Abstract
This article describes an optical method based on the association of surface plasmon resonance (SPR) with chitosan (CS) film and its nanocomposites, including zinc oxide (ZnO) or graphene oxide (GO) for glyphosate detection. CS and CS/ZnO or CS/GO thin films were deposited on an Au chip using the spin coating technique. The characterization, morphology, and composition of these films were performed by Fourier-transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and contact angle technique. Sensor preparation conditions including the cross-linking and mobile phase (pH and salinity) were investigated and thoroughly optimized. Results showed that the CS/ZnO thin-film composite provides the highest sensitivity for glyphosate sensing with a low detection limit of 8 nM and with high reproducibility. From the Langmuir-type adsorption model and the effect of ionic strength, the adsorption mechanisms of glyphosate could be controlled by electrostatic and steric interaction with possible formation of 1:1 outer-sphere surface complexes. The selectivity of the optical method was investigated with respect to the sorption of glyphosate metabolite (aminomethylphosphonic acid) (AMPA), glufosinate, and one of the glufonisate metabolites (3-methyl-phosphinico-propionic acid) (MPPA). Results showed that the SPR sensor offers a very good selectivity for glyphosate, but the competition of other molecules could still occur in aqueous systems.
Highlights
Glyphosate (N-(phosphonomethyl)glycine), the world most widely applied herbicide in agriculture and urban areas [1], was first synthesized by the Swiss chemist Henri Martin in 1950 [2] as a potential pharmaceutical compound [3]
The thin films of CS, CS/Graphene oxide (GO), and CS/zinc oxide (ZnO) composite deposited onto the Au chips and cross-linked by GA were characterized by Fourier-transform infrared spectroscopy (FTIR) (Figure 3)
Acidic medium, was favored by the presence of amino to the cross-linked improvement theinelectrostatic and the lower SPRproperties response could explained the of fact that aminocross-linked groups are partially hydrophobic of CS be sensor by EPI inblocked acidic medium, For that reason, GA wascould selected as a cross-linker for CS/ZnO
Summary
Glyphosate (N-(phosphonomethyl)glycine), the world most widely applied herbicide in agriculture and urban areas [1], was first synthesized by the Swiss chemist Henri Martin in 1950 [2] as a potential pharmaceutical compound [3]. A phosphonomethyl derivative of the amino acid glycine (Figure 1) [4], glyphosate is very polar, highly soluble in water (12 g/L or 71 mM at 25 ◦ C) and insoluble in non-polar organic solvents (acetone, ethanol, and xylene) [5,6,7,8,9]. The herbicidal activities of glyphosate were discovered in 1970 by John Franz and his co-workers at Monsanto [12]. Worldwide use of glyphosate in agriculture rose 13.3-fold, from 43 million kg in 1994 to 747 million kg in 2014 [2]. Glyphosate is widely used in agriculture because of its ability to control perennial weed species, overwintering rhizomes and tubers, its ability to bind to soil colloids, and to introduce transgenic, glyphosate-resistant crops [15,16]
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