Gum Arabic has previously been used to interact with pesticides and is known to attract an analyte in close proximity to the surface of the GA related nanoparticles. For this reason, Gum Arabic polymer layers helped to provide stable SERS signal and facilitated 2,4-D detection using Raman spectroscopy. It consists of polysaccharides rich in galactose and arabinose, and a small protein fraction associated with these polysaccharides, see SI Figure , and contains a large amount of -OH, -O-, and -CH bonds as well as a small portion of -NH2 and -SH group associated with the protein. Such functional groups may attract 2,4-D to the surface of the Ag-GA nano particles, through hydrogen bonding between -OH / -O-group on Ag-GA surface and the halogen (-Cl) group in 2,4-D moleculars, or π-π interactions between -CH and the benzene ring of 2,4-D. In our study, 2,4-D was detected in water by dropping a 10 µL aliquot on top of Ag-GA SERS substrates. See Figure for a schematic of the SERS device setup. Typical Raman spectra, taken in deionized water and in a 1 pM 2,4-D solution, are shown for S4 in Figure . Raman spectra of bare Ag -Gum Arabic SERS samples, acquired in the air, exhibited two broad peaks at 1400 cm -1 and 1601 cm -1 shown in SI Figure . Applying water onto the substrate did not change the spectral characteristics, but reduced the overall intensity, see also SI Figure . This could be attributed to the interaction between DI water and Gum Arabic which has numerous -OH bonds that have good affinity to water. After adding 2,4-D solution (1 pM) to a substrate, a distinct spectrum was observed after 30 minutes. Peaks ~ 230 cm -1 and ~1600 cm -1 gradually disappeared as the 2,4-D concentration increased, and new peaks at 293 cm -1 , 889 cm -1 , peaks at 926 cm -1 , 1400 cm -1 and three sharp peaks around 1129 cm -1 , appeared. In addition, we note that the 2,4-D solution did not dry during the course of our experiments. In the initial work, it was observed that structures S1, S2, S3 were able to detect 0.1 nM (0.0221 ppb) 2,4-D in 30 min; while structure S4 was able to detect as low as 1 pM (0.221 ppt) 2,4-D in the same time frame. An immediate response was observed for 0.1 nM 2,4-D (S4 substrates) which is more than four times lower than the MRL (0.1 ppb) required by EU water framework directive for individual pesticides. As a result, structure S4 substrates were selected for further sensing studies reported below. The better sensing capabilities of S4 could be attributed to the morphology and size difference of the Ag-GA nano particles.