Imaging of micropatterned self-assembled monolayers with adsorbed liquid crystals

We report a novel method of imaging micropatterned self-assembled monolayers (SAMs) using adsorbed films of thermotropic (smectic or nematic) mesophase which can then be studied by optical microscopy. Three alkylthiols, functionalized with CH3, OH and COOH groups, were used in various combinations to form patterned SAMs. Two alkylcyanobiphenyls (7CB and 9CB) were used as the liquid crystal imaging reagents. The images are formed by the contrast generated by the different alignments of adsorbed smectic or nematic films induced by different regions of the pattern. The spatial resolution is at least to 4 μm.     

Phosphonate self-assembled monolayers on aluminum surfaces

Substrates of aluminum (Al) deposited by physical vapor deposition onto Si substrates and then chemically reacted with perfluorodecylphosphonic acid (PFDPAlSi), decylphosphonic acid (DPAlSi), and octadecylphosphonic acid (ODPAlSi) were studied by x-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM), and friction force microscopy, a derivative of AFM, to characterize their surface chemical composition, roughness, and micro-/nanotribological properties. XPS analysis confirmed the presence of perfluorinated and nonperfluorinated alkylphosphonate molecules on the PFDPAlSi, DPAlSi, and ODPAlSi. The sessile drop static contact angle of pure water on PFDPAlSi was typically more than 130 degrees and on DPAlSi and ODPAlSi typically more than 125 degrees indicating that all phosphonic acid reacted AlSi samples were very hydrophobic. The surface roughness for PFDPAlSi, DPAlSi, ODPAlSi, and bare AlSi was approximately 35 nm as determined by AFM. The surface energy for PFDPAlSi was determined to be approximately 11 mNm by the Zisman plot method compared to 21 and 20 mNm for DPAlSi and ODPAlSi, respectively. Tribology involves the measure of lateral forces due to friction and adhesion between two surfaces. Friction, adhesion, and wear play important roles in the performance of micro-/nanoelectromechanical systems. PFDPAlSi gave the lowest adhesion and coefficient of friction values while bare AlSi gave the highest. The adhesion and coefficient of friction values for DPAlSi and ODPAlSi were comparable.     

3-methylthiophene self-assembled monolayers on planar and nanoparticle Au surfaces

In this article the adsorption of 3-methylthiophene on planar and nanoparticle Au surfaces is investigated. The resulting systems are compared with a benchmark system based on 1-decanethiol. The characterization data collected evidence the formation of a packed 3-methylthiophene SAM on the planar surface. In particular, spectroscopic investigations suggest that 3-methylthiophene aromatic system is not adsorbed on the surface through the pi-electron system but rather through the S atom alone. On the other hand, the behavior of 3-methylthiophene on nanoparticle surfaces is notably different from that of the alkanethiol. Only a limited fraction of the surface of Au nanoparticles results to be actually coated after purification; this notwithstanding, the nanoparticle growth seems to be strongly influenced by the presence of such a labile encapsulating agent.     

Carboranedithiols: building blocks for self-assembled monolayers on copper surfaces

Two different positional isomers of 1,2-dicarba-closo-dodecaboranedithiols, 1,2-(HS)(2)-1,2-C(2)B(10)H(10) (1) and 9,12-(HS)(2)-1,2-C(2)B(10)H(10) (2), have been investigated as cluster building blocks for self-assembled monolayers (SAMs) on copper surfaces. These two isomers represent a convenient system in which the attachment of SH groups at different positions on the skeleton affects their acidic character and thus also determines their reactivity with a copper surface. Isomer 1 exhibited etching of polycrystalline Cu films, and a detailed investigation of the experimental conditions showed that both the acidic character of SH groups and the presence of oxygen at the copper surface play crucial roles in how the surface reaction proceeds: whether toward a self-assembled monolayer or toward copper film etching. We found that each positional isomer requires completely different conditions for the preparation of a SAM on copper surfaces. Optimized conditions for the former isomer required the exposure of a freshly prepared Cu surface to vapor of 1 in vacuum, which avoided the presence of oxygen and moisture. Adsorption from a dichloromethane solution afforded a sparsely covered Cu(0) surface; isomer 1 effectively removes the surface copper(I) oxide, forming a soluble product, but apparently binds only weakly to the clean Cu(0) surface. In contrast, adsorption of the latter, less volatile isomer proceeded better from a dichloromethane solution than from the vapor phase. Isomer 2 was even able to densely cover the copper surface cleaned up by the dichloromethane solution of 1. Both isomers exhibited high capacity to remove oxygen atoms from the surface copper(I) oxide that forms immediately after the exposure of freshly prepared copper films to ambient atmosphere. Isomer 2 showed suppression of Cu film oxidation. A number of methods including X-ray photoelectron spectroscopy (XPS), X-ray Rutherford back scattering (RBS), proton-induced X-ray emission (PIXE) analysis, atomic force microscopy (AFM), cyclic voltammetry, and contact angle measurements were used to investigate the experimental conditions for the preparation of SAMs of both positional isomers on copper surfaces and to shed light on the interaction between these molecules and a polycrystalline copper surface.     

Supported bilayer lipid membrane arrays on photopatterned self-assembled monolayers

This work demonstrates the use of photocleavable cholesterol derivatives to create supported bilayer lipid membrane arrays on silica. The photocleavable cholesteryl tether is attached to the surface by using the reaction of an amine-functionalized self-assembled monolayer (SAM) and the N-hydroxysuccinimide-based reagent 9. The resultant SAM contains an ortho-nitrobenzyl residue that can be cleaved by photolysis by using soft (365 nm) UV light regenerating the original amine surface, and which can be patterned using a mask. The photoreaction yield was approximately 75 % which was significantly higher than previously found for related ortho-nitrobenzyl photochemistry on gold substrates. The SAMs were characterized by means of contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy. Patterned surfaces were characterized with SEM and AFM. After immersing the patterned surface into a solution containing small unilamellar vesicles of egg phosphatidylcholine (PC), supported lipid membranes were formed comprised of lipid bilayer over the amine functionalized "hydrophilic" regions and lipid monolayer over the cholesteryl "hydrophobic" regions. This was confirmed by fluorescence microscopy and AFM. FRAP studies yielded a lateral diffusion coefficient for the probe molecule of 0.14+/-0.05 microm(2) s(-1) in the bilayer regions and approximately 0.01 microm(2) s(-1) in the monolayer regions. This order of magnitude difference in diffusion coefficients effectively serves to isolate the bilayer regions from one another, thus creating a bilayer array.     

Self-organization of self-assembled tetrameric porphyrin arrays on surfaces

The incorporation of designed self-assembled supramolecular structures into devices requires deposition onto surfaces with retention of both structure and function. This remains a challenge and can present a significant barrier to developing devices using self-organizing materials. To examine the role of peripheral groups in the self-organization of self-assembled multiporphyrinic arrays on surfaces, Pd(II)-linked square and Pt(II)-linked trapezoidal tetrameric porphyrin arrays with peripheral tert-butylphenyl or dodecyloxyphenyl functionalities were investigated using various spectroscopies and atomic force microscopy. The Pd(II) assembled squares disassemble upon deposition on glass surfaces, while the Pt(II) assembled trapezoids are more robust and can be routinely cast on these surfaces. The orientation and length of the peripheral alkyl substituents influence the resultant structures on surfaces. The tert-butylphenyl-substituted porphyrin array forms discrete columnar stacks, which assemble in a vertical direction via pi-stacking interactions among the macrocycles. The tetrameric porphyrin array with dodecyloxyphenyl groups forms a continuous film via van der Waals interactions among the peripheral hydrocarbon chains. The super-molecules with liquid crystal-forming moieties also form three-dimensional crystalline structures at higher deposition concentrations. These observations clearly demonstrate that the number, position, and nature of the peripheral groups and the supramolecular structure and dynamics, as well as the energetics of interactions with the surface, are of key importance to the two-dimensional and three-dimensional self-organization of assemblies such as porphyrin arrays on surfaces.     

Template-assisted patterning of nanoscale self-assembled monolayer arrays on surfaces

We report a general, simple, and inexpensive approach to pattern features of self-assembled monolayers (SAMs) on silicon and gold surfaces using porous anodic alumina films as templates. The SAM patterns, with feature sizes down to 30 nm and densities higher than 10(10)/cm(2), can be prepared over large areas (>5 cm(2)). The feature dimensions can be tuned by controlling the alumina template structure. These SAM patterns have been successfully used as resists for fabricating gold and silicon nanoparticle arrays on substrates by wet-chemical etching. In addition, we show that arrays of gold features can be patterned with 10-nm gaps between the dots.     

Nanometric protein arrays on protein-resistant monolayers on silicon surfaces

We present a novel approach for preparation of nanometric protein arrays, based on binding of avidin molecules to nanotemplates generated by conductive AFM lithography on robust oligo(ethylene glycol)-terminated monolayers on silicon (111) surfaces that are protein-resistant. We showed that only biotinated-BSA but not the native BSA bind to the avidin arrays and that the resulting arrays of biotinated BSA could bind avidin to form protein dots with a feature size of approximately 30 nm. This result demonstrates that the avidin array may serve as templates for preparation of nanoarrays of a wide variety of biotin-tagged proteins for studying their interactions with other protein molecules at nanoscale.     

Carbon nanomembranes from self-assembled monolayers: Functional surfaces without bulk

In this topical review we describe the fabrication, characterization and applications of 1 nm thick, mechanically stable carbon nanomembranes (CNMs). They represent a new type of functional two-dimensional (2D) materials, which can be concisely described as “surfaces without bulk”. Because CNMs are made by electron-induced crosslinking of aromatic self-assembled monolayers (SAMs), we start with an overview of SAMs with a special emphasis on aromatic SAMs. We describe the chemical modification of SAMs by electron, ion and photon irradiation, introduce the concepts of irradiation-induced crosslinking and chemical nanolithography of aromatic SAMs and discuss the underlying physical and chemical mechanisms. We present examples for applications of these phenomena in the engineering of complex surface architectures, e.g., nanopatterns of proteins, fluorescent dyes or polymer brushes. Then we introduce a transfer procedure to release cross-linked aromatic SAMs from their original substrates and to form free-standing CNMs. We discuss mechanical and electrical properties of CNMs and demonstrate that they can be converted into graphene upon annealing. This transformation opens an original and flexible molecular route towards the large-scale synthesis of graphene sheets with tunable properties. Finally, we demonstrate the lithographic and chemical tailoring of CNMs to fabricate novel functional 2D carbon materials: supports for high resolution transmission electron microscopy (HRTEM) and nanolithography, nanosieves, Janus nanomembranes, polymer carpets, complex layered structures. Prospects of combining different types of nanomembranes made of SAMs (CNMs, graphene, nanosieves, Janus nanomembranes) towards the engineering of novel functional nanomaterials for a variety of electronic, optical, lab-on-a-chip and micro-/nanomechanical (MEMS/NEMS) devices are discussed.     

Multiscale modeling of self-assembled monolayers of thiophenes on electronic material surfaces

Computer simulation programs, spanning different time and length scales, are used to describe the fundamentals of thin film growth morphology in organic self-assembled monolayers using thiophenes on gold as representative systems. Ab initio calculations created a catalog of the energetics between two N-[4-(thien-2ylethynyl)phenyl] hydroxyl ("1P" molecules) in vacuum and interactions in three orthogonal orientations (parallel, perpendicular, and gamma-phase) to a Au (111) surface. This energetic dataset was supplied as the input for kinetic Monte Carlo simulations of dimer and trimer representations of small organic molecules to describe both sub-monolayer and multilayer growth on a series of hypothetical model substrates. On strongly binding metallic-like substrates, sub-monolayers of the model organic molecules formed ordered phases in the x and y directions at high temperatures and a disordered polycrystalline structure at low temperatures with the molecules lying down. Only at high temperatures was a "phase inversion" observed from a completely flat to an upright structure, suggesting the upright phase to be kinetically limited. Results for multilayer deposition of 1P molecules on three substrates which differ in their binding energy to the molecule (from non-interacting to strongly binding substrates) provided a rich view of the polymorphism that can result from differing choices of temperature and flux conditions. Irrespective of the binding energy of the molecule to the substrate, on highly corrugated surfaces we always observed 3D-island growth of multiple layers of the thiophenes, in contrast to Stranski-Krastanov or Frank-van der Merwe growth on more uniform substrates. The qualitative picture we obtained agrees with the growth habits of other small organic molecule systems like the acene series. Finally, molecular dynamics studies were used to understand the packing structures of stable polymorphs of thiophene SAMs. Different deposition conditions and substrate-molecule binding captured different regimes of growth morphology, some of which have already been observed experimentally.     

Multichannel surface plasmon resonance imaging and analysis of micropatterned self-assembled monolayers and protein affinity interactions

Multichannel images of 11-mercaptoundecanoic acid and 11-mercapto-1-undecanol self-assembled monolayers together with a biospecific interferon-gamma (IFN-gamma)/anti-IFN-gamma antibody immunoaffinity interaction were observed by the two-dimensional surface plasmon resonance (2D-SPR) imaging system. With the fabricated 2D-SPR imaging system, adopting a white light source in combination with a narrow band-pass filter, sharp images were resolved, minimizing the diffraction patterns on the resulting images. Micropatterning of self-assembled monolayers was acheived by exploiting the UV photolysis of thiol bonding, instead of conventional photolithography. The line profile calibration of the image contrast with ellipsometric analysis enabled us to discriminate the change in monolayer thickness within a sub-nanometer scale. For the protein interactions on the surface, the biospecific affinity recognition reaction of IFN-gamma antigen with surface-immobilized antibody was analyzed. Through the signal amplification strategy based on the enzyme-catalyzed precipitation reaction in a sandwich-type immunoassay, biospecific antigen binding was found detectable down to a concentration of 1 ng/mL.     

Probing the protein orientation on charged self-assembled monolayers on gold nanohole arrays by SERS

In this work, surface-enhanced Raman scattering (SERS) was applied to probe the orientation of cytochrome c (Cyt-c) on gold nanohole arrays functionalized with self-assembled monolayers (SAMs) of alkane thiols with positively (-NH2) and negatively (-COOH) charged terminal groups. Square grid gold nanohole arrays with a nanohole diameter of 270 nm and a grating of 350 nm were fabricated by electron beam lithography (EBL) and were used as the SERS substrates. The SERS intensities of the nontotally symmetric mode (B(1g) mode nu(11)) and the totally symmetric mode (A(1g) mode nu(4)) and their ratios were used to determine the orientation of Cyt-c on surfaces. The results indicate that the heme group is close and perpendicular to the negatively charged surface but is far from and oriented at an angle to the positively charged surface. Cyt-c has a random or more flat orientation on the bare Au nanoholes surface.     

Contact-line friction of liquid drops on self-assembled monolayers: chain-length effects

The static and dynamic wetting properties of self-assembled alkanethiol monolayers of increasing chain length were studied. The molecular-kinetic theory of wetting was used to interpret the dynamic contact angle data and evaluate the contact-line friction on the microscopic scale. Although the surfaces had a similar static wettability, the coefficient of contact-line friction zeta0 increased linearly with alkyl chain length. This result supports the hypothesis of energy dissipation due to a local deformation of the nanometer-thick layer at the contact line.     

Decaborane thiols as building blocks for self-assembled monolayers on metal surfaces

Three nido-decaborane thiol cluster compounds, [1-(HS)-nido-B(10)H(13)] 1, [2-(HS)-nido-B(10)H(13)] 2, and [1,2-(HS)(2)-nido-B(10)H(12)] 3 have been characterized using NMR spectroscopy, single-crystal X-ray diffraction analysis, and quantum-chemical calculations. In the solid state, 1, 2, and 3 feature weak intermolecular hydrogen bonding between the sulfur atom and the relatively positive bridging hydrogen atoms on the open face of an adjacent cluster. Density functional theory (DFT) calculations show that the value of the interaction energy is approximately proportional to the number of hydrogen atoms involved in the interaction and that these values are consistent with a related bridging-hydrogen atom interaction calculated for a B(18)H(22)·C(6)H(6) solvate. Self-assembled monolayers (SAMs) of 1, 2, and 3 on gold and silver surfaces have been prepared and characterized using X-ray photoelectron spectroscopy. The variations in the measured sulfur binding energies, as thiolates on the surface, correlate with the (CC2) calculated atomic charge for the relevant boron vertices and for the associated sulfur substituents for the parent B(10)H(13)(SH) compounds. The calculated charges also correlate with the measured and DFT-calculated thiol (1)H chemical shifts. Wetting-angle measurements indicate that the hydrophilic open face of the cluster is directed upward from the substrate surface, allowing the bridging hydrogen atoms to exhibit a similar reactivity to that of the bulk compound. Thus, [PtMe(2)(PMe(2)Ph)(2)] reacts with the exposed and acidic B-H-B bridging hydrogen atoms of a SAM of 1 on a gold substrate, affording the addition of the metal moiety to the cluster. The XPS-derived stoichiometry is very similar to that for a SAM produced directly from the adsorption of [1-(HS)-7,7-(PMe(2)Ph)(2)-nido-7-PtB(10)H(11)] 4. The use of reactive boron hydride SAMs as templates on which further chemistry may be carried out is unprecedented, and the principle may be extended to other binary boron hydride clusters.     

Characterization of transparent conducting oxide surfaces using self-assembled electroactive monolayers

The electronic properties of various transparent conducting oxide (TCO) surfaces are probed electrochemically via self-assembled monolayers (SAMs). A novel graftable probe molecule having a tethered trichlorosilyl group and a redox-active ferrocenyl functionality (Fc(CH2) 4SiCl3) is synthesized for this purpose. This molecule can be self-assembled via covalent bonds to form monolayers on various TCO surfaces. On as-received ITO, saturation coverage of 6.6 x 10(-10) mol/cm2 by a close-packed monolayer and an electron-transfer rate of 6.65 s(-1) is achieved after 9 h of chemisorption, as determined by cyclic voltammetry (CV) and synchrotron X-ray reflectivity. With this molecular probe, it is found that O2 plasma-treated ITO has a significantly greater electroactive coverage of 7.9 x 10 (-10) mol/cm2 than as-received ITO. CV studies of this redox SAM on five different TCO surfaces reveal that MOCVD-derived CdO exhibits the greatest electroactive coverage (8.1 x 10(-10) mol/cm2) and MOCVD-derived ZITO (ZnIn2.0Sn1.5O) exhibits the highest electron transfer rate (7.12 s(-1)).     

Stepwise synthesis of Gly-Gly-His on gold surfaces modified with mixed self-assembled monolayers

Peptide-modified electrode surfaces have been shown to have excellent recognition properties for metal ions. An efficient method of screening a potential peptide for its selectivity for a given metal would involve the synthesis of the peptide directly on the electrode surface. This paper outlines a procedure in which the tripeptide Gly-Gly-His was synthesized one amino acid at a time on a gold surface modified with a self-assembled monolayer of the mixed alkanethiolates 3-mercaptopropionic acid (MPA) and 3-mercaptopropane (MP). Electrochemistry and high-resolution mass spectrometry were used to elucidate the structure of the adsorbed species and follow the synthesis. The amino acids can be attached only to MPA, but the presence of a diluting unreactive molecule of MP reduces steric crowding about the reaction center. The maximum coverage of synthesized tripeptide occurs at a ratio of MPA/MP of 1:1.     

Reversible covalent patterning of self-assembled monolayers on gold and silicon oxide surfaces

This paper describes the generation of reversible patterns of self-assembled monolayers (SAMs) on gold and silicon oxide surfaces via the formation of reversible covalent bonds. The reactions of (patterned) SAMs of 11-amino-1-undecanethiol (11-AUT) with propanal, pentanal, decanal, or terephthaldialdehyde result in dense imine monolayers. The regeneration of these imine monolayers to the 11-AUT monolayer is obtained by hydrolysis at pH 3. The (patterned) monolayers were characterized by Fourier transform infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, contact angle and electrochemical measurements, and atomic force microscopy. Imines can also be formed by microcontact printing of amines on terephthaldialdehyde-terminated substrates. Lucifer Yellow ethylenediamine was employed as a fluorescent amine-containing marker to visualize the reversible covalent patterning on a terephthaldialdehyde-terminated glass surface by confocal microscopy. These experiments demonstrate that with reversible covalent chemistry it is possible to print and erase chemical patterns on surfaces repeatedly.     

SECM characterization of immobilised enzymes by self-assembled monolayers on titanium dioxide surfaces

SECM in generator-collector mode was used to detect the presence of immobilised enzymes on titanium dioxide layers which were chemically or electrochemically generated with possible application as chemical sensors and biosensors. Glucose oxidase (GOx) and horseradish peroxidase (HRP) were immobilised by SAM generation using aminopropyltriethoxysilane (APTES) and ascorbic acid. The enzymes were successfully immobilised on two different TiO(2) surfaces. A simple test of durability of the system was made and a model of SAM organisation is presented.     

Interaction of self-assembled squalenoyl gemcitabine nanoparticles with phospholipid-cholesterol monolayers mimicking a biomembrane

Gemcitabine (dFdC or Gem) is a water-soluble cytotoxic drug, with poor cellular uptake in the absence of a nucleoside transporter. To improve its diffusion through membranes, it was modified by grafting of a squalenoyl moiety. In water, this derivative is able to form stable and monodispersed nanoparticles made of inverse hexagonal phases. The formation and interfacial properties of the squalenoyl gemcitabine (SQ-Gem) nanoparticles, and their ability to interact with phospholipid and cholesterol monolayers modeling a biomembrane, was assessed from surface tension measurements and Brewster angle microscopy. To get a better insight into the mechanisms of SQ-Gem interaction with the various lipids, the interfacial behavior of SQ-Gem and squalene was also studied by surface pressure and surface potential measurements, in the absence and in the presence of phospholipids and cholesterol. The results showed that SQ-Gem nanoparticles adsorbed at the free air/water interface and disrupted to form a monolayer. SQ-Gem molecules released from the adsorbed nanoparticles were also able to penetrate into condensed phospholipid-cholesterol mixed monolayers. The kinetics of this penetration was apparently controlled by intermolecular interactions between the drug and the adsorbed lipids. Whereas distearoylphosphatidylcholine (DSPC) hindered SQ-Gem penetration, cholesterol favored it, which could have important implications in the therapeutic field since cholesterol targeting could alter lipid raft composition and cancer cell survival.     

Preparing metal-complex surfaces based on self-assembled monolayers of thiols and disulfides on gold

The complexation of monolayers of sulfur-containing ligands self-assembled on surface of gold with Co(II) and Cu(II) ions is studied using quartz crystal microbalance (QCM) and wetting measurements. The optimum conditions for obtaining metal-complex surfaces and the compositions of the resulting monolayers are determined.