Polymer brushes (PBs) are assembled by anchoring polymeric chains onto a surface, triggering an increasing interest due to the unique chemical and structural characteristics of polymeric chains. 1-2 So far, PBs have made significant contributions to a broad range of traditional and emerging applications such as biological lubrication , thermal and electrical stimuli-responsive surface , colloidal stabilization and surface protection , novel substrate modification, 6-7 biosensing and nanomaterial synthesis, etc. These applications are highly dependent on the interfacial microstructure of PBs and straightforwardly benefited from its precise tuning. 9-10 Thus, accurate density distribution of PBs on the molecular level become a focus of attention in surface modification.