Cellulose-Based Sensor Containing Phenanthroline for the Highly Selective and Rapid Detection of Fe²⁺ Ions with Naked Eye and Fluorescent Dual Modes

Iron ions play a vital role in many biological processes, and their concentrations are responsible for human health. Therefore, it is essential to detect the concentration of iron ions by a rapid, accurate, highly selective, and practical method. Herein, we have synthesized a cellulose-based fluorescent sensor (Phen-MDI-CA) for the highly selective and rapid detection of Fe²⁺ ions via chemically bonding 1,10-phenanthroline-5-amine (Phen) onto cellulose acetate (CA) using 4,4'-methylene diphenyl diisocyanate (MDI) as a cross-linker. Benefiting from the anchoring and diluting effect of a cellulose skeleton, the resultant Phen-MDI-CA displays excellent fluorescence properties in both solution and solid state. More interestingly, a cellulose-based polymer chain significantly improves the sensitivity of phenanthroline to Fe²⁺ ions. Upon meeting Fe²⁺ ions, a red, insoluble, and nonfluorescent Fe-(Phen-MDI-CA) complex appears immediately; thus, Phen-MDI-CA can work as a multimode chromogenic sensor for the highly selective, sensitive, and rapid detection of Fe²⁺ ions. In the instrument-free visual mode, the detection limit for Fe²⁺ ions is 50 ppb, and in fluorescence mode, the detection limit is 2.6 ppb. To our knowledge, this is the first time that such a low detection limit for Fe²⁺ ions in aqueous media has been observed by the naked eye. In addition, Phen-MDI-CA has good solubility and processability in common organic solvents, which facilitates its use in different material forms, e.g., printing ink, coating, and film. Therefore, the Fe²⁺-responsive and chromogenic Phen-MDI-CA exhibits a huge potential in the detection and extraction of Fe²⁺ ions.