Quantification of Carboxy Group. The pH dependence of the zeta potential was measured to evaluate the surface charge of Ox-80 (Figure ). The zeta potentials of the non-treated PPs have been reported with a transition from 5 to -50 mV (pH 3.5-9.0). In contrast, Ox-80 showed a more negative zeta potential value. The introduced carboxy groups (-COOH) may be attributed to the surface charge. Therefore, the surface of Ox-80 could interact with the cationic polymer, polyethyleneimine (PEI). Ox-80 was coated with PEI by soaking it in a 5 mg/mL PEI aqueous solution for 20 min at room temperature. The zeta potential of the PEI-coated sample was measured after washing with distilled water and drying in vacuo at room temperature. The PEI coating showed a significant increase in the zeta potential. This result indicated that the carboxy groups on Ox-80 conjugated with the positively charged amino (-NH2) groups of PEI via electrostatic interactions.