Small-Angle X-ray Scattering Study on Internal Microscopic Structures of Poly(N-isopropylacrylamide-co-tris(2,2'-bipyridyl))ruthenium(II) Complex Microgels

The Belousov-Zhabotinsky Reaction in Thermoresponsive Core-Shell Hydrogel Microspheres with a Tris(2,2'-bipyridyl)ruthenium Catalyst in the Core

The Belousov-Zhabotinsky (BZ) reaction shows temporal or spatiotemporal structures such as redox oscillation of the catalyst, [ruthenium(II)tris(2,2'-bipyridine)][PF₆]₂ ([Ru(bpy)₃][PF₆]₂). In this study, autonomously oscillating hydrogel microspheres (microgels) were investigated, which show swelling/deswelling oscillation induced by the redox oscillation of the BZ reaction inside the gel. Despite the periodically and autonomously induced oscillation that does not require an external stimulus, it has not been possible to perform any manipulation of the oscillatory behavior over time. The results of the present study show that it is possible to reversibly switch the microgel oscillations from an "on" active state of the BZ reaction to an "off" inactive state by changing the temperature in combination with thermoresponsive microgels. To realize on-demand switching, the construction of double-shell structures is crucial; the thermoresponsive first shell allows the microgels to modulate the diffusion of the substrates or intermediates in the BZ reaction, while the second shell maintains colloidal stability under high temperatures and high ion concentrations. The functionalized double-shell microgels were prepared via multistep seeded precipitation polymerization. The oscillatory switching behavior of the BZ reaction was observed directly and evaluated by ultraviolet-visible (UV-vis) spectroscopy. The central concept of this study, i.e., "on-off switching" can be expected to benefit the development of advanced bioinspired materials.

Room temperature crystallization of amorphous polysiloxane using photodimerization

Bulk crystallization in flexible polymeric systems is difficult to control due to the random orientation of the chains. Here we report a photo cross-linking strategy that results in simultaneous cross-linking and crystallization of polysiloxane chains into millimeter sized leaf-like polycrystalline structures. Polymers containing pendant anthracene groups are prepared and undergo [4+4] photocycloaddition under 365 nm irradiation at room temperature. The growth and morphology of the crystalline structures is studied using polarized optical microscopy (POM) and atomic force microscopy and is found to progress through three unique stages of nucleation, growth, and constriction. The mobility of the individual chains is probed using pulsed-field gradient (PFG) NMR to provide insights into the diffusion processes that may govern chain transport to the growing crystal fronts. The room temperature crystallization of this conventionally amorphous polymer system may allow for a new level of morphological control for silicone materials.

Effects of charge contrast and composition on microgel formation and interactions with bacteria-mimicking liposomes

Microgels offer opportunities for improved delivery of antimicrobial peptides (AMP). To contribute to a foundation for rational design of such systems, we here study the effects of electrostatics on the generation of peptide-carrying microgels. For this, alginate microgels loaded with polymyxin B and cross-linked by Ca²⁺, were formed by electrostatic complexation using a hydrodynamic focusing three-dimensional (3D)-printed micromixer, varying pH and component concentrations. The structure of the resulting composite nanoparticles was investigated by small-angle X-ray scattering, dynamic light scattering, and z-potential measurements, whereas peptide encapsulation and release was monitored spectrophotometrically. Furthermore, membrane interactions of these systems were assessed by dye leakage assays in model lipid vesicles. Our results indicate that charge contrast between polymyxin B and alginate during microgel formation affects particle size and network dimensions. In particular, while microgels prepared at maximum polymyxin B-alginate charge contrast at pH 5 and 7.4 are characterized by sharp interfaces, those formed at pH 9 are characterized by a more diffuse core, likely caused by a weaker peptide-polymer affinity, and a shell dominated by alginate that shrinks at high CaCl₂ concentrations. Quantitatively, however, these effects were relatively minor, as were differences in peptide encapsulation efficiency and electrolyte-induced peptide release. This demonstrates that rather wide charge contrasts allow efficient complexation and particle formation, with polymyxin B encapsulated within the particle interior at low ionic strength, but released at high electrolyte concentration. As a consequence of this, peptide-mediated membrane destabilization were suppressed by microgel incorporation at low ionic strength, but regained after microgel disruption. After particle disruption at high ionic strength, however, some polymyxin B was found to remain bound to alginate chains from the disrupted composite microgel particles, resulting in partial loss in membrane interactions, compared to the free peptide.

Monitoring Thermoresponsive Morphological Changes in Individual Hydrogel Microspheres

Real-time morphology/structure changes in individual hydrogel microspheres (microgels) were directly visualized at high spatiotemporal resolution using high-speed atomic force microscopy (HS-AFM) under temperature control ranging from room temperature to ∼40 °C. The recorded HS-AFM movies demonstrate that the size and morphology of thermoresponsive poly(N-isopropyl acrylamide)-based microgels change with increasing temperature at the individual microgel level. Specifically, the height of the microgels gradually decreases and domain structures appeared even below the volume phase transition temperature. Moreover, the domain structure is retained, even after the microgels have fully collapsed. The present study thus demonstrates that temperature-controlled HS-AFM is a useful tool for monitoring stimulus-responsiveness of microgels. In the near future, it should furthermore be possible to extend this temperature-controlled HS-AFM to other stimulus-responsive materials, including autonomously oscillating microgels.

Seeded Emulsion Polymerization of Styrene in the Presence of Water-Swollen Hydrogel Microspheres

In a previous study, we have ascertained that the charge distribution in hydrogel microspheres (microgels) plays a crucial role in controlling the nanocomposite structure of the polystyrene obtained from the seeded emulsion polymerization (SEP) of styrene in the presence of microgels. However, all these polymerizations were conducted at high temperature, where most of these microgels were dehydrated and deswollen. In the present study, we initially verified that the nanocomposite microgels can be synthesized even when the seed microgels are swollen and hydrated during the SEP of styrene. These highly swollen microgels were used as the nucleation sites for the polystyrene, and subsequently the propagation of the hydrophobic polystyrenes proceeded within water-swollen microgels.

Time-resolved structural evolution during the collapse of responsive hydrogels: The microgel-to-particle transition

Adaptive hydrogels, often termed smart materials, are macromolecules whose structure adjusts to external stimuli. Responsive micro- and nanogels are particularly interesting because the small length scale enables very fast response times. Chemical cross-links provide topological constraints and define the three-dimensional structure of the microgels, whereas their porous structure permits fast mass transfer, enabling very rapid structural adaption of the microgel to the environment. The change of microgel structure involves a unique transition from a flexible, swollen finite-size macromolecular network, characterized by a fuzzy surface, to a colloidal particle with homogeneous density and a sharp surface. In this contribution, we determine, for the first time, the structural evolution during the microgel-to-particle transition. Time-resolved small-angle x-ray scattering experiments and computer simulations unambiguously reveal a two-stage process: In a first, very fast process, collapsed clusters form at the periphery, leading to an intermediate, hollowish core-shell structure that slowly transforms to a globule. This structural evolution is independent of the type of stimulus and thus applies to instantaneous transitions as in a temperature jump or to slower stimuli that rely on the uptake of active molecules from and/or exchange with the environment. The fast transitions of size and shape provide unique opportunities for various applications as, for example, in uptake and release, catalysis, or sensing.

The deformation of hydrogel microspheres at the air/water interface

The deformation of soft hydrogel microspheres (microgels) adsorbed at the air/water interface was investigated for the first time using large poly(N-isopropyl acrylamide)-based microgels synthesized by a modified aqueous precipitation polymerization method.

Impact of Spatial Distribution of Charged Groups in Core Poly(N-isopropylacrylamide)-Based Microgels on the Resultant Composite Structures Prepared by Seeded Emulsion Polymerization of Styrene

A series of raspberry-shaped composite microgels were synthesized by seeded emulsion polymerization of styrene in the presence of hydrogel particles with different distributions of charged groups. Unlike microgels whose charged groups are localized in their center,29 polystyrene nanoparticles were formed inside the core microgels when the microgels whose charged groups were localized on their surface were used as cores for seeded emulsion polymerization. The effects of the surface charge densities of the core microgels and the concentration of styrene monomer during the polymerization on the resultant structures of composite microgels were investigated. The surface structures of obtained composite microgels were mainly evaluated by electron microscopy, and their stimuli responsiveness was evaluated by dynamic light scattering and laser Doppler velocimetry. The internal structures of the composite microgels were visualized from ultrathin cross sections observed by transmission electron microscopy (TEM). Cryo-TEM was used to clarify the microscopic structures of composite microgels when they were in hydrated states. Through a series of characterizations, we summarize the effects of structures of core microgels on the resultant composite structures.

Investigation of Changes in the Microscopic Structure of Anionic Poly(N-isopropylacrylamide-co-Acrylic acid) Microgels in the Presence of Cationic Organic Dyes toward Precisely Controlled Uptake/Release of Low-Molecular-Weight Chemical Compound

Changes in a microscopic structure of an anionic poly(N-isopropylacrylamide-co-acrylic acid) microgel were investigated using small- and wide-angle X-ray scattering (SWAXS). The scattering profiles of the microgels were analyzed in a wide scattering vector (q) range of 0.07 ≤ q/nm(-1) ≤ 20. In particular, the microscopic structure of the microgel in the presence of a cationic dye rhodamine 6G (R6G) was characterized in terms of its correlation length (ξ), which represents the length scale of the spatial correlation of the network density fluctuations, and characteristic distance (d*), which originated from the local packing of isopropyl groups of two neighboring chains. In the presence of cationic R6G, ξ exhibited a divergent-like behavior, which was not seen in the absence of R6G, and d* was decreased with decreasing the volume of the microgel upon increasing temperature. At the same time, the amount of R6G adsorbed per unit mass of the microgel increased upon heating. These results suggested that a coil-to-globule transition of the poly(N-isopropylacrylamide) chains in the present anionic microgel occurred because of efficiently screened, thus, short ranged electrostatic repulsion between the charged groups, and hydrophobic interaction between the isopropyl groups in the presence of cationic R6G. The combination of hydrophobic and electrostatic interaction between the cationic dye and the microgel affected the separation and volume transition behavior of the microgel.

Saccharide-induced modulation of photoluminescence lifetime in microgels

Sugar-responsive microgels based on boronic acid derivative and incorporating [Ru(bpy)₃]²⁺ as a luminescent reporter, exhibit very long lifetimes and unusually high quantum yields, which decrease upon saccharide addition.