Simultaneous determination of Hg(II) and Cu(II) in water samples using fluorescence quenching sensor of N-doped and N,K co-doped graphene quantum dots

The present study was aimed to use of N doped graphene quantum dots (N-GQDs) and N,K co-doped graphene quantum dots (N,K-GQDs) as a fluorescence quenching sensor to determine both mercury and copper in water sample, simultaneously using simple fluorescence protocol. Each of N-GQDs or N,K-GQDs was optimized separately with 15% (w/v) HNO3 or KNO3, respectively, and their quantum yields were determined and compared. It was found that N-GQDs, obtained from 3% (w/v) HNO3 doped resulted higher fluorescence intensity at the maximum excitation and emission wavelengths of 370 and 460 nm, respectively, with higher quantum yield (QY = 83.42%) compared with that of undoped GQDs (QY = 16.35%). While N,K-GQDs obtained from 5%(w/v) KNO3 gave somewhat different fluorescence spectrum, but still had the same maximum excitation and emission wavelengths with rather highest QY (94.07%). However, it is interesting that detection sensitivity expressed as slope of their calibration curve (y = 5.43x - 19.48; r(2) = 0.9971) of the N-GQDs is rather higher than that (y = 1.29x + 17.66; r(2) = 0.9977) of the N,K-GQDs for Hg2+ fluorescence quenching sensor, and the fluorescence intensity of N-GQDs had better selectively quenching effect only by both Hg2+ and Cu2+. Thus, their quenching effects were selected to develop the fluorescence turn-off sensor for trace level of both metal ions in real water samples. For method validation, the N-GQDs exhibited high sensitivity to detect both Hg2+ and Cu2+ with wide linear ranges of 20100 mu M and 100500 mu M, respectively. Limit of detection (LOD) and limit of quantitation (LOQ) were 0.42 mu M & 1.41 mu M for Hg2+ and 13.19 mu M & 43.97 mu M for Cu2+, respectively, with their precision expressed as an intra-day and an inter-day analysis of 6.98% & 11.35% for Hg2+ and 11.78% & 9.43% for Cu2+, respectively. Also the study of matrix analysis of the water samples (drinking water and tap water), was carried out using N-GQDs and N,K-GQDs resulted good percentage recoveries in comparison with those using undoped GQDs under the same optimum conditions. (C) 2020 The Author(s). Published by Elsevier B.V. on behalf of King Saud University.