Polymerization-induced emission and selective detection to Fe³⁺/ Fe²⁺ of triazine-containing polyureas

In this study, four polyureas with triazine moiety (PUAs) were successfully synthesized through the polymerization of triazine-containing diamine and diisocyanate. The intramolecular aggregation of triazine rings and urea groups along the macromolecular backbone gives PUAs a significant polymerization-induced emission (PIE). Among the four PUAs, PUA-LP shows a significant fluorescent emission at 450 nm, compared to non/weak fluorescent 2,4-diamino-6-phenyl-1,3,5-triazine and L-Lysine diisocyanate ethyl ester monomers. Additionally, the external factors such as solution concentration, excitation wavelength, and precipitants also play a crucial role in the fluorescence of PUAs. As expected, PUA-LP can selectively recognize and detect Fe³⁺/Fe²⁺ ions even in the presence of 12 other metal ions and 10 anions. The limit of detection of PUA-LP to Fe^3+/Fe²⁺ is as low as 1.02 μM (0.06 mg/L) and 0.86 μM (0.05 mg/L), respectively, and far below 0.3 mg/L of the allowable national standard for drinking water by WHO. Furthermore, the quenching mechanism of Fe^3+/Fe²⁺ to PUA-LP is attributed to static quenching caused by the coordination of Fe³⁺/Fe²⁺ ions with a coordination ratio of 2:1. Based on PIE, the fluorescent PUA-LP was made into an observable and portable testing paper for detecting Fe^3+/Fe²⁺ ions. Finally, we measured the recovery rate of the actual tap water samples and compared the performance of PIE-active PUA-LP with the other reported fluorescent probes to Fe³⁺/Fe²⁺ ions.