Trevor J. Simmons

Stable Aqueous Dispersions of Noncovalently Functionalized Graphene from Graphite and their Multifunctional High-Performance Applications

We present a scalable and facile technique for noncovalent functionalization of graphene with 1-pyrenecarboxylic acid that exfoliates single-, few-, and multilayered graphene flakes into stable aqueous dispersions. The exfoliation mechanism is established using stringent control experiments and detailed characterization steps. Using the exfoliated graphene, we demonstrate highly sensitive and selective conductometric sensors (whose resistance rapidly changes >10,000% in saturated ethanol vapor), and ultracapacitors with extremely high specific capacitance (∼ 120 F/g), power density (∼ 105 kW/kg), and energy density (∼ 9.2 Wh/kg).

Conductive Cable Fibers with Insulating Surface Prepared by Coaxial Electrospinning of Multiwalled Nanotubes and Cellulose

Core-sheath multiwalled carbon nanotube (MWNT)-cellulose fibers of diameters from several hundreds of nanometers to several micrometers were prepared by coaxial electrospinning from a nonvolatile, nonflammable ionic liquid (IL) solvent, 1-methyl-3-methylimidazolium acetate ([EMIM][Ac]). MWNTs were dispersed in IL to form a gel solution. This gel core solution was electrospun surrounded by a sheath solution of cellulose dissolved in the same IL. Electrospun fibers were collected in a coagulation bath containing ethanol-water to remove the IL completely and dried to form core-sheath MWNT-cellulose fibers having a cable structure with a conductive core and insulating sheath. Enzymatic treatment of a portion of a mat of these fibers with cellulase selectively removed the cellulose sheath exposing the MWNT core for connection to an electrode. These MWNT-cellulose fiber mats demonstrated excellent conductivity because of a conductive pathway of bundled MWNTs. Fiber mat conductivity increased with increasing ratio of MWNT in the fibers with a maximum conductivity of 10.7 S/m obtained at 45 wt % MWNT loading.

Electrospun polyvinylpyrrolidone fibers with high concentrations of ferromagnetic and superparamagnetic nanoparticles.

Electrospun polymer fibers were prepared containing mixtures of different proportions of ferromagnetic and superparamagnetic nanoparticles. The magnetic properties of these fibers were then explored using a superconducting quantum interference device. Mixed superparamagnetic/ferromagnetic fibers were examined for mesoscale magnetic exchange coupling, which was not observed as theoretically predicted. This study includes some of the highest magnetic nanoparticle loadings (up to 50 wt%) and the highest magnetization values (≈ 25 emu/g) in an electrospun fiber to date and also demonstrates a novel mixed superparamagnetic/ferromagnetic system.

High-Sensitivity Bolometers from Self-Oriented Single-Walled Carbon Nanotube Composites

In this work, films of horizontally aligned single-walled carbon nanotubes were thermally and electrically characterized in order to determine the bolometric performance. An average thermal time constant of τ = 420 μs along with a temperature coefficient of resistance of TCR = -2.94% K(-1) were obtained. The maximum voltage responsivity and detectivity obtained were R(V) =230 V/W and D* = 1.22 × 10(8) cm Hz(1/2)/W, respectively. These values are higher than the maximum voltage responsivity (150 V/W) and maximum temperature coefficient of resistance (1.0% K(-1)) previously reported for carbon nanotube films at room temperature. The maximum detectivity was obtained at a frequency of operation of 1.25 kHz.

Preparation of synthetic wood composites using ionic liquids

Synthetic wood composite films containing cellulose, hemicelluloses, and lignin, the three major components of natural wood, were prepared in a room temperature ionic liquid solvent, 1-ethyl-3-methylimidazolium acetate, [EMIM][Ac]. Various synthetic wood composites were obtained by dissolution of individual wood components together with additives, including polyethylene glycol (PEG), chitosan, and multi-wall carbon nanotubes (MWNTs) in [EMIM][Ac]. The addition of water affords a gel that was dried in either a low humidity environment or under vacuum. Synthetic wood films showed smoother surface textures, higher water resistance, and higher tensile strengths than cellulose films formed by the same methods. Tailor-made synthetic wood composites were also prepared having a variety of desirable properties, including antimicrobial activities, controlled hydro-phobicity/philicity, high relative dielectric constant, and a high degree of cohesiveness.

Composite polysaccharide fibers prepared by electrospinning and coating

Composite polysaccharide fibers composed two oppositely charged natural polysaccharides, chitosan and hyaluronic acid, were prepared by electrospinning and subsequent coating. The fiber size distribution was characterized by scanning electron microscopy. Chitosan/hyaluronic acid composite fibers were stable in water but showed controlled release of hyaluronic acid into phosphate buffered saline, and the presence of 3-wt% hyaluronic acid coating improved the swelling ratio to 30%. The resulting composite polysaccharide fibers have a number of potential biomedical applications in wound healing applications and in drug delivery systems.

Uniform nanoparticle coating of cellulose fibers during wet electrospinning

This work outlines a newly developed method that allows electrospun cellulose fibers to be coated with nanoparticles during dry-jet wet electrospinning. Cellulose fibers were wet electrospun from a room temperature ionic liquid solvent into a coagulation bath containing an aqueous suspension of magnesium hydroxide nanoparticles to prepare composites of nanofibers coated with functional nanoparticles. Flame retardant cellulose–magnesium hydroxide coated composite fibers were prepared to demonstrate this novel electrospinning method. The placement of the nanoparticles exclusively on the surface of the cellulose fibers dramatically impacted the functionality of the fibers. Electrospun cellulose fibers exhibited an onset of combustion in air at 239 °C and a maximum mass loss at 302 °C. Cellulose fibers with Mg(OH)2 nanoparticles ( 100 nm avg. diameter) were used, the onset of combustion was 185 °C and the maximum mass loss was at 216 °C when nanoparticles were inside the fibers, and the onset of combustion was 263 °C and the maximum mass loss was at 317 °C for Mg(OH)2 nanoparticle coated cellulose fibers. Simple flame tests showed a similar trend, with nanoparticle-coated fibers being fire resistant and fibers with nanoparticles inside burned rapidly upon exposure to an open flame.

Can natural fibers be a silver bullet? Antibacterial cellulose fibers through the covalent bonding of silver nanoparticles to electrospun fibers

Natural cotton was dissolved in a room-temperature ionic liquid 1-ethyl-3-methyl acetate and wet-jet electrospun to obtain nanoscale cotton fibers with a substantially reduced diameter-and therefore an increased surface area-relative to natural cotton fibers. The resulting nano-cotton fibers were esterified with trityl-3-mercaptopropionic acid, which after selective de-tritylation afforded nano-cotton fibers containing reactive thiol functionality. Silver nanoparticles that were covalently attached to these sulfhydryl groups were assembled next. The microstructure of the resulting nanocomposite was characterized, and the antibacterial activity of the resulting nano-cotton Ag-nanoparticle composite was also studied. This nanocomposite showed significant activity against both Gram-negative and Gram-positive bacteria.

Functionalization of nitrogen-doped carbon nanotubes with gallium to form Ga-CN(x)-multi-wall carbon nanotube hybrid materials.

In an effort to combine group III-V semiconductors with carbon nanotubes, a simple solution-based technique for gallium functionalization of nitrogen-doped multi-wall carbon nanotubes has been developed. With an aqueous solution of a gallium salt (GaI(3)), it was possible to form covalent bonds between the Ga(3+) ion and the nitrogen atoms of the doped carbon nanotubes to form a gallium nitride-carbon nanotube hybrid at room temperature. This functionalization was evaluated by x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy.

Flexible Electrospun Cellulose Fibers as an Affinity Packing Material for the Separation of Bovine Serum Albumin

Flexible, well-dispersed and continuous 100-nm diameter cellulose fibers were prepared from an ionic liquid solvent by a novel dry-jet wet-electro spinning process. The ribbon fibers formed were chemically activated and an affinity dye, cibacron blue (CB) was immobilized at 0.22 g CB/g dry fibers loading to the surface of these fibers. The resulting affinity matrix was packed into a chromatography column and the adsorption, desorption and specificity of this matrix for bovine serum albumin (BSA) was studied. These electrospun fibers had a BSA binding capacity of 230 mg/g, nearly twice that of CB-immobilized 100-?m beads and over ten-fold higher capacity that CB-immobilized cellulose fibers prepared by a conventional electrospinning process. The results of this work suggest that chromatography supports of flexible, well-dispersed, continuous nanofibers may offer advantages over conventional supports in affinity separations.