Contamination by trace elements including arsenic (As) is a pressing environmental issue that causes serious threat to human health. The presence of As has been well documented in multiple environmental matrices including groundwater, soil and sediment, and foods. The toxicity and mobility of As depend on its speciation. The need for rapid and sensitive field As speciation analysis has motivated great research efforts for decades.

Surface enhanced Raman scattering (SERS) is a rapid and ultrasensitive spectroscopic technique in chemical analysis, taking advantage of the explosive growth in nanofabrication. Herein, we present a portable SERS sensor for on site monitoring of arsenic speciation. Fe₃O₄@Ag core-shell magnetic NPs, which are capable of fast preconcentration, rapid separation, and high SERS sensitivity, were used as a reliable SERS substrate for determination of As(III) and As(V). The entire process can be completed within 2 min. Moreover, the active SERS substrate can be used for As sensing in complex media such as juice, wine, and soils.

The electromagnetic (EM) mechanism and short-range chemical (CT) effect simultaneously contribute to the overall SERS enhancement. The former does not involve the formation of chemical bonds between the analyte and the substrate, and thus the Raman bands do not shift significantly. In the CT mechanism the Raman peaks are selectively enhanced when excited by different laser lines. Moreover, the CT mechanism depends sensitively on the particular chemical bonds between the analyte and the substrate, resulting in shifts in peak positions. The magnetic SERS platform provides a robust and versatile approach in rapid detection of multiple trace elements in the field using a portable Raman spectrometer.