Critical role of intercalated water for electrocatalytically active nitrogen-doped graphitic systems.

Graphitic materials are essential in energy conversion and storage because of their excellent chemical and electrical properties. The strategy for obtaining functional graphitic materials involves graphite oxidation and subsequent dissolution in aqueous media, forming graphene-oxide nanosheets (GNs). Restacked GNs contain substantial intercalated water that can react with heteroatom dopants or the graphene lattice during reduction.

Controlled disulfonated poly(arylene ether sulfone) multiblock copolymers for direct methanol fuel cells.

Structure-property-performance relationships of disulfonated poly(arylene ether sulfone) multiblock copolymer membranes were investigated for their use in direct methanol fuel cell (DMFC) applications. Multiple series of reactive polysulfone, polyketone, and polynitrile hydrophobic block segments having different block lengths and molecular composition were synthesized and reacted with a disulfonated poly(arylene ether sulfone) hydrophilic block segment by a coupling reaction. Large-scale morphological order of the multiblock copolymers evolved with the increase of block size that gave notable influence on mechanical toughness, water uptake, and proton/methanol transport.

Mechanistic understanding of surface plasmon assisted catalysis on a single particle: cyclic redox of 4-aminothiophenol.

Surface plasmon assisted catalysis (SPAC) reactions of 4-aminothiophenol (4ATP) to and back from 4,4'-dimercaptoazobenzene (DMAB) have been investigated by single particle surface enhanced Raman spectroscopy, using a self-designed gas flow cell to control the reductive/oxidative environment over the reactions. Conversion of 4ATP into DMAB is induced by energy transfer (plasmonic heating) from surface plasmon resonance to 4ATP, where O₂ (as an electron acceptor) is essential and H₂O (as a base) can accelerate the reaction. In contrast, hot electron (from surface plasmon decay) induction drives the reverse reaction of DMAB to 4ATP, where H₂O (or H₂) acts as the hydrogen source.

Single-Nanocrystal Photoluminescence Spectroscopy Studies of Plasmon-Multiexciton Interactions at Low Temperature.

Using thick-shell or "giant" CdSe/CdS nanocrystal quantum dots (g-NQDs), characterized by strongly suppressed Auger recombination, we studied the influence of plasmonic interactions on multiexciton emission. Specifically, we assessed the separate effects of plasmonic absorption and plasmonic emission enhancement by a systematic analysis of the pump fluence dependence of low-temperature photoluminescence (low-T PL) derived from individual CdSe/CdS g-NQDs deposited on nanoroughened silver films. Our study reveals that (1) the multiexciton (MX) emissions in g-NQD coupled to silver films were enhanced not only through the creation of more excitons via enhancement of absorption but also through the direct modification of the competition between the radiative and nonradiative recombination processes of MXs; (2) strong enhancement in absorption is not necessary for strong multiexciton emission; and (3) the emission of MXs can become stronger with the increase of multiexciton order.

Super-Poissonian statistics of photon emission from single CdSe-CdS core-shell nanocrystals coupled to metal nanostructures.

We demonstrate that photon antibunching observed for individual nanocrystal quantum dots (NQDs) can be transformed into photon bunching characterized by super-Poissonian statistics when they are coupled to metal nanostructures (MNs). This observation indicates that, while the quantum yield of a biexciton (Q(2X)) is lower than that of a single exciton (Q(1X)) in freestanding NQDs, Q(2X) becomes greater than Q(1X) in NQDs coupled to MNs. This unique phenomenon is attributed to metal-induced quenching with a rate that scales more slowly with exciton multiplicity than the radiative decay rate and dominates over other nonradiative decay channels for both single excitons and biexcitons.

Fabrication of thorny Au nanostructures on polyaniline surfaces for sensitive surface-enhanced Raman spectroscopy.

Here we demonstrate, for the first time, the fabrication of Au nanostructures on polyaniline (PANI) membrane surfaces for surface enhanced Raman spectroscopy (SERS) applications, through a direct chemical reduction by PANI. Introduction of acids into the HAuCl(4) solution leads to homogeneous Au structures on the PANI surfaces, which show only sub-ppm detection levels toward the target analyte, 4-mercaptobenzoic acid (4-MBA), because of limited surface area and lack of surface roughness. Thorny Au nanostructures can be obtained through controlled reaction conditions and the addition of a capping agent poly (vinyl pyrrolidone) (PVP) in the HAuCl(4) solution and the temperature kept at 80 °C in an oven.

Nitrogen-doped graphene-rich catalysts derived from heteroatom polymers for oxygen reduction in nonaqueous lithium-O2 battery cathodes.

In this work, we present a synthesis approach for nitrogen-doped graphene-sheet-like nanostructures via the graphitization of a heteroatom polymer, in particular, polyaniline, under the catalysis of a cobalt species using multiwalled carbon nanotubes (MWNTs) as a supporting template. The graphene-rich composite catalysts (Co-N-MWNTs) exhibit substantially improved activity for oxygen reduction in nonaqueous lithium-ion electrolyte as compared to those of currently used carbon blacks and Pt/carbon catalysts, evidenced by both rotating disk electrode and Li-O(2) battery experiments. The synthesis-structure-activity correlations for the graphene nanostructures were explored by tuning their synthetic chemistry (support, nitrogen precursor, heating temperature, and transition metal type and content) to investigate how the resulting morphology and nitrogen-doping functionalities (e.

Direct measurement of coherency limits for strain relaxation in heteroepitaxial core/shell nanowires.

The growth of heteroepitaxially strained semiconductors at the nanoscale enables tailoring of material properties for enhanced device performance. For core/shell nanowires (NWs), theoretical predictions of the coherency limits and the implications they carry remain uncertain without proper identification of the mechanisms by which strains relax. We present here for the Ge/Si core/shell NW system the first experimental measurement of critical shell thickness for strain relaxation in a semiconductor NW heterostructure and the identification of the relaxation mechanisms.

Mechanistic study of silver nanoparticle formation on conducting polymer surfaces.

Conducting polymer (polyaniline) sheets are shown to be active substrates to promote the growth of nanostructured silver thin films with highly tunable morphologies. Using the spontaneous electroless deposition of silver, we show that a range of nanostructured metallic features can be controllably and reproducibly formed over large surface areas. The structural morphology of the resulting metal-polymer nanocomposite is demonstrated to be sensitive to experimental parameters such as ion concentration, temperature, and polymer processing and can range from densely packed oblate nanosheets to bulk crystalline metals.

Efficient synthesis of tailored magnetic carbon nanotubes via a noncovalent chemical route.

We report here an efficient noncovalent chemical route to dense and uniform assembly of magnetic nanoparticles onto multi-walled carbon nanotubes within a single-layer configuration. While preserving the electrical conduction behavior of the nanotube network itself, the resulting carbon nanotube derivatives exhibit a distinct superparamagnetism, and can be magnetically manipulated via a quick and reversible mode.

Titanium dioxide-supported non-precious metal oxygen reduction electrocatalyst.

A new non-precious metal oxygen reduction catalyst was developed via heat treatment of in situ polymerized polyaniline onto TiO(2) particles in the presence of Fe species. The TiO(2) provides for improved performance relative to a carbon black-based catalyst and, at a high catalyst loading, allows for reducing the performance gap between non-precious-metal catalyst and Pt/C to ca. 20 mV in RDE testing.

Field-assisted synthesis of SERS-active silver nanoparticles using conducting polymers.

A gradient of novel silver nanostructures with widely varying sizes and morphologies is fabricated on a single conducting polyaniline-graphite (P-G) membrane with the assistance of an external electric field. It is believed that the formation of such a silver gradient is a synergetic consequence of the generation of a silver ion concentration gradient along with an electrokinetic flow of silver ions in the field-assisted model, which greatly influences the nucleation and growth mechanism of Ag particles on the P-G membrane. The produced silver dendrites, flowers and microspheres, with sharp edges, intersections and bifurcations, all present strong surface enhanced Raman spectroscopy (SERS) responses toward an organic target molecule, mercaptobenzoic acid (MBA).

Bifunctional polyacrylamide based polymers for the specific binding of hexahistidine tagged proteins on gold surfaces.

We describe a modified bifunctional analogue of polyacrylamide that spontaneously forms self-assembled polymeric thin films on Au surfaces. The film is engineered to specifically bind histidine tagged proteins (6His), while simultaneously remaining inherently resistant to the non-specific adsorption of proteins in solution. The backbone of a polyacrylamide-co-n-acryloxysuccinimide copolymer is functionalized via tandem active ester (NHS) couplings with 3-(methylthio)propylamine (MTP) and nitrilotriacetic acid (NTA).

Facile fabrication of homogeneous 3D silver nanostructures on gold-supported polyaniline membranes as promising SERS substrates.

We report a facile synthesis of large-area homogeneous three-dimensional (3D) Ag nanostructures on Au-supported polyaniline (PANI) membranes through a direct chemical reduction of metal ions by PANI. The citric acid absorbed on the Au nuclei that are prefabricated on PANI membranes directs Ag nanoaprticles (AgNPs) to self-assemble into 3D Ag nanosheet structures. The fabricated hybrid metal nanostructures display uniform surface-enhanced Raman scattering (SERS) responses throughout the whole surface area, with an average enhancement factor of 10(6)-10(7).

Self-assembly approach to multiplexed surface-enhanced Raman spectral-encoder beads.

We present a strategy for the synthesis of multiplexed spectral encoder beads based on combinations of different surface enhanced Raman (SERS) signatures generated by dye-functionalized Ag nanoparticle tags. A key problem in SERS-based multiplexing arises in balancing the competitive binding of different signal generating dyes to the nanoparticle surfaces, which leads to difficulty in generating final summation spectra by design. We avoid this complication by decoupling the formation of individual tags from multiplexing of their spectra by self-assembly of different tag combinations onto SiO(2) microbead supports via biotin-avidin binding.

Femtosecond laser-nanostructured substrates for surface-enhanced Raman scattering.

We present a new type of surface-enhanced Raman scattering (SERS) substrate that exhibits extremely large and uniform cross-section enhancements over a macroscopic (greater than 25 mm2) area. The substrates are fabricated using a femtosecond laser nanostructuring process, followed by thermal deposition of silver. SERS signals from adsorbed molecules show a spatially uniform enhancement factor of approximately 10(7).

Optical transduction of chemical forces.

We describe a plasmonic crystal device possessing utility for optically transducing chemical forces. The device couples complex plasmonic fields to chemical changes via a chemoresponsive, surface-bound hydrogel. We find that this architecture significantly enhances the spectroscopic responses seen at visible wavelengths while enabling capacities for sensitive signal transduction, even in cases that involve essentially no change in refractive index, thus allowing analytical detection via colorimetric assays in both imaging and spectroscopic modes.

Quantitative multispectral biosensing and 1D imaging using quasi-3D plasmonic crystals.

We developed a class of quasi-3D plasmonic crystal that consists of multilayered, regular arrays of subwavelength metal nanostructures. The complex, highly sensitive structure of the optical transmission spectra of these crystals makes them especially well suited for sensing applications. Coupled with quantitative electrodynamics modeling of their optical response, they enable full multiwavelength spectroscopic detection of molecular binding events with sensitivities that correspond to small fractions of a monolayer.

Quantitative imaging of protein adsorption on patterned organic thin-film arrays using secondary electron emission.

Secondary electron emission is developed as a means to quantify and image protein binding to Au surfaces modified with patterned organic thin-film arrays. Alkane thiols were patterned via microcontact printing on gold, and their effects on the secondary electron (SE) yield of the surface, systematically quantified. We show that a self-assembled monolayer (SAM) of hexadecane thiol significantly increases the SE yield over the native gold surface, a yield that increases as a function of alkane chain length (C8-C16).

Origin of bulklike structure and bond length disorder of Pt37 and Pt6Ru31 clusters on carbon: comparison of theory and experiment.

We describe a theoretical analysis of the structures of self-organizing nanoparticles formed by Pt and Ru-Pt on carbon support. The calculations provide insights into the nature of these metal particle systems-ones of current interest for use as the electrocatalytic materials of direct oxidation fuel cells-and clarify complex behaviors noted in earlier experimental studies. With clusters deposited via metallo-organic Pt or PtRu(5) complexes, previous experiments [Nashner et al.

High performance plasmonic crystal sensor formed by soft nanoimprint lithography.

This paper describes a new type of plasmonic sensor fabricated by imprint lithography using a soft, elastomeric mold. Angle-dependent, zero-order transmission experiments demonstrate the sensing potential of this device, which uses a two dimensional plasmonic crystal. Full angle-dependent mapping shows that the sensitivity to surface chemical binding events reaches maxima near regions of the plasmonic Brillouin zone where the dispersion curves of multiple surface plasmon polariton modes converge.

pH-dependent photophysical behavior of rhenium complexes containing hydroxypyridine ligands.

Data related to the pH-dependent photophysics of a class of rhenium complexes containing the hydroxypyridine ligand are presented. Data include ground-state pK(a) values, emission energies, and lifetimes. The complexes all have ground-state pK(a) values near 7.

Multicomponent lifetime-based pH sensors utilizing constant-lifetime probes.

A multicomponent luminescent sensor system is described that uses probe species with constant lifetimes to generate an analyte-dependent change in the apparent sensor lifetime. This new sensing scheme not only allows for lifetime-based measurement techniques to be applied to sensors that employ static quenching interactions but also provides the ability to vary the sensitivity of the sensor system with simple changes in instrumental parameters. A model for the multicomponent sensor is presented, followed by data measured using a prototype pH sensor based on the model.