Lateral diffusion of single polymer molecules at interfaces between water and oil.

Lateral diffusion of polymer molecules at the interfaces between immiscible oil and water is investigated at the single molecular level. The interfaces between water and alkanes are chosen as the model systems and polyethylene oxide (PEO) is the model polymer. Fluorescence correlation spectroscopy is used to measure the interfacial diffusion of fluorescence-labeled PEO with its molecular weight ranging over more than an order of magnitude. It is discovered that the interfacial diffusion coefficient scales with the molecular weight by the exponent of -0.5. Detailed analysis shows that the PEO chain takes an ideal two-dimensional random coil conformation at these fluidic interfaces and the bigger contribution from water's hydrodynamic friction is discovered.

Preparation of Metalloporphyrin-Bound Superparamagnetic Silica Particles via "Click" Reaction.

A facile approach using click chemistry is demonstrated for immobilization of metalloporphyrins onto the surface of silica-coated iron oxide particles. Oleic-acid stabilized iron oxide nanocrystals were prepared by thermal decomposition of iron(III) acetylacetonate. Their crystallinity, morphology, and superparamagnetism were determined using X-ray diffraction, transmission electron microscopy, and a superconducting quantum interference device. Monodisperse core-shell particles were produced in the silica-coating of iron oxide via microemulsion synthesis. Surface modification of these particles was performed in two steps, which included the reaction of silica-coated iron oxide particles with 3-bromopropyltrichlorosilane, followed by azido-functionalization with sodium azide. Monoalkylated porphyrins were prepared using the Williamson ether synthesis of commercially available tetra(4-hydroxyphenyl) porphyrin with propargyl bromide in the presence of a base. (1)H NMR and matrix-assisted laser desorption ionization confirmed the identity of the compounds. The prepared monoalkyne porphyrins were zinc-metalated prior to their introduction to azide-functionalized, silica-coated iron oxide particles in the click reaction. X-ray photoelectron spectroscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy were used to characterize the surface chemistry after each step in the reaction. In addition, particle size was determined using dynamic light scattering and microscopy. The presented methodology is versatile and can be extended to other photoreactive systems, such as phthalocyanines and boron-dipyrromethane, which may lead to new materials for optical, photonic, and biological applications.

From repulsive to attractive glass: A rheological investigation.

Linear rheological properties and yielding behavior of polystyrene core and poly (N-isopropylacrylamide) (PNIPAM) shell microgels were investigated to understand the transition from repulsive glass (RG) to attractive glass (AG) and the A3 singularity. Due to the volume phase transition of PNIPAM in aqueous solution, the microgel-microgel interaction potential gradually changes from repulsive to attractive. In temperature and frequency sweep experiments, the storage modulus (G') and loss modulus (G″) increased discontinuously when crossing the RG-to-AG transition line, while G' at low frequency exhibited a different volume fraction (Φ) dependence. By fitting the data of RG and AG, and then extrapolating to high volume fraction, the difference between RG and AG decreased and the existence of A3 singularity was verified. Dynamic strain sweep experiments were conducted to confirm these findings. RG at 25 °C exhibited one-step yielding, whereas AG at 40 °C showed a typical two-step yielding behavior; the first yielding strain remained constant and the second one gradually decreased as the volume fraction increased. By extrapolating the second yield strain to that of the first one, the predicted A3 singularity was at 0.61 ± 0.02. At 37 °C, when Φeff = 0.59, AG showed one step yielding as the length of the attractive bond increased. The consistency and agreement of the experimental results reaffirmed the existence of A3 singularity, where the yielding behavior of RG and AG became identical.

Coupling of gelation and glass transition in a biphasic colloidal mixture-from gel-to-defective gel-to-glass.

The state transition from gel to glass is studied in a biphasic mixture of polystyrene core/poly(N-isopropylacrylamide) shell (CS) microgels and sulfonated polystyrene (PSS) particles. At 35 °C, the interaction between CS microgels is due to short-range van der Waals attraction, while that between PSS particles is from long-range electrostatic repulsion. During the variation of the relative ratio of the two species at a fixed apparent total volume fraction, the mixture exhibits a gel-to-defective gel-to-glass transition. When small amounts of PSS are introduced into the CS gel network, some of them are kinetically trapped, causing a change in its fractal structure, and act as defects to weaken the macroscopic gel strength. An increase of the PSS content in the mixture promotes the switch from the gel to the defective gel, e.g., the typical two-step yielding gel merges into one-step yielding. This phenomenon is an indication that inter-cluster bond breakage coincides with intra-cluster bond fracture. As the relative volume fraction of PSS exceeds a critical threshold, the gel network can no longer be formed; hence, the mixture exhibits characteristics of glass. A state diagram of the biphasic mixture is constructed, and the landscapes of the different transitions will be described in future studies.

Effects of particle softness on shear thickening of microgel suspensions.

A series of microgel particles composed of a polystyrene (PS) core and a thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) shell with different shell thicknesses were investigated to elucidate the effect of microgel softness on its shear thickening behavior. Since the softness of the microgels increases with decreasing temperature through the volume phase transition effect of PNIPAM shell, the measured softness parameter, n, which is derived from the Zwanzig-Mountain equation, was used to measure and describe the combined influences of temperature and shell thickness. Confocal microscopy is used to investigate the interaction potential between microgel particles with different softness parameters. According to the obtained results, the softness parameter can provide an estimate for the shear thickening behavior of microgel suspensions, at least semi-quantitatively.

Yielding behavior in colloidal glasses: comparison between "hard cage" and "soft cage".

Rheological measurements are utilized to examine the yielding behavior of a polystyrene (PS) core and poly(N-isopropylacrylamide) (PNIPAM) shell microgel system with varying shell/core ratio. For a shell/core ratio of 0.15 at high concentrations, the suspensions show a typical hard sphere (HS) yielding response where the loss modulus (G″) exhibits a single peak due to cage breaking. As a result of tighter cages and less cage distortion prior to yielding, the peak location of G″ decreases with volume fraction. For a shell/core ratio of 1.10, which behaves like a soft jammed glass at high concentration, the suspensions exhibit a one-step yielding behavior similar to that of HS glass. However, the location of the peak in G″ increases with volume fraction, demonstrating the important role of particle deformation in the breakage of cages. For an intermediate shell/core ratio of 0.34, the system displays a two-step yielding behavior, as observed in previous reports for attractive glasses. By increasing the volume fraction, the strain of the first peak increases while the second one decreases. In addition, as the effective volume fraction increases to 112%, the two peaks merge into one broad peak. It is demonstrated that the first peak of G″ is due to deformation of the shell, and the second peak of G″ is attributed to cage breaking as a result of the cores colliding with each other. Combining these results, a yielding state diagram from typical HS to soft jammed glass is demonstrated.

Phase transitions in the assembly of multivalent signalling proteins.

Cells are organized on length scales ranging from ångström to micrometres. However, the mechanisms by which ångström-scale molecular properties are translated to micrometre-scale macroscopic properties are not well understood. Here we show that interactions between diverse synthetic, multivalent macromolecules (including multi-domain proteins and RNA) produce sharp liquid-liquid-demixing phase separations, generating micrometre-sized liquid droplets in aqueous solution. This macroscopic transition corresponds to a molecular transition between small complexes and large, dynamic supramolecular polymers. The concentrations needed for phase transition are directly related to the valency of the interacting species. In the case of the actin-regulatory protein called neural Wiskott-Aldrich syndrome protein (N-WASP) interacting with its established biological partners NCK and phosphorylated nephrin, the phase transition corresponds to a sharp increase in activity towards an actin nucleation factor, the Arp2/3 complex. The transition is governed by the degree of phosphorylation of nephrin, explaining how this property of the system can be controlled to regulatory effect by kinases. The widespread occurrence of multivalent systems suggests that phase transitions may be used to spatially organize and biochemically regulate information throughout biology.

Characterization of the self-assembly of meso-tetra(4-sulfonatophenyl)porphyrin (H(2)TPPS(4-)) in aqueous solutions.

The aggregation of meso-tetra(4-sulfonatophenyl)porphyrin (H(2)TPPS(4-)) in phosphate solutions was investigated as a function of pH, concentration, time, ionic strength, and solution preparation (either from dilution of a freshly prepared 2 mM stock or by direct preparation of µM solution concentrations) using a combination of complementary analytical techniques. UV-vis and fluorescence spectroscopy indicated the formation of staggered, side-by-side (J-type) assemblies. Their size and self-associative behavior were determined using analytical ultracentrifugation and small-angle X-ray scattering. Our results indicate that in neutral and basic solutions of H(2)TPPS(4-), porphyrin dimers and trimers are formed at micromolar concentrations and in the absence of NaCl to screen any ionic interactions. At these low concentrations and pH 4, the protonated H(4)TPPS(2-) species self-assembles, leading to the formation of particularly stable aggregates bearing 25 ± 3 macrocycles. At higher concentrations, these structures further organize or reorganize into tubular, rod-like shapes of various lengths, which were imaged by cryogenic and freeze-fracture transmission electron microscopy. Micron-scale fibrillar aggregates were obtained even at micromolar concentrations at pH 4 when prepared from dilution of a 2 mM stock solution, upon addition of NaCl, or both.