Quantitative detection of dithiocarbamate pesticides by surface-enhanced Raman spectroscopy combined with an...

Surface-enhanced Raman spectroscopy (SERS) based on nanosilver colloid substrates has great potential for rapid detection of pesticide residues because of its advantages of sensitivity, rapidity, simplicity, low cost, etc. However, its poor repeatability and narrow linear quantitative range limit its practical application. In this paper, a silver colloid SERS analysis method combined with an exhaustive peak-seeking method was introduced for quantitative determination of thiram and ziram. This method can establish a linear quantitative relationship in a wide range by use of an own characteristic peak of analysis as an internal standard (IS) which is found via judging the linear correlation between the intensity ratios of two SERS peaks of analytes and the concentrations. Combined with improving the preparation method of silver colloids, adding suitable activators and optimizing the detection process, a liquid detection system with good repeatability and a wide linear quantitation range was obtained. The relative standard deviation (RSD) of the strongest SERS peak is no more than 8.98%, which is better than the general case of the silver colloid SERS substrate. The ratio of I₁₃₈₄/I₁₁₄₈ has a good linear relationship with the concentration of thiram solution, and the 1148 cm^-1 characteristic peak was utilized as the IS to establish the standard curve equation for the determination of thiram concentration. The equation is I₁₃₈₄/I₁₁₄₈ = -1.7930 × lg[c_thiram (ppm)] + 6.0078 with a linear range of 10^-2 to 10² ppm (4.16 × 10^-8 to 4.16 × 10^-4 mol L^-1) and a limit of detection (LOD) of 10^-2 ppm. The peak of IS for the determination of ziram concentration is at 938 cm^-1, and the equation is I₁₃₈₄/I₉₃₈ = 4.5531 × lg[c_ziram (ppm)] + 6.4792 with a linear range of 10^-1 to 10² ppm (3.27 × 10^-7 to 3.27 × 10^-4 mol L^-1) and a LOD of 10^-4 ppm. Thiram or ziram in apple juice was successfully detected by using this liquid detection system. This analysis system effectively solves the problem of poor repeatability and a narrow linear quantification range in SERS analysis based on silver colloid substrates, and the linear quantification range meets the requirements of the national standard (GB-2763-2019).