Gold nanoparticles modified with isothiouronium-derived amphiphile units 1 were prepared by the reduction of AuCl4-(n-C8H17)4N+ with NaBH4 in the presence of bis(5-(N-(2-(2-methoxyethoxy)ethyl)-N-methyl-S-isothiouronio)pentyl) disulfide 2. The obtained nanoparticles 1 showed a selective coloration for hydrophobic anions in water, possibly due to an anion exchange and subsequent aggregation. On the basis of this observation, the detection of F− in water has been achieved using 3-nitrophenylboronic acid as a mediator at pH 5.5, thereby providing a new strategy for the design of a F− sensory system.