Colloidal assemblies on patterned silane layers

Wetting-Controlled Localized Placement of Surface Functionalities within Nanopores

In the context of sensing and transport control, nanopores play an essential role. Designing multifunctional nanopores and placing multiple surface functionalities with nanoscale precision remains challenging. Interface effects together with a combination of different materials are used to obtain local multifunctionalization of nanoscale pores within a model pore system prepared by colloidal templating. Silica inverse colloidal monolayers are first functionalized with a gold layer to create a hybrid porous architecture with two distinct gold nanostructures on the top surface as well as at the pore bottom. Using orthogonal silane- and thiol-based chemistry together with a control of the wetting state allows individual addressing of the different locations within each pore resulting in nanoscale localized functional placement of three different functional units. Ring-opening metathesis polymerization is used for inner silica-pore wall functionalization. The hydrophobized pores create a Cassie-Baxter wetting state with aqueous solutions of thiols, which enables an exclusive functionalization of the outer gold structures. In a third step, an ethanolic solution able to wet the pores is used to self-assemble a thiol-containing initiator at the pore bottom. Subsequent controlled radical polymerization provides functionalization of the pore bottom. It is demonstrated that the combination of orthogonal surface chemistry and controlled wetting states can be used for the localized functionalization of porous materials.

Surface Preparation for Single-Molecule Chemistry

Immobilization procedures, intended to enable prolonged observation of single molecules by fluorescence microscopy, may generate heterogeneous microenvironments, thus inducing heterogeneity in the molecular behavior. On that account, we propose a straightforward surface preparation procedure for studying chemical reactions on the single-molecule level. Sensor fluorophores were developed, which exhibit dual-emissive characteristics in a homogeneously catalyzed showcase reaction. These molecules undergo a shift of fluorescence wavelength of about 100 nm upon Pd(0)-induced deallylation in the Tsuji-Trost reaction, allowing for separate visualization of the starting material and product. Whereas a simultaneous immobilization of dye and inert silane leads to strongly polydisperse reaction kinetics, a consecutive immobilization routine with deposition of dye molecules as the last step provides substrates underlying the kinetics of ensemble experiments. Also, the found kinetics are unaffected by the chemical variation of inert silanes, nearly uniform, and therefore well reproducible. Additional parameters like photostability, signal-to-noise ratio, dye-molecule density, and spatial distribution of dye molecules are, as well, hardly affected by surface modification in the successive immobilization scheme.

Multi-photon patterning of photoactive o-nitrobenzyl ligands bound to gold surfaces

We quantitatively investigate lithographic patterning of a thiol-anchored self-assembled monolayer (SAM) of photocleavable o-nitrobenzyl ligands on gold through a multi-photon absorption process at 1.7 eV (730 nm wavelength). The photocleaving rate increases faster than the square of the incident light intensity, indicating a process more complex than simple two-photon absorption. We tentatively ascribe this observation to two-photon absorption that triggers the formation of a long-lived intermediate aci-nitro species whose decomposition yield is partially determined either by absorption of additional photons or by a local temperature that is elevated by the incident light. At the highest light intensities, thermal processes compete with photoactivation and lead to damage of the SAM. The threshold is high enough that this destructive process can largely be avoided, even while power densities are kept sufficiently large that complete photoactivation takes place on time scales of tens of seconds to a few minutes. This means that this type of ligand can be activated at visible and near infrared wavelengths where plasmonic resonances can easily be engineered in metal nanostructures, even though their single-photon reactivity at these wavelengths is negligible. This will allow selective functionalization of plasmon hotspots, which in addition to high resolution lithographic applications would be of benefit to applications such as Surface Enhanced Raman Spectroscopy and plasmonic photocatalysis as well as directed bottom-up nanoassembly.

Assembly of colloidal particles in solution

Advances in both top-down and bottom-up syntheses of a wide variety of complex colloidal building blocks and also in methods of controlling their assembly in solution have led to new and interesting forms of highly controlled soft matter. In particular, top-down lithographic methods of producing monodisperse colloids now provide precise human-designed control over their sub-particle features, opening up a wide range of new possibilities for assembly structures that had been previously limited by the range of shapes available through bottom-up methods. Moreover, an increasing level of control over anisotropic interactions between these colloidal building blocks, which can be tailored through local geometries of sub-particle features as well as site-specific surface modifications, is giving rise to new demonstrations of massively parallel off-chip self-assembly of specific target structures with low defect rates. In particular, new experimental realizations of hierarchical self-assembly and control over the chiral purity of resulting assembly structures have been achieved. Increasingly, shape-dependent, shape-complementary, and roughness-controlled depletion attractions between non-spherical colloids are being used in novel ways to create assemblies that go far beyond early examples, such as fractal clusters formed by diffusion-limited and reaction-limited aggregation of spheres. As self-assembly methods have progressed, a wide variety of advanced directed assembly methods have also been developed; approaches based on microfluidic control and applying structured electromagnetic fields are particularly promising.

Fabricating Reactive Surfaces with Brush-like and Crosslinked Films of Azlactone-Functionalized Block Co-Polymers

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl dimethyl azlactone) (PGMA-b-PVDMA), are presented. Due to the high reactivity of azlactone groups towards amine, thiol, and hydroxyl groups, PGMA-b-PVDMA surfaces can be modified with secondary molecules to create chemically or biologically functionalized interfaces for a variety of applications. Previous reports of patterned PGMA-b-PVDMA interfaces have used traditional top-down patterning techniques that generate non-uniform films and poorly controlled background chemistries. Here, we describe customized patterning techniques that enable precise deposition of highly uniform PGMA-b-PVDMA films in backgrounds that are chemically inert or that have biomolecule-repellent properties. Importantly, these methods are designed to deposit PGMA-b-PVDMA films in a manner that completely preserves azlactone functionality through each processing step. Patterned films show well-controlled thicknesses that correspond to polymer brushes (~90 nm) or to highly crosslinked structures (~1-10 μm). Brush patterns are generated using either the parylene lift-off or interface directed assembly methods described and are useful for precise modulation of overall chemical surface reactivity by adjusting either the PGMA-b-PVDMA pattern density or the length of the VDMA block. In contrast, the thick, crosslinked PGMA-b-PVDMA patterns are obtained using a customized micro-contact printing technique and offer the benefit of higher loading or capture of secondary material due to higher surface area to volume ratios. Detailed experimental steps, critical film characterizations, and trouble-shooting guides for each fabrication method are discussed.

Showing particles their place: deterministic colloid immobilization by gold nanomeshes

The defined immobilization of colloidal particles on a non-close packed lattice on solid substrates is a challenging task in the field of directed colloidal self-assembly. In this contribution the controlled self-assembly of polystyrene beads into chemically modified nanomeshes with a high particle surface coverage is demonstrated. For this, solely electrostatic interaction forces were exploited by the use of topographically shallow gold nanomeshes. Employing orthogonal functionalization, an electrostatic contrast between the glass surface and the gold nanomesh was introduced on a sub-micron scale. This surface charge contrast promotes a highly site-selective trapping of the negatively charged polystyrene particles from the liquid phase. AFM force spectroscopy with a polystyrene colloidal probe was used to rationalize this electrostatic focusing effect. It provides quantitative access to the occurring interaction forces between the particle and substrate surface and clarifies the role of the pH during the immobilization process. Furthermore, the structure of the non-close packed colloidal monolayers can be finely tuned by varying the ionic strength and geometric parameters between colloidal particles and nanomesh. Therefore one is able to specifically and selectively adsorb one or several particles into one individual nanohole.

Fabrication of planar colloidal clusters with template-assisted interfacial assembly

The synthesis of nanoparticle clusters, also referred to as colloidal clusters or colloidal molecules, is being studied intensively as a model system for small molecule interactions as well as for the directed self-assembly of advanced materials. This paper describes a technique for the interfacial assembly of planar colloidal clusters using a combination of top-down lithographic surface modification and bottom-up Langmuir-Blodgett deposition. Micrometer sized polystyrene latex particles were deposited onto a chemically modified substrate from a decane-water interface with Langmuir-Blodgett deposition. The surface of the substrate contained hydrophilic domains of various size, spacing, and shape, while the remainder of the substrate was hydrophobic. Particles selectively deposited onto hydrophilic regions from the decane-water interface. The number of deposited particles depended on the size of each patch, thereby demonstrating that tuning cluster size is possible by engineering patch geometry. Following deposition, the clusters were permanently bonded with temperature annealing and then removed from the substrate via sonication. The permanently bonded planar colloidal clusters were stable in an aqueous environment and at a decane-water interface laden with isotropic colloidal particles. The method is a simple and fast way to synthesize colloidal clusters with few limitations on particle chemistry, composition, and shape.

Quantitative real-time imaging of protein-protein interactions by LSPR detection with micropatterned gold nanoparticles

Localized surface plasmon resonance (LSPR) offers powerful means for sensitive label-free detection of protein-protein interactions in a highly multiplexed format. We have here established self-assembly and surface modification of plasmonic nanostructures on solid support suitable for quantitative protein-protein interaction analysis by spectroscopic and microscopic LSPR detection. These architectures were obtained by layer-by-layer assembly via electrostatic attraction. Gold nanoparticles (AuNP) were adsorbed on a biocompatible amine-terminated poly(ethylene glycol) (PEG) polymer brush and further functionalized by poly-l-lysine graft PEG (PLL-PEG) copolymers. Stable yet reversible protein immobilization was achieved via tris(nitrilotriacetic acid) groups incorporated into the PLL-PEG coating. Thus, site-specific immobilization of His-tagged proteins via complexed Ni(II) ions was achieved. Functional protein immobilization on the surface was confirmed by real-time detection of LSPR scattering by reflectance spectroscopy. Association and dissociation rate constants obtained for a reversible protein-protein interaction were in good agreement with the data obtained by other surface-sensitive detection techniques. For spatially resolved detection, AuNP were assembled into micropatterns by means of photolithographic uncaging of surface amines. LSPR imaging of reversible protein-protein interactions was possible in a conventional wide field microscope, yielding detection limits of ∼30 protein molecules within a diffraction-limited surface area.

Line Patterning with Microparticles at Different Positions in a Single Device Based on Negative Dielectrophoresis

We report on control of line pattern positioning with particles fabricated by negative dielectrophoresis (n-DEP) using the applied intensity and phase of an AC electric field. Line patterns were fabricated in a microfluidic device consisting of upper conductive indium-tin-oxide (ITO) substrates and lower ITOinterdigitated microband array (IDA) electrodes used as the template. A 6-µm-diameter polystyrene particles suspension was introduced into the device between upper ITO and the bottom ITO-IDA substrate. An AC electric signal of a typically 20 peak-to-peak voltage and 1.0 MHz was then applied to upper ITO and bands on lower IDA, resulting in the formation of line patterns with low electric-field gradient regions. AC voltage was applied to bands A and B on lower IDA with the opposite phase and the same frequency and intensity. When the signal identical to band A was applied to upper ITO, particles were aligned above band A because relatively lower electric fields were produced in these regions. In contrast, the application of a signal identical to band B formed line patterns with particles aligned above band B due to the generation of a strong electric field between band A and upper ITO and the disappearance of the strong electric field between band B and upper ITO. The decrease in applied intensity to upper ITO shifted the accumulated position of particles to the center between bands A and B because of the balance of electric fields generated between band A or B and upper ITO. We thus fabricated line patterns with particles at desired positions in the fluidic device.

As flat as it gets: ultrasmooth surfaces from template-stripping procedures

In an experimentally simple replica process, the natural flatness of mica or polished silicon wafers can be transferred to metal films, resulting in metal surfaces with topographic features in Angstrom dimensions over large areas. Two decades after its invention, the template-stripping process continues to appeal to scientists from diverse research backgrounds primarily due to its simplicity, cost-effectiveness and ability to yield high quality substrates and structures. This article introduces the basic construction process for template-stripped substrates, and reports on a variety of extensions of the process, including the generation of materials contrasts and the design of tailored topographies. It also highlights the use of such substrates in a variety of research fields in nanoscience and technology ranging from surface force measurement and high definition imaging to the self-assembly of model membranes and plasmonics.

Multiphoton reactive surfaces using ruthenium(II) photocleavable cages

Photoreactive surfaces derived from a new photocleavable surface modification agent and with photosensitivity in the Vis and IR region are described. A ruthenium(II) caged aminosilane, [Ru(bpy)(2)(PMe(3))(APTS)](PF(6))(2), was synthesized and attached to silica surfaces. Light irradiation removed the cage and generated surface patterns with reactive amine groups. The photosensitivity of this compound under single (460 nm) and two-photon (900) excitation is demonstrated. Functional patterns with site-selective attachment of other molecular species are described.

Surfactant-induced formation of honeycomb pattern on micropipette with curvature gradient

Breath figure (BF) process is a facile method to prepare honeycomb structures by dynamic movements of condensed micrometer-sized water droplets at the interface of volatile fluid. Here, we aim to find answers to understand how the BF process occurs on micropipettes with curvature gradient and to understand the role of the surfactant in obtaining honeycomb patterns. Poly (L-lactic acid) (PLLA) chloroform solution with dioleoylphosphatidylethanolamine (DOPE) as surfactant was utilized. It is found that the honeycomb structure formed on the micropipettes changes remarkably with the gradually increased surface curvature. The variation trends of the arrangement and diameter of pores on the micropipettes with the increasing curvature are similar to the different time stages of BF process: smaller and sparse pores formed at higher curvature are similar to those formed at early stage of BF; regular honeycomb patterns formed at lower curvature are similar to those formed at the late stage of BF. Especially, the "semi-coalescence" hemispherical pores strings are found at high curvatures on PLLA-DOPE films, indicating the surfactant-induced coalescence of water droplets in BF process. The differences of drying speed of polymer solvent on micropipette with gradually increased curvatures make the printing of the pores at different BF stages on polymer film possible. These findings not only strongly support the mechanism of BF array formation, but also elucidate the surfactant-induced coalescence.

Modulating surface density of proteins via caged surfaces and controlled light exposure

We demonstrate the possibility of tuning the degree of functionalization of a surface using photoactivatable chemistries and controlled light exposure. A photosensitive organosilane with a protected amine terminal group and a tetraethyleneglycol spacer was synthesized. A o-nitrobenzyl cage was used as the photoremovable group to cage the amine functionality. Surfaces with phototunable amine densities were generated by controlled irradiation of silica substrates modified with the photosensitive anchor. Protein layers with different densities could be obtained by successive coupling and assembly steps. Protein surface concentrations were quantified by reflectance interference. Our results demonstrate that the protein density correlates with the photogenerated ligand density. The density control was proved over four coupling steps (biotin, SAv, (BT)tris-NTA, MBP, or GFP), indicating that the interactions between underlying layer and soluble targets are highly specific and the immobilized targets at the four levels maintain their full functionality. Protein micropatterns with a gradient of protein density were also obtained.

Functional immobilization and patterning of proteins by an enzymatic transfer reaction

Functional immobilization and lateral organization of proteins into micro- and nanopatterns is an important prerequisite for miniaturizing bioanalytical and biotechnological devices. Here, we report an approach for efficient site-specific protein immobilization based on enzymatic phosphopantetheinyl transfer (PPT) from coenzyme A (CoA)-functionalized glass-type surfaces to specific peptide tags. We devised a bottom-up surface modification approach for coupling CoA densely to a molecular poly(ethylene glycol) polymer brush. Site-specific enzymatic immobilization of proteins fused to different target peptides for the PPTase Sfp was confirmed by real-time label-free detection. Quantitative protein-protein interaction experiments confirmed that significantly more than 50% of the immobilized protein was fully active. The method was successfully applied with different proteins. However, different immobilization efficiencies of PPT-based immobilization were observed for different peptide tags being fused to the N- and C-termini of proteins. On the basis of this immobilization method, we established photolithographic patterning of proteins into functional binary microstructures.

Tunable pyramidal assemblies of nanoparticles by convective/capillary deposition on hydrophilic patterns made by AFM oxidation lithography

Close-packed pyramidal assemblies of 100 nm latex nanoparticles were made by convective/capillary deposition on hydrophilic patterns created by oxidation lithography using atomic force microscopy (AFM). We demonstrated that the substrate temperature during convective/capillary assembly is a key experimental parameter in finely tuning the geometry of these pyramids and thus the total number of nanoparticles forming each 3D assembly. The volume and shape of these nanoparticle assemblies are discussed and compared to simulations.

Parallel preparation of densely packed arrays of 150-nm gold-nanocrescent resonators in three dimensions

Metallic nanostructures show interesting optical properties due to their plasmonic resonances, and when arranged in three-dimensional (3D) arrays hold promise for optical metamaterials with negative refractive index. Towards this goal a simple, cheap, and parallel method to fabricate large-area, ordered arrays of 150-nm gold nanocrescents supporting plasmonic resonances in the near-infrared spectral range is demonstrated. In this process hexagonally ordered monolayers of monodisperse colloids are prepared by a simple floating technique, and subsequently the individual particles are size-reduced in a plasma process and used as a shadow mask with the initial lattice spacing. The resulting two-dimensional array of plasmonic resonators is coated with a transparent silica layer, which serves as a support for a second layer prepared by the identical process. The mutual orientation of the nanostructures between the individual layers can be freely adjusted, which determines the polarization-dependent absorption of the array and opens the possibility to introduce chirality in this type of 3D metamaterial. The iteration of this simple and efficient methodology yields 3D arrays with optical features as sharp as those of the individual nanocrescents, and shows strong potential for large-scale production of high-quality optical metamaterials.

Photoactive branched and linear surface architectures for functional and patterned immobilization of proteins and cells onto surfaces: a comparative study

Molecular architecture affects the properties of surface layers. Photosensitive silanes with branched architectures allow patterning and coupling of proteins and cells on surfaces while maintaining their biofunctional state. Attachment can be directed to the activated regions of irradiated substrates with high selectivity (see image of mouse fibroblasts). Novel photosensitive silanes with a branched molecular architecture combining three end-functionalized oligoethylene glycol (OEG) and alkyl arms are presented. These molecules are synthesized and applied to the modification of silica surfaces. The resulting layers are tested in their ability for the selective, patterned and functional immobilization of proteins and cells. The results demonstrate and accurately quantify the benefits of branched OEG structures against linear analogues for preventing non-specific interactions with the biological material. Linear structures guarantee high selectivity for the attachment of proteins, however, they fail in the case of cells. Branched structures provide good antifouling properties in both cases and allow the formation of protein patterns with higher densities of the target protein, as well as cell patterns. The results demonstrate the careful balance between surface functionality, composition and architecture that is required for maximizing the performance of any surface-based assay in biology.

Combining convective/capillary deposition and AFM oxidation lithography for close-packed directed assembly of colloids

We combine convective/capillary deposition and oxidation lithography by atomic force microscopy to direct the close-packed assembly of colloids on SiOx patterns fabricated on silicon substrates previously functionalized with a hydrophobic monolayer of octadecyltrimethoxysilane. The efficiency of this original generic method, which is well adapted to integrate colloids into silicon devices, is demonstrated for 100 nm colloidal latex nanoparticles and Escherichia coli bacteria in aqueous suspensions. A three-step mechanism involving convective flow and capillary forces appears to be responsible for these close-packed assemblies of colloids onto SiOx patterns.

Photoresponsive surfaces with two independent wavelength-selective functional levels

Two photoremovable protecting groups, namely, nitroveratryloxycarbonyl (NVo) and diethylamino-coumarin-4-yl (DEACM), have been tested for wavelength-selective, independent removal. The chromophores were attached to the amine group of aminopropyltriethoxysilane and used for the modification of silica surfaces. A photolytic experiment on the photosensitive layers allowed us to identify the irradiation conditions for the selective cleavage of the chromophores. UV measurements revealed that the photolabile DEACM group can be cleaved off with UV light at 412 nm without damaging the NVo group. The NVo group could then be removed at 365 nm. Masked irradiation of substrates modified with a 1:1 molar mixture of both silanes allowed the generation of bifunctional patterns after the selective cleavage of DEACM and NVo in a sequential irradiation process. The deprotection reaction was confirmed by coupling two different fluorescent dyes to the liberated amine groups. The expected two-color pattern could be observed by fluorescence microscopy.

Rings of nanoparticle-decorated honeycomb-structured polymeric film: the combination of pickering emulsions and capillary flow in the breath figures method

The self-assembly of nanoparticles at the fluid/fluid interface (Pickering emulsions) in the breath figures (BF) method have been explored to direct nanoparticles onto BF microarrays and adjust the BF assembly in microsize. Circular rings of nanoparticle-decorated honeycomb-structured polymeric film can be obtained by a one-step process. The combination of Pickering emulsions and capillary flow in the BF method may be responsible for the formation of this intriguing structure.

Poly(ethylene glycol) monolayer formation and stability on gold and silicon nitride substrates

Poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) are extensively used to modify substrates to prevent nonspecific protein adsorption and to increase hydrophilicity. X-ray photoelectron spectroscopy analysis, complemented by water contact angle measurements, is employed to investigate the formation and stability upon aging and heating of PEG monolayers formed on gold and silicon nitride substrates. In particular, thiolated PEG monolayers on gold, with and without the addition of an undecylic spacer chain, and PEG monolayers formed with oxysilane precursors on silicon nitride have been probed. It is found that PEG-thiol SAMs are degraded after less than two weeks of exposure to air and when heated at temperatures as low as 120 degrees C. On the contrary, PEG-silane SAMs are stable for more than two weeks, and fewer molecules are desorbed even after two months of aging, compared to those desorbed in two weeks from the PEG-thiol SAMs. A strongly bound hydration layer is found on PEG-silane SAMs aged for two months. Heating PEG-silane SAMs to temperatures as high as 160 degrees C improves the quality of the monolayer, desorbing weakly bound contaminants. The differences in stability between PEG-thiol SAMs and PEG-silane SAMs are ascribed to the different types of bonding to the surface and to the fact that the thiol-Au bond can be easily oxidized, thus causing desorption of PEG molecules from the surface.

Photopatterned surfaces for site-specific and functional immobilization of proteins

Photosensitive silanes containing nitroveratryl (Nvoc)-caged amine groups and protein repellent tetraethylene glycol units were synthesized and used for modification of silica surfaces. Functional surface layers containing different densities of caged amine groups were prepared and activated by UV-irradiation of the surface. The performance of these layers for functional and site-selective immobilization of proteins was tested. For this purpose, biotin and tris-nitrilotriacetic acid (tris-NTA) were fist coupled to the activated surface, and the interaction of streptavidin and His-tagged proteins with the functionalized surfaces was monitored by real-time label-free detection. After optimizing the coupling protocols, highly selective functionalization of the deprotected amine groups was possible. Furthermore, the degree of functionalization (and therefore the amount of immobilized protein) was controlled by diluting the surface concentration of the amine-functionalized silane with a nonreactive (OMe-terminated) tetraethylene glycol silane. Immobilized proteins were highly functional on these surfaces, as demonstrated by protein-protein interaction assays with the type I interferon receptor. Protein micropatterns were successfully generated after masked irradiation and functionalization of the caged surface following the optimized coupling protocols.

A lithography-free method for directed colloidal crystal assembly based on wrinkling

A simple, novel, and completely lithography-free assembly strategy is reported for directed colloidal crystal assembly on optically transparent substrates. The templates are stable, well-controlled relaxation-wrinkles of ultrathin multilayer films in a non-stretched state, which are fabricated by layer-by-layer self-assembly of polymeric films on soft elastomeric substrates followed by uniaxial plastic deformations. The results show that the wrinkles can be used efficiently to topographically direct colloidal crystal assembly in dip coating. Remarkably highly regular 1- and 2-dimensional patterned colloidal crystals with controlled structures have been obtained. Furthermore, the concept is rather universal and applicable to various particle types provided surface interactions between particles and template are suitable.

An in situ study of the adsorption behavior of functionalized particles on self-assembled monolayers via different chemical interactions

The formation of particle monolayers by convective assembly was studied in situ with three different kinds of particle-surface interactions: adsorption onto native surfaces, with additional electrostatic interactions, and with supramolecular host-guest interactions. In the first case carboxylate-functionalized polystyrene (PS-COOH) particles were assembled onto native silicon oxide surfaces, in the second PS-COOH onto protonated amino-functionalized (NH3+) self-assembled monolayers (SAMs), and in the third beta-CD-functionalized polystyrene (PS-CD) particles onto beta-CD SAMs with pre-adsorbed ferrocenyl-functionalized dendrimers. The adsorption and desorption behaviors of particles onto and from these surfaces were observed in situ on a horizontal deposition setup, and the packing density and order of the adsorbed particle lattices were compared. The desorption behavior of particles from surfaces was evaluated by reducing the temperature below the dew point, thus initiating water condensation. Particle lattices on native oxide surfaces formed the best hexagonal close packed (hcp) order and could be easily desorbed by reducing the temperature to below the dew point. The electrostatically modified assembly resulted in densely packed, but disordered particle lattices. The specificity and selectivity of the supramolecular assembly process were optimized by the use of ferrocenyl-functionalized dendrimers of low generation and by the introduction of competitive interaction by native beta-CD molecules during the assembly. The fine-tuned supramolecularly formed particle lattices were nearly hcp packed. Both electrostatically and supramolecularly formed lattices of particles were strongly attached to the surfaces and could not be removed by condensation.

Negative dielectrophoretic patterning with colloidal particles and encapsulation into a hydrogel

Microparticle patterns have been fabricated on a nonconductive glass substrate and a conductive indium tin oxide (ITO) substrate using negative dielectrophoresis (n-DEP). The patterned microparticles on the substrate were immobilized by covalent bonding or embedded into polymer sheets or strings. The patterning device consisted of an ITO interdigitated microband array (IDA) electrode as the template, a glass or ITO substrate, and a polyester film (10-microm thickness) as the spacer. A suspension of 2-microm-diameter polystyrene particles was introduced into the device between the upper IDA and the bottom glass or ITO support. An ac electrical signal (typically 20 Vpp, 3 MHz) was then applied to the IDA, resulting in the formation of line patterns with low electric field gradient regions on the bottom support. When the glass substrate was used as the bottom support, the particles aligned under the microband electrodes of the IDA within 5 s because the aligned areas on the support were regions with the weakest electric field; however, for the ITO support, the particles were directed to the regions under the electrode gap and aligned on the support because these regions had the weakest electric field. The width of the particle lines could be roughly controlled by regulating the initial concentration of the suspended particles. The particles forming the line and grid patterns with single-particle widths were immobilized by using a cross-linking reaction between the amino groups on the aligned particles and N-hydroxysuccinimide-activated ester on the glass substrate activated by succinimidyl 4-(p-maleimidophenyl)-butyrate (SMPB). The patterned particles were also embedded in a photoreactive hydrogel polymer. A prepolymer solution of poly(ethylene glycol) diacrylate (PEG-DA) was used as the suspension medium to maintain the particle patterns in the polymerized hydrogel sheet and string following photopolymerization. The hydrogel sheets with particle patterns were fabricated by ultraviolet (UV) irradiation through the ITO-IDA template for 120 s. Hydrogel strings with the aligned particles were fabricated by using a conductive ITO support and a Pt-IDA template. Pt-IDA was used as a template as well as a photomask to block UV transmission. The present procedure affords extremely simple, rapid, and highly reproducible fabrication of particle arrays. The reusability of the template IDA electrode is also a substantial advantage over previous methods.

Spontaneous pattern formation by dip coating of colloidal suspensions on homogeneous surfaces

We study the slow withdrawal of a partially wet vertical plate at velocity U from a suspension of well-wet particles. Periodic horizontal striped assemblies form spontaneously at the three-phase contact line on energetically uniform surfaces. Stripe width and spacing depend on the withdrawal velocity U relative to a transition velocity Ut. Thick stripes separated by large spaces form for U<Ut. For U>Ut, thin stripes separated by small spaces form. The stripe spacing is reduced by an order of magnitude and varies weakly with U until a maximum velocity is reached at which the stripes fail to form. A partially wet surface can entrain a meniscus. For U<Ut, the meniscus forms a finite contact angle wedge with a pinned contact line. As the plate moves upward, it stretches the meniscus until it becomes too heavy to be retained by the wet, porous network provided by the particles at the contact line. The contact line then jumps backward to find a new equilibrium location, and the process begins anew. For U>Ut, we infer that a film of thickness h is entrained above the meniscus. When h is smaller than the particle diameter D, particles aggregate where the entrained film thickens to match up to the wetting meniscus. When an entrained particle becomes exposed to air by evaporation, it becomes the new pinning site from which the next film is entrained. The film thickness h increases with U; at some velocity, h becomes comparable to D. Particles flow into the film and deposit there in a disordered manner. A diagram summarizing particle deposition is developed as a function of D, U, and h.

Photoelectron spectroscopy studies of the functionalization of a silicon surface with a phosphorylcholine-terminated polymer grafted onto (3-aminopropyl)trimethoxysilane

The structure of a biomimetic phosphorylcholine (PC)-functionalized poly(trimethylene carbonate) (PC-PTMC-PC), linked to a silicon substrate through an aminolysis reaction at 120 degrees C with (3-aminopropyl)trimethoxysilane (APTMS), was studied using photoelectron spectroscopy. Two chemical states were found for the unreacted APTMS amine, a neutral state and a protonated state, where the protonated amine on average was situated closer to the silicon substrate than the neutral amine. The experiments also indicated the presence of a third chemical state, where amines interact with unreacted silanol groups. The PTMC chains of the grafted films were found to consist of only 2-3 repeat units, with the grafted chains enriched in the zwitterionic end group, suggesting that these groups are attracted to the surface. This was further supported by the experiments showing that the PC groups were situated deeper within the film.

Real-time and Single Fibril Observation of the Formation of Amyloid β Spherulitic Structures

In Alzheimer disease, amyloid beta, a 39-43-residue peptide produced by cleavage from a large amyloid precursor protein, undergoes conformational change to form amyloid fibrils and deposits as senile amyloid plaques in the extracellular cerebral cortices of the brain. However, the mechanism of how the intrinsically linear amyloid fibrils form spherical senile plaques is unknown. With total internal reflection fluorescence microscopy combined with the use of thioflavin T, an amyloid-specific fluorescence dye, we succeeded in observing the formation of the senile plaque-like spherulitic structures with diameters of around 15 microm on the chemically modified quartz surface. Real-time observation at a single fibrillar level revealed that, in the absence of tight contact with the surface, the cooperative and radial growth of amyloid fibrils from the core leads to a huge spherulitic structure. The results suggest the underlying physicochemical mechanism of senile plaque formation, essential for obtaining insight into prevention of Alzheimer disease.

Soft-UV Photolithography using Self-Assembled Monolayers

We report thiol-on-gold self-assembled monolayers (SAMs) that can be photodeprotected using soft UV irradiation (lambda = 365 nm) to yield CO(2)H functionalized surfaces complementing those reported previously, which yielded NH(2) functionalized surfaces. The photolysis of these SAMs were monitored using a combination of surface sensitive techniques. In the SAM environment the photodeprotection yields are lower than those obtained for equivalent reactions in dilute solution. The protected carboxylic acids SAMs are shown to have a low yield approximately 50% due to competing photoreduction reactions of the nitro group. The results from infrared studies show that, as the photolysis progresses, the long chain protected residues reorganize and shield the functional COOH groups, thereby reducing the hydrophilic character of the surface.