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Papers - Oaki, Yuya

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A hydrophobic adsorbent based on hierarchical porous polymers derived from morphologies of a biomineral

K. Sato, Y. Oaki, H. Imai

Chemical Communications 51 ( 37 ) 7919 - 7922 2015

Research paper (scientific journal), Joint Work, Accepted, ISSN 1359-7345

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Hierarchical CaCO3 Chromatography: A Stationary Phase Based on Biominerals

K. Sato, D. Takahashi, Y. Oaki, K. Toshima, H. Imai

Chemistry- A European Journal 21 5034 - 5040 2015

Research paper (scientific journal), Accepted

Incorporation of organic crystals in the interspace of oriented nanocrystals: morphologies and properties

Y. Munekawa, Y. Oaki, H. Imai

Nanoscale 7 3466 - 3473 2015

Research paper (scientific journal), Joint Work, Accepted

Solvent-free Synthesis, Coating and Morphogenesis of Conductive Polymer Materials through Spontaneous Generation of Activated Monomers

Ryo Muramatsu, *Yuya Oaki, Kento Kuwabara, Kosei Hayashi, *Hiroaki Imai

Chem. Commun. 50 ( 80 ) 11840 - 11843 2014

Research paper (scientific journal), Joint Work, Accepted, ISSN 1359-7345

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An Experimental Study on the Processes of Hierarchical Morphology Replication by Means of a Mesocrystal: A Case Study of poly(3,4-ethylenedioxythiophene)

Yurika Munekawa, Yuya Oaki and Hiroaki Imai

Langmuir 30 ( 11 ) 3236 - 3242 2014

Research paper (scientific journal), Joint Work, Accepted, ISSN 0743-7463

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Bioinspired stiff and flexible composites of nanocellulose-reinforced amorphous CaCO3

Tsuguyuki Saito, Yuya Oaki, Tatsuya Nishimura, Akira Isogai and Takashi Kato

Materials Horizons 1 321 - 325 2014

Research paper (scientific journal), Joint Work, Accepted

Biogenic design of silicious architectures on Moso bamboo culm

Kazama M., Watanabe H., Hasekura C., Wakabayashi T., Ishigure T., Oaki Y., Imai H.

Scientific Reports 15 ( 1 ) 2025.12

Biosilicas that are produced in vascular plants (plant opal), such as Poaceae, have a variety of shapes and functions and are regarded as an excellent model for the architectural design of artificial amorphous materials. In this work, we studied the micro- and nanostructures and mechanical and optical functions of plant opals on the bamboo culm, which is available as an important natural material. The surface of the culm wall is totally covered with silicified epidermal cells containing silica wedges. The biogenic silicious architectures, such as silicified cell walls and wedges, are composed of nanoscale particles ~ 20–80 nm in diameter with cellulose nanofibrils. Silica wedges, which have a relatively low organic content and relatively high hardness and Young’s modulus, are initially formed on cellulose nanofibrils in an organic frame as a scaffold within a few weeks after the emergence of a bamboo shoot. Several months after the formation of the wedges, the epidermal cell walls, which protect the culm surface, are lightly silicified with cellulose nanofibrils. According to a numerical simulation, the silica wedges would have an optical function delivering sunlight to chloroplasts located under the epidermal cells.

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Twin-like chiral configuration of a calcium oxalate monohydrate mesocrystal regulated with an organic framework in grape leaves

Tanaka R., Watanabe H., Oaki Y., Imai H.

Crystengcomm 27 ( 19 ) 3128 - 3133 2025.04

Biogenic calcium oxalate monohydrate (COM, CaC<inf>2</inf>O<inf>4</inf>·H<inf>2</inf>O) needle crystals (raphides) having a pointed tip and two tails are arranged as a bundle in grape leaves. In this work, the specific hierarchical architecture was studied at the micrometric and nanometric scales. Raphides were macroscopically characterized as having a twin-like chiral configuration of COM divided by an organic central wall and covered with an organic enveloping membrane. The COM of the raphides has a mesocrystalline structure that consists of small grains ∼10 nm in diameter with a distorted single-crystal nature. The initially produced organic framework was suggested to control the specific crystal growth of COM under the construction of a raphide in a specific cell.

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Synthesis of Amorphous Graphene and Graphene Oxide Analogues

Sakuma T., Sato R., Yamaguchi A., Imai H., Arai N., Oaki Y.

Journal of the American Chemical Society 147 ( 13 ) 11564 - 11573 2025.04

ISSN 00027863

Graphene and graphene oxide (GO) are promising two-dimensional nanomaterials. An ultimate goal is to achieve large-scale bottom-up syntheses of perfect graphene and GO. However, controlled syntheses of perfect graphitic structures still remain challenges in chemistry and materials science. Moreover, amorphous types have not received much attention. The present work shows syntheses, structures, and applications of amorphous graphene and GO analogues alternative to the ideal ones. The simultaneous multiple reactions of two conjugated monomers provide amorphous conjugated polymer networks containing low-crystalline graphitic domains and their stacking. The stacked amorphous graphene and GO are exfoliated into thin nanosheets including few-layers and monolayers. Moreover, in situ syntheses of the amorphous GO analogues are applied to obtain a reinforced plastic with high mechanical strength. The present work implies that various functional nanocarbons can be designed and synthesized by tailored combinations of conjugated monomers.

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Free Volume Space of Polymers as a New Functional Nanospace: Synthesis of Guest Polymers

Hirai S., Sakuma T., Tokura Y., Imai H., Seishima R., Shigeta K., Okabayashi K., Oaki Y.

Macromolecular Rapid Communications 46 ( 8 ) 2025.04

ISSN 10221336

Nanospace has been used as a specific field for syntheses and assemblies of molecules, polymers, and materials. Free volume space among polymer chains is related to their properties, such as permeation of gas and small molecules. However, the void has not been used as a functional nanospace in previous works. The present work shows synthesis of guest conductive polymers in free volume space of conventional synthetic resins and rubbers as a new nanospace. Vapor of heteroaromatic monomer and oxidative agent is diffused into the soft dynamic nanospace among the polymer chains under ambient pressure at low temperature. The oxidative polymerization provides the conductive polymers, such as polypyrrole (PPy), in the free volume space of poly(methyl methacrylate) (PMMA), polypropylene (PP), silicone rubber (SR), and polyurethane rubber (PU). The ratio of the free volume decreases with the infiltration of the conductive polymers. The composites exhibit the improved mechanical and gas barrier properties. The rubbers containing PPy are used as mechanical-stress sensors with both the conductivity and flexibility. The free volume space of resins and rubbers can be used as a new dynamic nanospace for synthesis of functional polymer composites.

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Janus Particles Synthesized via Vapor-Phase Coupling Polymerization Protocol

Kuroiwa K., Takeuchi K., Hirai T., Nakamura Y., Oaki Y., Fujii S.

Langmuir 41 ( 8 ) 5439 - 5448 2025.03

ISSN 07437463

The vapor-phase polymerization of pyrrole in the presence of polystyrene (PS) particles adsorbed at the air-water interface successfully leads to the formation of PS/polypyrrole (PPy) Janus particles where only the surface in contact with the air phase is regioselectively covered with the PPy nanolayer. The coverage area of the PPy nanolayer on the Janus particles, and therefore the contact angle of Janus particles at the air-liquid interface, decreases with a decrease in surface tension of liquid by the addition of isopropanol. The contact angle of particles at the interface decreases after the polymerization, which should be because the pyrrole monomer dissolved in the water phase from the gas phase promotes the wetting of the liquid to the PS particles by decreasing the surface tension of the liquid. Controlling the PS particle size realizes the formation of PS/PPy Janus particles with sizes ranging between 5 and 1000 μm. Furthermore, the Janus particles are demonstrated to be oriented at the air-water interface with the hydrophilic PS side toward the water phase and the hydrophobic PPy side toward the air phase, realizing stabilization of an armored bubble in the water medium.

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Two-dimensional conjugated polymers with weakened interlayer interaction for highly sensitive visible responsiveness to compression stresses

Takeuchi Y., Imai H., Oaki Y.

Journal of Materials Chemistry C 13 ( 7 ) 3209 - 3214 2025.01

ISSN 20507526

Detection of mechanical stresses requires microscopic molecular motion triggered by macroscopically applied force. Polydiacetylene (PDA), a stimuli-responsive color-changing material, generally has no responsiveness to compression stresses. We found that a layered PDA with weakened interlayer interactions exhibits direct visible color changes in response to compression stresses. An amphiphilic diacetylene (DA) monomer, 1-(10,12-pentacosadiynyl) pyridinium bromide (PCPy+Br−), formed a lamellar structure with weakened interlayer and intermolecular interactions originating from the bulky cations and anions in the interlayer space. The resultant PDA exhibited blue-to-red color changes in response to compression stresses (P) in the range of 2.5-125 MPa. In previous works, visible detection of P < 50 MPa without excitation light has not been achieved using only mechanoresponsive materials. Moreover, the compression-stress distribution was visualized using the PDA-coated paper substrate.

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Data-scientific validation of prediction models for the controlled syntheses of exfoliated nanosheets

Kitamura Y., Namiuchi Y., Imai H., Igarashi Y., Oaki Y.

Nanoscale Advances 7 ( 15 ) 4620 - 4627 2025

Exfoliated nanosheets have attracted considerable interest as two-dimensional (2D) building blocks. In general, the yield, size, and size distribution of the exfoliated nanosheets cannot be easily controlled or predicted because of the complexity in the processes. Our group studied the prediction models of the yield, size, and size distribution based on the small experimental data available. Sparse modeling for small data (SpM-S) combining machine learning (ML) and chemical insight was used for the construction of predictors. In SpM-S, the weight diagram visualizing the significance of explanatory variables plays an important role in variable selection to construct the models. However, the processes of variable selection were not validated in a data-scientific manner. In the present work, the significance of data size, visualization method, and chemical insight for variable selection was studied to validate the processes of model construction. The data size had a lower limit to extract appropriate descriptors. The weight diagram had an appropriate visualizing range for variable selection. Chemical insight as domain knowledge supplemented the limitation caused by the data size. These studies indicated that SpM-S can be applied to construct predictors, straightforward linear regression models, for the controlled syntheses of other 2D materials, even based on small data.

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Performance Prediction Models with Improved Accuracy and Generalizability for Organic Cathode-Active Materials of Lithium-Ion Battery

Yamamoto R., Igarashi Y., Imai H., Sakata T., Miyakawa S., Yoshizaki S., Saito T., Oaki Y.

Batteries and Supercaps 2025

Development of organic energy storage requires enhancing performances of active materials. In particular, reaction potential and specific capacity of cathode-active materials have significant impact on energy density of organic lithium-ion battery. However, discovery of new compounds for active materials based on professional experience and intuition meets the limitation of huge search space of organic molecules. The performance predictors enable efficient discovery of new potential compounds. Although the predictors of potential, capacity, and energy density (models G1) are prepared in the previous work, these become older and have problems. In the present work, the updated models G2 have been constructed to improve the accuracy, usability, and generalizability. The models G2 are prepared by sparse modeling for small data combining machine learning and chemical insight on the training data set with adding new data. The updated models are validated using a new test data set and data-scientific methods. The improved predictors contribute to efficient exploration of new cathode-active materials to realize high-performance batteries.

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Polypyrrole microtubes synthesized by vapor-phase coupling polymerization

Sakabe R., Sugiura R., Mitamura K., Watase S., Hirai T., Nakamura Y., Oaki Y., Fujii S.

Emergent Materials 2025

ISSN 25225731

Simple and environmentally friendly synthetic method of polypyrrole (PPy) microtubes were proposed. First, sucrose microfibers coated by PPy overlayer were synthesised by solvent-free vapor-phase coupling polymerization of pyrrole (Py) monomer using solid oxidant in the presence of sucrose microfibers with a diameter of approximately 4 μm without any liquid media. Next, selective extraction of sucrose core component from the sucrose-core/PPy-sheath microfibers using water resulted in the formation of PPy microtubes. The sheath thickness of PPy microtubes increased from 168 nm to 209 nm by controlling relative humidity during the polymerization from 10 to 40%. Furthermore, the sheath thickness increased from 73 nm to 207 nm with an increase of the polymerization time from 4 h to 24 h. The capacitance of PPy microtubes (157.0 F g−1) was approximately three times higher than that of the bulk PPy films (52.2 F g−1), thanks to good accessibility of ions in the electrolyte to the PPy surface due to their hollow structure. The PPy microtubes synthesized in this study are expected to be applied as electrodes for energy storage devices such as capacitors and lithium-ion secondary batteries and soft micropump.

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Synthesis of polypyrrole and its derivative nanoparticles via a surfactant-free coupling polymerization protocol

Atsuta Y., Takeuchi K., Sakuma T., Mitamura K., Watase S., Song Y., Hirai T., Nakamura Y., Oaki Y., Fujii S.

Polymer Journal 57 ( 7 ) 723 - 735 2025

ISSN 00323896

Surfactant-free coupling polymerization of pyrrole (Py) and its derivatives, namely, N-methylpyrrole (MPy) and N-ethylpyrrole (EPy), was conducted using solid Fe(NO<inf>3</inf>)<inf>3</inf> in the presence of an aqueous medium, resulting in aqueous dispersions of polymer particles. Dynamic light scattering studies revealed the production of colloidally stable polymer nanoparticles with diameters of 153–206 nm, 262–294 nm and 273–278 nm in aqueous media for the Py, MPy and EPy systems, respectively. The particle sizes of poly(N-methylpyrrole) (PMPy) and poly(N-ethylpyrrole) (PEPy) were larger than those of polypyrrole (PPy), which could be due to the greater hydrophobicity of MPy and EPy than Py. The particles could achieve colloidal stability through an electrostatic stabilization mechanism, as the polymerization process introduces cationic charges to the polymers via doping. Larger amounts of hydroxy and carbonyl groups were introduced into PMPy and PEPy because of the easier overoxidation of MPy and EPy due to their lower redox potentials than that of Py. Furthermore, the resulting particles could adsorb on oil‒water interfaces and work as effective Pickering-type emulsifiers. Suspension polymerization of vinyl monomer-in-water Pickering emulsions stabilized with PPy and PMPy nanoparticles resulted in the production of nanoparticle-coated polymer microparticles with diameters of 25 μm and 154 μm, respectively.

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Data-driven finding of organic anode active materials for lithium-ion batteries from natural flower scent products using capacity predictors

Tobita H., Sakano K., Imai H., Yamashita Y., Oaki Y.

Journal of Materials Chemistry A 2025

ISSN 20507488

Organic electrode-active materials are important for the development of metal-free, high-performance energy storage devices without resource consumption. Further exploration at an accelerated pace is necessary to find new compounds that exhibit high performances in an infinite number of organic molecules. Exploration based on professional experience, research expertise and intuition has its limitations. In the present work, potential compounds for organic anode active materials are efficiently found in the natural products of flower scents. A total of 65 potential compounds with conjugated moieties are extracted from the original data by the initial screening. Prior to the experiments, eight compounds are selected as candidates using the capacity predictor. Two compounds, 1,4-dichlorobenzene and 6-methyl-2-pyridinecarboxyaldehyde, actually exhibit higher specific capacities of 532 and 293 mA h g−1 with subtraction of the capacity originating from conductive carbon at the current density of 100 mA g−1. Polymerizable structural analogues are found from further exploration. The polymer of pyrrole-2-carboxyaldehye as an analogue exhibits a specific capacity of 934 mA h g−1 at 100 mA g−1. Thus, a couple of new potential anode active materials have been successfully found by efficient exploration using the capacity predictor in natural products.

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Mechanochromic Chameleon Packaging Based on Polydiacetylene

Das B., Uchikura Y., Matsuhisa N., Oaki Y., Pennington M., Sugihara K.

ACS Sensors 9 ( 12 ) 6844 - 6851 2024.12

We demonstrated a mechanochromic chameleon packaging that quantitatively visualizes damage based on the polydiacetylene blue-to-red color transition. The applied pressures on the damage can be conveniently read by taking a photograph via a homemade mobile app called the “Pressure Analysis App”. The key aspects of the development were 1) the adjustment of the sensitivity by adding guest molecules into the PDA matrix, 2) quantitative calibration, and 3) the accurate reading of the pressure from RGB images based on the calibration. The force sensitivity of polydiacetylene on cellulose was characterized in the range of 30-100 nN (2.59-8.86 MPa) at the nanoscale by dual friction force/fluorescence microscopy, and in the range of 0.7-2.2 N (0.11-0.35 MPa) at the macroscopic scale by a friction tester. The scanning speed was found to be one of the key parameters, where the force sensitivity above 50 mm/s was 6.45 times larger than that below 10 mm/s. The developed mechanochromic packaging system can be used for transporting electronics and medical tools, such as vaccine vials, food, etc., more safely and reliably in the future.

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A biogenic geodesic dome of the silica skeleton in Phaeodaria

Yamaguchi M., Nakamura Y., Watanabe H., Kimoto K., Oaki Y., Shimode S., Imai H.

Scientific Reports 14 ( 1 ) 2024.12

Unique architectures of microbial skeletons are viewed as a model for the architectural design of artificial structural materials. In particular, the specific geometric arrangement of a spherical skeleton 0.5–1.5 mm in diameter of shell-bearing protists, Phaeodaria (Aulosphaera sp.), is remarkably interesting because of its similarity to a geodesic polyhedron, which is a hollow framework with 6-branched nodes that requires minimal building material for maximal strength. A phaeodarian skeleton composed of silica rods 5–10 µm in diameter was characterized as a distorted dome that is based on an icosahedron sectioned with a 7-frequency subdivision. The major difference of the biogenic architecture from the ideal geodesic dome is the coexistence of 7- and 5-branched nodes with the distortion of the frames and the presence of radial spines. From a microscopic perspective, the frames and radial spines were revealed to be hollow tubes having inner fibers and lamellar walls consisting of silica nanoparticles 4–8 nm in diameter with interlayer organic matter. The high degradability of the silica skeleton in seawater after cell mortality is ascribed to the specific nanometric composite structure. The biological architectonics sheds light on the production of environmentally friendly, lightweight structural materials and microdevices.

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Silver to Gold Metallic Luster Changes in Stimuli-Responsive Diacetylene Derivatives Uniquely Arranged within Crystals

Tachibana K., Kojima Y., Masu H., Ichikuni N., Takahashi H., Akiyama K., Nakamura K., Kobayashi N., Ichikawa S., Kondo Y., Oaki Y., Matsui J., Okada S., Omatsu T., Kishikawa K., Kohri M.

ACS Applied Materials and Interfaces 16 ( 46 ) 63904 - 63913 2024.11

ISSN 19448244

Eye-catching metallic luster materials, especially those whose colors can be controlled by external stimuli, have many potential applications. Here, we present a silver luster material that changes color to gold upon UV irradiation. Diacetylene (DA) derivatives with stilbenes introduced via linkers at both ends (DS-DAn (n = 1-6)) exhibited significantly different metallic luster and color change behaviors depending on the linker carbon number (n). The results revealed that the stacked structure of platelet crystals consisting of DS-DA1 with the shortest linker carbon chain exhibited a silver luster and changed its appearance to gold upon UV irradiation; this was an exceptional property of this material. More importantly, we found a unique crystal structure formed by DS-DA1, where the two assembled states coexisted. Partial topochemical polymerization of DA within this unique crystal structure dramatically changed its color from silver to gold. The findings of this study not only contribute to the development of the basic science of DA polymerization but also facilitate the development of new applications of metallic luster materials due to their attractive features that are adaptable to photomask patterning and UV laser lithography.

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