Nitrogen-doped carbon dot threads as a “turn-off” fluorescent probe for permanganate ions and its hydrogel hybrid as a naked eye sensor for gold(III) ions

Highly fluorescent nitrogen-doped carbon dot (NCD) threads were synthesized via simple pyrolysis of citric acid, p -hydroxybenzoic acid, and ammonia. The NCDs show excitation-independent behavior with maximum excitation and emission wavelengths of 350 nm and 435 nm, respectively. The developed probe was used as a turn-off fluorescent sensor for the selective and sensitive determination of permanganate ions in aqueous media. The probe’s hydrogel hybrid displayed a beautiful purple color demonstrating its potential as a naked eye sensor for gold detection. The ratiometric sensor exhibited excellent selectivity towards permanganate ions over 27 other ions with a linear range of 510 nM to 2 μM, a detection limit of 170 nM, and a linear regression value ( R 2 ) of 0.9944. Similarly, the linear range and limit of detection for gold ions was 3.89–20 μM and 1.285 μM, respectively. The synthesized NCDs were also used as a fluorescent ink as well as a naked eye marker in association with a gold solution demonstrating its potential forensic and anti-counterfeiting applications. Graphical abstract