Interaction of soil humic acids with herbicide paraquat analyzed by surface-enhanced Raman scattering and fluorescence spectroscopy on silver plasmonic nanoparticles

A study of the interaction between paraquat (methyl viologen) and humic acids, extracted from a soil amended over 30 years with crop residues, cow slurries and cattle manure, was carried out by two emission spectroscopies based on plasmonic effects: surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF). To carry out this study Ag nanoparticles were used. The complex formation was tested by analyzing the effect of the herbicide on humic acids, and by varying experimental parameters such as the pH and the laser excitation wavelength. The study of the vibrational bands led to infer information about the interaction mechanism of paraquat with humic acids and to find a correlation between this interaction and the humic acids structural modification induced by the different amendments added to soil.

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► The interaction between paraquat and soil humic acids amended over 30 years with different substrates is studied.
► SERS and SEF provide significant selective information from the pesticide and the organic matter.
► The aggregation, coiling/uncoiling and the adsorption of humic acids in the presence of paraquat on metallic surfaces can be investigated.
► The presence of carboxylate, aliphatic chains and aromatic moieties determines the interaction mechanism with the pesticide.