Magnetophoretic Velocimetry of Manganese(II) in a Single Emulsion Droplet at the Femtomole Level

We developed a new experimental technique named magnetophoretic velocimetry to determine a small amount of paramagnetic species in a single microdroplet. The magnetophoretic velocity of an aqueous droplet containing paramagnetic metal ion dispersed in an organic medium could response to a very small amount of the metal ion under an inhomogeneous magnetic field. The paramagnetic droplet (2∼8 μm diam) used as a test sample in this study was the aqueous droplet of manganese(II) chloride dispersed in ethylbenzoate whose density was nearly equal to water. A pair of small Nd−Fe−B magnets placed with a gap of 400 μm generated an inhomogeneous magnetic field between the edges, at which the product of the magnetic flux density and the gradient, B(∂B/∂x), was as high as 410 T² m^-1. When a silica capillary containing the emulsion was inserted into the gap between the magnets, the magnetophoretic migration of the droplets was observed with a video microscope. The magnetophoretic velocity divided by the squared radius of the droplet was proportional to the MnCl₂ concentration in the droplet, as predicted by a theoretical calculation. The estimated detection limit in this simple method was lower than 10^-16 mol for manganese(II).