The Preparation,Properties And Applications Of Mono-/Multi-carboxy Cellulose Nanocrystals

Cellulose nanocrystal?CNC?is widely applied in optical materials because of its excellent mechanical properties,optical properties,biocompatibility,biodegradability and abundant reserves.What's more,the rich hydroxyl groups on the surface of CNC contribute to a good flexibility towards facile modification.Roman et al.first made the fluorescein-5-isothiocyanate?FITC?grafted to CNC surface using the three-step method to synthesize fluorescence cellulose nanocrystals?fCNC?,opening the door for the applications of CNC in optical sensing and biomarker.However,the fluorescent molecules are apt to quench with the change of its density,so it is very important to control the amount of fluorescent molecules on the surface of fCNC to optimize the fluorescence intensity.In this work,fCNC is synthesized by a two-step method,and the fluorescent molecule contents on fCNC surface can be controlled by adjusting the carboxyl groups on the surface of CNC.Firstly,mono-carboxy and multi-carboxy cellulose nanocrystals are obtained by using TEMPO-oxidation and EDTAD-esterification respectively,which provide different active sites for fluorescent molecules.Then,fluorescent cellulose nanocrystals A-TOCN and A-ECNC are synthesized by amidation reaction between 7-amino-4-methylcoumarin?AMC?and TOCN or ECNC respectively.The effects of synthesize conditions on the performance of A-TOCN and A-ECNC are also investigated.Based on the fluorescence characteristics of fCNC,its application in copper ion sensing is further researched.Results of this work are as follows:?1?The monocarboxycellulose nanocrystalline?TOCN?obtained by TEMPO oxidation has a good dispersibility and stability,but the carboxyl content and degree of hydroxyl substitution can't be controlled.The multi-carboxy cellulose nanocrystals?ECNC?with controllable carboxyl contents and degree of hydroxyl substitutionare successfully obtained by EDTAD esterification,which maintain the original morphological and crystal structure of CNC and improve the decentralization of CNC.?2?The fluorescent CNC is obtained by grafting AMC to the surface of TOCN and ECNC using DMTMM as catalyst.The prepared fCNC has fluorescence stability in the range of surface fluorescence molecule concentration of 0.14-3.45 mmol/L,which effectively prevents the fluorescence quenching.A-ECNC/2 had better fluorescence performance than other fCNC.?3?The fCNC has good selectivity and sensing characteristics for Cu²⁺and can quantitatively detect Cu²⁺concentration.The fluorescence intensity of A-ECNC/2shows a good linear correlation with Cu²⁺concentrations of 0.5-16 ppm and 16-160ppm,and the limit of detection?LOD?is 0.5 ppm.Such LOD meets the requirement of the Guide-line of Drinking-Water Quality?GDWQ?issued by the World Health Organization and Chinese GB5749-2006 standard.Thus,A-ECNC/2 can be used for rapid detection and visual evaluation of water quality.This work provides the method to synthesize a new fluorescent nanomaterial that can be used in the detection of toxic metal ion.The nontoxicity,biodegradability and environment-friendliness of cellulose nanocrystals might contribute to the additional advantages for the application of environmental surveillance.