This article is protected by copyright. All rights reserved Fluorescent cellulose nanocrystals with carbazole and coumarin functionalities were synthesised in a one-step esterification reaction using carbazole-9-yl-acetic acid and coumarin-3-carboxylic acid respectively, using a p-toluenesulfonyl 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 DS surf = 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. This

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IntroductionChitosan has become one of the most interesting polymers in different fields due to a rare combination of biological, mechanical and physical activities. It is one of the main products of chitin, which is commercially obtained by chitin deacetylation produced by crustaceans. Reducing the molecular weight and thus decreasing the crystallinity, it is improved its solubility in diluted acids at pH = 5 where the free amine groups are protonated. In the synthesis of chitosan derivatives, three methods have been developed to produce chitosan compounds functionalized with polyphenols: ester-mediated modification; enzyme-mediated strategy; grafting induced by free radicals. The main purpose of experimental studies is the synthesis of chitosan derivatives by grafting it with certain compounds that exhibit antioxidant properties and represent part of the flavonoid class and studying, comparing the antioxidant properties of the obtained copolymers.

This paper is dedicated to the synthesis of a copolymer with reducing properties obtained by functionalizing chitosan with quercetin and determining the antioxidant activity of the derivatives obtained depending on the molar mass of the polymer. For this purpose, low molecular weight chitosan was obtained by oxidizing commercial chitosan with hydrogen peroxide and further functionalization with quercetin by the covalent grafting method. The functionalization process was performed through the following steps: functionalization of chitosan with ethyl chloroformate to increase the reactivity of the amine group to the hydroxyl group of quercetin and grafting the quercetin molecule to the synthesized intermediate. The comparative antioxidant properties of the composite obtained by grafting technical chitosan with quercetin and by grafting low molecular weight chitosan were studied by the DPPH (2,2-diphenyl-1-picrylhydrazyl radical) method. The obtained results indicate that a decrease in the molecular weight of chitosan contributed to its grafting with quercetin. As a result, the functionalized polymer composite acquired a higher antioxidant activity and can be successfully used to inhibit the oxidation of various organic substrates in the cosmetic, food and pharmaceutical industries.

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