1,3-dialkyl imidazolium salts are one of the most popular and investigated classes of room temperature ionic liquids. Although in various cases the physical-chemical properties and/or the outcome of the processes in these liquids significantly differ from those performed in "classical" dipolar organic solvents, they are still regarded as merely homogeneous solvents. In this brief overview it is developed the concept that pure 1,3-dialkylimidazolium ionic liquids are better described as hydrogen-bonded polymeric supramolecules of the type {[(DAI)x(X)x-n)] n+ [(DAI)x-n(X)x)] n-}n where DAI is the 1,3-dialkylimidazolium cation and X the anion. This structural pattern is a general trend for the solid phase and is maintained to a great extent in the liquid phase and even in the gas phase. The introduction of other molecules and macromolecules occurs with a disruption of the hydrogen bond network and in some cases can generate nano-structures with polar and non-polar regions where inclusion-type compounds can be formed.

Ionic liquids (ILs), in particular imidazolium-based ILs, have proven to be suitable media for the generation and stabilisation of soluble metal nanoparticles (NPs). Indeed, transition-metal NPs with small sizes, narrow size distribution and different shapes have been prepared by reduction of organometallic compounds with molecular hydrogen, decomposition of transition-metal complexes in the zero-valent state, metal bombardment or simple transfer for previously prepared water- or classical organic solvent-soluble colloids to the ILs. The formation and stabilisation of NPs in these highly hydrogen bonded organised supramolecular fluids occur with the re-organisation of the hydrogen bond network and the generation of nanostructures with polar and non-polar regions, including the NPs. The IL forms a protective layer, which is probably composed of imidazolium aggregates located immediately adjacent to the nanoparticle surface, which provides both steric and electronic protection against aggregation and/or agglomeration. These stable transition-metal NPs immobilised in the ILs have proven to be efficient green catalysts for several reactions in multiphase conditions and also novel materials for chemical sensors. In this critical review, the structural/surface properties of these soluble metal NPs dispersed in ILs and their application in catalysis and as chemical sensors are discussed, with particular attention paid to the stabilisation models proposed to explain the stability and properties of these metal NPs (219 references).