Distinguishing Surface versus Bulk Hydroxyl Groups of Cellulose

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Distinguishing Surface Versus Bulk Hydroxyl Groups of Cellulose Nanocrystals using Vibrational Sum Frequency Generation Spectroscopy Mohamadamin Makarem, Christopher M. Lee, Daisuke Sawada, Hugh M. O'Neill, and Seong H. Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02729 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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The Journal of Physical Chemistry Letters

Distinguishing Surface Versus Bulk Hydroxyl Groups of Cellulose Nanocrystals using Vibrational Sum Frequency Generation Spectroscopy

Mohamadamin Makarem,1 Christopher M. Lee,1 Daisuke Sawada,2 Hugh M. O’Neill,2 and Seong H. Kim1* 1. Department of Chemical Engineering, Materials Research Insitute, Pennsylvania State University, University Park, PA, 16802, USA. 2. Biology and Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 * Corresponding author: [email protected]

Abstract In plant cell walls and cellulose-containing composites, nanocrystalline cellulose interacts with water molecules or matrix polymers through hydrogen bonding of the hydroxyl groups at the cellulose surface. These interactions play key roles in cellulose assembly in plant cell walls and mechanical properties of cellulose-composites; however, they could not be studied properly due to the spectroscopic difficulty of selectively detecting the surface hydroxyl groups of nanocrystalline domains.

This study employed the sum frequency scattering principle to

distinguish the hydroxyl groups inside the crystalline nanodomain of cellulose and those exposed at the surface of crystalline domains. The comparison of the spectra at various scattering angles revealed that the OH peak near ~3450 cm-1 comes from the weakly hydrogen-bonded OH-groups at the surface of crystalline cellulose. Also, a time-delay measurement found that the sharp vibrational features observed near 3700 cm-1 can be attributed to isolated OH groups not accessible by ambient water molecules. These findings allow the distinction of surface versus bulk OH-groups in sum frequency generation vibrational spectroscopy. 1 ACS Paragon Plus Environment

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Cellulose is probably the most abundant nanocrystalline material on earth. It is produced in microfibril forms with diameters ranging from a few nm’s to tens of nm in most plant cell walls, certain bacterial pellicles, and tunics of Halocynthia.1 Different biological species produce cellulose microfibrils with a varying degree of cellulose chain packing in two crystalline polymorphs – Iα and Iβ – or mixture with the amorphous phase.1-2 The interaction of these crystalline microfibrils with surrounding water molecules and other matrix polymers (or with other microfibrils) in plant cell walls plays important roles in cell wall expansion during the cell growth and mechanical properties of cell walls at specific growth stages.3-5 There has been a growing interest to utilize cellulose nanocrystals (CNCs) isolated from plant cell walls and bacterial pellicles as structural reinforcement components in engineered composite materials.1 In such applications, not only the size, crystallinity, and volume fraction of CNCs, the hydrogen bonding interactions at the interface between CNCs and matrix components are a key factor determining the composite integrity and mechanical property. Thus, it is important to selectively detect and investigate the physical state of the hydroxyl groups exposed at the crystalline cellulose surface for better understanding of the roles of interfacial hydrogen bonding interactions in these natural and man-made materials. Detecting and distinguishing functional groups at the surface or interface in the presence of the same functional groups in the bulk phase (either substrate or surrounding environment) using a linear spectroscopy (such as infrared and Raman spectroscopy) is challenging because the amount of surface species is several orders of magnitude less than that of the bulk species in the probe beam path. This problem can be circumvented in a non-linear spectroscopy such as second harmonic generation (SHG) and sum frequency generation (SFG) spectroscopy as long as the bulk phase is amorphous or centrosymmetric because SHG and SFG at the phase matching



condition require noncentrosymmetry and only interfaces can meet such a requirement.6-10 However, this principle was not sufficient to confidently distinguish the surface versus bulk OH groups of the crystalline cellulose. In the case of cellulose microfibrils and CNCs, they consist of linear chains of β(14) linked D-glucopyranose units packed with a noncentrosymmetric order through a combination of intra- and inter-chain hydrogen bonds and van der Waals interactions.11-13 So, the bulk phase of cellulose microfibrils and CNCs can be SFG-active. Thus, SFG signals at the phase matching direction are dominated by the bulk contribution of the crystalline cellulose.14 This property gives a unique opportunity to investigate the nano- to meso-scale structural order of cellulose interspersed in plant cell walls and composite materials.15-17 In previous SFG studies of cellulose in plant cell walls, the OH SFG spectral features below 3400 cm-1 could be ascribed to the collective vibrations of OH groups inside the crystalline cellulose;18-19 but, additional peaks that could not be attributed to the bulk OH groups were often detected as a shoulder near 3450 cm-1 and sharp peaks near 3700 cm-1.16-17, 20-25 Based on the peak position, the 3450 cm-1 component must be weakly hydrogen-bonded OH species and the 3700 cm-1 species are free OH groups.26-27 Because such OH groups are not expected to be present inside the crystalline cellulose, they are speculated to be surface OH groups; but, independent physical evidence was lacking to support this tentative assignment of the peaks at >3400 cm-1. In this study, we have applied the newly developed SFG scattering technique to confirm that the SFG peaks near ~3450 cm-1 and ~3700 cm-1 indeed originate from the OH groups exposed at the surface of crystalline cellulose. Based on the Rayleigh-Gans-Debye scattering theory28, Roke and colleagues demonstrated that SFG scattering at non-phase matching angles can be used to study the interface of submicron particles in suspension (Figure 1).29-35 This study


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first confirms the preferential detection of surface OH species at a scattering angle away from the phase-matching direction using a model sample prepared by controlled OD/OH exchange the surface of CNCs. Then, the relative intensities of the 3450 cm-1 (surface) vs. 3320 cm-1 (bulk) peaks are compared at the phase-matching and scattering directions. Lastly, a time-delay measurement is used to confirm that the sharp OH peaks at ~3700 cm-1 is due to free OH groups with a much longer relaxation time than the broad peaks appearing at 3600 cm-1 have been observed in SFG studies of naturally dried plant cell walls.21-22, 25 The OH peaks above 3600 cm-1 are usually assigned to free-OH groups without hydrogen bonding interactions with surrounding molecules.26-27 To prove the same is applicable to the SFG analysis of cellulose, SFG spectra of a CNC-containing fiber at θ = 90o (see Figure 4 inset) were collected as a function of the delay time (td) between the IR and 800 nm pulses. Measuring the decay of vibrational polarization can shed light on dephasing dynamics of functional groups.44-46 Another purpose of the time delay experiment was to see if the surface OH groups of the nanocrystalline domain have dephasing dynamics significantly slower than the bulk (interior) OH groups within the domain. At θ = 90o, the surface contribution of nanocrystalline domains in the sample would be the largest(Figure 1b). The decay of vibrational polarization for free OH would be much longer than the OH groups involved in hydrogen bonding interactions.47 This is confirmed in the time-delay data shown in Figure 4; the dephasing time of the sharp peak at 3690 cm-1 is much longer than that of the hydrogen-bonded broad peak at 3320 cm-1. The dephasing time of the 3320 cm-1 peak in the time delay plot is comparable to thetemporal width of the 800 nm pulse (2.1-2.4 ps);48 Although the exact value cannot be determined, this data suggests that the dephasing time of the 3320 cm-1 vibration mode would not be significantly longer than 2.1-2.4 ps. Even the weakly-hydrogenbonded surface OH groups responsible for the peak at ~3450 cm-1 appears to have a faster dephasing rate than the 800 nm pulse width. It means that if this peak is to be deconvoluted by fitting, the linewidths of fit components in this surface OH region would be similar to those in the bulk OH region.17,19



Figure 4. SFG spectra of a uniaxially-aligned CNC film measured at a 90o scattering angle (reflection direction) as a function of the delay time (td) between the ~85 fs broadband IR pulse and the 2.1-2.4 ps 800 nm pulse. A schematic representation of the detection geometry is shown in the inset. The hydrogenbonded and free OH regions were collected separately at broadband IR pulses centered at 3350 cm-1 and 3690 cm-1, respectively. The polarization combination of SFG signal, 800nm, and mid-IR pulses was ssp. The IR beam profile was normalized with the nonresonance background collected with a quartz substrate. The spectrum shown in the left is the data collected at td = 0 ps. The plot shown at the top is the intensities at 3320 cm-1, 3450 cm-1, and 3690 cm-1 as a function of td. The SFG scattering geometry and the full SFG spectrum at psp and ssp polarizations are shown in Figure S6 of the Supporting Information.

The SFG scattering data shown in Figure 4 were collected for the CNC incased in the fibrin matrix (as in Figure 2); but, similar free OH peaks were observed for naturally-dried plant cell walls too.21-22, 25 If all surface OH groups are exposed to humid air, then physisorption of water molecules on the OH-terminated CNC surface would suppress the free OH signal. The 10 ACS Paragon Plus Environment

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presence of the free OH signal in the SFG spectrum implies that some OH groups at the interface of nanocrystalline domains and the amorphous matrix in the sample are not accessible by ambient water molecules. In summary, the SFG scattering of CNC confirms that the OH signals at >3400 cm-1 can be attributed to the hydroxyl groups exposed at the CNC surface. The broad peak or shoulder centered near 3450 cm-1 is the surface OH species whose hydrogen bonding interactions are weaker than those of the OH species in bulk. The sharp peaks at >3600 cm-1 can be attributed to the free OH groups at the cellulose surface which are not accessible by water molecules in ambient air. In previous SFG studies, the 3450 cm-1 / 3320 cm-1 relative intensities are found to vary drastically among plant cell walls at different species and developmental stages.22,

49

The

finding of this paper provides physical bases needed to interpret such spectral changes in terms of subtle differences in structural order at the surface and bulk of crystalline cellulose. The method demonstrated in this work can be applied for the distinction of surface versus bulk functional groups of other crystalline biopolymers.

Acknowledgements. This work was supported by The Center for Lignocellulose Structure and Formation, Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0001090. The authors thank Ms. Riddhi Shah for assistance in producing deuterated bacterial cellulose. DS acknowledges the support of the Genomic Science Program, Office of Biological and Environmental Research (OBER), U. S. Department of Energy, under Contract FWP ERKP752.

SI description. Experimental details describing the broadband SFG system with detection at three scattering angles, the preparation of the uniaxially-aligned CNC fibers, 2D-XRD of the model samples used for SFG scattering in Figure 2, and the sample mounting geometry with respect to the laser incidence and scattering plane for the fiber and thin film experiments. 11 ACS Paragon Plus Environment


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Distinguishing Surface versus Bulk Hydroxyl Groups of Cellulose

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