Positively charged, branched polyethylenimine (PEI) adsorbed onto silicon wafers and silica surfaces are attacked by free hydroxyl radicals. With AFM colloid probe technique, the surface forces between PEI layers are measured. Force profiles show that an electrostatic repulsion dominates the surface forces between freshly deposited PEI layers. After radical attack, both surface potential and surface charge density are reduced by a factor of about 0.5, while the Debye length remains unchanged. Adsorbed gold nanoparticles and force volume measurements show a homogeneous distribution of the surface charge on length scales between 40 nm and 30 μm. For radical attacked PEI layers, we find a 10% decrease of saturation coverage of gold nanoparticles. This is consistent with the decreased surface charge density, if the electrostatic three-body interaction is taken into account. Nevertheless, the NP adsorption kinetics are slowed down suggesting that the PEI layer is inhomogeneous on the nm-scale after radical attack.