The stabilization of the NPs by repulsive electrostatic interactions originates from a diffuse double layer at the surface of NPs coated with charged molecules. Presumably, K,Na-PHI, KMel, KCya, and KCA establish a hydrophilic protective layer around the growing metal nuclei that is permeable to tetrachloroaurate ions (to allow for self-limiting growth) while preventing the diffusional coupling of the particles. A representative model for the electrostatic stabilization of Au NPs by K, Na-PHI is shown in Fig. . These considerations are mainly based on the TEM and XPS results revealing that (i) the particle size of Au NPs stabilized by K, Na-PHI is larger than the estimated cavity size of heptazines (ESI, Fig. ), excluding the confinement of Au NPs into the heptazine cages, and (ii) the presence of K,Na-PHI does not affect the binding energy of Au 4f7/2 and Au 4f5/2 electrons of the Au(0) species, revealing that the stabilizer functional groups only weakly interact with the surface of the Au NPs.