Ultrasensitive detection of Hg(II) by small-sized Mn3O4 loaded on g-C3N4 nanosheets: Heterojunction facilitates electron transfer and Mn(II)/Mn(III)/Mn(IV) cycle

Abstract

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 Mn₃O₄/g-C₃N₄ composite (named as MO-CN) with small-sized of Mn₃O₄ 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 Mn₃O₄ becomes small in size and well disperses on g-C₃N₄, which solves the adverse effect of agglomeration and also lead to a good conductivity. And g-C₃N₄ 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-C₃N₄ to Mn₃O₄. This results in transforming Mn(IV) to Mn(II) and Mn(III) in Mn₃O₄, 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^-1 cm^-2) 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.

Keywords: Electron transfer; Heterojunction; Hg(II) detection; Mn(3)O(4)/g-C(3)N(4) composites; Valence cycle.