Countercation Engineering of Graphene-Oxide Nanosheets for Imparting a Thermoresponsive Ability

Graphene-oxide (GO) nanosheets, which are oxidized derivativesof graphene, are regarded as promising building blocks for functionalsoft materials. Especially, thermoresponsive GO nanosheets have beenwidely employed to develop smart membranes/surfaces, hydrogel actuators,recyclable systems, and biomedical applications. However, currentsynthetic strategies to generate such thermoresponsive GO nanosheetshave exclusively relied on the covalent or non-covalent modificationof their surfaces with thermoresponsive polymers, such as poly(N-isopropylacrylamide). To impart a thermoresponsive abilityto GO nanosheets themselves, we focused on the countercations of thecarboxy and acidic hydroxy groups on the GO nanosheets. In this study,we established a general and reliable method to synthesize GO nanosheetswith target countercations and systematically investigated their effectson thermoresponsive behaviors of GO nanosheets. As a result, we discoveredthat GO nanosheets with Bu4N+ countercationsbecame thermoresponsive in water without the use of any thermoresponsivepolymers, inducing a reversible sol-gel transition via theirself-assembly and disassembly processes. Owing to the sol-geltransition capability, the resultant dispersion can be used as a directwriting ink for constructing a three-dimensionally designable gelarchitecture of the GO nanosheets. Our concept of countercationengineering can become a new strategy for imparting a stimuli-responsiveability to various charged nanomaterials for the development of next-generationsmart materials.

Automated summary

This article describes a method to impart thermoresponsive ability to graphene oxide nanosheets by changing the countercations on their surfaces, resulting in a reversible sol-gel transition in water. The nanosheets can be used as a direct writing ink for constructing a three-dimensionally designable gel architecture.