Localized generation of attoliter protein solution droplets by electrofocused liquid-liquid separation

We develop a simple technique for the generation of droplets of a high-concentration protein solution of attoliter volume and a diameter of >or=500 nm, containing several thousand protein molecules, and their accurate deposition on micron-size electrodes, standalone or in an array. The technique is based on liquid-liquid phase separation in the homogeneous region of the phase diagram, induced by a nonuniform weak electric field, which also accurately directs the generated droplets toward the electrode. We show that the protein molecules are not chemically modified and retain their integrity, conformation, and activity post deposition. The technique is applicable to the manufacture of various types of protein/microelectrode arrays and is tunable, scalable, and applicable to a broad range of proteins.