Coumarin and carbazole fluorescently modified cellulose nanocrystals using a one-step esterification procedure

Fluorescent cellulose nanocrystals with carbazole and coumarin functionalities were synthesized in a one-step esterification reaction using carbazole-9-yl-acetic acid and coumarin-3-carboxylic acid respectively, using a p-toluenesulphonyl chloride/pyridine system. Characterization with elemental analysis, X-ray diffraction, X-ray photoelectron spectroscopy, infrared, absorption and emission spectroscopy, and AFM and TEM imaging confirmed that a high degree of modification could be achieved (up to DSsurf=1.33) while retaining the CNC core crystallinity and nanoparticle dimensions. The present grafting method gives a straightforward way when compared with previously reported fluorescent labelling methodologies and gives an extremely high grafting density while retaining the CNC crystallinity. UV spectroscopy also indicates fluorescence quenching at high grafting densities so that optimal fluorescence does not necessarily mean maximum grafting density.