8 research outputs found
Highly Hydrated Deformable Polyethylene Glycol-Tethered Lipid Bilayers
The
realization of a solid-supported lipid bilayer acting as a
workbench for the study of membrane processes is a difficult task.
For robustness, the bilayer has to be tethered to the substrate. At
the same time, diffusion of the lipids and plastic deformations of
the membrane should not be obstructed. Furthermore, a highly hydrated
surrounding is mandatory. Here, we show that grafting of a polyethylene
glycol–lipid construct (PEG2000–DSPE) to a silicon oxide
surface via multiple-step silane chemistry and subsequent deposition
of lipids by spin-coating result in a cushioned membrane that has
the desired properties. Neutron and X-ray reflectometry measurements
are combined to access thickness, density, and hydration of the bilayer
and the PEG cushion. We observe a spacer of 55 Å thickness between
lipid bilayer and silicon-oxide surface with a rather high hydration
of up to 90 ± 3% water. While 11.5 ± 3% of the lipids are
grafted to the surface, as determined from the neutron data, the diffusion
constant of the lipids, as probed by diffusion of 0.5% Texas Red labeled
lipids, remains rather large (<i>D</i> = 2.1 ± 0.1 μm<sup>2</sup>/s), which
is a reduction
of only 12% compared to a supported lipid bilayer reference without
immobilized lipids. Finally, AFM indentation confirms the plastic
behavior of the membrane against deformation. We show that rupture
of the bilayer does not occur before the deformation exceeds 40 Å.
Altogether, the presented PEG-tethered lipid bilayer mimics the deformability
of natural cell membranes much better than standard solid-supported
lipid bilayers
Nanostructures in <i>n</i>‑Octanol Equilibrated with Additives and/or Water
Fluid
fatty alcohols are believed to be nanostructured but broadly
amorphous (i.e., noncrystalline) fluids and solvents, including the
most popular fatty tissue mimetic, hydrated <i>n</i>-octanol
(i.e., hydro-octanol). To check this premise, we studied dry octanol
and hydro-octanol as a model of relatively short fluid <i>n</i>-alkanols with small-angle X-ray scattering (SAXS). We also combined
this alkanol with the matching alkane (i.e., octane) and with a common
anti-inflammatory pain killer (ketoprofen). This revealed that (hydro-)octanol
and arguably any other short fatty alcohol form a mesophase. Its basic
structural motif are regularly packed polar nanoclusters, reflected
in the inner peak in the SAXS diffractogram of (hydro-)octanol and
other fluid <i>n</i>-alkanols. The nanoclusters arguably
resemble tiny, (inverse) hydrated bilayer fragments, located on a
thermally smeared para-crystalline lattice. Additives to hydro-octanol
can change the nanoclusters only moderately, if at all. For example,
octane and the drug ketoprofen added to hydro-octanol enlarge the
nanoclusters only little because of the mixture’s packing frustration.
To associate with and to bring more water into hydro-octanol, an additive
must hence transform the nanoclusters: it expands them into irregularly
distributed aqueous lacunae that form a proto-microemulsion, reflected
in the previously unknown Guinier’s SAXS signal. A “weak”
(i.e., a weakly polar or nonpolar) additive can moreover create only
size-limited lacunae. Coexistence of nanoclusters and lacunae as well
as size variability of the latter in hydro-octanol subvert the concept
of octanol–water partition coefficient, which relies on the
studied compartment homogeneity. In turn, it opens new possibilities
for interfacial catalysis. Reinterpreting “octanol–water
partition coefficient” data in terms of octanol–water
association or binding constant(s) could furthermore diminish the
variability of molecular lipophilicity description and pave the ground
toward a more precise theoretical quantification and prediction of
molecular properties
Lipid Monolayer Formation and Lipid Exchange Monitored by a Graphene Field-Effect Transistor
Anionic and cationic lipids
are key molecules involved in many cellular processes; their distribution
in biomembranes is highly asymmetric, and their concentration is well-controlled.
Graphene solution-gated field-effect transistors (SGFETs) exhibit
high sensitivity toward the presence of surface charges. Here, we
establish conditions that allow the observation of the formation of
charged lipid layers on solution-gated field-effect transistors in
real time. We quantify the electrostatic screening of electrolyte
ions and derive a model that explains the influence of charged lipids
on the ion sensitivity of graphene SGFETs. The electrostatic model
is validated using structural information from X-ray reflectometry
measurements, which show that the lipid monolayer forms on graphene.
We demonstrate that SGFETs can be used to detect cationic lipids by
self-exchange of lipids. Furthermore, SGFETs allow measuring the kinetics
of layer formation induced by vesicle fusion or spreading from a reservoir.
Because of the high transconductance and low noise of the electrical
readout, we can observe characteristic conductance spikes that we
attribute to bouncing-off events of lipid aggregates from the SGFET
surface, suggesting a great potential of graphene SGFETs to measure
the on–off kinetics of small aggregates interacting with supported
layers
Shape and Interhelical Spacing of DNA Origami Nanostructures Studied by Small-Angle X‑ray Scattering
Scaffolded
DNA origami nanostructures enable the self-assembly of arbitrarily
shaped objects with unprecedented accuracy. Yet, varying physiological
conditions are prone to induce slight structural changes in the nanoscale
architecture. Here, we report on high precision measurements of overall
shape and interhelical distance of three prototypic DNA origami structures
in solution using synchrotron small-angle X-ray scattering. Sheet-,
brick-, and cylinder-shaped DNA constructs were assembled and the
shape factors determined with angstrom resolution from fits to the
scattering profiles. With decreasing MgCl<sub>2</sub> concentration
electrostatic swelling of both shape cross section and interhelical
DNA spacing of the DNA origami structures is observed. The structures
tolerate up to 10% interhelical expansion before they disintegrate.
In contrast, with increasing temperature, the cylinder-shaped structures
show no thermal expansion in a wide temperature window before they
abruptly melt above 50 °C. Details on molecular structure of
DNA origami can also be obtained using in-house X-ray scattering equipment
and, hence, allow for routine folding and stability testing of DNA-based
agents that are designed to operate under varying salt conditions
Position Accuracy of Gold Nanoparticles on DNA Origami Structures Studied with Small-Angle X‑ray Scattering
DNA origami objects allow for accurate
positioning of guest molecules
in three dimensions. Validation and understanding of design strategies
for particle attachment as well as analysis of specific particle arrangements
are desirable. Small-angle X-ray scattering (SAXS) is suited to probe
distances of nano-objects with subnanometer resolution at physiologically
relevant conditions including pH and salt and at varying temperatures.
Here, we show that the pair density distribution function (PDDF) obtained
from an indirect Fourier transform of SAXS intensities in a model-free
way allows to investigate prototypical DNA origami-mediated gold nanoparticle
(AuNP) assemblies. We analyze the structure of three AuNP-dimers on
a DNA origami block, an AuNP trimer constituted by those dimers, and
a helical arrangement of nine AuNPs on a DNA origami cylinder. For
the dimers, we compare the model-free PDDF and explicit modeling of
the SAXS intensity data by superposition of scattering intensities
of the scattering objects. The PDDF of the trimer is verified to be
a superposition of its dimeric contributions, that is, here AuNP-DNA
origami assemblies were used as test boards underlining the validity
of the PDDF analysis beyond pairs of AuNPs. We obtain information
about AuNP distances with an uncertainty margin of 1.2 nm. This readout
accuracy in turn can be used for high precision placement of AuNP
by careful design of the AuNP attachment sites on the DNA-structure
and by fine-tuning of the connector types
Molecular Architecture: Construction of Self-Assembled Organophosphonate Duplexes and Their Electrochemical Characterization
Self-assembled monolayers
of phosphonates (SAMPs) of 11-hydroxyundecylphosphonic acid, 2,6-diphosphonoanthracene,
9,10-diphenyl-2,6-diphosphonoanthracene, and 10,10′-diphosphono-9,9′-bianthracene
and a novel self-assembled organophosphonate duplex ensemble were
synthesized on nanometer-thick SiO<sub>2</sub>-coated, highly doped
silicon electrodes. The duplex ensemble was synthesized by first treating
the SAMP prepared from an aromatic diphosphonic acid to form a titanium
complex-terminated one; this was followed by addition of a second
equivalent of the aromatic diphosphonic acid. SAMP homogeneity, roughness,
and thickness were evaluated by AFM; SAMP film thickness and the structural
contributions of each unit in the duplex were measured by X-ray reflection
(XRR). The duplex was compared with the aliphatic and aromatic monolayer
SAMPs to determine the effect of stacking on electrochemical properties;
these were measured by impedance spectroscopy using aqueous electrolytes
in the frequency range 20 Hz to 100 kHz, and data were analyzed using
resistance–capacitance network based equivalent circuits. For
the 11-hydroxyundecylphosphonate SAMP, <i>C</i><sub>SAMP</sub> = 2.6 ± 0.2 μF/cm<sup>2</sup>, consistent with its measured
layer thickness (ca. 1.1 nm). For the anthracene-based SAMPs, <i>C</i><sub>SAMP</sub> = 6–10 μF/cm<sup>2</sup>,
which is attributed primarily to a higher effective dielectric constant
for the aromatic moieties (ε = 5–10) compared to the
aliphatic one; impedance spectroscopy measured the additional capacitance
of the second aromatic monolayer in the duplex (2ndSAMP) to be <i>C</i><sub>Ti/2ndSAMP</sub> = 6.8 ± 0.7 μF/cm<sup>2</sup>, in series with the first
Exciton Dynamics in MoS<sub>2</sub>‑Pentacene and WSe<sub>2</sub>‑Pentacene Heterojunctions
We measured the exciton dynamics in van der Waals heterojunctions
of transition metal dichalcogenides (TMDCs) and organic semiconductors
(OSs). TMDCs and OSs are semiconducting materials with rich and highly
diverse optical and electronic properties. Their heterostructures,
exhibiting van der Waals bonding at their interfaces, can be utilized
in the field of optoelectronics and photovoltaics. Two types of heterojunctions,
MoS2-pentacene and WSe2-pentacene, were prepared
by layer transfer of 20 nm pentacene thin films as well as MoS2 and WSe2 monolayer crystals onto Au surfaces.
The samples were studied by means of transient absorption spectroscopy
in the reflectance mode. We found that A-exciton decay by hole transfer
from MoS2 to pentacene occurs with a characteristic time
of 21 ± 3 ps. This is slow compared to previously reported hole
transfer times of 6.7 ps in MoS2-pentacene junctions formed
by vapor deposition of pentacene molecules onto MoS2 on
SiO2. The B-exciton decay in WSe2 shows faster
hole transfer rates for WSe2-pentacene heterojunctions,
with a characteristic time of 7 ± 1 ps. The A-exciton in WSe2 also decays faster due to the presence of a pentacene overlayer;
however, fitting the decay traces did not allow for the unambiguous
assignment of the associated decay time. Our work provides important
insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions
Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets
Organometal
halide perovskites have recently emerged displaying a huge potential
for not only photovoltaic, but also light emitting applications. Exploiting
the optical properties of specifically tailored perovskite nanocrystals
could greatly enhance the efficiency and functionality of applications
based on this material. In this study, we investigate the quantum
size effect in colloidal organometal halide perovskite nanoplatelets.
By tuning the ratio of the organic cations used, we can control the
thickness and consequently the photoluminescence emission of the platelets.
Quantum mechanical calculations match well with the experimental values.
We find that not only do the properties of the perovskite, but also
those of the organic ligands play an important role. Stacking of nanoplatelets
leads to the formation of minibands, further shifting the bandgap
energies. In addition, we find a large exciton binding energy of up
to several hundreds of meV for nanoplatelets thinner than three unit
cells, partially counteracting the blueshift induced by quantum confinement.
Understanding of the quantum size effects in perovskite nanoplatelets
and the ability to tune them provide an additional method with which
to manipulate the optical properties of organometal halide perovskites