Cellulose nanomaterial metrology: microscopy measurements

Cellulose nanomaterials are increasingly used for a wide variety of applications. Adequate characterization of these materials is required for quality control during production, to distinguish between materials synthesized by different methods, by different suppliers or from difference cellulose biomass sources, to facilitate development of applications and for regulatory purposes. Here we review recent microscopy measurements for the three main types of cellulose nanomaterials: cellulose nanocrystals, individual cellulose nanofibrils and cellulose nanofibrils. Atomic force microscopy and both scanning and transmission electron microscopy are covered with a focus on recent studies that have metrological rigor, rather than qualitative investigations. In some cases results are compared to those obtained by other methods that are more likely to see widespread use for routine quality control measurements. Detailed studies that use microscopy to provide insight on fundamental material properties (e.g., chiral properties) are also included. Particle size and morphology are important properties but are challenging to measure for cellulose nanomaterials due to the rod or fibril shaped particles, their propensity to agglomerate and aggregate, their low contrast for electron microscopy and, for cellulose nanofibrils, the complex branched and interconnected structures. Overall, the results show that there are now a number of studies in which attention to metrological detail has resulted in measurements that allow one to compare and distinguish between different materials, although there are still examples for which it is not possible to draw conclusions on size differences. The use of detailed microscopy protocols that yield accurate and reliable results will be beneficial in material production and addressing regulatory requirements and will allow the validation of other methods that are more amenable to routine measurements.