Direct patterning of silanized-biomolecules on semiconductor surfaces

A novel approach to pattern silanized-biomolecules directly onto glass (SiO(x)) substrates via Dip-Pen nanolithography (DPN) and microcontact printing (μCP) is presented. Subsequent hybridization reactions of DPN patterned silanized-DNA with its complementary strands provide "proof-of-concept" that the patterned oligonucleotides maintain their biological activities. The fabrication strategy does not require premodification of substrates and offers a cheap and robust way to immobilize molecules on electronically important semiconductor surfaces.     

Lithographic patterning on polydimethylsiloxane surfaces using polydimethylglutarimide

We present a method for high fidelity lithographic patterning on polydimethylsiloxane (PDMS) surfaces employing traditional cleanroom equipment and commercially available materials that overcomes previous problems in PDMS processing. To illustrate this method, an electrostatically actuated microfluidic pump and rectangular diffraction gratings were fabricated on PDMS.     

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.     

Electrochemically patterning sol-gel structures on conducting and insulating surfaces

A new approach for the local deposition of sol-gel films on conducting and insulating surfaces using scanning electrochemical microscopy (SECM) via the feedback and direct modes is presented. Patterning is based on enhancing sol-gel condensation by altering the local pH due to water electrolysis as a result of applying negative potentials.     

Molecular patterning on carbon based surfaces through photobiotin activation

We have demonstrated the site-specific adhesion of photobiotin as a method of producing protein micropatterns. These patterns were created by the selective UV irradiation of a thin film of deposited photobiotin. The UV activated areas of photobiotin were then developed using fluorescently labelled avidin. The size of pattern produced is an order of magnitude smaller than those previously reported by this method. The patterns were characterised, using atomic force microscopy (AFM) to determine their microstructure. It was found that the AFM could discriminate between the areas of protein immobilised to the surface through the activated photobiotin, and the bare substrate surface where the inactivated photobiotin had been removed during the washing process. The potential of these patterns as sensing surfaces is demonstrated through the creation of a spatially patterned immunosensing surface. In this case, a biotinylated antibody was bound to the surface and the pattern developed using a second antibody specific to the immobilised biotinylated antibody. This technique could thus provide a simple and efficient method of producing high density immunoassay systems.     

Light activated recombination

Many genes elicit their actions through their expression in precise spatial patterns in tissues. Photoregulated expression systems offer a means to remotely pattern gene expression in tissues. Using currently available photopatterning methods, gene expression is only transient. Herein is described a general method to permanently alter a cell's genome under the control of light. The photocaged estrogen receptor (ER) antagonists, nitroveratryl-hydroxytamoxifen (Nv-HTam) and nitroveratryl-hydroxytamoxifen aziridine (Nv-HTaz), mediate exposure-dependent recombination in cells expressing the Cre-ER, a fusion of the site-specific recombinase Cre and ER. Both Nv-HTam and Nv-HTaz only activate recombination by Cre-ER after exposure to light. When released only intracellularly, the covalent-modifying Taz can mediate significant amounts of recombination in an exposure-dependent manner. Nv-HTaz and Cre-ER represent perhaps the first compound that can be used to photopattern gene expression through recombination.     

Light driving force for surface patterning on azobenzene-containing polymers

In this paper, we investigated the effect of light driving force induced surface deformation on azobenzene-containing polymers. The surface deformation is attributed to light-induced mass migration inside the polymers. Circular cap arrays are firstly fabricated by high power laser ablation via polarization controlled three-beam interference. The circular caps are subsequently exposed to polarization controlled two-beam interfering field. The results illuminate that when the interfering laser beams are both set to P polarization, the circular caps are deformed. While the laser beams are of other interfering modes like (S, S) and (+45° , -45°), the caps are seldom deformed. The circular caps are also exposed to single intensity-homogeneous linearly polarized laser beam. The deformation of the caps keeps the same direction as the irradiating polarization. A model based on the focusing effect of the circular caps is addressed to explain the origin of the light driving force for mass migration in azopolymers. The all-optical approach for the production of deformed caps can be used to generate aspherical lens, which may be applied to many domains.     

Spreading of polymer molecules on solid surfaces

When a homopolymer adsorbs from dilute solution onto a solid surface it first attaches and then maximises its number of contacts with the substrate by means of a spreading process. Evidence for this spreading process can be obtained from experiments on the adsorption kinetics. We report on a case where the adsorption kinetics depend on the rate at which the polymer was supplied to the surface (a protein adsorbing onto silica). Also, we discuss competitive adsorption experiments in which one kind of chain molecule attempts to displace another one from the surface. In these experiments, the desorption rate of the displaced species reflects the spreading rate of the displacer. When this rate is slower than the supply of displacer molecules, oversaturated layers result that spontaneously eject polymer. We have measured the rate of these displacement-driven desorption processes in various cases and conclude that it depends strongly on the energy of the segment-surface bond. A model involving the diffusion of defects over the surface may account for this finding.     

Properties of large organic molecules on metal surfaces

The adsorption of large organic molecules on surfaces has recently been the subject of intensive investigation, both because of the molecules’ intrinsic physical and chemical properties, and for prospective applications in the emerging field of nanotechnology. Certain complex molecules are considered good candidates as basic building blocks for molecular electronics and nanomechanical devices. In general, molecular ordering on a surface is controlled by a delicate balance between intermolecular forces and molecule–substrate interactions. Under certain conditions, these interactions can be controlled to some extent, and sometimes even tuned by the appropriate choice of substrate material and symmetry. Several studies have indicated that, upon molecular adsorption, surfaces do not always behave as static templates, but may rearrange dramatically to accommodate different molecular species. In this context, it has been demonstrated that the scanning tunnelling microscope (STM) is a very powerful tool for exploring the atomic-scale realm of surfaces, and for investigating adsorbate–surface interactions. By means of high-resolution, fast-scanning STM unprecedented new insight was recently achieved into a number of fundamental processes related to the interaction of largish molecules with surfaces such as molecular diffusion, bonding of adsorbates on surfaces, and molecular self-assembly. In addition to the normal imaging mode, the STM tip can also be employed to manipulate single atoms and molecules in a bottom–up fashion, collectively or one at a time. In this way, molecule-induced surface restructuring processes can be revealed directly and nanostructures can be engineered with atomic precision to study surface quantum phenomena of fundamental interest. Here we will present a short review of some recent results, several of which were obtained by our group, in which several features of the complex interaction between large organic molecules and metal surfaces were revealed. The focus is on experiments performed using STM and other complementary surface-sensitive techniques.     

Anisotropic behavior of organic molecules on prepatterned surfaces

The nucleation of organic molecules on surfaces, prepatterned with stripes, is investigated with emphasis on anisotropy effects. Representing the molecules as ellipsoids, the related particle-particle interaction is modeled by means of a generalized Gay-Berne potential for similar biaxial particles. The orientation behavior of these ellipsoidal molecules induced by the stripe pattern is studied for the first monolayer by performing kinetic Monte Carlo simulations. It is shown how the properties of the particle alignment depend on energy scales, temperature, and flux. Based on the fact the particles strictly arrange in rows, it is furthermore instructive to analyze the orientation behavior within the different rows. Finally, the transfer of orientation from a preset row of molecules with fixed orientation to other nucleating particles is examined.     

Protein patterning based on electrochemical activation of bioinactive surfaces with hydroquinone-caged biotin

We report a protein attachment and patterning method based on a hydroquinone-caged biotin surface that generates bioactive biotin by mild electrochemical perturbation. The electrochemical activation proceeds under the buffered aqueous environment at neutral pH. It also allows site-selective generation of bioactive biotin for the immobilization of target protein by using prepatterned electrode arrays.     

Asymmetric charge patterning on surfaces and interfaces: formation of hexagonal domains

The structure of soft matter systems at interfaces is of utmost importance in the fields of nanopatterning and self-assembly. It has been shown that lamellar and hexagonal patterns can form on interfaces, for a wide variety of systems. The asphericity of charged domains is considered here for different strengths of the electrostatics, determined by the interface media, relative to the short range van der Waals interactions between the molecular components. The phase behavior of the surface structure is explored by using molecular dynamics simulations, including some dynamical aspects of the interaction between neighboring domains, using the Lindemann criterion [F. Lindemann, Z. Phys. 11, 609 (1910)]. The charge ratio of the electrostatic components influences the shape of the domains, as well as the degree of local order in the interdomain structure.     

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.     

Excitation spectrum of neutral molecules adsorbed on dielectric surfaces

It is shown that the excitation spectrum of neutral molecules physisorbed on a dielectric surface consists of two symmetric and two antisymmetric energy modes. The spectral functions of these modes are represented respectively by two lorentzian lines whose spectral widths are described by the radiative decay of the energy modes in question. Numerical results are derived for the energies of excitation and spectral widths for the rare-gas atoms adsorbed on graphite.     

Site-selective patterning of organic luminescent molecules via gas phase deposition

In this paper, we present a bottom-up approach to pattern organic luminescent molecules with a feature size down to sub-100 nm over wafer-sized areas. This method is based on the selective gas deposition of organic molecules on self-organized patterned structures, which consist of an organic monolayer with two different phases rather than different materials. The site selectivity is controllable by deposition rate and the pattern features. The reason for the site selectivity may be due to the nucleation and diffusion behaviors of the deposited organic molecules on different monolayer phases.     

Microscale patterning of thermoplastic polymer surfaces by selective solvent swelling

A new method for the fabrication of microscale features in thermoplastic substrates is presented. Unlike traditional thermoplastic microfabrication techniques, in which bulk polymer is displaced from the substrate by machining or embossing, a unique process termed orogenic microfabrication has been developed in which selected regions of a thermoplastic surface are raised from the substrate by an irreversible solvent swelling mechanism. The orogenic technique allows thermoplastic surfaces to be patterned using a variety of masking methods, resulting in three-dimensional features that would be difficult to achieve through traditional microfabrication methods. Using cyclic olefin copolymer as a model thermoplastic material, several variations of this process are described to realize growth heights ranging from several nanometers to tens of micrometers, with patterning techniques include direct photoresist masking, patterned UV/ozone surface passivation, elastomeric stamping, and noncontact spotting. Orogenic microfabrication is also demonstrated by direct inkjet printing as a facile photolithography-free masking method for rapid desktop thermoplastic microfabrication.     

Dissociative chemisorption dynamics of small molecules on metal surfaces

Much progress has been achieved for both experimental and theoretical studies on the dissociative chemisorption of molecules on surfaces.Quantum state-resolved experimental data has provided unprecedented details for these fundamental steps in heterogeneous catalysis,while the quantitative dynamics is still not fully understood in theory.An in-depth understanding of experimental observations relies on accurate dynamical calculations,in which the potential energy surface and adequate quantum mechanical implementation are desired.This article summarizes the current methodologies on the construction of potential energy surfaces and the quantum mechanical treatments,some of which are promising for future applications.The challenges in this field are also addressed.     

Chiral close-packing of achiral star-shaped molecules on solid surfaces

From the interplay of scanning tunneling microscopy and theoretical calculations, we study the chiral self-assembly of achiral HtB-HBC molecules upon adsorption on the Cu(110) surface. We find that chirality is expressed at two different levels: a +/-5 degrees rotation of the molecular axis with respect to the close-packed direction of the Cu(110) substrate and a chiral close-packed arrangement expected for star-shaped molecules in 2D. Out of the four possible chiral expressions, only two are found to exist due the effect of van der Waals (vdW) interactions forcing the molecules to simultaneously adjust to the atomic template of the substrate geometry and self-assemble in a close-packed geometry.