Commercially available PEI (Wako Pure Chemical Industries, Ltd.) was used without further purification. The average molecular weight of PEI used was 600, 1,800, and 10,000. In this study, the concentration of the PEI aqueous solution is expressed as the concentration of the monomer unit (-C 2 H 5 N-, 46 g/monomer). The PEI stock solution (2.0 mol/l) was prepared by dissolving 4.6 g of PEI in 50 mL of deionized distilled water in a graduated cylinder. At the time of use, the stock solution was diluted with water to the desired concentration.