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.

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

Hydration of biological membranes is essential to a wide range of biological processes. In particular, it is intrinsically linked to lipid thermodynamic properties, which in turn influence key cell functions such as ion permeation and protein mobility. Experimental and theoretical studies of the surface of biomembranes have revealed the presence of an interfacial repulsive force, which has been linked to hydration or steric effects. Here, we directly characterise the atomic-scale structure of water near supported lipid membranes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine in their gel and liquid phase through three-dimensional atomic force microscopy (3D AFM). First, we demonstrate the ability to probe the morphology of interfacial water of lipid bilayers in both phases with sub-molecular resolution by using ultrasharp tips. We then visualise the molecular arrangement of water at the lipid surface at different temperatures. Our experiments reveal that water is organised in multiple hydration layers on both the solid-ordered and liquid-disordered lipid phases. Furthermore, we observe a monotonic repulsive force, which becomes relevant only in the liquid phase. These results offer new insights into the water structuring near soft biological surfaces, and demonstrate the importance of investigating it with vertical and lateral sub-molecular resolution.

In operando nanomechanical mapping of PEDOT:PSS thin films in electrolyte solutions with bimodal AFM.

Understanding the interplay between the nanomechanical properties of organic electronic materials and their electronic properties is central to developing sensors and transducers for applications ranging from immunosensing to e-skin. Controlling organic device operations in aqueous electrolyte solutions and their mechanical compliance with the host tissue or living systems, as for instance in active implants for the recording or stimulation of neural signals, is still largely unexplored. Here, we implemented bimodal AFM to map the nanomechanical and structural properties of thin films made from poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS), the most widely used conducting polymer blend, during operation as a microelectrode in an electrolyte solution. Nanomechanical maps showed the film consisting of a granular structure made from PEDOT:PSS regions embedded in the PSS matrix. The film swelled upon immersion in an aqueous solution. In operando bimodal AFM data obtained by applying a sequence of doping/de-doping bias cycles showed a significant decrease in the modulus (70%) that saturated after about 10 cycles. A similar sequence of biases at the opposite polarity did not significantly influence the mechanical behaviour of PEDOT:PSS. The decrease in the modulus was explained by the development of persistent hydration, which was enhanced by the cations trapped inside the organic electronic material.

Tip Charge Dependence of Three-Dimensional AFM Mapping of Concentrated Ionic Solutions.

A molecular scale understanding of the organization and structure of a liquid near a solid surface is currently a major challenge in surface science. It has implications across different fields from electrochemistry and energy storage to molecular biology. Three-dimensional AFM generates atomically resolved maps of solid-liquid interfaces. The imaging mechanism behind those maps is under debate, in particular, for concentrated ionic solutions. Theory predicts that the observed contrast should depend on the tip's charged state. Here, by using neutrally, negatively, and positively charged tips, we demonstrate that the 3D maps depend on the tip's polarization. A neutral tip will explore the total particle density distribution (water and ions) while a charged tip will reveal the charge density distribution. The experimental data reproduce the key findings of the theory.

Atomically resolved interfacial water structures on crystalline hydrophilic and hydrophobic surfaces.

Hydration layers are formed on hydrophilic crystalline surfaces immersed in water. Their existence has also been predicted for hydrophobic surfaces, yet the experimental evidence is controversial. Using 3D-AFM imaging, we probed the interfacial water structure of hydrophobic and hydrophilic surfaces with atomic-scale spatial resolution. We demonstrate that the atomic-scale structure of interfacial water on crystalline surfaces presents two antagonistic arrangements. On mica, a common hydrophilic crystalline surface, the interface is characterized by the formation of 2 to 3 hydration layers separated by approximately 0.3 nm. On hydrophobic surfaces such as graphite or hexagonal boron nitride (h-BN), the interface is characterized by the formation of 2 to 4 layers separated by about 0.5 nm. The latter interlayer distance indicates that water molecules are expelled from the vicinity of the surface and replaced by hydrocarbon molecules. This creates a new 1.5-2 nm thick interface between the hydrophobic surface and the bulk water. Molecular dynamics simulations reproduced the experimental data and confirmed the above interfacial water structures.

High-Speed Nanomechanical Mapping of the Early Stages of Collagen Growth by Bimodal Force Microscopy.

High-speed atomic force microscopy (AFM) enabled the imaging of protein interactions with millisecond time resolutions (10 fps). However, the acquisition of nanomechanical maps of proteins is about 100 times slower. Here, we developed a high-speed bimodal AFM that provided high-spatial resolution maps of the elastic modulus, the loss tangent, and the topography at imaging rates of 5 fps. The microscope was applied to identify the initial stages of the self-assembly of the collagen structures. By following the changes in the physical properties, we identified four stages, nucleation and growth of collagen precursors, formation of tropocollagen molecules, assembly of tropocollagens into microfibrils, and alignment of microfibrils to generate microribbons. Some emerging collagen structures never matured, and after an existence of several seconds, they disappeared into the solution. The elastic modulus of a microfibril (∼4 MPa) implied very small stiffness (∼3 × 10-6 N/m). Those values amplified the amplitude of the collagen thermal fluctuations on the mica plane, which facilitated microribbon build-up.

Multi-scale characterisation of a ferroelectric polymer reveals the emergence of a morphological phase transition driven by temperature.

Ferroelectric materials exhibit a phase transition to a paraelectric state driven by temperature - called the Curie transition. In conventional ferroelectrics, the Curie transition is caused by a change in crystal symmetry, while the material itself remains a continuous three-dimensional solid crystal. However, ferroelectric polymers behave differently. Polymeric materials are typically of semi-crystalline nature, meaning that they are an intermixture of crystalline and amorphous regions. Here, we demonstrate that the semi-crystalline morphology of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) strongly affects its Curie transition, as not only a change in crystal symmetry but also in morphology occurs. We demonstrate, by high-resolution nanomechanical measurements, that the semi-crystalline microstructure in the paraelectric state is formed by crystalline domains embedded into a softer amorphous phase. Using in situ X-ray diffraction measurements, we show that the local electromechanical response of the crystalline domains is counterbalanced by the amorphous phase, effectively masking its macroscopic effect. Our quantitative multi-scale characterisations unite the nano- and macroscopic material properties of the ferroelectric polymer P(VDF-TrFE) through its semi-crystalline nature.

Fast, quantitative and high resolution mapping of viscoelastic properties with bimodal AFM.

Quantitative mapping of viscoelastic properties of soft matter with a nanoscale spatial resolution is an active and relevant research topic in atomic force microscopy (AFM) and nanoscale science characterization. The AFM has demonstrated its accuracy to measure the energy dissipated on a sample surface with an atomic-scale resolution. However, the transformation of energy dissipation values associated with viscoelastic interactions to a material property remains very challenging. A key issue is to establish the relationship between the AFM observables and some material properties such as viscosity coefficient or relaxation time. Another relevant issue is to determine the accuracy of the measurements. We demonstrate that bimodal atomic force microscopy enables the accurate measurement of several viscoelastic parameters such as the Young's modulus, viscosity coefficient, retardation time or loss tangent. The parameters mentioned above are measured at the same time that the true topography. We demonstrate that the loss tangent is proportional to the viscosity coefficient. We show that the mapping of viscoelastic properties neither degrades the spatial resolution nor the imaging speed of AFM. The results are presented for homogeneous polymer and block co-polymer samples with Young's modulus, viscosity and retardation times ranging from 100 MPa to 3 GPa, 10 to 400 Pa s and 50 to 400 ns, respectively. Numerical simulations validate the accuracy of bimodal AFM to determine the viscoelastic parameters.

Fast and high-resolution mapping of elastic properties of biomolecules and polymers with bimodal AFM.

Fast, high-resolution mapping of heterogeneous interfaces with a wide elastic modulus range is a major goal of atomic force microscopy (AFM). This goal becomes more challenging when the nanomechanical mapping involves biomolecules in their native environment. Over the years, several AFM-based methods have been developed to address this goal. However, none of these methods combine sub-nanometer spatial resolution, quantitative accuracy, fast data acquisition speed, wide elastic modulus range and operation in physiological solutions. Here, we present detailed procedures for generating high-resolution maps of the elastic properties of biomolecules and polymers using bimodal AFM. This requires the simultaneous excitation of the first two eigenmodes of the cantilever. An amplitude modulation (AM) feedback acting on the first mode controls the tip-sample distance, and a frequency modulation (FM) feedback acts on the second mode. The method is fast because the elastic modulus, deformation and topography images are obtained simultaneously. The method is efficient because only a single data point per pixel is needed to generate the aforementioned images. The main stages of the bimodal imaging are sample preparation, calibration of the instrument, tuning of the microscope and generation of the nanomechanical maps. In addition, with knowledge of the deformation, bimodal AFM enables reconstruction of the true topography of the surface. It takes ~9 h to complete the whole procedure.

Exploiting interfacial phenomena in organic bioelectronics: Conformable devices for bidirectional communication with living systems.

A novel fully organic bioelectronic device is presented and validated as electronic transducer and current stimulator for brain implants. The device integrates polymeric electrodes made of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) on paper thin foils, resulting in a high surface-to-volume ratio architecture that exhibits high sensitivity to interfacial ionic transport phenomena. The prototyping technique herein presented yields devices for the bidirectional communication with biological systems whose dimensionality can be controlled according to the desired application. Transduction of ultra-low local-field potentials and delivery of voltage pulse-trains alike those used in deep-brain stimulation are herein assessed, paving the way towards novel theranostic strategies for the treatment of Parkinson's Disease and other severe neurodegenerative and/or traumatic pathologies of the central nervous system.