5 research outputs found
Complete Exchange of the Hydrophobic Dispersant Shell on Monodisperse Superparamagnetic Iron Oxide Nanoparticles
High-temperature synthesized monodisperse
superparamagnetic iron
oxide nanoparticles are obtained with a strongly bound ligand shell
of oleic acid and its decomposition products. Most applications require
a stable presentation of a defined surface chemistry; therefore, the
native shell has to be completely exchanged for dispersants with irreversible
affinity to the nanoparticle surface. We evaluate by attenuated total
reflectance−Fourier transform infrared spectroscopy (ATR−FTIR)
and thermogravimetric analysis/differential scanning calorimetry (TGA/DSC)
the limitations of commonly used approaches. A mechanism and multiple
exchange scheme that attains the goal of complete and irreversible
ligand replacement on monodisperse nanoparticles of various sizes
is presented. The obtained hydrophobic nanoparticles are ideally suited
for magnetically controlled drug delivery and membrane applications
and for the investigation of fundamental interfacial properties of
ultrasmall core–shell architectures
Evaluation of High-Yield Purification Methods on Monodisperse PEG-Grafted Iron Oxide Nanoparticles
Fundamental research
on nanoparticle (NP) interactions and development
of next-generation biomedical NP applications relies on synthesis
of monodisperse, functional, core–shell nanoparticles free
of residual dispersants with truly homogeneous and controlled physical
properties. Still, synthesis and purification of e.g. such superparamagnetic
iron oxide NPs remain a challenge. Comparing the success of different
methods is marred by the sensitivity of analysis methods to the purity
of the product. We synthesize monodisperse, oleic acid (OA)-capped,
Fe<sub>3</sub>O<sub>4</sub> NPs in the superparamagnetic size range
(3–10 nm). Ligand exchange of OA for poly(ethylene glycol)
(PEG) was performed with the PEG irreversibly grafted to the NP surface
by a nitrodopamine (NDA) anchor. Four different methods were investigated
to remove excess ligands and residual OA: membrane centrifugation,
dialysis, size exclusion chromatography, and precipitation combined
with magnetic decantation. Infrared spectroscopy and thermogravimetric
analysis were used to determine the purity of samples after each purification
step. Importantly, only magnetic decantation yielded pure NPs at high
yields with sufficient grafting density for biomedical applications
(∼1 NDA-PEG(5 kDa)/nm<sup>2</sup>, irrespective of size). The
purified NPs withstand challenging tests such as temperature cycling
in serum and long-term storage in biological buffers. Dynamic light
scattering, transmission electron microscopy, and small-angle X-ray
scattering show stability over at least 4 months also in serum. The
successful synthesis and purification route is compatible with any
conceivable functionalization for biomedical or biomaterial applications
of PEGylated Fe<sub>3</sub>O<sub>4</sub> NPs
System-Dependent Signatures of Electronic and Vibrational Coherences in Electronic Two-Dimensional Spectra
In this work, we examine vibrational coherence in a molecular monomer,
where time evolution of a nuclear wavepacket gives rise to oscillating
diagonal- and off-diagonal peaks in two-dimensional electronic spectra.
We find that the peaks oscillate out-of-phase, resulting in a cancellation
in the corresponding pump–probe spectra. Our results confirm
the unique disposition of two-dimensional electronic spectroscopy
(2D-ES) for the study of coherences. The oscillation pattern is in
excellent agreement with the diagrammatic analysis of the third-order
nonlinear response. We show how 2D-ES can be used to distinguish between
ground- and excited-state wavepackets. On the basis of our results,
we discuss coherences in coupled molecular aggregates involving both
electronic and nuclear degrees of freedom. We conclude that a general
distinguishing criterion based on the experimental data alone cannot
be devised
Vibronic and Vibrational Coherences in Two-Dimensional Electronic Spectra of Supramolecular J‑Aggregates
In J-aggregates of cyanine dyes,
closely packed molecules form
mesoscopic tubes with nanometer-diameter and micrometer-length. Their
efficient energy transfer pathways make them suitable candidates for
artificial light harvesting systems. This great potential calls for
an in-depth spectroscopic analysis of the underlying energy deactivation
network and coherence dynamics. We use two-dimensional electronic
spectroscopy with sub-10 fs laser pulses in combination with two-dimensional
decay-associated spectra analysis to describe the population flow
within the aggregate. Based on the analysis of Fourier-transform amplitude
maps, we distinguish between vibrational or vibronic coherence dynamics
as the origin of pronounced oscillations in our two-dimensional electronic
spectra
Doping Method Determines Para- or Superparamagnetic Properties of Photostable and Surface-Modifiable Quantum Dots for Multimodal Bioimaging
Semiconductor
quantum dots (QDs) are widely used for optical applications
and bioimaging. In comparison to organic dyes used for fluorescent
labeling, QDs exhibit very high photostability and can be further
surface modified. Equipping QDs with magnetic properties (mQDs) makes
it possible to combine fluorescence and magnetic resonance imaging
analyses. For this purpose, we have prepared water-dispersible and
magnetic CdTe/ZnS mQDs, whereby ferrous ions are selectively incorporated
in either their cores or their shells. This study aims at understanding
the differences in optical, structural, and magnetic properties between
these core- and shell-doped mQDs. Field-dependent isothermal magnetic
susceptibility measurements show that shell-doped mQDs exhibit paramagnetic
and their core-doped equivalents superparamagnetic behavior
near room temperature. Shell doping results in about 1.7 times higher
photoluminescence quantum yields and 1.4 times higher doping efficiency
than core doping. X-ray diffraction patterns reveal that core doping
leads to defects in the lattice and hence to a severe decrease in
crystallinity, whereas shell doping has no significant impact on the
crystal structure and consequently fewer disadvantages regarding the
mQD’s quantum yield. These selective doping approaches, particularly
shell doping, allow for the tailored design of paramagnetic QDs having
modifiable and biocompatible particle surfaces. The organic ligandsin
this study <i>N</i>-acetyl-l-cysteinesufficiently
prevent leakage of toxic metal ions, as shown by cytotoxicity assays
with HepG2 cells. Confocal laser scanning microscopy shows that mQDs
are internalized by these cells and accumulated near their nuclei.
This study shows that biocompatible, fluorescent, and paramagnetic
QDs are promising photostable labels for multimodal bioimaging