6 research outputs found
Graphene Oxide and Lipid Membranes: Interactions and Nanocomposite Structures
We have investigated the interaction between graphene
oxide and
lipid membranes, using both supported lipid membranes and supported
liposomes. Also, the reverse situation, where a surface coated with
graphene oxide was exposed to liposomes in solution, was studied.
We discovered graphene oxide-induced rupture of preadsorbed liposomes
and the formation of a nanocomposite, bio-nonbio multilayer structure,
consisting of alternating graphene oxide monolayers and lipid membranes.
The assembly process was monitored in real time by two complementary
surface analytical techniques (the quartz crystal microbalance with
dissipation monitoring technique (QCM-D) and dual polarization interferometry
(DPI)), and the formed structures were imaged with atomic force microscopy
(AFM). From a basic science point of view, the results point toward
the importance of electrostatic interactions between graphene oxide
and lipid headgroups. Implications from a more practical point of
view concern structure–activity relationship for biological
health/safety aspects of graphene oxide and the potential of the nanocomposite,
multilayer structure as scaffolds for advanced biomolecular functions
and sensing applications
Integration of Quartz Crystal Microbalance with Vibrational Sum Frequency Spectroscopy–Quantification of the Initial Oxidation of Alkanethiol-Covered Copper
We report the first integration of the interface sensitive
technique
vibrational sum frequency spectroscopy (VSFS) and the mass sensitive
technique quartz crystal microbalance (QCM). VSFS–QCM has been
applied in-situ to follow the formation of a thin Cu<sub>2</sub>O-like
oxide on octadecanethiol-covered copper in dry air at ambient pressure
conditions. We observed significant changes and an evolution of the
VSF spectra caused by alterations in the electronic properties of
the metal surface, and simultaneous shifts in the QCM resonance frequency
due to a mass change during the formation of the oxide. QCM and VSFS
exhibit a resolution corresponding to the formation of around 2% and
5% of an ideal monolayer of Cu<sub>2</sub>O, respectively. The successful
integration of QCM increases the versatility of VSFS in numerous applications,
where simultaneous in situ mass and spectroscopic information is desirable
Time-Resolved Indirect Nanoplasmonic Sensing Spectroscopy of Dye Molecule Interactions with Dense and Mesoporous TiO<sub>2</sub> Films
Indirect nanoplasmonic sensing (INPS) is an experimental
platform
exploiting localized surface plasmon resonance (LSPR) detection of
processes in nanomaterials, molecular assemblies, and films at the
nanoscale. Here we have for the first time applied INPS to study dye
molecule adsorption/impregnation of two types of TiO<sub>2</sub> materials:
thick (10 μm) mesoporous films of the kind used as photoanode
in dye-sensitized solar cells (DSCs), with particle/pore size in the
range of 20 nm, and thin (12–70 nm), dense, and flat films.
For the thick-film experiments plasmonic Au nanoparticles were placed
at the hidden, internal interface between the sensor surface and the
mesoporous TiO<sub>2</sub>. This approach provides a unique opportunity
to selectively follow dye adsorption locally in the hidden interface
region inside the material and inspires a generic and new type of
nanoplasmonic hidden interface spectroscopy. The specific DSC measurement
revealed a time constant of thousands of seconds before the dye impregnation
front (the diffusion front) reaches the hidden interface. In contrast,
dye adsorption on the dense, thin TiO<sub>2</sub> films exhibited
much faster, Langmuir-like monolayer formation kinetics with saturation
on a time scale of order 100 s. This new type of INPS measurement
provides a powerful tool to measure and optimize dye impregnation
kinetics of DSCs and, from a more general point of view, offers a
generic experimental platform to measure adsorption/desorption and
diffusion phenomena in solid and mesoporous systems and at internal
hidden interfaces
Nanofabricated Catalyst Particles for the Investigation of Catalytic Carbon Oxidation by Oxygen Spillover
The catalytic oxidation of carbon by molecular oxygen was studied
using C/Pt, Pt/C, Pt/Al<sub>2</sub>O<sub>3</sub>/C, Pt/CeO<sub>2</sub>/C, Al<sub>2</sub>O<sub>3</sub>/C, and CeO<sub>2</sub>/C model samples
prepared by hole-mask colloidal lithography. By this technique, the
degree of contact between platinum and carbon was controlled with
high precision. The oxidation of carbon was monitored using atomic
force microscopy and scanning electron microscopy. The results show
that Pt in direct contact with carbon catalyzes the oxidation of carbon
by spillover of dissociated oxygen from Pt to carbon. By physically
separating Pt and carbon with a 10 nm thin spacer layer of Al<sub>2</sub>O<sub>3</sub>, the oxygen spillover was entirely blocked.
However, through a corresponding spacer layer of CeO<sub>2</sub>,
carbon oxidation was still observed, either by oxygen spillover from
Pt to carbon or directly dissociated on the ceria, although at a slower
rate compared to the case with no spacer layer between Pt and carbon
Influence of Divalent Cations on Deformation and Rupture of Adsorbed Lipid Vesicles
The
fate of adsorbed lipid vesicles on solid supports depends on
numerous experimental parameters and typically results in the formation
of a supported lipid bilayer (SLB) or an adsorbed vesicle layer. One
of the poorly understood questions relates to how divalent cations
appear to promote SLB formation in some cases. The complexity arises
from the multiple ways in which divalent cations affect vesicle–substrate
and vesicle–vesicle interactions as well as vesicle properties.
These interactions are reflected, e.g., in the degree of deformation
of adsorbed vesicles (if they do not rupture). It is, however, experimentally
challenging to measure the extent of vesicle deformation in real-time.
Herein, we investigated the effect of divalent cations (Mg<sup>2+</sup>, Ca<sup>2+</sup>, Sr<sup>2+</sup>) on the adsorption of zwitterionic
1,2-dioleoyl-<i>sn</i>-glycero-3-phosphocholine (DOPC) lipid
vesicles onto silicon oxide- and titanium oxide-coated substrates.
The vesicle adsorption process was tracked using the quartz crystal
microbalance-dissipation (QCM-D) and localized surface plasmon resonance
(LSPR) measurement techniques. On silicon oxide, vesicle adsorption
led to SLB formation in all cases, while vesicles adsorbed but did
not rupture on titanium oxide. It was identified that divalent cations
promote increased deformation of adsorbed vesicles on both substrates
and enhanced rupture on silicon oxide in the order Ca<sup>2+</sup> > Mg<sup>2+</sup> > Sr<sup>2+</sup>. The influence of divalent
cations
on different factors in these systems is discussed, clarifying experimental
observations on both substrates. Taken together, the findings in this
work offer insight into how divalent cations modulate the interfacial
science of supported membrane systems
Combined in Situ Quartz Crystal Microbalance with Dissipation Monitoring, Indirect Nanoplasmonic Sensing, and Vibrational Sum Frequency Spectroscopic Monitoring of Alkanethiol-Protected Copper Corrosion
In
this study, we have applied three techniques to simultaneously
and in situ study the initial stage of corrosion of copper protected
by a self-assembled monolayer of octadecanethiol (ODT). We combined
quartz crystal microbalance with dissipation monitoring (QCM-D), indirect
nanoplasmonic sensing (INPS), and vibrational sum frequency spectroscopy
(VSFS) and obtained complementary information about mass uptake and
optical and spectroscopic changes taking place during the initial
corrosion phase. All three techniques are very sensitive to the formation
of a corrosion film (thickness in the range 0–0.41 nm) under
mildly corrosive conditions (dry air, <0.5% relative humidity).
The three techniques yield information about the viscoelasticity of
the corrosion film (QCM-D), the homogeneity of the corrosion reaction
on the surface (INPS), and the stability of the ODT protection layer
(VSFS). Furthermore, by also studying the corrosion process in humid
air (ca. 70% relative humidity), we illustrate how the combination
of these techniques can be used to differentiate between simultaneously
occurring processes, such as water adsorption and corrosion product
formation