Modified magnetite nanoparticles for colorimetric sensing of anionic surfactants in water

Determination of anionic surfactants in water is classically based on the ion pairing of anionic surfactants with the methylene blue dye. As the optical absorption peaks of the dye and the ion pair overlap each other at 662 nm, it is required that the ion pair should be separated from the dye by chloroform extraction. In this reported work, it was found that the magnetite nanoparticles coated with poly(acrylic acid) could rapidly adsorb the methyl blue dyes in water and result in an obvious shift in optical absorption from 662 to 590 nm, which realises the spectral separation of the dye and the ion pair. Accordingly, using methyl blue dyes and the modified magnetite nanoparticles, anionic surfactants in water can be directly determined without extraction and/or separation. The lower limit of detection was 0.10 mg/l, and improved to 30 μg/l by magnetic removal of the nanoparticles. The functionalised magnetite nanoparticles allowed the development of a simple procedure for colorimetric sensing of anionic surfactants without the use of hazardous chlorinated organic solvents.