Abstract
The indirect determination of the most used herbicide worldwide, glyphosate, was achieved by the SERS technique using hemin chloride as the reporter molecule. An incubation process between hemin and glyphosate solutions was required to obtain a reproducible Raman signal on SERS substrates consisting of silicon decorated with Ag nanoparticles (Si-AgNPs). At 780 nm of excitation wavelength, SERS spectra from hemin solutions do not show extra bands in the presence of glyphosate. However, the hemin bands increase in intensity as a function of glyphosate concentration. This allows the quantification of the herbicide using as marker band the signal associated with the ring breathing mode of pyridine at 745 cm−1. The linear range was from 1 × 10−10 to 1 × 10−5 M and the limit of detection (LOD) was 9.59 × 10−12 M. This methodology was successfully applied to the quantification of the herbicide in honey. From Raman experiments with and without silver nanoparticles, it was possible to state that the hemin is the species responsible for the absorption in the absence or the presence of the herbicide via vinyl groups. Likewise, when the glyphosate concentration increases, a subtle increase occurs in the planar orientation of the vinyl group at position 2 in the porphyrin ring of hemin over the silver surface, favoring the reduction of the molecule. The total Raman signal of the hemin-glyphosate incubated solutions includes a maximized electromagnetic contribution by the use of the appropriate laser excitation, and chemical contributions related to charge transfer between silver and hemin, and from resonance properties of Raman scattering of hemin. Incubation of the reporter molecule with the analyte before the conjugation with the SERS substrate has not been explored before and could be extrapolated to other reporter-analyte systems that depend on a binding equilibrium process.
Highlights
N-phosphonomethyl glycine, known as glyphosate (GLY), is the main active ingredient of one of the most used herbicides worldwide (Avino et al, 2020)
We report a SERS approach for indirect quantification using a reporter molecule (RM) under a previous incubation process with the analyte, a method that has not been reported previously
The key concept is based on implementing an incubation step that allows a binding equilibrium process between the RM and the analyte, since we demonstrated that this step may influence the Raman signal reproducibility, which is an important aspect pursued in this research area
Summary
N-phosphonomethyl glycine, known as glyphosate (GLY), is the main active ingredient of one of the most used herbicides worldwide (Avino et al, 2020). Since the 1970s, glyphosate has been used in an excessive and deregulate manner in agriculture. It is currently applied in field crops, including fruits, vegetables and cereals, for both conventional and genetically modified (GM). Glyphosate residues are commonly found in environmental, biological, and food samples (Steinborn et al, 2016; Avino et al, 2020). Despite the importance of glyphosate, its stability to hydrolysis and the lack of chromophore or fluorescent groups complicate its analytical determination, besides the difficulties of being separated in food matrices due to the similarity with natural amino acids
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