The analysis of cellulose nanocrystals (CNCs) via various spectroscopic techniques is useful to understand their physicochemical properties. The research community uses absorption methods, such as FTIR and solid-state NMR, to define the functional groups present on CNC surfaces and their interactions. X-ray and neutron elastic scattering are also employed to determine, for example, cellulose crystallinity, CNC size, shape, degree of aggregation/self-assembly and characteristic spacings within suspensions. Here, we propose the use of fluorescent spectroscopy, specifically utilising the native fluorescence of the CNCs, as a new method to rapidly determine critical CNC properties.<br/>The autofluorescent properties of CNCs arise from the intraparticle hydrogen bonds between individual cellulose chains, and the interparticle electronic space overlap between free electron pairs in oxygen and the π orbitals of double bonds present in the surface sulfate, or carboxylate, groups. We demonstrate that the ratio between fluorescent bands associated with these two types of intermolecular interaction is dependent on the number of CNCs present and the number of interactions between neighboring CNCs. This enables us to determine the critical packing concentration for CNCs, i.e. the onset of liquid crystalline phase formation, or gelation (Figure 1a). Furthermore, the surface charge content of the CNCs correlates well to the deconvoluted band ratio (Figure 1b) suggesting that an autofluorescence “calibration curve” could be used in real-time to determine sulfate/carboxylate content. These results support that the autofluorescence is directly related to the intermolecular interactions, presenting new CNC characterization opportunities.

Critical cellulose nanocrystal properties characterized via autofluorescent spectroscopy

ACS Spring 2022

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