Optical fiber-based sensors are expected to become key components in the control of industrial processes, and the tuning and the enhancement of their sensing properties are crucial for the further development of this technology. Atomic Layer Deposition (ALD), a vapor phase technique allowing for the deposition of conformal thin films, is particularly suited for the deposition of controllable thin films on challenging substrates. In this work, we report the tuning of fiber-optic microsphere sensors using an ALD process of zinc oxide (ZnO) based on diethylzinc (DEZ) and H2O at 90°C. Nanolayers of 50 and 100 nm were deposited on the fiber-optic microspheres sensors, using 250 and 500 ALD cycles respectively. The fabricated samples were characterized using Scanning Electron Microscopy (SEM), and the spectral responses of the devices were investigated theoretically and experimentally, by measuring the refractive index of different oils. It has been found that the biocompatible ZnO functional nanocoatings of the fiber-optic microsphere sensors allowed for a wider measurement range of refractive indexes, opening up new prospects for fiberbased sensing devices.