Zinc oxide has been prepared with crystallite size in nanodimension by the use of wet chemical technique. The crystalline nature of nano-ZnO with average crystallite size of 36–63 nm was confirmed by X-ray diffraction. The synthesized nano-ZnO was further characterized by ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy and energy-dispersive X-ray spectroscopy. The prominent spherical surface morphology of synthesized nano-ZnO was confirmed by scanning electron microscopy. The optical band gap of 3.12 eV was obtained for nano-ZnO. The prepared nano-ZnO were utilized for the treatment of dye as well as bacterial cultures found in aqueous phase. Excellent remediation by nano-ZnO was observed against the fast green dye and bacteria Bacillus subtilis and E. coli.