Sustainable and highly-stable carbon dots from cellulose as a fluorescent reference for visual ratiometric pH detection

Fluorescent materials, including organic molecules and quantum dots, usually respond to the environmental changes, such as acidity and basicity. It is challenging to construct highly-stable fluorescent materials. In this work, we fabricated novel carbon dots (CDs) with a high stability and a high quantum yield by using a solvothermal process, in which cellulose was the green precursor and 1-allyl-3-methylimidazolium chloride (AmimCl)/N,N-dimethylformamide (DMF) was the solvent. The resultant CDs had the cationic imidazolium groups on their surface. As a result, they exhibited the excellent fluorescence stability in strong-acidic, strong basic and high salt solutions. In addition, the CDs also presented a good resistance to the photobleaching under strong UV irradiation conditions. The highly stable CDs can be utilized as a fluorescent reference to significantly improve the responsiveness of the fluorescent materials. For example, the CDs were combined with cellulose acetate fluorescein isothiocyanate (CA-FITC) to obtain a series of ratiometric probes, which can visually detect pH values with a higher sensitivity. Furthermore, based on the outstanding processability and formability of cellulose derivatives, the CDs/CA-FITC was easily fabricated into responsive fluorescent coatings, films, and inks, indicating a huge potential in visual detection, solid-state luminescence, and anti-counterfeiting.