Facile size evaluation of cellulose nanofibrils adsorbed on polypropylene substrates using fluorescence microscopy

The current study proposes a facile method based on fluorescence microscopy (FM) for evaluating the size of cellulose nanofibrils (CNFs) prepared by aqueous counter collision (ACC). Prior to FM, a polypropylene film substrate was employed for the absorption and fixing of CNFs. Using FM, it is possible to detect a single ACC-CNF that is longer than 2 µm by staining with a fluorescent probe for cellulose (i.e., Calcofluor White). Comparison between FM and field emission scanning electron microscopy (FE-SEM) indicated that the lengths of the ACC-CNFs estimated from the FM images were in good agreement with the lengths from FE-SEM, although it was necessary for the ACC-CNFs to be over 20 nm in width for the FM. The full width at half maximum of the fluorescent intensity in the fluorescent images of ACC-CNFs correlated with the real widths obtained using FE-SEM, indicating that the current FM method also enables estimation of the widths of the CNFs using a regression line as a correction formula. Eventually this facile fluorescence microscopy technique could simultaneously provide reasonable size information with regard to both the lengths and widths of CNFs having over 20 nm in width and 2 µm in length with high aspect ratios over 100.