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,
Fe3O4 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)/nm2, 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 Fe3O4 NPs.
In this work we analyze how nuclear coherences modulate diagonal and off-diagonal peaks in two-dimensional electronic spectroscopy. 2D electronic spectra of pinacyanol chloride are measured with 8 fs pulses, which allows coherent excitation of the 1300 cm(-1) vibrational mode. The 2D spectrum reveals both diagonal and off-diagonal peaks related to the vibrational mode. On early time scales, up to 30 fs, coherent dynamics give rise to oscillations in the amplitudes, positions, and shapes of the peaks in the 2D spectrum. We find an anticorrelation between the amplitude and the diagonal width of the two diagonal peaks. The measured data are reproduced with a model incorporating a high frequency mode coupled to an electronic two-level-system. Our results show that these anticorrelated oscillations occur for vibrational wavepackets and not exclusively for electronic coherences as has been assumed previously.
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.
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.
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.
Magnetic polymersomes were prepared by self-assembly of the amphiphilic block copolymer poly(isoprene-b-N-isopropylacrylamide) with monodisperse hydrophobic superparamagnetic iron oxide nanoparticles (SPION). The specifically designed thermoresponsive block copolymer allowed for efficient incorporation of the hydrophobic nanoparticles in the membrane core and encapsulation of the water soluble dye calcein in the lumen of the vesicles. Magnetic heating of the embedded SPIONs led to increased bilayer permeability through dehydration of the thermoresponsive PNIPAM block. The entrapped calcein could therefore be released in controlled doses solely through exposure to pulses of an alternating magnetic field. This hybrid SPION-polymersome system demonstrates a possible direction for release applications that merges rational polymersome design with addressed external magnetic field-triggered release through embedded nanomaterials.
The interaction of exciton and charge transfer (CT) states plays a central role in photo-induced CT processes in chemistry, biology, and physics. In this work, we use a combination of two-dimensional electronic spectroscopy (2D-ES), pump-probe measurements, and quantum chemistry to investigate the ultrafast CT dynamics in a lutetium bisphthalocyanine dimer in different oxidation states. It is found that in the anionic form, the combination of strong CT-exciton interaction and electronic asymmetry induced by a counter-ion enables CT between the two macrocycles of the complex on a 30 fs timescale. Following optical excitation, a chain of electron and hole transfer steps gives rise to characteristic cross-peak dynamics in the electronic 2D spectra, and we monitor how the excited state charge density ultimately localizes on the macrocycle closest to the counter-ion within 100 fs. A comparison with the dynamics in the radical species further elucidates how CT states modulate the electronic structure and tune fs-reaction dynamics. Our experiments demonstrate the unique capability of 2D-ES in combination with other methods to decipher ultrafast CT dynamics.
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