Hierarchical self-assembly of designed 2 x 2-alpha-helix bundle proteins on Au(111) surfaces

Self-assembled monolayers of biomolecules on atomically planar surfaces offer the prospect of complex combinations of controlled properties, e.g., for bioelectronics. We have prepared a novel hemi-4-alpha-helix bundle protein by attaching two alpha-helical peptides to a cyclo-dithiothreitol (cyclo-DTT) template. The protein was de novo designed to self-assemble in solution to form a 4-alpha-helix bundle, whereas the disulfide moiety enables the formation of a self-assembled monolayer on a Au(111) surface by opening of the disulfide, thus giving rise to a two-step self-assembly process. The 2 x 2-alpha-helix bundle protein and its template were studied by X-ray photo electron spectroscopy (XPS), electrochemical methods, and electrochemical in situ scanning tunneling microscopy (in situ STM). XPS showed that the cyclo-DTT opens on adsorption to a gold surface with the integrity of the 2 x 2-alpha-helix bundle proteins retained. The surface properties of the DTT and 2 x 2-alpha-helix bundle protein adlayer were characterized by interfacial capacitance and impedance techniques. Reductive desorption was used to determine the coverage of the adlayers, giving values of 65 and 16 muC cm(-2) for DTT and 2 x 2-helix, respectively. The 2 x 2-alpha-helix bundle protein adlayers were imaged by in situ STM. The images indicated a dense monolayer according with the voltammetric data. No long-range order could be detected, but two clearly distinct STM contrasts were assigned to 2 x 2-alpha-helix bundle protein molecules oriented in parallel and antiparallel conformations. The template molecule DTT alone forms highly ordered 30-40 nm domains, giving an adlayer density which agreed well with the coverage determined by voltammetry. This could be exploited in STM imaging of mixed DTT/2 x 2-alpha-helix bundle protein monolayers, with clearly distinct STM patterns of the two components.     

Combined theoretical and experimental study on the adsorption of methanol on the ZnO(1010) surface

The structure, dynamics, and energetics of methanol adlayers on the nonpolar ZnO(1010) surface have been studied by He-atom diffraction (HAS), high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and density functional calculations. The experimental and theoretical data consistently show that at temperatures below 357 K methanol forms an ordered adlayer with a (2 × 1) periodicity and a coverage of one monolayer in which half of the methanol molecules are dissociated. The ordering of the methanol molecules is governed by repulsive interactions between the methyl groups of the methanol molecules. This repulsive interaction is also responsible for the formation of a second, low-density phase at higher temperatures with half monolayer coverage of undissociated methanol which is stable up to 440 K.     

On the formation kinetics of two-dimensional cytidine films

The kinetics of phase transitions of cytidine adsorbed on mercury are studied by chronoamperometry and capacitance measurements. Cytidine forms highly ordered two-dimensional adlayers in a broad range of pH. In acid solvent, only one kind of condensed layer is formed. In the alkaline solution, cytidine forms two different two-dimensional (2D) adlayers. The minimum capacitance value in adlayer II at pH 5 is 7.0 microF cm(-2) and, at pH 8.3, it is 5.1 microF cm(-2); in adlayer III, the minimum capacitance is 10.6 microF cm(-2). The formation of a physisorbed film of cytidine molecules adsorbed at the mercury surface proceeds by complex mechanisms. From j-t transients, it can be seen that the phase transformations from dilute adlayer Ia to condensed physisorbed film II is accompanied by the reorientation of cytidine molecules at the mercury surface (inverted current transient). The interfacial transformations of the cytidine film yield a sigmoidal C-t transient. This experimentally measured C-t transient were analysed by Avrami theorem. The rate of the transformations from dilute adlayer Ia to condensed film II of cytidine at pH 5 depends strongly on temperature but is only slightly affected by temperature at pH 8.3. The effect of pH and ionic composition of the supporting electrolyte on the rate of transformation of cytidine films was studied as well.     

Classification of hexagonal adlayer arrangements by means of collective geometrical properties

Unequal-sphere packing model is applied for the simulation of large number of hexagonal adlayer structures with surface coverage between theta=13 and theta=1 on the hexagonal substrate, with atomic radius of the adsorbate and substrate atoms as the only input. Each structure is characterized with respect to collective adlayer properties: the average adlayer height and the adlayer roughness. The distribution of hexagonal arrangements is presented in a special plot, which can be used for identification and characterization of hexagonal adlayers of different surface coverages and atomic registries. The most likely structures are related to the extreme values of our model parameters. The usefulness of this methodology is successfully demonstrated by comparison with some real adsorbate-substrate systems, i.e., halogens and rare gases adsorbed on (111) surface. Besides the agreement with experimental results, our model offers new insight into the formation of atomic adlayers and detailed analysis of the atomic registry. We believe that our approach will be of use for identification of probable structures among the large number of combinatorial possibilities in theoretical studies and for better interpretation of experimental results (i.e., scanning-tunneling microscopy images of atomic adlayers).     

Probing the microenvironment of surface-attached pyrene formed by a thermo-responsive oligomer

We have constructed a novel molecular assembly attached to quartz, oxidized silicon and indium-doped tin oxide coated substrates, where a tethered pyrene derivative is co-immobilized with oligo-N-isopropyl acrylamide (oligo-NIPAM). The addition of tethered oligo-NIPAM to the adlayer creates two different, temperature-dependent microenvironments for the surface-bound pyrene. X-ray photoelectron spectroscopy (XPS) and ellipsometry measurements demonstrate the covalent attachment of both oligo-NIPAM and pyrene in our adlayers. Contact angle results confirm the thermo-responsive nature of the oligo-NIPAM on the substrate surface. Steady-state fluorescence data show that the presence of oligo-NIPAM moieties reduces the extent of pyrene excimer formation and provides different environments for the chromophore at temperatures above and below the phase transition. Fluorescence lifetime decay data on surface-bound pyrene are biexponential, consistent with multiple local environments, regardless of whether tethered oligo-NIPAM is present or not. Quenching studies reveal that we can manipulate the sensing properties of this new film simply by adjusting the conformations of oligo-NIPAM.     

1-Dodecanethiol self-assembled monolayers on cobalt

Cobalt and its alloys are used in a broad range of application fields. However, the use of this metal is especially limited by its strongly oxidizable nature. The use of alkanethiol self-assembled monolayers (SAMs) is a very efficient way to protect against such oxidation and/or to inhibit corrosion. This surface modification method has been particularly applied to oxidizable metals such as copper or nickel, yet the modification of cobalt surfaces by alkanethiol SAMs received limited attention up to now. In this work, we study the influence of parameters by which to control the self-assembly process of 1-dodecanethiol monolayers on cobalt: nature of the surface pretreatment, solvent, immersion time, and concentration. Each of these parameters has been optimized to obtain a densely packed and stable monolayer able to efficiently prevent the reoxidation of the modified cobalt substrates. The obtained monolayers were characterized by X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection-absorption spectroscopy, and contact angle measurements. The stability of the optimized 1-dodecanethiol monolayer upon air exposure for 28 days has been confirmed by XPS.     

Preparation and electrochemical behavior of ordered rh adlayers on Pt(100) electrodes

Rhodium adlayers on Pt(100) substrates have been prepared by electrodeposition from dilute Rh(III) acidic solutions. The initially disordered layer is electrochemically annealed by applying a polarization program consisting of high-sweep-rate multicycle sequences between 0.05 and 0.78 V(RHE) in 0.1 M H(2)SO(4). In this way, a pseudomorphic Rh monolayer can be prepared on Pt(100) substrates. The degree of order of the electrochemically annealed layer has been evidenced not only through voltammetric experiments but also by means of scanning tunneling microscopy with atomic resolution for iodine-protected adlayers, which show a c(2 x 2) structure. The electrochemically induced ordering of the Rh adlayer appears to be a consequence of the repeated cycles of adsorption/desorption of H and, especially, oxygenated species. Voltammetry in sulfuric acid solutions permits examination of the energetics of H/anions and OH/O adsorption as a function of the Rh coverage. The first monolayer adsorbs both hydrogen and oxygenated species more strongly than the second one. This can be explained through an electronic effect caused by the underlying Pt(100) substrate.     

Synthesis of Au nanoparticles with Bi adlayers using glucose as dispersant

Well-dispersed Au/Bi nanoparticles with average size below 10 nm were prepared by using NaBH₄ to reduce HAuCl₄ with glucose as dispersant. The obtained Au/Bi NPs were well characterized by UV-Vis spectra, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and electrochemical measurements. The electrochemical study found that Bi adlayers on the surface of Au nanoparticles owns two kinds of surface structures, including a low coverage (2 × 2)-Bi adlayer and a close-packed (p × √3)-2 Bi adlayer due to the strong interaction between the two Bi layers and the below Au atoms, which is same with that bulk on Au surface.     

ToF-SIMS analysis of poly(l-lysine)-graft-poly(2-methyl-2-oxazoline) ultrathin adlayers

Understanding of the interfacial chemistry of ultrathin polymeric adlayers is fundamentally important in the context of establishing quantitative design rules for the fabrication of nonfouling surfaces in various applications such as biomaterials and medical devices. In this study, seven poly(l-lysine)-graft-poly(2-methyl-2-oxazoline) (PLL–PMOXA) copolymers with grafting density (number of PMOXA chains per lysine residue) 0.09, 0.14, 0.19, 0.33, 0.43, 0.56, and 0.77, respectively, were synthesized and characterized by means of nuclear magnetic resonance spectroscopy (NMR). The copolymers were then adsorbed on Nb₂O₅ surfaces. Optical waveguide lightmode spectroscopy method was used to monitor the surface adsorption in situ of these copolymers and provide information on adlayer masses that were then converted into PLL and PMOXA surface densities. To investigate the relationship between copolymer bulk architecture (as shown by NMR data) and surface coverage as well as surface architecture, time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis was performed. Furthermore, ToF-SIMS method combined with principal component analysis (PCA) was used to verify the protein resistant properties of PLL–PMOXA adlayers, by thorough characterization before and after adlayer exposure to human serum. ToF-SIMS analysis revealed that the chemical composition as well as the architecture of the different PLL–PMOXA adlayers indeed reflects the copolymer bulk composition. ToF-SIMS results also indicated a heterogeneous surface coverage of PLL–PMOXA adlayers with high grafting densities higher than 0.33. In the case of protein resistant surface, PCA results showed clear differences between protein resistant and nonprotein-resistant surfaces. Therefore, ToF-SIMS results combined with PCA confirmed that the PLL–PMOXA adlayer with brush architecture resists protein adsorption. However, low increases of some amino acid signals in ToF-SIMS spectra were detected after the adlayer has been exposed to human serum.

Light-induced structural transformation in self-assembled monolayer of 4-(amyloxy)cinnamic acid investigated with scanning tunneling microscopy

UV light irradiation effect on the structural transformation in a self-assembled monolayer of 4-(amyloxy)cinnamic acid (AOCA) on Au(111) has been investigated by using electrochemical scanning tunneling microscopy (ECSTM), cyclic voltammetry, and infrared (IR) spectroscopy. A well-defined 4-(amyloxy)cinnamic acid adlayer with a (4 x 11) symmetry was first prepared on Au(111). After UV-light irradiation onto the adlayer, a new adlayer is observed with different molecular arrangement and a symmetry of (5 x 8). On the basis of the results from high-resolution STM image and photochemical reaction, a dimerizaion of AOCA molecules in the adlayer with structural transformation is concluded. Schematic models have been proposed for the unirradiated and irradiated adlayers, respectively. The direct evidence at molecular level about photodimerization of cinnamic acid on metal substrate is presented.     

Structures of glycine, enantiopure alanine, and racemic alanine adlayers on Cu(110) and Cu(100) surfaces

We have determined the structures of dense adlayers of glycine and alanine on the Cu(110) and Cu(100) surfaces using plane wave density functional theory. These calculations resolve several experimental controversies regarding these structures. Glycine exists on Cu(110) as a single adlayer structure, while on Cu(100) two distinct glycine adlayers coexist. The glycine structures serve as useful starting points for constructing alanine adlayer structures. We considered separately the adsorption of enantiopure alanine and racemic alanine on each surface. Adlayers of enantiopure alanine are found to be closely related to the adlayers observed for glycine. Racemic alanine adlayers on Cu(110) are structurally analogous to those observed for glycine on this surface and adopt a pseudo-racemate ordering. On Cu(100), in contrast to glycine, racemic alanine is found to adopt a single adlayer structure that is an ordered racemate. Spontaneous segregation of molecular enantiomers does not occur in racemic adsorbed mixtures on either surface. Consideration of the orientationally distinct domains that may exist for each adlayer on these surfaces provides important information for the interpretation of the adlayer domain boundaries that are commonly observed in scanning tunneling microscopy images of amino acid adlayers. Examining this set of amino acid adlayers provides useful insight into the range of subtle behaviors that can arise in these and related systems where chiral molecules form ordered adlayers on flat metal surfaces.     

Long-range effects on palladium deposited on Pt(1 1 1)

Two-dimensional long-range effects have been pointed out electrochemically for palladium adlayers on Pt(1 1 1) in the absence of anion specific adsorption. The observation of these effects requires well ordered electrode substrates. In this way, the characteristic features are not observed on stepped electrodes with 20 atoms wide terraces. The formation of a homogeneous adlayer is also required and the effects are pointed out only when the palladium coverage is close to the completion of the full monolayer.     

Construction of mixed mercaptopropionic acid/alkanethiol monolayers of controlled composition by structural control of a gold substrate with underpotentially deposited lead atoms.

Mixed monolayers of 3-mercaptopropionic acid (MPA) and alkanethiols of various chain lengths have been constructed on Au based on a novel concept, namely, control of the composition of the component thiols in mixed monolayers by controlling the surface structure of the substrate. The Au substrate surface was first modified with underpotentially deposited Pb (UPD Pb) atoms, followed by the formation of a self-assembled monolayer (SAM) of alkanethiol. The UPD Pb atoms were then oxidatively stripped from the surface to create vacant site, on which MPA was adsorbed to finally form the mixed monolayers. The surface coverages of Pb, alkanethiol and MPA, and the total numbers of thiols were determined using an electrochemical quartz crystal microbalance, X-ray photoelectron spectroscopy, and reductive desorption voltammetry. These results demonstrate that the surface coverage of MPA in the mixed monolayers is determined by the initial coverage of UPD Pb. Fourier transform infrared spectra also support this conclusion. The observed single peak in the cyclic voltammogram for the reductive desorption shows that MPA and alkanethiol do not form their single-component domains. Scanning tunneling microscopy revealed the single-row pinstripe structure for all the thiol adlayers formed during each step of the preparation. This shows that the surface structure of the mixed monolayers is determined by the structure of the initially formed SAM on Au partially covered with UPD Pb.     

Functionalization of titanium oxide surfaces by means of poly(alkyl-phosphonates)

The use of a multiple attachment sites strategy is considered in order to improve the stability of monomolecular adlayers. The hypothesis was tested in the case of PEG-ylated compounds carrying phosphonate groups, known for their affinity toward titanium oxide surfaces. As a result, a new class of co- and terpolymers were synthesized by free-radical polymerization of three different monomers: dialkyl(methacryloyloxyalkyl)phosphonates, PEG methyl ether methacrylate, and/or butyl methacrylate monomers. Adlayers were formed following a simple dip-and-rinse protocol using diluted aqueous polymer solutions and were characterized by evaluating their thicknesses with variable angle spectroscopic ellipsometry (VASE) and their elemental compositions with X-ray photoelectron spectroscopy (XPS). The same techniques were used to determine changes of the adlayer as a function of exposure to electrolytes at different pH values and to monitor nonspecific protein adsorption upon serum exposures. The results indicated that the poly(alkyl-phosphonate)-based adlayers combine multiple site attachment of phosphonic groups and presentation of PEG side chains to the aqueous environment, resulting in both improved stability over a wide pH range in comparison to the tested reference surfaces and excellent resistance to protein adsorption when exposed to full human serum.     

Probing the effects of cholesterol on pyrene-functionalized interfacial adlayers

We have synthesized a novel set of pyrene-functionalized, covalently bound surface adlayers with and without cholesterol derivatives coadded to the adlayer. We have deposited these adlayers on quartz, oxidized silicon wafers, and indium-doped tin oxide coated substrates. The addition of tethered cholesterol to the adlayer creates a hydrophobic, likely disordered, microenvironment in which the surface-bound pyrene resides. X-ray photoelectron spectroscopy measurements demonstrate the covalent attachment of both cholesterol and pyrene in our adlayers. The presence of the cholesterol moieties gives rise to a reduction in film thickness, as measured ellipsometrically, and contact angle data indicate significant surface heterogeneity. Steady-state fluorescence data show that the presence of cholesterol moieties reduces the extent of pyrene excimer formation and provides a less polar environment for the chromophore. Fluorescence lifetime measurements on surface-bound pyrene were biexponential, consistent with multiple local environments, regardless of whether tethered cholesterol was present or not. Cyclic voltammetry reveals competition between the pyrene and cholesterol moieties for binding to available surface sites on the epoxide-terminated surface-binding layer we use.     

Adsorption and thermal decomposition of 2-octylthieno[3,4-b]thiophene on Au(111)

The adsorption and thermal stability of 2-octylthieno[3,4-b]thiophene (OTTP) on the Au(111) surfaces have been studied using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). UHV-STM studies revealed that the vapor-deposited OTTP on Au(111) generated disordered adlayers with monolayer thickness even at saturation coverage. XPS and TPD studies indicated that OTTP molecules on Au(111) are stable up to 450K and further heating of the sample resulted in thermal decomposition to produce H(2) and H(2)S via C-S bond scission in the thieno-thiophene rings. Dehydrogenation continues to occur above 600K and the molecules were ultimately transformed to carbon clusters at 900K. Highly resolved air-STM images showed that OTTP adlayers on Au(111) prepared from solution are composed of a well-ordered and low-coverage phase where the molecules lie flat on the surface, which can be assigned as a (9×2√33)R5° structure. Finally, based on analysis of STM, TPD, and XPS results, we propose a thermal decomposition mechanism of OTTP on Au(111) as a function of annealing temperature.     

In situ stress and nanogravimetric measurements during underpotential deposition of bismuth on (111)-textured Au

The surface stress associated with the underpotential deposition (upd) of bismuth on (111)-textured Au is examined, using the wafer curvature method, in acidic perchlorate and nitrate supporting electrolyte. The surface stress is correlated to Bi coverage by independent nanogravimetric measurements using an electrochemical quartz crystal nanobalance. The mass increase measured in the presence of perchlorate is consistent with the (2 x 2) and (p x square root 3)-2Bi adlayers reported in the literature. ClO(4)(-) does not play a significant role in the upd process. The complete Bi monolayer causes an overall surface stress change of about -1.4 N m(-1). We attribute this compressive stress to the formation of Bi-Au bonds which partially satisfy the bonding requirements of the Au surface atoms, thereby reducing the tensile surface stress inherent to the clean Au surface. At higher Bi coverage, an additional contribution to the compressive stress is due to the electrocompression of the (p x square root 3)-2Bi adlayer. In nitric acid electrolyte, NO(3)(-) coadsorbs with Bi over the entire upd region but has little fundamental impact on adlayer structure and stress.     

Solvent effects on the adsorption and self-organization of Mn12 on Au(111)

A sulfur-containing single molecule magnet, [Mn12O12(O2CC6H4SCH3)16(H2O)4], was assembled from solution on a Au(111) surface affording both submonolayer and monolayer coverages. The adsorbate morphology and the degree of coverage were inspected by scanning tunneling microscopy (STM), while X-ray photoelectron spectroscopy (XPS) allowed the determination of the chemical nature of the adsorbate on a qualitative and quantitative basis. The properties of the adsorbates were found to be strongly dependent on the solvent used to dissolve the magnetic complex. In particular, systems prepared from tetrahydrofuran solutions gave arrays of isolated and partially ordered clusters on the gold substrate, while samples prepared from dichloromethane exhibited a homogeneous monolayer coverage of the whole Au(111) surface. These findings are relevant to the optimization of magnetic addressing of single molecule magnets on surfaces.     

Surface-confined energy transfer in mixed self-assembled monolayers

We have fabricated a set of self-assembled monolayers consisting of naphthalene and dansyl derivatives in a range of surface loading ratios for the purpose of examining excitation transport in mixed self-assembled monolayer systems. Both tethered chromophores were immobilized on an epoxide-terminated adlayer on silica via an identical spacer, where the linking chemistry produced an amide linkage. X-ray photoelectron spectroscopy (XPS), ellipsometry, and contact angle measurements were used to characterize these chromophore-containing layers. The excitation transfer behavior of these monolayers has been examined using steady-state and time-resolved fluorescence spectroscopy. Steady-state fluorescence measurements show that excitation transfer from the naphthalene to dansyl chromophores occurs, with the efficiency of excitation transport scaling with chromophore surface loading densities, as expected. The donor lifetimes decrease with increasing acceptor loading density, and the functional form of the acceptor decay was independent of the donor/acceptor ratio. Our findings are not consistent with a homogeneous adlayer, but do provide information on the structural heterogeneity that is characteristic of these interfaces.     

Formation of Ultrathin Films at the Solid-Liquid Interface Studied by In Situ Ellipsometry

Ellipsometric investigations of self-assembled monolayers (SAMs) of alkylsiloxanes on native silicon substrates and of organothiols on gold substrates were performed under in situ conditions with the substrate in direct contact with the adsorbate solution. Specially designed liquid cells matched for different incidence angles were used to carry out measurements in a range of organic solvents with different refractive indices as the ambient medium. The observed shifts in the ellipsometric phase angles Delta upon monolayer formation were found to depend very sensitively on the incidence angle and the refractive indices of the adsorbate film and the ambient solvent, from which a rather simple method for determining the refractive index of the adsorbate film, based on a variation of the ambient refractive index, was derived. Time-resolved in situ measurements of SAM formation in different solvents and onto different substrates yielded accurate kinetic information on the monolayer growth process and revealed hitherto unknown strong solvent effects on the growth rate. Copyright 1999 Academic Press.