Synthesis and Characterization of New Mesoporous Polyurethane-Nitrogen Doped Carbon Dot Nanocomposites: Ultrafast, Highly Selective and Sensitive Turn-off Fluorescent Sensors for Fe3+ Ions.

A new fluorescent mesoporous polyurethane (PU) (9) was synthesized by reaction between 2,2'-(methylenebis(4,1-phenylene))bis(5-isocyanatoisoindoline-1,3-dione) (Diisocyanate) (5) and 4,4',4″-((1,3,5-triazine-2,4,6-triyl)tris (azanediyl))triphenol (Triol, TO) (8) (molar ratio 3:2). PU was characterized by using FT-IR, 1H-NMR, XRD, UV-Vis, TGA, Nitrogen adsorption-desorption isotherm, BET, FE-SEM and Photoluminescence (PL) analyses. To the best of our knowledge, this is the first time that a fluorescent polyurethane has been made without the use of commercial fluorescent materials. PU has high fluorescent intensity and it is ultrafast (about few seconds), highly selective and sensitive turn-off fluorescent sensor for Fe3+ ions. This chemosensor exhibited a wide concentration range of (10-250)×10-6 M Fe3+ with quenching efficiency (η) 97.50%. Limit of detection (LOD), limit of quantification (LOQ) and quenching constant (Ksv) values were calculated 10.10×10-6 M, 30.60×10-6 M and 6919.31 M-1, respectively. Nitrogen doped carbon dots (N-doped CDs) as fluorescent nanoparticles and with the aim of improving Fe3+ detecting were synthesized by microwave-assisted and using citric acid monohydrate (10) and ethylenediamine (11) as carbon and nitrogen sources, respectively. Fluorescent nanocomposites (FNCs) were prepared by using casting and in-situ methods. In both methods, two nanocomposites containing 5 and 10%w of N-doped CDs were prepared. FNCs were characterized by using FT-IR, UV-Vis, XRD, TGA, Nitrogen adsorption-desorption isotherm, BET, FE-SEM and PL analyses. All nanocomposites showed better thermal property and sensitivity and lower LOD values in lower concentration of Fe3+ related to PU. Among them, FNC10in exhibited the best results as η, LOD, LOQ, Ksv reached 99.80%, 1.15×10-6 M, 3.48×10-6 M and 53,551.48 M-1, respectively.