Fluorescent Polyamides with Piperazine and Valerolactam Structures Prepared by the Modified Ugi Reaction and Its Specific Detection to NO₂^– Ions

In this investigation, a nonconjugated fluorescent polyamide with piperazine and valerolactam moieties (PAM–PV) has been synthesized by the modified Ugi polymerization using 1,4-bis(3-aminopropyl)piperazine, levulinic acid, and 1,4-bis(3-isocyanopropyl)piperazine as monomers. In the polymerization, the special construction of levulinic acid leads to the formation of valerolactam on PAM–PV chains via the Mumm rearrangement. PAM–PV shows a strong fluorescence emission at 428 nm due to the intramolecular aggregation of piperazine and valerolactam moieties along the backbone. Impressively, PAM–PV can specifically detect NO 2 – ions via static quenching with a linear range of 10 –6 –10 –5 M and a detection limit of 0.6 μM (0.0276 mg/L), which was far less than the minimum allowable concentration of 3 mg/L for NO 2 – in drinking water specified by WHO. Furthermore, the several common anions and cations have no interference effects on the specific detection of PAM–PV to NO 2 – ions. The quenching mechanism of NO 2 – and PAM–PV by FTIR, UV, and 1 H NMR measurements indicates that the strong electrostatic interaction between NO 2 – and PAM–PV makes the macromolecular chains more relaxed and outstretched, leading to a formation of a nonfluorescent complex. Moreover, the practical detection of NO 2 – in the three water bodies, sauerkraut, and ham sausage and the fabrication of a portable detecting paper to NO 2 – ions were conducted in this experiment.