Ultrasensitive detection of Hg(II) by small-sized MnO loaded on g-CN nanosheets: Heterojunction facilitates electron transfer and Mn(II)/Mn(III)/Mn(IV) cycle.

Transition metal oxides are widely used in electrochemical detection because of the promotion of redox of heavy metal ions (HMIs) by valence change behavior. However, it is challenging to favorably promote the valence change to achieve the improvement of detection sensitivity. Herein, a MnO/g-CN composite (named as MO-CN) with small-sized of MnO and high proportion of Mn(II) and Mn(III) was prepared, which reveals an excellent performance on detecting mercury ion (Hg(II)). It is discovered that MnO becomes small in size and well disperses on g-CN, which solves the adverse effect of agglomeration and also lead to a good conductivity. And g-CN can provide more adsorption sites to enhance the adsorption on Hg(II). Heterojunction is proved to form in MO-CN and thus accelerates electrons to flow from g-CN to MnO. This results in transforming Mn(IV) to Mn(II) and Mn(III) in MnO, thereby promoting the cycle of Mn(II)/Mn(III)/Mn(IV) and furthermore facilitating the redox of Hg(II). Simultaneously, the obtained sensitivity (473.43 μA μM cm) and limit of detection (LOD, 0.003 μM) are as expected. The nanocomposites and heterojunction based on transition metal oxide and 2D nanomaterials is promising to boost the detection of HMIs.