Fabrication and characterization of fiber optic volatile organic compounds (VOCs) sensor using pristine and copper (Cu) functionalized zinc oxide (ZnO) nanoflakes have been reported. Pristine ZnO nanoflakes were synthesized by co-precipitation method and were functionalized with Cu nanoparticles. Further, the synthesized nanoflakes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy. The characterization results reveal that the nanoparticles are in flakes shape with wurtzite structure and also confirm the Cu functionalization. Cladding modification technology was adopted to fabricate the sensor probe followed by dip coating of Cu functionalized ZnO nanoflakes over un-cladded part of an optical fiber. Different VOC concentrations of 0−350 ppm (such as acetone, ammonia, ethanol, methanol, hexane and chloroform) were prepared and their spectral characteristics were recorded at ambient temperature. The sensing analysis revealed that Cu functionalized ZnO nanoflakes can significantly improve the sensitivity towards all VOCs than pristine ZnO and exhibited highest sensitivity (∼ 47.4 at 350 ppm) towards ammonia with the limit of detection 10 ppm, being 6 times higher than pristine ZnO. The response towards ammonia was linear (R2 = 0.9912) over the concentration range of 10 ppm–50 ppm. Further, the sensor showed shorter response/recovery time of 11 s and 13 s, stable repeatability along with the long-term stability. The remarkable sensing property advocates that Cu functionalized ZnO nanoflakes are propitious candidate for ammonia sensor. © 2020