Tunable self-assembled peptide amphiphile nanostructures

Peptide amphiphiles are capable of self-assembly into a diverse array of nanostructures including ribbons, tubes, and vesicles. However, the ability to select the morphology of the resulting structure is not well developed. We examined the influence of systematic changes in the number and type of hydrophobic and hydrophilic amino acids on the self-assembly of amphiphilic peptides. Variations in the morphology of self-assembled peptides of the form X(6)K(n) (X = alanine, valine, or leucine; K = lysine; n = 1-5) are investigated using a combination of transmission electron microscopy and dynamic light scattering measurements. The secondary structures of the peptides are determined using circular dichroism. Self-assembly is controlled through a combination of interactions between the hydrophobic segments of the peptide molecules and repulsive forces between the charged segments. Increasing the hydrophobicity of the peptide by changing X to a more lipophilic amino acid or decreasing the number of hydrophilic amino acids transforms the self-assembled nanostructures from vesicles to tubes and ribbons. Changes in the hydrophobicity of the peptides are reflected in changes in the critical micelle concentration observed using pyrene probe fluorescence analysis. Self-assembled materials formed from cationic peptide amphiphiles of this type display promise as carriers for insoluble molecules or negatively charged nucleic acids in drug or gene delivery applications.     

Nucleic acid amphiphiles: synthesis and self-assembled nanostructures

This review provides an overview of a relatively new class of bio-conjugates, DNA amphiphiles, which consist of oligonucleotides covalently bonded to synthetic hydrophobic units. The reader will find the basic principles for the structural design and preparation methods of the materials. Moreover, the self-assembly into superstructures of higher order will be highlighted. Finally, some potential applications will be described.     

Functional nanostructures from surface chemistry patterning

Nanoscale devices are expected to provide important advances for a number of applications. While many methods to generate nanoscale patterns exist, their use is confined to a relatively narrow range of materials. To fabricate nanoscale structures of a material with useful properties, the most convenient route is to transfer the geometry of an existing pattern into another material. Methods to achieve this pattern transfer are summarized and organized in this review. Methods to generate the original patterns, as well as applications of the final structure are also described.     

Preparation and photoinduced patterning of azidoformate-terminated self-assembled monolayers

This paper presents a new method for the patterning of self-assembled monolayers (SAMs) using UV light. Azidoformate-terminated SAMs starting from 18-acetoxy-octadecyltrichlorosilane SAMs on silicon, prepared for the first time, are electrophilic and photosensitive, and can be patterned by UV irradiation through a mask. The resulting structured surfaces are still electrophilic and can be reacted with nucleophilic functions, for example, primary amines.     

Catalytically driven colloidal patterning and transport

We recently reported the convection and pattern formation of tracers caused by a catalytically generated electric field. The electric field arises due to the heterogeneous electrochemical reduction and oxidation of hydrogen peroxide (H2O2) on silver (Ag) and gold (Au), respectively.1 Here we describe an electrokinetic model, developed in conjunction with experiments, that explains the details of the convection and pattern formation phenomenon. The model also enables the measurement of reaction kinetic parameters that are otherwise difficult to obtain. This quantitative model serves as a platform for the modeling of other catalytic redox systems and systems with broken symmetries.     

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.     

A templating approach for monodisperse self-assembled organic nanostructures

The precise structural control is known for self-assembly into closed spherical structures (e.g., micelles), but similar control of open structures is much more challenging. Inspired by natural tobacco mosaic virus, we present the use of a rigid-rod template to control the size of a one-dimensional self-assembly. We believe that this strategy is novel for organic self-assembly and should provide a general approach to controlling size and dimension.     

Photon antibunching from oriented semiconducting polymer nanostructures

Fluorescence intensity correlation measurements reveal that z-oriented nanostructures from single chains of a cyano-substituted polyphenylene vinylene (CN-PPV) polymer act as single-quantum emitters. Photon antibunching is observed for the first time on individual polymer nanostructures with a modulation depth exceeding 90%, providing definitive proof that radiative recombination of molecular excitons occurs at a single localized site within the folded polymer chain.     

Controlled chemical stabilization of self-assembled PS-P4VP nanostructures

Supramolecular self-assemblies in selective solvents give rise to many patterning possibilities. The diblock copolymer polystyrene-b-poly(4-vinylpyridine) (PS-P4VP) is one such polymer that self-assembles into neat nanostructures in toluene. These nanostructures once formed are highly susceptible to solvent influence. Unfortunately, for use as nanotemplates and in the synthesis of nanoparticles, the susceptibility of the films to solvents can be a problem. In this study, we present a method to stabilize the structures through chemical means in solution. We used 1,4-dibromobutane in solution to chemically crosslink the pyridine residues of each of PS-P4VP to yield a series of stable spherical aggregates. In this way, the cross-linking ratio can be precisely controlled. The solution properties were studied using dynamic light scattering and small angle X-ray scattering and the morphology of the resulting micellar film was studied using transmission electron microscopy (TEM). The size of the micelles formed was found to be dependent on the amount of cross-linking and the shape of the PS-P4VP micelles remains stable when exposed to a selective solvent for PS.     

Macroscopic arrays of magnetic nanostructures from self-assembled nanosphere templates

We have extended the widely used technique of nanosphere lithography to produce nanosphere templates with significantly improved long-range order. Single, ordered domains stretching over areas greater than 1 cm2 have been achieved by assembling spheres with the correct surface chemistry on a water/air interface. Self-assembly over macroscopic areas is facilitated by a combination of electrostatic and capillary forces. The presented technique is easily implemented, and the assembled monolayers can be transferred onto almost any surface, thus making the procedure applicable to a broad range of nanoscale research. We demonstrate this through the fabrication of hexagonally ordered, macroscopic arrays of magnetic nanostructures with modified magnetic properties.     

Charge driven, electrohydrodynamic patterning of thin films

In electrohydrodynamic patterning, electrical forces and surface tension acting at the interface between two fluids sandwiched between silicon wafers compete to set the period of pillar arrays, gratings, and concentric rings. Shrinking the period to deep submicron lengths requires a precise understanding of the source of the electric field. Previous modeling efforts have assumed that applied voltages, contact potentials, and static charge drive the flow. Here we show the location of that charge and the tangential stress it engenders to impact profoundly how the period and growth rate depend on the dielectric contrast and the relative film thickness. The pillar-to-pillar spacing scales inversely proportional to the charge density, and densities of approximately 1 mC/m(2) (approximately 1 charge/100 nm(2)) suffice to produce micron sized pillars.     

Toward reliable gold nanoparticle patterning on self-assembled DNA nanoscaffold

Assembly of gold nanoparticles (AuNP) into designer architectures with reliablity is important for nanophotonics and nanoelectronics applications. Toward this goal we present a new strategy to prepare AuNPs monofunctionalized with lipoic acid modified DNA oligos. This strategy offers increased bonding strength between DNA oligos and AuNP surface. These conjugates are further selectively mixed with other DNA strands and assembled into fixed sized DNA nanostructures carring a discrete number of AuNPs at desired positions. Atomic force microscopy imaging reveals a dramatically improved yield of the AuNPs on DNA tile structure compared to the ensembles using monothiolate AuNP-DNA conjugates.     

Self-assembly of highly oriented one-dimensional h-WO3 nanostructures

A new self-assembled and highly oriented one-dimensional single-crystal nanostructure of WO3 with hexagonal form was successfully prepared by a mild, solution-based colloidal approach.     

Templating silica nanostructures on rationally designed self-assembled peptide fibers

Nature presents exquisite examples of templating hard, functional inorganic materials on soft, self-assembled organic substrates. An ability to mimic and control similar processes in the laboratory would increase our understanding of fundamental science, and may lead to potential applications in the broad arena of bionanotechnology. Here we describe how self-assembled, alpha-helix-based peptide fibers of de novo design can promote and direct the deposition of silica from silicic acid solutions. The peptide substrate can be removed readily through proteolysis, or other facile means to render silica nanotubes. Furthermore, the resulting silica structures, which span the nanometer to micrometer range, can themselves be used to template the deposition of the cationic polyelectrolyte, poly-(diallyldimethylammonium chloride). Finally, the peptide-based substrates can be engineered prior to silicification to alter the morphology and mechanical properties of the resulting hybrid and tubular materials.     

Deformation of a "rigid" molecule in self-assembled nanostructures

A simple spirobifluorene molecule with pseudotetrahedral structure was investigated for its supposed conformational resilience upon adsorption. Through deposition at room temperature of this molecule on a Cu(111) surface and subsequent observation at 5 K with an ultrahigh vacuum scanning tunneling microscope, this "rigidity" upon physisorption is confirmed. However, an unexpected chemisorbed state was also found with the molecules arranged in trimers. The unique coexistence of physisorbed and chemisorbed states on the same substrate is thus demonstrated at the early stage of self-assembly.     

Molecular imprinting of biomineralized CdS nanostructures: crystallographic control using self-assembled DNA-membrane templates

A wide range of biomineralization and templating methods exist for organizing inorganic materials at a wide range of length-scales. Here, we show that crystallographic control of the inorganic nanostructures is possible using synthetic biomolecular templates comprised of anionic DNA and cationic membranes, which self-assemble into a multilamellar structure where a periodic one-dimensional (1D) lattice of parallel DNA chains is confined between stacked two-dimensional (2D) lipid sheets. We have organized Cd2+ ions within the interhelical pores between DNA strands and subsequently reacted them with H2S to form CdS nanorods of controllable widths and crystallographic orientation. The strong electrostatic interactions align the templated CdS (002) polar planes parallel to the negatively charged sugar-phosphate DNA backbone, which indicates that molecular details of the DNA molecule are imprinted onto the inorganic crystal structure. The resultant nanorods have (002) planes tilted by 60 degrees with respect to the rod axis, in contrast to all known II-VI semiconductor nanorods.     

Directed electroless growth of metal nanostructures on patterned self-assembled monolayers

The directed placement of Cu nanostructures on surfaces has been studied using a combination of scanning probe lithography and electroless metal deposition onto nanopatterned SAMs of 16-mercaptohexadecanoic acid (16-MHA) on Au. In situ studies using nanoscale molecular gradients reveal how controlling the areal density of the 16-MHA molecules dictates the nucleation and growth of the metal nanostructures. The influence of controlling pattern line spacing and tip path on pattern feature fidelity is also discussed.     

Redox-responsive morphology transformation of self-assembled nanostructures based on thiol-disulfide interconversion

One dimensional (1D) nanotubes and three dimensional (3D) flowerlike supernanostructures were transformed reversibly, which was controlled by the oxidation and reduction cycle of aromatic diamide-derived thiol 1 and disulfide 2, as evidenced by SEM study. Their self-assembling patterns were investigated by UV-vis, ¹H NMR, X-ray crystallographic, and powder X-ray diffraction experiments.     

Direct patterning of self-assembled monolayers on gold using a laser beam

The development of a methodology to manipulate surface properties of a self-assembled monolayer (SAM) of alkanethiol on a gold film using direct laser patterning is the objective of this paper. The present study demonstrates proof of the concept for the feasibility of laser patterning monolayers and outlines theoretical modeling of the process to predict the resulting feature size. This approach is unique in that it eliminates the need for photolithography, is noncontact, and can be extended to other systems such as SAMs on silicon wafers or potentially polymeric substrates. A homogeneous SAM made of 1-hexadecanethiol is formed on a 300-A sputtered film of gold (supported by a soda lime glass substrate). Localized regions are then desorbed by scanning the focal spot of a 488-nm continuous-wave argon ion laser beam under a nitrogen atmosphere. The desorption occurs as a result of a high substrate temperature produced by the moving laser beam with a Gaussian spatial profile at a constant speed of 200 microm/s. After completing the scans, the sample is dipped into a dilute solution of 16-mercaptohexadecanoic acid and a hydrophilic monolayer self-assembles along the previously irradiated regions. The resultant lines are viewed, and line widths are measured using both wetting with tridecane under a light microscope and scanning electron microscopy. Using the direct laser patterning method, we have produced straight line patterns with widths of 28-170 microm. A thermal model was constructed to predict the line width of the desorbed monolayer. The effect of the laser power, beam waist, and temperature dependence of the substrate conductivity on the theoretical predictions is considered. It is shown that the theoretical predictions are in good agreement with the experimental results, and, thus, the model can effectively be used to predict experimental results.