Recently, intrinsically photoluminescent polymers (IPPs) without any conventional chromophores have aroused increasing attention and been extensively investigated. Simple and green synthetic method is definitely preferred for the production of such polymer. In this work, intrinsically photoluminescent hyperbranched polyethylenimine (IP-HPEI) was facilely prepared through directly heating the viscous neat HPEI under air. Since none of solvents, organic modifiers, purification and pressure vessel is required, it can be claimed that this is a simpler and greener method to prepare IPPs in comparison with other reports. Moreover, this simple technique favors the large scale preparation of IP-HPEIs. H-1 NMR, FTIR, XPS, TGA and fluorescence characterizations supported that air oxidation led to the generation of luminogens in HPEIs and tertiary amine oxides were the real luminogens, not the oximes reported by others. At pH 6, IP-HPEI could be used as fluorescent sensor to selectively detect Cu2+ ions with an LOD value of 0.72 mu M.