Photoalignment of liquid crystals by a covalently attached self-assembled ultrathin film

In this paper, the polyanion-containing cinnamoyl group (PACSS-CF3) was self-assembled with diazoresin (DR) to form a kind of stable covalent ultrathin film by irradiation with 365?nm UV light. The photoalignment properties of the DR/PACSS-CF₃ covalent film were investigated. The covalent film was found to have anisotropy after irradiation by 297?nm linearly polarised ultraviolet light (LPUVL), and could induce uniform alignment of liquid crystals (LCs). The pretilt angle of the LC was 2.5°. The stability of the film was enhanced by the covalent bonds. The films were thermally stable to 180°C. Polarised UV-Vis spectroscopy was utilised to investigate the photochemical process of the covalent film. It was found that cinnamoyl moieties parallel to the polarisation direction of the LPUVL were consumed by the photoreaction faster than those perpendicular to the polarisation direction. It can be concluded that the selective photoreaction induced the anisotropy of the films. The anisotropic films induced the homogeneous alignment of LC.     

Two-dimensional crystal structure of a quaterthiophene-alkanethiol self-assembled monolayer on gold

We investigated the fine structure of a self-assembled monolayer of dodecanethiol functionalized by alpha-quaterthiophene on gold (alpha-4TC 12H 24SH). The molecular orientation, quantified using polarization modulation infrared reflection-absorption spectroscopy, was studied as a function of the adsorption time. The alpha-4T moieties arrange in the upright position on the surface as the adsorption time increases, while the alkyl chain organization remains poor. Here we quantify the orientation of the self-assembled monolayer and, more significantly, reveal through surface X-ray diffraction that after a long incubation period (12 h) the alpha-4T on the gold surface adopts a 2D crystal structure.     

Controlling molecular crystal polymorphism with self-assembled monolayer templates

The control of crystal polymorphism is a long-standing issue in solid-state chemistry, which has many practical implications for a variety of commercial applications. At least four different crystalline forms of 1,3-bis(m-nitrophenyl) urea (MNPU), a classic molecular crystal system, are known to crystallize from solution in various concomitant combinations. Herein we demonstrate that the introduction of gold-thiol self-assembled monolayers (SAMs) of substituted 4'-X-mercaptobiphenyls (X = H, I, and Br) into the crystallization solution can serve as an effective means to selectively template the nucleation and growth of alpha-, beta-, and gamma-MNPU phases, respectively. Polymorph control in the presence of SAM surfaces persists under a variety of solution conditions and consistently results in crystalline materials with high phase purity. The observed selectivity is rationalized on the basis of long-range two-dimensional geometric lattice matching and local complementary chemical interactions at the SAM/crystal interfaces.     

Supercritical self-assembled monolayer growth

The growth of octadecyltrimethylammonium bromide (C(18)TAB) monolayers on mica was investigated using atomic force microscopy and infrared spectroscopy. A critical temperature was identified below which the monolayer formed via an "islanding" mechanism, that is, nucleation and growth of densely packed two-dimensional (2D) islands within a matrix of a disordered dilute phase. However, above the critical temperature, there was no coexistence of 2D phases during film formation. Instead, the monolayer gradually became better ordered, remaining laterally homogeneous throughout. We show that this corresponds to a critical point in a 2D phase diagram of the monolayer. Additional evidence is provided by the in situ observation of 2D phase separation upon cooling an incomplete monolayer from the one-phase to the two-phase region. The lack of coexisting domains (and domain boundaries) during growth above the critical point provides a possible route for the preparation of essentially defect-free monolayers.     

Self-assembling monolayer formation of glucose oxidase covalently attached on 11-aminoundecanethiol monolayers on gold

Glucose oxidase (GOx) has been attached covalently to form uniform enzyme monolayers on self-assembled monolayers (SAMs) from 11-aminoundecanethiol (AUDT) by taking advantage of chemical oxidation of GOx carbohydrate residues followed by coupling the resulting 'aldehydic' enzyme with the terminal amino group in the SAM as characterized by AFM imaging, IR, QCM, and electrochemical measurements.     

Surface plasmon resonance spectroscopic chiral discrimination using self-assembled leucine derivative monolayer

Pirkle-type chiral stationary phases (CSPs) showed excellent enantiomeric separation for amino acid derivatives by forming energetically different two transient diastereomeric pi-pi donor-acceptor complexes with two enantiomers. A CSP derived from N-(3,5-dinitrobenzoyl) leucine with a thiol ending group for immobilization on Au was synthesized and self-assembled on Au surface as chiral sensing layer. The monolayer characterized by spectroscopic and microscopic methods such as AFM, FTIR reflection absorption spectroscopy (FTIR-RAS) and cyclic voltammetry (CV). The enantiospecific detection onto CSP of the leucine derivative was studied by surface plasmon resonance (SPR). (S)-CSP SAM showed high chiral differential detection for (S)-analyte in a range of 1.0x10(-9) to 1.0x10(-4) M. In combination with the SPR method, the leucine derivative monolayer provided a reliable and simple experimental platform for enantiospecific detection.     

Molecular dynamics simulations of liquid crystal molecules on a polyimide monolayer

Preliminary results are presented on the molecular dynamics simulations of alignment of the liquid crystal molecule, 4-n-octyl-4'-cyanobiphenyl (8CB), on a polyimide (pyromelltic dianhydride-p-phenylene diamine) oligomer monolayer. We actually simulated a three-layer system, i.e., liquid crystal molecule/polyimide oligomer/a basal plane of graphite. First, simulations of the oligomers adsorbed on graphite were done in order to obtain reasonable adsorption structures, as the pre-stage simulation of the three-layer system. Then, by placing a liquid crystal layer on top, the three-layer system was simulated. The stable liquid crystal alignment direction on the polyimide monolayer was found roughly to be the polyimide chain direction with zero pretilt in this combination of liquid crystal and polymer materials. The calculated adsorption energy of an 8CB molecule to the polyimide monolayer was 128 kJ mol^-1 and the carbonyl group of the polyimide was the main adsorption site.     

Zinc oxide growth morphology on self-assembled monolayer modified silver surfaces

Using organic molecules to direct inorganic crystal growth has opened up new avenues for controlled synthesis on surfaces. Combined with soft lithography to form patterned templates, self-assembled monolayers (SAMs) have been shown to be a powerful approach for the assembly of inorganic nanostructures. In this work, we show that the surface free energy of SAM-modified silver, which depends on end groups and deposition method of SAMs, has a dramatic effect on the nucleation and growth of crystalline ZnO, a technologically important material, from supersaturated solutions. For SAMs with inert methyl end groups, ZnO nucleation is inhibited. For SAMs with chemically active (carboxylic or thiol) end groups, the ZnO morphology is found to be three-dimensional nanorods on low-surface-energy surfaces and two-dimensional thin films on high-energy surfaces.     

Electrochemistry of a carotenoid self-assembled monolayer

A carotenoid self-assembled monolayer was prepared by dipping a gold electrode into a solution of 4^′-thioxo-β,β-caroten-4-one in acetonitrile. Electrochemistry of the surface layer was investigated by cyclic voltammetry in an aqueous solution. No electrochemical reaction was detected in the potential region between 0.5 and −0.6 V vs. SCE. The anodic reaction of adsorbed carotenoid occurs at 0.8 V, whereas the irreversible anodic desorption proceeds at 1.4 V in 0.01 M HClO₄. Formation of the surface layer resulted in a decrease of the charging current as well as in a strong inhibition of the electron transfer reaction for species such as Fe(CN)₆³⁻, Ru(NH₃)₆³⁺, and dissolved oxygen. Prolonged voltage cycling in the O₂ reduction range induced some changes in the surface layer characteristics that were tentatively accounted for by the cross-linking of adsorbed molecules under the effect of transient oxygen radicals.     

Protein nanopores with covalently attached molecular adapters

Molecular adapters are crucial for the stochastic sensing of organic analytes with alpha-hemolysin (alphaHL) protein nanopores when direct interactions between analytes and the pore cannot readily be arranged by conventional protein engineering. In our earlier studies, cyclodextrin adapters were lodged noncovalently within the lumen of the alphaHL pore. In the present work, we have realized the controlled covalent attachment of a beta-cyclodextrin (betaCD) adapter in the two possible molecular orientations inside alphaHL pores prepared by genetic engineering. There are two advantages to such a covalent system. First, the adapter cannot dissociate, which means there are no gaps during stochastic detection, a crucial advance for single-molecule exonuclease DNA sequencing where the continuous presence of a molecular adapter will be essential for reading individual nucleotides. Second, the ability to orient the adapter allows analytes to bind through only one of the two entrances to the betaCD cavity. We demonstrate that the covalently attached adapters can be used to alter the ion selectivity of the alphaHL pore, examine binding events at elevated temperatures, and detect analytes with prolonged dwell times.     

Poly(ethylene glycol) self-assembled monolayer island growth

Here, we report a study of the morphology and growth dynamics of a self-assembled monolayer (SAM) of the amide containing poly(ethylene glycol) (PEG) thiol (CH3O(CH2CH2O)17NHCO(CH2)2SH) on atomically flat Au(111) surfaces. SAM growth from a 20 muM ethanolic solution reveals island growth through three distinct steps: island nucleation, island growth, and coalescence. The coalescence-step, filling voids in the SAM, is by far slowest. The fine structure study reveals dendritic island formation, an observation which can be explained by attractive intermolecular interactions and surface diffusion-limited aggregation. We have also observed a change in the island height, which peaks during the island growth phase. This height change can be associated with a molecular conformational transition.     

A novel photoalignment film for ferroelectric liquid crystal based on self-assembled monolayer method

A novel strategy based on self-assembly technology was devised for design of photosensitive material as a ferroelectric liquid crystal (FLC) alignment layer. This development offers new tools for the study and control at the molecular level of the interaction of FLCs with solid surfaces. The photoreactive material was self-assembled to the substrate by covalent bond linkage due to a special chemical adsorption reaction. Through ester bond linkage, a cyano group with strong polarity was introduced to be terminus of the film. Under irradiation of linearly polarised ultraviolet light, an optically anisotropic self-assembled film was easily obtained. The irradiated film was demonstrated to result in homogenous alignment of FLC by optical transmittance measurements and polarising optical microscopy images of a FLC cell at different rotation angles. The alignment quality of the FLC on this self-assembled monolayer film is comparable to that of commercial rubbed polyimide film. Furthermore, it was also found that the fine alignment of the FLC may be related to the smoothness of the self-assembled film surface owing to its polar end.     

Enzymatic nanolithography of a self-assembled oligonucleotide monolayer on gold

This paper describes a simple strategy to biochemically manipulate a surface at the nanoscale by enzyme dip-pen nanolithography using an endonuclease (DNase I) that is directly patterned on a self-assembled monolayer presenting a terminal oligonucleotide. Physisorbed nanopatterns of DNase I carried out nanoscale enzymology at the surface creating oligonucleotide patterns with the fidelity of the patterned enzyme because of the affinity of the enzyme for the immobilized, oligonucleotide substrate.     

Biomimetics with a self-assembled monolayer of catalytically active tethered isoalloxazine on Au

A new biomimetic nanostructured electrocatalyst comprised of a self-assembled monolayer (SAM) of flavin covalently attached to Au by reaction of methylformylisoalloxazine with chemisorbed cysteamine is introduced. Examinations by Fourier transform infrared spectroscopy and scanning tunneling microscopy (STM) show that the flavin molecules are oriented perpendicular to the surface with a 2 nm separation between flavin molecules. As a result of the contrast observed in the STM profiles between areas only covered by unreacted cysteamine and those covered by flavin-cysteamine moieties, it can be seen that the flavin molecules rise 0.7 nm above the chemisorbed cysteamines. The SAM flavin electrocatalyst undergoes fast electron transfer with the underlying Au and shows activity toward the oxidation of enzymatically active beta-NADH at pH 7 and very low potential (-0.2 V vs Ag/AgCl), a requirement for use in an enzymatic biofuel cell, and a 100-fold increase in activity with respect to the collisional reaction in solution.     

Label-free detection of DNA molecules on the dendron based self-assembled monolayer by electrochemical impedance spectroscopy

We report preparation of a novel platform for effective DNA hybridization and its application to the detection of single mismatched DNA. Cone-shaped dendrimer molecules have been immobilized on the gold surface at equidistance, 3.1 nm, from each other with a probe DNA molecule attached to the top of each dendrimer so that enough space would be secured for effective hybridization. This arrangement allows each probe DNA molecule to form a natural DNA double helix upon hybridization with a target DNA molecule. The single nucleotide polymorphism at either the central or end position of the 25-mer target DNA has been shown to be effectively discriminated against on this platform from each other as well as from a complementary DNA by electrochemical impedance measurements. We also report adverse effects exerted by probe ions, Fe(CN)₆^3−/4−, on DNA hybridization reactions. The significance of the results for the use in DNA analysis is discussed.     

Selective growth of a less stable polymorph of 2-iodo-4-nitroaniline on a self-assembled monolayer template

Orthorhombic and triclinic crystals of 2-iodo-4-nitroaniline (INA) grow concomitantly from supersaturated ethanol solutions, but the less stable orthorhombic phase can be selectively grown on 3'-X-4-mercaptobiphenyl (X = NO(2), I) self-assembled monolayer templates.     

Electrochemical quartz crystal microbalance study of azurin adsorption onto an alkanethiol self-assembled monolayer on gold

A quartz crystal microbalance coupled with electrochemistry was used to examine the adsorption of azurin on a gold electrode modified with a self-assembled monolayer of octanethiol. Azurin adsorbed irreversibly to form a densely packed monolayer. The rate of azurin adsorption was related to the bulk concentration of azurin in solution within the concentration range studied. At a high azurin concentration (2.75 muM), adsorption was rapid with a stable adsorption maximum attained in 2-3 min. At a lower azurin solution concentration (0.35 muM), the time to reach a stable adsorption maximum was approximately 30 min. Interestingly, the maximum surface concentration attained for all solution concentrations studied by the QCM method was 25 +/- 1 pmol cm-2, close to that predicted for monolayer coverage. The dissipation was monitored during adsorption, and only small changes were detected, implying a rigid adsorption model, as needed when using the Sauerbrey equation. Cyclic voltammetric data were consistent with a one-electron, surface-confined CuII/CuI azurin process with fast electron-transfer kinetics. The electroactive surface concentration calculated using voltammetry was 7 +/- 1 pmol cm-2. The differences between the QCM and voltammetrically determined surface coverage values reflect, predominantly, the different measurement methods but imply that all surface-confined azurin is not electrochemically active on the time scale of cyclic voltammetry.     

Patterned growth of large oriented organic semiconductor single crystals on self-assembled monolayer templates

This work demonstrates a method for inducing site-specific nucleation and subsequent growth of large oriented organic semiconductor single crystals using micropatterned self-assembled monolayers (SAMs). We demonstrate growth of oriented, patterned, and large organic semiconductor single crystals for potential use in organic electronic devices. The control over multiple parameters in a single system has not yet been reported. The ability to control various aspects of crystal growth in one system provides a powerful technique for the bottom-up fabrication of organic single-crystal semiconductor devices.     

Stereo-electrochemistry by a self-assembled monolayer of sulfur-bridged calixthiophene on gold

Sulfur-bridged calixthiophene formed a self-assembled mono-molecular layer on polycrystalline gold, and it regulated an electrochemical electron transfer by the host–guest interaction between the cavity and reactants. 1,7,13,19,25-Tetrathia[1.₅](2,5)thiophenophane (thiacalix[5]thiophene) perfectly passivated the gold electrode for relatively large reversible metal complexes: [Fe(CN)₆]^4−/3− and [IrCl₆]^3−/2−. However, for mono-atomic ions, such as silver and some of the halogen ions, the electrode behaved reversibly. For copper reduction, a large activation overpotential was observed to induce an initial copper reduction in the cavity.