On-site preconcentration of pesticide residues in a drop of seawater by using electrokinetic trapping, and their determination by surface-enhanced Raman scattering.

The authors have developed a method for simultaneous quantification of several charged pesticides (as shown for amitrole, simazine, trichlorfon and bisultap). It is based on the use of a reduced graphene oxide-modified screen-printed electrode (RGO-SPE) and combines electrokinetic trapping (EKT) and surface-enhanced Raman scattering (SERS). When a 50 μL droplet containing negatively charged RGO and positively charged gold nanorods is placed on the SPE, the RGO and gold nanorods are selectively attracted on the surface of the SPE during EKT. This leads to the formation of sandwich-type hybrid substrates. The resulting substrates also contain Raman "hot spots" among the high-density gold nanorods. This, along with the excellent adsorption performance of RGO, makes it an excellent SERS substrate for on-site detection of the charged pesticides. The method is highly reproducible and long-term stable. The spot-to-spot variation of the intensity of the SERS is <15%, and the performance of SERS activity is maintained over a period of 6 weeks. The method works over a wide range of concentrations (0.5 nM to 4 μM) for charged pesticides under optimal conditions, with a sub-nanomolar detection limit (at a signal to noise ratio of 3). The EKT-SERS method requires only microliter volumes and takes only minutes for completion. Therefore, the method provides high sensitivity for detection while preserving the selectivity and stability required for reliable quantitative analysis. Graphical abstract A method combining electrokinetic trapping and SERS can be used for simultaneous detection of charged pesticides in a drop of seawater.