Ultrasensitive and selective electrochemical sensing of Hg(ii) ions in normal and sea water using solvent exfoliated MoS2: affinity matters

Abstract

In this work, solvent exfoliated molybdenum disulphide (MoS₂) has been exploited for the electrochemical (EC) sensing of mercury(II) ions (Hg²⁺). A 30-fold increase in peak current compared to that of a glassy carbon electrode (GCE) and a detection limit (LOD) of 0.000001 nM, i.e. in parts per quadrillion (0.2 ppq) levels, which satisfactorily meets the sensitivity requirement of the U.S. Environmental Protection Agency (EPA), were exhibited by the MoS₂/GCE. Furthermore, the MoS₂ electrode demonstrated an excellent selectivity towards Hg²⁺ with almost nil current response for most other metal ions except Ag⁺. No interference was observed from Ag⁺ during simultaneous determination of Hg²⁺ and Ag⁺ as the latter peak was observed at a higher peak potential with a ΔE of ∼0.4 V. The MoS₂/GCE extended the remarkable sensing of Hg²⁺ to real water samples such as tap and sea water. The sensitivity of MoS₂ for Hg²⁺ was found to be superior than that of reduced graphene oxide (rGO) and graphene oxide (GO) (∼4 & 30 times, respectively). The superior sensing of Hg²⁺ by MoS₂ was assigned to the synergy of the affinity (between the Hg²⁺ and S²⁻ groups) and the semiconducting properties of MoS₂. It is notable that this performance of MoS₂ is without any modifications except for simple solvent ultrasonication.