10 research outputs found
Influence of Cholesterol on the Phase Transition of Lipid Bilayers: A Temperature-Controlled Force Spectroscopy Study
Cholesterol (Chol) plays the essential function of regulating
the
physical properties of the cell membrane by controlling the lipid
organization and phase behavior and, thus, managing the membrane fluidity
and its mechanical strength. Here, we explore the model system DPPC:Chol
by means of temperature-controlled atomic force microscopy (AFM) imaging
and AFM-based force spectroscopy (AFM-FS) to assess the influence
of Chol on the membrane ordering and stability. We analyze the system
in a representative range of compositions up to 50 mol % Chol studying
the phase evolution upon temperature increase (from room temperature
to temperatures high above the <i>T</i><sub>m</sub> of the
DPPC bilayer) and the corresponding (nano)Āmechanical stability. By
this means, we correlate the mechanical behavior and composition with
the lateral order of each phase present in the bilayers. We prove
that low Chol contents lead to a phase-segregated system, whereas
high contents of Chol can give a homogeneous bilayer. In both cases,
Chol enhances the mechanical stability of the membrane, and an extraordinarily
stable system is observed for equimolar fractions (50 mol % Chol).
In addition, even when no thermal transition is detected by the traditional
bulk analysis techniques for liposomes with high Chol content (40
and 50 mol %), we demonstrate that temperature-controlled AFM-FS is
capable of identifying a thermal transition for the supported lipid
bilayers. Finally, our results validate the AFM-FS technique as an
ideal platform to differentiate phase coexistence and transitions
in lipid bilayers and bridge the gap between the results obtained
by traditional methods for bulk analysis, the theoretical predictions,
and the behavior of these systems at the nanoscale
AFM-Based Force-Clamp Monitors Lipid Bilayer Failure Kinetics
The lipid bilayer rupture phenomenon is here explored
by means
of atomic force microscopy (AFM)-based force clamp, for the first
time to our knowledge, to evaluate how lipid membranes respond when
compressed under an external constant force, in the range of nanonewtons.
Using this method, we were able to directly quantify the kinetics
of the membrane rupture event and the associated energy barriers,
for both single supported bilayers and multibilayers, in contradistinction
to the classic studies performed at constant velocity. Moreover, the
affected area of the membrane during the rupture process was calculated
using an elastic deformation model. The elucidated information not
only contributes to a better understanding of such relevant process,
but also proves the suitability of AFM-based force clamp to study
model structures as lipid bilayers. These findings on the kinetics
of lipid bilayers rupture could be extended and applied to the study
of other molecular thin films. Furthermore, systems of higher complexity
such as models mimicking cell membranes could be studied by means
of AFM-based force-clamp technique
Template-Assisted Lateral Growth of Amyloid-Ī²42 Fibrils Studied by Differential Labeling with Gold Nanoparticles
Amyloid-Ī² protein (AĪ²) aggregation into amyloid
fibrils
is central to the origin and development of Alzheimerās disease
(AD), yet this highly complex process is poorly understood at the
molecular level. Extensive studies have shown that AĪ² fibril
growth occurs through fibril elongation, whereby soluble molecules
add to the fibril ends. Nevertheless, fibril morphology strongly depends
on aggregation conditions. For example, at high ionic strength, AĪ²
fibrils laterally associate into bundles. To further study the mechanisms
leading to fibril growth, we developed a single-fibril growth assay
based on differential labeling of two AĪ²42 variants with gold
nanoparticles. We used this assay to study AĪ²42 fibril growth
under different conditions and observed that bundle formation is preceded
by lateral interaction of soluble AĪ²42 molecules with pre-existing
fibrils. Based on this data, we propose template-assisted lateral
fibril growth as an additional mechanism to elongation for AĪ²42
fibril growth
CurrentāVoltage Characteristics and Transition Voltage Spectroscopy of Individual Redox Proteins
Understanding how molecular conductance depends on voltage
is essential
for characterizing molecular electronics devices. We reproducibly
measured currentāvoltage characteristics of individual redox-active
proteins by scanning tunneling microscopy under potentiostatic control
in both tunneling and wired configurations. From these results, transition
voltage spectroscopy (TVS) data for individual redox molecules can
be calculated and analyzed statistically, adding a new dimension to
conductance measurements. The transition voltage (TV) is discussed
in terms of the two-step electron transfer (ET) mechanism. Azurin
displays the lowest TV measured to date (0.4 V), consistent with the
previously reported distance decay factor. This low TV may be advantageous
for fabricating and operating molecular electronic devices for different
applications. Our measurements show that TVS is a helpful tool for
single-molecule ET measurements and suggest a mechanism for gating
of ET between partner redox proteins
Disruption of the Chemical Environment and Electronic Structure in p-Type Cu<sub>2</sub>O Films by Alkaline Doping
In this work we present an experimental and theoretical
study of
Cu<sub>2</sub>O films doped with alkaline ions (Li<sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>, and Cs<sup>+</sup>) prepared by Cu anodization.
By X-ray photoelectron spectroscopy we determined dopant incorporation
as high as 1% for Na<sup>+</sup>. Three oxygen species were found:
O<sup>2ā</sup> ions in the bulk cuprite structure, adsorbed
OH<sup>ā</sup> and oxygen
in hydroxylated dopant sites. The main effects of the alkaline doping
on the optical properties were a reduction in the direct band gap
and an approach of the acceptor level edge to the maximum of the valence
band. Electrochemical tunneling microscopy experiments confirmed that
the valence band maximum energy position is almost invariant. Additional
electrochemical impedance, photoelectrochemical activity, and current
sensing atomic force microscopy measurements showed an increase of
the carrier density and electrical conductivity and a reduction in
the photocurrent response with the dopant ion size. Urbach tail parameter
analysis suggested additional interaction between copper vacancy derived
states and dopant states. From first-principles calculations with
the B3LYP hybrid functional on models for the alkaline-doped Cu<sub>2</sub>O systems we determined that the main effect of the alkaline
substitution of copper atoms consists of polarizing the O states,
which causes a reduction in the insulating gap and splitting of the
density of states just below the Fermi level. The nature of the oxygenādopant
interaction was also calculated: there is a net attractive interaction
for LiāO, a slightly repulsive interaction for NaāO,
and a net repulsive interaction for KāO and CsāO. The
repulsive interactions between K<sup>+</sup> or Cs<sup>+</sup> and
O cause an accumulation of the dopant at the surface of the crystallites,
whereas for Na<sup>+</sup> and Li<sup>+</sup> the doping ions are
more uniformly distributed in the film bulk. It was found that the
surface accumulation of K<sup>+</sup> and Cs<sup>+</sup> hinders vacancy
diffusion and therefore blocks film growth, leading to a reduction
of roughness and thickness as the ion size increases
Metal-Controlled Magnetoresistance at Room Temperature in SingleāMolecule Devices
The
appropriate choice of the transition metal complex and metal
surface electronic structure opens the possibility to control the
spin of the charge carriers through the resulting hybrid molecule/metal <i>spinterface</i> in a single-molecule electrical contact at room
temperature. The single-molecule conductance of a Au/molecule/Ni junction
can be switched by flipping the magnetization direction of the ferromagnetic
electrode. The requirements of the molecule include not just the presence
of unpaired electrons: the electronic configuration of the metal center
has to provide occupied or empty orbitals that strongly interact with
the junction metal electrodes and that are close in energy to their
Fermi levels for one of the electronic spins only. The key ingredient
for the metal surface is to provide an efficient <i>spin texture</i> induced by the spināorbit coupling in the topological surface
states that results in an efficient spin-dependent interaction with
the orbitals of the molecule. The strong magnetoresistance effect
found in this kind of single-molecule wire opens a new approach for
the design of room-temperature nanoscale devices based on spin-polarized
currents controlled at molecular level
Metal-Controlled Magnetoresistance at Room Temperature in SingleāMolecule Devices
The
appropriate choice of the transition metal complex and metal
surface electronic structure opens the possibility to control the
spin of the charge carriers through the resulting hybrid molecule/metal <i>spinterface</i> in a single-molecule electrical contact at room
temperature. The single-molecule conductance of a Au/molecule/Ni junction
can be switched by flipping the magnetization direction of the ferromagnetic
electrode. The requirements of the molecule include not just the presence
of unpaired electrons: the electronic configuration of the metal center
has to provide occupied or empty orbitals that strongly interact with
the junction metal electrodes and that are close in energy to their
Fermi levels for one of the electronic spins only. The key ingredient
for the metal surface is to provide an efficient <i>spin texture</i> induced by the spināorbit coupling in the topological surface
states that results in an efficient spin-dependent interaction with
the orbitals of the molecule. The strong magnetoresistance effect
found in this kind of single-molecule wire opens a new approach for
the design of room-temperature nanoscale devices based on spin-polarized
currents controlled at molecular level
Thermoplastic Polyurethane:Polythiophene Nanomembranes for Biomedical and Biotechnological Applications
Nanomembranes
have been prepared by spin-coating mixtures of a polythiophene (P3TMA)
derivative and thermoplastic polyurethane (TPU) using 20:80, 40:60,
and 60:40 TPU:P3TMA weight ratios. After structural, topographical,
electrochemical, and thermal characterization, properties typically
related with biomedical applications have been investigated: swelling,
resistance to both hydrolytic and enzymatic degradation, biocompatibility,
and adsorption of type I collagen, which is an extra cellular matrix
protein that binds fibronectin favoring cell adhesion processes. The
swelling ability and the hydrolytic and enzymatic degradability of
TPU:P3TMA membranes increases with the concentration of P3TMA. Moreover,
the degradation of the blends is considerably promoted by the presence
of enzymes in the hydrolytic medium, TPU:P3TMA blends behaving as
biodegradable materials. On the other hand, TPU:P3TMA nanomembranes
behave as bioactive platforms stimulating cell adhesion and, especially,
cell viability. Type I collagen adsorption largely depends on the
substrate employed to support the nanomembrane, whereas it is practically
independent of the chemical nature of the polymeric material used
to fabricate the nanomembrane. However, detailed microscopy study
of the morphology and topography of adsorbed collagen evidence the
formation of different organizations, which range from fibrils to
pseudoregular honeycomb networks depending on the composition of the
nanomembrane that is in contact with the protein. Scaffolds made of
electroactive TPU:P3TMA nanomembranes are potential candidates for
tissue engineering biomedical applications
Highly Conductive Single-Molecule Wires with Controlled Orientation by Coordination of Metalloporphyrins
Porphyrin-based
molecular wires are promising candidates for nanoelectronic
and photovoltaic devices due to the porphyrin chemical stability and
unique optoelectronic properties. An important aim toward exploiting
single porphyrin molecules in nanoscale devices is to possess the
ability to control the electrical pathways across them. Herein, we
demonstrate a method to build single-molecule wires with metalloporphyrins
via their central metal ion by chemically modifying both an STM tip
and surface electrodes with pyridin-4-yl-methanethiol, a molecule
that has strong affinity for coordination with the metal ion of the
porphyrin. The new flat configuration resulted in single-molecule
junctions of exceedingly high lifetime and of conductance 3 orders
of magnitude larger than that obtained previously for similar porphyrin
molecules but wired from either end of the porphyrin ring. This work
presents a new concept of building highly efficient single-molecule
electrical contacts by exploiting metal coordination chemistry
Large Conductance Switching in a Single-Molecule Device through Room Temperature Spin-Dependent Transport
Controlling the spin of electrons
in nanoscale electronic devices is one of the most promising topics
aiming at developing devices with rapid and high density information
storage capabilities. The interface magnetism or <i>spinterface</i> resulting from the interaction between a magnetic molecule and a
metal surface, or <i>vice versa</i>, has become a key ingredient
in creating nanoscale molecular devices with novel functionalities.
Here, we present a single-molecule wire that displays large (>10000%)
conductance switching by controlling the spin-dependent transport
under ambient conditions (room temperature in a liquid cell). The
molecular wire is built by trapping individual spin crossover Fe<sup>II</sup> complexes between one Au electrode and one ferromagnetic
Ni electrode in an organic liquid medium. Large changes in the single-molecule
conductance (>100-fold) are measured when the electrons flow from
the Au electrode to either an Ī±-up or a Ī²-down spin-polarized
Ni electrode. Our calculations show that the current flowing through
such an interface appears to be strongly spin-polarized, thus resulting
in the observed switching of the single-molecule wire conductance.
The observation of such a high spin-dependent conductance switching
in a single-molecule wire opens up a new door for the design and control
of spin-polarized transport in nanoscale molecular devices at room
temperature