Ordered gelation of chemically converted graphene for next-generation electroconductive hydrogel films.

From a chemistry point of view, graphene is essentially a conducting polymer with a giant, two-dimensional (2D) molecular configuration. Research on this unique macromolecule has previously been centered on its physics. Although the study on the chemistry of graphene can be traced back to over a century ago, graphene chemistry did not receive much interest until very recently. Nevertheless, recent studies on the chemistry of graphene and its derivatives have been very fruitful, and a number of exciting chemical properties and chemistry-enabled applications have been reported. We have been particularly interested in studying the colloidal chemistry of graphene because understanding the colloidal behavior is highly desirable for developing new methods for efficient and controllable assembly of graphene into new useful materials. We previously demonstrated that chemically converted graphene (CCG) can form a stable aqueous dispersion without the need for any surfactants. Herein, we reveal an unusual gelation behavior of CCG. We demonstrate that a combination of 2D configuration, ultralarge molecular size, and self-contained functional groups makes CCG sheets self-gel at the solid–liquid interface during filtration, leading to a new class of oriented, conductive hydrogel films with unprecedented mechanical, electrical, and anisotropic stimuli-responsive properties. Hydrogels are polymeric or supramolecular cross-linked networks that absorb large quantities of water without dissolving and are traditionally formed by physical or chemical cross-linking of natural or synthetic polymers. Besides being widely consumed in our everyday life (e.g. fruit jellies, contact lenses, and hair gel), hydrogels have been extensively explored as functional soft materials for use in tissue engineering, sensors, actuators, drug delivery, and smart separation membranes. By blending with electrically conductive additives, including graphene, hydrogels can be made conductive for potential use in implantable electrochemical biosensors, electrostimulated drug release devices, and neural prosthetics. We have previously observed that CCG can gel in water without the need for additional gelators if the concentration of its dispersion exceeds a certain value. Shi and co-workers have recently demonstrated that graphene hydrogels can be readily formed when a high concentration of graphene oxide dispersion is hydrothermally reduced. The graphene hydrogels formed in the bulk solutions using both the methods are composed of a randomly cross-linked 3D graphene network, which is similar to the structure of many conventional polymer hydrogels. Filtration is a routine technique widely used for separating suspended particles from liquids. It has been recently demonstrated that ultrastrong graphene paper can be formed simply by vacuum filtration of graphene dispersions, followed by drying. However, the structural evolution mechanism of CCG sheets during the filtration process has remained unclear. Given that the concentration of the remaining CCG solution usually increases as water is being filtered out, we surmised that CCG could gel during the filtration process. Our experiments have revealed that gelation does occur during filtration, but surprisingly not in the bulk solution. The concentration of the CCG dispersion does not increase as the water is drained out (Figure 1b). A uniform black film is