Developing a new ultra-sensitive interface to detect As(III) is highly desirable, due to its seriously toxic and low concentration in drinking water. Recently, Fe3O4 nanoparticles of high adsorption toward As(III) have become very promising to be such an interface. However, it is still limited by the poor understanding of their surface physicochemical properties.

Collaborating with Prof. ZHANG Lizhi from Central China Normal University, Prof. HUANG Xingjiuin from Institute of Intelligent Machines, Chinese Academy of Sciences, designed ultra-highly sensitive electrochemical sensing of arsenic(III) promoted by surface Fe(II)/Fe(III) cycle on dumbbell-like Au/Fe3O4nanoparticles. The corresponding article has been accepted for publication by Analytical Chemistry (http://pubs.acs.org/journal/ancham).

Aiming at this, sub-20 nm dumbbell-like Au/Fe3O4 nanoparticle are synthesized and applied to As(III) detection under nearly neutral conditions with square wave anodic strippingvoltammetry (SWASV).

The excellent performance of Au/Fe3O4 modifies screen-printed carbonelectrode (SPCE) for As(III) analysis by combining the catalytic properties of Au NPs with the adsorption ability of the ~10 nm Fe3O4 NPs, as well as the enhanced redox activity by Fe(II)/Fe(III) cycling on the surface of Au/Fe3O4 nanoparticle is found.

Furthermore, the X-ray photoelectron spectroscopy(XPS)evidence of As and Fe on Au/Fe3O4 NPs after electro-adsorption and stripping of 10 ppm As(III) strongly proves that the redox of As(0)/As(III) directly occurs on sub-20 nm Au/Fe3O4NPs with the assistance of Fe(II)/(III) cycle.

Meanwhile, a more distorted local coordination of Fe-O and Fe-Fe in 10 nm Fe3O4 NPs, which has been proved by extended X-ray absorption fine structure (EXAFS), offers its higher adsorption and surface atom activity.

Combining the data of XPS with EXAFS and electrochemistry, this work reveals that the Fe(II)/Fe(III) cycle at ~ 10 nm Fe3O4 and Au/Fe3O4 NPs surface can highly enhance the electroanalysis performance toward As(III).

This work proposes a sensitive sensors and provides novel insights and new theoretical and experimental investigations in understanding the mechanism of electroanalysis from the viewpoint of surface active atoms.

The study was financially supported by the National Natural Science Foundation of China (21735005, 21475133, U1532123, 61573334, 61474122 and U1532120), Natural Science Funds for Distinguished Young Scholars (Grant 21425728), and National Natural Science Fund of China (11405256).