Nanoparticles having a core formed of a metal selected from Ag, Cu, Sn and Zn and a shell formed of a plurality of molecules of quaternary ammonium compounds NR1R2R3R4+X-, where X is selected from Cl, Br and I and where NR1R2R3R4+ is a quaternary ammonium compound wherein at least one R is different from the others and is an alkyl chain, linear or functionalised or provided with branches or aromatic lateral groups with length preferably between 8 and 18 atoms of carbon.