Hongcai Gao

High-performance asymmetric supercapacitor based on graphene hydrogel and nanostructured MnO2.

We have successfully fabricated an asymmetric supercapacitor with high energy and power densities using graphene hydrogel (GH) with 3D interconnected pores as the negative electrode and vertically aligned MnO(2) nanoplates on nickel foam (MnO(2)-NF) as the positive electrode in a neutral aqueous Na(2)SO(4) electrolyte. Because of the desirable porous structure, high specific capacitance and rate capability of GH and MnO(2)-NF, complementary potential window of the two electrodes, and the elimination of polymer binders and conducting additives, the asymmetric supercapacitor can be cycled reversibly in a wide potential window of 0-2.0 V and exhibits an energy density of 23.2 Wh kg(-1) with a power density of 1.0 kW kg(-1). Energy density of the asymmetric supercapacitor is significantly improved in comparison with those of symmetric supercapacitors based on GH (5.5 Wh kg(-1)) and MnO(2)-NF (6.7 Wh kg(-1)). Even at a high power density of 10.0 kW kg(-1), the asymmetric supercapacitor can deliver a high energy density of 14.9 Wh kg(-1). The asymmetric supercapacitor also presents stable cycling performance with 83.4% capacitance retention after 5000 cycles.

Mussel-inspired synthesis of polydopamine-functionalized graphene hydrogel as reusable adsorbents for water purification.

We present a one-step approach to polydopamine-modified graphene hydrogel, with dopamine serving as both reductant and surface functionalization agents. The synthetic method is based on the spontaneous polymerization of dopamine and the self-assembly of graphene nanosheets into porous hydrogel structures. Benefiting from the abundant functional groups of polydopamine and the high specific surface areas of graphene hydrogel with three-dimensional interconnected pores, the prepared material exhibits high adsorption capacities toward a wide spectrum of contaminants, including heavy metals, synthetic dyes, and aromatic pollutants. Importantly, the free-standing graphene hydrogel can be easily removed from water after adsorption process, and can be regenerated by altering the pH values of the solution for adsorbed heavy metals or using low-cost alcohols for synthetic dyes and aromatic molecules.

One-step electrochemical synthesis of PtNi nanoparticle-graphene nanocomposites for nonenzymatic amperometric glucose detection.

We report a facile one-step ultrasonication-assisted electrochemical method to synthesize nanocomposites of graphene and PtNi alloy nanoparticles (NPs) and their uses for highly selective nonenzymatic glucose detection. We have demonstrated that the obtained nanocomposites exhibit a collection of unique features including well-dispersed NPs with alloy features, high NP loading, and effective reduction of graphene oxide (GO). And the resulting nanoelectrocatalyst shows significantly improved electrochemical performance in nonenzymatic amperometric glucose detection, compared to a number of control electrode materials including the PtNi NP-chemically reduced GO nanocomposites fabricated in two steps (chemical reduction of GO followed by the electrodeposition of metal NPs). Under the physiological condition, the response current of the sensor is linear to glucose concentration up to 35 mM with a sensitivity of 20.42 μA cm(-2) mM(-1) at a substantially negative potential (i.e., -0.35 V). Operation under this potential eliminates the impact from the oxidation of common interfering species. This sensor with excellent sensitivity and selectivity also allows for reproducible detection of glucose in human urine samples.

Flexible All-Solid-State Asymmetric Supercapacitors Based on Free-Standing Carbon Nanotube/Graphene and Mn3O4 Nanoparticle/Graphene Paper Electrodes

We report the design of all-solid-state asymmetric supercapacitors based on free-standing carbon nanotube/graphene (CNTG) and Mn(3)O(4) nanoparticles/graphene (MG) paper electrodes with a polymer gel electrolyte of potassium polyacrylate/KCl. The composite paper electrodes with carbon nanotubes or Mn(3)O(4) nanoparticles uniformly intercalated between the graphene nanosheets exhibited excellent mechanical stability, greatly improved active surface areas, and enhanced ion transportation, in comparison with the pristine graphene paper. The combination of the two paper electrodes with the polymer gel electrolyte endowed our asymmetric supercapacitor of CNTG//MG an increased cell voltage of 1.8 V, a stable cycling performance (capacitance retention of 86.0% after 10,000 continuous charge/discharge cycles), more than 2-fold increase of energy density (32.7 Wh/kg) compared with the symmetric supercapacitors, and importantly a distinguished mechanical flexibility.

Coating graphene paper with 2D-assembly of electrocatalytic nanoparticles : a modular approach toward high-performance flexible electrodes

The development of flexible electrodes is of considerable current interest because of the increasing demand for modern electronics, portable medical products, and compact energy devices. We report a modular approach to fabricating high-performance flexible electrodes by structurally integrating 2D-assemblies of nanoparticles with freestanding graphene paper. We have shown that the 2D array of gold nanoparticles at oil-water interfaces can be transferred on freestanding graphene oxide paper, leading to a monolayer of densely packed gold nanoparticles of uniform sizes loaded on graphene oxide paper. One major finding is that the postassembly electrochemical reduction of graphene oxide paper restores the ordered structure and electron-transport properties of graphene, and gives rise to robust and biocompatible freestanding electrodes with outstanding electrocatalytic activities, which have been manifested by the sensitive and selective detection of two model analytes: glucose and hydrogen peroxide (H(2)O(2)) secreted by live cells. The modular nature of this approach coupled with recent progress in nanocrystal synthesis and surface engineering opens new possibilities to systematically study the dependence of catalytic performance on the structural parameters and chemical compositions of the nanocrystals.

2D and 3D graphene materials: Preparation and bioelectrochemical applications

The attractive properties of graphene materials have stimulated intense research and development in the field of bioelectrochemistry. In particular, the construction of 2D and 3D graphene architectures provides new possibilities for developing flexible and porous carbon scaffolds, which not only inherit some of the key properties of individual graphene sheets, but also develop additional functions that are of considerable interest for bioelectrochemical applications. In this review article, we will first summarize the recently developed approaches to preparing graphene sheets, and then focus on the methods to assemble them into macroscopic 2D and 3D structures. Furthermore, we will highlight the potential applications of these materials in electrochemical biosensors and biological fuel cells.

Cytotoxicity Evaluation of Oxidized Single-Walled Carbon Nanotubes and Graphene Oxide on Human Hepatoma HepG2 cells: An iTRAQ-Coupled 2D LC-MS/MS Proteome Analysis

Because of their attractive chemical and physical properties, graphitic nanomaterials and their derivatives have gained tremendous interest for applications in electronics, materials, and biomedical areas. However, few detailed studies have been performed to evaluate the potential cytotoxicity of these nanomaterials on living systems at the molecular level. In the present study, our group exploited the isobaric tagged relative and absolute quantification (iTRAQ)-coupled two-dimensional liquid chromatography-tandem mass spectrometry (2D LC-MS/MS) approach with the purpose of characterizing the cellular functions in response to these nanomaterials at the proteome level. Specifically, the human hepatoma HepG2 cells were selected as the in vitro model to study the potential cytotoxicity of oxidized single-walled carbon nanotubes (SWCNTs) and graphene oxide (GO) on the vital organ of liver. Overall, 30 differentially expressed proteins involved in metabolic pathway, redox regulation, cytoskeleton formation, and cell growth were identified. Based on the protein profile, we found oxidized SWCNTs induced oxidative stress and interfered with intracellular metabolic routes, protein synthesis, and cytoskeletal systems. Further functional assays confirmed that oxidized SWCNTs triggered elevated level of reactive oxygen species (ROS), perturbed the cell cycle, and resulted in a significant increase in the proportion of apoptotic cells. However, only moderate variation of protein levels for the cells treated with GO was observed and functional assays further confirmed that GO was less cytotoxic in comparison to oxidized SWCNTs. These finding suggested that GO was more biocompatible and could be a promising candidate for bio-related applications.

Growth of coral-like PtAu–MnO2 binary nanocomposites on free-standing graphene paper for flexible nonenzymatic glucose sensors

The growing demand for compact point-of-care medical devices and portable instruments for on-site environmental sampling has stimulated intense research on flexible sensors that can be miniaturized and function under considerable physical deformation. We report a new type of flexible electrochemical biosensors based on free-standing graphene paper carrying binary nanocomposites of PtAu alloy and MnO(2). The coral-like PtAu-MnO(2) nanocomposites are grown on the substrate through one-step template-free electrodeposition, leading to an intimate contact between the PtAu alloy and MnO(2) matrix. The flexible electrode exhibits a unique set of structural and electrochemical properties such as better uniformity, larger active surface areas, and faster electron transfer in comparison with the control electrode prepared by tandem growth of MnO(2) network and PtAu alloy in two steps. In nonenzymatic amperometric glucose detection, the PtAu-MnO(2) binary nanostructure-decorated graphene paper has shown greatly enhanced sensing performance such as wide liner range (0.1 mM to 30.0 mM), high sensitivity (58.54 μA cm(-2) mM(-1)), low detection limit (0.02 mM, S/N=3), satisfactory selectivity, excellent reproducibility and stability, and tolerability to mechanical stress. The strategy of co-growth of metal and metal oxides on freestanding carbon substrates opens new possibility to develop high-performance flexible electrochemical sensors.

Comparative protein profile of human hepatoma HepG2 cells treated with graphene and single-walled carbon nanotubes: An iTRAQ-coupled 2D LC–MS/MS proteome analysis

Graphitic nanomaterials are promising candidates for applications in electronics, energy, materials and biomedical areas. Nevertheless, few detailed studies related to the mechanistic understanding of these nanomaterials with the living systems have been performed to date. In the present study, our group applied the iTRAQ-coupled 2D LC-MS/MS approach to analyze the protein profile change of human hepatoma HepG2 cells treated with graphene and single-walled carbon nanotubes (SWCNTs), with the purpose of characterizing the interactions between living system and these nanomaterials at molecular level. Overall 37 differentially expressed proteins involved in metabolic pathway, redox regulation, cytoskeleton formation and cell growth were identified. Based on the protein profile, we found SWCNTs severely interfered the intracellular metabolic routes, protein synthesis and cytoskeletal systems. Moreover, our data suggested that SWCNTs might induce oxidative stress, thereby activating p53-mediated DNA damage checkpoint signals and leading to apoptosis. However, only moderate variation of protein levels for the cells treated with graphene was observed, which indicated graphene was less toxic and might be promising candidate for biomedical applications. We envision that this systematic characterization of cellular response at protein expression level will be of great importance to evaluate biocompatibility of nanomaterials.

Cytotoxicity of single-walled carbon nanotubes on human hepatoma HepG2 cells: An iTRAQ-coupled 2D LC–MS/MS proteome analysis

Single-walled carbon nanotubes (SWCNTs) and its derivatives are promising candidates for applications in electronics, energy, materials and biomedical areas. However, with the growing potential biomedical applications and the rising societal concerns on nanosafety, mechanistic understanding of the interactions between nanomaterials and living systems has become imperative. In the present study, our group applied the iTRAQ-coupled 2D LC-MS/MS approach to analyze the protein profile change of mammalian cells in response to SWCNTs. Specifically, the human hepatoma HepG2 cells were chosen as the in vitro model to study the potential cytotoxicity of SWCNTs on the vital organ of liver. Overall 51 differentially expressed proteins that involved in metabolic pathway, redox regulation, signaling pathway, cytoskeleton formation and cell growth were identified. We found SWCNTs triggered the up-regulation of metabolic enzymes, heat shock proteins and proteins involved in redox regulation, which indicated SWCNTs could induce oxidative stress, perturb protein synthesis and interfere cellular metabolism. Our data also suggested that SWCNTs might induce the activation of apoptosis signal-regulating kinase 1, and finally lead to stress-induced apoptosis. The comparative protein profile obtained here provided molecular evidence on the cellular functions in response to SWCNTs, which should very useful to elucidate the cytotoxicity caused by those nanomaterials.