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Scientific Reports
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10.1038/s41598-024-71649-z
Article
Additive-assisted macroscopic self-assembly and control of the shape of assemblies based on host–guest interaction
http://orcid.org/0000-0003-0061-3432
Hashidzume Akihito hashidzume@chem.sci.osaka-u.ac.jp

1
Itami Takahiro 1
Nakahata Masaki 1
Kamon Yuri 1
http://orcid.org/0000-0002-4801-5071
Yamaguchi Hiroyasu 1
Harada Akira harada@chem.sci.osaka-u.ac.jp

2
1 https://ror.org/035t8zc32 grid.136593.b 0000 0004 0373 3971 Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043 Japan
2 https://ror.org/035t8zc32 grid.136593.b 0000 0004 0373 3971 The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047 Japan
5 9 2024
5 9 2024
2024
14 2067619 7 2024
29 8 2024
© The Author(s) 2024
2024
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In these decades, considerable attention has focused on supramolecular polymers due to their unique structures and properties. More recently, macroscopic supramolecular polymers have attracted increasing interest from not only biologists but also materials scientists inspired by the sophisticated structures and functions of living organisms. Since the functions of supramolecular polymers are strongly dependent on their shape, control of the shape is an important issue in controlling the functions of supramolecular polymers. However, the control of shape in macroscopic supramolecular assemblies has not yet been sufficiently investigated. Previously, we studied the macroscopic self-assembly behavior of super absorbent polymer (SAP) microparticles modified with β-cyclodextrin (βCD) and adamantane (Ad) residues (βCD(x)-SAP and Ad(y)-SAP microparticles, where x and y are the mol% contents of βCD and Ad residues, respectively). More elongated assemblies were formed at higher y, indicating that the shape of assemblies can be controlled by varying the interaction strength. The noteworthy is that 1-adamantanamine hydrochloride (AdNH3Cl) assisted the formation of assemblies from βCD(x)-SAP and Ad(y)-SAP microparticles, indicating that AdNH3Cl acts as a chemical stimulus for macroscopic assemblies of βCD(x)-SAP and Ad(y)-SAP microparticles. In this study, we have thus studied the assembling behavior of βCD(x)-SAP microparticles with Ad(y)-SAP microparticles and unmodified SAP microparticles assisted by AdNH3Cl, as well as the shape of the resulting macroscopic assemblies. AdNH3Cl assisted the formation of assemblies from βCD(16.2)-SAP and Ad(15.1)-SAP microparticles, in which AdNH3Cl crosslinked the SAP microparticles through the formation of inclusion complexes of βCD residues with the Ad residue and the electrostatic interaction of ammonium and carboxylate residues. Assemblies of βCD(26.7)-SAP and unmodified SAP microparticles were formed at the concentrations of AdNH3Cl ([AdNH3Cl]0) higher than a certain level (ca. 0.05 mM). The aspect ratio (a/b) of assemblies showed a maximum at [AdNH3Cl]0 ~ 0.10 mM, indicating that the chemical stimulus, i.e., addition of AdNH3Cl, controls the shape of assemblies formed from βCD(26.7)-SAP and unmodified SAP microparticles. This study suggests that other stimuli, e.g., heat, pH, light, redox, and force, can be utilized to control the shape of macroscopic assemblies based on supramolecular interactions.

Keywords

Macroscopic self-assembly
Host–guest interaction
Super absorbent polymer microparticles
Cyclodextrin
Amines
Subject terms

Supramolecular polymers
Polymers
issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Living organisms are macroscopic supramolecular polymers with sophisticated functions for sustaining life activities. In living organisms, supramolecular structures are formed by molecular recognition through non-covalent bonds (e.g., hydrogen bonding, electrostatic interaction, and hydrophobic interaction) using biological molecules (e.g., nucleic acids, proteins, polysaccharides, and lipids) as building blocks. These supramolecular structures are further accumulated to construct macroscopic supramolecular polymers1–3. Inspired by the functions of living organisms, researchers have synthesized supramolecular polymers composed of artificial molecules, and a variety of functions have been reported, including the formation and dissociation in response to external stimuli, and the control of physical properties through conformational changes4–25. More recently, a number of macroscopic supramolecular polymers have been constructed utilizing millimeter-scale scaffolds (i.e., gel and polymer) through molecular recognition of interaction residues26–39. Macroscopic assemblies with controlled sequences or stimuli-responsiveness have been also realized40–47. The formation of these macroscopic assemblies is based on microscopic interactions of interacting residues at the molecular level. Since the microscopic interactions should be in equilibrium, the formation of macroscopic assemblies is partly explained by equilibrium27. However, some points cannot be explained by equilibrium in the macroscopic assemblies presumably because of a kinetically-frozen state based on the slow dynamics of crosslinked polymer chains and multivalency. It is important to bridge the gap between macroscopic and microscopic interactions for precise control of the formation of macroscopic assemblies through molecular design.

Not only in supramolecular structures in living organisms but also in artificial supramolecular polymers, their functions are based on the shape. Thus, control of the shape is an important issue in controlling the function of supramolecular polymers. However, the control of shape in macroscopic supramolecular assemblies has not yet been sufficiently investigated. Previously, we selected super absorbent polymer (SAP) microparticles (i.e., crosslinked poly(sodium acrylate) microparticles) as a scaffold and studied the macroscopic self-assembly behavior of SAP microparticles modified with β-cyclodextrin (βCD) and adamantane (Ad) residues (βCD(x)-SAP and Ad(y)-SAP microparticles, where x and y are the mol% contents of βCD and Ad residues, respectively)48. Among the βCD(x)-SAP microparticles of x = 16.2, 22.3, and 26.7, only βCD(26.7)-SAP microparticles formed assemblies with all the Ad(y)-SAP microparticles of y = 5.2, 7.1, 10.7, and 15.1. The shape of assemblies was estimated using the aspect ratio (a/b, where a and b are the longer and shorter axes, respectively) determined by ellipse fit in the Image-J software49. The a/b value of assemblies increased with increasing y, indicating that the shape of assemblies can be controlled by varying the interaction strength.

In the previous study, βCD and sodium 1-adamantanecarboxylate (AdC) were utilized as competitors to assess the interaction strength of βCD(26.7)-SAP and Ad(y)-SAP microparticles, in which βCD and AdC capped Ad and βCD residues on the SAP microparticles, respectively, to reduce the effective concentration of the interaction residues. Analysis using a simplified model50 confirmed that the interaction between βCD(26.7)-SAP and Ad(y)-SAP microparticles became stronger as y increased. The noteworthy is that 1-adamantanamine hydrochloride (AdNH3Cl) assisted the formation of assemblies from βCD(x)-SAP and Ad(y)-SAP microparticles when similar interaction experiments were conducted in the presence of AdNH3Cl. This observation indicates that AdNH3Cl acts as a chemical stimulus for macroscopic assemblies of βCD(x)-SAP and Ad(y)-SAP microparticles. In this study, we have thus studied the self-assembly behavior of βCD(x)-SAP microparticles with Ad(y)-SAP microparticles and unmodified SAP microparticles assisted by AdNH3Cl, as well as the shape of the resulting macroscopic assemblies.

Results

The interaction experiments were performed for a mixture of βCD(16.2)-SAP and Ad(5.2)-SAP microparticles, which did not form assemblies48, using AdNH3Cl as an additive (Figs. 1a and b). In the presence of 0.05 mM AdNH3Cl, βCD(16.2)-SAP and Ad(5.2)-SAP microparticles did not form assemblies. In the presence of 0.50 mM or higher concentrations of AdNH3Cl, on the other hand, the microparticles formed assemblies. These observations indicate that AdNH3Cl assisted the formation of assemblies from βCD(16.2)-SAP and Ad(5.2)-SAP microparticles.Fig. 1 Interaction of βCD(x)-SAP and Ad(y)-SAP microparticles in the presence of AdNH3Cl. (a) Chemical structures of βCD(x)-SAP and Ad(y)-SAP microparticles, and AdNH3Cl. (b) Typical examples of optical micrograms for assemblies formed from βCD(16.2)-SAP and Ad(15.1)-SAP microparticles in the presence of varying concentrations of AdNH3Cl ([AdNH3Cl]). The bars indicate 100 μm. (c) Conceptual illustration of the formation of assemblies from βCD(x)-SAP and Ad(y)-SAP microparticles assisted by addition of AdNH3Cl.

It should be noted here that the diameters of βCD(16.2)-SAP and Ad(5.2)-SAP microparticles decreased with increasing AdNH3Cl concentration ([AdNH3Cl]0) presumably because of a reduced osmotic pressure of SAP microparticles upon addition of AdNH3Cl (Fig. 1b). The decrease in the diameters of βCD(x)-SAP and Ad(y)-SAP microparticles increased the concentrations of βCD and Ad residues on the surface of the SAP microparticles, which might induce the formation of assemblies. To test how effective the increased surface concentrations of βCD and Ad residues were, the interaction of βCD(16.2)-SAP and Ad(15.1)-SAP microparticles was investigated in the presence of varying concentrations of NH4Cl ([NH4Cl]0). As can be seen in Figure S1 in Supplementary Information, the SAP microparticles became smaller, but no assemblies were formed, indicating that the increased surface concentrations of βCD and Ad residues alone did not promote the formation of assemblies.

The interaction of βCD(16.2)-SAP and Ad(15.1)-SAP microparticles was also investigated in the presence of several ammonium chlorides as additives. The results are summarized in Table 1. Neither tetramethylammonium chloride (TMAC) nor n-butylammonium chloride (nBuNH3Cl) assisted the formation of assemblies. On the other hand, t-butylammonium chloride (tBuNH3Cl), dodecyltrimethylammonium chloride (DTAC), and AdNH3Cl assisted βCD(16.2)-SAP and Ad(15.1)-SAP microparticles to form assemblies. Table 1 also contains the binding constants (K) for the formation of inclusion complexes of βCD with these ammonium chlorides. Notably, even though tBuNH3Cl shows a rather smaller binding constant (5.3 M−1), tBuNH3Cl assisted the formation of assemblies from βCD(16.2)-SAP and Ad(15.1)-SAP microparticles. These observations indicate that the formation of inclusion complexes of βCD residues with the guest residues in ammonium chlorides assisted the formation of assemblies51,52.Table 1 Interaction of βCD(16.2)-SAP and Ad(15.2)-SAP microparticles in the presence of additives.

additive	assemblya	K/M−1	
AdNH3Clb	 + 	8.43 × 103c	
AdCd	–	4.2 × 104c	
nBuNH3Cle	–	3.3 × 10−1f	
tBuNH3Clg	 + 	5.3 h	
TMACi	–	–	
DTACj	 + 	1.3 × 103h	
a. + indicates assemblies are formed and—indicates no assemblies are formed. b. Admantanamine hydrochloride. c. From ref.53. d. Sodium adamantanecarboxylate. e. n-Butylamine hydrochloride. f. From ref.54. g. t-Butylamine hydrochloride. h. Determined in this study (see Figure S2 in Supplementary Information). i. Tetramethylammonium chloride. j. Dodecyltrimethylammonium chloride.

In our previous study48, we conducted adsorption experiments of mono-(6-amino-6-deoxy)-βCD (βCD-NH2) and AdNH3Cl onto SAP microparticles to characterize how SAP microparticles were modified with βCD-NH2 and AdNH3Cl. We observed that the amount of AdNH3Cl adsorbed was markedly smaller than that of βCD-NH2, indicating that AdNH3Cl molecules were adsorbed onto the outer layer of SAP microparticles because AdNH3Cl did not penetrate the interior of microparticles for the hydrophobicity of Ad residue. Apparent ζ-potential data were measured by electrophoretic light scattering (ELS) for unmodified SAP microparticles in the presence of varying [AdNH3Cl]0 (Figure S2 in Supplementary Information). The data show that the apparent ζ-potential data are negative and the absolute value of apparent ζ-potential decreases with increasing [AdNH3Cl]0, indicating that AdNH3Cl molecules are adsorbed onto the surface of SAP microparticles. These observations indicate that βCD(16.2)-SAP and Ad(15.1)-SAP microparticles form assemblies in the presence of ammonium chlorides carrying guest residue, in which the ammonium chlorides crosslink the SAP microparticles through the formation of inclusion complexes of βCD residues with the guest residue and the electrostatic interaction of ammonium and carboxylate residues (Fig. 1c). It should be noted here that the ammonium residues interact with three types of carboxylate residues: (1) carboxylate residues in the βCD(x)-SAP microparticle carrying the βCD residue forming inclusion complex with the guest residue in the additive molecule, (2) carboxylate residues in βCD(x)-SAP microparticles other than (1), and (3) carboxylate residues in Ad(y)-SAP microparticles. When the ammonium residue of AdNH3Cl interacts with carboxylate residues of type (1), the AdNH3Cl does not act as a crosslinker between SAP microparticles. On the other hand, when the ammonium residue of AdNH3Cl interacts with carboxylate residues of types (2) and (3), the AdNH3Cl crosslinks SAP microparticles, resulting in promotion of the formation of assemblies.

More interestingly, AdNH3Cl assisted βCD(26.7)-SAP and unmodified SAP microparticles to form assemblies (Fig. 2a and Figure S3 in Supplementary Information). The interaction was investigated for a mixture of βCD(26.7)-SAP and unmodified SAP microparticles in the presence of varying concentrations of AdNH3Cl. Assemblies of βCD(26.7)-SAP and unmodified SAP microparticles were formed at [AdNH3Cl]0 ≥ ca. 0.05 mM. As in our previous study48, the aspect ratios (a/b) were determined by ellipse fit using an ImageJ software49 and plotted in Fig. 2b against [AdNH3Cl]0. The a/b value increases from 1.36 to 2.06 as [AdNH3Cl]0 increases from 0.050 to 0.10 mM, and the a/b value then decreases to 1.86 as [AdNH3Cl]0 further increases to 0.17 mM. Assemblies of a larger aspect ratio (a/b) are likely formed through stronger interactions. Thus, in the ternary system of βCD(26.7)-SAP and unmodified SAP microparticles, and AdNH3Cl, the interaction is likely strongest at [AdNH3Cl]0 ~ 0.10 mM. As [AdNH3Cl]0 increases from 0 to 0.10 mM, the number of crosslinks increases through the formation of inclusion complexes of βCD residues with the Ad residues and the electrostatic interaction between ammonium and carboxylate residues in SAP microparticles. However, at [AdNH3Cl]0 ≥ 0.10 mM, the electrostatic interaction is weaker presumably because of shielding of the electrostatic interactions at higher [AdNH3Cl]0, leading to the reduced strength of interaction.Fig. 2 Interaction of βCD(26.7)-SAP and unmodified SAP microparticles in the presence of AdNH3Cl. (a) Typical examples of optical micrograms for assemblies formed from βCD(26.7)-SAP and unmodified SAP microparticles in the presence of varying concentrations of AdNH3Cl ([AdNH3Cl]0). The bars indicate 100 μm. (b) The aspect ratio (a/b) for the assemblies formed from βCD(26.7)-SAP and unmodified SAP microparticles in the presence of varying concentrations of AdNH3Cl ([AdNH3Cl]0).

Discussion

Here, we discuss the formation of assemblies assisted by ammonium additives based on a simplified equilibrium model to compare macroscopic and microscopic interactions. As can be seen in Fig. 3a, we assume that the three equilibria are established. Here βCDSAP and AdSAP are the βCD and Ad residues on the βCD(x)-SAP and Ad(y)-SAP microparticles, respectively, and COO–SAP denotes the carboxylate residues on the βCD(x)-SAP and Ad(y)-SAP microparticles. RNH3+ indicates an ammonium additive. We assume that equilibria (i) and (ii) compete whereas equilibrium (iii) is independent. KCD•Ad, KCD•R, and KNH3•COO are the binding constants in the respective equilibria. [βCDSAP∙AdSAP], [βCDSAP∙RNH3+], and [RNH3+∙COO−SAP] denote the molar concentrations of the complexes, respectively. [βCDSAP], [AdSAP], [RNH3+], and [COO−SAP] are the molar concentrations of the respective free components, and [βCDSAP]0, [AdSAP]0, and [COO−SAP]0 are the total molar concentrations of the respective components. As described in Supplementary Information, if KCD•Ad, KCD•R, KNH3•COO, [βCDSAP]0, [AdSAP]0, [COO−SAP]0, and [RNH3+] are given, [βCDSAP•AdSAP], [βCDSAP•RNH3+], and [RNH3+•COO−SAP] can be calculated. Since [RNH3+]0 = [RNH3+] + [βCDSAP•RNH3+], [βCDSAP•AdSAP], [βCDSAP•RNH3+], and [RNH3+•COO−SAP] can be plotted against [RNH3+]0. Here, βCDSAP•AdSAP acts as a crosslinking point for microparticles, and RNH3+ which forms a complex with both βCDSAP and COO–SAP also acts as a crosslinking point for microparticles. (Although there are three types of COO–SAP as described above, the COO–SAP types are ignored here for simplicity.) Thus, the concentration of crosslinking points (CCL) for βCD(x)-SAP and Ad(y)-SAP microparticles is given as1 CCL=βCDSAP·AdSAP+βCDSAP·RNH3+·COO-SAPRNH3+0

Fig. 3 Simplified equilibrium model. (a) Equilibria of βCDSAP, AdSAP, RNH3+, and COO–SAP. The concentration of cross-liking point (CCL) as a function of [RNH3+]0 for (b) βCD(16.2)-SAP and Ad(15.1)-SAP microparticles, and (c) βCD(26.7)-SAP and unmodified SAP microparticles; the swelling ratio of SAP microparticles was fixed at 30: KCD•Ad = KCD•R = 104 M−1: KNH3•COO = 10 M−1.

Since the βCDSAP/AdSAP and RNH3+/COO–SAP interactions occur on the interface of SAP microparticles, [βCDSAP]0, [AdSAP]0, and [COO–SAP]0 can be considered as the concentrations in the SAP microparticles. Thus, these total concentrations are calculated, given x, y, and the swelling ratio of SAP microparticles, for which 30 is chosen as a typical value here. KCD•Ad = KCD•R = 104 M–1 when AdNH3+ is chosen as RNH3+53. For convenience, the binding constant for the NH3+/COO−SAP interaction (KNH3•COO) was assumed to be 10 M–1. The relationships between CCL and [RNH3+]0 are shown in Figs. 3b and c for the combinations of x = 16.2 and y = 15.1, corresponding to the data in Fig. 1, and of x = 26.7 and y = 0, corresponding to the data in Fig. 2, respectively (see also Figure S4 in Supplementary Information). In both cases, CCL increases with increasing [RNH3+]0, indicative of an enhanced interaction of SAP microparticles. (In the real systems, the dissociation of equilibrium (i) may be less likely because of the multivalent interaction of βCD(x)-SAP and Ad(y)-SAP microparticles.) These observations are consistent with the formation of assemblies assisted by the additive. It should be noted that CCL values are much higher for βCD(16.2)-SAP and Ad(15.1)-SAP microparticles than those for βCD(26.7)-SAP and unmodified SAP microparticles. At present, we do not have any reasonable explanation for this discrepancy. But it is likely that the onset of the formation of assemblies cannot be explained based on equilibrium27.

Conclusions

This study dealt with the self-assembly behavior of βCD(x)-SAP microparticles with Ad(y)-SAP and unmodified SAP microparticles assisted by AdNH3Cl, i.e., a chemical stimulus, as well as the shape of the resulting macroscopic assemblies. The interaction experiments were performed for a mixture of βCD(16.2)-SAP and Ad(5.2)-SAP microparticles, which did not form assemblies, using AdNH3Cl as an additive. In the presence of 0.05 mM AdNH3Cl, βCD(16.2)-SAP and Ad(5.2)-SAP microparticles did not form assemblies. In the presence of 0.50 mM or higher concentrations of AdNH3Cl, on the other hand, the microparticles formed assemblies. These observations indicate that AdNH3Cl assists the formation of assemblies from βCD(16.2)-SAP and Ad(5.2)-SAP microparticles. The interaction of βCD(16.2)-SAP and Ad(15.1)-SAP microparticles was also investigated in the presence of several ammonium chlorides as additives. The formation of inclusion complexes of βCD residues with the guest residues in ammonium chlorides assisted the formation of assemblies, in which the ammonium chlorides crosslinked the SAP microparticles through the formation of inclusion complexes of βCD residues with the guest residue and the electrostatic interaction of ammonium and carboxylate residues. The interaction was further investigated for a mixture of βCD(26.7)-SAP and unmodified SAP microparticles in the presence of varying concentrations of AdNH3Cl. Assemblies of βCD(26.7)-SAP and unmodified SAP microparticles were formed at [AdNH3Cl]0 ≥ ca. 0.05 mM. The a/b value of assemblies increased from 1.36 to 2.06 as [AdNH3Cl]0 increased from 0.050 to 0.10 mM, and then decreased to 1.86 as [AdNH3Cl]0 further increased to 0.17 mM. These observations indicate that the chemical stimulus, i.e., addition of AdNH3Cl, somehow controls the shape of assemblies formed from βCD(26.7)-SAP and unmodified SAP microparticles. These observations suggest that other external stimuli, e.g., heat, light, pH, redox, and force can be also utilized to control the formation and shape of macroscopic assemblies. This study should likely provide a significant insight into control of macroscopic assemblies based on microscopic interactions.

Methods

Materials

Superabsorbent polymer 10SH-NF (SAP) microparticles were kindly supplied from Sumitomo Seika Chemicals Co., Ltd. (Osaka, Japan). AdNH3Cl was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). n-Butylamine, t-butylamine, TMAC, and DTAC were purchased from FUJIFILM Wako Pure Chemical Corp. (Osaka, Japan). Pararosaniline was purchased from Nacalai Tesque, Inc. (Kyoto, Japan). Other reagents were used without further purification.

The βCD(x)-SAP and Ad(y)-SAP microparticles prepared in our previous study were also used in this study48. For visual discrimination, the Ad(y)-SAP and unmodified SAP microparticles were dyed by immersing in an aqueous solution of pararosaniline (0.05 M) for 1 h.

n-BuNH3Cl and t-BuNH3Cl were prepared from the corresponding amines and an equimolar amount of hydrochloric acid and recovered by lyophilization.

Electrophoretic light scattering (ELS)

Apparent electrophoretic mobilities for unmodified SAP microparticles were determined using a Zeta-potential & Particle size Analyzer ELSZ-2000ZS (Otsuka Electronics Co., Ltd., Osaka, Japan). The system utilizes a diode laser (λ = 662.80 nm), and data are reported for scattered intensity at an effective scattering angle of 15°, corrected from the mechanical angle for refraction. Since the applied electric field produces flow in the sample cell due to electroosmosis, measurements were made in the stationary layer, determined by locating the cell wall and moving a predetermined distance from it. Apparent values of ζ-potential were calculated using the average values of electrophoretic mobility with Smoluchowski's equation. Sample suspensions were prepared by mixing unmodified SAP microparticles with water or an aqueous solution of AdNH3Cl, which was filtrated by a membrane filter (0.2 µm).

Interaction of βCD(x)-SAP microparticles with Ad(y)-SAP and unmodified SAP microparticles

Suspensions of βCD(x)-SAP and Ad(y)-SAP (or unmodified SAP) microparticles (1 μL each) were placed and mixed on a glass plate. After a predetermined amount of additive was added to the mixture, the mixture was agitated at ca. 500 rpm using an EYELA CM-1000 cute mixer. The assemblies formed were observed on an EVOS optical microscope.

Determination of the binding constants

The binding constants were determined for βCD/DTAC and βCD/tBuNH3Cl binary mixtures by 1H NMR. 1H NMR spectra were obtained for mixtures containing a constant concentration of the guest (or the host) and varying concentrations of the host (or the guest) by a JEOL JNM ECA500 (or ECS400) spectrometer using D2O as a solvent at 25.0 °C. Chemical shifts were referenced to 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt (0 ppm), which was used as an external standard.

Supplementary Information

Supplementary Information.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-71649-z.

Acknowledgements

The authors express their acknowledgment to Professor Yoshinori Takashima, Graduate School of Science, Osaka University, for his fruitful suggestions. The authors thank Professor Shinji Sakai, Graduate School of Engineering Science, Osaka University for his kind support on the ELS measurements.

Author contributions

A. Harada supervised the project; A. Hashidzume, T.I., H.Y., and A. Harada designed the project; T.I., M.N., and Y.K. performed the experiments; A. Hashidzume, T.I., M.N., and Y.K. analyzed data; all the authors discussed the results; A. Hashidzume and A. Harada wrote the paper.

Data availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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