Over the years, several examples of cellulose impregnation with dyes (including photochromes) have been presented. Liu et al. adsorbed negatively charged stilbene derivative on cellulose using a divalent cation, Ca 2+ . Iamazaki and Atvars facilitated the sorption of Tinopal CBS on cellulose fibers using surfactants. Araki showed that sorption of cationic dyes on cellulose is so efficient it can be used as a quantitative analytical method for assessing the number of cellulose surface groups. Our strategy for preparing a functional plasmonic-photochromic composite via the use of CNFs was developed independently, yet it corresponds very well with the literature examples. In our case, Au nanocrystals do not mediate AzoGly-CNFs interactions, as we proved in control experiments with bare cellulose nanofibers (Figure ). Bare CNFs, upon impregnation with AzoGly, yielded composites exhibiting distinct photochromic bands, indicating high efficiency of dye adsorption on the fibers (Figure ). In principle, the higher the concentration of AzoGly upon impregnation, the better the photochromic properties of the resulting composite. The increase of ~3.8 times in dye concentration gave ~26 times higher intensity of the π→π * band (based on the corrected values in Figure , comparison between concentrations 0.69 mM and 2.62 mM), emphasizing profits from impregnations performed at higher concentrations of AzoGly.