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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.


Papers
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Journal ArticleDOI
TL;DR: It is demonstrated that the rEPADs provide a simple system for direct and accurate voltammetric measurements that are referenced by an electrode with a constant, well-defined potential.
Abstract: This paper describes the development of a referenced Electrochemical Paper-based Analytical Device (rEPAD) comprising a sample zone, a reference zone, and a connecting microfluidic channel that includes a central contact zone. We demonstrated that the rEPADs provide a simple system for direct and accurate voltammetric measurements that are referenced by an electrode with a constant, well-defined potential. The performance of the rEPADs is comparable to commercial electrochemical cells, and the layout can be easily integrated into systems that permit multiplexed analysis and pipette-free sampling. The cost of this portable device is sufficiently low that it could be for single-use, disposable applications, and its method of fabrication is compatible with that used for other paper-based systems.

92 citations

Journal ArticleDOI
TL;DR: In this article, a model 3-D microstructure of hexagonal elements was fabricated using a liquid drop as a template to direct the self-assembly of the hexagonal rings.
Abstract: Microfabrication, as it is now practiced, is based on photolithography and is an intrinsically two-dimensional technology.1 Many important objectives in microfabrication, microelectronic devices having new architectures,2 photonic band-gap crystals,3 and biomimetic structures,4 require 3-D fabrication. This communication describes the fabrication of a model 3-D microstructure, an open, spherical, metal lattice composed of hexagonal elements ∼100 μm in size (Figure 1a).5 The procedure used to fabricate this structure combined the following three elements that will, we believe, be widely applicable in 3-D microfabrication: (i) the surface of a drop of liquid to act as a template for the structure, (ii) self-assembly using capillary forces at a liquidliquid interface6,7 to order the components on the surface of the spherical drop, and (iii) microelectrodeposition of metal to form and weld together the small, regular metallic structural components. The components on which this structure is based are open hexagons designed to support the modification of their surface properties. Using capillary forces8 for self-assembly requires careful control of the wettability of the surfaces of these hexagonal rings. Hexagons A are designed to assemble around a drop of water suspended in heptane. All sides and one hexagonal face are hydrophobic; the other hexagonal face is hydrophilic. Hexagons B have all sides plus one hexagonal face hydrophilic and one hexagonal face hydrophobic; these hexagons assemble around a drop of perfluorodecalin (PFD), chloroform, or chlorobenzene suspended in water. Figure 2 outlines the preparation of hexagonal rings. An array of 6-μm deep hexagonal molds was prepared using standard photolithographic techniques. Gold was electroplated into these molds.9 To fabricate hexagons A, a thin layer of Si (50 nm) was evaporated on top of the wafer; this Si formed a hydrophilic layer of native SiO2. The photoresist was removed by rinsing with acetone. The Si layer was removed simultaneously, leaving Si only on top of the hexagons. The wafers were briefly (<15 min) sonicated in ethanol to lift off the hexagons. Addition of hexadecanethiol to the suspension of hexagons rendered the gold surfaces of the hexagons hydrophobic. To fabricate hexagons B, the photoresist was removed prior to evaporation of Si or Ti. The Si and Ti both formed native oxide layers and covered all sides except for the bottom; this process created hexagons with all sides and one hexagonal face hydrophilic. We used a liquid drop as a template to direct the self-assembly of the hexagonal rings. The use of a liquid-liquid interface allowed the hexagons to adjust their positions relative to one another without frictional impediment under the influence of capillary forces.6 The procedure used to assemble the spherical lattice relied on several stages of self-assembly. The hexagons were prepared and suspended in the continuous phase. On addition of a drop of the discontinuous phase, an approximately spherical liquid-liquid interface formed spontaneously. The hexagons segregated and ordered on this interface. Hexagons A were suspended in heptane, and a small drop of water (diameter ∼1-2 mm, 50 μL) was added. The hexagons assembled on the surface of the drop (Figure 1b). In all experiments, an excess of hexagons was used, which remained suspended in heptane. These hexagons did not interfere with the assembled spheres. The inside of the sphere was pink; this pattern of coloration demonstrated that the hydrophilic Si/SiO2 sides faced the water phase. Assembly of hexagons B around a droplet of PFD in water gave similar spherical assemblies but with a pink exterior surface (Figure 1c). Because the hexagons had a low aspect ratio (∼1:16) and a large center hole, some overlapped, giving double layers. Similar effects have been observed in other mesoscale self-assembling systems.10 Other defects that can be seen in Figure 1a-c are voids that are due to poor packing of the hexagons.11 When self-assembly of the hexagons at the surface of the drop was complete, the resulting structure was, of course, held together only by capillary forces. These structures were surprisingly robust; pressing together two drops of hexane, suspended in water and covered with hexagons, * To whom correspondence should be addressed. Tel: (617) 495-9430. Fax: (617) 495-9857. E-mail: gwhitesides@gmwgroup.harvard.edu. (1) Moreau, W. M. Semiconductor Lithography; Plenum Press: New York, 1988. (2) Lent, C. S.; Tougaw, P. D.; Porod, W.; Bernstein, G. H. Nanotechnology 1993, 4, 49. (3) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, 1995. (4) Gopel, W. Biosens. Bioelectron. 1995, 10, 35. (5) Initial experiments showed that the size of the hexagons can be reduced to 10 μm without fundamentally changing the assembly process. Hexagons with sizes smaller than 10 μm could be fabricated, but the agitation of these particles might be difficult. The capillary forces become small (in an absolute sense), and the assemblies will be easily disturbed. (6) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233. (7) Terfort, A.; Bowden, N.; Whitesides, G. M. Nature 1997, 386, 162. (8) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. Simpson, A. W.; Hodkinson, P. H. Nature 1972, 237, 320. (9) We used a commercial gold sulfite plating solution (Technigold 25 E.; Technic, Inc. Cranston, RI), with current densities of 1-5 mA/cm2 and solution temperatures of 45-60 °C. (10) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349. Figure 1. (a) Porous sphere composed of self-assembled hexagonal rings welded by electrodeposition of Ag. The rings had approximately 100μm sides and were assembled on a drop of chlorobenzene in an aqueous silver-plating solution. Defects are indicated by arrows: A, void; B, double layer; C, point at which the electrode was attached. (b) Mesoscale selfassembly of hexagons; hexagons A assembled on a drop of water in heptane. (c) Hexagons B assembled on a drop of perfluorodecalin in water. (d) Two spheres pressed against each other do not fuse; the assemblies have substantial stability even without welding to connect the particles. Scale bar ) 100 μm. 8267 J. Am. Chem. Soc. 1998, 120, 8267-8268

91 citations

Journal ArticleDOI
TL;DR: A pneumatically actuated soft robot capable of navigating the inside of a tube that has the potential to be collaborative (i.e., work with humans) and also to interact safely with delicate environments.
Abstract: This article demonstrates a pneumatically actuated soft robot capable of navigating the inside of a tube. This robot was built using buckling pneumatic actuators (vacuum-actuated muscle-inspired pneumatic structures, or VAMPs). The tube climber can navigate through a tube with turns, inclines, and varying diameters. The robot is also able to remove obstacles (of more than 10 times its own weight) from tubes to perform a clearing function. It maintains climbing and clearing performance in wet conditions and under water. The tube climber is lightweight and completely soft and thus has the potential to be collaborative (i.e., work with humans) and also to interact safely with delicate environments.

91 citations

Patent
13 Jan 2006
TL;DR: In this article, a plate defining a surface and a plurality of cytophilic islands that adhere cells, isolated by cytophobic regions to which cells do not adhere, contiguous with the plate, are formed of a self-assembled monolayer.
Abstract: The invention provides a device for adhering cells in a specific and predetermined position, and associated methods. The device includes a plate defining a surface and a plurality of cytophilic islands that adhere cells, isolated by cytophobic regions to which cells do not adhere, contiguous with the cytophilic islands. The islands or the regions or both may be formed of a self-assembled monolayer (SAM).

90 citations

Journal ArticleDOI
TL;DR: In this paper, the crystal structures of a series of seven l:1 complexes between N,Nrbis(z-X-phenyl)melamine and 5,5-diethyl barbituric acid (X = H, F, Cl, Br, I, CH3, and CF3) were described.
Abstract: Abstracf This paper describes the crystal structures of a series of seven l:1 complexes between N,Nrbis(z-X-phenyl)melamine and 5,5-diethylbarbituric acid (X = H, F, Cl, Br, I, CH3, and CF3). This series provides small perturbations on the structures of rnolecules (l{,Nrbis(p-X-phenyl)melamines) used in a previous study (Zerkowski, J. A.; MacDonald, J. C.; Seto, C. T.; Wierda, D. A.; Whitesides, G. M. J. Am. Chem. Soc. In press). Both the meta andparc complexes crystallize as hydrogen-bonded *tapes'. With the phenyl substituent in the metaposition, however, the melamines can adopt a greater number of molecular conformations; this feature leads to a greater variety of orientations of packing. The meta series packs in both linear and crinkled tape motifs, and four of the seven complexes are solvates. By contrast, the para series, which used the same set of phenyl substituents, always yielded linear tapes and crystals without solvent inthelattice. Inclusionofsolventincreasesthepackingcoefficientsofmembersofthezeraseriestovaluesapproximately equal to those of the para series. The multiplicity of molecular conformations available to the meta series is probably largely responsible for the clathration of solvent, which does not rely upon specific non-covalent interactions except in the case of the z-iodo complex with acetonitrile. The wider range of crystalline architecture in themeta series attests only to the kinetic accessibility of these packing formats; polymorphic phases of greater thermodynamic stability may occur. Polymorphism has not been investigated in the series of meta-substituted compounds. 4305

90 citations


Cited by
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08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
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 …

33,785 citations

28 Jul 2005
TL;DR: PfPMP1)与感染红细胞、树突状组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�ly.
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。

18,940 citations

Journal ArticleDOI
TL;DR: Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols used xiii 1.
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

14,171 citations

Journal ArticleDOI
05 Feb 2009-Nature
TL;DR: The direct synthesis of large-scale graphene films using chemical vapour deposition on thin nickel layers is reported, and two different methods of patterning the films and transferring them to arbitrary substrates are presented, implying that the quality of graphene grown by chemical vapours is as high as mechanically cleaved graphene.
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.

10,033 citations

Journal ArticleDOI
29 Aug 1997-Science
TL;DR: In this article, a general approach for multilayers by consecutive adsorption of polyanions and polycations has been proposed and has been extended to other materials such as proteins or colloids.
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.

9,593 citations