16th Interview | Assistant Professor Ami Nishijima, Department of Applied Chemistry

Interviewer: Professor Yukitoshi Motome (Assistant Dean)

Design nanomaterials at will

- Opening up new possibilities for two-dimensional polymer sheets -

Motome: Nishijima-sensei is working on synthesis, design, and construction of molecular nanospace materials in the field of coordination chemistry. You are creating new nanomaterials by controlling molecular nanospaces, but what exactly are you researching?

Nishijima: In the field of complex chemistry, which is the subject of my research, we are working on manufacturing using a reaction in which an organic ligand is mixed with a metal ion to form a complex that has a shape. In my current laboratory, we are particularly interested in utilizing the nanospaces created by complexes.

Coordination chemistry is a field that studies compounds in which organic molecules and charged compounds called ligands are linked around metal ions by a bonding mode called coordination bonds. By making good use of coordination bonds, we can produce an interesting phenomenon (called the “self-assembly phenomenon”) in which metal ions and ligands gather on their own to form a specific shape. If you prepare materials skillfully and wisely, you can design the shapes of materials you want, such as triangles and squares, for example.

I am currently working with layered complex materials, in which the space between the layers is spread out. When monomers, which are the building blocks of polymers (raw materials for plastics), are introduced into the slit-like gaps between the layers, a polymerization reaction takes place to form polymers within the gaps. The result is that the thinness of the gaps between the coordination complex’s layers gets copied by the polymers, forming ultra-thin, two-dimensional polymers. Ordinary polymers are chain-like or string-like, but when they are made into two-dimensional, ultra-thin sheets, various differences in physical properties become apparent.

Motome: It is interesting that when complexes are manipulated this way, molecules come together on their own to form a specific shape. So, you can create a kind of mold for polymer synthesis, and by pouring molecules into it, you can create the form you want. You mentioned that in your research, it is possible for substances to self-assemble in a two-dimensional shape. Have you established a trick or recipe for creating a desired shape?

Nishijima: Half of it has been established, but we are still trying to find a synthesis strategy through trial and error. What is established is the synthesis of organic ligands that we can design ourselves. We can make organic molecules that form coordination bonds to metals according to our own design by connecting molecular parts to each other one by one. For example, we can synthesize L-shaped ligands or banana-shaped ligands. On the other hand, it is difficult to predict the self-assembled form of metal ions and ligands.

We can predict that if we stretch the ligand a little, the same shape will be formed, but we cannot predict the self-assembly from scratch. We leave it to the forces of nature to self-assemble and find the environment and conditions under which two-dimensional self-assembly can occur through repeated trial and error.

Motome: So you can control the material part to some extent, but for the overall design, trial and error is important. Do you use computational chemistry to design materials?

Nishijima: We are trying to do so, but from a practical perspective, it is faster and more interesting to come up with various structures through trial and error rather than predicted structures.

Motome: So now your main focus is on actually making things through experimentation. You are creating a two-dimensional gap and polymerizing monomers poured into it, and in fact, low-dimensional systems are very attractive. In the field of condensed matter physics, which is my specialty, one- and two-dimensional materials are also attracting attention because they exhibit a variety of physical properties that differ from those of ordinary three-dimensional materials, such as quantum phenomena. What interesting physical properties do you see in two-dimensional membranes in the field of polymers?

Nishijima: Polymers are characterized by the fact that their physical properties are determined by the entanglement of molecular chains when they become solid materials. Therefore, when polymers are made to become two-dimensional, they form two-dimensional sheets like fishing nets in which the polymers are cross-linked to form a mesh, making it difficult for the strings to entangle with each other.

In this case, when vibration or stress is applied, a polymer material system with one-dimensional entanglement is elastic, whereas in a system without two-dimensional entanglement, there is no force that causes the polymers to hook together, so the polymer becomes silky and liquid-like. The difference in the shape of the polymers can produce differences in the mechanical properties of the material.

Motome: When you say two-dimensional shape, do you mean each of the pieces that peel off one by one? What is the size and is there any variety in the material?

Nishijima: The thickness is uniformly about 0.7 nanometers, and the size of the layers can range from 100 nanometers to 2 micrometers.

As long as we use raw materials that can fit into the holes, we can produce a wide variety, such as cationic and anionic sheets.

Motome: Two-dimensional materials are very popular in the world of physics, and various materials have been discovered, with graphene being the typical example of a completely new artificial material created by combining them. Similar efforts are being made in the field of polymers, aren't they? What are some of the highlights of your recent research?

Nishijima: Our recent hot topic is anion (carboxylic acid) polymer sheets. They exhibit pH responsiveness: when shaken to the anionic side (base side), it becomes an anion, and when shaken to the acidic side, it becomes a neutral functional group. It is now becoming possible to see that they show structural changes, such as becoming flat in solution, or becoming crumpled, paper-like structures. Ordinary inorganic sheets also roll up when peeled off, but they do not curl up like crumpled paper. It is precisely because two-dimensional sheets have the softness of polymers that they undergo structural changes such as curling and opening, and I believe that this will open up new possibilities for them as materials.

Motome: It is very interesting that controlling pH makes it possible to control the dynamics. How did you come to realize this? Also, what are the possible applications?

Nishijima: I thought that if I chemically treated the polymers I had made, I would be able to produce variations while maintaining their two-dimensionality, and when I tried it, it worked.

Among other applications, polymers with an electrical charge, such as polyanions, have potential for use in drug delivery. We are also thinking about the possibility of coating surfaces in a flat state. With ordinary chain polymers, you would have to build up a very thick layer to cover the surface, but I think that a two-dimensional sheet could cover the surface quite thinly and efficiently.

Friends I met in my quest to become a researcher

Motome: Nishijima-sensei, how did you become interested in the research field of coordination chemistry?

Nishijima: When I was an undergraduate student, I was very interested in graphene and did research on the topic of processing it to make porous materials. While I conducted my undergraduate research, I felt that porous materials made of complexes would be interesting from the perspective of functionality and design. So, I moved to a different university for graduate school and started my research on making porous materials with coordination chemistry.

Motome: Some people say that there are few women in engineering and that there is an atmosphere of difficulty for women to advance in the field. Have you ever felt any kind of resistance like that?

Nishijima: Thanks to my family, I have always had an environment where I could feel science close to me, such as frequent visits to science museums, so I naturally always liked science. I also thought that being a woman in science would be a path that would allow me to be unique and do my own thing.

I myself thought that this was my path, so I don't think I was particularly inconvenienced. It is true that there are many men, but on the other hand, the few women who are in my field can get along well.

Motome: That is encouraging for those who are thinking of entering the field of science and engineering. For you, Nishijima-sensei, what was it that supported you in your decision to pursue a doctoral degree in science and engineering and to become a researcher?

Nishijima: I participated in the MERIT program (Materials Education Program for the Future Leaders in Research, Industry, and Technology), and the friends I met there encouraged and inspired me a lot. Therefore, I think it is important to find friends. When I look back on my days in junior high school and high school, I had friends who were somewhat good at science. If you talk with such friends, you will be able to encourage each other by sharing your ideas. I think you should be proactive and make a conscious effort to find good friends and form connections.

Motome: MERIT is an educational program that provides financial support for students who wish to pursue doctoral studies through an integrated master's and doctoral program, and also aims to broaden their research and career perspectives through interaction with students from different fields and people in industry.

Nishijima: To me, MERIT was the best program. Normally, doctoral students from different fields do not have many opportunities to interact with each other, but MERIT gave me many such opportunities. Talking about my research gave me a chance to polish my own language, and I met excellent colleagues who I am sure will be the ones I can turn to when I need help in the future.

There were people in the electrical engineering field who were attaching devices to the brains of rats, and others in the theoretical physics field who were running calculations on computers day and night. Even now, I consult with my friends from back then about my research.

Motome: I hope that you will continue to make use of such a wonderful network after graduation. Besides connecting you with people who support you, are there any other ways in which the MERIT program helps your research?

Nishijima: It helped me in my decision to go to Karlsruhe Institute of Technology in Germany to study for about half a year. Fortunately, I found an opportunity to apply for a scholarship in Germany and studied there for half a year. I am really glad that I was able to learn new things during my study abroad. Although that did not directly lead to the research results of my doctoral thesis, I think it was definitely useful for my current research.

Motome: How is it connected to your research?

Nishijima: One aspect of synthetic techniques is that it is difficult to create what you want to create without already having experience. The experience I gained while studying in Germany has been helpful in learning new synthetic techniques and being able to make the ligands I want to make.

I want to design polymers at will

Motome: What are your dreams for your own research in the future?

Nishijima: The molecular shape of polymers is like a tangled string, so as a material they inherently lack regularity. My desire is to freely create shapes with those polymers. Until now, I have been studying self-assembly, or self-organization, in which small molecules organize themselves to form a specific shape, but my major goal is to not only entangle long-chain molecules, such as polymers, but also to arrange them systematically and, if possible, achieve self-assembly to create the desired shape.

I think that finding a way to control the shape of these polymers using two-dimensional space, as I was able to do with polyanion sheets, is one stepping stone towards my goal. The next step is to try to change the spatial shape of the coordination materials used in the synthesis. For example, if I synthesize a polymer in a three-dimensional space, I may be able to create a polymer that replicates the shape of its gaps in three dimensions, not just two.

Furthermore, by making the structure of the coordination materials chiral and helical, I may be able to introduce helicity to the polymers as well. My goal is to make use of the space to give the polymers a specific shape. At first, my plan is to discover various methods of shaping polymers using the frame of nanospace as a tool.

Motome: It sounds like quite a big dream to design the shape and structure of polymers at will. Which area is the most difficult or challenging?

Nishijima: There is some research that tries to shape polymers in various ways, but the polymer chains themselves do not have a single molecular weight and have a statistical distribution.

It is not easy to make an orderly structure out of such a disordered sort of polymer. First of all, I need a synthesis method that can precisely match the size of the polymers. To achieve this, I need to control the synthesis method and the polymer growth reaction.

In addition, by synthesizing polymers in nanospace, I would like to weave polymer chains together and create unique structures, and I would like to find new functions derived from those structures.

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Note: All affiliations and positions are as of the time of the interview.