Variable-Angle Surface Spectroscopy Reveals the Water Structure in the Stern Layer at Charged Aqueous Interfaces.

At charged aqueous interfaces, the second-order nonlinear optical response originates from water molecules within the diffuse part of the electrical double layer, which are ordered by the surface field and from water that additionally experiences chemical and physical interactions with the surface in the Stern layer. These two environments can either reinforce or diminish the overall signal and can be disentangled by varying the coherence length of their interaction with external laser fields. Here, we demonstrate a method in which the angle of incidence is varied to afford a significant change in the coherence length. When this technique was applied to the silica-water interface, it was observed that water molecules in the Stern and diffuse layers direct their hydrogen atoms toward the mineral surface at a low ionic strength and neutral pH. A decrease in the signal with increasing ionic strength is attributed to hydrated cation adsorption that competes with free water for deprotonated silanol sites.

Graphene oxide based crosslinker for simultaneous enhancement of mechanical toughness and self-healing capability of conventional hydrogels.

Extraordinary self-healing efficiency is rarely observed in mechanically strong hydrogels, which often limits the applications of hydrogels in biomedical engineering. We have presented an approach to utilize a special type of graphene oxide-based crosslinker (GOBC) for the simultaneous improvement of toughness and self-healing properties of conventional hydrogels. The GOBC has been prepared from graphene oxide (GO) by surface oxidation and further introduction of vinyl groups. It has been designed in such a way that the crosslinker is able to form both covalent bonds and noncovalent interactions with the polymer chains of hydrogels. To demonstrate the efficacy of GOBC, it was incorporated in a conventional polyacrylamide (PAM) and polyacrylic acid (PAA) hydrogel matrix, and the mechanical and self-healing properties of the prepared hydrogels were investigated. In PAM-GOBC hydrogels, it has been observed that the mechanical properties such as tensile strength, Young's modulus, and toughness are significantly improved by the incorporation of GOBC without compromising the self-healing efficiency. The PAM-GOBC hydrogel with a modulus of about 0.446 MPa exhibited about 70% stress healing efficiency after 40 h. Whereas, under the same conditions a PAM hydrogel with commonly used crosslinker N,N'-methylene-bis(acrylamide) of approximately the same modulus demonstrated no self-healing at all. Similar improvement of self-healing properties and toughness in PAA-GOBC hydrogel has also been observed which demonstrated the universality of the crosslinker. This crosslinker-based approach to improve the self-healing properties is expected to offer the possibility of the application of commonly used hydrogels in many different sectors, particularly in developing artificial tissues.

Mechanically tough and highly stretchable poly(acrylic acid) hydrogel cross-linked by 2D graphene oxide.

The mechanical performances of hydrogels are greatly influenced by the functionality of cross-linkers and their covalent and non-covalent interactions with the polymer chains. Conventional chemical cross-linkers fail to meet the demand of large toughness and high extensibility for their immediate applications as artificial tissues like ligaments, blood vessels, and cardiac muscles in human or animal bodies. Herein, we synthesized a new graphene oxide-based two-dimensional (2D) cross-linker (GOBC) and exploited the functionality of the cross-linker for the enhancement of toughness and stretchability of a poly(acrylic acid) (PAA) hydrogel. The 2D nanosheets of GO were modified in such a way that they could provide multifunctional sites for both physical and chemical bonding with the polymer chains. Carboxylic acid groups at the surfaces of the GO sheets were coupled with the acrylate functional groups for covalent cross-linking, while the other oxygen-containing functional groups are responsible for physical cross-linking with polymers. The GOBC had been successfully incorporated into the PAA hydrogel and the mechanical properties of the GOBC cross-linked PAA hydrogel (PAA-GOBC) were investigated at various compositions of cross-linker. Seven times enhancement in both toughness and elongation at break has been achieved without compromising on the modulus for the as-synthesized PAA-GOBC compared to the conventional N,N'-methylenebis(acrylamide) (MBA) cross-linked PAA hydrogel. This facile and efficient way of GO modification is expected to lead the development of a high-performance nanocomposite for cutting-edge applications in biomedical engineering.

Magnetic nanocomposite based on polyacrylic acid and carboxylated cellulose nanocrystal for the removal of cationic dye.

The development of safe and cost-effective methods for the treatment of dye polluted wastewater has been a great concern among researchers. Herein, we developed a nanocomposite (M3D-PAA-CCN) based on polyacrylic acid (PAA) crosslinked with magnetic 3D crosslinkers (M3D) and carboxylated cellulose nanocrystals (CCN), for the removal of cationic dyes from aqueous solutions. Acrylic-functionalized Fe3O4 nanoparticles were covalently linked to the polymer chains via the form of the 3D crosslinker to introduce magnetic properties into the as-synthesized nanocomposite. The addition of highly dispersive CCN reduced the gel-like properties of the nanocomposite and instead incorporated a diffusive nature, which was more desirable for adsorbents. The surface morphology of the nanocomposite was analyzed by FESEM and the size of the nanocomposite particles was found to be in the range of 60-90 nm. The chemical functionalities and compositions were determined by XPS, FTIR, and EDX analyses whereas TGA confirmed the thermal stability of M3D-PAA-CCN. The maximum adsorption capacity of the M3D-PAA-CCN (332 mg g-1) was measured higher than that of M3D-PAA (114 mg g-1) to a cationic methylene blue (MB) dye indicating the significant contribution of CCN. The adsorption capacity of the as-synthesized M3D-PAA-CCN was found to be highly pH-dependent and the adsorption capacity increased with the increase of pH owing to the greater negative charge as indicated by the higher zeta potential. The adsorption kinetics of MB on the composites was found to follow the pseudo-second-order model. The adsorption capacity was also investigated as a function of concentration to figure out the adsorption mechanism using Langmuir and Freundlich isotherm models. The Langmuir model fitted the adsorption process better as suggested by the relatively smaller nonlinear chi-square value obtained from the fitting parameters.