6 research outputs found
Adsorption of the Herbicide 4āChloro-2-methylphenoxyacetic Acid (MCPA) by Goethite
Interaction
between the goethite surface and 4-chloro-2-methylphenoxyacetic
acid (MCPA) herbicide was studied using density functional theory
(DFT) calculations combined with molecular dynamics (MD). The important
step made here lies in the use of a periodic DFT method enabling the
study of a mineral surface of different protonation states, in strong
contrast with previous molecular modeling studies limited to single
protonation state corresponding to the point of zero charge. Different
surface OH groups and MCPA proton states were used to mimic the strong
effects of pH on the outer- and inner-sphere surface complexes that
are theoretically possible, together with their binding energies,
and their bond lengths. Modeling both a solvated and a protonated
(110) goethite surface provided a major breakthrough in the acidic
adsorption regime. An outer-sphere complex and a monodentate inner-sphere
complex with the neutral MCPA molecule were found to be the most energetically
stable adsorbate forms. MD modeling predicted that the latter forms
via the sharing of the carbonyl oxygen between the MCPA carboxylate
group and a singly coordinated surface hydroxyl group, releasing an
H<sub>2</sub>O molecule. All the other complexes, including the bidentate
inner-sphere complex, had higher relative energies and were therefore
less likely. The two most likely DFT-optimized structures were used
to constrain a surface complexation model applying the charge distribution
multisite complexation (CD-MUSIC) approach. The adsorption constants
for the complexes were successfully fitted to experimental batch equilibrium
data
HelixāCoil Transition in Cylindrical Brush Polymers with Poly-l-lysine Side Chains
Cylindrical brush polymers with poly-l-lysine
side chains
were prepared by grafting lysine NCA from a macroinitiator via living
ring-opening polymerization. The main chain degree of polymerization
of the methacrylate main chain was <i>P</i><sub>w</sub> =
870, the side chains consisted of 25 and 55 lysine repeat units, respectively.
Upon deprotection, the cylindrical brush polymers in 0.005 M NaBr
exhibited an almost rodlike conformation with a Kuhn statistical segment
length of several hundred nanometers. Cryo-TEM as well as AFM in aqueous
solution clearly demonstrated pronounced undulations along the main
chain at low ionic strength which could not be detected at higher
salt concentrations. With increasing concentration of NaClO<sub>4</sub> the PLL side chains underwent a coil-to-helix transition as revealed
by CD measurements. The effect of the side chain coil-to-helix transition
on the main chain stiffness could not be followed by light scattering
due to intramolecular attraction (āfoldingā) of the
cylindrical brushes at high salt concentration, which is somewhat
more pronounced for the helical as compared to the coiled PLL side
chain conformation. Comparison with linear PLL revealed the coil-to-helix
transition to be hardly affected by the high grafting density of the
PLL side chains in the cylindrical brush structures
Plasmonic CoreāSatellite Assemblies as Highly Sensitive Refractive Index Sensors
Highly sensitive and spectrally tunable
plasmonic nanostructures
are of great demand for applications such as SERS and parallel biosensing.
However, there is a lack of such nanostructures for the midvisible
spectral regions as most available chemically stable nanostructures
offer high sensitivity in the red to far red spectrum. In this work,
we report the assembly of highly sensitive nanoparticle structures
using a hydroxylamine mediated coreāsatellite assembly of 20
nm gold nanoparticle satellites onto 60 nm spherical gold cores. The
average number of satellites allows tuning the plasmon resonance wavelength
from 543 to 575 nm. The coreāsatellite nanostructures are stable
in pH ranges from 5 to 9 and show about 2-fold higher plasmonic sensitivity
than similar sized gold nanospheres
Unexpected Multivalent Display of Proteins by Temperature Triggered Self-Assembly of Elastin-like Polypeptide Block Copolymers
We report herein the unexpected temperature triggered
self-assembly
of proteins fused to thermally responsive elastin-like polypeptides
(ELPs) into spherical micelles. A set of six ELP block copolymers
(ELP<sub>BC</sub>) differing in hydrophilic and hydrophobic block
lengths were genetically fused to two single domain proteins, thioredoxin
(Trx) and a fibronectin type III domain (Fn3) that binds the Ī±<sub>v</sub>Ī²<sub>3</sub> integrin. The self-assembly of these proteināELP<sub>BC</sub> fusions as a function of temperature was investigated by
UV spectroscopy, light scattering, and cryo-TEM. Self-assembly of
the ELP<sub>BC</sub> was unexpectedly retained upon fusion to the
two proteins, resulting in the formation of spherical micelles with
a hydrodynamic radius that ranged from 24 to 37 nm, depending on the
protein and ELP<sub>BC</sub>. Cryo-TEM images confirmed the formation
of spherical particles with a size that was consistent with that measured
by light scattering. The bioactivity of Fn3 was retained when presented
by the ELP<sub>BC</sub> micelles, as indicated by the enhanced uptake
of the Fn3-decorated ELP<sub>BC</sub> micelles in comparison to the
unimer by cells that overexpress the Ī±<sub>v</sub>Ī²<sub>3</sub> integrin. The fusion of single domain proteins to ELP<sub>BC</sub>s may provide a ubiquitous platform for the multivalent presentation
of proteins
Synthesis and Magnetic Properties of FePt@MnO Nano-heteroparticles
Monodisperse FePt@MnO nano-heteroparticles
with different sizes and morphologies were prepared by a seed-mediated
nucleation and growth technique. Both size and morphology of the individual
domains could be controlled by adjustment of the synthetic parameters.
As a consequence, different particle constructs, including dimers,
dumbbell-like particles, and flowerlike particles, could be obtained
by changing the polarity of the solvent. The FePt@MnO nano-heteroparticles
were thoroughly characterized by high resolution transmission electron
microscopy (HR-TEM) and X-ray diffraction (XRD) analyses and superconducting
quantum interference device (SQUID) magnetometry. Due to a sufficient
lattice match, the MnO nanoparticles (NPs) preferentially grow on
the (111) surfaces of the <i>fcc</i>-FePt seeds. Furthermore,
the surface spins of the antiferromagnetic MnO domains pin the magnetic
moments of the ferromagnetic FePt NPs, which leads to an exchanged
biased magnetic hysteresis
Wet Chemical Synthesis and a Combined X-ray and MoĢssbauer Study of the Formation of FeSb<sub>2</sub> Nanoparticles
Understanding how solids form is a challenging task, and few strategies allow for elucidation of reaction pathways that are useful for designing the synthesis of solids. Here, we report a powerful solution-mediated approach for formation of nanocrystals of the thermoelectrically promising FeSb<sub>2</sub> that uses activated metal nanoparticles as precursors. The small particle size of the reactants ensures minimum diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solidāsolid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. A time- and temperature-dependent study of formation of nanoparticular FeSb<sub>2</sub> by X-ray powder diffraction and iron-57 MoĢssbauer spectroscopy showed the incipient formation of the binary phase in the temperature range of 200ā250 Ā°C