Atomic-scale structure of interfacial water on gel and liquid phase lipid membranes

Quantification of solvation forces with amplitude modulation AFM

HYPOTHESIS

Interfacial solvation forces arise from the organisation of liquid molecules near solid surfaces. They are crucial to fundamental phenomena, spanning materials science, molecular biology, and technological applications, yet their molecular details remain poorly understood. Achieving a complete understanding requires imaging techniques, such as three-dimensional atomic force microscopy (3D AFM), to provide atomically resolved images of solid-liquid interfaces (SLIs). However, converting 3D AFM data into accurate tip-sample forces remains challenging, as the process of translating observables into forces is not straightforward.

EXPERIMENTS/SIMULATIONS

This study compares standard amplitude modulation AFM (AM-AFM) force reconstruction methods (FRMs) and identifies their limitations in reconstructing SLI forces. A novel numerical matrix-based FRM specifically designed for AM-AFM is then introduced, aiming to overcome the limitations and inaccuracies found in standard approaches. The new method is validated through simulations and experimental data obtained at the SLI of silicon oxide and water with 3D AFM.

FINDINGS

The proposed matrix-based FRM, differently from standard FRMs, can reconstruct the full SLI interaction at the atomic scale, with no loss of information deriving from the specific choice of AFM experimental parameters or the force functional form. This method unlocks the full spectrum of physical phenomena encoded in the tip-sample interaction at the SLI in AFM experiments, greatly advancing our understanding of interfacial properties and their effects on colloid science, including nanoparticle interactions and molecular self-assembly.

What does an ion feel at the electrochemical interface? Revisiting electrosorption through nonlocal electrostatics

The traditional Gouy-Chapman-Stern theory has been effective in explaining the behavior of dilute electrolytes in the electrical double layer but falls short when it comes to describing how ions behave at the metal/electrolyte interface. This is because it overlooks key factors such as the molecular structure of water at the interface and the effects of electron screening in the metal. To address these gaps, we revisit ion adsorption at the metal/electrolyte interface. The approach combines the method of images with a field-theoretic framework for dilute electrolytes and metals described by the Thomas-Fermi model. Nonlocal polarization correlations in water are described by a first-order gradient expansion in the Landau free energy functional. Unlike earlier approaches that relied on the "specular reflection approximation," our method provides a less constrained way to handle the complex electrostatic boundary conditions at the interface. Analyzing the behavior of a test charge near the interface, an electrostatic energy minimum is found. This minimum depends on the metal's screening properties and the overall potential drop across the double layer. In addition, the alignment of water dipoles at the interface creates an asymmetry in the energy experienced by positively and negatively charged ions. Finally, we derived an expression for the electrosorption isotherm by describing both the distribution of the electrostatic potential and the lateral interactions between charges along the interface. Our findings highlight how the structure of interfacial water can drive processes such as underpotential deposition by creating favorable electrostatic conditions for ion adsorption.

Structure and thermodynamics of supported lipid membranes on hydrophobic van der Waals surfaces

Understanding the adsorption and physical characteristics of supported lipid membranes is crucial for their effective use as model cell membranes. Their morphological and thermodynamic properties at the nanoscale have traditionally been studied on hydrophilic substrates, such as mica and silicon oxide, which have proved to facilitate the reconstruction of biomembranes. However, in more recent years, with the advent of the van der Waals crystals technology, two-dimensional crystals such as graphene have been proposed as potential substrates in biosensing devices. Membranes formed on these crystals are expected to behave differently owing to their intrinsic hydrophobicity, however thus far knowledge of their morphological and thermodynamic properties is lacking. Here we present a comprehensive nanoscale analysis of the adsorption of phosphatidylcholine lipid monolayers on two of the most commonly used van der Waals crystals, graphite and hexagonal boron nitride. Both morphological and thermodynamic properties of the lipid membranes were investigated using temperature-controlled atomic force microscopy. Our experiments show that the lipids adsorb onto the crystals, forming monolayers with their orientation dependent upon their concentration. Furthermore, we found that the hydrophobicity of van der Waals crystals determines a strong increase in the transition temperature of the lipid monolayer compared to that observed on hydrophilic substrates. These results are important for understanding the properties of lipid membranes at solid surfaces and extending their use to novel drug delivery and biosensing devices made of van der Waals crystals.

Interfacial water on collagen nanoribbons by 3D AFM

Collagen is the most abundant structural protein in mammals. Type I collagen in its fibril form has a characteristic pattern structure that alternates two regions called gap and overlap. The structure and properties of collagens are highly dependent on the water and mineral content of the environment. Here, we apply 3D AFM to characterize at angstrom-scale resolution the interfacial water structure of collagen nanoribbons. For a neutral tip, the interfacial water structure is characterized by the oscillation of the water particle density distribution with a value of 0.3 nm (hydration layers). The interfacial structure does not depend on the collagen region. For a negatively charged tip, the interfacial structure might depend on the collagen region. Hydration layers are observed in overlap regions, while in gap regions, the interfacial solvent structure is dominated by electrostatic interactions. These interactions generate interlayer distances of 0.2 nm.