⁻⁴ mol l⁻¹ and low detection limit of 1.0 × 10⁻⁷ mol l⁻¹.     

Electrochemiluminescent biosensor based on multi-wall carbon nanotube/nano-Au modified electrode

The electrochemiluminescent (ECL) behavior of lucigenin on a multi-wall carbon nanotube/nano-Au modified glassy carbon electrode (MWNT/nano-Au/GCE) was studied in this paper. Compared with the bare GCE, the ECL intensity of lucigenin can be greatly enhanced at MWNT/nano-Au/GCE. Based on the fact that superoxide dimutase (SOD) could obviously inhibit the ECL of lucigenin at MWNT/nano-Au/GCE, a sensitive ECL biosensor for determination of SOD was developed with a wide linear range of 5.0 × 10⁻⁸–5.0 × 10⁻⁶ mol/L with detection limit of 2.5 × 10⁻⁸ mol/L.     

Performance of silver nanocubes based on electrochemical surface area for catalyzing oxygen reduction reaction

Ag nanocubes that are 45 nm in size are synthesized and successfully used as catalysts in oxygen electroreduction. Electrochemical surface areas (ESAs) are considered to determine the effect on HO₂⁻ production, which is found to be in the following order: nanocubes < nanoparticles. Comparative data generated using Tafel analyses in 0.1 M NaOH electrolyte without and with methanol show that unchanged slopes on the prepared cubic catalysts can indicate high resistance of Ag nanocubes for methanol oxidation during oxygen reduction reaction. Among these Ag catalysts, nanocubes exhibit 9.29 × 10^− 2 mA cm^− 2 (at − 0.15 V vs. Ag/AgCl), the better activity in the oxygen reduction reaction.     

Photo-induced degradation of some flavins in aqueous solution

The blue-light induced photo-degradation of FMN, FAD, riboflavin, lumiflavin, and lumichrome in aqueous solution at pH 8 is studied by measurement of absorption coefficient spectral changes due to continuous excitation at 428 nm. The quantum yields of photo-degradation determined are ϕ_D(riboflavin, pH 8) ≈ 7.8 × 10⁻³, ϕ_D(FMN, pH 5.6) ≈ 7.3 × 10⁻³, ϕ_D(FMN, pH 8) ≈ 4.6 × 10⁻³, ϕ_D(FAD, pH 8) ≈ 3.7 × 10⁻⁴, ϕ_D(lumichrome, pH 8) ≈ 1.8 × 10⁻⁴, and ϕ_D(lumiflavin, pH 8) ⩽ 1.1 × 10⁻⁵. In a mass-spectroscopic analysis, the photo-products of FMN dissolved in water (solution pH is 5.6) were identified to be lumichrome and the lumiflavin derivatives dihydroxymethyllumiflavin, formyllumiflavin, and lumiflavin-hydroxy-acetaldehyde. An absorption and emission spectroscopic characterisation of the primary photoproducts of FMN at pH 8 is carried out.     

The determination of cysteine at Bi-powder carbon paste electrodes by cathodic stripping voltammetry

The determination of cysteine by means of square wave cathodic stripping voltammetry (SWCSV) is reported here for the first time at Bi-modified carbon paste electrodes (CPEs). The modified electrodes are 17% w/w metallic Bi powder mixed with CP (Bi-CPEs) and the technique is based on the enhancement of Bi surface oxidation in the presence of cysteine at a carefully chosen accumulation potential and the subsequent reduction-stripping of the product (proposed to be bismuth(III) cysteinate) by potential scanning to more negative values. The wide concentration range of 1 × 10⁻⁶–5 × 10⁻⁵ M for cysteine can be assessed by SWCSV using Bi-CPEs and, by appropriate choice of accumulation times, two linear response concentration regimes could be identified: 1 × 10⁻⁶–1 × 10⁻⁵ M (accumulation for 600 s) and 1 × 10⁻⁵–5 × 10⁻⁵ M (accumulation for 100 s), with estimated detection limits of 3 × 10⁻⁷ and 2 × 10⁻⁶ M, respectively.     

Sulfur tolerant redox stable layered perovskite SrLaFeO4 − δ as anode for solid oxide fuel cells

Redox stable K₂NiF₄ type layered perovskite SrLaFeO_4 − δ(SLFO_4 − δ) has been prepared and evaluated as anode for solid oxide fuel cell (SOFC). The SLFO_4 − δ shows linear thermal expansion behavior with TEC of 14.3 × 10^− 6 K^− 1. It also demonstrates excellent catalytic activity for various fuels. A scandia stabilized zirconia (ScSZ, 180 μm) electrolyte supported SOFC with the anode achieves maximum power densities (P_max) of 0.93, 0.76, 0.63, and 0.46 Wcm^− 2 at 900–750 °C, respectively, in wet H₂. P_maxs of cells supported by 250 μm ScSZ reach 0.57, 0.60 and 0.50 Wcm^− 2 in H₂, H₂ + 50 ppm H₂S and propane, respectively, at 800 °C. Moreover, the cells show stable power output during ~ 100 h operation at 800 °C under 0.7 V in various fuels. The P_max at 800 °C in wet H₂ even increases by ~ 11% in the subsequent two thermal cyclings, indicating that SLFO_4 − δ is a promising anode candidate for SOFC with good electro-catalytic activity, high stability and resistance to sulfur and coking.