Vibrational spectral signatures of crystalline cellulose using high resolution broadband sum frequency generation vibrational spectroscopy (HR-BB-SFG-VS)
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Both the C–H and O–H region spectra of crystalline cellulose were studied using the sub-wavenumber high-resolution broadband sum frequency generation vibrational spectroscopy (HR-BB-SFG-VS) for the first time. The resolution of HR-BB-SFG-VS is about 10-times better than conventional scanning SFG-VS and has the capability of measuring the intrinsic spectral lineshape and revealing many more spectral details. With HR-BB-SFG-VS, we found that in cellulose samples from different sources, including Avicel and cellulose crystals isolated from algae Valonia (Iα) and tunicates (Iβ), the spectral signatures in the O–H region were unique for the two allomorphs, i.e. Iα and Iβ, while the spectral signatures in the C–H regions varied in all samples examined. Even though the origin of the different spectral signatures of the crystalline cellulose in the O–H and C–H vibrational frequency regions are yet to be correlated to the structure of cellulose, these results lead to new spectroscopic methods and opportunities to classify and to understand the basic crystalline structures, as well as variations in polymorphism of the crystalline cellulose.