A facile, environmentally friendly route is demonstrated for the synthesis of Pt-Au dendrimer-like nanoparticles on the surface of polydopamine (PDA)-wrapped reduced graphene oxide (RGO), in which Pt-Au alloy nanoparticles are synthesized by the reduction of H₂PtCl₆ and HAuCI₄ with ascorbic acid. The effects of support material and chemical composition on the catalytic activity for the reduction of 4-nitrophenol (4-NP) are investigated in detail. Pt nanoparticles supported on PDA/RGO (Pt-PDA/RGO) exhibit significantly higher catalytic activity as compared to those exhibited by Pt nanop articles deposited on pristine graphene sheets (Pt-RGO) and commercial Pt/C catalyst. Furthermore, the chemical composition seriously affects the catalytic ability of the catalysts. With Pt-to-Au molar ratios of 3/1 and 1/1, significantly enhanced catalytic activities are observed, outperforming the support decorated with each single constituent. The high activity of Pt-Au-PDA/RGO can be explained by electronic effect involving in two types of electron transfers: (1) from the PDA coating to both Au and Pt atoms; (2) from Au to Pt atoms. Moreover, the Pt3Au1-PDA/RGO composite keeps a stable conversion efficiency of around 100% over six successive reduction reaction cycles. Through an experimental device of "filtering and catalyzing," the Pt₃Aul- PDA/RGO sample exhibits superior efficiency for the purification of water containing 4-NP. Within 8 s, the water becomes colorless.