MCPA is a widely used herbicide used to control weed growth in agricultural fields that has been found at exceeded level (limit of 0.1 ppb or 0.1 g/L in EU' WFD) in water bodies in Ireland and Europe. The measured bulk Raman spectrum of MCPA with the associated peak assignment are presented in SI (Figure ). However, initial SERS experiments failed to detect MCPA even at elevated concentrations ~100 mM. To understand why this was the case, a DFT simulation study was undertaken to explore the adsorption of MCPA at the silver surfaces. In the most energetically favored configuration Figure (a), with an adsorption energy of -0.15 eV, the molecule-surface interactions seem to be mediated by the acid and methyl groups in MCPA. We simulated an O-Ag distance (between the terminating O and the closest Ag atom at the surface) of 2.55 Å, and H-Ag distances (between H in methyl and their closest surface Ag) in a range between 2.73-3.44 Å. In Figure (b) the methyl group stays far from the surface, and the acid group mainly mediates the adsorption. In this case, the computed adsorption energy is weaker than in (a), with a computed adsorption energy of -0.07 eV and bond distances between MCPA and the surface of 3.14 Å for O and 2.61 Å for the terminating H.