Grafting of organophilic silane in the interlayer space of acid-treated smectite: Application to the direct electrochemical detection of glyphosate

The internal and external surface of smectite was successfully modified by triethoxyoctylsilane. The material was first submitted to a soft acid pre-treatment to provide abundant and available hydroxyl functions capable of reacting with the silane. Physicochemical characterizations (XRD, TGA, solid-state C-13 NMR, FTIR and specific surface analysis) showed that HCl treatment led to the moderate modification of the clay structures. This resulted in a slight increase of d(001) (from 10.1 angstrom to 11.1 angstrom) and the specific surface area (from 102.8 m(2) g(-1) to 113.9 m(2) g(-1)). Silane grafting was more effective in ethylene glycol (EG) as solvent compared to toluene, as showed by the high amount of silane present in the functionalized clay and the significant increase of d(001) following grafting (14.9 angstrom). On the other hand, despite the high amount of grafted silane, the electrochemical characterization clearly showed that the clay still retains some cation exchange capacity, although negligible. The materials were then evaluated for their performance as modifiers of a carbon paste electrode applied to the electroanalysis of glyphosate, certainly the most used pesticide in the world. The material functionalized in EG was found to be the most efficient. After the optimization of some experimental parameters (5% of modifier and pH of the electrolytic solution set at 6.8), a calibration curve was plotted in the concentration range of 10 mu M to 100 mu M. A sensitivity of 0.15 mu A mu M-1 and a detection limit of 0.98 mu M were obtained. This electrode presents the advantage of being able to perform direct detection of glyphosate and offers exceptional reproducibility (variation of about 1.1% for eight consecutive measurements). It was therefore successfully used for the determination of trace amount of glyphosate present in contaminated soil.