Showing papers by "George M. Whitesides published in 1992"

How to make water run uphill.

Abstract: A surface having a spatial gradient in its surface free energy was capable of causing drops of water placed on it to move uphill. This motion was the result of an imbalance in the forces due to surface tension acting on the liquid-solid contact line on the two opposite sides ("uphill" or "downhill") of the drop. The required gradient in surface free energy was generated on the surface of a polished silicon wafer by exposing it to the diffusing front of a vapor of decyltrichlorosilane, Cl(3)Si(CH(2))(9)CH(3). The resulting surface displayed a gradient of hydrophobicity (with the contact angle of water changing from 97 degrees to 25 degrees ) over a distance of 1 centimeter. When the wafer was tilted from the horizontal plane by 15 degrees , with the hydrophobic end lower than the hydrophilic, and a drop of water (1 to 2 microliters) was placed at the hydrophobic end, the drop moved toward the hydrophilic end with an average velocity of approximately 1 to 2 millimeters per second. In order for the drop to move, the hysteresis in contact angle on the surface had to be low (

Self-assembled monolayers of n-alkanethiolates on copper are barrier films that protect the metal against oxidation by air

Abstract: Self-assembled monolayers (SAMs) derived from adsorption of n-alkanethiols onto the surfaces of copper slow the oxidation of the copper surface by reaction with atmospheric dioxygen. Angstrom-level changes in the thickness of the monolayer result in readily observable differences (by X-ray photoelectron spectroscopy, XPS) in the rates of oxidation of the copper and adsorbed thiolates. The rates of oxidation of the copper and the thiolates can be decreased by ∼50% by increasing the length of the adsorbate, and thus of the SAM, by four CH 2 units

Correlation Between Surface Free Energy and Surface Constitution

Abstract: Self-assembled monolayers (SAMs) of alkylsiloxanes on elastomeric PDMS (polydimethylsiloxane) were used as model systems to study interactions between surfaces. Surface free energies (γsv) of these chemically modified surfaces were estimated by measuring the deformations that resulted from the contact between small semispherical lenses and flat sheets of the elastomer under controlled loads. The measured surface free energies correlated with the surface chemical compositions of the SAMs and were commensurate with the values estimated from the measurements of contact angles. This study provides direct experimental evidence for the validity of estimates of the surface free energies of low-energy solids obtained from contact angles.

Manipulation of the Wettability of Surfaces on the 0.1 to 1-Micrometer Scale Through Micromachining and Molecular Self-Assembly

Abstract: Micromachining allows the formation of micrometer-sized regions of bare gold on the surface of a gold film supporting a self-assembled monolayer (SAM) of alkanethiolate. A second SAM forms on the micromachined surfaces when the entire system-the remaining undisturbed gold-supported SAM and the micromachined features of bare gold-is exposed to a solution of dialkyl disulfide. By preparing an initial hydrophilic SAM from HS(CH(2))(15)COOH, micromachining features into this SAM, and covering these features with a hydrophobic SAM formed from [CH(3)(CH(2))(11)S](2), it is possible to construct micrometer-scale hydrophobic lines in a hydrophilic surface. These lines provide new structures with which to manipulate the shapes of liquid drops.

Structure of monolayers formed by coadsorption of two n-alkanethiols of different chain lengths on gold and its relation to wetting

Abstract: We have characterized and correlated the structure and wetting properties of self-assembled monolayers (SAMs) on gold derived from two different mixtures of n-alkanethiols (C12SH and CI3SH; Cr2SH and C22SH); in each of these SAMs, one thiol had a perdeuterated alkyl chain and one a perprotonated chain to allow the two alkanethiols to be distinguished in the SAM by polarized infrared external reflectance spectroscopy (PIERS). The hydrocarbon parts of the SAMs consist of well-defined regions exhibiting structural characteristics reminiscent of those formed in crystalline and liquidlike states. In each SAM, both the shorter alkanethiolate and the section of the longer alkanethiolate closest to the gold contain very low densities of gauche conformations (and presumably are ordered) over all compositions of the SAM. The terminal portion of the longer alkanethiolate-that which extends beyond the terminus of the shorter alkanethiolate-is disordered. The density of gauche conformers present in this mmponent decreases as its concentration in the SAM is increased. The advancing and receding contact angles (0" and d,, respectively) of water and hexadecane (HD) respond differently to the disordered region: for hexadecane, 0.(HD) and d,(HD) reach minima when the interface is present in the disordered, liquidlike state; for water, d"(H2O) and 0,(H2O) appear insensitive to the presentr of this disorder. The IR spectra of these mixed SAMs are consistent with previous inferences from wetting that the components of the SAMs, as prepared here (adsorption of 1 mM thiol from ethanolic solutions over 24-48 h at -25 oC), do not phase segregate into macroscopic islands. The larger interpretation which follows from the data, however, is that this result reflects a structural architecture derived from a kinetically controlled growth process. Under suitable conditions, phase segregation is important as might be expected from consideration of elementary thermodynamics. The importance of kinetic and thermodynamic factors in the structural characteristics of multicomponent SAMs is discussed.

The use of self-assembled monolayers and a selective etch to generate patterned gold features

Abstract: on the surface of the gold block this etching. Using a number of techniques-micromachining, microwriting, electron-beam lithography, ion-beam lithography-it is possible to form patterns of SAMs on the Au surface. By combining these techniques for forming patterns with selective etching using the CN-/O2 solution, high-resolution patterns of gold on silicon can be fabricated with dimensions as small as I pm.5 One procedure used a pen to write patterns of hexadecanethiolate6 as monolayers on Au substrates. The pen, filled with

Affinity capillary electrophoresis can simultaneously measure binding constants of multiple peptides to vancomycin

Abstract: Affnity capillary electrophoresis is a sensitive and convenient technique for studying molecular recognition involving low molecular weight receptors

Substrate specificity and carbohydrate synthesis using transketolase

Abstract: This paper describes the use of the enzyme transketolase as a catalyst in organic synthesis. The properties of transketolase from both yeast and spinach were investigated. The yeast enzyme was found to be more convenient for routine use. Examination of the substrate specificity of yeast transketolase demonstrated that the enzyme accepts a wide variety of 2-hydroxy aldehydes as substrates. A practical protocol for transketolase-catalyzed condensation of hydroxypyruvic acid with these aldehydes has been developed and used for the synthesis of four carbohydrates: L-idose, L-gulose, 2-deoxy-L-xylohexose, an L-xylose

Gram-scale synthesis of uridine 5'-diphospho-N-acetylglucosamine: comparison of enzymic and chemical routes

Abstract: practical chemoenzymatic and chemical routes to uridine 5'-diphospho-N'acetylglucosamine (UDP-GlcNAc) on r !ruscale have been developed. The chemoenzymatic synthesis provided convenient access to glucosaminigpnosphate and N-acetylgluiosamine&phosphatc (GlcNAc-GP) in )lGmmol quantities. The condensation betweenblcNAc-6-P and UTi, was catalyzed by readily available crude enzyme extracts from dried cells of the ieit Co\"aida utitis and afforde d a LTVo yield of UDP-GlcNAc from GlcNAc-6-P. The otherwise straightforward .h.-*n y-\"tic sequence was hampered by the need to punfy the product from the final complex reaction mixture. The chemical synthesis of UDP-GlcNAc proceeded through five steps in an overall yield of 15% from pentaacetvlgluco.\"-i.,. with the selective formation of tetraacetylglucosamine-a-1-phosphate as the key reaction.

Practical enzyme-based syntheses of uridine 5'-diphosphogalactose and uridine 5'-diphospho-N-acetylgalactosamine on a gram scale

Abstract: Practical enzyme-based routes for the syntheses of uridine 5'-diphosphogalactose (UDP-Gal) and uridine 5'-diphospho-N-acetylgalactosamine (UDP-GalNAc) on millimole scales have been developed. The activity of galactokinase (EC 2.7.L.6) in crude enzyme extracts from galactose-adapted yeast, coupled to a regenerating system for ATP, provides convenient and economical access to galactose-a-1-phosphate (Gal-1-P) and galactosaminea-l-phosphate (GalN-l-P). The transfer of UMP to the sugar-l-phosphates was also accomplished enzymatically bv Gal-1-P uridyltransferase (EC 2.7.7.12) using uridine 5'-diphosphoglucose (UDP-Glc) as the UMP donor. UDP-Glc was in turn regenerated in situ from glucose-l-phosphate and UTP using UDP-Glc pyrophosphorylase (EC 2.7.7.il. The only chemical step in the sequence was the acetylation of UDP-GaIN to afford UDP-GalNAc using N-acetoxysuccinimide. The moderate overall yields (a3% and34Vo for UDP-Gal and UDP-GalNAc from Gal-l-P and Galll-l-P, respectively) were compensated by the straightforward preparation of the starting materials, UTP and the corresponding sugar-1-P.

Self-assembled monolayers of alkanethiols on gold: the adsorption and wetting properties of monolayers derived from two components with alkane chains of different lengths

Abstract: This paper describes the preparation and wetting properties of two-component self-assembled monolayers (SAMs) obtained by the competitive adsorption of one short-chain (HS(CH2)10Sh) and one long-chain (HS(CH2)21Lg) alkanethiol onto gold from dilute ethanolic solutions. The four possible combinations of the tail groups CH3 and CH2OH were investigated: Sh = CH3/Lg = CH2OH, Sh = CH2OH/Lg = CH3, Sh = CH2OH/Lg = CH2OH, and Sh = CH3/Lg = CH,. The compositions of these SAMs are not the same as the compositions of the solutions from which they were formed. Although the relationship between the composition of the SAM and the composition of the solution suggests that some phase separation may be occurring within the SAM, contact angles with water and hexadecane show that significant disorder still remains in the interfacial region.

Two-terminal voltammetric microsensors

Abstract: Self-assembly of a chemically insensitive redox material, such as ferrocenyl thiol, and a chemically sensitive redox material, such as a quinone thiol, onto microelectrodes forms the basis for a two-terminal, voltammetric microsensor having reference and sensor functions on the same electrode. Detection is based on measuring the potential difference of current peaks for oxidation and reduction of the reference (ferrocene) and indicator (quinone) in aqueous electrolyte in a two-terminal, linear sweep voltammogram using a counterelectrode of relatively large surface area. Use of microelectrodes modified with monolayer coverages of reference and indicator molecules minimizes the size of the counterelectrode and the perturbation of the solution interrogated. Key advantages are that the sensor requires no separate reference electrode and the sensor functions as long as current peaks can be located for reference and indicator molecules.

Heterogeneous, platinum-catalyzed hydrogenations of (diolefin)dialkylplatinum(II) complexes

Abstract: The heterogeneous hydrogenation of olefins is perhaps the most extensively studied reaction in heterogeneous ~atalysis.~ Despite intense scrutiny, the mechanism by which this reaction proceeds has not been unambiguously characterized. In 1934, Horiuti and Polanyi proposed the first plausible mechanism (Scheme I).3 This basic scheme still has wide support, although the individual steps are more complex than those depicted in this simple representation. Questions concerning the structures and reactions of the surface alkyls (e.g., A and B in Scheme I) are particularly complicated. Recent reviews summarize the structures of the many proposed intermediates and the pathways leading to them.4 As part of an effort to characterize the reactions of surface alkyls (R*) on platinum, we have developed a new type of heterogeneous reaction that generates R* of known initial structure under conditions similar to those employed in organic chemistry for the heterogeneous, catalytic hydrogenation of olefins, viz., the hydrogenation of (diolefin)dialkylplatinum(II) complexes ((DO)PtRJ, using platinum black suspended in organic solvents as catalyst (eq 1; Scheme II)."13 Adsorption

Thermal Reconstruction of the Functionalized Interface of Polyethylene Carboxylic Acid and Its Derivatives

Abstract: Chemical modification of the surface of a polymer changes its free energy of interaction with contacting solids or liquids while leaving its bulk physical properties largely unchanged. The ability to control interfacial interactions by chemical modification of a surface is important for applications involving properties, such as adhesion,(1–6)* biocompatability,(7–9) printability,(10) static discharge,(11) that depend on wetting or on hydrophilicity.(12, 13) We(14–20) and others(21–29) have studied the relations between the structures of the surfaces of surface-modified polymers and their wettability. The meaning of the phrase structure of the surface in this context is not sharply defined, but includes those characteristics of the solid—liquid interface—functional group types, orientation, and distribution; surface topology; bulk polymer properties—that influence wetting. These studies have led to a useful understanding of the factors that control wetting, phrased in the qualitative terms of physical organic chemistry.

Orthogonal Self-Assembly of Redox Active Molecules on Pt and Au: Selective Reaction of Disulfide with Au and Isonitrile with Pt

Abstract: : Surfaces of platinum and gold adsorb ferrocenyl-12-dodecyl isocyanide, I, and di(10-ferrocenecarbonyl-decyl) disulfide, II, and yield self-assembled, redox active monolayers incorporating ferrocene groups. The values of E 1/2 for alkyl ferrocenyl centers from I are approximately 300 mV more negative than acyl ferrocenyl centers from II. This difference allows easy electrochemical measurement of surface concentrations of ferrocene centers from I and II on Pt or Au. I adsorbs selectively on Pt or Au compared to II, but with different degrees of selectivity. Further, for Pt the selectivity depends on the surface pretreatment. The affinity of II for a Pt surface first exposed to an O2 plasma (oxidized Pt) is lower than that for a Pt surface exposed first to an O2 plasma and then to a H2 plasma (reduced Pt). The same two plasma treatments of Pt do not affect the binding of I on Pt. Thus, the selectivity of Pt for I vs. II is higher for the oxidized Pt approximately (200:1) compared to that for the reduced Pt approximately (20:1). The two plasma treatments of Au do not influence the selectivity for binding I vs. II; the selectivity for binding I vs II. on Au is approximately 8.1. By using the O2 plasma pretreatment and a 40:1 solution of II:I, orthogonal self-assembly of II onto Au and I onto Pt can be effected when the two samples are derivatized in the same solution.