Raman spectra were acquired with a Renishaw InVia microscope equipped with a 514 nm Ag ion laser. For the measurements, SERS substrates were placed in a dedicated holder, shown in SI Figure ). A 10 μL aliquot of 2,4-D solution was dropped over Ag-GA SERS substrate and covered with a cover glass slide. All measurements were acquired using a 20x lens (NA 0.4). All the spectra presented herein were averaged from ten measurements obtained at different random points on the substrate except for the reproducibility study where 25 measurements were employed. All spectra were obtained using a laser power of 0.07 mW and an integration time of 10 s.