Bong June Sung

Enhancement of electrical and thermomechanical properties of silver nanowire composites by the introduction of nonconductive nanoparticles: experiment and simulation.

Electrically conductive polymer nanocomposites have been applied extensively in many fields to develop the next generation of devices. Large amounts of conductive nanofillers in polymer matrices are, however, often required for a sufficiently high electrical conductivity, which in turn deteriorates the desired thermomechanical properties. We illustrate a novel but facile strategy to improve the electrical conductivity and the thermomechanical property of silver nanowire/polymer nanocomposites. We find that one may increase the electrical conductivity of silver nanowire/polymer nanocomposites by up to about 8 orders of magnitude by introducing silica nanoparticles with nanocomposites. The electrical percolation threshold volume fraction of silver nanowires decreases from 0.12 to 0.02. Thermomechanical properties also improve as silica nanoparticles are introduced. We carry out extensive Monte Carlo simulations to elucidate the effects of silica nanoparticles at a molecular level and find that van der Waals attractive interaction between silica nanoparticles and silver nanowires dominates over the depletion-induced interaction between silver nanowires, thus improving the dispersion of silver nanowires. Without silica nanoparticles, silver nanowires tend to aggregate, which is why additional silver nanowires are required for a desired electrical conductivity. On the other hand, with silica nanoparticles mixed, the electrical percolating network is likely to form at a smaller volume fraction of silver nanowires.

High-concentration boron doping of graphene nanoplatelets by simple thermal annealing and their supercapacitive properties.

For the utilization of graphene in various energy storage and conversion applications, it must be synthesized in bulk with reliable and controllable electrical properties. Although nitrogen-doped graphene shows a high doping efficiency, its electrical properties can be easily affected by oxygen and water impurities from the environment. We here report that boron-doped graphene nanoplatelets with desirable electrical properties can be prepared by the simultaneous reduction and boron-doping of graphene oxide (GO) at a high annealing temperature. B-doped graphene nanoplatelets prepared at 1000 °C show a maximum boron concentration of 6.04 ± 1.44 at %, which is the highest value among B-doped graphenes prepared using various methods. With well-mixed GO and g-B2O3 as the dopant, highly uniform doping is achieved for potentially gram-scale production. In addition, as a proof-of-concept, highly B-doped graphene nanoplatelets were used as an electrode of an electrochemical double-layer capacitor (EDLC) and showed an excellent specific capacitance value of 448 F/g in an aqueous electrolyte without additional conductive additives. We believe that B-doped graphene nanoplatelets can also be used in other applications such as electrocatalyst and nano-electronics because of their reliable and controllable electrical properties regardless of the outer environment.

Disulfide bond cleavage in TEMPO-free radical initiated peptide sequencing mass spectrometry

The gas-phase free radical initiated peptide sequencing (FRIPS) fragmentation behavior of o-TEMPO-Bz-conjugated peptides with an intra- and intermolecular disulfide bond was investigated using MS(n) tandem mass spectrometry experiments. Investigated peptides included four peptides with an intramolecular cyclic disulfide bond, Bactenecin (RLCRIVVIRVCR), TGF-α (CHSGYVGVRC), MCH (DFDMLRCMLGRVFRPCWQY) and Adrenomedullin (16-31) (CRFGTCTVQKLAHQIY), and two peptides with an intermolecular disulfide bond. Collisional activation of the benzyl radical conjugated peptide cation, which was generated through the release of a TEMPO radical from o-TEMPO-Bz-conjugated peptides upon initial collisional activation, produced a large number of peptide backbone fragments in which the S-S or C-S bond was readily cleaved. The observed peptide backbone fragments included a-, c-, x- or z-types, which indicates that the radical-driven peptide fragmentation mechanism plays an important role in TEMPO-FRIPS mass spectrometry. FRIPS application of the linearly linked disulfide peptides further showed that the S-S or C-S bond was selectively and preferentially cleaved, followed by peptide backbone dissociations. In the FRIPS mass spectra, the loss of •SH or •SSH was also abundantly found. On the basis of these findings, FRIPS fragmentation pathways for peptides with a disulfide bond are proposed. For the cleavage of the S-S bond, the abstraction of a hydrogen atom at C(β) by the benzyl radical is proposed to be the initial radical abstraction/transfer reaction. On the other hand, H-abstraction at C(α) is suggested to lead to C-S bond cleavage, which yields [ion ± S] fragments or the loss of •SH or •SSH.

The effect of matrix structure on the diffusion of fluids in porous media

The effect of matrix structure on the transport properties of adsorbed fluids is studied using computer simulations and percolation theory. The model system consists of a fluid of hard spheres diffusing in a matrix of hard spheres fixed in space. Three different arrangements of the fixed spheres, random, templated, and polymeric, are investigated. For a given matrix volume fraction the diffusion coefficient of the fluid, D, is sensitive to the manner in which the matrix is constructed, with large differences between the three types of matrices. The matrix is mapped onto an effective lattice composed of vertices and bonds using a Voronoi tessellation method where the connectivity of bonds is determined using a geometric criterion, i.e., a bond is connected if a fluid particle can pass directly between the two pores the bond connects, and disconnected otherwise. The percolation threshold is then determined from the connectivity of the bonds. D displays universal scaling behavior in the reduced volume fraction, i.e., D approximately (1-phi(m)phi(c))(gamma), where phi(m) is the matrix volume fraction and phi(c) is the matrix volume fraction at the percolation threshold. We find that gamma approximately 2.2, independent of matrix type, which is different from the result gamma approximately 1.53 for diffusion in lattice models, but similar to that for conduction in Swiss cheese models. Lattice simulations with biased hopping probabilities are consistent with the continuous-space simulations, and this shows that the universal behavior of diffusion is sensitive to details of local dynamics.

One-Step Peptide Backbone Dissociations in Negative-Ion Free Radical Initiated Peptide Sequencing Mass Spectrometry

Peptide dissociation behavior in TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-based FRIPS (free radical initiated peptide sequencing) mass spectrometry was analyzed in both positive- and negative-ion modes for a number of peptides including angiotensin II, kinetensin, glycoprotein IIb fragment (296-306), des-Pro(2)-bradykinin, and ubiquitin tryptic fragment (43-48). In the positive mode, the ·Bz-C(O)-peptide radical species was produced exclusively at the initial collisional activation of o-TEMPO-Bz-C(O)-peptides, and two consecutive applications of collisional activation were needed to observe peptide backbone fragments. In contrast, in the negative-ion mode, a single application of collisional activation to o-TEMPO-Bz-C(O)-peptides produced extensive peptide backbone fragmentations as well as ·Bz-C(O)-peptide radical species. This result indicates that the duty cycle in the TEMPO-based FRIPS mass spectrometry can be reduced by one-half in the negative-ion mode. In addition, the fragment ions observed in the negative-ion experiments were mainly of the a-, c-, x-, and z-types, indicating that radical-driven tandem mass spectrometry was mainly responsible for the TEMPO-based FRIPS even with a single application of collisional activation. Furthermore, the survival fraction analysis of o-TEMPO-Bz-C(O)-peptides was made as a function of the applied normalized collision energy (NCE). This helped us to better understand the differences in FRIPS behavior between the positive- and negative-ion modes in terms of dissociation energetics. The duty-cycle improvement made in the present study provides a cornerstone for future research aiming to achieve a single-step FRIPS in the positive-ion mode.

Insertion mechanism of cell-penetrating peptides into supported phospholipid membranes revealed by X-ray and neutron reflection

X-Ray and neutron reflectivity measurements on systems composed of a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer and transcription-activating-factor derived peptides (TDPs) have allowed us to determine the mechanism of membrane translocation. By monitoring the structural changes of the bilayers caused by the binding of TDPs while systemically varying temperature and TDP concentration, our results revealed the detailed molecular structures of the stepwise interactions that occurred during the translocation of TDP across the lipid bilayers. While little indication of membrane perturbation was observed at low TDP concentrations, we found that the TDP movement across the membrane induced defect formations in the membrane at higher TDP concentrations.

Effects of silica particles on the electrical percolation threshold and thermomechanical properties of epoxy/silver nanocomposites

We experimentally and theoretically demonstrated that the addition of silica particles could improve both the electrical conductivity and the thermomechanical properties of epoxy/silver nanocomposites. As silica particles were added, the electrical percolation threshold concentration of silver nanoparticles decreased and the electrical resistivity of the composite decreased by about 8 orders of magnitude. The coefficient of thermal expansion also decreased with increasing volume fraction of silica particles. Molecular simulations showed that the additional silica particles made the effective intermolecular interactions between silver nanoparticles attractive, thereby enhancing the formation of an electrical percolation network.

Monte Carlo simulations and integral equation theory for the structure of telechelic polymers

The structure of telechelic polymers is investigated using off-lattice Monte Carlo simulations and the polymer reference interaction site model (PRISM) integral equation theory. The polymer molecules are modeled as tangent-sphere freely-jointed chains where all beads interact via a hard sphere potential and end beads interact via an additional short-ranged attractive potential. The static properties, i.e., conformational properties, end-bead aggregation, intermolecular pair correlations, and partial static structure factors are investigated as a function of density and temperature. For a given density, as the temperature is lowered, the chain ends aggregate to form multiplets. For a given temperature, this tendency is greater at higher densities. Predictions of the PRISM theory for the pair correlation functions and partial static structure factors are compared to the simulation results. Three different closure approximations, the reference-Molecular mean spherical approximation (R-MMSA), the reference-mo...

Molecular-dynamics simulations for nonclassical kinetics of diffusion-controlled bimolecular reactions.

Molecular-dynamics simulations are presented for the diffusion-controlled bimolecular reaction A+B<==>C in two and three dimensions. The reactants and solvent molecules are modeled as spheres interacting via continuous potential-energy functions. The interaction potential between two reactants contains a deep well that results in a reaction. When the solvent concentration is low and the reactant dynamics is essentially ballistic, the system reaches equilibrium rapidly, and the reaction follows classical kinetics with exponential decay to the equilibrium. When the solvent concentration is high the particles enter the normal diffusion regime quickly and nonclassical behavior is observed, i.e., the reactant concentrations approach equilibrium as t(-d/2) where d is the dimensionality of space. When the reaction well depth is large, however, the reaction becomes irreversible within the simulation time. In this case the reactant concentrations decay as t(-d/4). Interestingly this behavior is also observed at intermediate times for reversible reactions.

Accuracy of the energy partitioning data obtained by classical trajectory calculations on potential energy surfaces constructed by interpolation: H2CO→H2 + CO as an example

Capability of the classical trajectory calculation on a potential energy surface (PES) constructed by interpolation in reproducing experimental energy partitioning data has been tested. The title reaction, a prototype polyatomic dissociation which has been heavily investigated over the years, has been taken as the example. The product energy partitioning data obtained by scaling the classical trajectory results on PESs constructed at the moderately high quantum chemical levels of Hartree–Fock (HF), second-order Mo/ller–Plesset (MP2), quadratic configuration interaction single double (QCISD), and B3LYP were nearly level-independent and reproduced the experimental data almost quantitatively. The overall scheme, which is systematic, may become an important quantitative tool for the study of the exit channel dynamics in favorable cases.