Immiscible oil-water interface: dual function of electrokinetic concentration of charged molecules and optical detection with interfacially trapped gold nanorods.

In this paper, we report that an immiscible oil-water interface can achieve the dual function of electrokinetic molecular concentration without external electric fields and sensitive optical detection without a microscope. As a proof-of-concept, we have shown that the concentration of positively charged molecules at the oleic acid-water interface can be increased significantly simply by controlling the pH. Three-dimensional phase field simulation suggests that the concentration of positively charged rhodamine 6G can be increased by about 10-fold at the interface. Surface-enhanced Raman spectroscopy (SERS) is utilized for label-free detection by taking advantage of this molecular accumulation occurring at the interface, since gold nanorods can be spontaneously trapped at the interface via electrostatic interaction. SERS measurements suggest that the immiscible oleic acid-water interface allows the limit of detection to be improved by 1-3 orders of magnitude.