The stability of metal nanoparticles in solutions of high ionic strength is crucial for their application in catalysis and biology. Therefore, the effect of ionic strength on the stability of Au10, K,NaPHI2, KMel0.3, KCya0. Moreover, the optical images of these samples demonstrate a complete agglomeration in the presence of 0.1 M NaCl after less than 24 h (Fig. ). Similar behaviour is well-known for Au NPs stabilized by electrostatic repulsion, such as citrate-capped Au NPs, and can be ascribed to the increased screening of the surface charge and compression of the double layer, resulting in a more attractive particle-particle van-der-Waals interaction and agglomeration. In this context, the strongly enhanced stability of Au NPs stabilized by K,Na-PHI and KMel can be likely explained by the ability of these stabilizers to act as a hydrated protective layer that is, due to the more pronounced delocalization of charges (see the discussion below), less prone to the double layer compression effects and thus more effectively diminishes the attractive forces between the particles at high ionic strength. 55