Dual detection of Malation and Hg (II) by fluorescence switching of graphene quantum dots

The present study investigates sensitive and selective “off-on” method to analyze and detect mercury ions (Hg2+) and Malation pesticide using graphene quantum dots (GQDs) as a new fluorescent (FL) probe. When GQDs are present, Hg2+ cations are capable of being coordinated on the GQD surface by their negatively charged surface. The process of absorption occurs using electrostatic interactions. Hence, the FL of the GQDs would be remarkably quenched (turn-off) at 463 nm for 1 μM Hg2+ as a result of FL charge transfer. Nevertheless, when 40 μM of Malation is added to the GQDs-Hg2+ solution, the FL of the GQDs enhances significantly at 463 nm (turn-on), since Hg2+ is combined with SR- group in the Malation rather than with the GQDs in the aqueous solution. Instead of the weak electrostatic interaction, a strong metal-thiol bond interaction is formed between Hg2+ and Malation and an enhancement in the FL of GQDs is observed. Finally, acceptable linearity for Hg2+ and Malation detection is obtained in the concentration ranges of 1-10 μM and 10-80 for Hg2+ and linear range of 80-120 μM for Malation with an LOD of 0.5 μM and R2 = 0.99.