Jianfeng Shen

Synthesis of Amphiphilic Graphene Nanoplatelets

Graphene, a flat monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice, has attracted a great deal of attention in recent years for potential applications in many technological fields, such as nanoelectronics, nanocomposites, and hydrogen supercapacitors. One possible route to harnessing these properties would be to incorporate graphene sheets into composite materials. The manufacturing of such composites requires not only that the graphene sheets are produced on a sufficient scale, but also that they are incorporated, and homogeneously distributed, into various matrices. Graphite, which consists of a stack of flat graphene sheets, is available in large quantities from natural sources. It is likely the most readily available and least expensive source for the production of bulk graphene sheets. Graphite oxide (GO) is a layered material produced by the oxidation of graphite. In contrast to pristine graphite, the graphene-derived sheets in GO are heavily oxygenated and bear hydroxyl and epoxide functional groups on their basal planes, in addition to carbonyl and carboxyl groups located at the sheet edges. Unfortunately, owing to their hydrophilic nature, graphene oxide sheets can only be dispersed in aqueous media that are incompatible with most organic solvents and polymers. In addition, GO is electrically insulating, which limits its usefulness for the synthesis of composites. However, it has been demonstrated that the electrical conductivity of GO can be significantly increased and very thin graphene-like sheets can be obtained through the chemical reduction of exfoliated GO. Just like carbon nanotubes, a key challenge in the synthesis and processing of bulk-quantity graphene sheets is aggregation. Graphene sheets, due to their high specific surface area, tend to form irreversible agglomerates or even restack to form graphite through van der Waals interactions. As with carbon nanotubes, full utilization of graphene sheets in polymer nanocomposite applications will inevitably depend on their ability to achieve complete dispersion in the polymer matrix of choice.

Layer-by-layer self-assembly of graphene nanoplatelets.

In this report, graphene nanoplatelets were self-assembled through the layer-by-layer (LBL) method. The graphene surface was modified with poly(acrylic acid) and poly(acryl amide) by covalent bonding, which introduced negative and positive charge on the surface of graphene, respectively. Through electrostatic interaction, the positively and negatively charged graphene nanoplatelets assembled together to form a multilayer structure. Thermogravimetric analysis, Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy were used to demonstrate the modification of graphene nanoplatelets. Fourier transform infrared spectroscopy and SEM proved this method is feasible for preparing graphene-containing films. Ultraviolet-visible spectroscopy confirmed that the adsorption technique resulted in uniform film growth.

Covalent attaching protein to graphene oxide via diimide-activated amidation.

In this paper, graphene oxide nanosheets (GOS) are functionalized by bovine serum albumin (BSA) via diimide-activated amidation under ambient conditions. The obtained GOS-BSA conjugate is highly water-soluble. Results of atomic force microscopy (AFM), Raman spectra and Fourier transform infrared spectroscopy analysis confirm that GOS-BSA conjugate contains both GOS and BSA protein. AFM image shows that GOS are fully exfoliated. Results of cyclic volatammograms show that the protein in the GOS-BSA conjugate retains its bioactivity. The present method may also provide a way to synthesize graphene-based composites with other biomolecules.

Covalent synthesis of organophilic chemically functionalized graphene sheets.

In this study, we report a scalable, fast, and easy method for preparation of organophilic chemically functionalized graphene (OCFG) sheets. The basic strategy involved the preparation of graphite oxide (GO) and the complete exfoliation of GO into graphene oxide sheets, followed by reacting with 1-bromobutane to obtain OCFG sheets. Thermogravimetric analysis, Raman spectroscopy, and Fourier transform infrared spectroscopy indicated the functionalization of GO. Transmission electron microscopy and atomic force microscopy were used to demonstrate the structure of produced graphene oxide and OCFG sheets. Ultraviolet-visible spectroscopy confirmed that OCFG sheets disperse well in organic solvents and the solutions obey Beers law. The resulting organic dispersions are homogeneous, exhibit long-term stability, and are made up of graphene sheets a few hundred nanometers large. The ability to prepare graphene dispersions in organic media facilitates their combination with polymers to yield homogeneous composites.

Synthesis of hydrophilic and organophilic chemically modified graphene oxide sheets.

In this work, hydrophilic and organophilic chemically modified graphene oxide (CMGO) sheets were prepared through a two-step diimide-activated amidation. The hydrophilic and organophilic products were characterized by atomic force microscopy, transmission electron microscopy and ultraviolet-visible spectroscopy. The resulted dispersions are homogeneous and exhibit long-term stability, which will facilitate the combination of CMGO sheets with polymers to yield homogeneous composites.

Layer-by-layer self-assembly of multiwalled carbon nanotube polyelectrolytes prepared by in situ radical polymerization.

Anionic and cationic multiwalled carbon nanotube polyelectrolytes, prepared by covalent modification of multiwalled carbon nanotubes (MWCNTs) with poly(acrylic acid) and poly(acrylamide), were used for the layer-by-layer (LBL) self-assembly of MWCNTs on different substrates with polyelectrolytes, such as poly(diallyldimethylammonium chloride) and sodium poly(styrenesulfonate). Thermogravimetric analysis, Raman spectroscopy, and scanning electron microscopy (SEM) were used to demonstrate the modification of MWCNTs. Investigations using Fourier transform infrared spectroscopy, atomic force microscopy, SEM, and ultraviolet-visible spectroscopy proved this method to be practicable for preparing LBL films.

Synthesis of graphene oxide-based biocomposites through diimide-activated amidation

In this work, a novel and facile method for covalent attachment of biomaterials to graphene oxide sheets (GOS) was developed. Four conjugates were obtained via the diimide-activated amidation reaction under ambient conditions. Final products were characterized by FT-IR spectroscopy, atomic force microscopy and transmission electron microscopy. Electrochemical characterization of the composite showed that the covalently bonded biomaterial retained its bioactivity. This method may provide a way for further preparation of graphene-based biodevices.