Optical sensing of salinity of water using magnetic nanoemulsions

We report a highly sensitive and simple optical method to measure salinity of water using magnetic nanoemulsion. The proof of concept of salinity sensing using magnetic nanoemulsion is demonstrated by measuring the concentration of NaCl in water. Under the influence of a magnetic field, nanoemulsion drops form one dimensional periodic structures that diffract visible light. With increasing salt concentration the interdroplet distance decreases resulting in a blue shift in the diffraction peak wavelength. The linear shift in the diffraction peak wavelength changes with the ionic concentrations is exploited to develop a salinity sensor. The cause for the Bragg peak shift with NaCl is understood by measuring the intermolecular forces using a force apparatus. As the emulsion used is easy to produce, inexpensive, portable and allows rapid detection of several metal ions, our new approach offer promising application for ion recognition. The current efforts are aimed at engineering the properties of magnetically polarizable drops to improve its visual detection capability and specificity.