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.

Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers

Abstract: An electrical junction formed by mechanical contact between two self-assembled monolayers (SAMs)--a SAM formed from an dialkyl disulfide with a covalently linked tetracyanoquinodimethane group that is supported by silver (or gold) and a SAM formed from an alkanethiolate SAM that is supported by mercury-rectifies current. The precursor to the SAM on silver (or gold) was bis(20-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl)) disulfide and that for the SAM on mercury was HS(CH(2))(n-1)CH(3) (n = 14, 16, 18). The electrical properties of the junctions were characterized by current-voltage measurements. The ratio of the conductivity of the junction in the forward bias (Hg cathodic) to that in the reverse bias (Hg anodic), at a potential of 1 V, was 9 +/- 2 when the SAM on mercury was derived from HS(CH(2))(15)CH(3). The ratio of the conductivity in the forward bias to that in the reverse bias increased with decreasing chain length of the alkanethiol used to form the SAM on mercury. These results demonstrate that a single redox center asymmetrically placed in a metal-insulator-metal junction can cause the rectification of current and indicate that a fixed dipole in the insulating region of a metal-insulator-metal junction is not required for rectification.

Phase behavior of two-component self-assembled monolayers of alkanethiolates on gold

Abstract: This paper examines the relationship between the composition of two-component self-assembled monolayers (SAMs) of alkanethiolates on gold and the composition of the solutions from which they were formed. The SAMs were prepared by competitive adsorption of a long-chain alkanethiol (HS(CHz)zrCHl) and a short-chain alkanethiol (HS(CHz)rrOH) from solutions in ethanol. Under conditions in which the alkanethiolates in a SAM and the alkanethiols in solution are close to equilibrium, the relationship between the composition of the solution and the composition of the SAM suggests that the monolayer tends, thermodynamically, to exist as a single phase predominantly composed of either long-chain or short-chain thiolates. A derivation of the thermodynamic relationship betwecn the compositions of the SAM and solution is described that includes intermolecular interactions between components in the SAM; theory and experiment agre,equalitatively. This analysis concludes that, for a two-cornponent system of alkanethiolates on gold well-equilibrated with alkanethiols in solution, a single phase is preferred at equilibrium; phase-separated, two-component monolayers of the sort extensively studied in Langmuir systems are not observed. Introduc6on "Mired SAMs" on gold--elf-assembled monolayers (SAMs) compnsing two alkanethiolates prepared by cochemisorption from a solution containing two alkanethiols-are useful in studying phenomena involving organic surfaces.2-I3 A number of techniques can establish the auerage composition of a mixed SAM.2-I t X-ray photoelectron spectroscopy, XPS, can ascertain the composition over an area thesize of the X-rayspot (approximately I mm2), and scanning electron microscopy can sometimes detect heterogeneities in composition of a SAM on thedimension of the order of I prn;tl phase separation has not been observed on thesc scales. In short, neither has thc extcnt to which the components of a mixed SAM segregate into separate phases within a SAM been established nor has the question of the relationship betwecn the composition of the solution used to form the SAM and its heterogeneity bcen addressed experimentally or theoretically. Thc problem is a difficult one experimentally, because there are few techniques well adapted for dircctvisualization of phase-separated regions with small sizes in an organic monolayer, especially on optically opaque sub,strates. Scanning probe microscopies (especially atomic and lateral force microscopies)17 are techniques that are applicable but only to certain types of SAMs.rT-re In this psper, we investigate the phase behavior of twocomponent SAMs from both theoretical and experimental perspectives. The approach we have taken is thermodynamic-to scarch for characteristic signatures of phase separation in the dependencc of average composition on temperature, con@ntration, and tim-rather than spcctroscopic and based on irraging. We focused on the question of whether two-component SAMs form phasc-separated domains. In the theoretical section, we derive a thermodynamic relationship between the composition of the solution and the composition of the sAM in terms of interactions bctween nearest neighbors within the SAM. This relationsbip prcdicts that phase separation in a SAM by a mechanism involving theequilibrium of species in thc sAM with those in solution will be detectable through a sharp transition between the properties characteristic of one monolayer to those characteristic of thc second.

A microfabricated array of clamps for immobilizing and imaging C. elegans

Abstract: This paper describes the fabrication of a microfluidic device for rapid immobilization of large numbers of live C. elegans for performing morphological analysis, microsurgery, and fluorescence imaging in a high-throughput manner. The device consists of two principal elements: (i) an array of 128 wedge-shaped microchannels, or clamps, which physically immobilize worms, and (ii) a branching network of distribution channels, which deliver worms to the array. The flow of liquid through the device (driven by a constant pressure difference between the inlet and the outlet) automatically distributes individual worms into each clamp. It was possible to immobilize more than 100 worms in less than 15 min. The immobilization process was not damaging to the worms: following removal from the array of clamps, worms lived typical lifespans and reproduced normally. The ability to monitor large numbers of immobilized worms easily and in parallel will enable researchers to investigate physiology and behavior in large populations of C. elegans.

Enzyme-catalyzed organic synthesis: NAD(P)H cofactor regeneration by using glucose-6-phosphate and the glucose-5-phosphate dehydrogenase from Leuconostoc mesenteroides

Abstract: Glucose-6-phosphate dehydrogenase (from Leuconostoc mesenteroides) and glucose 6-phosphate comprise a useful system for regeneration of reduced nicotinamide cofactors for use in enzyme-catalyzed organic synthesis. This enzyme is approximately equally active in reduction of NAD and NADP. [t is commercially available, inexpensive, stable, and easily immobilized. Glucose 6-phosphate can be prepared in quantity by hexokinase-catalyzed phosphorylation of glucose by ATP (coupled with ATP regeneration) or by other methods. The operation of this regeneration system is illustrated by syntheses of enantiomerically enriched o-lactic acid (0.4 mol, enantiomeric excess 95%) and (S)-benzyl-a-dl alcohol (0.4 mol, enantiomeric excess95%)andbyasynthesisof threo-o,(*)-isocitricacid(0.l7mol). FactorsinfluencingthestabilityofNAD(P)(H)in solution have been explored. Enzyme-ca talyzed reactions that require reduced nicotinamide cofactors (NADH, NADPH) are not widely used in preparative chemistry.2'3 Although many of these rcactions are potentially valuable in synthesis, the cofactors required are expensive and significantly unstable in solution and can be used economically in stoichiometric reactions only on a small scale. A number of procedures for regenerating the reduced cofactors have been proposed and tested.4 Of these, the use of formate and formate dehydrogenase (EC 1.2.1.2) to reduce NAD to NADH is, in principle, one of the most practical,s'6 although the enzyme used in this procedure (from Candida boidinii) is still expensive when purchased commercially. This formate dehydrogenase does not reduce NADP,7 although formate dehydrogenase from Clostri-dium thermoacticum does do so. This paper describes a regeneration system based on dehy-drogenation of glucose 6-phosphate (G-6-P) catalyzed by glucose -6-phosphate dehydrogenase (G-6-PDH, G-6-P, NAD(P) oxidoreductase, EC LI.I.49, from Leuconostoc mesenteroides\\. The G-6-P required can be prepared by several procedures. The most convenient for syntheses carried out on a-1 mol scale is the hexokinase-catalyzed phosphorylation of glucose with ATP, with coupled acetate kinase catalyzed regeneration of ATP by using acetyl phosphate. The preparations of acetyl phosphates and glucose 6-phosphatee have been described previously. For larger scale work, routes based on conversion of starch and phosphate to glucose 6-phosphate (using phosphorylase a and phosphoglucose mutase) may also prove useful. The advantages of the G-6-PHD from Leuconostoc mesenteroides as a catalyst for use in organic synthetic procedures are that it is almost equally effective in reducing NAD and NADPTO'rr and that it is stable,

Attenuation Lengths of Photoelectrons in Hydrocarbon Films

Abstract: : Quantitative analysis of data obtained by X-ray photoelectron spectroscopy (XPS) requires a knowledge of the escape depths of electrons from the surface of a sample. In order to derive the composition of a homogeneous material from the intensities of the photoelectrons originating from different elements, one needs to know not only the relative atomic cross-sections, but also the variation of the attenuation length, gamma, with the energy of the photoelectrons. The ability to derive an elemental depth profile of a layered material from the variation in the photoelectron intensity with the angle of emission requires a knowledge of the absolute value of gamma. The recent growth of interest in thin organic films has generated an immediate need for accurate, reliable values of gamma in organic materials in general, and in thin, densely packed hydrocarbon films in particular. In this paper we have determined the attenuation length of electrons with energies in the range 940-1400 eV in self- assembled monolayers of n-alkanethiols adsorbed on gold. (JS)

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.