Enhancement of the Hydrogen-Bonding Network of Water Confined in

Article pubs.acs.org/Langmuir

Enhancement of the Hydrogen-Bonding Network of Water Confined in a Polyelectrolyte Brush Kosuke Yamazoe,† Yuji Higaki,‡,§,∥ Yoshihiro Inutsuka,‡ Jun Miyawaki,†,⊥,# Yi-Tao Cui,# Atsushi Takahara,‡,§,∥ and Yoshihisa Harada*,†,⊥,# †

Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan ⊥ Institute for Solid State Physics, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277-0882, Japan ‡ Graduate School of Engineering, §Institute for Materials Chemistry and Engineering, and ∥International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku Fukuoka 819-0395, Japan # Synchrotron Radiation Research Organization, The University of Tokyo, 1-490-2, Kouto, Shingu-cho, Tatsuno, Hyogo 679-5165, Japan S Supporting Information *

ABSTRACT: Water existing in the vicinity of polyelectrolytes exhibits unique structural properties, which demonstrate key roles in chemistry, biology, and geoscience. In this study, X-ray absorption and emission spectroscopy was employed to observe the local hydrogen-bonding structure of water confined in a charged polyelectrolyte brush. Even at room temperature, a majority of the water molecules confined in the polyelectrolyte brush exhibited one type of hydrogen-bonding configuration: a slightly distorted, albeit ordered, configuration. The findings from this study provide new insight in terms of the correlation between the function and local structure of water at the interface of biological materials under physiological conditions.

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hydrogen-bonding network.11 The formation of a hydration layer of polyelectrolytes and the resulting hydrogen-bonding network is expected to be crucial to controlling the properties of biomolecules, such as lubrication12 and antifouling.13,14 In this study, a novel experimental approach for investigating the hydrogen-bonding structure of water confined in a polyelectrolyte brush is developed on the basis of synchrotron-based soft X-ray absorption (XAS) and X-ray emission spectroscopy (XES). XAS and XES probe element-specific unoccupied and occupied electronic structures, respectively, through the excitation of a core level and the de-excitation of the thus-created core hole of a particular element.15,16 The combination of XAS and XES has long been applied to solids to study the origin of their electronic and/or magnetic properties in terms of the electronic structure but has recently expanded its use to liquid systems.17 As shown in Figure 1, the O 1s XAS/ XES spectrum of water in the gas phase exhibits five characteristic molecular orbitals, two in the unoccupied state and three in the occupied, which are strongly modulated upon condensation to liquid and ice phases. Detailed analyses of the distinct electronic structures from XAS−XES spectra and

INTRODUCTION In the crowded environment of a living cell, several biomolecular polyelectrolytesespecially proteins, nucleic acids, and complex sugarsare compressed together. Water encapsulated between the aforementioned entities is no longer a simple space-filling medium but is believed to exhibit intriguing hydrogen-bonding networks depending on its molecular dimensions and interfacial properties.1 This unique hydrogen-bonding structure of water in the vicinity of polyelectrolytes should significantly affect the specific structure and functions of biomolecules and their assemblies.2−5 Hence, it is imperative to devise a novel method for investigating the local hydrogen-bonding structure of water near polyelectrolytes, which can aid in the understanding of the functions of biological systems. A polyelectrolyte brush6−10a surface-tethered polymer layeris a proper model interface of polymeric soft materials, such as proteins.10 In highly concentrated or nearly saturated aqueous solutions, neighboring polyelectrolytes are separated by a distance of only a few nanometers. In such an environment, a layer of water is confined in the space between the crowded polyelectrolytes, which have numerous hydrophilic and hydrophobic sites. A recent study using microscopic infrared (IR) spectroscopy indicated that water molecules are confined in a polyelectrolyte brush with a highly ordered © XXXX American Chemical Society

Received: January 26, 2017 Revised: March 20, 2017 Published: March 30, 2017 A

DOI: 10.1021/acs.langmuir.7b00243 Langmuir XXXX, XXX, XXX−XXX

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SPring-8 synchrotron radiation facility using the BL07LSU31 HORNET station.32 A silicon carbide (SiC) membrane with polyelectrolyte brushes was used to separate the flow of liquid or water vapor from high vacuum and to transmit incident and emitted soft X-rays, as shown in Figure 2 and Figure 3S. Water vapor was supplied by steam-generation equipment (HUM-1, Rigaku Corporation, Tokyo, Japan), and nitrogen was used as the carrier gas. Ultrapure water (Direct-Q, Millipore Inc., Billerica, MA) was used as the water source. Moisture was passed over the polyelectrolyte brush and then drained from the outlet port.

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RESULTS AND DISCUSSION Figure 3a compares the O 1s XES spectra of the water confined in the PMTAC brush, bulk liquid water, and ice Ih.33 The raw O

Figure 1. O 1s XAS and XES spectra for three forms of H2O referring to XES spectra of ice,33 liquid, and gas18 and XAS spectra of ice, liquid, and gas.22

temperature-dependence data provide unique, constructive information about the hydrogen-bonding network of liquid water.18−30 Among them, the most intriguing feature is the presence of two 1b1-derived peaks (1b1′ and 1b1″) in the O 1s XES spectrum of liquid water, whose origin has long been debated. However, there is a clear consensus for the interpretation of ice and gas-phase water; for the case of fully hydrogen-bonded ice Ih,33 the lower-energy 1b1′ peak dominates, whereas for the less-hydrogen-bonded water in water/acetonitrile mixtures34 or non-hydrogen-bonded gasphase molecules,18,20 the higher-energy 1b1″ peak dominates. This study sheds light on the local hydrogen-bonding structure of water confined in a charged polyelectrolyte brush and reveals that the hydrogen-bonding structure of water confined in the polyelectrolyte brush is very similar to the ice Ih, even at room temperature.

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EXPERIMENTAL SECTION

Synthesis of a Polyelectrolyte Brush. The polyelectrolyte brush was prepared by surface-initiated atom-transfer radical polymerization (SI-ATRP) on a 150-nm-thick SiC membrane with a Au layer on the top (NTT Advanced Technology Co., Kanagawa, Japan). First, a commercially available surface initiator, bis[2-(2-bromoisobutyryloxy)undecyl] disulfide (Sigma-Aldrich, Tokyo, Japan), was immobilized on the Au-coated SiC membrane by immersing it in an ethanol solution of the initiator (5 mM) at 40 °C for 12 h. Next, SI-ATRP of 2(methacryloyloxy)ethyltrimethylammonium chloride (MTAC) was performed in the presence of the surface-modified SiC membrane according to a reported procedure.9 Figure 2 shows the chemical structure of the poly-MTAC (PMTAC) brush. XAS and XES Experiments. O 1s XAS and XES experiments of water confined in the polyelectrolyte brush were performed at the

Figure 3. (a) O 1s soft X-ray emission spectra of water confined in the polyelectrolyte brush, the dry brush measured in vacuum, liquid H2O, and ice Ih.33 The excitation energy is 550.3 eV, which is well above the ionization threshold. (b) Soft X-ray absorption spectra of water confined in the polyelectrolyte brush, dry brush measured in vacuum, liquid H2O22 at room temperature, and Ih ice.36 (c) Resonant XES spectra of water confined in the polyelectrolyte brush at room temperature across the O 1s edge excitation. Excitation energies for resonant XES measurements are indicated by the letters in part b and denoted by labels on the left side of each spectrum. The XES spectrum of the PMTAC brush in a humidified environment is obtained by selective excitation. At the excitation energies of A−E, the XES spectra of the confined water alone are shown by subtracting the contribution of the dry PMTAC brush from the raw XES spectra of the water in the PMTAC brush at each energy. All of the spectra are normalized by the area under the curve. (d) Schematic of the hydrogen-bonding structure of water confined in the PMTAC brush.

1s XES spectrum of the water in the PMTAC brush is the summation of X-ray emission from oxygens in the confined water and PMTAC brush. Hence, the XES spectrum of the confined water alone is shown in Figure 3a as “Confined water” by subtracting the XES spectrum of the dry (under high vacuum state) PMTAC brush (represented by “Dry brush”) from the raw XES spectrum of the water in the PMTAC brush. The intensities of the XES spectra of the confined water and

Figure 2. Schematic of the experimental setup for the XES of water confined in the PMTAC brush. Also shown is the chemical structure of the PMTAC brush on a Au-coated SiC substrate. B


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specific XAS features representing the characteristic hydrogenbonding configuration, to directly combine the XES and XAS spectra.19,38 Again, the pre- (535 eV) and main-edge (537−538 eV) peaks are attributed to distorted hydrogen bonds, whereas the post-edge (540−541 eV) peaks are attributed to tetrahedral hydrogen bonds. Figure 3c shows the resonant XES spectra of water confined in the PMTAC brush, obtained by tuning the excitation energy across the XAS spectrum in Figure 3b, i.e., nonresonant (550.3 eV), post-edge (539.9 eV), above or below the main edge (537.9 eV/536.25 eV), and pre-edge (534.7 eV) excitations. Similar to the nonresonant excitation shown in Figure 3a, when the excitation energy is tuned to the onset of the post-edge (539.9 eV), only one peak (1b1′) is observed in the lone-pair region. At lower excitation energies (534.7−537.9 eV), this 1b1′ peak splits into two subpeaks: one remains at the same energy, and the other shifts to lower energy. The latter is typical of soft X-ray Raman scattering, although the energy shift is significantly less than the excitation energy. This result can be explained by the screening effect of the excited electron similar to the case of the 1b1″ component of bulk liquid water.26 The relative intensity of the shifted 1b1′ component increases with decreasing excitation energy, which can be simply explained by the effective lifetime of the excited electron. At a resonance excitation of around 539.9 eV, the excited electron dissipates within the lifetime of the core hole, resulting in a fluorescencelike component, which remains at the same emission energy. On the other hand, detuning from the resonance at 539.9 eV decreases the effective lifetime of the excited electron such that the electron does not have sufficient time to dissipate; this results in the predominance of the Raman-like component. As there is only one peak in the lone-pair region (peak 1b1′) at nonresonant excitation, the two split components are attributed to the water molecules in the same (tetrahedrally coordinated but slightly distorted) hydrogen-bonding configuration. Thus, the dependence on excitation energy also indicates that water confined in the PMTAC brush has an almost homogeneous hydrogen-bonding configuration, i.e., tetrahedrally coordinated, albeit slightly distorted. We will not exclude a possible contribution of the 1b1″ component that would be enhanced at the pre-edge resonance as appearing around 526 eV on the high-energy side of the 1b1′ component, although its contribution is at least 1 order of magnitude smaller than that of the bulk liquid water. It is noted that the XES spectra are totally dominated by the water because there is a clear difference in the XAS of the dry brush over the energy range from 534.7 to 550.3 eV compared to the XAS spectrum of confined water. One possible explanation for the absence of 1b1″ in Figure 3a is that the geometrical restrictions imposed by the interspace of the polymer chain enhances the formation of hydrogen bonds among confined water molecules. For the same reason or under an electric field created by an effective charge on the polymer surface as mentioned above, a certain hydrogen-bonding distortion may be introduced. Water molecules located near the hydrophobic groups in the polymers can form a distorted hydrogen bond structure with perturbed molecular dynamics.39 Sum-frequency vibrational spectroscopy has indicated a distorted icelike hydrogen-bonding network formed by the water molecules at the silica/water surface.40 A previous neutron diffraction study has suggested that the hydrogen bonding between water molecules is enhanced in ordered mesoporous silica.41 An IR study of water encapsulated in reverse micelles has revealed unique freezing behavior with the

dry PMTAC brush reflect the proportions in the raw spectrum. The XES spectra of the bulk liquid water and ice Ih are shown with the same intensity of peak 1b1′ as that of confined water to compare the spectral shape. The XES spectra of the confined water and ice Ih are very similar; the 1b1′′ peak, which is observed in the spectrum of bulk liquid water, is negligible. The similarity between the two XES spectra is quite surprising as a significant enhancement in the hydrogen-bonding structures is observed in the water confined in the PMTAC brush even at room temperature. Enhanced spectral intensity is observed around the 3a1 region for the confined water, as compared to that in ice Ih and bulk liquid water. The 3a1 peak, which is attributed to the dipole-forbidden transition (3a1 → 1a1) in tetrahedral symmetry, is not observed for ice Ih and is smeared out in the case of bulk liquid water because of the high sensitivity of the 3a1 orbital to the distribution of the hydrogenbond strength. Therefore, the enhancement of the 3a1 peak observed for the confined water can be explained by the fact that the hydrogen bonds are distorted (not straight O−H···O bonds), albeit somewhat ordered, in the PMTAC brush. An additional faint structure at 517.5 eV is attributed to the electronic states resulting from the bonding between the 1b2 orbital of H2O and Cl− 3s.35 Figure 3b shows a comparison of the XAS spectrum of water confined in the PMTAC brush with the spectra of bulk water and ice Ih.22,36 The XAS spectra of the confined water alone and the dry PMTAC brush are shown in Figure 3b as “Confined water” and “Dry brush”, respectively, in the same way as for the XES spectra. The XAS spectra of the bulk liquid water and ice Ih are shown with the same intensity of XAS as that of the confined water at a cross-point of around 545 eV. The XAS preedge at 535 eV is mainly attributed to weakened hydrogen bonds,21,22 and the XAS post-edge at 540−542 eV is attributed to tetrahedrally coordinated hydrogen bonds.19,22,24 The main edge at around 537 eV is enhanced by the formation of various high-density-water24-like crystalline phases of high-pressure ice, such as IIII, IVI, IVII, and IVIII.25 The XAS spectrum of confined water almost lacks the pre-edge feature that serves as an indicator of the presence of strongly distorted hydrogen bonds in bulk liquid water.19,21,22 Similar to the discussion for XES, the absence of the XAS pre-edge is indicative of a fully hydrogen-bonded structure for water in the PMTAC brush. The post-edge, which is observed at 540.5 eV in ice Ih, is shifted to 539.5−540.0 eV for water confined in the PMTAC brush. The post-edge shift in the PMTAC brush also indicates a slightly distorted but somewhat ordered hydrogen-bonding configuration,25 as expected from the XES results. According to Odelius et al., the post-edge position should follow the bond length by 8 eV/Å, and the approximately 0.5−1 eV shift would correspond to an average bond elongation of ∼0.1 Å at the maximum or at a slight distortion from the tetrahedral coordination.37 We speculate that the elongation or distortion can be introduced thermally or by an electric field created by an effective charge on the polymer surface but is not so strong as to lead to enhanced pre-edge intensity. Thus, the results obtained from XAS and XES analyses indicate the complete hydrogen-bonding configuration of water in the PMTAC brush with slight distortion, which serves as clear evidence for the structure of water in PMTAC brushes. The hydrogen-bonding configuration is also supported by the red shift of the IR absorption peak for the OH stretching vibration.11 In addition, resonance excitation was utilized for the XES measurements, which allows for the selective excitation of C


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Langmuir decreasing size of the water pool.42 Murakami et al.11 have reported that structural water in the PMTAC brush is one of the factors responsible for the prevention of complete water droplet wetting of the surface. The estimated dimensions of water between PMTAC polymers are 1.5 nm2 on average (Supporting Information Table 1S). This value is consistent with that reported in the study by Ohba43 where predominant icelike cluster formation is induced in a 2−3 nm carbon nanotube, as observed by hybrid reverse Monte Carlo simulations. Thus, the results obtained herein are in good agreement with those reported previously and will provide more details about the hydrogen-bonding configuration of water by the comparison of appropriate XAS−XES calculations. Recently, a strong hydrogen-bonding configuration has been reported for water at the interface between polymer and water.44 Strong hydrogen-bonding structures of water in swollen polymer brushes can possibly suppress protein adsorption, affording biocompatibility.45 For antifreeze protein Maxi,46 the interior of the four bundled helices is filled with an unconventional amount of strongly hydrogen-bonded water well connected to the external surface. Thus, the presence of highly structured interfacial water is correlated to the specific functional states of proteins. We believe that XAS−XES can also contribute to the common understanding of protein folding by shedding light on the network structure of hydrated water.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yi-Tao Cui: 0000-0001-7104-0059 Yoshihisa Harada: 0000-0002-4590-9109 Author Contributions

All authors contributed to the research design. K.Y., J.M., Y.T.C., and Yo.H. conducted X-ray emission spectroscopy and Xray absorption spectroscopy measurements. K.Y. analyzed experimental data. Y.I. and Yu.H. synthesized and characterized the polyelectrolyte brush. All authors discussed the results and commented on the manuscript. K.Y., Yu.H., and Yo.H. wrote the article. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan. This work was performed under the Research Program for Next Generation Young Scientists of “Network Joint Research for Materials and Devices: Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials.” Experiments at SPring-8 BL07LSU were performed jointly by the Synchrotron Radiation Research Organization and the University of Tokyo (proposal nos. 2015B7403 and 2016A7507).

CONCLUSIONS

The local structure of water confined in a polyelectrolyte brush is observed by XAS−XES spectroscopy. Estimated from the signal intensity, the average dimension of water confined in the polyelectrolyte brush is calculated to be approximately 1.5 nm2. Our XAS−XES results confirm that these techniques are extremely sensitive to the hydrogen-bonding states. A majority of the water molecules in the polyelectrolyte brush are held together by one type of hydrogen-bonding configuration: possibly a slightly distorted but ordered hydrogen-bonding configuration even at room temperature. The distorted picture is only a speculation from the emergence of the 3a1 state and the consideration of the possible hydrogen-bond structure in the vicinity of the charged site, which needs further study from both experimental and theoretical aspects. With the improvement in theory, we will be able to obtain a detailed hydrogenbonding structure of water using these XAS-XES technique, which will develop a basic understanding of the relationship between the local structure of water confined in the polyelectrolyte brush and the characteristics of polyelectrolytes, such as proteins. Thus, we believe that XAS−XES will pave the way for novel, important research fields in the near future.

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of water in the polymer brush. Experimental XES and XAS setup using a liquid flow cell. (PDF)

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ABBREVIATIONS XAS, X-ray absorption spectroscopy; XES, X-ray emission spectroscopy; IR, infrared; PMTAC, poly[2-(methacryloyloxy)ethyltrimethylammonium chloride]; SI-ATRP, surface-initiated atom-transfer radical polymerization

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00243. Synthesis of a polyelectrolyte brush. Estimation of the dimensions of water in the PMTAC brush. Comparison of calculated and experimental data from integrated XES intensities. O 1s soft X-ray emission spectra of confined water in a polyelectrolyte brush, dry brush, and liquid H2O. Schematic of a polymer brush and the dimensions D


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