Single-Molecule Tracking of Polymer Surface Diffusion

Michael J. Skaug , Joshua N. Mabry , and Daniel K. Schwartz *

Department of Chemical and Biological Engineering,University of Colorado Boulder, Boulder, Colorado 80309, United States

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/ja407396v

Publication Date (Web): November 10, 2013

Copyright © 2013 American Chemical Society

The dynamics of polymers adsorbed to a solid surface are important in thin-film formation, adhesion phenomena, and biosensing applications, but they are still poorly understood. Here we present tracking data that follow the dynamics of isolated poly(ethylene glycol) chains adsorbed at a hydrophobic solid–liquid interface. We found that molecules moved on the surface via a continuous-time random walk mechanism, where periods of immobilization were punctuated by desorption-mediated jumps. The dependence of the surface mobility on molecular weight (2, 5, 10, 20, and 40 kg/mol were investigated) suggested that surface-adsorbed polymers maintained effectively three-dimensional surface conformations. These results indicate that polymer surface diffusion, rather than occurring in the two dimensions of the interface, is dominated by a three-dimensional mechanism that leads to large surface displacements and significant bulk–surface coupling.

The Role of Surface Oxygen in the Growth of Large Single-Crystal Graphene on Copper

Published Online October 24 2013Science 8 November 2013: 
Vol. 342 no. 6159 pp. 720-723 
DOI: 10.1126/science.1243879

The growth of high-quality single crystals of graphene by chemical vapor deposition on copper (Cu) has not always achieved control over domain size and morphology, and the results vary from lab to lab under presumably similar growth conditions. We discovered that oxygen (O) on the Cu surface substantially decreased the graphene nucleation density by passivating Cu surface active sites. Control of surface O enabled repeatable growth of centimeter-scale single-crystal graphene domains. Oxygen also accelerated graphene domain growth and shifted the growth kinetics from edge-attachment–limited to diffusion-limited. Correspondingly, the compact graphene domain shapes became dendritic. The electrical quality of the graphene films was equivalent to that of mechanically exfoliated graphene, in spite of being grown in the presence of O.

Folding Paper-Based Lithium-Ion Batteries for Higher Areal Energy Densities

Qian Cheng †, Zeming Song †, Teng Ma ‡, Bethany B. Smith †, Rui Tang §, Hongyu Yu §, Hanqing Jiang ‡, and Candace K. Chan *†

Nano Lett., 2013, 13 (10), pp 4969–4974
DOI: 10.1021/nl4030374
Publication Date (Web): September 23, 2013
Copyright © 2013 American Chemical Society

Paper folding techniques are used in order to compact a Li-ion battery and increase its energy per footprint area. Full cells were prepared using Li4Ti5O12 and LiCoO2 powders deposited onto current collectors consisting of paper coated with carbon nanotubes. Folded cells showed higher areal capacities compared to the planar versions with a 5 × 5 cell folded using the Miura-ori pattern displaying a 14× increase in areal energy density.

Lithographically Defined Macroscale Modulation of Lateral Fluidity and Phase Separation Realized via Patterned Nanoporous Silica-Supported Phospholipid Bilayers

Eric L. Kendall †, Viviane N. Ngassam ‡, Sean F. Gilmore §, C. Jeffrey Brinker , and Atul N. Parikh *†‡§


J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/ja408434r
Publication Date (Web): October 10, 2013
Copyright © 2013 American Chemical Society

Using lithographically defined surfaces consisting of hydrophilic patterns of nanoporous and nonporous (bulk) amorphous silica, we show that fusion of small, unilamellar lipid vesicles produces a single, contiguous, fluid bilayer phase experiencing a predetermined pattern of interfacial interactions. Although long-range lateral fluidity of the bilayer, characterized by fluorescence recovery after photobleaching, indicates a nominally single average diffusion constant, fluorescence microscopy-based measurements of temperature-dependent onset of fluidity reveals a locally enhanced fluidity for bilayer regions supported on nanoporous silica in the vicinity of the fluid–gel transition temperature. Furthermore, thermally quenching lipid bilayers composed of a binary lipid mixture below its apparent miscibility transition temperature induces qualitatively different lateral phase separation in each region of the supported bilayer: The nanoporous substrate produces large, microscopic domains (and domain-aggregates), whereas surface texture characterized by much smaller domains and devoid of any domain-aggregates appears on bulk glass-supported regions of the single-lipid bilayer. Interestingly, lateral distribution of the constituent molecules also reveals an enrichment of gel-phase lipids over nanoporous regions, presumably as a consequence of differential mobilities of constituent lipids across the topographic bulk/nanoporous boundary. Together, these results reveal that subtle local variations in constraints imposed at the bilayer interface, such as by spatial variations in roughness and substrate adhesion, can give rise to significant differences in macroscale biophysical properties of phospholipid bilayers even within a single, contiguous phase.

Villification: How the Gut Gets Its Villi

Amy E. Shyer1,*, Tuomas Tallinen2,3,*, Nandan L. Nerurkar1, Zhiyan Wei2, Eun Seok Gil4, David L. Kaplan4, Clifford J. Tabin1,†, L. Mahadevan2,5,6,7,8,†

Published Online August 29 2013Science 11 October 2013: 
Vol. 342 no. 6155 pp. 212-218 
DOI: 10.1126/science.1238842

The villi of the human and chick gut are formed in similar stepwise progressions, wherein the mesenchyme and attached epithelium first fold into longitudinal ridges, then a zigzag pattern, and lastly individual villi. We find that these steps of villification depend on the sequential differentiation of the distinct smooth muscle layers of the gut, which restrict the expansion of the growing endoderm and mesenchyme, generating compressive stresses that lead to their buckling and folding. A quantitative computational model, incorporating measured properties of the developing gut, recapitulates the morphological patterns seen during villification in a variety of species. These results provide a mechanistic understanding of the formation of these elaborations of the lining of the gut, essential for providing sufficient surface area for nutrient absorption.

Hybrid and Nonhybrid Lipids Exert Common Effects on Membrane Raft Size and Morphology

Frederick A. Heberle *†, Milka Doktorova ‡#, Shih Lin Goh ‡, Robert F. Standaert †§, John Katsaras†, and Gerald W. Feigenson *‡

†Biology and Soft Matter and §Biosciences Divisions,Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

‡Department of Molecular Biology and Genetics and#Tri-Institutional Training Program in Computational Biology and Medicine, Cornell University, Ithaca, New York 14853, United States

Departments of Biochemistry and Molecular & Cellular Biology and Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996,United States

 Joint Institute for Neutron Sciences, Oak Ridge, Tennessee 37831, United States

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/ja407624c

Publication Date (Web): September 16, 2013

Copyright © 2013 American Chemical Society

Nanometer-scale domains in cholesterol-rich model membranes emulate lipid rafts in cell plasma membranes (PMs). The physicochemical mechanisms that maintain a finite, small domain size are, however, not well understood. A special role has been postulated for chain-asymmetric or hybrid lipids having a saturated sn-1 chain and an unsaturated sn-2 chain. Hybrid lipids generate nanodomains in some model membranes and are also abundant in the PM. It was proposed that they align in a preferred orientation at the boundary of ordered and disordered phases, lowering the interfacial energy and thus reducing domain size. We used small-angle neutron scattering and fluorescence techniques to detect nanoscopic and modulated liquid phase domains in a mixture composed entirely of nonhybrid lipids and cholesterol. Our results are indistinguishable from those obtained previously for mixtures containing hybrid lipids, conclusively showing that hybrid lipids are not required for the formation of nanoscopic liquid domains and strongly implying a common mechanism for the overall control of raft size and morphology. We discuss implications of these findings for theoretical descriptions of nanodomains.