A boundary between plasmas and liquids, which activates physical and chemical reactions, has successfully been generated under a low gas pressure condition by utilizing ionic liquids with unique properties such as fully ionized plasma state, extremely low vapor pressure and high heat capacity. Due to the formation and control of a sheath electric field just above the ionic liquid, the plasma-ion behavior can be manipulated and the effects of the ion irradiation on the ionic liquid are quantitatively revealed. In connection with the plasma-ion irradiation, the secondary electron emission is found to be enhanced by introducing the ionic liquid as a cathode electrode. The control of the plasma-ion irradiation flux and energy to the ionic liquid leads to the creation of various kinds of metal nanoparticles by the reduction in metal chlorides and the realization of efficient synthesis. Furthermore, the metal nanoparticles are synthesized with carbon nanotubes as a template in the ionic liquid using the plasma irradiation method. It is found that the high density, mono-dispersed and isolated metal nanoparticles are synthesized between or inside the carbon nanotubes by controlling the gas-liquid interfaced plasmas.