Polydopamine-Coated Magnetite as a Sensing Material for the Optical Detection of Cationic Surfactants

The sensing of surfactants is a topic of interest for industrial and environmental purposes. Polydopamine-coated magnetite (Fe3O4@PDA) can be a relevant support for the detection of cationic surfactants in water samples. The negative charge in the surface of the PDA material favors the interaction with cationic molecules and allows the design of a chemoreagent for the detection of cationic surfactants by displacement or competition with methylene blue (MB). Magnetite nanoparticles with single and double PDA coating have been prepared and characterized. The PDA surface effectively coats magnetite nanoparticles with a thickness of 5 or 19 nm and a Z potential of −30 mV. The adsorption of MB follows second-order kinetics, and up 33 mg of dye can be loaded in 1 g of the support. The cationic surfactants can displace MB from the Fe3O4@PDA surface, coloring the solution. Thus, it can be applied for the analysis of water samples. The system is selective towards cationic molecules with long alkyl chains, but the response is influenced by high concentrations of divalent cations. The material can be used following diverse sensing protocols with a detection range from 4 × 10−6 to 2 × 10−4 M. The simplicity of its handling together with the naked eye detection allows its application in kits for field analysis with screening purposes.