One-pot microwave-assisted synthesis of blue emissive multifunctional N-S-P co-doped carbon dots as a nanoprobe for sequential detection of Cr(VI) and ascorbic acid in real samples, fluorescent ink and logic gate operation

Abstract

In this communication, blue light-emitting fluorescent N-S-P co-doped carbon dots (PGP-CDs) were synthesized by a simple, yet rapid procedure using microwave charring. Low cost, easily accessible precursors like glutathione, polyethylene glycol (PEG-400), and o-phosphoric acid (H₃PO₄) were used in the synthesis following which PGP-CDs obtained with a good quantum yield (15%) were well characterized. The HRTEM analysis showed that PGP-CDs were well distributed and spherical, with an average size of 4.5 nm. XPS analysis confirmed the chemical composition of doped N-S-P-CDs, and fluorescence spectroscopy revealed that they exhibit pH-independent and excitation wavelength-dependent fluorescence with excitation/emission maxima at 380/455 nm. Blue fluorescence of the PGP-CDs was exclusively quenched by the addition of carcinogenic Cr(VI) in the presence of other metal ions within a linear range of 0.1–100 µM which can be explained by the inner filter effect (IFE). The regeneration of carbon dots could be achieved by the addition of ascorbic acid (AA) and thus, PGP-CDs act as a sensitive fluorescent turn “on–off-on” nano-sensor in water. The limit of detection (LOD) for Cr(VI) was found to be 4 nM which is lesser than the permissible limit (0.9 µM) as prescribed by World Health Organisation (WHO) in drinking water. PGP-CDs show reversibility up to four cycles and on this basis, a combinatorial molecular logic gate consisting of a special combination of logic functions NAND & NOT has been constructed. Furthermore, PGP-CDs were also utilized as fluorescent ink without any chemical alteration. For real sample applications, PGP-CDs were also successfully applied to determine Cr(VI) in real water samples and PGP-CD + Cr(VI) complex to detect AA levels in food samples. Results obtained from real sample analysis suggest that PGP-CDs can be used as a potential nanoprobe that offers outstanding precision in detecting the trace amount of Cr(VI) and AA with a low detection limit.