Fluorescent Polyamides with Piperazine and Valerolactam\nStructures Prepared by the Modified Ugi Reaction and Its Specific\nDetection to NO<sub>2</sub><sup>–</sup> Ions

In this investigation, a nonconjugated\nfluorescent polyamide with\npiperazine and valerolactam moieties (PAM–PV) has been synthesized\nby the modified Ugi polymerization using 1,4-bis(3-aminopropyl)piperazine,\nlevulinic acid, and 1,4-bis(3-isocyanopropyl)piperazine as monomers.\nIn the polymerization, the special construction of levulinic acid\nleads to the formation of valerolactam on PAM–PV chains via\nthe Mumm rearrangement. PAM–PV shows a strong fluorescence\nemission at 428 nm due to the intramolecular aggregation of piperazine\nand valerolactam moieties along the backbone. Impressively, PAM–PV\ncan specifically detect NO<sub>2</sub><sup>–</sup> ions via\nstatic quenching with a linear range of 10<sup>–6</sup>–10<sup>–5</sup> M and a detection limit of 0.6 μM (0.0276 mg/L),\nwhich was far less than the minimum allowable concentration of 3 mg/L\nfor NO<sub>2</sub><sup>–</sup> in drinking water specified\nby WHO. Furthermore, the several common anions and cations have no\ninterference effects on the specific detection of PAM–PV to\nNO<sub>2</sub><sup>–</sup> ions. The quenching mechanism of\nNO<sub>2</sub><sup>–</sup> and PAM–PV by FTIR, UV, and <sup>1</sup>H NMR measurements indicates that the strong electrostatic\ninteraction between NO<sub>2</sub><sup>–</sup> and PAM–PV\nmakes the macromolecular chains more relaxed and outstretched, leading\nto a formation of a nonfluorescent complex. Moreover, the practical\ndetection of NO<sub>2</sub><sup>–</sup> in the three water\nbodies, sauerkraut, and ham sausage and the fabrication of a portable\ndetecting paper to NO<sub>2</sub><sup>–</sup> ions were conducted\nin this experiment.