George M. Whitesides

Bio: George M. Whitesides is an academic researcher from Harvard University. The author has contributed to research in topics: Microcontact printing & Self-assembled monolayer. The author has an hindex of 240, co-authored 1739 publications receiving 269833 citations. Previous affiliations of George M. Whitesides include University of California, Davis & University of Texas at Austin.

Differentiating Functional Groups with the Scanning Tunneling Microscope

Abstract: Scanning tunneling microscopy (STM) images of l-docosanol, 1-docosanethiol, didocosyl disulfide, and 1-chlorooctadecane on graphite are compared. The images of the I -docosanethiol and didocosyl disulfide show bright spots which are attributed to the positions of the S-H and S-S functional groups. The STM images of the l-docosanol and l-chlorooctadecane do not show such bright spots. Thb fact that both the S-H and S-S groups appear bright in the STM images indicates that the presence of an S atom on graphite results in a higher tunneling current when the tip scans over it compared to the tunneling current over a C, O. or Cl atom. The diff'erent behavior of the S atoms conrpiired to the O. C. and Cl atoms is discussed in terrns of the interactions between these atoms and the underlving graphite substrate. The persistent brightness of S atoms in the images of molecular adsorbates sugsests that sult'ur nrav serve as a useful "chromophore" for molecules imased bv STM.

Electrostatic Self-Assembly of Polystyrene Microspheres by Using Chemically Directed Contact Electrification

Abstract: Herein we describe a process—based on contact electrification and electrostatic interactions—that directs the selfassembly of chemically modified polystyrene microspheres to form three-dimensional microstructures. When two solid surfaces are brought into contact and separated, charge is often transferred from one surface to the other in a process known as contact electrification. 2] We can predictably and rationally control the contact electrification of polystyrene microspheres, and use the resulting charged materials for electrostatic self-assembly. We control the contact electrification of these microspheres by introducing immobilized ions and mobile counterions: the choice of these ions determines the electrostatic charges that these beads acquire through contact before and during the assembly process. Oppositely charged microspheres assemble into uniform spherical microstructures under the influence of electrostatic forces. Sequential steps of self-assembly can create multilayered microstructures. There are many examples of electrostatic self-assembly of charged ions, polyelectrolytes, and colloids in solution. Xerography, an example of dry electrostatic self-assembly, uses corona discharge from a high-voltage electrode to create a charge on the imaging drum, and contact electrification to create an opposite charge on the toner particles. Contact electrification can also direct the self-assembly of millimetersized spheres into ordered two-dimensional lattices. (That process used the inherent differences in contact electrification of various polymers, in contrast to the rational, chemically directed contact electrification we describe herein.) Patterns of charge, created by electron-beam writing, an atomic force microscopy (AFM) tip, or electrical microcontact printing on a dielectric surface, can guide the self-assembly of microor nanoparticles with sub-100 nm lateral resolution. Although contact electrification is a familiar phenomenon, the detailed mechanisms of contact electrification are not known, and it is likely that different mechanisms may be involved depending on the specific materials and environmental conditions. There is one class of materials that exhibits predictable contact electrification and for which Diaz and coworkers have proposed a plausible mechanism of chargetransfer: the contact charging of ionomers (polymers with covalently bound ionic functional groups) is believed to result from the transfer of mobile ions from the ionomer to another material (Figure 1).

Oscillations with uniquely long periods in a microfluidic bubble generator

Abstract: Understanding spatiotemporal complexity1,2,3 is important to many disciplines, from biology4,5 to finance6. However, because it is seldom possible to achieve complete control over the parameters that determine the behaviour of real complex systems, it has been difficult to study such behaviour experimentally. Here we demonstrate a simple microfluidic bubble generator that shows stable oscillatory patterns (both in space and time) of unanticipated complexity and uniquely long repetition periods. At low flow rates, the device produces a regular stream of bubbles of uniform size. As the flow increases, the system shows intricate dynamic behaviour typified by a stable limit cycle of order 29 bubbles per period, which repeats without change over intervals of up to 100 periods and more. As well as providing an example of a well-characterized and experimentally tractable model system with which to study complex, nonlinear dynamics, such behaviour demonstrates that it is possible to observe complex and stable limit cycles without active external control.

Mesoscale Self-Assembly: Capillary Bonds and Negative Menisci

Abstract: This paper describes the self-assembly of hexagonal plates (with 2.7 mm wide sides) at the interface between perfluorodecalin (PFD) and water. All 14 different hexagons that can be made by permuting the number and location of the hydrophobic and hydrophilic faces were examined. The plates attracted one another by lateral capillary forces involving the menisci on the hydrophilic faces. The plates were made of poly(dimethylsiloxane) (PDMS) containing aluminum oxide and had a density of 1.86 g/cm3, close to the density of PFD (ρ = 1.91 g/cm3). This work complements a previous paper (Bowden, N.; Choi, I. S.; Grzybowski, B. A.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 5373) that examined the self-assembly of hexagonal plates of PDMS (ρ = 1.05 g/cm3) that had a density close to that of water, and were attracted through menisci on the hydrophobic faces. The arrays that formed from the heavy (ρ = 1.86 g/cm3) hexagons with a particular pattern of hydrophilic faces were analogous to the arrays that formed fro...

Patterned Block-Copolymer-Silica Mesostructures as Host Media for the Laser Dye Rhodamine 6G

Abstract: Rhodamine 6G-doped mesostructured silica is prepared by an acidic sol−gel route using poly-b-poly(propylene oxide)-b-poly(ethylene oxide) (EOx−POy−EOx) block copolymer surfactants. Using low-refrac...

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

The Theory of Island Biogeography

Abstract: Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

Large-scale pattern growth of graphene films for stretchable transparent electrodes

Abstract: Problems associated with large-scale pattern growth of graphene constitute one of the main obstacles to using this material in device applications. Recently, macroscopic-scale graphene films were prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films was found to be much larger than theoretically expected values. Here we report the direct synthesis of large-scale graphene films using chemical vapour deposition on thin nickel layers, and present two different methods of patterning the films and transferring them to arbitrary substrates. The transferred graphene films show very low sheet resistance of approximately 280 Omega per square, with approximately 80 per cent optical transparency. At low temperatures, the monolayers transferred to silicon dioxide substrates show electron mobility greater than 3,700 cm(2) V(-1) s(-1) and exhibit the half-integer quantum Hall effect, implying that the quality of graphene grown by chemical vapour deposition is as high as mechanically cleaved graphene. Employing the outstanding mechanical properties of graphene, we also demonstrate the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics.

Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites

Abstract: Multilayer films of organic compounds on solid surfaces have been studied for more than 60 years because they allow fabrication of multicomposite molecular assemblies of tailored architecture. However, both the Langmuir-Blodgett technique and chemisorption from solution can be used only with certain classes of molecules. An alternative approach—fabrication of multilayers by consecutive adsorption of polyanions and polycations—is far more general and has been extended to other materials such as proteins or colloids. Because polymers are typically flexible molecules, the resulting superlattice architectures are somewhat fuzzy structures, but the absence of crystallinity in these films is expected to be beneficial for many potential applications.