The second hypothesis concerns the role of CTAB as a molecular glue. CTAB mediates Au-CNFs interactions starting from the pre-composite formation. The 𝐶 -𝐻 symmetric stretching band of -𝐶𝐻 2group (blush highlight) is indicative of CTAB presence when comparing CNFs (no CTAB, no peak) and pre-composite (CTAB present, peak at 2858 cm -1 ). In the final formulation -𝐶𝐻 2group is present due to both CTAB and AzoGly and hence, another band needed to be selected for surfactant identification. We analyzed the evolution of -𝐶𝐻 3 band of ammonium salt appearing in the final material at 1456 cm -1 (vivid red highlight) and the appearance of -𝐶 -𝑁 + stretching band at 906 cm -1 (very weak shoulder of the β-glycosidic band) instead. Their presence (particularly clear for the 1456 cm -1 bandsee Figure ) indicates that the AzoGly-Au-2-CNFs composite contains residual CTAB molecules. Albeit a thorough washing process (see Materials and methods section in the SI) excludes free surfactant molecules in bulk solution, it is reasonable to assume that CTAB molecules remain intercalated in the CTAB-CNFs system. The surfactant remains either (1) solely on the surface of CNFs or (2) as a residual bilayer with lower integrity, mediating Au-CNFs interactions (Figure ). Thus, CTAB serves as molecular glue in the plasmonic-photochromic formulation and further supports the interactions between Au nanocrystals and cellulose nanofibers.