Haixin Chang

Graphene Fluorescence Resonance Energy Transfer Aptasensor for the Thrombin Detection

Combining nanomaterials and biomolecule recognition units is promising in developing novel clinic diagnostic and protein analysis techniques. In this work, a highly sensitive and specific fluorescence resonance energy transfer (FRET) aptasensor for thrombin detection is developed based on the dye labeled aptamer assembled graphene. Due to the noncovalent assembly between aptamer and graphene, fluorescence quenching of the dye takes place because of FRET. The addition of thrombin leads to the fluorescence recovery due to the formation of quadruplex-thrombin complexes which have weak affinity to graphene and keep the dyes away from graphene surface. Because of the high fluorescence quenching efficiency, unique structure, and electronic properties of graphene, the graphene aptasensor exhibits extraordinarily high sensitivity and excellent specificity in both buffer and blood serum. A detection limit as low as 31.3 pM is obtained based on the graphene FRET aptasensor, which is two orders magnitude lower than those of fluorescent sensors based on carbon nanotubes. The excellent performance of FRET aptasensor based on graphene will also be ascribed to the unique structure and electronic properties of graphene.

Graphene Oxide Amplified Electrogenerated Chemiluminescence of Quantum Dots and Its Selective Sensing for Glutathione from Thiol-Containing Compounds

Here we report a graphene oxide amplified electrogenerated chemiluminescence (ECL) of quantum dots (QDs) platform and its efficient selective sensing for antioxidants. Graphene oxide facilitated the CdTe QDs*+ production and triggered O2*- generation. Then, a high yield of CdTe QDs* was formed due to the combination of CdTe QDs*+ and O2*-, leading to an approximately 5-fold ECL amplification. Glutathione is the most abundant cellular thiol-containing peptide, but its selective sensing is an intractable issue in analytical and biochemical communities because its detection is interfered with by some thiol-containing compounds. This platform showed a detection limit of 8.3 microM (S/N = 3) for glutathione and a selective detection linear dependence from 24 to 214 microM in the presence of 120 muM cysteine and glutathione disulfide. This platform was also successfully used for real sample (eye drug containing glutathione) detection without any pretreatment with a wide linear range from 0.04 to 0.29 microg mL(-1).

Graphene-based nanocomposites: preparation, functionalization, and energy and environmental applications

Graphene, a two-dimensional carbon sheet with one atom thickness and one of the thinnest materials in universe, has inspired huge interest in physics, materials science, chemistry and biology. However, pure graphene sheets are limited for many applications despite their excellent characteristics and scientists face challenges to induce more and controlled functionality. Therefore graphene nanocomposites or hybrids are attracting increasing efforts for real applications in energy and environmental areas by introducing controlled functional building blocks to graphene. In this Review, we first give a brief introduction of graphenes unique physical and chemical properties followed by various preparation and functionalization methods for graphene nanocomposites in the second section. We focus on recent energy-related progress of graphene nanocomposites in solar energy conversion (e.g., photovoltaic and photoelectrochemical devices, artificial photosynthesis) and electrochemical energy devices (e.g., lithium ion battery, supercapacitor, fuel cell) in the third section. We then review the advances in environmental applications of functionalized graphene nanocomposites for the detection and removal of heavy metal ions, organic pollutants, gas and bacteria in the fourth section. Finally a conclusion and perspective is given to discuss the remaining challenges for graphene nanocomposites in energy and environmental science.

Graphene and Graphene-like Two-Dimensional Materials in Photodetection: Mechanisms and Methodology

Graphene and graphene-like two-dimensional (2D) materials have attracted much attention due to its extraordinary electronic and optical properties, which accommodate a large potential in optoelectronic applications such as photodetection. However, although much progress has been made, many challenges exist in fundamental and practical aspects hindering graphene and graphene-like 2D materials from photodetector and other photonic and optoelectronic applications. Here, we review the recent progress in photodetection based on graphene and graphene-like 2D materials and start with the summary of some most important physical mechanisms, including photoelectric, photo-thermoelectric, and photo-bolometric regimes. Then methodology-level discussions are given from viewpoints of state-of-the-art designs in device geometry and materials. It is worth emphasizing that emerging photodetection and photodetectors based on graphene-like 2D materials such as metal chalcogenide nanosheets are reviewed systematically. Finally, we conclude this review in a brief discussion with remaining challenges in photodetection of two-dimensional photonics and optoelectronics (2D POE) and note that complete understandings of 2D materials and 2D POE may inspire solar energy conversion and other new applications.

Thin Film Field‐Effect Phototransistors from Bandgap‐Tunable, Solution‐Processed, Few‐Layer Reduced Graphene Oxide Films

Thin film field-effect phototransistors (FETs) can be developed from bandgap-tunable, solution-processed, few-layer reduced graphene oxide (FRGO) films. Large-area FRGO films with tunable bandgaps ranging from 2.2 eV to 0.5 eV can be achieved readily by solution-processing technique such as spin-coating. The electronic and optoelectronic properties of FRGO FETs are found to be closely related to their bandgap energy. The resulting phototransistor has great application potential in the field of photodetection.

Hydrogen evolution from water using semiconductor nanoparticle/graphene composite photocatalysts without noble metals

Semiconductor nanoparticle/graphene composite photocatalysts containing semiconductor CdS or TiO2 nanoparticles are fabricated by one-pot solution methods and their structures are characterized. The photocatalytic hydrogen-generating capabilities of the composite photocatalysts are investigated in the presence of sacrificial reagent and compared with those of the same semiconductor materials with platinum as a co-catalyst under the same conditions. The results obtained by the measurements of time-resolved emission spectra, photocurrent generated response and electrochemical impedance spectra revealed that graphene attached to semiconductor surfaces can efficiently accept and transport electrons from the excited semiconductor, suppressing charge recombination and improving interfacial charge transfer processes. The semiconductor nanoparticle/graphene photocatalysts displayed higher activity for photocatalytic hydrogen evolution, which can be compared with the hydrogen-generating efficiency of systems containing the well-known Pt co-catalyst. This work provides an inexpensive means of harnessing solar energy to achieve highly efficient hydrogen evolution without noble metals.

Nanoporous Gold Based Optical Sensor for Sub-ppt Detection of Mercury Ions

Precisely probing heavy metal ions in water is important for molecular biology, environmental protection, and healthy monitoring. Although many methods have been reported in the past decade, developing a quantitative approach capable of detecting sub-ppt level heavy metal ions with high selectivity is still challenging. Here we report an extremely sensitive and highly selective nanoporous gold/aptamer based surface enhanced resonance Raman scattering (SERRS) sensor. The optical sensor has an unprecedented detection sensitivity of 1 pM (0.2 ppt) for Hg(2+) ions, the most sensitive Hg(2+) optical sensor known so far. The sensor also exhibits excellent selectivity. Dilute Hg(2+) ions can be identified in an aqueous solution containing 12 metal ions as well as in river water and underground water. Moreover, the SERRS sensor can be reused without an obvious loss of the sensitivity and selectivity even after 10 cycles.

Polyelectrolyte-bridged metal/cotton hierarchical structures for highly durable conductive yarns

A novel, facile, and versatile approach for preparing highly durable, electrically conductive cotton yarns is reported. Polyelectrolyte brushes, a polymer that covalently tethers one end on a surface, are first grown from cotton surfaces by surface-initiated atomic transfer radical polymerization. Subsequent electroless deposition of metal particles onto the brush-modified cotton yarns yields electrically conductive yarns, which have conductivity as high as approximately 1 S/cm and can be used as electrical wires in wearable, flexible electronic devices. Importantly, the formation of polymer brush-bridged metal/cotton hierarchical structures provides robust mechanical and electrical durability to the yarns under many stretching, bending, rubbing, and washing cycles. With proper selection of metal, the conductivity of the samples remains stable after they are stored in air for a few months. This chemical approach can be extended as a general method for making conductive yarns and fabrics from all kinds of natural fibers.

Facile Synthesis of Wide‐Bandgap Fluorinated Graphene Semiconductors

The bandgap opening of graphene is extremely important for the expansion of the applications of graphene-based materials into optoelectronics and photonics. Current methods to open the bandgap of graphene have intrinsic drawbacks including small bandgap openings, the use hazardous/harsh chemical oxidations, and the requirement of expensive chemical-vapor deposition technologies. Herein, an eco-friendly, highly effective, low-cost, and highly scalable synthetic approach is reported for synthesizing wide-bandgap fluorinated graphene (F-graphene or or fluorographene) semiconductors under ambient conditions. In this synthesis, ionic liquids are used as the only chemical to exfoliate commercially available fluorinated graphite into single and few-layer F-graphene. Experimental and theoretical results show that the bandgap of F-graphene is largely dependent on the F coverage and configuration, and thereby can be tuned over a very wide range.

Regulating Infrared Photoresponses in Reduced Graphene Oxide Phototransistors by Defect and Atomic Structure Control

Defects play significant roles in properties of graphene and related device performances. Most studies of defects in graphene focus on their influences on electronic or luminescent optical properties, while controlling infrared optoelectronic performance of graphene by defect engineering remains a challenge. In the meantime, pristine graphene has very low infrared photoresponses of ~0.01 A/W due to fast photocarrier dynamics. Here we report regulating infrared photoresponses in reduced graphene oxide phototransistors by defect and atomic structure control for the first time. The infrared optoelectronic transport and photocurrent generation are significantly influenced and well controlled by oxygenous defects and structures in reduced graphene oxide. Moreover, remarkable infrared photoresponses are observed in photoconductor devices based on reduced graphene oxide with an external responsivity of ~0.7 A/W, at least over one order of magnitude higher than that from pristine graphene. External quantum efficiencies of infrared devices reach ultrahigh values of ~97%, which to our knowledge is one of the best efficiencies for infrared photoresponses from nonhybrid, pure graphene or graphene-based derivatives. The flexible infrared photoconductor devices demonstrate no photoresponse degradation even after 1000 bending tests. The results open up new routes to control optoelectronic behaviors of graphene for high-performance devices.