Adsorption of the Herbicide 4‑Chloro-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 H₂O 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

Helix–Coil 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>_w = 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 NaClO₄ 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 (“folding”) 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 Core–Satellite 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 core–satellite 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 core–satellite 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_BC) 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 α_vβ₃ integrin. The self-assembly of these protein–ELP_BC fusions as a function of temperature was investigated by UV spectroscopy, light scattering, and cryo-TEM. Self-assembly of the ELP_BC 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_BC. 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_BC micelles, as indicated by the enhanced uptake of the Fn3-decorated ELP_BC micelles in comparison to the unimer by cells that overexpress the α_vβ₃ integrin. The fusion of single domain proteins to ELP_BCs 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 Mössbauer Study of the Formation of FeSb₂ 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 FeSb₂ 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 solid–solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. A time- and temperature-dependent study of formation of nanoparticular FeSb₂ by X-ray powder diffraction and iron-57 Mössbauer spectroscopy showed the incipient formation of the binary phase in the temperature range of 200–250 °C