Atomic Force Microscopy Characterization Research Articles

Impact of Polymer Residue Level on the In-Plane Thermal Conductivity of Suspended Large-Area Graphene Sheets.

The presence of polymer transfer residues on graphene surfaces is a major bottleneck to overcome for the commercial and industrial viability of devices incorporating graphene layers. In particular, how clean the surface must be to recover high (>2500 W/mK) thermal conductivity and maximize the heat spreading capability of graphene for thermal management applications remains unclear. Here, we present the first systematic study of the impact of different levels of polymer residues on the in-plane thermal conductivity (κr) of single-layer graphene (SLG) fabricated by chemical vapor deposition (CVD). Control over the quantity of surface residue was achieved by varying the length of time each sample was rinsed in toluene to remove the poly(methyl methacrylate) (PMMA) support layer. The level of residue contamination was assessed using atomic force microscopy (AFM) and optical characterization. The thermal conductivity of the suspended SLG was measured using an optothermal Raman technique. We observed that the presence of polymer surface residue has a significant impact on the thermal properties of SLG, with the most heavily contaminated sample exhibiting a κr as low as (905 +155/-100) W/mK. Even without complete eradication of surface residues, a thermal conductivity as high as (3100 +1400/-900) W/mK was recovered, where the separation between adjacent clusters was sufficiently large (>700 nm). The proportion of the SLG surface covered by residues and the mean separation distance between clusters were found to be key factors in determining the level of κr suppression. This work has important implications for future large-scale graphene fabrication and transfer, particularly where graphene is to be used as a heat spreading layer in devices. The possibility of new opportunities for manipulation of the thermal properties of SLG via PMMA nanopatterning is also raised.

Lipid Phase Influences the Dynamic Interactions between Graphene Oxide Nanosheets and a Phospholipid Membrane.

To understand the possible perturbations of graphene oxide (GO) nanosheets on cell membranes is the first step to evaluate their cytotoxicity, while the membrane heterogeneity such like lipid phase separation complicates such interactions. Using the dynamic giant unilamellar vesicle leakage assays, atomic force microscopy characterizations, and molecular dynamics simulations, we demonstrated the structural and property disturbance of GO on a lipid bilayer membrane in a low ionic strength and neutral pH condition, specifically the influence of lipid phase on this process. GO tends to obliquely insert into and even be sandwiched between leaflets of a liquid-phase membrane, inducing formidable flaw in lipid packing states and fast transmembrane leakage. However, GO adopts parallel adsorption or vertical insertion on/into a gel-phase bilayer, while permeabilization occurs only when the disturbance is strong enough. Our results are helpful to understand the fundamental interaction mechanism between GO nanosheets and cells.

GUIDELINES TO SIMULATE LINEAR VISCOELASTIC MATERIALS WITH AN ARBITRARY NUMBER OF CHARACTERISTIC TIMES IN THE CONTEXT OF ATOMIC FORCE MISCROSCOPY

We provide guidelines for modeling linear viscoelastic materials containing an arbitrary number of characteristic times, under atomic force microscopy (AFM) characterization. Instructions are provided to set up the governing equations that rule the deformation of the material by the AFM tip. Procedures are described in detail in the spirit of providing a simple handbook, which is accompanied by open-access code and workbook (Excel) sheets. These guidelines seek to complement the existing literature and reach out to a larger audience in the awareness of the interdisciplinary nature of science. Examples are given in the context of force-distance curves characterization within AFM, but they can be easily extrapolated to other types of contact characterization techniques at different length scales. Despite the simplified approach of this document, the algorithms described herein are built upon rigorous classical linear viscoelastic theory.

Axial localization and tracking of self-interference nanoparticles by lateral point spread functions

Sub-diffraction limited localization of fluorescent emitters is a key goal of microscopy imaging. Here, we report that single upconversion nanoparticles, containing multiple emission centres with random orientations, can generate a series of unique, bright and position-sensitive patterns in the spatial domain when placed on top of a mirror. Supported by our numerical simulation, we attribute this effect to the sum of each single emitter’s interference with its own mirror image. As a result, this configuration generates a series of sophisticated far-field point spread functions (PSFs), e.g. in Gaussian, doughnut and archery target shapes, strongly dependent on the phase difference between the emitter and its image. In this way, the axial locations of nanoparticles are transferred into far-field patterns. We demonstrate a real-time distance sensing technology with a localization accuracy of 2.8 nm, according to the atomic force microscope (AFM) characterization values, smaller than 1/350 of the excitation wavelength.

Continuum level-set model for anisotropic wet etching of patterned sapphire substrates

We present a continuum modeling approach to simulate anisotropic wet etching of single-crystal sapphire employing mixtures of sulfuric acid and phosphoric acid. Wet etching of sapphire leads to the formation of crystal facets with high Miller–Bravais indices. The resulting complex three-dimensional topographies can be exploited to optimize the patterning of sapphire substrates which are employed for gallium nitride based light-emitting diodes. Due to the strong impact of the three-dimensional sapphire topography on the light extraction efficiency of the final device, precise control over the wet etching process, in particular etchant mixture, etch time, and temperature, is highly important. We present our model in the context of process technology computer-aided design, where we use the level-set method to track the evolution of the three-dimensional etch profile over time. In order to describe the intricate anisotropy of wet etched sapphire, we propose a flexible interpolation method for the etch rate distribution, which incorporates experimentally characterized crystal facets and deduces local extrema in the distribution based on local convexity/concavity considerations. The developed model has been calibrated and evaluated based on scanning electron microscopy and atomic force microscopy characterizations from the literature. Our model enables accurate sapphire etching simulations, where the emerging and disappearing crystal facets show a good agreement with experimental observations for several etchant mixtures and temperatures.

Bioelectrocatalytic Activity of W-Formate Dehydrogenase Covalently Immobilized on Functionalized Gold and Graphite Electrodes.

The decrease of greenhouse gases such as CO2 has become a key challenge for the human kind and the study of the electrocatalytic properties of CO2-reducing enzymes such as formate dehydrogenases is of importance for this goal. In this work, we study the covalent bonding of Desulfovibrio vulgaris Hildenborough FdhAB formate dehydrogenase to chemically modified gold and low-density graphite electrodes, using electrostatic interactions for favoring oriented immobilization of the enzyme. Electrochemical measurements show both bioelectrocatalytic oxidation of formate and reduction of CO2 by direct electron transfer (DET). Atomic force microscopy and quartz crystal microbalance characterization, as well as a comparison of direct and mediated electrocatalysis, suggest that a compact layer of formate dehydrogenase was anchored to the electrode surface with some crosslinked aggregates. Furthermore, the operational stability for CO2 electroreduction to formate by DET is shown with approximately 100% Faradaic yield.

DNA Electrochemical Biosensors for In Situ Probing of Pharmaceutical Drug Oxidative DNA Damage.

Deoxyribonucleic acid (DNA) electrochemical biosensors are devices that incorporate immobilized DNA as a molecular recognition element on the electrode surface, and enable probing in situ the oxidative DNA damage. A wide range of DNA electrochemical biosensor analytical and biotechnological applications in pharmacology are foreseen, due to their ability to determine in situ and in real-time the DNA interaction mechanisms with pharmaceutical drugs, as well as with their degradation products, redox reaction products, and metabolites, and due to their capacity to achieve quantitative electroanalytical evaluation of the drugs, with high sensitivity, short time of analysis, and low cost. This review presents the design and applications of label-free DNA electrochemical biosensors that use DNA direct electrochemical oxidation to detect oxidative DNA damage. The DNA electrochemical biosensor development, from the viewpoint of electrochemical and atomic force microscopy (AFM) characterization, and the bottom-up immobilization of DNA nanostructures at the electrode surface, are described. Applications of DNA electrochemical biosensors that enable the label-free detection of DNA interactions with pharmaceutical compounds, such as acridine derivatives, alkaloids, alkylating agents, alkylphosphocholines, antibiotics, antimetabolites, kinase inhibitors, immunomodulatory agents, metal complexes, nucleoside analogs, and phenolic compounds, which can be used in drug analysis and drug discovery, and may lead to future screening systems, are reviewed.

Formation of Novel Octuplex DNA Molecules from Guanine Quadruplexes.

Guanine quadruplex (G4)-DNA structures have sparked the interest of many scientists due to their important biological roles and their potential use in molecular nanoelectronics and nanotechnology. The high guanine content in G4-DNA endows it with mechanical stability, robustness, and improved charge transport properties-attractive attributes for a molecular nanowire. The self-driven formation of a novel G4-DNA-based nanostructure, coined guanine octuplex (G8)-DNA, is reported herein. Atomic force microscopy and scanning tunneling microscopy characterization of this molecule reveal its organized coiled-coil structure, which is found to be stable under different temperatures and surrounding conditions. G8-DNA exhibits enhanced stiffness, mechanical and thermodynamic stability when compared to its parent G4-DNA. These, along with its high guanine content, make G8-DNA a compelling new molecule, and a highly prospective candidate for molecular nanoelectronics.

Optical anisotropy of self-organized gold quasi-blazed nanostructures based on a broad ion beam.

To circumvent elaborate conventional lithographic methods for realizing metallic nanostructures, it is necessary to develop self-organized nanofabrication methods for suitable template structures and their optical characterization. We demonstrate the potential of ion bombardment with impurity co-deposition to fabricate terraced or quasi-blazed nanostructure templates. Self-organized terraced nanostructures on fused silica were fabricated using Ar+ ion bombardment with iron impurity co-deposition and subsequent Au shadow deposition. The aspect ratios are enhanced threefold, and the range of nanostructure period variation is significantly increased with respect to that of conventional nanostructures realized by pure ion bombardment. We reveal the key features of the method via atomic force microscopy and optical characterization. Variable-profile quasiperiodic nanostructures with periods of 100-450 nm, heights of 25-180 nm, and blaze angles of 10°-25° were fabricated over an area of 20×40mm2, and these exhibited tunable and broadening optical anisotropy across the nanostructured area. Thus, the proposed method is a viable technique for rapid, cost-effective, and deterministic fabrication of variable nanostructure templates for potential optical applications.

Thiolated polymer nanocarrier reinforced with glycyrrhetinic acid for targeted delivery of 5-fluorouracil in hepatocellular carcinoma.

The present work investigates the targeting efficacy of a novel thiolated polymer-based nanocomposite reinforced with glycyrrhetinic acid (GA) and loaded with 5-fluorouracil in hepatocellular carcinoma (HCC). The thiolated polymers were synthesized by EDAC-mediated conjugation reactions and lyophilization. The nanoparticles were prepared by solvent diffusion and high-pressure homogenization method. The prepared nanocomposite was characterized by Fourier transform infrared (FTIR) radiation, x-ray diffraction (XRD), dynamic light scattering (DLS), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. Pharmacological evaluation of the formulation was carried out on a rat model of diethylnitrosamine (DEN), and carbon tetrachloride (CCl4)-induced HCC and MTT assay was carried out with HEP-G2 cell line. In silico studies were conducted to investigate the probable mechanistic pathway of the nanocomposite. FTIR and XRD analysis indicated the successful thiolation of the polymers and confirmed the formation of the nanocomposite without any incompatibilities. DLS, SEM/EDX and AFM characterization confirmed that the nanoparticles were within the nano-size range. MTT assay implied the cytotoxic nature of the nanocomposite against hepatic carcinoma cells. The in vivo study revealed that serum SGOT, SGPT, ALP, GGT and total bilirubin levels were significantly reduced, in comparison with disease control and the result was confirmed by histopathology studies. The results of the HPLC analysis of liver homogenate confirmed the liver targeting ability of the nanocomposite. In silico studies exhibited significant binding affinity of GA and thiolated Eudragit towards liver homolog receptor-1 (LRH-1) suggesting that the developed nanocomposite could be a potential material for the treatment of HCC.

Atomic Force Microscopy Reveals that the Drosophila Telomere-Capping Protein Verrocchio Is a Single-Stranded DNA-Binding Protein.

Atomic force microscopy (AFM) is a scanning probe technique that allows visualization of biological samples with a nanometric resolution. Determination of the physical properties of biological molecules at a single-molecule level is achieved through topographic analysis of the sample adsorbed on a flat and smooth surface. AFM has been widely used for the structural analysis of nucleic acid-protein interactions, providing insights on binding specificity and stoichiometry of proteins forming complexes with DNA substrates. Analysis of single-stranded DNA-binding proteins by AFM requires specific single-stranded/double-stranded hybrid DNA molecules as substrates for protein binding. In this chapter we describe the protocol for AFM characterization of binding properties of Drosophila telomeric protein Ver using DNA constructs that mimic the structure of chromosome ends. We provide details on the methodology used, including the procedures for the generation of DNA substrates, the preparation of samples for AFM visualization, and the data analysis of AFM images. The presented procedure can be adapted for the structural studies of any single-stranded DNA-binding protein.

Atomic force microscopy characterization of β-casein nanoparticles on mica and graphite

The morphology and size distribution of β-casein nanoparticles adsorbed on mica and graphite were determined using atomic force microscopy. The height of the dried nanoparticles on graphite was about twice as smaller as that on mica, and the cross-sectional profiles of particles were different for these substrates. It is due to the hydrophobic interaction of nanoparticles with the graphite surface.

Facile Li-Al layered double hydroxide films on Al alloy for enhanced hydrophobicity, anti-biofouling and anti-corrosion performance

In this work, lithium (Li)-aluminum (Al) layered double hydroxide (LDH) films modified by 4-amino-2-((hydrazine methylene) amino)-4-oxobutanoic acid (denoted as AOA acid) and/or 1H,1H,2H,2H-perfluorooctyltriethoxysilane were prepared on 6N01 Al alloy by a facile, in-situ growth method with enhanced hydrophobicity, anti-biofouling and anti-corrosion performance. The preparation is low energy consumptive and environment friendly, relying on self-assembly at ambient temperature. The structure, molecular weight and functional groups of the synthesized AOA acid were characterized by NMR spectrometer, ESI-MS spectrometer and Fourier transform infrared (FT-IR) spectroscopy. And the compositions, structure and morphology of the films were characterized by Fourier transform infrared (FT-IR) spectroscopy, glancing-angle X-ray diffraction (GA-XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) and energy-dispersive x-ray spectrum (EDS). Water contact angle measurements (CA) and atomic force microscopy (AFM) characterization show that the films possess a micro/nanostructure with an improved hydrophobicity. Immersion test, neutral salt tests (NSS) and electrochemical impedance spectroscopy (EIS) conducted in 3.5 wt.% NaCl solutions demonstrate the improved corrosion resistance of the films over bare Al alloy. Meanwhile, the films also possess an excellent anti-bacterial property to Escherichia coli, Bacillus subtilis and sulfate-reducing bacteria.

Effect of nitrogen (N2) flow rate over the tribological, structural and mechanical properties diamond-like carbon (DLC) thin film

In this article, nitrogen (N2) doped diamond-like carbon (N-DLC) coatings are deposited by Atmospheric pressure chemical vapor deposition (APCVD) technique at varying N2 flow rates and are investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, nanoindentation (NI) and nano scratch (NS) characterization. The wear rate (WR) and coefficient of friction (μ) of the N-DLC thin films increase from 1.32 × 10−4 to 3.11 × 10−4 μm2 and 0.05 to 0.15 with the increase of load from 40 to 70 mN at 70 sccm N2 flow rate. The results showed that there is a significant improvement in the tribological and mechanical properties of the APCVD synthesized DLC film as compare to the other PVD deposited DLC film.

Ultrasensitive detection of interleukin 1α using 3-phosphonopropionic acid modified FTO surface as an effective platform for disposable biosensor fabrication

In this study, we utilized a carboxyalkylphosphonic acid covered fluorine doped tin oxide (FTO) as an electrode material for fabrication of Interleukin 1α (IL-1α) immunosensor. For this aim, anti-IL-1α antibodies were attached on the 3-phosphonopropionic acid (PHP) modified FTO surface covalently. Electrochemical (electrochemical impedance spectroscopy and cyclic voltammetry) and morphological (scanning electron microscopy and atomic force microscopy) characterizations were performed to monitor the successful fabrication of immunoelectrodes. After incubation of anti-IL-1α antibody immobilized FTO electrodes in IL-1α antigen solutions, increases were seen in impedimetric responses. IL-1α antigen was determined in a linear detection range from 0.02 to 2 pg/mL by EIS. The detection limit of the suggested immunosensor was 6 fg/mL. The applicability of the designed biosensor was tested by using human serum and saliva samples and acceptable results were obtained. In addition, high sensitivity, good specificity, low detection limit made the proposed immunosensor a potential technique for IL-1α antigen determination in routine clinical analysis.

Hierarchical Self-Assembly Mechanism of Ladder-Like Orientated Aβ40 Single-Stranded Protofibrils into Multistranded Mature Fibrils.

The complex self-assembly processes in three dimensions of Alzheimer's β-peptide (Aβ) amyloid protofibrils into polymorphic mature fibrils, particularly the relative protofibril orientation and packing mechanism, are poorly understood. We report here the identification and quantification of the hierarchical self-assembly details among distinct Aβ40 fibrils, particularly the winding pictures of two, three, and four individual single-stranded protofibrils into two-, three-, and four-stranded mature fibrils, respectively, via cross-sectional analysis of atomic force microscopy (AFM) images. The statistical polymer physics analysis of fibril flexibilities from AFM characterizations as well as molecular dynamics (MD) simulations reveal a ladder-like packing mechanism rather than a closed-packing manner for the interprotofibril association into Aβ40 mature fibrils. Moreover, our MD results show atomic packing polymorphism at the well-packing interfaces even within the same multistranded fibril. This work provides mechanistic insights into the polymorphic transition of single-stranded Aβ40 protofibrils into multistranded mature fibrils at the mesoscopic level, which is useful for a more comprehensive understanding of Alzheimer's β-peptide amyloidosis.

Synthesis of large-area MoS2 films by plasma-enhanced chemical film conversion of solution-processed ammonium tetrathiomolybdate

The synthesis of large-area molybdenum disulfide (MoS2) films is desired for many emerging electronic and optical applications. A typical approach is to deposit films from vapor-phase precursors either by thermal- or plasma-activated chemistry. Here, we show that MoS2 films exhibiting excellent crystallinity and smooth surfaces are produced by plasma conversion of solution-deposited precursor films. Films containing a single-source precursor for MoS2, ammonium tetrathiomolybdate, were spin coated and converted by an atmospheric-pressure dielectric barrier discharge with substrate heating at 500 °C. X-ray diffraction and micro-Raman spectroscopy show that the plasma is necessary to successfully convert the precursor to MoS2. Further enhancements in the crystallinity and surface roughness were obtained by annealing the films, as revealed by high-resolution transmission electron microscopy and atomic force microscopy (AFM). Insights into the role of the plasma were provided by AFM characterization and a “plasma-trigger” experiment, which suggest that a nanocone surface morphology facilitates the growth of the films.

Replica molding of cicada wings: The role of water at point of synthesis on nanostructure feature size.

Many natural surfaces, including the wings of cicada insects, have shown to display bactericidal properties as a result of surface topography. Moreover, the size and distribution of the surface features (on the nano- and microscale) are known to influence the efficacy of the surface at inhibiting bacterial cell growth. While these types of natural surfaces illustrate the effect of structure on the bactericidal activity, a deeper understanding can be achieved by creating surfaces of different feature sizes. This is essential in order to understand the effects of changes of surface topography on bacteria-surface interactions. To this end, we have performed a series of replica molding processes of the wings of the Megapomponia Intermedia cicada to prepare wing replicas in polyethylene glycol (PEG), which possess the topographical features of the wing surface, with a minimum loss of feature resolution. Atomic force microscopy characterization of these patterned surfaces in both air and aqueous environments shows that by controlling the swelling characteristics of the PEG, we can control the ultimate swollen dimensions of the nanopillar structures on the surface of PEG. As a result, by using a single wing with an average nanopillar height of 220 nm, different patterned PEG samples with nanopillar heights ranging from 180 to 307 nm were produced.

Damage mechanism of graphene nanoribbons field effect transistors during focused ion beam etching

The fabrication of graphene nanoribbons (GNRs) is a familiar method to open the band gap of graphene. As one of the most novel means, focused ion beam (FIB) etching is expected to become the mainstream of GNRs production. Monte Carlo analysis is devoted to simulate the process of ion scattering in FIB etching GNRs. In the process of scattering, charged ions collide with solid materials continuously, which changes the direction and energy of ions in solid. Therefore, the information such as the motion track of the scattered ions, the damage of recoil atoms and the sputtering of surface atoms can be obtained, through the calculation of the scattering angle as well as nuclear energy loss of the charged ions. Combined with Raman spectroscopy and atomic force microscope (AFM) characterization, it is revealed that FIB etching indirectly induces the structure defects on GNRs and affects electrical performance.

Facile preparation of self-assembled black phosphorus-based composite LB films as new chemical gas sensors

In recent years, the researches on the modification and functionalization of black phosphorus (BP) materials have attracted more and more attention. In this study, few-layers BP nanosheets (BPNS) were prepared by a simple liquid exfoliation method. However, BPNS has the disadvantages of easy oxidation and poor stability, which will also affect the performance of BP. Therefore, 4-azidobenzoic acid was used to modify the BPNS to improve the stability of BP. In addition, 4-azidobenzoic acid modified BPNS (f-BPNS) and dye molecules can form a composite Langmuir-Blodgett (LB) film through electrostatic interaction or π-π stacking forces. UV–vis, Atomic force microscopy (AFM) and other characterizations were used to prove the successful preparation of the f-BPNS-dye composite LB film. It should be noted that the obtained uniform and dense composite films showed good sensitivity to acid and alkali gas. Moreover, the f-BPNS-dye composite films can still maintain high performance after 10 cycles of acid-alkali tests, indicating that the prepared films have high stability. This work provides a new direction for the preparation of new two-dimensional composite materials and their application in gas sensors.