This paper discusses the biogenic synthesis of zinc oxide (ZnO) nanoparticles by using aqueous solutions of zinc acetate dihydrate and Ocimum tenuiflorum leaf extract. Field emission scanning electron microscopy (FE-SEM) confirms the formation of fine ZnO aggregates. Crystallographic structure and functional group of the nanoparticles are determined by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) techniques. UV–visible and fluorescence spectroscopies are employed to assess optical behaviour of ZnO nanoparticles.The ZnO nanoparticles show nearly 100 % catalytic efficiency in the sodium borohydride (NaBH4)-mediated reduction of 4-nitrophenol within 60 s. Also, the catalyst can be reused for five cycles without losing 100 % regeneration efficiency. This study demonstrates that the prepared ZnO nanoparticles can effectively detoxify 4-nitrophenol from an aqueous solution.