This study aims to use graphene quantum dots (GQDs) as a fluorescence switching sensor (turn on–off) for the simultaneous detection of cyanide (CN–) and ferricyanide [Fe­(CN)6]3– in wastewater samples. The GQDs were synthesized by pyrolyzing solid citric acid. The intrinsic blue color of the solution was observed under ultraviolet irradiation. The fluorescence spectrum was maximized at both excitation and emission wavelengths of 370 and 460 nm, respectively. The fluorescence intensity of GQDs decorated with Hg2+ (turn-off mode as the starting baseline) could be selectively turned on in the presence of CN– and once back to turn-off mode by [Fe­(CN)6]3–. The fluorescence switching properties were used to develop a fluorescence turn-on–off sensor that could be used to detect trace amounts of CN– and [Fe­(CN)6]3– in water samples. For highly sensitive detection under optimum conditions (Britton–Robinson buffer solution in the pH range of 8.0–9.0, linearity ranges of 5.0–15.0 μM (R 2 = 0.9976) and 10.0–50.0 μM (R 2 = 0.9994), respectively, and detection limits of 3.10 and 9.48 μM, respectively), good recoveries in the ranges of 85.89–112.66% and 84.88–113.92% for CN– and [Fe­(CN)6]3–, respectively, were recorded. The developed methods were successfully used for the simultaneous and selective detection of CN– and [Fe­(CN)6]3– in wastewater samples obtained from local municipal water reservoirs.

This study aimed to synthesize dimethylglyoxime (DMG) (N-source)-doped graphene quantum dots (N-GQDs) via simultaneous pyrolysis of citric acid and 1.0% (w/v) DMG. The maximum excitation wavelength (λ max , ex = 380 nm) of the N-GQD solution (49% quantum yield (QY)) was a red shift with respect to that of bare GQDs (λ max , ex = 365 nm) (46% QY); at the same maximum emission wavelength (λ max , em = 460 nm), their resonance light scattering (RLS) intensity peak was observed at λ max , ex/em = 530/533 nm. FTIR, X-ray photoelectron spectroscopy, XRD, energy-dispersive X-ray spectroscopy, and transmission electron microscopy analyses were performed to examine the synthesized materials. The selective and sensitive detection of Ni 2+ using the RLS intensity was performed at 533 nm under the optimum conditions consisting of both 25 mg L –1 N-GQDs and 2.5 mg L –1 DMG in the ammonium buffer solution of pH 9.0. The linearity of Ni 2+ was 50.0–200.0 μg L –1 with a regression line, y = 5.031 x – 190.4 ( r 2 = 0.9948). The limit of detection (LOD) and the limit of quantitation (LOQ) were determined to be 20.0 and 60.0 μg L –1 , respectively. The method precision expressed as % RSDs was 4.90 for intraday ( n = 3 × 3) and 7.65 for interday ( n = 5 × 3). This developed method afforded good recoveries of Ni 2+ in a range of 85–108% when spiked with real water samples. Overall, this innovative method illustrated the identification and detection of Ni 2+ as a DMG complex with N-GQDs, and the detection was highly sensitive and selective.

The goal of this work was to use the pyrolysis process to synthesize graphene quantum dots doped with garlic extract (as N,S-GQDs) and simultaneously co-doped with iodine (as I-GQDs).