Improvement of selectivity via the surface modification of carbon nanodots towards the quantitative detection of mercury ions

Highly fluorescent carbon nanodots are promising fluorophores for biochemical, pharmaceutical, and environmental analysis due to their facile preparation, biocompatibility, tunability, and low-cost precursors. The selectivity improvements towards an environmentally interesting analyte are challenging in analytical chemistry. Herein, a surface modification using a mixed solvent was used to prepare fluorescent carbon dots (CDs) as selective fluorophores for the detection of mercury ions (Hg2+). The solvothermal method was used for the fabrication of 4-6 nm CDs using xylose as a carbon precursor and ethanol-H2O as a mixed solvent. Moreover, optical, morphological, and surface characterizations were thoroughly conducted. The fluorescence emission of the CDs was selectively quenched after the addition of mercury ions and restored by the addition of cysteine. The CDs were successfully used for the quantitative detection of Hg2+ ions without interferences. The fabricated nanoprobe exhibited the good linearity range of 50-800 nM (R-2 = 0.9947), and a low detection limit down to 10 nM for Hg2+ was calculated. The selectivity experiments showed that the fluorescent probe was specific for Hg2+ even in the presence of interferences. The CDs were stable under rigorous conditions such as extreme ionic strength and low and high pH values. In addition, the CDs were photo- and thermostable; this made the CDs a promising fluorophore for the fabrication of a rugged and robust nanoprobe towards the detection of Hg2+. The Hg2+ ions in tap and wastewater were determined quantitatively with statistically good spike recoveries and standard deviations.