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 μM, 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 μM, respectively; these are both considerably lower than the allowable content of Cu2+ (20 μM) 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.