Graphene Quantum Dots Doped with Sulfur and Nitrogen as Versatile Electrochemical Sensors for Heavy Metal Ions Cd(II), Pb(II), and Hg(II)

A sulfur co-doped nitrogen-graphene quantum dot (S,N-GQD) was synthesized from polyaniline using sulfuric acid as an acid catalyst and S-doping agent. Thus, through a simple hydrothermal synthesis method, we could achieve nanosized (similar to 4 nm) highly crystalline and aromatic S,N-GQDs. The S,N-GQDs, for the first time, exhibited simultaneous sensing toward three of the top 10 toxic metal ions: Cd(II), Pb(II), and Hg(II) with highly sharp peaks and adequate peak-to-peak separation, while the control N-GQD (no S co-doping) exhibited current response only for Cd(II) and the current response of Cd(II) on S,N-GQD was similar to 7-fold higher than that of on N-GQD. The detection limit values were lowest hitherto for Cd(II), Pb(II), and Hg(II) on S,N-GQD with 1, 10, and 1 pM, respectively, for the simultaneous sensing of the metal ions. The significantly enhanced sensitivity compared to that of N-GQD for Cd(II) and the versatile simultaneous sensing capability of S,N-GQD is assigned to the co-doping with S, which enabled the sensing of Pb(II) and Hg(II) through the M(II)-S interactions and the enhanced electronic properties, respectively. The sensing characteristics were effectively stretched to actual environmental water samples such as groundwater, seawater, and wastewater, spiked with Cd(II), Pb(II), and Hg(II), and accomplished similar to 100% recovery in all the tested samples with a relative standard deviation of <0.5%.

Automated summary

A sulfur co-doped nitrogen-graphene quantum dot was synthesized using sulfuric acid as a catalyst and S-doping agent from polyaniline, resulting in nanosized highly crystalline and aromatic S,N-GQDs. These S,N-GQDs exhibited simultaneous sensing towards Cd(II), Pb(II), and Hg(II) with sharp peaks and adequate peak-to-peak separation, while the control N-GQD only responded to Cd(II). The detection limit values for Cd(II), Pb(II), and Hg(II) on S,N-GQD were the lowest reported so far. The enhanced sensitivity of S,N-GQD for Cd(II) and its versatile simultaneous sensing capability are attributed to the co-doping with S, which enables the sensing of Pb(II) and Hg(II) through M(II)-S interactions and enhanced electronic properties, respectively. The sensing characteristics were successfully applied to actual environmental water samples and achieved near 100% recovery with a relative standard deviation of less than 0.5%.