The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

State of the art in gold nanoparticle synthesis

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters. Over the last decade, different nanomaterials have been exploited to design highly efficient biosensors for the detection of analyte biomolecules. The discovery of graphene has spectacularly accelerated research on fabricating low-cost electrode materials because of its unique physical properties, including high specific surface area, high carrier mobility, high electrical conductivity, flexibility, and optical transparency. Graphene and its oxygenated derivatives, including graphene oxide (GO) and reduced graphene oxide (rGO), are becoming an important class of nanomaterials in the field of biosensors. The presence of oxygenated functional groups makes GO nanosheets strongly hydrophilic, facilitating chemical functionalization. Graphene, GO and rGO nanosheets can be easily combined with various types of inorganic nanoparticles, including metals, metal oxides, semiconducting nanoparticles, quantum dots, organic polymers and biomolecules, to create a diverse range of graphene-based nanocomposites with enhanced sensitivity for biosensor applications. This review summarizes the advances in two-dimensional (2D) and three-dimensional (3D) graphene-based nanocomposites as emerging electrochemical and fluorescent biosensing platforms for the detection of a wide range of biomolecules with enhanced sensitivity, selectivity and a low limit of detection. The biofunctionalization and nanocomposite formation processes of graphene-based materials and their unique properties, surface functionalization, enzyme immobilization strategies, covalent immobilization, physical adsorption, biointeractions and direct electron transfer (DET) processes are discussed in connection with the design and fabrication of biosensors. The enzymatic and nonenzymatic reactions on graphene-based nanocomposite surfaces for glucose- and cholesterol-related electrochemical biosensors are analyzed. This review covers a very broad range of graphene-based electrochemical and fluorescent biosensors for the detection of glucose, cholesterol, hydrogen peroxide (H2O2), nucleic acids (DNA/RNA), genes, enzymes, cofactors nicotinamide adenine dinucleotide (NADH) and adenosine triphosphate (ATP), dopamine (DA), ascorbic acid (AA), uric acid (UA), cancer biomarkers, pathogenic microorganisms, food toxins, toxic heavy metal ions, mycotoxins, and pesticides. The sensitivity and selectivity of graphene-based electrochemical and fluorescent biosensors are also examined with respect to interfering analytes present in biological systems. Finally, the future outlook for the development of graphene based biosensing technology is outlined.

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A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors

Graphene production via electrochemical reduction of graphene oxide: Synthesis and characterisation

Due to the outstanding physicochemical properties arising from its truly two-dimensional (2D) planar structure with a single-atom thickness, graphene exhibits great potential for use in sensors, catalysts, electrodes, and in biological applications, etc. With further developments in the theoretical understanding and assembly techniques, graphene should enable great changes both in scientific research and practical industrial applications. By the look of development, it is of fundamental and practical significance to translate the novel physical and chemical properties of individual graphene nanosheets into the macroscale by the assembly of graphene building blocks into macroscopic architectures with structural specialities and functional novelties. The combined features of a 2D planar structure and abundant functional groups of graphene oxide (GO) should provide great possibilities for the assembly of GO nanosheets into macroscopic architectures with different macroscaled shapes through various assembly techniques under different bonding interactions. Moreover, macroscopic graphene frameworks can be used as ideal scaffolds for the incorporation of functional materials to offset the shortage of pure graphene in the specific desired functionality. The advantages of light weight, supra-flexibility, large surface area, tough mechanical strength, and high electrical conductivity guarantee graphene-based architectures wide application fields. This critical review mainly addresses recent advances in the design and fabrication of graphene-based macroscopic assemblies and architectures and their potential applications. Herein, we first provide overviews of the functional macroscopic graphene materials from three aspects, i.e., 1D graphene fibers/ribbons, 2D graphene films/papers, 3D network-structured graphene monoliths, and their composite counterparts with either polymers or nano-objects. Then, we present the promising potential applications of graphene-based macroscopic assemblies in the fields of electronic and optoelectronic devices, sensors, electrochemical energy devices, and in water treatment. Last, the personal conclusions and perspectives for this intriguing field are given.

Graphene-based macroscopic assemblies and architectures: an emerging material system.

We report a simple, facile, and reproducible method for the fabrication of electrochemically reduced graphene oxide (ERGO) films on glassy carbon electrode (GCE) by the self-assembly method. The graphene precursor, graphene oxide (GO), was self-assembled on GCE through a diamine linker which was preassembled on GCE by electrostatic interaction between the positively charged amine and the negatively charged layers of graphene oxide (GO). The oxygen functional groups present on the surface of GO were electrochemically reduced to retain the aromatic backbone of graphene. The attachment of GO followed by its electrochemical reduction was confirmed by ATR-FT-IR spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Raman spectra show that the intensity ratio of D and G bands was increased after the electrochemical reduction of GO. XPS results reveal that the carbon-to-oxygen ratio was increased af...

Fabrication of Electrochemically Reduced Graphene Oxide Films on Glassy Carbon Electrode by Self-Assembly Method and Their Electrocatalytic Application

Simultaneous determination of uric acid, xanthine, hypoxanthine and caffeine in human blood serum and urine samples using electrochemically reduced graphene oxide modified electrode.

The attachment of nitrogen-doped graphene (NG) on glassy carbon electrode (GCE) followed by electrodeposition of copper nanostructures (CuNSs) is described in this paper. Nitrogen-doped graphene oxide (N-GO) was prepared by intercalating melamine into graphene oxide (GO) by sonication. The doping of nitrogen was confirmed from the characteristic peaks at 285.3 and 399 eV in the XPS corresponding to the C–N bond and nitrogen, respectively. The presence of amine groups on the N-GO was exploited to attach them on GCE via Michael’s reaction. Subsequently, N-GO was electrochemically reduced to form NG by reducing the oxygen functionalities present on the N-GO. Then, the CuNSs on the NG modified electrode was prepared by electrodeposition at various applied potentials with different deposition times. The homogeneous deposition of cubic, spherical, quasidendritic, and dendritic NS at the applied potentials of 0, −0.10, −0.30, and −0.40 V, respectively, was evidenced from scanning electron microscopy (SEM) studie...

Nitrogen-Doped Graphene as a Robust Scaffold for the Homogeneous Deposition of Copper Nanostructures: A Nonenzymatic Disposable Glucose Sensor

In this paper, electrochemical determination of norepinephrine (NE) and theophylline (TP) was demonstrated using an electrochemically reduced graphene oxide (ERGO) modified glassy carbon electrode (GCE). The ERGO was fabricated on a GCE by self-assembling graphene oxide (GO) on 1,6-hexadiamine (HDA) which was preassembled on the GCE, followed by the electrochemical reduction of GO. SEM imaging shows a flake-like structure for the GO modified substrate, whereas an aggregated structure was observed for the ERGO modified substrate. Then, the ERGO modified GCE was used for the determination of NE and TP. It was found that the ERGO modified electrode not only enhanced the oxidation currents of NE and TP, but also prevented a surface fouling effect caused by their oxidation products. In contrast, the oxidation currents of both NE and TP were low at bare and GO modified GCEs and very importantly, their oxidation signals were not stable due to the surface fouling effect. The ERGO modified GCE separated the oxidation signals of NE and TP in a mixture and was hence used for their simultaneous determination. Selective determination of NE in the presence of a higher concentration of TP was also achieved at an ERGO modified GCE. Using amperometry, a detection of 40 nM NE and 50 nM TP was obtained. The current responses of NE and TP were increased linearly while increasing their concentrations from 4 × 10−8 to 1 × 10−4 M and 5 × 10−8 to 4 × 10−5 M and the detection limits were found to be 8.4 × 10−10 M and 2.9 × 10−9 M (S/N = 3), respectively. The practical application of the present modified electrode was demonstrated by measuring the amounts of NE and TP in injections and tablets.

Graphene layer modified glassy carbon electrode for the determination of norepinephrine and theophylline in pharmaceutical formulations

In this paper, an electrochemically reduced graphene oxide–gold nanoparticle (ERGO–AuNP) composite film was fabricated on a glassy carbon electrode (GCE) by a simple electroless deposition method using a solution containing HAuCl4 and NH2OH. The deposition of AuNPs on the ERGO film was achieved via two different approaches. The first approach involves the electroless deposition of AuNPs on the graphene oxide (GO) modified GCE followed by the electrochemical reduction. The second approach is the electroless deposition of AuNPs on the ERGO film modified GCE which was fabricated by self-assembling GO on a 1,6-hexadiamine modified GCE followed by the electrochemical reduction. The particle coverage estimated from cyclic voltammetry (CV) showed that the particle coverage of AuNPs deposited on the ERGO film (22%) was higher than that of the GO film (17%) and bare GCE (7%) under identical conditions. The obtained higher coverage is attributed to the ERGO film's ability to spontaneously reduce Au3+ ions. Although AuNP deposition was observed at the ERGO surface in the absence of NH2OH, the particle coverage was much less (2%) and hence the electroless deposition was carried out in the presence of NH2OH. SEM and CV studies showed that the particle coverage and density of AuNPs were increased while increasing the electroless deposition time.

M. Amal Raj

Papers

Fabrication of Electrochemically Reduced Graphene Oxide Films on Glassy Carbon Electrode by Self-Assembly Method and Their Electrocatalytic Application

Simultaneous determination of uric acid, xanthine, hypoxanthine and caffeine in human blood serum and urine samples using electrochemically reduced graphene oxide modified electrode.

Nitrogen-Doped Graphene as a Robust Scaffold for the Homogeneous Deposition of Copper Nanostructures: A Nonenzymatic Disposable Glucose Sensor

Graphene layer modified glassy carbon electrode for the determination of norepinephrine and theophylline in pharmaceutical formulations

Assembly of gold nanoparticles on graphene film via electroless deposition: spontaneous reduction of Au3+ ions by graphene film