Sum frequency generation study of hydrated cellulose

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Sum frequency generation (SFG) spectra of cotton cellulose fibers were successfully obtained with IR wavenumber from 2800 to 3400 cm-1 . The spectra showed the two peaks at 2840 cm-1 and 2941 cm-1 assigned to symmetric and asymmetric CH2 stretching modes, respectively. There was also a peak obtained at wavenumber 2959 cm-1 assigned to the overtone of H-O-C bending at Fermi resonance with 2941 cm-1 peak.

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Nội dung Text: Sum frequency generation study of hydrated cellulose

Nghiên cứu khoa học công nghệ SUM FREQUENCY GENERATION STUDY OF HYDRATED CELLULOSE Nguyen Duc Long1, Vuong Thi Quynh Huong1, Nguyen Van Kien2, Chu Văn Biên3, Nguyen The Binh1, Hoang Chi Hieu1* Abstract: Sum frequency generation (SFG) spectra of cotton cellulose fibers were successfully obtained with IR wavenumber from 2800 to 3400 cm-1. The spectra showed the two peaks at 2840 cm-1 and 2941 cm-1 assigned to symmetric and asymmetric CH2 stretching modes, respectively. There was also a peak obtained at wavenumber 2959 cm-1 assigned to the overtone of H-O-C bending at Fermi resonance with 2941 cm-1 peak. A spectral band of OH range appeared at 3322 cm-1. Hydration effects on cellulose were demonstrated on the SFG spectra of cellulose samples hydrated in different conditions. These results are the decrease in the peaks’ intensities and moreover the disappearance of two peaks. Explanation and discussion are shown in this paper. Keywords: Nonlinear optics, Sum frequency generation, SFG spectroscopy, Cellulose, Hydrated cellulose, Hydration, Hydrogen bonds. 1. INTRODUCTION Nonlinear optical spectroscopy has developed considerably in recent decades [1]. There have been a number of studies using Second Harmonic Generation (SHG) and Sum Frequency Generation (SFG) to probe surfaces and interfaces as nonlinear effects are forbidden in centrosymmetric environment [2][3]. On the other hand, SFG has high selectivity in molecular vibrational modes with orientation and sensitivity for chirality, thus it is a powerful tool to study biological molecules in the biomaterials [4][5][6]. Sum Frequency Generation is a second-order nonlinear optical response from a system where optical centrosymmetry is broken [7]. In SFG spectroscopy, the frequency of SFG signal is the sum of frequencies of two incoming beams, visible beam (VIS) and infrared beam (IR). The intensity is given by: [5] ( ) ( ) ( )∼ ( ) ( ) ( ) Here, intensity is proportional to the effective 2nd order susceptibility squared ( ) and the product of the intensities of the incoming beams. The frequencies are given as ωVIS and ωIR. The effective second order susceptibility can be expressed by [5]: ∑ , , 〈 〉 ( ) = − − Г Where N is the density in unit volume, Mαβ and Aγ are the Raman and IR tensors, respectively. 〈 〉 is the macroscopic average of molecular hyperpolarizability, ωq is the frequency of a normal vibrational mode, each q is a resonance mode, and Г is the damping or line-width coefficient. When ωIR matches ωq, the effective second order susceptibility reaches a maximum, the resonance is observed as a peak appears. Cellulose, a linear homopolymer which is composed of (1–4)-β-glucopyranose and is the most abundant polymer in nature, was chosen for our study. As the individual polymers are aligned in parallel, cellulose forms crystalline structure [8]. Hydrogen atoms of hydroxyl groups and oxygen atoms form hydrogen bonds not only between two adjacent glucopyranose units of one chain (interchain hydrogen bonds) but also between Tạp chí Nghiên cứu KH&CN quân sự, Số 37, 06- 2015 151 Vật lý two glucopyranose units from neighbor chains (intrachain hydrogen bonds), this leads to the crystalline structure of cellulose in which cellulose chains are highly ordered [9]. There are several polymorphs of cellulose: I, II, III and IV. Cellulose I, with two distinct crystal phases, Iα and Iβ, is naturally produced by a variety of organisms [8]. Cellulose Iα is found in the cell walls of some algae and bacteria, while cellulose Iβ is widely found in higher plants, which is more abundant [9]. Cellulose Iβ chains are arranged in a monoclinic P21 symmetry, a chiral space group, and SFG can be active in cellulose Iβ. Hydrated cellulose is chemically identical with cellulose, but different in some properties such as crystal modulus, tensile strength and thermal conductivity [10]. This is because of the hydration effects, which is the formation of hydrogen bonds of glucopyranose units with oxygen and hydrogen atoms in the solution [11]. In reality, an example for the hydration of cellulose happens when clothes, which are made mostly from cotton, are washed. The fact that after several times of washing, the clothes become softer and more warmth retaining, is an evidence of the change in tensile strength and thermal conductivity. Moreover, washing powder with large content of soap, composed of derivatives of fatty acids with alkali hydroxide, creates an amount of hydroxyl groups in solution. This may lead to some changes of the hydration process. Furthermore, hydrated cellulose in jute fiber is quite suitable for being used as a cheap substitution for wool in making warm fabrics [10]. Miyauchi et al. [7] reported a study applying SFG microscopy to observe the colorless and transparent starch granules in a plant. However, plants cell wall is composed of cellulose, is also sensitive with SFG spectroscopy. Besides, water inside the cell leads to some changes in the spectra so that the study should involve cellulose and hydrated cellulose. Hence, the investigation of this material as well as the hydration process on it using SFG spectroscopy is expected to offer useful information. 2. EXPERIMENTAL SECTION 2.1. SFG Spectroscopy System Measurements were carried out using EKSPLA Picosecond Vibrational SFG Spectrometer. The visible light (VIS) at wavelength of 532 nm is obtained from the frequency-doubled output of the Harmonics Unit driven by mode-locked Nd:YAG with 1064 nm wavelength, pulse duration of 30 picosecond, operating at repetition rate of 50 Hz. The wavelength-tunable infrared light (IR) is obtained from the output of an OPG/OPA system driven by the two laser beams of wavelength 1064 nm and 532 nm, the possible range of IR is 2.3-10 µm. Half-wave plates are used to configure the polarization of the beams. Using appropriate filters and monochromator, we can obtain and analyze the SFG signal with IR wavenumber. (Figure 1). 2.2. Samples Filter Papers (Advantec MFS, Inc) with 100% cotton linter cellulose were used as samples. This material has a large content of crystalline cellulose Iβ. X-ray Diffraction of the filter paper had been carried out. The spectrum highly matched the XRD standard database of cellulose Iβ, thus this material is appropriate for experiments with cellulose Iβ. The spectrum also gives out the P21 symmetry space group of cellulose Iβ. Samples have thickness of 0.23 mm were cut in 10 mm x 10 mm size, and stuck on glass plates to prevent from movement. 152 N. D. Long, V.T.Q. Huong,…,“Sum frequency generation study of hydrated Cellulose.” Nghiên cứu khoa học công nghệ Figure 1. SFG Spectroscopy Optical layout. Samples for hydrated cellulose are filter papers having been immerged in pure water for different times which are 1 hours, 3 hours and 6 hours. Before immerging, the samples were cut in different shapes to distinguish from each other. After the process, samples were dried using microwave oven for 1.5 hours. Hydration in NaOH 30% solution with 3 hours in steep was also included to compare and discuss. 3. RESULTS AND DISCUSSION 3.1. Sum Frequency Generation Spectroscopy of Cellulose Figure 2: a) SFG spectra of cellulose Iβ in different polarization configurations: psp, ppp, spp, ssp. Lines are multiple Gaussian functions fitted to the experiment. The zero levels of the spectra are displaced in the vertical direction for convenience to the eyes. (The letters give the polarization of SFG signal, VIS beam and IR beam, correspondingly) b) Scheme figure of two glucopyranose units in cellulose chain. Figure 2.a shows the SFG spectra of cellulose in different polarization configurations. The spectra are quite similar but different in intensities. There are 4 visible peaks at IR wavenumber of 2840, 2941, 2959 and 3322 cm-1. The assignments are shown and compare in Table 3.1. The 3322 cm-1 peak position is very close to the stretch peak of O-H (Figure Tạp chí Nghiên cứu KH&CN quân sự, Số 37, 06- 2015 153 Vật lý 2.b). Yet the O-H groups are located at the opposite sides of the glycosidic linkage, the SFG peak gives the evidence that they are not symmetrically arranged. One explanation had been proposed that these O-H groups interact with different O atoms, 3O-H is hydrogen-bonded to 5 O and 2O-H is hydrogen bonded to 6O (Figure 2.b). Thus, they don’t cancel each other and SFG will be detected. The 2840 cm-1 (weak) and 2941 cm-1 (strong) peaks are assigned respectively to symmetric and asymmetric stretching modes of CH2, of the exocyclic 6 CH2OH group. This assignment matches with some previous studies [1][5]. The C-H stretching peak at 2900 cm-1, which appears in both Raman and IR spectra of cellulose [12][13], is absent in SFG spectra because of the noncentrosymmetry selection rule. Table 1: Assignments of vibrational modes in spectral CH region 2800 cm-1 to 3000 cm-1 Wavenumber Atalla [13] Zimmerly [14] Barnette [5] This study (cm-1) (Raman) (Raman) (SFG) (SFG) (approx) FR of 2959 νa (CH2) ν (CH) H-O-C 2941 νa (CH2) ν (CH) νa (CH2) νa (CH2) 2885 ν (CH) νs (CH2) 2868 ν (CH) 2840 νs (CH2) νs (CH2) (νa (CH2) : symmetric stretching mode of CH2; νa (CH2): asymmetric stretching mode of CH2; ν(CH): stretching mode of CH; FR of H-O-C: Fermi resonance of H-O-C bending overtone) The 2959 cm-1 peak is supposed to be an overtone of H-O-C bending mode (fundamental H-O-C bending mode at 1475 cm-1 had been observed in previous studies using Raman or SFG spectroscopy [5][12][13]) at Fermi resonance with 2941 cm-1 peak. This peak is hardly observable because of the overlap with 2941 cm-1 peak. Some SFG study could not have found this peak because the resolution of infrared light is not high enough [5] (Table 1). The proof is given and discussed in the next sections. 3.2. Sum Frequency Generation Spectroscopy of cellulose hydrated in water Figure 3 shows the SFG spectra of three cellulose samples with different times of water steep: 1 hour, 3 hours and 6 hours. As shown on the spectra, all the peaks become smaller after the hydration process. This can be explained by the fact that water is still remained in the sample. Hence, only a fraction of the incoming beams reaches the sample, due to the different reflectance and transmittance with different polarizations of the beams, which has been describe in Fresnel equations. On the other hand, because of the presence of water, hydrogen bonds are formed with H atoms in cellulose structure [11]. This influences the orientation of C-H and O-H and makes the spectra change [14]. Moreover, it is apparent that the longer the time of water steep is, the smaller the 2941 cm-1 peak’s intensity is (Figure 3). This can be explained by the assumption that CH2 groups are affected by hydrogen bonds with water more than OH are. The more detailed explanation is discussed in next section. 154 N. D. Long, V.T.Q. Huong,…,“Sum frequency generation study of hydrated Cellulose.” Nghiên cứu khoa học công nghệ Figure 3: SFG spectra of non-hydrated cellulose and 3 samples with different hydrating time: 1h, 3h and 6h (Lines are multiple Gaussian functions fitted to the experiment). The fact that the intensity of the third peak at 2959 cm-1 does not change much is a proof that this peak is not contributed by the asymmetric stretching mode of CH2. 3.3. Sum Frequency Generation Spectroscopy of cellulose hydrated in NaOH solution Figure 4: SFG spectra of hydrated cellulose using pure water and NaOH solution with the same steep time of 3 hours. (Lines are multiple Gaussian functions fitted to the experiment). Since NaOH solution creates more hydroxyl groups, hydrogen bonds are created more, thus the process of hydration in NaOH solution is expected to be stronger than that of H2O. [11]. From Figure 4, it is clear that with the same time of hydration, the intensity of sample hydrated using NaOH 30% solution is remarkably smaller than that of sample hydrated using pure water, which is a proof that hydration process in NaOH solution is stronger. Moreover, in the SFG spectrum of samples hydrated by NaOH for 3 hours, the two peaks of CH2 stretching vanish, this had never been observed before. The peaks of the overtone at Fermi resonance and O-H stretching remain with smaller intensities. The peak Tạp chí Nghiên cứu KH&CN quân sự, Số 37, 06- 2015 155 Vật lý at 2959 cm-1 becomes easily to be observed as it no longer overlaps with CH2 asymmetric stretching peak. The disappearance of these two peaks is the evidence for the hydration effect. When cellulose is immerged in NaOH solution, hydrogen bonds are formed hydroxyl groups within the solution. The medium in NaOH solution with random positions of hydroxyl groups, similar to an amorphous medium, is centrosymmetric. Hence, the centrosymmetry of CH2 groups is enhanced. As the hydration is strong enough, CH2 groups are totally in centrosymmetric configuration, due to hydrogen bonds. Consequently, the peaks CH2 at 2840 and 2941 cm-1 are canceled. This result assures the assumption that the third peak does not belong to asymmetric stretching mode of CH2, but the overtone of H-O-C bending at Fermi resonance as we proposed. There was also a small difference in wavenumber of the third peak, 2959 cm-1 and 2961 cm-1. This might be because of the difference in glucopyranose units structure after hydration in pure water and NaOH solution, which leads to the difference of vibrational levels. This orienting effect might be the main reason why crystalline structure changes after hydration, which leads to some changes in properties of hydrated cellulose [10][14]. 4. CONCLUSION The experiments have been carried out to measure SFG spectra of cellulose and hydrated cellulose in different polarization configurations, different hydration conditions. The results give some useful information. Discussion and explanations have been proposed to analyze the spectra, which are the non-centrosymmetry of groups, and the orienting influence of hydration on them. The disappearance of two peaks in the spectrum of cellulose hydrated in NaOH solution, which has not been observed before, leads to a new assignment for one peak, as it is different with some previous, the others are consistent with literature. Acknowledgement: This work is supported by Project QG.13.04 Granted by Vietnam National University Hanoi (VNU). REFERENCES [1]. Hoang Chi Hieu, Nguyen Anh Tuan, Hongyan Li, Yoshihiro Miyauchi, and Goro Mizutani, “Sum Frequency Generation Microscopy Study of Cellulose Fibers”, Applied Spectroscopy. (2011) 65(11): 1254-1259. [2]. D. Epperlein, B. Dick, and G. Marowsky, “Second-Harmonic Generation in Centro- Symmetric Media”, Appl. Phys. (1987) B 44, 5-10. [3]. Li Fu, Zhuguang Wang and Elsa C.Y. Yan, “Chiral Vibrational Structures of Proteins at Interfaces Probed by Sum Frequency Generation Spectroscopy”, Int. J. Mol. Sci. (2011), 12, 9404-9425. [4]. Hoang Chi Hieu, Hongyan Li, Yoshihiro Miyauchi, Goro Mizutani, Naoko Fujita, Yasunori Nakamura, “Wetting effect on optical sum frequency generation (SFG) spectra of D-glucose, D-fructose, and sucrose”, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015) Mar 5;138:834-9. [5]. Anna L. Barnette, Laura C. Bradley, Brandon D. Veres, Edward P. Schreiner, Yong Bum Park, Junyeong Park, Sunkyu Park, and Seong H. Kim, “Selective Detection of Crystalline Cellulose in Plant Cell Walls with Sum-Frequency-Generation (SFG) Vibration Spectroscopy”, Biomacromolecules (2011) Jul 11;12(7):2434-9. [6]. Caitlin Howell, Ronny Schmidt, Volker Kurz, and Patrick Koelsch, “Sum-frequency- generation spectroscopy of DNA films in air and aqueous environments”, Biointerphases 3, FC47 (2008). 156 N. D. Long, V.T.Q. Huong,…,“Sum frequency generation study of hydrated Cellulose.” Nghiên cứu khoa học công nghệ [7]. Yoshihiro Miyauchi, Haruyuki Sano, and Goro Mizutani, “Selective observation of starch in a water plant using optical sum-frequency microscopy”, Vol. 23, No. 7/July (2006)/J. Opt. Soc. Am. A [8]. Robert J. Moon, Ashlie Martini, John Nairn, John Simonsen and Jeff Youngblood, “Cellulose nanomaterials review: structure, properties and nanocomposites”, Chem. Soc. Rev., (2011), 40, 3941–3994. [9]. Masahisa Wada, Yoshiharu Nishiyama, Henri Chanzy, Trevor Forsyth, and Paul Langan, “The structure of celluloses”, JCPDS-International Centre for Diffraction Data (2008) ISSN 1097-0002. [10]. S.C.Sirkar, D.Sc, F.N.I., and N.N.Saha, “On the structure of hydrated cellulose obtained from raw jute fibre”, Nature 157, 839-839 (22 June 1946). [11]. Brooks D. Rabideau and Ahmed E. Ismail, “Mechanisms of hydrogen bond formation between ionic liquids and cellulose and the influence of water content”, Phys.Chem.Chem.Phys., (2015), 17,5767. [12]. John Blackwell, “Infrared and Raman Spectroscopy of Cellulose”, Cellulose Chemistry and Technology Chapter 14, (1977) pp 206–218. [13]. James H. Wiley and Rajai H. Atalla, “Band assignments in the Raman Spectra of Celluloses”, Carbohydrate Research Volume 160, 15 February (1987), Pages 113. [14]. M. Zimmerley, R. Younger, T. Valenton, D. C. Oertel, J. L. Ward, and E. O. Potma, “Molecular orientation in dry and hydrated cellulose ﬁbers: Acoherent anti-stokes Raman scattering microscopy study”, J.Phys.Chem.B (2010) Aug 12;114(31):10200-8. TÓM TẮT NGHIÊN CỨU SỰ PHÁT TẦN SỐ TỔNG TRÊN XENLULÔZƠ HYDRAT HÓA Phổ tần số tổng (SFG) của xenlulôzơ từ sợi bông thu được trong vùng bước sóng hồng ngoại từ 2800 đến 3400 cm-1. Các đỉnh phổ ở vị trí 2840 cm-1 và 2941 cm-1 được gán cho dao động kéo dãn đối xứng và phản đối xứng của CH2. Ngoài ra còn xuất hiện một đỉnh ở vị trí 2959 cm-1 được cho là bội tần số (tần số gấp đôi tần số cơ bản) của dao động uốn cong H-O-C ở cộng hưởng Fermi với đỉnh 2941 cm-1. Vùng dao động OH cũng được quan sát quanh đỉnh 3322 cm-1. Ảnh hưởng của sự hydrat hóa lên xenlulôzơ được thể hiện trên phổ SFG của các mẫu xenlulôzơ được hydrat hóa trong các điều kiện khác nhau. Kết quả thu được là sự giảm cường độ của các đỉnh, đáng chú ý là sự biến mất của 2 đỉnh phổ. Các đề xuất giải thích và thảo luận cũng được đưa ra trong bài báo này. Từ khóa: Quang học phi tuyến, Sự phát tần số tổng, Phổ tần số tổng SFG, Xenlulôzơ, Xenlulôzơ hydrat hóa, Sự hydrat hóa, Liên kết hydro. Nhận bài ngày 15 tháng 5 năm 2015 Hoàn thiện ngày 11 tháng 6 năm 2015 Chấp nhận đăng ngày 12 tháng 6 năm 2015 Contact address: 1 Faculty of Physics, VNU University of Science, *Email: hieuhc@gmail.com 2 Institute of Chemical-Biological technique and Security documents, 3 Hong Duc University, Thanh Hoa, Vietnam. Tạp chí Nghiên cứu KH&CN quân sự, Số 37, 06- 2015 157