Sulfur and nitrogen co-doped carbon quantum dots/polyurethane nanocomposite as a new fluorescent sensor for highly sensitive and selective detection of toxic mercury (II) ions

Abstract

Here in, new nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs)/polyurethane (PU) nanocomposites have been prepared as efficiently sensitive and selective fluorescent sensors to detect mercury ions. At first, N,S-CQDs as fluorescent nanoparticles, 1, 2-bis (4-isocyanatophenoxy) ethane and bis (2-hydroxy ethyl) terephthalate (BHET) containing two hydroxyl groups were synthesized. The N,S-CQD_S nanoparticles were synthesized through microwave-assisted carbonization of PU wastes at the presence of sulfuric acid and thiourea. Then, new fluorescent polyurethane nanocomposites were synthesized through an in situ polymerization reaction between new fabricated diisocyanate, BHET and 5 and 10% (w/w) of the N,S-CQDs nanoparticles. The N,S-CQDs/PU-a (6a) and N,S-CQDs/PU-b (6b) nanocomposites containing 5 and 10% (w/w) of fluorescent nanoparticles showed a broad concentration range of 0–250 and 0–200 µM of mercury ions with quenching efficiencies (η) of 95% and 97%, respectively. The limit of detection (LOD), limit of quenching (LOQ), and quenching constant (K_sv) of two nanocomposites were calculated. According to the obtained results, the N,S-CQDs/PU-b nanocomposite (6b) exhibited better performance in Hg⁺² sensing because of lower LOD and LOQ and higher K_sv at lower concentrations of mercury ions. Therefore, an increase in the fluorescent nanoparticles loaded in the nanocomposite, led to an enhancement of the selectivity of mercury (II) ions and sensitivity to its detection. The N,S-CQDs/PU-b nanocomposite (6b) exhibited the LOD, LOQ, and K_sv values at 2.03 µM, 6.77 µM, and 6694.6 M⁻¹, respectively. So, the synthesized N,S-CQDs/PU-b nanocomposite (6b) is an excellent sensor for mercury ions.

Graphical abstract