S-doped reduced graphene oxide: a novel peroxidase mimetic and its application in sensitive detection of hydrogen peroxide and glucose.

This article presents a novel peroxidase mimetic by doping S atoms into reduced graphene oxide (rGO), which was synthesized through a facile hydrothermal reaction without any templates or surfactants. The peroxidase-like activity of S-doped rGO (S-rGO) is greatly boosted compared with the pristine rGO, demonstrating the peroxidase-like active sites are dominantly originated in sulfur-containing groups. The steady-state kinetic studies further indicate that S-rGO obeys the typical Michaelis-Menten curves and has a much smaller Michaelis constant (Km) for hydrogen peroxide (H2O2) and 3, 3', 5, 5'-tetramethylbenzidine (TMB). In view of the outstanding performance of S-rGO as a peroxidase mimetic, an efficient and sensitive colorimetric detection platform for H2O2 and glucose has been successfully established. The linear detection for H2O2 is obtained in a range of 0.1-1 µM with an extremely lower detection limit of 0.042 µM, and glucose can be measured in a linear range of 1-100 µM, giving a detection limit of 0.38 µM. This study not only provides a new avenue for the reasonable design of heteroatom-doped carbon-based nanomaterials but also offers meaningful reference for detecting the important biomolecules in biotechnology. Graphical abstract.

The influence of oxidation debris containing in graphene oxide on the adsorption and electrochemical properties of 1,10-phenanthroline-5,6-dione.

It is gradually accepted that graphene oxide, which is derived from the exfoliation of graphite oxide that is synthesized by the chemical oxidation of graphite, actually consists of partially oxidized graphene sheets and highly oxidized carbonaceous debris. The quantity of oxidation debris comprises around one third of the total mass of the graphene oxide. The presence of oxidation debris has a significant impact on the physical and chemical properties of graphene oxide. In this article, we address the influence of the oxidation debris on the surface properties of graphene oxide. We discovered that the surface adsorption of organic molecules on graphene oxide was improved greatly after the elimination of the oxidation debris. A typical redox mediator, 1,10-phenanthroline-5,6-dione, was studied as the model adsorbed molecule in terms of its adsorption quantity and electrochemical response. After removing the oxidation debris, a five-fold increase in adsorption capacity is achieved on the same amount of graphene oxide. Correspondingly, the electrochemical response for the oxidation of NADH mediated by the adsorbed 1,10-phenanthroline-5,6-dione was enhanced as well, which led to improved analytical performances in terms of the sensitivity, linear range and detection limit for the purified graphene oxide modified electrode.

Electrocatalytic Activities of Chemically Reduced Graphene Are Essentially Dominated by the Adhered Carbonaceous Debris.

Due to its simple, scalable, and facile qualities, the chemical reduction of graphene oxide seems to be the most popular approach to prepare graphene. We show that such prepared graphene is strongly adhered with carbonaceous debris that has been produced during the synthesis of graphene oxide by the chemical exfoliation of graphite and still remain on graphene sheets through the chemical reduction steps. Interestingly, the presence of the carbonaceous debris causes a significant impact on the electrochemical behavior of the chemical reduced graphene. Herein, we demonstrate that the electrocatalytical activities of the graphene are greatly boosted by the adhered carbonaceous debris. After the removal of the carbonaceous debris, the electrocatalysis of graphene is not superior to conventional graphite.

The significant role of carboxylated carbonaceous fragments in the electrochemistry of carbon nanotubes.

Carbon nanotubes (CNTs) have been widely employed as electrode materials in diverse branches of electrochemistry, which are claimed to display dramatically improved electrochemical behaviour compared to the conventional carbon materials. But a series of recent publications have demonstrated that the electrocatalysis of CNTs might be due to the presence of some impurities, such as metallic catalysts, nanographitic particles and amorphous carbon. For this reason, CNTs are usually purified or treated with nitric acid or nitric and sulphuric acid prior to their versatile applications. However, the strong acidic and oxidative conditions are so aggressive that serious erosion of the tube structures has inevitably taken place, which creates defects on the sidewalls and gives rise to numerous molecular byproducts, commonly referred as carboxylated carbonaceous fragments (CCFs). The adsorption of CCFs on CNTs greatly alters the surface conditions of CNTs which may significantly impact on their electrochemical properties. To this end, we wish to disclose whether the electrocatalysis of the nitric acid purified CNTs is affected by the adsorption of the CCFs. Ascorbic acid (AA) and ß-nicotinamide adenine dinucleotide (NADH) as selected as the targeting benchmarks that are known to be insensitive to the presence of metallic impurities, which may guarantee the preclusion of the promoting contributions from the metallic catalysts resident in CNTs. We have demonstrated that the electrocatalytic activities of the CNTs are actually dominated by the adsorbed CCFs generated during the acidic pre-treatment. After removal of the CCFs by base rinse, the electrocatalytic properties of CNTs are greatly deteriorated and degraded to the level similar to the conventional graphite powder. We believe this finding is particularly meaningful to uncover the mysterious electrocatalysis of CNTs.

Electrochemical biosensors based on redox carbon nanotubes prepared by noncovalent functionalization with 1,10-phenanthroline-5,6-dione.

To improve the electrocatalytic activities of carbon nanotubes (CNT) towards the oxidation of nicotinamide adenine dinucleotide (NADH), we derive them with a redox mediator, 1,10-phenanthroline-5,6-dione (PD), by the noncovalent functionalization method. The redox carbon nanotubes (PD/CNT/GC) show excellent electrocatalytic activities towards the oxidation of NADH (catalytic reaction rate constant, k(h) = 7.26 × 10(3) M(-1) s(-1)), so the determination of NADH can be achieved with a high sensitivity of 8.77 µA mM(-1) under the potential of 0.0 V with minimal interference. We also develop an amperometric ethanol biosensor by integration of alcohol dehydrogenase (ADH) within the redox carbon nanotubes (PD/CNT/GC). The ethanol biosensor exhibits a wide linear range up to 7 mM with a lower detection limit of 0.30 mM as well as a high sensitivity of 10.85 nA mM(-1).

Use of cyclodextrin-modified gold nanoparticles for enantioseparations of drugs and amino acids based on pseudostationary phase-capillary electrochromatography.

The application of chemical-modified gold nanoparticles (GNPs) as chiral selector for the enantioseparation based on pseudostationary phase-CEC (PSP-CEC) is presented. GNPs modified by thiolated beta-CD were characterized by NMR and FT-IR. The nanoparticle size was determined to be of 9.5 nm (+2.5 nm) by Transmission Electron Microscopy (TEM) and UV spectra. Four pairs of dinitrophenyl-labeled amino acid enantiomers (DL-Val, Leu, Glu and Asp) and three pairs of drug enantiomers (RS-chlorpheniramine, zopiclone and carvedilol) were analyzed by using modified GNPs as the chiral selector in PSP-CEC. Good theoretical plate number (up to 2.4x10(5) per meter) and separation resolution (up to 4.7) were obtained even with low concentration of modified GNPs (0.8-1.4 mg/mL). The corresponding concentration of beta-CD in the buffer was only 0.30-0.53 mM, which was much lower than the optimum concentration of 15 mM if pure beta-CD was used as chiral selector. Our results showed that thiolated beta-CD modified GNPs have more sufficient interaction with the analytes, resulting in significant enhancement of enantioseparation. The study shed light on potential usage of chemical modified GNPs as chiral selector for enantioseparation based on PSP-CEC.

Dual-enzyme, co-immobilized capillary microreactor combined with substrate recycling for high-sensitive glutamate determination based on CE.

A new method for high-sensitive determination of glutamate was developed and evaluated based on CE by using dual-enzyme co-immobilized capillary microreactor combined with substrate recycling. The capillary microreactor was prepared by covalently co-immobilizing glutamate dehydrogenase (GDH) and glutamic pyruvic transaminase (GPT) on the inner surface of a capillary and was characterized by SEM, ultraviolet-visible spectroscopy, and fluorescence spectroscopy. The GDH-GPT co-immobilized capillary microreactor showed great stability and reproducibility. The apparent K(m) for glutamate with GDH-GPT coupled reaction was determined to be 0.61+/-0.06 mM but 2.56+/-0.24 mM when only GDH was immobilized. Glutamate determination was based on on-column monitoring UV absorption at 340 nm of the reaction product reduced nicotinamide adenine dinucleotide, of which peak area was directly related to the glutamate concentration. The response of the present co-immobilized GDH-GPT assay for glutamate is greatly enhanced over single enzyme system, and a 15.7-fold improvement in sensitivity was obtained. The detection limit of the proposed method is 0.15 muM glutamate (S/N=3). Selectivity for glutamate is good over most of the 20 amino acids. Finally, this method was successfully applied to determine the glutamate content in rat plasma and serum samples.

Bimetal- and nitrogen-codoped spherical porous carbon with efficient catalytic performance towards oxygen reduction reaction in alkaline media.

Nonprecious-metal electrocatalysts have been intensively investigated, but how to keep a balance between their sustainability and competitive performance for the oxygen reduction reaction (ORR) is still a big challenge for energy applications. In this work, a type of bimetal- and nitrogen-codoped mesoporous carbon electrocatalyst (FeCo-N/C-800) was successfully synthesized via a simple hydrothermal method, followed by a calcination process at 800 °C in nitrogen atmosphere and an acid-etching process. The obtained FeCo-N/C-800 can act as a high-performance ORR catalyst with high onset and half-wave potentials (0.94 and 0.85 V), a large limiting current density (5.94 mA cm-2) and a four-electron reaction process in 0.1 M KOH solution, which can be comparable with commercial Pt/C catalyst. Additionally, the FeCo-N/C-800 exhibited superior long-term durability and high methanol tolerance. The excellent electrocatalytic performance of the FeCo-N/C-800 can be ascribed to the mesoporous structure (bimodal pores system), the synergetic interaction of the multiple ORR active sites, suitable N-doping level and the highly conductive carbon matrix. These structural features can promote efficiently the mass and electron transfer, provide abundant active sites for the adsorption and reaction of oxygen molecules, and thus improve the reaction kinetics. The present study not only provides a strategy for the synthesis of carbon-based electrocatalyst with high ORR catalytic activity and stability, but also demonstrates that the bimetal- and nitrogen-codoped carbon materials could be a class of competitive candidate for non-noble metal-based electrocatalysts.

Bienzymatic glucose biosensor based on co-immobilization of peroxidase and glucose oxidase on a carbon nanotubes electrode.

A bienzymatic glucose biosensor was proposed for selective and sensitive detection of glucose. This mediatorless biosensor was made by simultaneous immobilization of glucose oxidase (GOD) and horseradish peroxidase (HRP) in an electropolymerized pyrrole (PPy) film on a single-wall carbon nanotubes (SWNT) coated electrode. The amperometric detection of glucose was assayed by potentiostating the bienzymatic electrode at -0.1 versus Ag/AgCl to reduce the enzymatically produced H(2)O(2) with minimal interference from the coexisting electroactive compounds. The single-wall carbon nanotubes, sandwiched between the enzyme loading polypyrrole (PPy) layer and the conducting substrate (gold electrode), could efficiently promote the direct electron transfer of HRP. Operational characteristics of the bienzymatic sensor, in terms of linear range, detection limit, sensitivity, selectivity and stability, were presented in detail.

Electrocatalytic oxidation of NADH with Meldola's blue functionalized carbon nanotubes electrodes.

Meldola's blue (MB) functionalized carbon nanotubes (CNT) nanocomposite film (MB/CNT) electrode was prepared by non-covalent adsorbing MB on the surface of a carbon nanotubes modified glassy carbon electrode (CNT/GCE). Electrochemical behaviors of the resulting electrode were investigated thoroughly with cyclic voltammetry in the potential range of -0.6 to 0.2V, and two well-defined redox couples were clearly visualized. We also studied the electron transfer kinetics of MB loaded on CNT (MB/CNT) in comparison with that of MB on conventional graphite powder (MB/GP). The heterogeneous electron transfer rate constant (k(s)) of MB/CNT was calculated to be about three times larger than that of MB/GP. The accelerated electron transfer kinetics was attributed to the unique electrical and nanostructural properties of CNT supports as well as the interaction between MB and CNT. In connection with the oxidation of nicotinamide adenine dinucleotide (NADH), excellent electrocatalytic activities were observed at MB/CNT/GCE compared with MB/GP modified glassy carbon electrode (MB/GP/GCE). Based on the results, a new NADH sensor was successfully established using the MB/CNT/GCE. Under a lower operation potential of -0.1V, NADH could be detected linearly up to a concentration of 500 microM with an extremely lower detection limit of 0.048+/-0.02 microM estimated at a signal-to-noise ratio of 3. Sensitivity, selectivity, reproducibility and stability of the NADH sensor were also investigated and the main analytical data were also compared with those obtained with the MB/GP/GCE.

Electrochemiluminescent determination of L-cysteine with a flow-injection analysis system.

A flow-injection electrochemiluminescent method for L-cysteine determination has been developed based on its enhancement of the electrochemiluminecence of luminol at a glassy carbon electrode. This method is simple and sensitive for cysteine determination. Under the selected experimental parameters, the linear range for cysteine concentration was 1.0 x 10(-6) - 5.0 x 10(-5) mol/l, and the detection limit was 0.67 micromol/l (S/N = 3). The relative standard deviation for 11 measurements of 1.0 x 10(-5) mol/l cysteine was 4.5%. The proposed method has been applied to the detection of cysteine in pharmaceutical injections with satisfactory results.

Metal organic frameworks/macroporous carbon composites with enhanced stability properties and good electrocatalytic ability for ascorbic acid and hemoglobin.

The thermal, water and electrochemical stability of Cu-based metal organic frameworks (Cu-MOFs) confined in macroporous carbon (MPC) hybrids has been investigated. Thermogravimetric analyses, X-Ray diffraction, scanning electron microscopy, and cyclic voltammetry were employed to confirm the stability of pure Cu-MOFs, MPC, and Cu-MOFs-MPC. As compared to pure Cu-MOFs, the porous composite materials of MPC and Cu-MOFs interact and seem to form new materials having homogenous structure and chemistry, which show structural stability in aqueous media and electrochemical stability in phosphate buffer solution (PBS pH 7.4). The detection of ascorbic acid and hemoglobin is performed as an electrochemical probe, indicating Cu-MOFs-MPC holds great promise for the design of electrochemical sensors.