Sensitive fluorometric determination of picric acid and antioxidant stabilizers in propellant compositions using amine-rich nitrogen-doped carbon quantum dots

Determination of both trace explosives in soil samples and antioxidant stabilizers in propellant compositions using the same quantum dots is an important challenge in analytical chemistry. For this purpose, unmodified carbon quantum dots (CQDs) were prepared with surface heteroatom (nitrogen and oxygen) doped, watersoluble and highly fluorescent properties. The CQDs were synthesized by reflux method using citric acid (CA) and N-(4-methylpyridin-2-yl)ethane-1,2-diamine (NMPE) as carbon- and nitrogen-sources, respectively, and displayed a quantum yield as high as 48.30%. Surface morphology of CQDs was determined by STEM, IR, Raman and XPS techniques, and optical properties were determined by fluorescence spectroscopy, fluorescence lifetime measurement, and UV-vis spectroscopy. CQDs, excited at 310 nm, have a spherical structure with an average diameter of 2.0 nm, and emit strong blue fluorescence at a wavelength of 380 nm. With the static fluorescence quenching phenomenon based on the internal filter effect (IFE) mechanism, CQDs can precisely detect picric acid (PA), curcumin, and amine-type antioxidants used as stabilizers (i.e. N-phenyl-p-phenylenediamine, N-phenyl-1naphthylamine, N-phenyl-2-naphthylamine, and diphenylamine). With detection limits of 0.5 nmol L-1 for picric acid, 70 nmol L-1 for curcumin, and 600 nmol L-1 for N-phenyl-p-phenylenediamine, CQDs can detect analytes with good recovery in the range of 95% to 105% in the presence of different explosives (i.e., tetryl, TNT, PETN, HMX, and RDX for picric acid), metal ions (Cd2+, Fe2+, Mg2+, Cu2+, Zn2+, Mn2+, and Ca2+ for picric acid and curcumin), anions (Cl-, NO3 -, NO2-, SO4 2-, and CO32- for picric acid and curcumin), biomolecules (glucose, trypsin, lysine, L-phenylalanine, and DL-alanine for curcumin), and camouflage materials (i.e. paracetamol, glucose, detergent, aspartame, and acetyl salicylic acid for picric acid). The developed method was validated against LC-MS/MS (for picric acid) and spectrophotometric CUPRAC (for curcumin) by comparatively analyzing picric acid in contaminated sandy soil and curcumin in synthetically prepared propellant composition.