Dynamic Measurements with Scanning Probe Microscopy: Surface Studies Using Nanostructured Test Platforms of Metalloporphyrins, Nanoparticles and Amyloid Fibrils

A hybrid imaging mode for characterization of magnetic nanomaterials has been developed, using atomic force microscopy (AFM) combined with electromagnetic sample actuation. Instead of using a coated AFM probe as a magnetic sensor; our strategy is to use a nonmagnetic probe with contact mode AFM to characterize the vibration of magnetic and superparamagnetic nanomaterials responding to the flux of an AC electromagnetic field. We refer to the hybrid imaging mode as magnetic sample modulation (MSM-AFM). An oscillating magnetic field is produced by applying an AC current to a wire coil solenoid placed under the sample stage for tuning selected parameters of driving frequency and strength of the magnetic field. When the AC field is on, the AFM probe is scanned in contact with the sample to sense periodic changes in the force and motion of vibrating nanomaterials. With MSM, responses of both the amplitude and phase signal along with spatial maps of the topography channel can be collected simultaneously. A requirement for MSM is that the samples can be free to vibrate, yet remain attached to the surface. Particle lithography was used to prepare well-defined test platforms of ring structures of magnetic or superparamagnetic nanomaterials. Capillary filling of polydimethylsiloxane (PDMS) molds was applied to generate stripes of FeNi3 nanoparticles with microscale dimensions as test platforms. The MSM-AFM imaging mode was used successfully to characterize nanomaterials of FeNi3 nanoparticles, cobalt nanoparticles, octa-substituted porphyrin nanocrystals and ionic liquid nanoGUMBOS with dimensions ranging from 1 to 200 nm. Dynamic MSM-AFM measurements can be obtained by placing the tip on a vibrating nanoparticle and sweeping the frequency or field strength. Changes in frequency spectra and vibrational amplitude can be mapped for nanoparticles of different sizes, shapes and composition. The MSM-AFM imaging mode provides a useful tool for investigating changes in size dependent magnetic properties of materials at the nanoscale. Samples of designed amyloid proteins were characterized ex situ using scanning probe microscopy. The progressive growth and fibrillization of amyloid â over extended time intervals was visualized with high resolution using AFM

Characterization of designed cobaltacarborane porphyrins using conductive probe atomic force microscopy

© 2016 Jayne C. Garno, et al. Porphyrins and metalloporphyrins have unique chemical and electronic properties and thus provide useful model structures for studies of nanoscale electronic properties. The rigid planar structures and π-conjugated backbones of porphyrins convey robust electrical characteristics. For our investigations, cobaltacarborane porphyrins were synthesized using a ring-opening zwitterionic reaction to produce isomers with selected arrangements of carborane clusters on each macrocycle. Experiments were designed to investigate how the molecular structure influences the selforganization, surface assembly, and conductive properties of three molecular structures with 2, 4, or 8 cobaltacarborane substituents. Current versus voltage (I-V) spectra for designed cobaltacarborane porphyrins deposited on conductive gold substrates were acquired using conductive probe atomic force microscopy (CP-AFM). Characterizations with CP-AFM provide capabilities for obtaining physical measurements and structural information with unprecedented sensitivity. We found that the morphology of cobaltacarborane porphyrin structures formed on surfaces depends on a complex interplay of factors such as the solvent used for dissolution, the nature of the substrate, and the design of the parent molecule. The conductive properties of cobaltacarborane porphyrins were observed to change according to the arrangement of cobaltacarborane substituents. Specifically, the number and placement of the cobaltacarborane ligands on the porphyrin macrocycle affect the interactions that drive porphyrin self-assembly and crystallization. Interestingly, coulombic staircase I-V profiles were detected for a porphyrin with two cobaltacarborane substituents

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. © 2008 Wiley Periodicals, Inc

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 Alzheimer\u27s 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 Alzheimer\u27s disease. Microsc. Res. Tech., 2011. © 2010 Wiley-Liss, Inc

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

Considerable research effort has focused on the discovery of mitigators that block the toxicity of the Β-amyloid peptide (AΒ) by targeting a specific step involved in AΒ fibrillogenesis and subsequent aggregation. Given that aggregation intermediates are hypothesized to be responsible for AΒ 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 AΒ 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 AΒ1-40 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 AΒ1-40 fibrillization in vitro. © 2010 American Chemical Society

Dynamic magnetic characterizations at the nanoscale: A new mode for AFM imaging with magnetic sample modulation (MSM-AFM)

© 2014 by Nova Science Publishers, Inc. Magnetic sample modulation combined with contact mode atomic force microscopy (AFM) provides a sensitive way to detect the vibration of magnetic nanomaterials at the level of individual nanoparticles or ferroproteins. Rather than using a coated AFM probe as a magnetic sensor, our strategy is to use a non-magnetic probe for contact mode AFM to characterize the vibration of superparamagnetic nanomaterials responding to the flux of an AC electromagnetic field. We refer to this hybrid imaging and measurement mode as magnetic sample modulation AFM (MSM-AFM). An alternating electromagnetic field induces the actuation of magnetic and superparamagnetic nanomaterials that are attached to surfaces, and the AFM tip is used to detect the sample vibration. For MSM-AFM, an oscillating magnetic field is produced by applying an AC current to a wire coil solenoid placed under the sample stage. The AFM is configured for contact mode imaging, and the probe is scanned slowly across the vibrating samples. Selected parameters of frequency and magnetic field strength can be tuned to study dynamic changes in the vibrational response of samples. The MSM-AFM imaging mode has been used successfully to detect the vibration of nanomaterials with dimensions less than 1.0 nm, and has been applied for measurements with ferritin, an iron-containing protein. Dynamic measurements can be accomplished by changing the modulation frequency and the strength of the applied electromagnetic field. With MSM-AFM, responses of both the amplitude and phase signal along with spatial maps of the topography channel can be collected simultaneously. The hybrid approach of combining MSM with contact mode AFM provides nanoscale characterizations for detecting the changes in vibrational resonance signatures, differences in vibrational amplitude versus size, and coupling effects of the magnetic response due to the proximity of adjacent magnetic nanomaterials

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

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 mice, 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. © 2008 Future Medicine Ltd