Different 2,4-D concentrations ranging from 0.2 mM down to 1 pM in DI water were used for the calibration study, see Figure (C). The peak ratios I293 cm-1 / I1400 cm-1 and I1130 cm-1 / I1400 cm-1 increased with increasing 2,4-D concentration, while the broad peak near 1600 cm -1 decreased because of the overall coverage of 2,4-D molecules as the concentration increased. Figure (D) showed the plot of I293 cm-1 / I1400 cm-1 and I1130 cm-1 / I1400 cm-1 against the logarithm of the concentrations of the 2,4-D solutions. Each spectrum was obtained by averaging ten spectra acquired from different random spots on the same substrate. In the range of 2,4-D concentration studied, a linear calibration plot of y = 0.6296 + 0.0266*log10(x), with an R 2 = 0.9530 was obtained. The data did show variability; the relation between I293 cm-1 / I1400 cm-1 (y) and log10 (concentration (in nM)) (x) was: y = 0.4078 + 0.0197*log10(x), R 2 = 0.8990; which, with a low slope in the calibration curve, limits the accuracy in quantitative detection. However, we note that SERS sensors can be used to detect the presence of 2,4-D and trigger sample collection for further analysis in dedicated laboratories; or to determine the 2,4-D concentration by standard addition methods. This SERS platform offers a rapid approach to monitoring the presence of 2,4-D pesticide at ultra-low concentrations. Similar experiments were undertaken with Gum Arabic that was purchased from Amazon, as shown in SI figure and exhibited similar results. Figure (A) Waterfall spectra of 25 random points for 2 µM 2,4-D on S4 SERS substrate. (B) Histogram of I293 cm -1 / I1400 cm -1 , I1130 cm -1 / I1400 cm -1 at 25 different random points; (C) Spectra of 2,4-D concentration from 1 pM to 0.2 mM, offset for clarity. (D) Linear fitting line for I1130 cm -1 / I1400 cm -1 , I293 cm -1 / I1400 cm -1 VS log10 (concentration) (in nM) for a S4 substrate. The error bars were obtained from the standard deviation (SD) of ten measurements at random points.