Fabrication of fluorescent cellulose nanocrystal via controllable chemical modification towards selective and quantitative detection of Cu(II) ion

Fluorescent cellulose nanocrystals (fCNCs) have shown a great potential as optical probe for the applications of bioimaging or metal ion detection. In this work, by a controllable esterification strategy, the cellulose nanocrystal (CNC) surface was modified with ethylenediaminetetraacetic dianhydride (EDTAD) to achieve highly carboxylation degree, and, meanwhile, keep surface integration and crystalline structure in comparison with the TEMPO oxidation method. Then, the controllability of carboxylation degree was applied to produce a series of fCNCs with various conjugating densities of fluorescent 7-amino-4-methylcoumarin (AMC). The steric effect of AMCs on the fCNC surface could inhibit self-quenching, resulting in a more stable fluorescence intensity independent upon the change of fCNC concentration and a relatively high quantum efficiency in the solid state. Moreover, the fluorescence signals of fCNC can be selectively affected by Cu²⁺, and even high Cu²⁺ concentration resulted in the visualized color change from blue to purple under ultraviolet radiation. Hence, two linear correlations with the Cu²⁺ concentration, i.e. the fluorescence attenuation ratio at 390 nm in 0.5–16 ppm of Cu²⁺ concentration and the fluorescence intensity ratio of 440 nm versus 390 nm in 16–160 ppm of Cu²⁺ concentration, contributed to a potential of quantitative Cu²⁺ detection. Especially, the on–off behavior of the fCNC fluorescence at the critical Cu²⁺ concentration of 0.5 ppm was well consistent with the requirement of the Guideline of Drinking-Water Quality (GDWQ) administrated by World Health Organization (WHO), and hence might be used to rapid and visualized evaluation of drinking water quality. This work provides a new fluorescent nanomaterial that can be used in metal ion detection. Furthermore, the nontoxicity, biodegradability and environment-friendliness of cellulose nanocrystals might also contribute to the additional advantages for the application of environmental surveillance.