Development of a facile rolling method to amplify an analyte's weak SERS activity and its application for chlordane detection†
The surface-enhanced Raman scattering (SERS) technique has been extensively explored as a rapid analytical tool for various pesticides. However, not all pesticides exhibit similar SERS activities; in particular, certain organochloride pesticides, such as chlordane, have relatively weaker SERS signals when interacting with citrate-coated gold nanoparticles. Herein, a simple rolling method was developed to amplify the SERS signal of chlordane and to facilitate its detection in a livestock oil sample. The rolling method was performed by mixing a 2.5 microliter chlordane methanolic solution with a gold nanoparticle solution with a pipette on a piece of parafilm for 20 seconds. Once the half volume size of solvents was evaporated, another 2.5 microliter chlordane solution was added to the mixture and was pipetted for another 20 seconds. This action was continuously repeated to enrich the chlordane concentration, allow a better interaction between chlordane and gold nanoparticles, and achieve Raman signal amplification. For example, for the 2.5 ppm chlordane sample, after four times of rolling, the Raman signals were enhanced by approximately four times. Later on, a mathematic model was established to predict the concentration of chlordane in a complex crude oil sample based on the obtained intensity reading and the number of repeats. Compared to the spiked concentration (i.e., 1 ppm) of chlordane, the recovery value reached 95% approximately. The rolling method is a simple enrichment approach without the need for the use of concentrating instruments. Combined with the prediction model, it can determine the concentration of hard-to-detect analytes in food samples based on the amplification of their weak SERS signals.