Room temperature ionic liquids (RTILs) are unique in many respects. The preparation method reported here is based on their extremely low vapour pressure that allows for their usage in vacuum. By magnetron sputtering of nickel or by co-sputtering of Ni-Fe or Ag-Cu on a surface of several imidazolium-based RTILs, we prepared nanocolloids containing metallic nanoparticles ranging in size from 6 to 30 nm, depending on the kind of RTIL, with low size dispersion. The nanocolloids are stable in time, no sedimentation or agglomeration was observed after two years. The method can be used also for preparation of the nanoparticles from semiconducting or insulating materials. As the next step, nanocolloids prepared in this way can be used for decoration of thin-film gas sensors by proper nanoparticles aiming at enhancement of their sensitivity and selectivity. Two ways of immobilization are presented mild heating of a drop of colloid containing Ni-Fe nanoparticles placed on TEM mesh, and Ag-Cu nanoparticles that were immobilized on the surface of gold thin-film electrode by electrophoresis.