This paper focuses on the recent works conducted by my research group at United Arab Emirates University which are related to design and fabrication of conductometric sensors decorated with metallic nanoclusters for selective detection of mercury ions. These sensors are designed with field effect transistor structure and are fabricated by deposition of interdigitated gold electrodes on the surface of substrates by thermal evaporation through a shadow mask. Metallic nanoclusters are then synthesized in an ultra-high vacuum system (UHV) using magnetron sputtering and inert gas condensation and are self-assembled directly on graphene/graphite oxide layer. Metallic nanoclusters increase the surface to volume ratio and hence enhance the absorption and interaction between mercury ions and the sensor. Consequently, these sensors present optimum behavior with high sensitivity and selectivity to mercury ions.