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1. Correlation of Atomic Force Microscopy Tapping Forces to Mechanical Properties of Lipid Membranes

   https://doi.org/10.1115/DETC2012-70233  

   Shamitko-Klingensmith, Nicole ; Wambaugh, Kelley M. ; Burke, Kathleen A. ; Magnone, George J. ; Legleiter, Justin ( August 2012 , ASME 2012 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference)

   There is considerable interest in measuring, with nanoscale spatial resolution, the physical properties of lipid membranes because of their role in the physiology of living systems. Due to its ability to nondestructively image surfaces in solution, tapping mode atomic force microscopy (TMAFM) has proven to be a useful technique for imaging lipid membranes. However, further information concerning the mechanical properties of surfaces is contained within the time-resolved tip/sample force interactions. The tapping forces can be recovered by taking the second derivative of the cantilever deflection signal and scaling by the effective mass of the cantilever; this technique is referred to as scanning probe acceleration microscopy. Herein, we describe how the maximum and minimum tapping forces change with surface mechanical properties. Furthermore, we demonstrate how these changes can be used to measure mechanical changes in lipid membranes containing cholesterol. 

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2. Acceleration of imaging in atomic force microscopy working in sub-resonance tapping mode

   https://doi.org/10.1063/5.0089806  

   Echols-Jones, Piers ; Messner, William ; Sokolov, Igor ( August 2022 , Review of Scientific Instruments)

   Sub-resonance tapping (SRT) mode of atomic force microscopy (AFM) enables researchers to image surfaces with well-controlled load forces and to collect maps of multiple physical properties of samples. The major bottleneck of this mode is a relatively low scan speed compared to other scanning modes. This paper presents a novel control algorithm that substantially improves the scanning speed over the standard SRT. We propose naming the new modality Trajectory Tracking SRT (TT-SRT). In contrast with the standard SRT control, TT-SRT uses the feedback within every single touch of the sample by the AFM probe. To demonstrate the advantage of TT-SRT, we conduct scans on a variety of samples with differing topologies, roughnesses, and mechanical properties. Each sample region is scanned with both standard SRT and TT-SRT at the same set of speeds. The control gains are tuned before each scan for maximum performance in each mode. Performance is evaluated by selecting a given level of image quality and finding the maximum speed that can be achieved by each algorithm. We find that with increased demand for data quality, the utility of TT-SRT becomes more apparent; for example, the speed of TT-SRT can be ten times faster or more than standard SRT for a reasonable expectation of data quality.

    

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3. Measurement of the surface hydrophobicity of engineered nanoparticles using an atomic force microscope

   https://doi.org/10.1039/c8cp04676j  

   Fu, Wanyi ; Zhang, Wen ( January 2018 , Physical Chemistry Chemical Physics)

   Determination of the surface hydrophobicity or wettability of nanomaterials and nanoparticles (NPs) is often challenged by the heterogeneous properties of NPs that vary with particle size, shape, surface charge, aggregation states, and surface sorption or coating. This study first summarized inherent limitations of the water contact angle, octanol–water partition coefficient ( K ow ) and surface adsorption of probe molecules in probing nanomaterial hydrophobicity. Then, we demonstrated the principle of a scanning probe method based on atomic force microscopy (AFM) for the local surface hydrophobicity measurement. Specifically, we measured the adhesion forces between functionalized AFM tips and self-assembled monolayers (SAMs) to establish a linear relationship between the adhesion forces and water contact angles based on the continuum thermodynamic approach (CTA). This relationship was used to determine the local surface hydrophobicity of seven different NPs ( i.e. , TiO 2 , ZnO, SiO 2 , CuO, CeO 2 , α-Fe 2 O 3 , and Ag), which agreed well with bulk contact angles of these NPs. Some discrepancies were observed for Fe 2 O 3 , CeO 2 and SiO 2 NPs, probably because of surface hydration and roughness effects. Moreover, the solution pH and ionic strength had negligible effects on the adhesion forces between the AFM tip and MWCNTs or C 60 , indicating that the hydrophobicity of carbonaceous nanomaterials is not influenced by pH or ionic strength (IS). By contrast, natural organic matter (NOM) appreciably decreased the hydrophobicity of MWCNTs and C 60 due to surface coating of hydrophilic NOM. This scanning probe method has been proved to be reliable and robust toward the accurate measurement of the nanoscale hydrophobicity of individual NPs or nanomaterials in liquid environments. 

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4. Nuclear Stiffness Decreases with Disruption of the Extracellular Matrix in Living Tissues

   https://doi.org/10.1002/smll.202006699  

   McCreery, Kaitlin P. ; Xu, Xin ; Scott, Adrienne K. ; Fajrial, Apresio K. ; Calve, Sarah ; Ding, Xiaoyun ; Neu, Corey P. ( January 2021 , Small)

   Reciprocal interactions between the cell nucleus and the extracellular matrix lead to macroscale tissue phenotype changes. However, little is known about how the extracellular matrix environment affects gene expression and cellular phenotype in the native tissue environment. Here, it is hypothesized that enzymatic disruption of the tissue matrix results in a softer tissue, affecting the stiffness of embedded cell and nuclear structures. The aim is to directly measure nuclear mechanics without perturbing the native tissue structure to better understand nuclear interplay with the cell and tissue microenvironments. To accomplish this, an atomic force microscopy needle‐tip probe technique that probes nuclear stiffness in cultured cells to measure the nuclear envelope and cell membrane stiffness within native tissue is expanded. This technique is validated by imaging needle penetration and subsequent repair of the plasma and nuclear membranes of HeLa cells stably expressing the membrane repair protein CHMP4B‐GFP. In the native tissue environment ex vivo, it is found that while enzymatic degradation of viable cartilage tissues with collagenase 3 (MMP‐13) and aggrecanase‐1 (ADAMTS‐4) decreased tissue matrix stiffness, cell and nuclear membrane stiffness is also decreased. Finally, the capability for cell and nucleus elastography using the AFM needle‐tip technique is demonstrated. These results demonstrate disruption of the native tissue environment that propagates to the plasma membrane and interior nuclear envelope structures of viable cells.

    

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5. Morphology and kinetics of asphalt binder microstructure at gas, liquid and solid interfaces

   https://doi.org/10.1111/jmi.12842  

   RAMM, A. ; DOWNER, M. C. ; SAKIB, N. ; BHASIN, A. ( November 2019 , Journal of Microscopy)

   We combined optical and atomic force microscopy to observe morphology and kinetics of microstructures (typically referred to as bees) that formed at free surfaces of unmodified Performance Graded (PG) 64‐22 asphalt binders upon cooling from 150°C to room temperature (RT) at 5°C min^–1, and changes in these microstructures when the surface was terminated with a transparent solid (glass) or liquid (glycerol) overlayer. The main findings are: (1) at free binder surfaces, wrinkled microstructures started to form near the crystallization temperature (∼45°C) of saturates such as wax observed by differential scanning calorimetry, then grew to ∼5 µm diameter, ∼25 nm wrinkle amplitude and 10–30% surface area coverage upon cooling to RT, where they persisted indefinitely without observable change in shape or density. (2) Glycerol coverage of the binder surface during cooling reduced wrinkled area and wrinkle amplitude three‐fold compared to free binder surfaces upon initial cooling to RT; continued glycerol coverage at RT eliminated most surface microstructures within ∼4 h. (3) No surface microstructures were observed to form at binder surfaces covered with glass. (4) Submicron bulk microstructures were observed by near‐infrared microscopy beneath the surfaces of all binder samples, with size, shape and density independent of surface coverage. No tendency of such structures to float to the top or sink to the bottom of mm‐thick samples was observed. (5) We attribute the dependence of surface wrinkling on surface coverage to variation in interface tension, based on a thin‐film continuum mechanics model.

   Asphalt binder, or bitumen, is the glue that holds aggregate particles together to form a road surface. It is derived from the heavy residue that remains after distilling gasoline, diesel and other lighter products out of crude oil. Nevertheless, bitumen varies widely in composition and mechanical properties. To avoid expensive road failures, bitumen must be processed after distillation so that its mechanical properties satisfy diverse climate and load requirements. International standards now guide these mechanical properties, but yield varying long‐term performance as local source composition and preparation methods vary.In situdiagnostic methods that can predict bitumen performance independently of processing history are therefore needed. The present work focuses on one promising diagnostic candidate: microscopic observation of internal bitumen structure. Past bitumen microscopy has revealed microstructures of widely varying composition, size, shape and density. A challenge is distinguishing bulk microstructures, which directly influence a binder's mechanical properties, from surface microstructures, which often dominate optical microscopy because of bitumen's opacity and scanning‐probe microscopy because of its inherent surface specificity. In previously published work, we used infrared microscopy to enhance visibility of bulk microstructure. Here, as a foil to this work, we use visible‐wavelength microscopy together with atomic‐force microscopy (AFM) specifically to isolatesurfacemicrostructure, to understand its distinct origin and morphology, and to demonstrate its unique sensitivity to surface alterations. To this end, optical microscopy complements AFM by enabling us to observe surface microstructures form at temperatures (50°C–70°C) at which bitumen's fluidity prevents AFM, and to observe surface microstructure beneath transparent, but chemically inert, liquid (glycerol) and solid (glass) overlayers, which alter surface tension compared to free surfaces. From this study, we learned, first, that, as bitumen cools, distinctly wrinkled surface microstructures form at the same temperature at which independent calorimetric studies showed crystallization in bitumen, causing it to release latent heat of crystallization. This shows that surface microstructures are likely precipitates of the crystallizable component(s). Second, a glycerol overlayer on the cooling bitumen results in smaller, less wrinkled, sparser microstructures, whereas a glass overlayer suppresses them altogether. In contrast, underlying smaller bulk microstructures are unaffected. This shows that surface tension is the driving force behind formation and wrinkling of surface precipitates. Taken together, the work advances our ability to diagnose bitumen samples noninvasively by clearly distinguishing surface from bulk microstructure.

    

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