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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
29 Aug 2001-Langmuir
TL;DR: There seems to be little or no correlation between the adsorption of protein (fibrinogen and lysozyme) and the adhesion of cells.
Abstract: This paper examines the hypothesis that surfaces resistant to protein adsorption should also be resistant to the adhesion of bacteria (Staphylococcus aureus, Staphylococcus epidermidis) and the attachment and spreading of mammalian cells (bovine capillary endothelial (BCE) cells). The surfaces tested were those of self-assembled monolayers (SAMs) terminated with derivatives of tri(sarcosine) (Sarc), N-acetylpiperazine, permethylated sorbitol, hexamethylphosphoramide, phosphoryl choline, and an intramolecular zwitterion (−CH2N+(CH3)2CH2CH2CH2SO3-) (ZW); all are known to resist the adsorption of proteins. There seems to be little or no correlation between the adsorption of protein (fibrinogen and lysozyme) and the adhesion of cells. Surfaces terminated with derivatives of Sarc and N-acetylpiperazine resisted the adhesion of S. aureus and S. epidermidis as well as did surfaces terminated with tri(ethylene glycol). A surface that presented Sarc groups was the only one that resisted the adhesion of BCE cells a...

587 citations

Journal ArticleDOI
TL;DR: The formation of conformal electrodes from the fluid metal eutectic, Ga–In (which the authors abbreviate “EGaIn” and pronounce “e-gain”) are described and their use in studying charge transport across self-assembled monolayers (SAMs) is described.
Abstract: Herein we describe the formation of conformal electrodes from the fluid metal eutectic, Ga–In (which we abbreviate “EGaIn” and pronounce “e-gain”; 75% Ga, 25% In by weight, m.p.= 15.5 8C), and their use in studying charge transport across self-assembled monolayers (SAMs). Although EGaIn is a liquid at room temperature, it does not spontaneously reflow into the shape with the lowest interfacial free energy as do liquids such as Hg and H2O: as a result, it can be formed into metastable, nonspherical structures (e.g., cones, and filaments with diameters 1 mm). This behavior, along with its high electrical conductivity (3.4 4 10 Scm ) and its tendency to make low contact-resistance interfaces with a variety of materials, makes EGaIn useful for forming electrodes for thin-film devices. We discuss the convenience and precision of measurements of current density (J, Acm ) versus applied voltage (V, V) through SAMs of n-alkanethiolates on template-stripped, ultraflat Ag (Ag–SCn Ag–SCnH2n+1, n= 10, 12, 14, 16) using EGaIn. An ideal electrode for physical-organic studies of SAMs would 1) make conformal, but nondamaging, physical contacts, 2) readily form small-area (micrometer diameter) contacts, to minimize the contribution of defects in the SAM to J, 3) form without specialized equipment, and 4) be nontoxic. Point 3 is particularly important: elimination of procedures such as evaporating metals or lithographic patterning would allow a wide range of laboratories—including those without access to clean rooms—to survey relationships between structure and electrical conductivity. There are currently three general techniques for forming top contacts for large-area (i.e., more than a few molecules) electrical measurements on SAMs of organic molecules: 1) The direct deposition of metals such as Au or Ti by using electron-beam or thermal evaporation ensures atomic-level contact, but results in low yields of devices owing to damage to the organic monolayer by reaction with hot metal vapors, and in the formation of metal filaments that short the junctions. 2) The installation of an electrically conducting polymer between the SAM and a metallic top contact inhibits formation of metal filaments, but the instability of SAMs of alkanethiolates to the temperatures required to anneal most electroactive polymers limits the broad application of this approach. 3) The use of Hg allows formation of conformal contacts at room temperature, but Hg is toxic, amalgamates with metals, tends to form junctions that short, is difficult to form into small contacts, and measurements with Hg must be performed under a solvent bath. EGaIn does not flow until it experiences a critical surface stress (0.5 Nm ), at which point it yields (i.e., flows). EGaIn 1) makes conformal, nondamaging contacts at room temperature, 2) can be molded into nonspherical shapes with micrometer-scale (or larger) dimensions, 3) is commercially available, 4) can be deposited with a pipette or syringe without high temperatures or vacuum, 5) has a low vapor pressure, and 6) is nontoxic. The work function of EGaIn (4.1–4.2 eV) is close to that of Hg (4.5 eV), but EGaIn does not alloy with many metals. It is therefore an ideal replacement for Hg, especially in devices that incorporate SAMs (which are generally formed on Au or Ag). Auger spectroscopy on samples of EGaIn in air show that its surface is principally composed of oxides of Ga (see the Supporting Information); gallium oxide is an n-type semiconductor. There is undoubtedly an adsorbed film of water on this surface, as EGaIn has a high surface free energy (ca. 630 dynescm ), as do oxides formed from similar metals. During our measurements, there were no observable changes in the average magnitude or range of J when EGaIn was allowed to sit in air for extended periods before we deposited it on the SAM, or when we performed the measurements using the same drop of EGaIn to form between three and five junctions, or while we flowed dry N2 over the sample: therefore the contribution of the surface oxide to J was probably constant for the duration of the experiments. We formed EGaIn electrodes by suspending a drop of EGaIn from a metal 26s-gauge needle affixed to a 10-mL syringe, bringing the drop into contact with the bare surface of a sacrificial film of Ag using a micromanipulator, and retracting the needle slowly (ca. 50 mms ); the EGaIn adhered to both the needle and the Ag (Figure 1). The drop of EGaIn pinched into to an hour-glass shape until it bifurcated into two structures, one attached to the syringe (a cone approximately 0.05 mL in volume) and one (which was discarded) attached to the Ag. We produced conical tips of EGaIn with diameters ranging from less than 1 mm to 100 mm; the larger the bore of the needle, and the more rapidly we [*] Dr. R. C. Chiechi, Dr. E. A. Weiss, Dr. M. D. Dickey, Prof. G. M. Whitesides Department of Chemistry and Chemical Biology Harvard University 12 Oxford St., Cambridge, MA 02138 (USA) Fax: (+1)617-495-9857 E-mail: gwhitesides@gmwgroup.harvard.edu

586 citations

Journal ArticleDOI
01 Mar 1988-Langmuir
TL;DR: In this paper, a self-assembled, supported organic monolayer films are used to study problems in the physical-organic chemistry and materials science of organic surfaces, especially the relation between the molecular-level structure of the film constituents and the macroscopic properties of the assembled monolayers.
Abstract: : Exposure of evaporated gold films supported on silicon wafers to solutions of dialkyl sulfides (R(CH2)m-S-(CH2)n-R'; R and R'=CH3 or CO2H) or alkyl thiols (R(CH2)nSH, R=CO2H or CH3) in methanol or ethanol results in rapid formation of a monolayer of the organosulfer compound adsorbed onto the gold. The resulting films have been characterized using a number of techniques, including X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IRS), ellipsometry, and wetting. These self-assembled, supported organic monolayer films are systems that can be used to study problems in the physical-organic chemistry and materials science of organic surfaces, especially the relation between the molecular-level structure of the film constituents and the macroscopic properties of the assembled monolayers. Keywords: Thin film, Monolayer, Surface spectroscopy, electronic materials

579 citations

Journal ArticleDOI
TL;DR: In this article, the authors describe an experimentally straightforward procedure for preparing and screening surfaces for their ability to resist the adsorption of proteins from solution, which is referred to as protein resistant surfaces.
Abstract: This contribution describes an experimentally straightforward procedure for preparing and screening surfaces for their ability to resist the adsorption of proteins from solution. For brevity, we call surfaces “protein resistant” when they are resistant to the adsorption of proteins from solution. We have used this procedure to identify several functional groups that had not previously been recognized as protein resistant. This work both identifies a number of functional groups that will be useful in designing resistance to the adsorption of biomolecules into devices used in sensing and in cell biology,1-3 and contributes to an understanding of the mechanism of action of protein resistant surfaces by correlating this property with molecular-scale structure.4-6 We combined self-assembled monolayers (SAMs)7,8 and surface plasmon resonance (SPR) spectroscopy9 into a system that enabled us to screen a number of functional groups rapidly for their ability to resist the adsorption of proteins. The surfaces were prepared by the “anhydride method” (Figure 1).10,11 This reaction generates a “mixed” SAM that comprises an ∼1:1 mixture of -CONRR′ and CO2H/CO2 groups.10,12 (We have not defined the state of the ionization of the CO2H groups in these SAMs.) The ease with which this class of mixed SAMs can be prepared by the anhydride method (relative to the synthesis of the functionalized alkanethiols HS(CH2)nR′ normally used for the preparation of singlecomponent SAMs) makes this route efficient for exploratory and screening work.11,13 We have examined the adsorption of two proteins to these surfaces: fibrinogen, a large (340 kD) blood plasma protein that adsorbs strongly to hydrophobic surfaces, and lysozyme, a small protein (14 kD, pI ) 12) that is positively charged under the conditions of our experiment (phosphate buffered saline, PBS, pH 7.4). Fibrinogen is used as a model for “sticky” serum proteins;11,14,15 lysozyme is often used in model studies of electrostatic adsorption of proteins to surfaces.16,17 Since lysozyme has a substantial net positive charge (Zp ) + 7.5 at pH 7.4, 100 mM KCl),18 it allowed us to examine attractive electrostatic interactions with CO2 groups on the surface. We have prepared more than 50 surfaces, each presenting a different functional group, using the anhydride procedure, and surveyed them for protein resistance.19 Table 1 summarizes selected results from this survey. The amount of protein adsorbed (∆RU ) change in response units) as measured by SPR was determined by subtracting the value of RU prior to the injection of protein from the value of RU measured 10 min after the completion of the protein injection; for clarity, these points are each labeled with a vertical dashed line in Figure 2. The value of ∆RU was used to calculate the percentage of a monolayer (%Monolayer) of that protein using eq 1.11,20 We define “%ML” according to eq 1 strictly to simplify the comparison between surfaces.

576 citations

Journal ArticleDOI
TL;DR: In this paper, the authors describe an experimentally simple system for measuring rates of electron transport across organic thin films having a range of molecular structures, which uses a metal−insulator−metal junction based on self-assembled monolayers (SAMs).
Abstract: This paper describes an experimentally simple system for measuring rates of electron transport across organic thin films having a range of molecular structures. The system uses a metal−insulator−metal junction based on self-assembled monolayers (SAMs); it is particularly easy to assemble. The junction consists of a SAM supported on a silver film (Ag-SAM(1)) in contact with a second SAM supported on the surface of a drop of mercury (Hg-SAM(2))that is, a Ag-SAM(1)SAM(2)-Hg junction. SAM(1) and SAM(2) can be derived from the same or different thiols. The current that flowed across junctions with SAMs of aliphatic thiols or aromatic thiols on Ag and a SAM of hexadecane thiol on Hg depended both on the molecular structure and on the thickness of the SAM on Ag: the current density at a bias of 0.5 V ranged from 2 × 10-10 A/cm2 for HS(CH2)15CH3 on Ag to 1 × 10-6 A/cm2 for HS(CH2)7CH3 on Ag, and from 3 × 10-6 A/cm2 for HS(Ph)3H (Ph = 1,4-C6H4) on Ag to 7 × 10-4 A/cm2 for HSPhH on Ag. The current density increase...

576 citations


Cited by
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Journal ArticleDOI

[...]

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