11 research outputs found
Potential Controls the Interaction of Liposomes with Octadecanol-Modified Au Electrodes: An in Situ AFM Study
The formation of supported lipid
bilayers using liposomes requires
interaction with the solid surface, rupture of the liposome, and spreading
to cover the surface with a lipid bilayer. This can result in a less-than-uniform
coating of the solid surface. Presented is a method that uses the
electrochemical poration of an adsorbed lipid-like layer on a Au electrode
to control the interaction of 100 nm DOPC liposomes. An octadecanol-coated
Au-on-mica surface was imaged using tapping-mode AFM during the application
of potential in the presence or absence of liposomes. When the substrate
potential was made negative enough, defects formed in the adsorbed
layer and new taller features were observed. More features were observed
and existing features increased in size with time spent at this negative
poration potential. The new features were 1.8–2.0 nm higher
than the octadecanol-coated gold surface, half the thickness of a
DOPC bilayer. These features were not observed in the absence of liposomes
when undergoing the same potential perturbation. In the presence of
liposomes, the application of a poration potential was needed to initiate
the formation of these taller features. Once the applied potential
was removed, the features stopped growing and no new regions were
observed. The size of these new regions was consistent with the footprint
of a flattened 100 nm liposome. It is speculated that the DOPC liposomes
were able to interact with the defects and became soluble in the octadecanol,
creating a taller region that was limited in size to the liposome
that adsorbed and became incorporated. This AFM study confirms previous
in situ fluorescence measurements of the same system and illustrates
the use of a potential perturbation to control the formation of these
regions of increased DOPC content
What Happens to the Thiolates Created by Reductively Desorbing SAMs? An in Situ Study Using Fluorescence Microscopy and Electrochemistry
In situ examination of the reductive desorption process
for Au
microelectrodes modified with a thiol self-assembled monolayer (SAM)
using fluorescence microscopy enabled the study of the fate of the
desorbed thiolate species. The Bodipy labeled alkyl-thiol SAM, when
adsorbed, is not fluorescent due to quenching by the Au surface. Once
reductively desorbed, the thiolate molecules fluoresce and their direction
and speed are monitored. At moderately negative reduction potentials,
the thiolate species hemispherically diffuse away from the microelectrode.
Also observed is the influence of a closely positioned counter electrode
on the direction of the desorbed thiolate movement. As the potential
becomes more negative, the molecules move in an upward direction,
with a speed that depends on the amount of dissolved H<sub>2</sub> produced by water reduction. Shown is that this motion is controlled,
in large part, by the change in the electrolyte density near the electrode
due to dissolved H<sub>2</sub>. These results should help in explaining
the extent of readsorption at oxidative potentials observed in cyclic
voltammetry (CV) reductive desorption measurements, as well as improving
the general understanding of the SAM removal process by reductive
desorption. The electrogenerated H<sub>2</sub> was also shown to be
able to reductively remove the thiol SAM from the Pt/Ir particles
that decorate the microelectrode glass sheath
What Happens to the Thiolates Created by Reductively Desorbing SAMs? An in Situ Study Using Fluorescence Microscopy and Electrochemistry
In situ examination of the reductive desorption process
for Au
microelectrodes modified with a thiol self-assembled monolayer (SAM)
using fluorescence microscopy enabled the study of the fate of the
desorbed thiolate species. The Bodipy labeled alkyl-thiol SAM, when
adsorbed, is not fluorescent due to quenching by the Au surface. Once
reductively desorbed, the thiolate molecules fluoresce and their direction
and speed are monitored. At moderately negative reduction potentials,
the thiolate species hemispherically diffuse away from the microelectrode.
Also observed is the influence of a closely positioned counter electrode
on the direction of the desorbed thiolate movement. As the potential
becomes more negative, the molecules move in an upward direction,
with a speed that depends on the amount of dissolved H<sub>2</sub> produced by water reduction. Shown is that this motion is controlled,
in large part, by the change in the electrolyte density near the electrode
due to dissolved H<sub>2</sub>. These results should help in explaining
the extent of readsorption at oxidative potentials observed in cyclic
voltammetry (CV) reductive desorption measurements, as well as improving
the general understanding of the SAM removal process by reductive
desorption. The electrogenerated H<sub>2</sub> was also shown to be
able to reductively remove the thiol SAM from the Pt/Ir particles
that decorate the microelectrode glass sheath
What Happens to the Thiolates Created by Reductively Desorbing SAMs? An in Situ Study Using Fluorescence Microscopy and Electrochemistry
In situ examination of the reductive desorption process
for Au
microelectrodes modified with a thiol self-assembled monolayer (SAM)
using fluorescence microscopy enabled the study of the fate of the
desorbed thiolate species. The Bodipy labeled alkyl-thiol SAM, when
adsorbed, is not fluorescent due to quenching by the Au surface. Once
reductively desorbed, the thiolate molecules fluoresce and their direction
and speed are monitored. At moderately negative reduction potentials,
the thiolate species hemispherically diffuse away from the microelectrode.
Also observed is the influence of a closely positioned counter electrode
on the direction of the desorbed thiolate movement. As the potential
becomes more negative, the molecules move in an upward direction,
with a speed that depends on the amount of dissolved H<sub>2</sub> produced by water reduction. Shown is that this motion is controlled,
in large part, by the change in the electrolyte density near the electrode
due to dissolved H<sub>2</sub>. These results should help in explaining
the extent of readsorption at oxidative potentials observed in cyclic
voltammetry (CV) reductive desorption measurements, as well as improving
the general understanding of the SAM removal process by reductive
desorption. The electrogenerated H<sub>2</sub> was also shown to be
able to reductively remove the thiol SAM from the Pt/Ir particles
that decorate the microelectrode glass sheath
What Happens to the Thiolates Created by Reductively Desorbing SAMs? An in Situ Study Using Fluorescence Microscopy and Electrochemistry
In situ examination of the reductive desorption process
for Au
microelectrodes modified with a thiol self-assembled monolayer (SAM)
using fluorescence microscopy enabled the study of the fate of the
desorbed thiolate species. The Bodipy labeled alkyl-thiol SAM, when
adsorbed, is not fluorescent due to quenching by the Au surface. Once
reductively desorbed, the thiolate molecules fluoresce and their direction
and speed are monitored. At moderately negative reduction potentials,
the thiolate species hemispherically diffuse away from the microelectrode.
Also observed is the influence of a closely positioned counter electrode
on the direction of the desorbed thiolate movement. As the potential
becomes more negative, the molecules move in an upward direction,
with a speed that depends on the amount of dissolved H<sub>2</sub> produced by water reduction. Shown is that this motion is controlled,
in large part, by the change in the electrolyte density near the electrode
due to dissolved H<sub>2</sub>. These results should help in explaining
the extent of readsorption at oxidative potentials observed in cyclic
voltammetry (CV) reductive desorption measurements, as well as improving
the general understanding of the SAM removal process by reductive
desorption. The electrogenerated H<sub>2</sub> was also shown to be
able to reductively remove the thiol SAM from the Pt/Ir particles
that decorate the microelectrode glass sheath
Direct Mapping of Heterogeneous Surface Coverage in DNA-Functionalized Gold Surfaces with Correlated Electron and Fluorescence Microscopy
The
characterization of biofunctionalized surfaces such as alkanethiol
self-assembled monolayers (SAMs) on gold modified with DNA or other
biomolecules is a challenging analytical problem, and access to a
routine method is desirable. Despite substantial investigation from
structural and mechanistic perspectives, robust and high-throughput
metrology tools for SAMs remain elusive but essential for the continued
development of these devices. We demonstrate that scanning electron
microscopy (SEM) can provide image contrast of the molecular interface
during SAM functionalization. The high-speed, large magnification
range, and ease of use make this widely available technique a powerful
platform for measuring the structure and composition of SAM surfaces.
This increased throughput allows for a better understanding of the
nonideal spatial heterogeneity characteristic of SAMs utilized in
real-world conditions. SEM image contrast is characterized through
the use of fluorescently labeled DNA, which enables correlative SEM
and fluorescence microscopy. This allows identification of the DNA-modified
regions at resolutions that approach the size of the biomolecule.
The effect of electron beam irradiation dose is explored, which leads
to straightforward lithographic patterning of DNA SAMs with nanometer
resolution and with control over the surface coverage of specifically
adsorbed DNA
Potential-Dependent Interaction of DOPC Liposomes with an Octadecanol-Covered Au(111) Surface Investigated Using Electrochemical Methods Coupled with in Situ Fluorescence Microscopy
The
potential-controlled incorporation of DOPC liposomes (100 nm
diameter) into an adsorbed octadecanol layer on Au(111) was studied
using electrochemical and in situ fluorescence microscopy. The adsorbed
layer of octadecanol included a small amount of a lipophilic fluorophoreî—¸octadecanol
modified with BODIPYî—¸to enable fluorescence imaging. The deposited
octadecanol layer was found not to allow liposomes to interact unless
the potential was less than −0.4 V/SCE, which introduces defects
into the adsorbed layer. Small increases in the capacitance of the
adsorbed layer were measured after introducing the defects, allowing
the liposomes to interact with the defects and then annealing the
defects at 0 V/SCE. A change in the adsorbed layer was also signified
by a more positive desorption potential for the liposome-modified
adsorbed layer as compared to that for an adsorbed layer that was
porated in a similar fashion but without liposomes present in the
electrolyte. These subtle changes in capacitance are difficult to
interpret, so an in situ spectroscopic study was performed to provide
a more direct measure of the interaction. The incorporation of liposomes
should result in an increase in the fluorescence measured because
the fluorophore should become further separated from the gold surface,
reducing the efficiency of fluorescence quenching. No significant
increase in the fluorescence of the adsorbed layer was observed during
the potential pulses used in the poration procedure in the absence
of liposomes. In the presence of liposomes, the fluorescence intensity
was found to depend on the potential and time used for poration. At
0 V/SCE, no significant change in the fluorescence was observed for
defect-free adsorbed layers. Changing the poration potential to −0.4
V/SCE caused significant increases in the fluorescence and the appearance
of new structural features in the adsorbed layers that were more easily
observed during the desorption procedure. The extent of fluorescence
changes was found to be strongly dependent on the nature of the adsorbed
layer under investigation, which suggests that the poration and liposome
interaction are dependent on the quality of the adsorbed layer and
its ease of poration through changes in the electrode potential
Controlling Nanoparticle Interconnectivity in Thin-Film Platinum Catalyst Layers
The
optimization of conventional hydrogen fuel cell catalyst layers
suffers from a poor understanding of their composite nanostructure
during both initial preparation and its evolution during use. We demonstrate
how highly active, ultralow loading platinum (Pt) catalyst layers
can be fabricated in a single, solution-processable step using electroless
deposition. Growing Pt nanoparticles directly in the surface of a
polyelectrolyte Nafion membrane yields a mechanically robust film
with tunable optical reflectance and electronic conductivity. Small
changes in the polymer hydration and Pt film thickness critically
modulate nanoparticle interconnectivity near the percolation threshold.
Conductive atomic force microscopy (AFM) and electron microscopy reveal
how the film’s dynamic nanoscale morphology allows control
over bulk electrochemical and optical properties. Well-defined composition
and structure make these layers an experimentally accessible model
system for studying thin-film electrocatalyst architectures
Influence of Surface Structure on Single or Mixed Component Self-Assembled Monolayers via in Situ Spectroelectrochemical Fluorescence Imaging of the Complete Stereographic Triangle on a Single Crystal Au Bead Electrode
The
use of a single crystal gold bead electrode is demonstrated
for characterization of self-assembled monolayers (SAM)Âs formed on
the bead surface expressing a complete set of face centered cubic
(fcc) surface structures represented by a stereographic projection.
Simultaneous analysis of many crystallographic orientations was accomplished
through the use of an in situ fluorescence microscopic imaging technique
coupled with electrochemical measurements. SAMs were prepared from
different classes of molecules, which were modified with a fluorescent
tag enabling characterization of the influence of electrical potential
and a direct comparison of the influence of surface structure on SAMs
adsorbed onto low index, vicinal and chiral surfaces. The assembly
of alkylthiol, Aib peptide and DNA SAMs are studied as a function
of the electrical potential of the interface revealing how the organization
of these SAMs depend on the surface crystallographic orientation,
all in one measurement. This approach allows for a simultaneous determination
of SAMs assembled onto an electrode surface onto which the whole fcc
stereographic triangle can be mapped, revealing the influence of intermolecular
interactions as well as the atomic arrangement of the substrate. Moreover,
this method enables study of the influence of the Au surface atom
arrangement on SAMs that were created and analyzed, both under identical
conditions, something that can be challenging for the typical studies
of this kind using individual gold single crystal electrodes. Also
demonstrated is the analysis of a SAM containing two components prepared
using thiol exchange. The two component SAM shows remarkable differences
in the surface coverage, which strongly depends on the surface crystallography
enabling estimates of the thiol exchange energetics. In addition,
these electrode surfaces enable studies of molecular adsorption onto
the symmetry related chiral surfaces since more than one stereographic
triangle can be imaged at the same time. The ability to observe a
SAM modified surface that contains many complete fcc stereographic
triangles will facilitate the study of the single and multicomponent
SAMs, identifying interesting surfaces for further analysis
Influence of Surface Structure on Single or Mixed Component Self-Assembled Monolayers via in Situ Spectroelectrochemical Fluorescence Imaging of the Complete Stereographic Triangle on a Single Crystal Au Bead Electrode
The
use of a single crystal gold bead electrode is demonstrated
for characterization of self-assembled monolayers (SAM)Âs formed on
the bead surface expressing a complete set of face centered cubic
(fcc) surface structures represented by a stereographic projection.
Simultaneous analysis of many crystallographic orientations was accomplished
through the use of an in situ fluorescence microscopic imaging technique
coupled with electrochemical measurements. SAMs were prepared from
different classes of molecules, which were modified with a fluorescent
tag enabling characterization of the influence of electrical potential
and a direct comparison of the influence of surface structure on SAMs
adsorbed onto low index, vicinal and chiral surfaces. The assembly
of alkylthiol, Aib peptide and DNA SAMs are studied as a function
of the electrical potential of the interface revealing how the organization
of these SAMs depend on the surface crystallographic orientation,
all in one measurement. This approach allows for a simultaneous determination
of SAMs assembled onto an electrode surface onto which the whole fcc
stereographic triangle can be mapped, revealing the influence of intermolecular
interactions as well as the atomic arrangement of the substrate. Moreover,
this method enables study of the influence of the Au surface atom
arrangement on SAMs that were created and analyzed, both under identical
conditions, something that can be challenging for the typical studies
of this kind using individual gold single crystal electrodes. Also
demonstrated is the analysis of a SAM containing two components prepared
using thiol exchange. The two component SAM shows remarkable differences
in the surface coverage, which strongly depends on the surface crystallography
enabling estimates of the thiol exchange energetics. In addition,
these electrode surfaces enable studies of molecular adsorption onto
the symmetry related chiral surfaces since more than one stereographic
triangle can be imaged at the same time. The ability to observe a
SAM modified surface that contains many complete fcc stereographic
triangles will facilitate the study of the single and multicomponent
SAMs, identifying interesting surfaces for further analysis