Mónica Luna

Nanogeometry Matters: Unexpected Decrease of Capillary Adhesion Forces with Increasing Relative Humidity

The sticking effect between hydrophilic surfaces occurring at increasing relative humidity (RH) is an everyday phenomenon with uncountable implications. Here experimental evidence is presented for a counterintuitive monotonous decrease of the capillary adhesion forces between hydrophilic surfaces with increasing RH for the whole humidity range. It is shown that this unexpected result is related to the actual shape of the asperity at the nanometer scale: a model based on macroscopic thermodynamics predicts this decrease in the adhesion force for a sharp object ending in an almost flat nanometer-sized apex, in full agreement with experiments. This anomalous decrease is due to the fact that a significant growth of the liquid meniscus formed at the contact region with increasing humidity is hindered for this geometry. These results are relevant in the analysis of the dynamical behavior of nanomenisci. They could also have an outstanding value in technological applications, since the undesirable sticking effect between surfaces occurring at increasing RH could be avoided by controlling the shape of the surface asperities at the nanometric scale.

Creating Biomimetic Surfaces through Covalent and Oriented Binding of Proteins

This manuscript describes a novel method for the biofunctionalization of glass surfaces with polyhistidine-tagged proteins. The main innovation of this methodology consists of the covalent binding between the nitrilotriacetic acid (NTA) moiety and the proteins, ensuring not only orientation, but also stability of the recombinant proteins on NTA-covered surfaces. In this work, as C-terminal polyhistidine tagged cadherin extracellular fragments have been used, this methodology guarantees the proper orientation of these proteins, by mimicking their insertion into cell plasma membranes. These biofunctionalized surfaces have been characterized by confocal microscopy, X-ray photoelectron spectroscopy, contact angle, and atomic force microscopy, showing a high density of cadherins on the glass surfaces and the stability of the linkage. The prepared materials exhibited a high tendency to promote cell spreading, demonstrating the functionality of the protein and the high utility of these biomaterials to promote cell adhesion events. Interestingly, differences in the cytoskeleton organization have been observed in cells adhering to surfaces with no cadherins or with nonoriented cadherins, in comparison to surfaces functionalized with well-oriented cadherins. This method, which allows the robust immobilization of polyhistidine tagged proteins due to their covalent binding and with a defined orientation, may also find particular usefulness in the making of protein biochips, for analysis of protein-protein interactions, as well as structural and single-molecule studies.

Carbohydrate-carbohydrate interaction prominence in 3D supramolecular self-assembly.

Self-association in water of biologically significant carbohydrate molecules is a controversial topic due to the strong solvation of these molecules in this solvent and the difficulty to experimentally detect these very weak intermolecular forces by biophysical techniques. Herein we report the tremendous ability of amphiphilic carbohydrate molecules to form complex three-dimensional architectures. We have experimentally observed the 3D self-assembly into multilayers of disaccharide neoglycolipid dimers on graphite by means of noncontact AFM and we have also theoretically modeled the interaction between two dimers in order to learn about the structure and composition of these layers. A simple bilayer structure as observed for many amphiphilic lipids was discarded by the experiments. Instead, based on the good agreement between experiments and calculations, we propose that multilayer formation takes place through the assembly of building blocks consisting of two dimers each. The fundamental key in the formation of this supramolecular structure is the complementarity between the van der Waals surfaces of the amphiphilic carbohydrate molecules, a result which differs from the most common idea that H-bonding interactions are prominent in carbohydrate-mediated interactions.

A study of the topographic and electrical properties of self-assembled islands of alkylsilanes on mica using a combination of non-contact force microscopy techniques

We use a combination of non-contact scanning force microscope operation modes to study the changes in topographic and electrostatic properties of self-assembled monolayer islands of alkylsilanes on mica. The combined technique uses simultaneous electrical and mechanical modulation and feedback modes to produce four images that reveal the topography, phase, surface potential and dielectric constant. The results show significant advantages with this combined method. As an example we show that the interaction of water with self-assembled monolayer islands of alkylsilanes produces changes in the surface potential of the system but not in the topography.

Structure and support induced structure disruption of soft nanoparticles obtained from hydroxylated fatty acids

Soft and spherical nanoparticles, named as cutinsomes, have been prepared from concentrated 9(10),16-dihydroxypalmitic acid (diHPA) in aqueous solution. After isolation, cutinsomes have been chemically and structurally characterized by ATR-FTIR, TEM and dynamic atomic force microscopy (dynamic AFM). The nanoparticle can be described as a lipidic, liquid-like and mostly esterified core surrounded by a polar shell of carboxylate/carboxylic acid molecules. Molecular dynamic (MD) simulations have been used to support this model. The structural stability of soft cutinsomes has been tested by deposition on both non-polar (HOPG) and polar (mica) flat substrates. It has been found that the magnitude of the interaction between the polar shell of cutinsomes and the support determines their structure conservation or its spreading or rupture and spill out of the liquid-like content. The structural consistence of these nanoparticles as a function of the polarity of substrate is of interest in elucidating the formation mechanism of cutin, the most abundant biopolyester in nature and a very interesting biomaterial to be mimetized.

Structural characterization of polyhydroxy fatty acid nanoparticles related to plant lipid biopolyesters

In the present work, we report the physico-chemical properties and structural characteristics of special polyhydroxy fatty acid nanoparticles after their fusion by means of attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electronic microscopy (SEM), atomic force microscopy (AFM), and light microscopy. All the characteristics and properties investigated have an important degree of similarity to the native plant cutin, the main biopolymer present in the plant cuticles. The supramolecular organization of these polymerized prime nanoparticles after their interaction on cellulose substrate and isolated cuticle samples, simulating the in vivo conditions in epidermal plant cells, strongly suggests a growth of these nanoparticles after a self-assembly process.

Gold nanoparticle coated silicon tips for Kelvin probe force microscopy in air

The tip apex dimensions and geometry of the conductive probe remain the major limitation to the resolution of Kelvin probe force microscopy (KPFM). One of the possible strategies to improve the spatial resolution of surface potential images consists in the development of thinner and more durable conductive tips. In an effort to improve the lateral resolution of topography and surface potential maps, we have evaluated high aspect ratio conductive tips created by depositing gold nanoparticles on standard silicon tips. Besides the already known general topographic resolution enhancement offered by these modified tips, an improvement of surface potential lateral resolution and signal-to-noise ratio is reported here for a variety of samples as compared to other regular conductive probes. We have also observed that the modified conductive tips have a significant auto-regeneration capability, which stems from a certain level of mobility of the nanoparticle coating. This property makes the modified tips highly resistant to degradation during scanning, thus increasing their durability. As demonstrated by the heterogeneous set of structures measured in the present study performed in air, the nanoparticle coated tips are suitable for KPFM analysis. In particular, surface potential difference determination on graphene deposited on silicon, gold sputtered on a salt surface, large and mildly rough areas of ZnO films and small DNA molecules on insulating mica have been achieved with enhanced resolution.

Ammonia induces aquaporin-4 rearrangement in the plasma membrane of cultured astrocytes

Aquaporin-4 (AQP4) is a water channel protein mainly located in the astroglial plasma membrane, the precise function of which in the brain edema that accompanies hepatic encephalopathy (HE) is unclear. Since ammonia is the main pathogenic agent in HE, its effect on AQP4 expression and distribution in confluent primary astroglial cultures was examined via their exposure to ammonium chloride (1, 3 and 5 mM) for 5 and 10 days. Ammonia induced the general inhibition of AQP4 mRNA synthesis except in the 1 mM/5 day treatment. However, the AQP4 protein content measured was dependent on the method of analysis; an apparent increase was recorded in treated cells in in-cell Western assays, while an apparent reduction was seen with the classic Western blot method, perhaps due to differences in AQP4 aggregation. Ammonia might therefore induce the formation of insoluble AQP4 aggregates in the astroglial plasma membrane. The finding of AQP4 in the pellet of classic Western blot samples, plus data obtained via confocal microscopy, atomic force microscopy (using immunolabeled cells with gold nanoparticles) and scanning electron microscopy, all corroborate this hypothesis. The effect of ammonia on AQP4 seems not to be due to any osmotic effect; identical osmotic stress induced by glutamine and salt had no significant effect on the AQP4 content. AQP4 functional analysis (subjecting astrocytes to a hypo-osmotic medium and using flow cytometry to measure cell size) demonstrated a smaller water influx in ammonia-treated astrocytes suggesting that AQP4 aggregates are representative of an inactive status; however, more confirmatory studies are required to fully understand the functional status of AQP4 aggregates. The present results suggest that ammonia affects AQP4 expression and distribution, and that astrocytes change their expression of AQP4 mRNA as well as the aggregation status of the ensuing protein depending on the ammonia concentration and duration of exposure.

Nanoparticle-mediated monitoring of carbohydrate-lectin interactions using Transient Magnetic Birefringence.

The development of sensitive and easy-to-use biosensors that allow an adequate characterization of specific weak biological interactions like carbohydrate-lectin interactions still remains challenging today. Nanoparticles functionalized with carbohydrates are one of the most powerful systems for studying carbohydrate-lectin interactions, because they mimic the multivalent presentation of carbohydrates encountered in nature, for example when viruses and bacteria bind to cells. On the basis of the model system glucose-Concanavalin A (ConA), we explore the application of Transient Magnetic Birefringence (TMB) to study these weak interactions, using glucose-functionalized colloidal magnetite nanoparticles (NPs) as probes. We demonstrate that the binding dynamics can be monitored and derive a model to obtain the apparent cooperativity. For our studies, we use nanoparticles of 6 and 8 nm in diameter. The ConA-generated response shows apparent cooperativity, due to the cross-linking of nanoparticles by the ConA tetramer which has four binding sites. Cooperativity is higher for 6 nm NPs, possibly due to a better accessibility of all four ConA binding sites on smaller NPs, enhancing cross-linking. For this system, we find a detection limit of 3-23 nM.

Magnetostriction-driven cantilevers for dynamic atomic force microscopy

An actuation mode is presented to drive the mechanical oscillation of cantilevers for dynamic atomic force microscopy. The method is based on direct mechanical excitation of the cantilevers coated with amorphous Fe–B–N thin films, by means of the film magnetostriction, i.e., the dimensional change in the film when magnetized. These amorphous magnetostrictive Fe–B–N thin films exhibit soft magnetic properties, excellent corrosion resistance in liquid environments, nearly zero accumulated stress when properly deposited, and good chemical stability. We present low noise and high resolution topographic images acquired in liquid environment to demonstrate the method capability.