Interfacial superassembly of flower-like NiMn-LDH@poly-l-lysine composites for selective electrochemical sensing of tryptophan.

Development of sensitive, selective, and facile electrochemical analytical approaches to monitor tryptophan in different samples is of high significance. For such approaches, efficient electrodes play the key role. Herein, a NiMn-layered double hydroxide (LDH)@poly-l-lysine (PLL) composite has been prepared through in-situ electro-polymerization and further utilized to monitor tryptophan in medicament and biological systems. The as-synthesized NiMn-LDH@PLL composite is highly stable and features a flower-like morphology. It is found that the electron transfer rate of NiMn-LDH@PLL is greatly improved due to the introduction of PLL. The existing Lewis acid-base interaction between LDH and PLL contributes to the increase of unsaturated metal sites and endows this heterojunction composite with high electrocatalytic activity. Especially, the NiMn-LDH (2:1)@PLL composite displays much improved electrochemical response toward tryptophan sensing. This is due to reasonable balance between the outstanding electrocatalytic ability of nickel and the conductivity improvement caused by manganese species. On the other hand, the coordination promotion between NiMn-LDH and PLL is also conducive to improve the sensitivity of electrochemical response. The fabricated sensor exhibits wide linear response in the concentration ranges of 0.1-40 µM and 40-130 µM for tryptophan detection, along with a low detection limit of 52.7 nM. It further displays a high anti-interference ability for tryptophan monitoring even in the presence of other coexistent amino acids and small-sized biological molecules. This study provides a new way to employ LDHs as brilliant sensing platform to ensemble electrochemical sensors for the monitoring of biomolecules.

Coupling W18 O49 /Ti3 C2 Tx MXene Pseudocapacitive Electrodes with Redox Electrolytes to Construct High-Performance Asymmetric Supercapacitors.

A pseudocapacitive electrode with a large surface area is critical for the construction of a high-performance supercapacitor. A 3D and interconnected network composed of W18 O49 nanoflowers and Ti3 C2 Tx MXene nanosheets is thus synthesized using an electrostatic attraction strategy. This composite effectively prevents the restacking of Ti3 C2 Tx MXene nanosheets and meanwhile sufficiently exposes electrochemically active sites of W18 O49 nanoflowers. Namely, this self-assembled composite owns abundant oxygen vacancies from W18 O49 nanoflowers and enough active sites from Ti3 C2 Tx MXene nanosheets. As a pseudocapacitive electrode, it shows a big specific capacitance, superior rate capability and good cycle stability. A quasi-solid-state asymmetric supercapacitor (ASC) is then fabricated using this pseudocapacitive anode and the cathode of activated carbon coupled with a redox electrolyte of FeBr3 . This ASC displays a cell voltage of 1.8 V, a capacitance of 101 F g-1 at a current density of 1 A g-1 , a maximum energy density of 45.4 Wh kg-1 at a power density of 900 W kg-1 , and a maximum power density of 18 000 W kg-1 at an energy density of 10.8 Wh kg-1 . The proposed strategies are promising to synthesize different pseudocapacitive electrodes as well as to fabricate high-performance supercapacitor devices.

Morphology-dependent sensing performance of CuO nanomaterials.

The morphology of nanomaterials affects their properties and further their applications. Herein, CuO nanomaterials with different morphologies are synthesized, including CuO nanostrips, nanowires and microspheres. After their characterization by means of electron microscopy and X-ray powder diffraction, these CuO nanomaterials are further mixed with graphene nanoplates (GNP) to explore their performance towards electrochemical detection of glucose and tetrabromobisphenol A (TBBPA). Among three composites, the composite of CuO nanostrips and GNP exhibits the largest active surface area, the lowest charge transfer resistance, and the highest accumulation efficiency toward TBBPA. Meanwhile, this composite based non-enzymatic sensor shows superior performance for the glucose monitoring. Since these sensors for the monitoring of both glucose and TBBPA possesses long-term stability, high reproducibility, and wide linear ranges and low detection limits, this work provides a strategy to tune the sensing performance of nanomaterials by means of tailoring the morphologies of nanomaterials.

A UiO-66-NH2/carbon nanotube nanocomposite for simultaneous sensing of dopamine and acetaminophen.

Carbon nanomaterials are quite promising to be combined with metal-organic frameworks (MOFs) to enhance the sensing ability of both materials. In this work, a MOF nanoparticle of UiO-66-NH2 is integrated with carbon nanotubes (CNTs) (UiO-66-NH2/CNTs) with a facile solvothermal method. The morphology, surface area and properties of this UiO-66-NH2/CNTs nanocomposite was investigated using electron microscopy, XRD, XPS, BET analysis and electrochemical techniques. Catalytic oxidation of dopamine (DA) and acetaminophen (AC) on this nanocomposite was achieved, owing to a 3D hybrid structure or a large electroactive surface area, excellent electrical conductivity, a large number of active sites of this nanocomposite. It was further utilized as a sensing platform to establish an electrochemical sensor for the monitoring of both DA and AC. The enhanced oxidation signals led to the voltametric sensing of DA and AC in a broad linear range from 0.03 to 2.0 µM and low detection limits (S/N = 3) of 15 and 9 nM for DA and AC, respectively. The proposed sensor also possessed good reproducibility, repeatability, long-term stability, selectivity, and satisfactory recovery in serum samples analysis. Therefore, it has the great potential for the accurate quantification of DA and AC in complex matrixes.

Morphology-controlled electrochemical sensing of environmental Cd2+ and Pb2+ ions on expanded graphite supported CeO2 nanomaterials.

Heavy metal ions (e.g., Cd2+ and Pb2+) are widely existed in environment and highly toxic. Their convenient, sensitive, and selective determination is thus desirable. In this study, a novel electrochemical sensing interface is developed using the composite of CeO2 nanomaterials supported on expanded graphite as the sensitive materials. As-prepared CeO2 nanomaterials through a hydrothermal method feature different morphologies (e.g., nanocube, nanopolyhedra, and nanorod-shape). Electrocatalytic ability of this interface and sensing performance towards the monitoring of Cd2+ and Pb2+ ions rely on the morphology and structure of used CeO2 nanomaterial. The interface using nanorod-shape CeO2 nanomaterials supported on expanded graphite exhibits superior electrochemical activity, namely remarkable signal enhancement for the monitoring of Cd2+ and Pb2+ ions. The developed electrochemical sensor with this r-CeO2/EG composite as the sensitive material delivers the detection limits of 0.39 and 0.21 µg L-1 for Cd2+ and Pb2+ ions, respectively.

Tailoring the CeO2 morphology and its electrochemical reactivity for highly sensitive and selective determination of dopamine and epinephrine.

Four CeO2 nanomaterials with the morphologies of a nanoplate (CeO2-p), a nanocube (CeO2-c), a porous triangular microplate (CeO2-t), and of a porous hierarchical rod-stacked nanobundle (CeO2-b) were synthesized using a hydrothermal method. They were characterized by scanning and transmission electron microscopies, X-ray diffraction and X-ray photoelectron spectroscopy. Electrochemical characterizations reveal the tuning of their morphology and the presence of exposed crystal planes of CeO2 that can be realized by changing the alkali sources. Among these materials, the CeO2-b features the largest specific surface and lowest electron transfer resistance towards the redox probe Fe(CN)63-/4-. The best voltammetric response to dopamine and epinephrine is thus achieved by using the Nafion-CeO2-b coated electrode. A sensitive and selective method was developed that can voltammetrically detect dopamine (with a peak near 0.13 V vs. SCE), and epinephrine (with a peak near 0.25 V vs. SCE). The detection limits are 2.9 and 0.67 nM, respectively. Graphical abstractSchematic representation of morphology tailoring of CeO2 and electrochemical sensing of dopamine and epinephrine on these CeO2 samples with different morphologies.

Graphene nanoplatelet supported CeO2 nanocomposites towards electrocatalytic oxidation of multiple phenolic pollutants.

To explore suitable sensing materials for sensitive and selective detection of phenolic pollutants, CeO2 nanocubes, nanopolyhedras, and nanorods were synthesized by a hydrothermal method. These CeO2 nanomaterials were further loaded on the support of graphene nanoplatelets. As-synthesized nanomaterials and nanocomposites were characterized using transmission electron microscopy, X-ray diffraction and Raman spectroscopy as well as electrochemical techniques including cyclic voltammetry, electrochemical impedance spectroscopy, and differential pulse voltammetry. The nanocomposite of graphene nanoplatelets with CeO2 nanorods shows the highest electrochemical activity towards soluble species. Highly sensitive and selective determination of tetrabromobisphenol A, catechol, diethylstilbestrol, and nonylphenol was thus achieved at this nanocomposite based electrode. Their limits of detection were as low as 1.8, 42, 1.5 and 2.7 nM, respectively. Such an electrochemical sensor is thus promising for simple, fast and sensitive electrochemical determining of trace-leveled phenolic pollutants in water samples.

Recent Advances of Porous Graphene: Synthesis, Functionalization, and Electrochemical Applications.

Graphene is a 2D sheet of sp2 bonded carbon atoms and tends to aggregate together, due to the strong π-π stacking and van der Waals attraction between different layers. Its unique properties such as a high specific surface area and a fast mass transport rate are severely blocked. To address these issues, various kinds of 2D holey graphene and 3D porous graphene are either self-assembled from graphene layers or fabricated using graphene related materials such as graphene oxide and reduced graphene oxide. Porous graphene not only possesses unique pore structures, but also introduces abundant exposed edges and accelerates mass transfer. The properties and applications of these porous graphenes and their composites/hybrids have been extensively studied in recent years. Herein, recent progress and achievements in synthesis and functionalization of various 2D holey graphene and 3D porous graphene are reviewed. Of special interest, electrochemical applications of porous graphene and its hybrids in the fields of electrochemical sensing, electrocatalysis, and electrochemical energy storage, are highlighted. As the closing remarks, the challenges and opportunities for the future research of porous graphene and its composites are discussed and outlined.

Three-dimensional catalyst systems from expanded graphite and metal nanoparticles for electrocatalytic oxidation of liquid fuels.

Cheap and high-performance electrocatalysts are required for fuel cells. Herein, we present the application of three-dimensional (3D) catalyst systems for electrocatalytic oxidation of formic acid and methanol. These systems consist of cost-effective boron-doped expanded graphite (B-EG) as the support and palladium nanoparticles (NPs) or platinum/palladium bimetal NPs as the catalysts. The characterization of these 3D catalyst systems using scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, and electrochemical techniques reveals that stable and efficient electrocatalytic methanol oxidation, achieved in a 3D catalyst system of B-EG and PdPt bimetal NPs (with a mass ratio of 1 : 1), is due to its big surface area, high conductivity, and an enhanced amount of exposed active sites from bimetal NPs. This price-reduced, stable, and efficient 3D catalyst system is thus promising to be employed for a large scale production of industrial direct methanol fuel cells.

Electrochemical Grafting of Graphene Nano Platelets with Aryl Diazonium Salts.

To vary interfacial properties, electrochemical grafting of graphene nano platelets (GNP) with 3,5-dichlorophenyl diazonium tetrafluoroborate (aryl-Cl) and 4-nitrobenzene diazonium tetrafluoroborate (aryl-NO2) was realized in a potentiodynamic mode. The covalently bonded aryl layers on GNP were characterized using atomic force microscopy and X-ray photoelectron spectroscopy. Electrochemical conversion of aryl-NO2 into aryl-NH2 was conducted. The voltammetric and impedance behavior of negatively and positively charged redox probes (Fe(CN)63-/4- and Ru(NH3)62+/3+) on three kinds of aryl layers grafted on GNP reveal that their interfacial properties are determined by the charge states of redox probes and reactive terminal groups (-Cl, -NO2, -NH2) in aryl layers. On aryl-Cl and aryl-NH2 garted GNP, selective and sensitive monitoring of positively charged lead ions as well as negatively charged nitrite and sulfite ions was achieved, respectively. Such a grafting procedure is thus a perfect way to design and control interfacial properties of graphene.

High-Performance Electrochemical Catalysts Based on Three-Dimensional Porous Architecture with Conductive Interconnected Networks.

The electrochemical applications of traditional carbon nanomaterials such as carbon nanotubes (CNTs) and graphene (G) powders are significantly impeded by their poor three-dimensional (3D) conductivity and lack of hierarchical porous structure. Here, we have constructed a 3D highly conductive CNTs networks and further combined it with mesoporous carbon (mC) for the creation of a core-shell structured (CNT@mC) composite sponge that featured 3D conductivity and hierarchical porous structure. In the composite sponge, interconnected CNTs efficiently eliminates the contact resistance and the hierarchical pores significantly facilitate the mass transport. The electron transfer rates, electroactive surface area and catalytic activity of the CNT@mC composite sponge based catalysts were tested in the direct methanol fuel cells (DMFCs) and electrochemical sensors. In DMFCs, the Pd nanoparticles deposited CNT@mC showed significantly improved catalytic activity and methanol oxidization current. As for amperometric sensing of endocrine disrupting compounds (EDCs), CNT@mC-based catalyst gave a liner range from 10 nM to 1 mM for bisphenol A (BPA) detection and showed great promise for simultaneous detection of multiple EDCs. BPA recovery from environmental water further indicated the potential practical applications of the sensor for BPA detection. Finally, the electrochemical performance of CNT@mC were also investigated in impedimetric sensors. Good selectivity was obtained in impedimetric sensing of BPA and the detection limit was measured to be 0.3 nM. This study highlighted the exceptional electrochemical properties of the CNT@mC composite sponge enabled by its 3D conductivity and hierarchical porous structure. The strategy described may further pave a way for the creation of novel functional materials through integrating multiple superior properties into a single nanostructure for future clean energy technologies and environmental monitoring systems.

Universal electrode interface for electrocatalytic oxidation of liquid fuels.

Electrocatalytic oxidations of liquid fuels from alcohols, carboxylic acids, and aldehydes were realized on a universal electrode interface. Such an interface was fabricated using carbon nanotubes (CNTs) as the catalyst support and palladium nanoparticles (Pd NPs) as the electrocatalysts. The Pd NPs/CNTs nanocomposite was synthesized using the ethylene glycol reduction method. It was characterized using transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, voltammetry, and impedance. On the Pd NPs/CNTs nanocomposite coated electrode, the oxidations of those liquid fuels occur similarly in two steps: the oxidations of freshly chemisorbed species in the forward (positive-potential) scan and then, in the reverse scan (negative-potential), the oxidations of the incompletely oxidized carbonaceous species formed during the forward scan. The oxidation charges were adopted to study their oxidation mechanisms and oxidation efficiencies. The oxidation efficiency follows the order of aldehyde (formaldehyde) > carboxylic acid (formic acid) > alcohols (ethanol > methanol > glycol > propanol). Such a Pd NPs/CNTs nanocomposite coated electrode is thus promising to be applied as the anode for the facilitation of direct fuel cells.

Versatile matrix for constructing enzyme-based biosensors.

A versatile matrix was fabricated and utilized as a universal interface for the construction of enzyme-based biosensors. This matrix was formed on the gold electrode via combining self-assembled monolayer of 2,3-dimercaptosuccinic acid with gold nanoparticles. Gold nanoparticles were electrochemically deposited. Electrochemistry of three redox enzymes (catalase, glucose oxidase, and horseradish peroxidase) was investigated on such a matrix. The electrocatalytic monitoring of hydrogen peroxide and glucose was conducted on this matrix after being coated with those enzymes. On them the monitoring of hydrogen peroxide and glucose shows rapid response times, wide linear working ranges, low detection limits, and high enzymatic affinities. This matrix is thus a versatile and suitable platform to develop highly sensitive enzyme-based biosensors.

In situ synthesized gold nanoparticles for direct electrochemistry of horseradish peroxidase.

In order to construct a three-dimensional electrode, in situ electrochemical deposition of gold nanoparticles onto a gold electrode coated with a self-assembled monolayer of 3-mercaptopropionic acid (Au NP/MPA/Au) was conducted. Horseradish peroxidase (HRP) was then immobilized into this three-dimensional electrode, leading to the realization of direct electron transfer of HRP. Scanning electron microscopy and electrochemical impedance spectroscopy were applied to characterize the electrode before and after HRP immobilization. The apparent Michaelis-Menten constant for immobilized HRP on a Au NP/MPA/Au electrode is 0.78 mM, indicating high enzymatic activity of HRP. The HRP modified electrode was further utilized as a sensing platform for the electrocatalytic reduction and detection of hydrogen peroxide. For the detection of hydrogen peroxide on this electrode, the sensitivity is 311.72 µA mM(-1) cm(-2), the detection limit is 0.16 µM, and the dynamic range is from 0.48 µM to 1.2 mM. Therefore this electrode is a promising device for sensitive and reproducible detection of H(2)O(2).

Graphene nanoplatelets: electrochemical properties and applications for oxidation of endocrine-disrupting chemicals.

In most graphene-based electrochemical applications, graphene nanoplatelets (GNPs) have been applied. Now, for the first time, electrochemical properties of GNPs, namely, its electrochemical activity, potential window, and double-layer capacitance, have been investigated. These properties are compared with those of carbon nanotubes (CNTs). GNP- and CNT-coated electrodes were then applied for electrochemical oxidation of endocrine-disrupting chemicals. The GNP-coated electrode was characterized by atomic force microscopy and electrochemical techniques. Compared with the CNT-coated electrode, higher peak current for the oxidation of 4-nonylphenol is achieved on the GNP-coated electrode, together with lower capacitive current. Electrochemical oxidation of 2,4-dichlorophenol, bisphenol A, and octylphenol in the absence or presence of 4-nonylphenol was studied on the GNP-coated electrode. The results suggest that GNPs have better electrochemical performance than CNTs and are thus more promising for electrochemical applications, for example, electrochemical detection and removal of endocrine-disrupting chemicals.

Bucky-gel coated glassy carbon electrodes, for voltammetric detection of femtomolar leveled lead ions.

Femtomolar (fM) leveled lead ions were electrochemically detected using a bucky-gel coated glassy carbon electrode and differential pulse anodic stripping voltammetry. The bucky-gel was composed of dithizone, ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate), and multi-walled carbon nanotubes (MWCNTs). The fabrication of the bucky-gel coated electrode was optimized. The modified electrode was characterized with voltammetry, electrochemical impedance spectroscopy, and chronoamperometry. After the accumulation of lead ions into the bucky-gel modified electrode at -1.2V vs. saturated calomel electrode (SCE) for 5 min in a pH 4.4 sodium acetate-acetate acid buffer solution, differential pulse anodic stripping voltammograms of the accumulated lead show an anodic wave at -0.58 V. The anodic peak current is detectable for lead ions in the concentration range from 1.0 µM down to 500 fM. The detection limit is calculated to be 100 fM. The proposed method was successfully applied for the detection of lead ions in lake water.

Voltammetric determination of mercury (II) in aqueous media using glassy carbon electrodes modified with novel calix[4]arene.

A novel calix[4]arene derivative containing benzothiazole group was coated on glassy carbon electrode (GCE) and then applied to the recognition of mercury ion. Cyclic and square wave voltammetric results showed that the modified electrode selectively recognizes Hg(2+) ion in aqueous media. A new anodic stripping peak at -0.3 V (vs. Ag/Ag(+)) can be obtained by scanning the potential from -0.6 to 0.6 V, and the peak currents are proportional to the Hg(2+) concentration. The modified electrode in a 0.1 M H(2)SO(4)+0.01 M NaCl solution shows linear voltammetric response in the range of 25-300 mug l(-1) and detection limit of 5 mug l(-1) (ca. 2.5x10(-8) M). This modified GCE does not present any significant interference from alkali, alkaline and transition metal ions except for Pb(2+), Ag(+) and Cu(2+) ions. Only 500, 50 and 100-fold molar excess of Pb(2+), Ag(+) and Cu(2+) ions, respectively, can lead to voltammetric response comparable with that of Hg(2+). The proposed method was successfully applied to determine mercury in natural water.

Electrochemical behavior of thiamine on a self-assembled gold electrode and its square-wave voltammetric determination in pharmaceutical preparations.

The electrochemical behavior of thiamine on a self-assembled electrode of L-cysteine (Cys/SAM/Au) has been investigated and Cys/SAM/Au can be used to detect thiamine using square-wave voltammetry (SWV). At pH 11.40 Britton-Robinson buffer, thiamine exhibits a well-defined anodic peak on Cys/SAM/Au. Under the optimized conditions, the anodic peak current of SWV was linear with the content of thiamine in the range of 1.1 x 10(-8) - 2.2 x 10(-6) mol/L; the detection limit was 5.5 x 10(-9) mol/L. The method was successfully applied to the determination of thiamine in pharmaceutical preparations.