Spectral characteristics of clad modified fiber optic gas sensor are studied for various concentrations (0–400 ppm) of acetone, ammonia and ethanol at room temperature. The ZnO nanorhombuses were synthesized by co-precipitation technique. The synthesized ZnO nanorhombuses were characterized by X-ray diffraction, high resolution transmission electron microscope, optical absorption studies and Fourier transform infrared spectroscopy. The analysis confirms the formation of ZnO nanorhombuses with wurtzite structure. The synthesized ZnO nanorhombuses were surface functionalized with thiourea to study the impact of functionalization on gas sensing characteristics. The small portion of cladding layer was replaced with nanorhombuses to study the gas sensing characteristics. It shows that the synthesized ZnO nanorhombuses have better selectivity towards acetone than ammonia and ethanol. The thiourea functionalized nanorhombuses showed enhanced sensitivity and fast response compared to the pristine ZnO nanorhombuses. Further, the wavelength dependent sensitivity analysis show that the fiber optic acetone sensor can be operated in visible wavelength with higher sensitivity and in infrared wavelength with moderate sensitivity. © 2017 Elsevier B.V.