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 NO2– 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 NO2– 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 NO2– ions. The quenching mechanism of NO2– and PAM–PV by FTIR, UV, and 1H NMR measurements indicates that the strong electrostatic interaction between NO2– and PAM–PV makes the macromolecular chains more relaxed and outstretched, leading to a formation of a nonfluorescent complex. Moreover, the practical detection of NO2– in the three water bodies, sauerkraut, and ham sausage and the fabrication of a portable detecting paper to NO2– ions were conducted in this experiment.