A methanol sensor based on stimulus-responsive magnetic nanoemulsions

We demonstrate the methanol detection ability of stimulus-responsive magnetically polarizable nanoemulsions. The oil-in-water type of nanoemulsions of droplet radius ∼100 nm is produced by emulsification of Fe3O4 based magnetic nanofluid in presence of water, oil and an anionic surfactant. The nano-emulsion droplets in a cylindrical vial or cuvette placed inside a small solenoid that produces a magnetic field of ∼100 Gauss constitute the sensor assembly. It is observed that the presence of methanol in ppm level produces strong changes in the repulsive forces between the nanoemulsion droplets, giving rise to a red shift in the incident white light and a visually perceptible color change of the nanoemulsion. The origin of the red shift is studied by measuring the subtle changes in the intermolecular forces between the emulsion droplets in presence of methanol molecules. We observe that the diffusion of methanol molecules significantly alters the onset of electrostatic repulsion between nanodroplets. The sensor showed a linear detection range of 0-2250 ppm and a response time of ∼1 s. The present approach is a simple, inexpensive and useful for sensitive detection of methanol in aqueous solution.