We have developed a straightforward synthetic route to prepare core-shell systems based on gold nanoparticles (NPs) surrounded with J-aggregates molecules. This synthesis allows the direct and efficient coating, at room temperature, of pretreated citrate-stabilized gold NPs with 5, 5', 6, 6'-tetrachloro-1-1(')-diethyl-3, 3'-di (4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), without supplementary adding of salts and bases during the synthesis. As the size of gold particle is tunable, the precise optimization of the strong coupling between the electronic transitions of organic components (TDBC) and the plasmon modes of the gold NPs is achieved corresponding to a Rabi energy of 220 meV, a value not yet obtained in such a system. (C) 2010 American Institute of Physics. [doi:10.1063/1.3456523]