In recent years, utilizing nanomaterials modified electrodes to realize highly sensitive and selective electrochemical detection of trace heavy metal ions in water has become a hot topic. However, it is generally believed that increased currents and analytical sensitivity were caused by an increased microscopic surface area. It was rarely reported about the essence of nanomaterials enhanced electrochemical response on the atomic scale.

　　Recently, a series of progresses have been made in the novel electroanalysis method through metal oxide nanostructures with exposed different facets modified electrodes by the research group which is under supervision of Prof. Jinhuai Liu and Prof. Xingjiu Huang who is one of the international renowned scholars at research center for biomimetic functional materials and sensing devices, Institute of intelligent machines, Chinese academy of sciences. They found that metal oxide nanostructures with different exposed crystal facets modified electrodes could result in the selective electrochemical response, and they also proposed the role of the effect of exposed nanocrystal facets in the electrochemical sensing of metal ions (Sci. Rep. 2013, 3, 2886; Electrochem. Commun. 2013, 34, 270).

　　Based on the achievements above, the research group further investigated the electrochemical sensing mechanism of three types of α-Fe₂O₃ nanostructures including nanocubes, nanoplates, and nanorods with crystallographically dominant facets of {012}, {001} and {110}, respectively, toward Pb²⁺ ion from the atomic-scale level with the combined experimental and theoretical efforts. Three types of α-Fe₂O₃ nanostructures including nanocubes, nanoplates and nanorods with crystallographically dominant facets of {012}, {001} and {110} were first designed and fabricated. It was found interestingly that the electrochemical performances (e.g., sensitivity, limit of detection, etc.) of these α-Fe₂O₃ nanostructures depend on their exposed facets, and especially, the sensitivity could be ranked as "nanocube < nanoplate < nanorod", which was consistent with the experimental results about adsorption. Furthermore, the research group, in cooperation with Prof. Qunxiang Li at Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China , carried out the simulation of density functional theory to better and scientifically understand different exposed facet with different surface energy to influence adsorption and diffusion behaviors of Pb²⁺ and result in different electrochemical performances. The results revealed that the selective adsorption of heavy metal ions on different exposed crystal facets was attributed to the selective electrochemical response, which provided a new route to realize the improved sensitivity in electrochemical sensing of toxic metal ions. The experimental results were published in Chemical Communications (Chem. Commun., 2014, 50, 5011-5013).

　　This research was supported by the National Basic Research Program of China (2011CB933700, and 2011CB921404), the Natural Science Foundation of China (21073197, 11074235, and 11034006), and the Strategic Priority Research Program (B) of the CAS (XDB01020000).　　 

　　The electrochemical performances toward Pb²⁺ of α-Fe₂O₃ nanocubes, nanoplates, and nanorods modified GCE strongly depend on their exposed facets