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. We demonstrate that removal of intercalated water using simple solvent treatments causes significant structural reorganization, substantially affecting the oxygen reduction reaction (ORR) activity and stability of nitrogen-doped graphitic systems. Amid contrasting reports describing the ORR activity of GN-based catalysts in alkaline electrolytes, we demonstrate superior activity in an acidic electrolyte with an onset potential of ~0.9 V, a half-wave potential (E ½) of 0.71 V, and a selectivity for four-electron reduction of >95%. Further, durability testing showed E ½ retention >95% in N2- and O2-saturated solutions after 2000 cycles, demonstrating the highest ORR activity and stability reported to date for GN-based electrocatalysts in acidic media.

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. Chemical structural changes of the hydrophobic blocks through polar group, fluorination, and bisphenol type allowed further control of the specific properties. DMFC performance was analyzed to elicit the impact of structural variations of the multiblock copolymers. Finally, DMFC performances of selected multiblock copolymers were compared against that of the industrial standard Nafion in the DMFC system.

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. More interestingly, the cyclic redox between 4ATP and DMAB by SPAC approach has been demonstrated. This SPAC methodology presents a unique platform for studying chemical reactions that are not possible under standard synthetic conditions.

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.

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. Axial and tangential strain relief is initiated by the formation of periodic a/2 <110> perfect dislocations via nucleation and glide on {111} slip-planes. Glide of dislocation segments is directly confirmed by real-time in situ transmission electron microscope observations and by dislocation dynamics simulations. Further shell growth leads to roughening and grain formation which provides additional strain relief. As a consequence of core/shell strain sharing in NWs, a 16 nm radius Ge NW with a 3 nm Si shell is shown to accommodate 3% coherent strain at equilibrium, a factor of 3 increase over the 1 nm equilibrium critical thickness for planar Si/Ge heteroepitaxial growth.

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. We also exploited the strong enhancement of MX emission to perform second-order photon correlation and cross-correlation experiments using very low pump fluences and observed a strong photon bunching that decays with increasing pump fluence.

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. Those thorny Au nanostructures, with higher surface areas and unique geometric feature, show a SERS detection sensitivity of 1 × 10(-9) M (sub-ppb level) toward two different analyte molecules, 4-MBA and Rhodamine B, demonstrating their generality for SERS applications. These highly sensitive SERS-active substrates offer novel robust structures for trace detection of chemical and biological analytes.

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.g., pyridinic, pyrrolic, and quaternary) influence the catalyst activity. In particular, an optimal temperature for heat treatment during synthesis is critical to creating a high-surface-area catalyst with favorable nitrogen doping. The sole Co phase, Co(9)S(8), was present in the catalyst but plays a negligible role in ORR. Nevertheless, the addition of Co species in the synthesis is indispensable for achieving high activity, due to its effects on the final catalyst morphology and structure, including surface area, nitrogen doping, and graphene formation. This new route for the preparation of a nitrogen-doped graphene nanocomposite with carbon nanotube offers synthetic control of morphology and nitrogen functionality and shows promise for applications in nonaqueous oxygen reduction electrocatalysis for Li-O(2) battery cathodes.

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. The deposition mechanisms are explained using a diffusion-limited aggregation (DLA) model to describe the semi-fractal-like growth of the metal nanostructures. We find these composite films to exhibit strong surface-enhanced Raman (SERS) activity, and the nanostructured features are optimized with respect to SERS activity using a self-assembled monolayer of mercapto-benzoic acid as a model Raman reporter. SERS enhancements are estimated to be on the order of 10(7). Through micro-Raman SERS mapping, these materials are shown to exhibit uniform SERS responses over macroscopic areas. These metal-polymer nanocomposites benefit from the underlying polymer's processability to yield SERS-active materials of almost limitless shape and size and show significant promise for future SERS-based sensing and detection schemes.

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). This facile field-assisted synthesis of Ag nanoparticles via chemical reduction presents an alternative approach to nanomaterial fabrication, which can yield a wide range of unique structures with enhanced optical properties that were previously inaccessible by other synthetic routes.

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). The resulting functionalized polymers form stable and exceptionally hydrophilic thin films that are approximately 2-5 nm thick, a mass coverage that varies with the MTP graft density. These films are characterized using a variety of techniques (X-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS), ellipsometry, surface plasmon resonance (SPR), and matrix assisted laser desorption ionization (MALDI)) to establish their structure and function. The protein resistance of the films, as demonstrated by their exposure to solutions of bovine serum albumin (BSA), can be modulated by the amount of MTP grafted to the polymer, which in turn, affects their mass coverage. We show that it is possible to specifically capture hexahistidine tagged proteins with low incidences of nonspecific adsorption using these materials, a discrimination quantified using surface plasmon resonance (SPR) at concentrations down to approximately 20 nM. These polymers also bind strongly to the surfaces of Au nanoparticles, stabilizing them against aggregation, providing them with a similar capacity to selectively bind 6His tagged proteins that can then be speciated using MALDI.

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). The nanocavities formed by the stereotypical stacking of these Ag nanosheets and the junctions and gaps between two neighboring AgNPs are believed to be responsible for the strong SERS response upon plasmon absorption. These homogeneous metal nanostructure decorated PANI membranes can be used as highly efficient SERS substrates for sensitive detection of chemical and biological analytes.

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. Linear combinations of individual nanoparticle tag spectra are generated in precursor solutions and are found to directly translate to the final encoder bead fingerprint spectrum in a 1:1 binding stoichiometry that preserves the original solution ratios. The result is an ability to multiplex spectral signatures in both frequency and intensity space to generate a large number of unique encoder signatures from a limited number of initial tag spectra. Raman microscopy of 75 individual beads shows that spectral response is highly uniform from bead-to-bead, making the encoder assemblies suitable for highly multiplexed bioassay applications and as model systems for cellular surface labeling studies for imaging and immunoassays.

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). Spectroscopic characterization of these substrates suggests their potential for use in few or single-molecule Raman spectroscopy.

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. The high degree of spatial uniformity of the crystals, formed by a soft nanoimprint technique, provides the ability to image binding events over large areas with micrometer spatial resolution. These features, together with compact form factors, low-cost fabrication procedures, simple readout apparatus, and ability for direct integration into microfluidic networks and arrays, suggest promise for these devices in label-free bioanalytical detection systems.

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). This effect is linearly correlated with the surface potentials and wetting properties of these SAMs. Surface layers comprised of poly(ethylene glycol) (PEG) grafted polyacrylamide polymers behave differently, affecting the SE yield by attenuation according to the polymer thickness. These results demonstrate the relative contributions of factors related to the adsorbate molecular structures that serve to strongly mediate the SE yield, providing a foundation for exploiting them as a quantitative electron imaging probe. The latter capability is demonstrated using a model microfluidic assay in which a series of proteins was spatially addressed to a SAM-based pixel array. The gray scale contrasts seen with protein adsorption are directly correlated with both protein molecular weight and mass coverage. These methods are used in two model protein assay experiments: (1) the measurement of the concentration dependent adsorption isotherm for a model protein (fibrinogen); and (2) the selective recognition of a biotinylated protein layer by avidin. These results demonstrate a unique approach to imaging protein binding processes on surfaces with both high analytical and spatial sensitivity.

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. J. Am. Chem. Soc. 1997, 119, 7760; Nashner et al. J. Am. Chem. Soc. 1998, 120, 8093; Frenkel et al. J. Phys. Chem. B 2001, 105, 12689] showed that the Pt and Pt-Ru based clusters are formed with fcc(111)-stacked cuboctahedral geometry and essentially bulklike metal-metal bond lengths, even for the smallest (few atom) nanoparticles for which the average coordination number is much smaller than that in the bulk, and that Pt in bimetallic [PtRu(5)] clusters segregates to the ambient surface of the supported nanoparticles. We explain these observations and characterize the cluster structures and bond length distributions using density functional theory calculations with graphite as a model for the support. The present study reveals the origin of the observed metal-metal bond length disorder, distinctively different for each system, and demonstrates the profound consequences that result from the cluster/carbon-support interactions and their key role in the structure and electronic properties of supported metallic nanoparticles.