Chemical and structural properties of polyethyleneimine film coated on a SiO2 substrate in different concentrations

Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and atomic force microscopy (AFM) were employed to investigate the detailed surface bonding structure and morphology of polyethyleneimine (PEI) layers coated on a SiO2 substrate in concentrations of 0.5-10.0 mass%. With an increase in the PEI concentration, the C-N bonds corresponding to the imine group (-NH) became dominant in the PEI layers, implying a characteristic feature to verify the immobilization of PEI on the SiO2 surface. The thickness of PEI layer calculated using ARXPS gradually increased with increasing PEI concentration up to a maximum 5 mass%, above which it became saturated. AFM results showed that an increase in PEI concentration led to a decrease in the root mean square (RMS) roughness of PEI layers. Such a saturation tendency of PEI thickness combined with the strongly dependence of the surface morphology behavior of PEI layers on the PEI concentration indicated that SiO2 surface was fully covered by monolayer PEI with a smooth surface morphology at PEI concentration in excess of 5 mass%.