Self-assembly of silanated poly(ethylene glycol) on silicon and glass surfaces for improved haemocompatibility

Surface immobilization of poly(ethylene glycol) (PEG) is an effective method to produce a material surface with protein repulsive property. This property could be made permanent by using covalent grafting of the PEG molecules onto material surfaces. In this study, self-assembled monolayers (SAMs) of PEG on silicon-containing materials (silicon chip and glassplate) were obtained through a one-step coating procedure of one kind of silanated PEG molecules made through the reaction between monomethoxy PEG and 3-isocyanatopropyltriethoxysilane. Atomic force microscopy (AFM) and water static contact angle measurement were employed to investigate the surface topography and wettability of the PEGylated material surfaces. The changes in the topography and the water contact angle of the surfaces with time of incubation in PBS solution were also measured. The results revealed that stable and uniform self-assembled monolayers of PEG could be formed on silicon or glass surfaces by simply soaking the substrates in the solution of silanated PEGs. The covalent coupling of PEGs to the substrates was also confirmed. In order to evaluate the stability of the SAMs, blood compatibility of the modified glassplate surface was evaluated by measuring full blood activated partial thromboplastin time (APTT), prothrombin time (PT), and thrombin time (TT), as well as by scanning electron microscopy (SEM) analysis of the appearance of adherence and denaturation of blood platelets onto the glassplate. The silanated PEGs were shown to have good effect on the protein-repulsion as well as haemocompatibility of the substrates.     

Thermal stability of thiol and silane monolayers: A comparative study

The stability of self-assembled monolayers (SAMs) at elevated temperatures is of considerable technological importance. The thermal stability of 1-octadecanethiol (ODT), 16-mercaptohexadecanoic acid (MHDA) and 1H,1H,2H,2H-perfluorodecanethiol (PFDT) SAMs on gold surfaces, and of 4-aminobutyltriethoxysilane (ABTES) and 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFDS) assembled on hydroxylated silicon surfaces, was studied by X-ray photoelectron spectroscopy (XPS). The samples were heated in ultrahigh vacuum to temperatures in excess of that required for SAM degradation. ODT monolayers were stable to ca. 110 °C, while MHDA and PFDT SAMs were stable to ca. 145 °C. ABTES SAMs were found to be indefinitely stable to 250 °C, while PFDS SAMs were stable to 350 °C. These studies demonstrate the advantages of using silane monolayers for moderate to high temperature applications and illustrate differences that arise due to the nature of the tail group. To demonstrate the feasibility of silanes for template-directed patterning, a hydroxylated silicon oxide surface containing microcontact-printed PFDS patterns was spin-coated with a mainly hydrophilic block copolymer. Annealing the surface at 90 °C for 2 h caused the block copolymer to dewet the hydrophobic PFDS-patterned regions and adsorb exclusively on the unpatterned regions of the surface.     

Monolayer vs. multilayer self-assembled alkylphosphonate films: X-ray photoelectron spectroscopy studies

Combining functional organic self-assembled monolayers (SAMs) with conventional semiconductor materials is a key step in the development of integrated electronics-based devices. T-BAG (Tethering by Aggregation and Growth) has been shown to be a simple and reliable method to grow SAMs of alkylphosphonates on oxide surfaces. However, distinguishing SAMs from ultra-thin multilayers is a challenge for most conventional surface characterization techniques.Self-assembled films of octadecylphosphonic acid (ODPA) were deposited on oxide-covered silicon coupons, converted to the corresponding phosphonates, and characterized by high resolution angularly resolved X-ray photoelectron spectroscopy (XPS). It was our goal to distinguish among different bonding configurations for phosphorous in the phosphonate head groups, namely, mono-dentate, bi-dentate or tri-dentate interactions with the oxide surface, as well as to assess quantitatively the near-surface layer composition.We also present an innovative method that allows us to distinguish between monolayer and multilayer films of ODPA on silicon oxide surfaces. This method is based on differential surface charging effects in XPS. It was found that variation in the ODPA film thickness causes differential responses of various spectral characteristics to an electrical bias applied to the sample during XPS measurements. Both positive and negative applied biases were found to affect the carbon core-level (C1s) line-shape and intensity in the case of the multilayer ODPA film, whereas line-shapes and intensities of all XPS lines measured for the monolayer film were unaffected by the application of a dc bias in the ±30 V range.     

Preparation and tribological tests of thin fluoroorganic films

Adhesion, friction and consequent wear of sliding surfaces are the basic problems that limit the performance and reliability of microelectromechanical devices. Lubrication of these nano- and microscale contacts is different from traditional lubricants. Self-assembled monolayers (SAMs) chemically bonded to the substrate are considered to be the best solution of lubrication. The majority of these monolayers are hydrophobic providing low friction, adhesion and wear.Chemical vapor deposition was used to grow a fluorosilane film on silicon Si(1 0 0) and a condensed monolayer of 3-mercaptopropyltrimethoxysilane (MPTMS) on Au(1 1 1). The films were characterized by means of a contact angle analyzer for hydrophobicity, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) for identification of thin fluoroorganic monolayers deposited on silica surfaces and condensed monolayer MPTMS. Adhesion and friction measurements were performed using atomic force microscopy (AFM) and compared with measurements performed using a microtribometer operating in millinewton (mN) normal load range. Nanotribological measurements indicated that silica and MPTMS modified by fluorosilanes have the lowest friction coefficient and indicated a decrease friction coefficient with increasing fluoric alkyl chain length.     

Vapour phase formation of amino functionalised Si3N4 surfaces

Self-assembled monolayer (SAM) formation of silanes on SiO₂ surfaces has been extensively studied. However, SAMs formed on silicon nitride (Si₃N₄) substrates have not been explored to the same level as SiO₂, even though they are of technological interest with a view to the chemical modification of microelectromechanical systems (MEMS). Therefore, this article presents the formation and characterisation of 3-aminopropyltrimethoxysilane (APTMS) SAMs on Si₃N₄ substrates from solution phase and vapour phase, compared to the well characterised APTMS SAMs formed on SiO₂ surfaces. Contact angle, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and ellipsometric data indicate the formation of APTMS SAMs (0.55 nm ellipsometric thickness) after 60 min immersion of either SiO₂ or Si₃N₄ substrates in APTMS solution (0.5 mM in EtOH). By comparison Si₃N₄ substrates exposed to APTMS vapour, at 168 mbar for 60 min, result in the formation of the equivalent of a bi or trilayer of APTMS.     

SIMS as a subnanometer probe: A new tool for chemical profile analysis of grafted molecules

The complexity of modern engineered surfaces requires the development of very powerful methods to analyze and characterize them. We demonstrate that it is possible to obtain chemical information about the skeleton of organic molecules constituting SAMs grafted on a silicon surface by using a new type of SIMS method. A profile can be achieved by the investigation of the temporal variation of secondary ion intensities that correspond to the fractional parts of the molecule constituting the SAMs. The equivalent ablation rate is less than 0.5 nm/min.     

An anchoring strategy for photoswitchable biosensor technology: azobenzene-modified SAMs on Si(111)

A mild and efficient procedure has been developed to obtain covalently attached self-assembled monolayers (SAMs) on Si(111) with photochromic azobenzene head-groups. Starting from neat or diluted carboxylic acid functionalized monolayers on-chip coupling reactions were applied to attach hydroxyl-functionalized azobenzene units to the SAMs by ester bond formation. The modified surfaces were characterized by high-resolution X-ray photoelectron spectroscopy (XPS), transmission Fourier transform infrared spectroscopy (FT-IR), and contact angle measurements. Reversible cis ↔ trans isomerizations of photoswitchable SAMs were monitored by wettability measurements. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users. Dedicated with respect and compliments to Professor Helmut Schwarz on the occasion of his 65th birthday.     

Ageing effects on the wettability behavior of laser textured silicon

In the present work we investigate the ageing of acid cleaned femtosecond laser textured 〈1 0 0〉 silicon surfaces. Changes in the surface structure and chemistry were analysed by Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS), in order to explain the variation with time of the water contact angles of the laser textured surfaces. It is shown that highly hydrophobic silicon surfaces are obtained immediately after laser texturing and cleaning with acid solutions (water contact angle > 120°). However these surfaces are not stable and ageing leads to a decrease of the water contact angle which reaches a value of 80°. XPS analysis of the surfaces shows that the growth of the native oxide layer is most probably responsible for this behavior.     

Molecular simulation studies of self-assembled monolayers of alkanethiols on Au(111)

A review is presented of this group's recent molecular simulation studies of self-assembled monolayers (SAMs) of alkanethiols on Au(111) surfaces. SAMs are very useful for the systematic alteration of the chemical and structural properties of a surface by varying chain length, tail group and composition. The scientific and technological importance of SAMs cannot be overestimated. The present work has been centred on studies of atomic scale surface properties of SAMs. First, configurational-bias Monte Carlo simulations were performed in both semigrand canonical and canonical ensembles to investigate the preferential adsorption and phase behaviour of mixed SAMs on Au(111) surfaces. Second, a novel hybrid molecular simulation technique was developed to simulate atomic force microscopy (AFM) over experimental timescales. The method combines a dynamic element model for the tip-cantilever system in AFM and a molecular dynamics relaxation approach for the sample. The hybrid simulation technique was applied to investigate atomic scale friction and adhesion properties of SAMs as a function of chain length. Third, dual-control-volume grand canonical molecular dynamics (DCV-GCMD) simulations were performed of transport diffusion of liquid water and methanol through a slit pore with both inner walls consisting of Au(111) surfaces covered by SAMs under a chemical potential gradient. Surface hydrophobicity was adjusted by varying the terminal group of CH₃ (hydrophobic) or OH (hydrophilic) of the SAMs. Finally, ab initio quantum chemical calculations were performed on both clusters and periodic systems of methylthiols on Au(111) surfaces. Based on the ab initio results, an accurate force field capable of predicting c(4×2) superlattice structures over a wide range of temepratures for alkanethiols on Au(111) was developed. The extension of current work is discussed briefly.     

基于机械-化学的方法在硅表面制造功能化结构

"利用机械与化学结合的方法在芳烃重氮盐溶液中用金刚石刀具切削硅片,使得芳香烃分子和硅之间以共价键连接,实现了对硅片的"成形并功能化"的一步完成．研究了在大气环境如何利用金刚石刀具在硅表面加工出表面质量较好的微结构,为下一步在溶液中"成形并功能化"硅提供好的基底．在溶液中对硅表面进行可控自组装实验,初步研究了切削速度和组装时间对切削处生成自组装膜质量的影响,总结出较适合膜生长的参数．用X射线光电子能谱对自组装膜进行了检测,用扫描电子显微镜和原子力显微镜对自组装膜的表面形貌进行了表征,并用原子力显微镜对自     

表面纳米结构及其自由能对滴状冷凝传热的影响

通过抛光和氧化刻蚀方法在基体壁面形成微米和纳米尺度的微观结构,然后制备十八烷基硫醇分子自组装膜,从而得到空气中表观接触角为160°的SAM-1表面和空气中表观接触角为116°的SAM-2表面.实验研究了常压条件下两类表面的滴状冷凝传热特性.结果表明两种表面都能够有效提高冷凝传热效果.但是,具有表面纳米结构的SAM-1表面的滴状冷凝传热特性低于SAM-2表面.分析了纳米结构和液固自由能差效应对滴状冷凝传热影响的共同作用机理.     

Effect of substrate elasticity on evaporation kinetics and evaporative deposition of aqueous polystyrene nanoparticles droplets

Evaporation of aqueous polystyrene(PS) nanoparticles droplets on silicon and polydimethylsiloxane(PDMS) surfaces was studied. Experimental results showed that softer PDMS surfaces yielded a longer constant contact radius(CCR) stage, which could be ascribed to surface deformation of PDMS induced by the vertical component of liquid-vapor interfacial tension. Ringlike depositions of nanoparticles with different crack patterns were found on both silicon and PDMS surfaces. In-situ observation of crack formation showed that nanoparticle movement on the silicon surface was impeded, resulting in radial cracks with periodic distribution. In contrast, nanoparticles were shown to move easily on the PDMS surface. This observation indicated the difference in crack patterns on surfaces could be attributed to the friction force between nanoparticles and the substrate. A large friction force between nanoparticles and the substrate prevented cracks from moving, resulting in a radial crack pattern with periodic distribution, while a small friction force produced multiple large cracks.     

单晶硅表面有机硅烷/Ag2O纳米微粒复合自组装膜的制备和表征

利用分子自组装成膜技术 ,在单晶硅表面制备了有机硅烷 /Ag2 O纳米微粒复合膜 .应用接触角测定仪、原子力显微镜和X射线光电子能谱仪分析表征了薄膜的组成和结构 .结果表明 ,通过硅烷偶联剂 3 氨丙基 三乙氧基硅烷在单晶硅基底表面的成功组装 ,获得了较为均匀的硅烷化表面 ,而Ag2 O纳米微粒可在硅烷化表面成功地进行组装 ,并呈亚单层排布     

Combined AFM and laser lithography on hydrogen-passivated amorphous silicon

We report a novel combination of AFM lithography and laser direct writing on hydrogen-passivated amorphous silicon surfaces to fabricate combined silicon milli-, micro- and nanostructures. Selective oxidation is performed by focusing a laser beam (λ=458 nm) on a hydrogen-terminated silicon surface, forming the millimetre-size contact pads for connection of nanometre-scale patterns. The nanostructures are made by electric-field-enhanced oxidation using a contact mode AFM equipped with a metal-coated tip. Both techniques are based on selective oxidation of hydrogen-passivated amorphous silicon, where the oxide is used as an etch mask in a single etch step. The lithographic process has also been demonstrated using a reflection mode scanning near-field optical microscope with an uncoated fiber probe.     

Molecular dynamics of contact behavior of self-assembled monolayers on gold using nanoindentation

Molecular dynamics simulation is used to study nanoindentation of the self-assembled monolayers (SAMs) on an Au surface. The interaction of SAM atoms is described by a general universal force field (UFF), the tight-binding second-moment approximation (TB-SMA) is used for Au substrate, and the Lennard-Jones potential function is employed to describe interaction among the indenter, the SAMs, and the Au substrate atoms. The model consists of a planar Au substrate with n-hexadecanethiol SAM chemisorbed to the substrate. The simulation results show that the contact pressure increases as the SAMs temperature increases. In addition, the contact pressure also increases as the depth and velocity of indentation increase.     

Growth kinetics of self-assembled monolayers of thiophene and terthiophene on Au(111): An infrared spectroscopic study

The growth kinetics of self-assembled monolayers (SAMs) of thiophene compounds on Au(111) surfaces was revealed by Fourier-transform infrared reflection absorption spectroscopy (FT-IR-RAS). Thiophene and terthiophene form well-ordered SAMs on Au(111) surfaces by immersing gold substrates into their ethanol solutions for ca. 15 h. Gibbs free energies for the adsorption processes of thiophene and terthiophene were found to be identical. However, the growth and molecular orientation of SAMs are different between two thiophene compounds. Terthiophene in SAMs orients parallel to the surface. The SAM growth of terthiophene obeys a time-dependent Langmuir scheme. On the other hand, the thiophene SAM undergoes a two-step growth process with unique molecular orientations. In the primary phase, thiophene assumes a parallel orientation on the Au(111) surface. In the second phase, thiophene is oriented close to the normal of the surface. The different growth process between thiophene and terthiophene is attributable to the topology of sulfur positions in the molecules. Received 23 May 2001 and Received in final form 11 February 2002     

Patterned self-assembled monolayers of alkanethiols on copper nanomembranes by submerged laser ablation

Self-assembled monolayers (SAMs) of alkanethiols are major building blocks for nanotechnology. SAMs provide a functional interface between electrodes and biomolecules, which makes them attractive for biochip fabrication. Although gold has emerged as a standard, copper has several advantages, such as compatibility with semiconductors. However, as copper is easily oxidized in air, patterning SAMs on copper is a challenging task. In this work we demonstrate that submerged laser ablation (SLAB) is well-suited for this purpose, as thiols are exchanged in-situ, avoiding air exposition. Using different types of ω-substituted alkanethiols we show that alkanethiol SAMs on copper surfaces can be patterned using SLAB. The resulting patterns were analyzed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Both methods indicate that the intense laser beam promotes the exchange of thiols at the copper surface. Furthermore, we present a procedure for the production of free-standing copper nanomembranes, oxidation-protected by alkanethiol SAMs. Incubation of copper-coated mica in alkanethiol solutions leads to SAM formation on both surfaces of the copper film due to intercalation of the organic molecules. Corrosion-protected copper nanomembranes were floated onto water, transferred to electron microscopy grids, and subsequently analyzed by electron energy loss spectroscopy (EELS).     

Differential friction force spectroscopy of micropatterned organosilane self-assembled monolayers

Frictional properties of organosilane self-assembled monolayers (SAMs) and hydrated silicon oxide (SiOH) surfaces on a single sample substrate were studied; the frictional force difference between the surfaces was measured by employing one as a standard. Using a lateral force microscope (LFM), differential frictional force microscopic data were obtained by measuring the difference in the friction forces of the two surfaces with respect to the vertical load force applied to the LFM probe. The SAMs were prepared from n-octadecyltrimethoxysilane [ODS, H₃C(CH₂)₁₇Si(OCH₃)₃], n-(6-aminohexyl) aminopropyltrimethoxysilane [AHAPS, H₂N(CH₂)₆NH(CH₂)₃Si(OCH₃)₃], 3,3,3-trifluoropropyltrimethoxysilane [FAS3, F₃C(CH₂)₂Si(OCH₃)₃] and heptadecafluoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane [FAS17, F₃C(CF₂)₇(CH₂)₂Si(OCH₃)₃] by chemical vapor deposition. In the vertical force range of 0 to 600 nN, the SAMs showed no damage at all, and frictional force on the SAM surfaces increased linearly with the vertical force. The order of the frictional force magnitudes determined with the SiOH-terminated probe was SiOH > AHAPS > FAS3 > FAS17 > ODS. In addition, the frictional force difference did not become zero even at a vertical force of 0 nN, that is, the frictional differences could even be imaged by LFM through probe-sample adhesion.     

Protein quantification on dendrimer-activated surfaces by using time-of-flight secondary ion mass spectrometry and principal component regression

Interaction between streptavidin and biotin on poly(amidoamine) (PAMAM) dendrimer-activated surfaces and on self-assembled monolayers (SAMs) was quantitatively studied by using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The surface protein density was systematically varied as a function of protein concentration and independently quantified using the ellipsometry technique. Principal component analysis (PCA) and principal component regression (PCR) were used to identify a correlation between the intensities of the secondary ion peaks and the surface protein densities. From the ToF-SIMS and ellipsometry results, a good linear correlation of protein density was found. Our study shows that surface protein densities are higher on dendrimer-activated surfaces than on SAMs surfaces due to the spherical property of the dendrimer, and that these surface protein densities can be easily quantified with high sensitivity in a label-free manner by ToF-SIMS.     

ToF-SIMS study on the cleaning methods of Au surface and their effects on the reproducibility of self-assembled monolayers

The production of reproducible self-assembled monolayers (SAMs) is essential to many nano(bio)technology applications. To check the effects of different cleaning methods on a reproducible SAMs formation, the cleaning methods were varied and then used for preparing each SAM. The reproducibility of each SAM was examined by ToF-SIMS analysis along with principal component analysis (PCA). Using what we found to be a superior method of cleaning gold surfaces, alkanethiol SAMs with different terminal groups such as 1-dodecanethiol (DDT), 11-mercaptoundecanoic acid (MUA), 11-mercapto-1-undecanol (MUD) were reproduced. Our statistical results show that reproducible alkanethiol SAMs on a well-cleaned gold surface can be produced within only a few standard deviation percentages obtained from point-to-point and sample-to-sample spectra.