Generally, nanographene is insoluble in organic solvents and water, but this did not stop researchers from Kumamoto University (KU) and Tokyo Institute of Technology (Tokyo Tech) from finding a way to dissolve it in water.
The researchers used “molecular containers” that enclose water-insoluble molecules to develop a formation process for a nanographene adlayer—a layer that interacts chemically with the underlying substance, by simply combining nanographene and the molecular containers together in water. It is believed that this technique will be useful for the analysis and development of advanced functional nanomaterials.
Graphene—a monolayer of carbon atoms—is organized in sheet form. Lighter than metal, graphene has excellent electrical properties and, as a result, has attracted a great deal of interest as a sophisticated material for electronics. Nanographene is a structurally defined nano-sized graphene that has physical characteristics different from graphene. While nanographene is a useful material for molecular devices and organic semiconductors, its molecular group is insoluble in a wide range of solvents; moreover, its central physical characteristics are not adequately known.
By using micelles, water-insoluble substances can be dissolved in water. A well-known example of a micelle is soap. When water and soap micelles are mixed together, bubbles that are hydrophilic on the outside and hydrophobic on the inside start to form. It is these bubbles that trap oil-based dirt, making it relatively easy to wash away with water.
Using this property of micelles, Dr Michito Yoshizawa of Tokyo Tech created amphipathic (molecules that possess both hydrophilic and hydrophobic characteristics) micelle capsules. Building upon Dr Yoshizawa’s work, KU scientists created a micelle capsule for insoluble nanographene compound groups.
Using micelle capsules containing particular chemical structures (anthracene) as molecular containers, the KU team deftly exploited molecular interactions to successfully intake nanographene molecules within the capsules. The micelle capsules behave like Santa Claus’ presents, and the highly hydrophobic nanographene molecules (the toy) within the capsule (the wrapping paper/box) are sent to the surface of the gold (Au) substrate underwater (the Christmas tree). In the acidic aqueous solution, the micelle capsules subsequently experience a change of molecular state or equilibrium, and the nanographene within these capsules is adsorbed and arranged on the Au substrate because it cannot dissolve in water without its “protective wrapping.”
With the help of an electrochemical scanning tunneling microscope, or EC-STM, which is capable of resolving material surfaces at the atomic level, the team was able to view three forms of nanographene molecules, such as dicoronylene, circobiphenyl, and ovalene, in molecular-scale resolution for the first time ever. Through the images, it was observed that the molecules that were adsorbed on the Au substrate were frequently aligned, forming a highly ordered 2D molecular adlayer.
While this technique of molecular adlayer fabrication employs molecules that have solubility constraints, it can also be applied to other different types of molecules. In addition, the method must attract attention as an eco-friendly technology because it certainly eliminates the use of dangerous organic solvents. The researchers believe that this can possibly pave the way for newer avenues in nanographene science research.
A couple of years ago, KU faced significant challenges due to the 2016 Kumamoto earthquakes. While we were recovering from this disaster, Tokyo Tech accepted senior undergraduate students from our laboratory as special auditors. This collaborative research project started from that point. The results of this work are a direct result of Tokyo Tech’s rapid response and kind cooperation during the difficult situation we faced here in Kumamoto. We really appreciate their generous assistance. The method we developed can also be applied to a group of molecules with a larger chemical structure. We expect to see this work lead to the development of molecular wires, new battery materials, thin film crystal growth from precise molecular designs, and the further elucidation of fundamental physical properties.
Soichiro Yoshimoto, Associate Professor and Project Leader, Kumamoto University
The results of the study were reported in the Angewandte Chemie International Edition on October 23rd, 2018.