Showing papers by "George M. Whitesides published in 1991"

Molecular self-assembly and nanochemistry: a chemical strategy for the synthesis of nanostructures.

Abstract: Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Abstract: Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Comparison of the Structures and Wetting Properties of Self-Assembled Monolayers of n- Alkanethiols on the Coinage Metal Surfaces, Cu, Ag, Au'

Abstract: Long-chain alkanethiols, HS(CH2),CH3, adsorb from solution onto.the surfaces of gold, silver, and copper and form monolayers. Reflection infrared spectroscopy indicates that monolayers on silver and on copper (when carefully prepared) have the chains in well-defined molecular orientations and in crystalline-like phase states, as has been observed on gold. Monolayers on silver are structurally related to those formed by adsorption on gold, but different in details of orientation. The monolayers formed on copper are structurally more complex and show a pronounced sensitivity to the details of the sample preparation. Quantitative analysis of the IR data using numerical simulations based on an average single chain model suggests that the alkyl chains in monolayers on silver are all-trans zig-zag and canted by - 12' from the normal to the surface. The analysis also suggests a twist of the plane containing the carbon backbone of -45' from the plane defined by the tilt and surface normal vectors. For comparison, the monolayers that form on adsorption of alkanethiols on gold surfaces, as judged by their vibrational spectra, are also trans zig-zag extended but, when interpreted in the context of the same single chain model, have a cant angle of -27O and a twist of the plane of the carbon backbone of -53'. The monolayers formed on copper (when they are obtained in high quality) exhibit infrared spectra effectively indistinguishable from those on silver and thus appear to have the same structure. Films on copper are also commonly obtained that are structurally ill-defined and appear to contain significant densities of gauche conformations. These spectroscopically based interpretations are compatible with inferences from wetting and XPS measurements. The structure of the substrate-sulfur interface appears to control molecular orientations of the alkyl groups in these films. An improved structural model, incorporating a two-chain unit cell and allowing for the temperature-dependent population of gauche conformations, is presented and applied to the specific case of the structures formed on gold.

Self-assembled organic monolayers: model systems for studying adsorption of proteins at surfaces

Abstract: Self-assembled monolayers (SAMs) of ω-functionalized long-chain alkanethiolates on gold films are excellent model systems with which to study the interactions of proteins with organic surfaces. Mon...

Direct measurement of interfacial interactions between semispherical lenses and flat sheets of poly(dimethylsiloxane) and their chemical derivatives

Abstract: The deformations resulting on contacting small (1-2 mm) semispherical lenses of elastomeric poly-, (dimethylsiloxane) (PDMS) *ith the flat sheets of this material were measured in air and in mixtures of water and methanol. The measurements in air were carried out in two ways: as a function of external loads, and under zero load but with variations in the sizes of the lenses. The measurements in liquids were carried out under zero load and varied the composition of the liquid mixtures. These experimental data were analyzed by using a theory of Johnson, Kendall and Roberts to obtain the works of adhesion between PDMS suifaces in the air and liquid media. The strength of interaction between PDMS surfaces decreased in mixtures of water and methanol as the concentration of methanol increased. A small interaction persisted even in pure methanol. The interfacial free energies (t.r) of the PDMS-liquid interfaces obtained from these measurements, together with the contact angles of these liquids on PDMS, were analyzed by using Young's equation. This analysis provided an estimate of the surface free energ-v of the polymer (y.") that was consistent both with the value obtained from measurements made in air and with the value estimated from the analysis of the contact angles of nonpolar liquids on PDMS using the Good-Girifalco-Fowkes equation. This research also developed ways to modify the surface of PDMS chemically and thus to control its properties. The chemically derivatized poly(dimethylsiloxanes), in the form of lenses and flat sheets, were subjected to load-deformation measurements similar to those used for unmodified PDMS. These functionalized PDMS surf'aces exhibited hysteresis in contact deformations, whereas no hysteresis was detected for unmodified PDMS. The origin of this hysteresis is not clear at present. The observation of hysteresis at solid-solid interfaces is relevant to understanding adhesion to these surfaces. 1013

Formation of self-assembled monolayers by chemisorption of derivatives of oligo(ethylene glycol) of structure HS(CH2)11(OCH2CH2)mOH on gold

Abstract: This paper describes the preparation of oligo(ethylene glycol)-terminated alkanethiols having structure HS- (CHz)'l(OCH2CHz),nOH (m = 3-7) and their use in the formation of self-assembled monolayers (SAMs) on gold. A combination of experimental evidence derived from X-ray photoelectron sp€ctroscopy (XPS), measurement of contact angles, and ellipsometry implies substantial disorder in the oligo(ethylene glycol)-containing segment. The order in the -(CHz)r- group is not defined by the available evidence. The SAMs are moderately hydrophilic: du(H2O) = 34-38o; d,(H2O) = 22-25". A study of monolayers containing mixtures of HS(CH2)rrCHr and HS(CH2)rr(OCH2CH 2)6OH suggests that the oligo(ethylene glycol) moieties are eflective at preventing underlying methylene groups from influencing wetting by water. A limited study demonstrates that these oligo(ethylene glycol)-containing SAMs resist the adsorption of protein from solution and suggests that SAMs will be a useful model system for studying the adsorption of proteins onto organic surfaces.

Selective reduction of disulfides by tris(2-carboxyethyl) phosphine

Abstract: Tris(2-carboxyethyl) phosphine (TCEP) reduces disulfides rapidly and completely in water at pH 4.5. It preferentially reduces more strained disulfides, in accordance with the usual mechanism postulated for reduction of disulfides by phosphines in water. The reagent can be synthesized conveniently in large quantities by acidic hydrolysis of the commercially available tris(2-cyanoethyl) phosphine

.omega.-Terminated alkanethiolate monolayers on surfaces of copper, silver, and gold have similar wettabilities

Abstract: : Long chain alkanethiols (HS(CH2)nX) adsorb from solution onto the surfaces of freshly evaporated copper, silver and gold films and form oriented monolayers. Both polar and non-polar tail groups (X) can be accommodated in these adsorptions. Adsorption on all three metals generates self-assembled monolayers (SAMs) exhibiting similar wetting properties, and thus, by inference, having related oriented structures. XPS data suggest that the omega-terminated eta-alkanethiolate monolayers, like those derived from simple alkanethiols, are composed of trans-extended chains having orientations on copper and silver that are closer to the perpendicular to the surface than are those on gold. We have prepared and characterized monolayers ( mixed monolayers ) by exposure of all three metals to mixtures of HS (CH2)11OH and HS9CH2)11CH3.

Attenuation of photoelectrons in monolayers of n-alkanethiols adsorbed on copper, silver, and gold

Abstract: and 22 A at 554 eV. Estimates of ), obtained by using SAMs as scattering layers are more consistent than those obtained by using thin organic films fabricated by other methods. The relationship between tr and the kinetic energy (KE) of photoelectrons, over the energy range studied by using SAMs, can be fit equally well by the expressions \ = 9.0 * 0.022K8 or tr o KEp where p = 0.67 + 0.11. Bethe's equation for electron scattering in matter also relates A to KE.

Molecular Self-Assembly of Two-Terminal, Voltammetric Microsensors with Internal References

Abstract: Self-assembly of a ferrocenyl thiol and a quinone thiol onto Au microelectrodes forms the basis for a new microsensor concept: a two-terminal, voltammetric microsensor with reference and sensor functions on the same electrode. The detection is based on measurement of 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 in which a counterelectrode of relatively large surface area is used. The quinone has a half-wave potential, E½, that is pH-sensitive and can be used as a pH indicator; the ferrocene center has an E1/2 that is a pH-insensitive reference. The key advantages are that such sensors require no separate reference electrode and function as long as current peaks can be located for reference and indicator molecules.

Polyacrylamides bearing pendant .alpha.-sialoside groups strongly inhibit agglutination of erythrocytes by influenza virus

Abstract: The initial step of invasion of a mammalian cell bv influenza virus is the binding of the viral membrane protein hernagglutinin (HA) to sialic acid (SA) residues of cell surface glvcoproteins and glycolipids,2 Tight-binding inhibitors of this associarion are potential inhibitors of infl uenza infection. Systematic examinations of monomeric derivatives of sialic acids have not, to date, revealed compounds binding significantly more tightlv to HA than rr-gl1-cosides of sialic acid.3'a A!though the binding of llA to a-sialosides is weak (K,.-t x l0-l M),3 the binding of influenza virus to celis appears to be strong. The qualitative difference between the strength of interaction between monomeric HA and rnethyl a-sialoside, and that. between virus and cell surface, probably reflecrs thc poly-valency of the latter interaction.s This inference is supported by the observation that certain glycoproteins. especiallv ,i,-macroglobulins. having high contents of sialic acrd itre stic,nF inhibitors of virus-induced agglutination of er',,throcytes.6'j Oi the known sialic acid containing givcoproteins^ onil' a ferv rrre capable of protecting ervthrocytcs from viral agglutination. and it is difficult to pinpoint the origin of this activirl. \\'c belicvc that the number and accessibility of sialic acid groups in these glvcoproteins play kev roles. The structures of these cornplex. naturalil' rrccurring. pxrlvvaieni hemagglutination inhibitors rre l;irgel-r' unknown. and ir ,' int-practical to prepare close analogues of thern by s lhesi\. ()r to study relations between their structure and strength of inhibrtrrtir. We therefore sought practical routes to synthetic macromolecule:; to which sialic acid groups could be attached, and in rvhich composition and strucxiure could be varied readily. Here we reflort that such substances can be prepared conveniently by' free-radical copolymerization of l, an acrvlamide derivative of sialic acid, *ith acrylamide and its de rivatives (Scheme l). T'he rnost activc of these copolynrers are powerful inhibitors of'hemagglurinttion bl' influenza virus.8 Polymerizations followed standard procedures.e We have not characterizcd these polymers full1,, but dialysis of representative (l) This research is part of a collaboration with (7) The inhibitory potency of some of these proteins has been increased by factors up to 600 by cross-linking them to form oligomeric protein clusters.

Comparisons of self-assembled monolayers on silver and gold: mixed monolayers derived from HS(CH2)21X and HS(CH2)10Y (X, Y = CH3, CH2OH) have similar properties

Abstract: : This paper describes the preparation of self-assembled monolayers (SAMs) by adsorption of alkanethiols on evaporated silver, and compares these SAMs with analogous SAMs prepared on gold. The work concentrated on SAMs derived from mixtures of alkanethiols having long and short chains and terminating in hydrophobic and hydrophilic tails: HS(CH2) 10CH3, HS(CH2) 21CH3, HS(CH2)10CH2OH, and HS(CH2)21CH2OH. The thickness and the compositions of these SAMs were established by X ray photoelectron spectroscopy (XPS). As is observed with self- assembled alkanethiolate monolayers on gold, the composition of the monolayers parallels but does not equal the solution composition. Less pronounced differences are observed between compositions in solution and on the surface on silver than have previously been reported on gold. Rates of exchange of surface thiolates with thiols in solution are similar on silver and gold.

Contact Adhesion of Thin Gold Films on Elastomeric Supports: Cold Welding Under Ambient Conditions

Abstract: Thin gold films placed in contact on compliant elastomeric poly(dimethylsiloxane) supports weld together. This ``cold welding99 is remarkable both for the low loads required and for the fact that it occurs under ambient laboratory conditions, conditions in which the gold surfaces are covered with films of weakly adsorbed organic impurities. These impurities are probably displaced laterally during the welding. Welding can be prevented by the presence of a self-assembled gold(I) alkylthiolate monolayer on the gold surfaces. The welded contacts have low electrical resistivity and can be made thin enough to transmit light. This system is a promising one with which to study interaction between interfaces.

X-ray Damage to CF3CO2-Terminated Organic Monolayers on Si/Au: Principal Effect of Electrons

Abstract: The relative importance of x-rays alone and of x-ray-generated primary and secondary electrons in damaging organic materials was explored by use of self-assembled monolayers (SAMs) on multilayer thin-film supports. The substrates were prepared by the deposit of thin films of silicon (0, 50, 100, and 200 angstroms) on thick layers of gold (2000 angstroms). These systems were supported on chromium-primed silicon wafers. Trifluoroacetoxy-terminated SAMs were assembled on these substrates, and the samples were irradiated with common fluxes of monochromatic aluminum Kα x-rays. The fluxes and energy distributions of the electrons generated by interactions of the x-rays with the various substrates, however, differed. The substrates that emitted a lower flux of electrons exhibited a slower loss of fluorine from the SAMs. This observation indicated that the electrons—and not the x-rays themselves—were largely responsible for the damage to the organic monolayer.

X-ray grazing incidence diffraction from alkylsiloxane monolayers on silicon wafers

Abstract: X‐ray reflection (both specular and off‐specular) and grazing incidence diffraction (GID) have been used to study the structure of alkylsiloxane monolayers (n‐C18H37SiO1.5) formed by self‐assembly from solution on silicon wafers. GID studies of complete monolayers reveal a single ring of scattering associated with the monolayer. The Lorentzian line shape of this ring indicates that the film is characterized by liquidlike order, with a typical translational correlation length of about 45 A. The thermal coefficient of expansion of the monolayer, as determined from the GID peak position, is approximately equal to the value for liquid n‐alkanes. Upon either heating or cooling, the monolayer correlation lengths decrease, suggesting that the differential thermal‐expansion coefficients of the film and substrate figure prominently in thermal changes of the molecular ordering. GID data for incomplete monolayers also reveal a single ring of scattering associated with the monolayer. While both the translational corr...

Selective functionalization of gold microstructures with ferrocenyl derivatives via reaction with thiols or disulfides: characterization by electrochemistry and Auger electron spectroscopy

Abstract: Auger electron spectroscopy and electrochemistry show that ferrocenyl thiols or disulfides can be used to modify selectively the Au microelectrodes (∼2 μm×90 μm×0.1 μm) on a Si 3 N 4 substrate. The Auger technique shows the selectivity of chemisorption of thiol (−SH) or disulfide (―SS―) groups on Au relative to their physisorption on Si 3 N 4 to be at least 100:1. Immersion of Au electrodes into solutions containing (1-mercapto-3,6-dithiaheptanyl)octamethylferrocene (I), 11-mercaptoundecanoylferrocene (II), or bis[10-(ferrocenylcarbonyl)decyl] disulfide (III) yields modified Au electrodes showing about one monolayer of a reversibly redox active ferrocene reagent

Selenols catalyze the interchange reactions of dithiols and disulfides in water

Abstract: The mechanism of thiol-disulfide interchange reaction involves the nucleophilic attack of thiolate along the S-S bond axis of the disulfide.t The reaction is kinetically second order: first order in thiolate and in disulfide.2{ At pH 7, only 0.1-l% of typical thiol groups is present as thiolate in water, and the apparent rate constant of the thiol-disulfide interchange reaction is small (ftou'a = 0.1 M-r s-1).2'5 Protein disulfide isomerase (EC 5.3.4.1) has been suggested to act as a catalyst for thiol-disulfide interchange in vivo, but its lack of specificity and the fact that it induces only moderate rate enhancements make its biological role uncertain.6-e As part of a program examining thiolate-disulfide interchange, we surveyed a number of types of compounds (aromatic thiols, nonthiol nucleophiles, and cations) as potential catalysts for this reaction. The only significant rate enhancement was obtained with phenylselenol.s Selenolate is a strong nucleophile toward diselenides and a good leaving group in selenolatediselenide interchange.lo The pK\" of selenols is =7,11 and at pH ? the attack of selenol on diselenide is rapid (ftoua = 1.65 x 10? M-l s-1).10 We expected selenolate to be a strong nucleophile toward disulfide and to be a good leaving group in the attack of thiolate on selenosulfide (RS-SeR). Scheme I shows the steps involved in the catalysis of a thiol-disulfide interchange reaction by selenol.

CMP-N-acetylneuraminic acid synthetase of Escherichia coli: high level expression, purification and use in the enzymatic synthesis of CMP-N-acetylneuraminic acid and CMP-neuraminic acid derivatives.

Abstract: The gene encoding CMP-N-acetylneuraminic acid (CMP-NeuAc) synthetase (EC 2.7.7.43) in Escherichia coli serotype O7 K1 was isolated and overexpressed in E.coli W3110. Maximum expression of 8-10% of the soluble E.coli protein was achieved by placing the gene with an engineered 5'-terminus and Shine-Dalgarno sequence into a pKK223 vector derivative behind the tac promoter. The overexpressed synthetase was purified to greater than 95% homogeneity in a single step by chromatography on high titre Orange A Matrex dye resin. Enzyme purified by this method was used directly for the synthesis of CMP-NeuAc and derivatives. The enzymatic synthesis of CMP-NeuAc was carried out on a multigram scale using equimolar CTP and N-acetylneuraminic acid as substrates. The resultant CMP-NeuAc, isolated as its disodium salt by ethanol precipitation, was prepared in an overall yield of 94% and was judged to be greater than 95% pure by 1H NMR analysis. N-Carbomethoxyneuraminic acid and N-carbobenzyloxyneuraminic acid were also found to be substrates of the enzyme; 5-azidoneuraminic acid was not a substrate of the enzyme. N-Carbomethoxyneuraminic acid was coupled to CMP at a rate similar to that observed with NeuAc, whereas N-carbobenzyloxyneuraminic acid was coupled greater than 100-fold more slowly. The high level of expression achieved with the E.coli synthetase, together with the high degree of purity readily obtainable from crude cell extracts, make the recombinant bacterial enzyme the preferred catalyst for the enzymatic synthesis of CMP-N-acetylneuraminic acid.

Acid-base interactions in wetting

Abstract: The study of the ionization of carboxylic acid groups at the interface between organic solids and water demonstrates broad similarities to the ionizations of these groups in homogeneous aqueous solution, but with important systematic differences. Creation of a charged group from a neutral one by protonation or deprotonation (whether -NH3 + from -NH2 or -CO2- from -CO2H) at the interface between surface-functionalized polyethylene and water is more difficult than that in homogeneous aqueous solution. This difference is probably related to the low effective dielectric constant of the interface (e≃9) relative to water (e≃80). It is not known to what extent this difference in e (and in other properties of the interphase, considered as a thin solvent phase) is reflected in the stability of the organic ions relative to their neutral forms in the interphase and in solution, and to what extent in differences in the concentration of H+ and OH- in the interphase and in solution. Self-assembled monolayers (SAMs)-esp...

Wetting films on chemically modified surfaces: An x-ray study

Abstract: The wetting of silicon wafers and silicon wafers coated with alkylsiloxane monolayers by saturated vapors of cyclohexane and methanol were studied using x-ray specular reflection. Differentially heating the substrate surface relative to the temperature of a liquid reservoir was used to probe the disjoining pressure as a function of the film thickness and surface chemistry. Uncoated silicon wafers wet completely. The variations in film thickness with \ensuremath{\Delta}T are explained exclusively in terms of the nonretarded van der Waals forces for films 10--120 \AA{} thick. Wafers coated with methyl terminated alkylsiloxane monolayers wet incompletely, with a microscopic film 1--3 \AA{} thick adsorbing on the surface. Changing the alkylsiloxane terminal group from -${\mathrm{CH}}_{3}$ to -${\mathrm{CH}}_{2}$OH converts the surface from incompletely to completely wet. Surfaces coated with partial monolayers of methyl terminated alkylsiloxane of greater than 50% coverage are incompletely wet by cyclohexane, with the monolayers ``swelling'' to a thickness close to that of fully extended alkane chains through incorporation of cyclohexane into the film structure. The data are consistent with a first-order transition to complete wetting upon reduction of the alkylsiloxane coverage below approximately 50%. The importance of the surface atomic layer in the promotion or suppression of complete wetting is explained in the context of van der Waals forces.

A reagent for reduction of disulfide bonds in proteins that reduces disulfide bonds faster than does dithiothreitol

Abstract: We have synthesized a new reagent―N,N'-dimethyl-N,N'-bis(mercaptoacetyl)hydrazine (DMH)―for the reduction of disulfide bonds in proteins. DMH reduces disulfide bonds 7 times faster than does dithiothreitol (DTT) in water at pH7. DMH reduces mixed disulfides of cystein proteases (papain and ficin) especially rapdily (30 times faster than DTT). DMH (e o =−0.300 V) reduces moncyclic disulfides completely, although it is less strongly reducing than DTT (e o =−0.356 V)

Heterogeneous catalysis on platinum and self-assembled monolayers on metal and metal oxide surfaces

Abstract: This paper describes research in two areas: heterogeneous catalysis and molecular self-assembly. The work in heterogeneous catalysis used the platinum-catalyzed hydrogenation of diolefin(dialkyl)platinum(II) complexes to generate platinum surface alkyls of known structure. The structure and reactivity of these suiface alkyls were inferred from characteristic processes occurring on the surface. The incorporation of deuterium from isotopically labelled protic solvents (e.g. EtOD) was particularly useful mechanistically. The technique of self-assembly was used to form organic monolayers by coordination of a ligand to a metal or metal oxide surface. In addition to the chemisorption of alkyl thiols on gold (as alkyl thiolates, RS-Au+), a number of other systems were demonstrated to givs ordered organic surface species.

Synthesis of glycosyl phosphates using the Fraser-Reid activation

Abstract: Glycosyl phosphates are biologically important, both as intermediates in metabolismr and as constituents of cell walls.2-5 Polymers of glycosyl phosphates are an immunologically active part of the capsule or cell wall of several microorganisms.a's A convenient synthetic route to this important class of compounds would be useful. Although enzymatic syntheses6 appear attractive in principle, they Eue now practical only in the galactose series. The enzymes involved in formation of most sugar phosphates catalyze equilibria unfavorable to the sug€lr l-phosphates,T although galactokinase (EC 2.7.1.6) catalyzes the direct phosphorylation at the anomeric center by ATP and is thermo-

Attenuation of Photoelectrons in Monolayers of n‐Alkanethiols Adsorbed on Copper, Silver, and Gold.

Abstract: and 22 A at 554 eV. Estimates of ), obtained by using SAMs as scattering layers are more consistent than those obtained by using thin organic films fabricated by other methods. The relationship between tr and the kinetic energy (KE) of photoelectrons, over the energy range studied by using SAMs, can be fit equally well by the expressions \ = 9.0 * 0.022K8 or tr o KEp where p = 0.67 + 0.11. Bethe's equation for electron scattering in matter also relates A to KE.