Fluorescent Spherical Sponge Cellulose Sensors for Highly Selective and Semiquantitative Visual Analysis: Detection of Hg2+ and Cu2+ ions

In this work, a novel sponge cellulose fluorescence spherical (CS-CDs) was prepared by an in situ microwave method using cellulose as a base material, polyethylenimine (PEI) as a nitrogen-doping reagent, and citric acid (CA) as an external carbon source. The CS-CDs were used for highly sensitive selective detection of Hg2+ and semiquantitative visual detection of Cu2+ in aquatic environments. Their excellent fluorescence properties and porous structure enable the CS-CDs to adsorb and detect these ions quickly and sensitively, and they can detect Hg2+ selectively with a detection limit of 26 nM. Additionally, an interesting phenomenon was observed where the color of the cellulose sphere changes continuously from inherent pale yellow to deep green as the Cu2+ adsorption increases from 3 to 60 mu m, indicating that the CS-CDs can be used for semiquantitative visual detection of Cu2+. The fluorescence and visual detection limits for Cu2+ are 0.11 and 3 mu M, respectively; these are both considerably lower than the allowable content of Cu2+ (20 mu M) in potable water. The discoloration mechanism of Cu2+ adsorption by CS-CDs was studied. It can be considered that after CS-CDs adsorb Cu2+, a large amount of Cu2+ amine complexes form, aggregate, and concentrate on the cellulose sphere substrate to develop the color. Finally, we used the CS-CDs to analyze tap and river water based on highly reproducible results. This cellulose sponge-based device could be used to protect probes from environmental interference and is sufficiently stable to withstand at least 22 cycles of exposure to ethylenediaminetetraacetic acid.