Investigation of Local Hydration Structures of Alkanethiol Self-Assembled Monolayers with Different Molecular Structures by FM-AFM

Insights in Cell Biomechanics through Atomic Force Microscopy

We review the advances obtained by using Atomic Force Microscopy (AFM)-based approaches in the field of cell/tissue mechanics and adhesion, comparing the solutions proposed and critically discussing them. AFM offers a wide range of detectable forces with a high force sensitivity, thus allowing a broad class of biological issues to be addressed. Furthermore, it allows for the accurate control of the probe position during the experiments, providing spatially resolved mechanical maps of the biological samples with subcellular resolution. Nowadays, mechanobiology is recognized as a subject of great relevance in biotechnological and biomedical fields. Focusing on the past decade, we discuss the intriguing issues of cellular mechanosensing, i.e., how cells sense and adapt to their mechanical environment. Next, we examine the relationship between cell mechanical properties and pathological states, focusing on cancer and neurodegenerative diseases. We show how AFM has contributed to the characterization of pathological mechanisms and discuss its role in the development of a new class of diagnostic tools that consider cell mechanics as new tumor biomarkers. Finally, we describe the unique ability of AFM to study cell adhesion, working quantitatively and at the single-cell level. Again, we relate cell adhesion experiments to the study of mechanisms directly or secondarily involved in pathologies.

Hydration Structure at the Calcite-Water (10.4) Interface in the Presence of Rubidium Chloride

Solid-liquid interfaces are of significant importance in a multitude of geochemical and technological fields. More specifically, the solvation structure plays a decisive role in the properties of the interfaces. Atomic force microscopy (AFM) has been used to resolve the interfacial hydration structure in the presence and absence of ions. Despite many studies investigating the calcite-water interface, the impact of ions on the hydration structure at this interface has rarely been studied. Here, we investigate the calcite-water interface at various concentrations (ranging from 0 to 5 M) of rubidium chloride (RbCl) using three-dimensional atomic force microscopy (3D AFM). We present molecularly resolved images of the hydration structure at the interface. Interestingly, the characteristic pattern of the hydration structure appears similar regardless of the RbCl concentration. The presence of the ions is detected in an indirect manner by more frequent contrast changes and slice displacements.

Local Cross-Coupling Activity of Azide-Hexa(ethylene glycol)-Terminated Self-Assembled Monolayers Investigated by Atomic Force Microscopy

Azide-oligo(ethylene glycol)-terminated self-assembled monolayers (N₃-OEG-SAMs) are promising interfacial structures for surface functionalization. Its many potential applications include chemical/bio-sensing and construction of surface models owing to its cross-coupling activity that originates from the azide group and oligo(ethylene glycol) (OEG) units for non-specific adsorption resistance. However, there are only a few studies and limited information, particularly on the molecular-scale structures and local cross-coupling activities of N₃-OEG-SAMs, which are vital to understanding its surface properties and interfacial molecular design. In this study, molecular-scale surface structures and cross-coupling activity of azide-hexa(ethylene glycol)-terminated SAMs (N₃-EG₆-SAMs) were investigated using frequency modulation atomic force microscopy (FM-AFM) in liquid. The N₃-EG₆-SAMs were prepared on Au(111) substrates through the self-assembly of 11-azido-hexa(ethylene glycol)-undecane-1-thiol (N₃-EG₆-C₁₁-HS) molecules obtained from a liquid phase. Subnanometer-resolution surface structures were visualized in an aqueous solution using a laboratory-built FM-AFM instrument. The results show a well-ordered molecular arrangement in the N₃-EG₆-SAM and its clean surfaces originating from the adsorption resistance property of the terminal EG₆ units. Surface functionalization by the cross-coupling reaction of copper(I)-catalyzed azide-alkyne cycloaddition was observed, indicating a structural change in the form of fluctuating structures and island-shaped structures depending on the concentration of the alkyne molecules. The FM-AFM imaging enabled to provide information on the relationship between the surface structures and cross-coupling activity. These findings provide molecular-scale information on the functionalization of the N₃-EG₆-SAMs, which is helpful for the interfacial molecular design based on alkanethiol SAMs in many applications.

Mapping the capacitance of self-assembled monolayers at metal/electrolyte interfaces at the nanoscale by in-liquid scanning dielectric microscopy

Organic self-assembled monolayers (SAMs) at metal/electrolyte interfaces have been thoroughly investigated both from fundamental and applied points of view. A relevant figure of merit of metal/SAM/electrolyte interfaces is the specific capacitance, which determines the charge that can be accumulated at the metal electrode. Here, we show that the specific capacitance of non-uniform alkanethiol SAMs at gold/electrolyte interfaces can be quantitatively measured and mapped at the nanoscale by in-liquid scanning dielectric microscopy in force detection mode. We show that sub-100 nm spatial resolution in ultrathin (<1 nm) SAMs can be achieved, largely improving the performance of current sensing characterization techniques. The present results provide access to study the dielectric properties of metal/SAM/electrolyte interfaces at scales that have remained unexplored until now.

Dynamic behaviors of interfacial water on the self-assembly monolayer (SAM) heterogeneous surface

Dynamic behaviors of water molecules near the surface with mixed hydrophobic and hydrophilic areas are studied by molecular dynamics simulation. More specifically, the diffusion coefficient and hydrogen bond lifetime of interfacial water on the self-assembly monolayer composed of hydrophobic and hydrophilic groups and their dependence on the mixing ratio are studied. The diffusion dramatically slows down, and the hydrogen bond lifetime considerably increases when a few hydrophilic groups are added to the hydrophobic surface. When the percentage of hydrophilic groups increases to 25%, the behavior of interfacial water is similar to the case of the pure hydrophilic surface. The sensitivity to the hydrophilic group can be attributed to the fact that the grafted hydrophilic groups can not only retard the directly bound water molecules but also affect indirectly bound water by stabilizing hydrogen bonds among interfacial water molecules.

Improvements in fundamental performance of in-liquid frequency modulation atomic force microscopy

In-liquid frequency modulation atomic force microscopy (FM-AFM) has been used for visualizing subnanometer-scale surface structures of minerals, organic thin films and biological systems. In addition, three-dimensional atomic force microscopy (3D-AFM) has been developed by combining it with a three-dimensional (3D) tip scanning method. This method enabled the visualization of 3D distributions of water (i.e. hydration structures) and flexible molecular chains at subnanometer-scale resolution. While these applications highlighted the unique capabilities of FM-AFM, its force resolution, speed and stability are not necessarily at a satisfactory level for practical applications. Recently, there have been significant advancements in these fundamental performances. The force resolution was dramatically improved by using a small cantilever, which enabled the imaging of a 3D hydration structure even in pure water and made it possible to directly compare experimental results with simulated ones. In addition, the improved force resolution allowed the enhancement of imaging speed without compromising spatial resolution. To achieve this goal, efforts have been made for improving bandwidth, resonance frequency and/or latency of various components, including a high-speed phase-locked loop (PLL) circuit. With these improvements, now atomic-resolution in-liquid FM-AFM imaging can be performed at ∼1 s/frame. Furthermore, a Si-coating method was found to improve stability and reproducibility of atomic-resolution imaging owing to formation of a stable hydration structure on a tip apex. These improvements have opened up new possibilities of atomic-scale studies on solid-liquid interfacial phenomena by in-liquid FM-AFM.