Highly efficient carbon quantum dot suspensions and membranes for sensitive/selective detection and adsorption/recovery of mercury ions from aqueous solutions

Nitrogen-doped carbon quantum dots (CQDs), synthesized through a solid-phase microwave-assisted pyrolysis of citric acid and urea at 250 °C, were employed as highly-selective/-sensitive probe as well as an efficient adsorbent toward Hg2+ ions that exist in aqueous solutions. The spherical CQDs possess a homogeneous dispersion with a narrow distribution, ranged from 2.5 to 5.5 nm. The fluorescence responses from CQD samples demonstrate triple band at ca. 450, 500 and 520 nm, influenced by the ratio of citric acid to urea in the carbon precursor. The fluorescence quenching ratio was found to be proportional to the Hg2+ concentration within the solution. Such a correlation was formulated using Stern–Volmer model. The detection limit of CQDs toward Hg2+ ions reached as high as 10 ppb, and the maximal adsorption capacity of Hg2+ onto CQDs was as high as 3.33 g/g, which is 1280, 6, and 16 times higher than the values reported for the active carbon, graphene, and carbon nanotubes, respectively. The CQD-coated polymeric membrane also exhibits the fluorescence quenching and it is easily regenerated by iodide anions for subsequent usages. Accordingly, the functionalized CQDs pave the pathway for engineering the adsorption and recovery of toxic Hg2+ contaminant that are present in aqueous solutions.