Johnpeter N. Ngunjiri

Investigation of the magnetic properties of ferritin by AFM imaging with magnetic sample modulation

AbstractIndividual ferritin molecules can be sensitively detected using magnetic sample modulation (MSM) combined with contact mode atomic force microscopy (AFM). To generate an oscillating magnetic field, an alternating current (AC) was applied to a solenoid placed within the base of the AFM sample stage. When a modulated electromagnetic field is applied to samples, ferromagnetic and paramagnetic nanomaterials are induced to vibrate. The flux of the AC electromagnetic field causes the ferritin samples to vibrate with corresponding rhythm and periodicity of the applied field. This motion can be detected and mapped using contact mode AFM with a soft, nonmagnetic cantilever. Changes in the phase and amplitude of the periodic motion of the sample are sensed by the tip to selectively map vibrating magnetic nanomaterials. Particle lithography was used to create nanopatterned test platforms of ferritin for MSM measurements. Regularly spaced structures of proteins provide precise reproducible dimensions for multiple successive surface measurements at dimensions of tens of nanometers. FigureRing patterns of ferritin were used as nanoscale test platforms to characterize magnetic properties at the level of individual proteins with AFM imaging

Electrochemical and thermal grafting of alkyl grignard reagents onto (100) silicon surfaces.

Passivation of (100) silicon surfaces using alkyl Grignard reagents is explored via electrochemical and thermal grafting methods. The electrochemical behavior of silicon in methyl or ethyl Grignard reagents in tetrahydrofuran is investigated using cyclic voltammetry. Surface morphology and chemistry are investigated using atomic force microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS). Results show that electrochemical pathways provide an efficient and more uniform passivation method relative to thermal methods, and XPS results demonstrate that electrografted terminations are effective at limiting native oxide formation for more than 55 days in ambient conditions. A two-electron per silicon mechanism is proposed for electrografting a single (1:1) alkyl group per (100) silicon atom. The mechanism includes oxidation of two Grignard species and subsequent hydrogen abstraction and alkylation reaction resulting in a covalent attachment of alkyl groups with silicon.

Studies of the growth, evolution, and self‐aggregation of β‐amyloid fibrils using tapping‐mode atomic force microscopy

Amyloid peptide (Aβ) is the major protein component of plaques found in Alzheimers disease, and the aggregation of Aβ 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 Aβ 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 β1‐40 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 Aβ 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 Aβ assembly process at the nanoscale will help elucidate the role of Aβ in the pathology of Alzheimers disease. Microsc. Res. Tech., 2011.

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.

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.

Nanografting versus Solution Self-Assembly of α,ω-Alkanedithiols on Au(111) Investigated by AFM

The solution self-assembly of alpha,omega-alkanedithiols onto Au(111) was investigated using atomic force microscopy (AFM). A heterogeneous surface morphology is apparent for 1,8-octanedithiol and for 1,9-nonanedithiol self-assembled monolayers (SAMs) prepared by solution immersion as compared to methyl-terminated n-alkanethiols. Local views from AFM images reveal a layer of mixed molecular orientations for alpha,omega-alkanedithiols, which evidence surface structures with heights corresponding to both lying-down and standing-up orientations. For dithiol SAMs prepared by solution self-assembly, the majority of alpha,omega-alkanedithiol molecules chemisorb with both thiol end groups bound to the Au(111) surface with the backbone of the alkane chain aligned parallel to the surface. However, AFM images disclose that there are also islands of standing molecules scattered throughout the surface. To measure the thickness of alpha,omega-alkanedithiol SAMs with angstrom sensitivity, methyl-terminated n-alkanethiols with known dimensions were used as molecular rulers. Under conditions of spatially constrained self-assembly, nanopatterns of alpha,omega-alkanedithiols written by nanografting formed monolayers with heights corresponding to an upright configuration.

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.

Electrochemical Patterning of Organic Monolayers on Silicon

Organic monolayers may be grafted onto silicon surfaces using an in situ electrochemical patterning method. In this technique, dielectric templates such as polystyrene spheres (e = 2.5) or poly(dimethylsiloxane) stamps (e = 2.3-2.8) are placed in close proximity to, or in direct contact with, silicon electrodes while a potential is applied to drive electrografting reactions. In this work, the authors describe methyl monolayer patterns created in anodic processes and phenylacetylene monolayer patterns created in cathodic processes. Both anodic and cathodic processes show similar chronoamperometric behavior, suggesting silicon passivation associated with the formation of monolayers. Atomic force microscopy shows the sizes, geometries, and thickness of patterned films. Comparison of experimental results with electric field simulations also shows that solution resistance controls the feature sizes, resulting from electrografting with proximal dielectric templates. Similarly, electrochemical impedance spectroscopy shows that the films are densely packed with relatively low levels of defects. The versatile technique is further demonstrated as a monolayer resist for patterned electrodeposition of copper on silicon.

Self-assembly of multiwalled carbon nanotubes from quench-condensed CNI3 films

Freestanding, vertical, multiwalled carbon nanotubes (MWCNTs) are formed during the vacuum deposition of thin films of the metastable carbides CT3 (T=Ni, Co) onto fire-polished glass substrates. In contrast to widely used chemical and laser vapor deposition techniques, we utilize direct e-beam evaporation of arc-melted CT3 targets to produce MWCNTs that are self-assembled out of the CT3-film matrix. The depositions are made in an ambient vapor pressure that is at least six orders of magnitude lower than the 1−100 Torr typically used in chemical vapor techniques. Furthermore, the substrates need not be heated, and, in fact, we observe a robust nanotube growth on liquid nitrogen cooled glass and sapphire substrates. High-resolution atomic force microscopy reveals that MWCNTs of heights 1−40 nm are formed in films with nominal thicknesses in the range of 5−60 nm. We show that the growth parameters of the nanotubes are very sensitive to the grain structure of the films. This is consistent with a precipitation...

Electrochemical Passivation of (100) Silicon in Alkyl Grignard Solutions

Surface modification of (100) silicon with methyl groups is analyzed using electrografting and thermal hydrosilation. The surface chemistry is investigated by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), voltammetry and atomic force microscopy (AFM). Surfaces anodically electrografted in methyl Grignard solutions show a smooth topology and improved passivation relative to passivation via hydrosilation. Functionalized surfaces are stable and hinder the formation of oxides up to 45 days after the electrografting as shown in the XPS results.