Solvent-responsive properties of octadecyltrichlorosiloxane nanostructures investigated using atomic force microscopy in liquid.

An emerging challenge for nanoscale measurements is to capture and quantify the magnitude of structural changes in response to environmental changes. Certain environmental parameters can affect the nanoscale morphology of samples, such as changing the pH, solvent polarity, ionic strength, and temperature. We prepared test platforms of n-octadecyltrichlorosilane ring nanostructures to study surface morphology changes at the nanoscale in selected liquid media compared to dry conditions in air. Particle lithography combined with organosilane vapor deposition was used to fabricate nanostructures of regular dimensions. Multilayer nanostructures of OTS were used as a test platform for scanning probe studies of solvent-responsive properties where the sides of designed ring structures expose a 3D interface for studying the interaction of solvents with molecular side groups. In dry, ambient conditions, nanostructures of OTS were first imaged using contact mode atomic force microscopy (AFM). Next, ethanol or buffer was introduced to the sample cell, and images were acquired using the same probe. We observed substantial changes in the lateral and vertical dimensions of the ring nanostructures in AFM topography frames; the sizes of the rings were observed to swell by tens of nanometers. Even after heat treatment of samples to promote cross-linking, the samples still evidenced swelling in liquid media. This research will have consequences for studies of the properties of nanomaterials, such as solvent-responsive organic films and polymers.

Applying AFM-based nanofabrication for measuring the thickness of nanopatterns: the role of head groups in the vertical self-assembly of omega-functionalized n-alkanethiols.

Molecules of n-alkanethiols with methyl head groups typically form well-ordered monolayers during solution self-assembly for a wide range of experimental conditions. However, we have consistently observed that, for either carboxylic acid or thiol-terminated n-alkanethiols, under certain conditions nanografted patterns are generated with a thickness corresponding precisely to a double layer. To investigate the role of head groups for solution self-assembly, designed patterns of omega-functionalized n-alkanethiols were nanografted with systematic changes in concentration. Nanografting is an in situ approach for writing patterns of thiolated molecules on gold surfaces by scanning with an AFM tip under high force, accomplished in dilute solutions of desired ink molecules. As the tip is scanned across the surface of a self-assembled monolayer under force, the matrix molecules are displaced from the surface and are immediately replaced with fresh molecules from solution to generate nanopatterns. In this report, side-by-side comparison of nanografted patterns is achieved for different matrix molecules using AFM images. The chain length and head groups (i.e., carboxyl, hydroxyl, methyl, thiol) were varied for the nanopatterns and matrix monolayers. Interactions such as head-to-head dimerization affect the vertical self-assembly of omega-functionalized n-alkanethiol molecules within nanografted patterns. At certain threshold concentrations, double layers were observed to form when nanografting with head groups of carboxylic acid and dithiols, whereas single layers were generated exclusively for nanografted patterns with methyl and hydroxyl groups, regardless of changes in concentration.

Achieving precision and reproducibility for writing patterns of n-alkanethiol self-assembled monolayers with automated nanografting.

Nanografting is a high-precision approach for scanning probe lithography, which provides unique advantages and capabilities for rapidly writing arrays of nanopatterns of thiol self-assembled monolayers (SAMs). Nanografting is accomplished by force- induced displacement of molecules of a matrix SAM, followed immediately by the self-assembly of n-alkanethiol ink molecules from solution. The feedback loop used to control the atomic force microscope tip position and displacement enables exquisite control of forces applied to the surface, ranging from pico to nanonewtons. To achieve high-resolution writing at the nanoscale, the writing speed, direction, and applied force need to be optimized. There are strategies for programing the tip translation, which will improve the uniformity, alignment, and geometries of nanopatterns written using open-loop feedback control. This article addresses the mechanics of automated nanografting and demonstrates results for various writing strategies when nanografting patterns of n-alkanethiol SAMs.

Long-chain 3,4-ethylenedioxythiophene/thiophene oligomers and semiconducting thin films prepared by their electropolymerization.

A series of soluble H-terminated conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) combined with a small number of thiophene units and ranging in length from four to eight EDOT/thiophene groups was prepared with the ultimate goal to investigate if facile formation of a reactive trication radical species would enable electrochemical polymerization of such long-chain oligomers. Spectroscopic and electrochemical studies of the oligomers revealed some general dependencies of their electronic properties on the total number and position of EDOT groups. It was the number of consecutive EDOT units rather than total number of these units which was found to have the most profound effect on electronic energy gap and conjugation length. This influence originates from the especially strong planarization induced in the conjugated backbone by the incorporation of EDOT units. In contrast, incorporation of thiophene units was found to result in loss of the conformational stabilization. This phenomenon was analyzed using the natural bond orbital computational approach, which revealed the predominantly hyperconjugative nature of the EDOT-induced conformational stabilization. Whereas shorter oligomers, in agreement with the general consensus, were found to be inert toward electrochemical polymerization due to low reactivity of electrochemically generated cation radical and dication species, the longest oligomer showed an unprecedentedly efficient electropolymerization to yield a stable thin film of an electroactive polymer. The efficient electropolymerization of the long-chain oligomer was found to be in agreement with the formation of a reactive trication radical species. The electronic and spectral properties of the resulting semiconducting polymer film were studied by various electrochemical and spectroelectrochemical methods, as well as conductive probe AFM technique, and revealed a number of unusual features (such as electrical rectifying switching behavior) consistent with the possibility of increased molecular order in this material.

Self-assembly of octadecyltrichlorosilane: Surface structures formed using different protocols of particle lithography.

Particle lithography offers generic capabilities for the high-throughput fabrication of nanopatterns from organosilane self-assembled monolayers, which offers the opportunity to study surface-based chemical reactions at the molecular level. Nanopatterns of octadecyltrichlorosilane (OTS) were prepared on surfaces of Si(111) using designed protocols of particle lithography combined with either vapor deposition, immersion, or contact printing. Changing the physical approaches for applying molecules to masked surfaces produced OTS nanostructures with different shapes and heights. Ring nanostructures, nanodots and uncovered pores of OTS were prepared using three protocols, with OTS surface coverage ranging from 10% to 85%. Thickness measurements from AFM cursor profiles were used to evaluate the orientation and density of the OTS nanostructures. Differences in the thickness and morphology of the OTS nanostructures are disclosed based on atomic force microscopy (AFM) images. Images of OTS nanostructures prepared on Si(111) that were generated by the different approaches provide insight into the self-assembly mechanism of OTS, and particularly into the role of water and solvents in hydrolysis and silanation.

Studies of the growth, evolution, and self-aggregation of ß-amyloid fibrils using tapping-mode atomic force microscopy.

Amyloid peptide (Aß) is the major protein component of plaques found in Alzheimer's disease, and the aggregation of Aß into oligomeric and fibrillic assemblies has been shown to be an early event of the disease pathway. Visualization of the progressive evolution of nanoscale changes in the morphology of Aß oligomeric assemblies and amyloid fibrils has been accomplished ex situ using atomic force microscopy (AFM) in ambient conditions. In this report, the size and the shape of amyloid ß(1-40) fibrils, as well as the secondary organization into aggregate structures were monitored at different intervals over a period of 5 months. Characterizations with tapping-mode AFM serve to minimize the strong adhesive forces between the probe and the sample to prevent damage or displacement of fragile fibrils. The early stages of Aß growth showed a predominance of spherical seed structures, oligomeric assemblies, and protofibrils; however the size and density of fibrils progressively increased with time. Within a few days of incubation, linear assemblies and fibrils became apparent. Over extended time scales of up to 5 months, the fibrils formed dense ensembles spanning lengths of several microns, which exhibit interesting changes due to self-organization of the fibrils into bundles or tangles. Detailed characterization of the Aß assembly process at the nanoscale will help elucidate the role of Aß in the pathology of Alzheimer's disease.

Effects of peptides derived from terminal modifications of the aß central hydrophobic core on aß fibrillization.

Considerable research effort has focused on the discovery of mitigators that block the toxicity of the ß-amyloid peptide (Aß) by targeting a specific step involved in Aß fibrillogenesis and subsequent aggregation. Given that aggregation intermediates are hypothesized to be responsible for Aß toxicity, such compounds could likely prevent or mitigate aggregation, or alternatively cause further association of toxic oligomers into larger nontoxic aggregates. Herein we investigate the effect of modifications of the KLVFF hydrophobic core of Aß by replacing N- and C-terminal groups with various polar moieties. Several of these terminal modifications were found to disrupt the formation of amyloid fibrils and in some cases induced the disassembly of preformed fibrils. Significantly, mitigators that incorporate MiniPEG polar groups were found to be effective against Aß(1-40) fibrilligonesis. Previously, we have shown that mitigators incorporating alpha,alpha-disubstituted amino acids (ααAAs) were effective in disrupting fibril formation as well as inducing fibril disassembly. In this work, we further disclose that the number of polar residues (six) and ααAAs (three) in the original mitigator can be reduced without dramatically changing the ability to disrupt Aß(1-40) fibrillization in vitro.

Structure-activity relationships in peptide modulators of ß-amyloid protein aggregation: variation in α,α-disubstitution results in altered aggregate size and morphology.

Neuronal cytotoxicity observed in Alzheimer's disease (AD) is linked to the aggregation of ß-amyloid peptide (Aß) into toxic forms. Increasing evidence points to oligomeric materials as the neurotoxic species, not Aß fibrils; disruption or inhibition of Aß self-assembly into oligomeric or fibrillar forms remains a viable therapeutic strategy to reduce Aß neurotoxicity. We describe the synthesis and characterization of amyloid aggregation mitigating peptides (AAMPs) whose structure is based on the Aß "hydrophobic core" Aß(17-20), with α,α-disubstituted amino acids (ααAAs) added into this core as potential disrupting agents of fibril self-assembly. The number, positional distribution, and side-chain functionality of ααAAs incorporated into the AAMP sequence were found to influence the resultant aggregate morphology as indicated by ex situ experiments using atomic force microscopy (AFM) and transmission electron microscopy (TEM). For instance, AAMP-5, incorporating a sterically hindered ααAA with a diisobutyl side chain in the core sequence, disrupted Aß(1-40) fibril formation. However, AAMP-6, with a less sterically hindered ααAA with a dipropyl side chain, altered fibril morphology, producing shorter and larger sized fibrils (compared with those of Aß(1-40)). Remarkably, ααAA-AAMPs caused disassembly of existing Aß fibrils to produce either spherical aggregates or protofibrillar structures, suggesting the existence of equilibrium between fibrils and prefibrillar structures.

Controlling the surface coverage and arrangement of proteins using particle lithography.

AIMS: The applicability of particle lithography with monodisperse mesospheres is tested with various proteins to control the surface coverage and dimensions of protein nanopatterns. METHODS & MATERIALS: The natural self-assembly of monodisperse spheres provides an efficient, high-throughput route to prepare protein nanopatterns. Mesospheres assemble spontaneously into organized crystalline layers when dried on flat substrates, which supply a structural frame or template to direct the placement of proteins. The template particles are displaced with a simple rinsing step to disclose periodic arrays of protein nanopatterns on surfaces. RESULTS & DISCUSSION: The proteins are attached securely to the surface, forming nanopatterns with a measured thickness of a single layer. The morphology and diameter of the protein nanostructures can be tailored by selecting the diameter of the mesospheres and choosing the protein concentration. CONCLUSIONS: Particle lithography is shown to be a practical, highly reproducible method for patterning proteins on surfaces of mica, glass and gold. High-throughput patterning was achieved with ferritin, apoferritin, bovine serum albumin and immunoglobulin-G. Depending on the ratio of proteins to mesospheres, either porous films or ring structures were produced. This approach can be applied for fundamental investigations of protein-binding interactions of biological systems in surface-bound bioassays and biosensor surfaces.