•GO sheets were separated by their thickness via density gradient centrifugation.•Thin layer sheets have more N element and higher oxygen reduction activity.•The physicochemical properties of GO sheets were morphology-related.Graphene oxide (GO) made from graphite oxidation by Hummer’s method is a mixture of GO sheets with different size and thickness. Previous work has shown that these GO sheets have different electrophoretic properties and can be separated via capillary electrophoresis according to their surface charge (J. Zhao et al., Anal. Chem. 83 (2011) 9100–9106). Herein, the GO sheets were separated by density gradient centrifugation to achieve preparation of bulk GO materials. GO sheets were separated by their thicknesses and the as-obtained sheets were characterized for their spectroscopic, electronic and electrochemical properties. After treated by hydrazine, GO was derivatized with N element and become active toward oxygen reduction reaction. The thin layer GO sheets incorporated with more hydrazine and the atomic ratio of N/C was higher than thick layer GO sheets. This work suggested that the physicochemical properties of GO sheets were N of layer related and it is necessary to separate GO sheets to prepare suitable materials for device fabrication or catalysts use.Graphical abstract

A glassy carbon electrode was modified with PdO-NiO composite nanofibers (PdO-NiO-NFs) and applied to the electrocatalytic reduction of hydrogen peroxide (H2O2). The PdO-NiO-NFs were synthesized by electrospinning and subsequent thermal treatment, and then characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Factors such as the composition and fraction of nanofibers, and of the applied potential were also studied. The sensor exhibits high sensitivity for H2O2 (583.43 μA · mM−1 · cm−2), a wide linear range (from 5.0 μM to 19 mM), a low detection limit (2.94 μM at an SNR of 3), good long term stability, and is resistant to fouling.

Palladium nanoparticles (PdNPs) modified indium tin oxide (ITO) electrode was fabricated via electrospinning followed by calcination. The results of scanning electron microscopy (SEM) images and X-ray photoelectron spectroscopy (XPS) indicated the PdNPs were immobilized on the ITO electrode surface successfully. The electrochemical behavior of the modified electrodes could be adjusted easily by changing the collected time of electrospinning. The prepared electrodes exhibited high sensitivity, good reproducibility and stability toward NO2−. The current responses were linear with NO2− concentrations in a wide range with low detection limit. The proposed method can be successfully applied to the determination of NO2− in real water samples.

A novel nanoporous array template with hexagonal nanorod-like morphology was developed with sacrificing nanostructured ZnO. ZnO hexagonal nanorod (NR) array was first electrochemically deposited on ITO, and then was spin-coated with photoresist (PR) followed by heating treatment to harden the PR film. After resolving the ZnO NRs in diluted hydrochloric acid solution, the template with ZnO NR-like nanopores was obtained. The as-prepared template has good stability and uniformity. Additionally, by controlling the deposition charge of ZnO, and the rinsing time in removing a PR layer to expose ZnO top end, the size and pore-density of the template can be controlled. In presence of glucose, an enhancement in oxidation current of Ni NRs was observed due to an electrochemical catalytic reaction scheme. So the Ni NR array modified ITO electrode can be applied for glucose sensing. And the Ni NR array with a higher density gave a higher sensitivity.Graphical abstractA novel nanoporous array template with hexagonal nanorod-like morphology was developed with sacrificing photoresist and nanostructured ZnO.Highlights► A nanoporous array template with hexagonal nanorod-like morphology was developed. ► It was fabricated with sacrificing photoresist and nanostructured ZnO. ► The as-prepared template has good stability and uniformity.

We presented a modified electrode based on electrospun PdO–Co3O4 nanofiber composite for electro-oxidation of methanol in alkaline media. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were adopted to characterize the structures and composition of the composite modified electrodes. The influential factors such as the component and amounts of nanofibers were also studied. After electrochemical pretreatment, the Nafion/PdO–Co3O4/GCE (the atomic ratio of Pd:Co = 2:1) exhibited the greatest electrocatalytic activity toward methanol electro-oxidation among the electrodes investigated. The present novel strategy is expected to reduce the cost of the catalyst of methanol electro-oxidation remarkably.Highlights▶ PdO–Co3O4 nanofibers were fabricated by electrospinning and following calcination. ▶ The PdO–Co3O4 nanofiber modified electrode was pretreated to dissolve surface Co3O4. ▶ The NA/PdO–Co3O4/GCE exhibited high catalytic activity and stability to methanol.

Using photocatalytic reactive nanoparticles and DNA composite modified gold electrodes, exploiting the sensitivity of DNA-mediated electron transfer to base pair stacking, we examined the effects of DNA oxidation damage by photocatalytically generated hydroxyl radicals (˙OH) on charge transfer efficiency and proposed an electrochemical technique to read the effective diffusing distance of ˙OH.

An amperometric glucose biosensor based on the direct electron transfer (DET) of glucose oxidase (GOx) was developed and the enhanced catalytic current in presence of substrate was used to quantify glucose. GOx was adsorbed onto nanostructured ZnO modified indium tin oxide (ITO) electrodes. The goal of this work was to study the effects of morphology of nanostructured ZnO on DET of GOx and the sensor’s analytical properties. Nanostructured ZnO materials including nanorods (NRs) and nanoplates (NPs) were electrochemical deposited onto ITO and were characterized using scanning electron microscopy and X-ray diffraction. Through adjusting the precursor of Zn ions, electrolytes and controlling deposition charge (Q, in Coulomb, C), different nanostructured ZnO with different size, shape and deposition density were obtained. GOx was adsorbed onto nanostructured ZnO via layer-by-layer assembling to form GOx and polylectrolyte multilayer films. Compared to bare ITO or ITO deposited with nanostructured ZnO with other morphologies, GOx immobilized onto ZnO NRs arrays (Q = 0.1 C) gave higher current signals, exhibited reversible DET (ks, 2.16 s−1), wide linear range (from 0.1 to 9 mM) with the limit of detection of 1.94 μM and low apparent Michaelis–Menten constant (KMapp, 3.12 mM).

Acidic dissolution of transition metals from Pt based alloy catalysts for oxygen reduction reaction (ORR) is an unavoidable process during fuel cell operation. In this work we studied the effect of acid treatment of graphene-supported Pt1Nix(x = 0, 0.25, 0.5, 1, and 2) alloys on the kinetics of the ORR in both alkaline and acidic solutions together with the generation of OH radicals in alkaline solutions. The alloy nanoparticles were synthesized through coimpregnation and chemical reduction. The electronic and structural features of the alloy were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. The ORR performances were studied using cyclic voltammetry and rotating ring disk electrode techniques in 0.05 M H2SO4 and 0.1 M NaOH, respectively. The alloy catalysts were more active than pure Pt toward ORR, and after acid treatment the ORR activity of Pt−Ni alloy was enhanced in both acidic and alkaline media. The maximum activity of the Pt-based catalysts was found with ca. 50 atom % Ni content in the alloys (Pt1Ni1@graphene). OH radicals were generated through dissociation of hydroperoxide at the catalysts’ surface and detected by fluorescence technique using terephthalic acid as capture reagent, which readily reacts with OH radical to produce highly fluorescent product, 2-hydroxyterephthalic acid. More OH radicals were found to be generated at Pt1Ni1@graphene catalyst. This work may be valuable in the design of electrocatalysts with higher ORR activity but lower efficiency of OH radical generation.

In this work, we synthesized TiO2–graphene composite as a novel preconcentration material. It was enclosed in a microcolumn in the on-line flow injection system to adsorb trace light (La), medium (Tb), and heavy (Ho) rare earth elements (REEs) prior to their determinations by microwave plasma torch-atomic emission spectrometry (MPT-AES). Various experimental parameters, such as sample loading time, sample flow rate, sample pH, eluent flow rate, eluent concentration, and interfering ions, were investigated systematically. Under the optimum conditions, the detection limits (three times of standard deviations of blank by 7 reiterations) of La, Tb, and Ho were found to be 2.2, 1.6, and 2.8 μg L−1, with enrichment factors of 17.1, 11.1, and 10.2, respectively. Relative standard deviations for the determination of the target REEs were 3.6%, 1.3%, and 1.4%, respectively (n = 7). The developed method was validated by the analysis of La, Tb, and Ho in certified reference material (GBW07313, marine sediment) and high purity REE oxide samples.Highlights► TiO2–graphene composite was synthesized and used for a rapid on-line pretreatment procedure coupled with MPT-AES. ► Experimental conditions were optimized to adsorb trace La, Tb, and Ho with the on-line preconcentration system. ► The on-line preconcentration system was applied to determine La, Tb, and Ho in certified reference material and high purity REE oxide samples.

Graphene oxide (GO) obtained from chemical oxidation of flake graphite was derivatized with sulfonic groups to form sulfonic-functionalized GO (GO–SO3−) through four sulfonation routes: through amide formation between the carboxylic group of GO and amine of sulfanilic acid (AA–GO–SO3−), aryl diazonium reaction of sulfanilic acid (AD–GO–SO3−), amide formation between the carboxylic group of GO and amine of cysteamine and oxidation by H2O2 (CA–GO–SO3−), and alkyl diazonium reaction of cysteamine and oxidation by H2O2 (CD–GO–SO3−). Results of Fourier transform infrared spectroscopy and X-ray photoelectrospectrocopy showed that –SO3− groups were attached onto GO. Thermo gravimetric analysis showed that derivatization with sulfonic groups improved thermo stability of GO. X-ray diffraction results indicated that GO–SO3− had more ordered π–π stacking structure than the original GO. GO–SO3− and cationic polyelectrote, poly (diallyldimethylammoniumchloride) (PDDA) were adsorbed at indium tin oxide (ITO) glass surface through layer-by-layer assembling to form (GO–SO3−/PDDA)n/ITO multilayers. After tris-(2,2′-bipyridyl) ruthenium (II) dichloride (Ru(bpy)32+) was incorporated into the multilayers, the obtained Ru(bpy)32+/(GO–SO3−/PDDA)n/ITO electrodes can be used as electrochemiluminescence sensors for detection of organic amine with high sensitivity (limit of detection of 1 nM) and stability.

A dependence of oxygen reduction reaction activity and generation of OH radicals upon the composition of Pt–Co alloy electrocatalyst supported on graphene was found in alkaline solutions.

Pt nanoparticles decorated TiO2 nanotubes (Pt/TiO2NTs) modified electrode has been successfully synthesized by depositing Pt in TiO2NTs, which were prepared by anodization of the Ti foil. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods were adopted to characterize their structures and properties. The Pt/TiO2NTs electrode shows excellent electrocatalytic activity toward methanol oxidation reaction (MOR) in alkaline electrolyte without UV irradiation.

Electrochemical behavior of nitrobenzene (NB) at a bismuth-film modified carbon paste electrode (BiF/CPE) in the presence of cetyltrimethylammonium bromide (CTAB) was investigated by square wave voltammetry. The electrochemical response of NB was apparently improved by CTAB due to the enhanced accumulation of NB at the electrode surface. Operational parameters including the deposition potential and time of bismuth film, pH of solutions and concentration of CTAB were investigated and optimized. Under optimized conditions, the cathodic peak current was proportional to the concentration of NB in the range of 1.0 × 10−6 to 1.0 × 10−4 mol L−1 with the detection limit of 8.3 × 10−7 mol L−1. The proposed BiF/CPE revealed good stability and reproducibility for rapid, simple and sensitive analysis of NB.

A bismuth-film modified graphite nanofibers–Nafion glassy carbon electrode (BiF/GNFs–NA/GCE) was constructed for the simultaneous determination of trace Cd(II) and Pb(II). The electrochemical properties and applications of the modified electrode were studied. Operational parameters such as deposition potential, deposition time, and bismuth ion concentration were optimized for the purpose of determination of trace metal ions in 0.10 M acetate buffer solution (pH 4.5). Under optimal conditions, based on three times the standard deviation of the baseline, the limits of detection were 0.09 μg L−1 for Cd(II) and 0.02 μg L−1 for Pb(II) with a 10 min preconcentration. In addition, the BiF/GNFs–NA/GCE displayed good reproducibility and selectivity, making it suitable for the simultaneous determination of Cd(II) and Pb(II) in real sample such as river water and human blood samples.

Bismuth film electrodes are widely used for determination of heavy metal ions in acidic solutions, while alkaline solutions are rarely employed. We have compared the deposition of Bi(III) and Pb(II) on a Nafion-coated glassy carbon electrode in alkaline and acidic solutions. The results indicate that both Bi(III) and Pb(II) can be deposited in either alkaline or acidic solution, but the quantity of Pb(II) deposited in alkaline solution is less than that in acidic solution. The modified electrode was used to determine heavy metal ions in both alkaline and acidic solutions, and the results of the method agree well with those of atomic absorption spectroscopy.

Here we investigated the analytical performances of the bismuth-modified zeolite doped carbon paste electrode (BiF-ZDCPE) for trace Cd and Pb analysis. The characteristics of bismuth-modified electrodes were improved greatly via addition of synthetic zeolite into carbon paste. To obtain high reproducibility and sensitivity, optimum experimental conditions for bismuth deposition were studied. Voltammetric responses of the BiF-ZDCPEs prepared with different ratios of zeolite, carbon powder, and silicone, were examined under same conditions. The in situ plated (zeolite/graphite powder/silicone, 10/190/80 w/w) BiF-ZDCPEs exhibited the most sensitive response to Cd and Pb in 0.10 M acetate buffer (pH 4.5). The detection limits of the modified electrode were 0.08 μg L−1 for Cd(II) and 0.10 μg L−1 for Pb(II) based on three times the standard deviation of the baseline with a preconcentration time of 120 s under optimal conditions, respectively. The modified electrode showed well linear response to both Cd(II) and Pb(II) over the concentration range from 1.0 to 20.0 μg L−1. The BiF-ZDCPEs were successfully applied to the determination of Cd(II) and Pb(II) in real samples, and the results were in agreement with those of atomic absorption spectroscopy (AAS).

Three-dimensional (3D) macroporous Pt (MPPt) with highly open porous walls has been successfully synthesized using the hydrogen bubble dynamic template synthesis and galvanic replacement reaction. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods were adopted to characterize their structures and properties. The resulting MPPt shows the same morphology as the initial 3D copper. The MPPt modified glassy carbon electrode (MPPt/GCE) exhibits excellent catalytic activity toward methanol oxidation. The present strategy is expected to reduce the cost of the Pt catalyst remarkably.

A novel hydrogen peroxide biosensor was fabricated that is based on horseradish peroxidase–Au nanoparticles immobilized on a viologen-modified glassy carbon electrode (GCE) by amino cation radical oxidation in basic solution. The immobilized BAPV acts as a mediator and a covalent linker between GCE and the Au nanoparticles. The biosensor exhibited fast response, good reproducibility, and long-term stability.

This report describes the preparation of Pt-nanoparticle-coated gold-nanoporous film (PGNF) on a gold substrate via a simple “green” approach. The gold electrode that has been anodized under a high potential of 5 V is reduced by freshly prepared ascorbic acid (AA) solution to obtain gold nanoporous film electrode. Then the Pt nanoparticle is grown on the electrode by cyclic voltammetry (CV). The resulting PGNF electrode has highly ordered arrangement and large surface area, as verified by scanning electron microscopy (SEM) and CV, suggesting that the nanoporous gold film electrode provides a good matrix for obtaining PGNF with high surface area. Furthermore, the as-prepared PGNF electrode exhibited high electrocatalytic activity toward methanol oxidation in a 0.5 M H2SO4 solution containing 1.5 M methanol. The present novel strategy is expected to reduce the cost of the Pt catalyst remarkably.

Using photocatalytic reactive nanoparticles and DNA composite modified gold electrodes, exploiting the sensitivity of DNA-mediated electron transfer to base pair stacking, we examined the effects of DNA oxidation damage by photocatalytically generated hydroxyl radicals (˙OH) on charge transfer efficiency and proposed an electrochemical technique to read the effective diffusing distance of ˙OH.

A dependence of oxygen reduction reaction activity and generation of OH radicals upon the composition of Pt–Co alloy electrocatalyst supported on graphene was found in alkaline solutions.

Electrochemical behavior of nitrobenzene (NB) at a bismuth-film modified carbon paste electrode (BiF/CPE) in the presence of cetyltrimethylammonium bromide (CTAB) was investigated by square wave voltammetry. The electrochemical response of NB was apparently improved by CTAB due to the enhanced accumulation of NB at the electrode surface. Operational parameters including the deposition potential and time of bismuth film, pH of solutions and concentration of CTAB were investigated and optimized. Under optimized conditions, the cathodic peak current was proportional to the concentration of NB in the range of 1.0 × 10−6 to 1.0 × 10−4 mol L−1 with the detection limit of 8.3 × 10−7 mol L−1. The proposed BiF/CPE revealed good stability and reproducibility for rapid, simple and sensitive analysis of NB.