Water-soluble K,Na-PHI has various functional groups attached to the heptazine units. Our structural analyses and theoretical calculations proposed that K,Na-PHI is characterized by a decreased number of terminal NHx-groups, as compared to conventional, water-insoluble PCN, that are exchanged with cyamelurate and melonate moieties. Furthermore, an incomplete condensation of the PCN precursor might result in the formation of triazine species within the polymer. Therefore, to obtain a deeper insight into the possible interactions between the Au NPs and K,Na-PHI, we also studied the stabilization effect of potassium melonate (KMel), potassium cyamelurate (KCya), and potassium cyanurate (KCA) at different concentrations (Fig. ). For all the proposed stabilizers, NaBH4 was used as a reducing agent for the synthesis of AuNPs. Notably, it is well established that the excess NaBH4 can electrostatically stabilize even very small (~ 5 nm) colloidal Au NPs without any additional stabilizing agent. Therefore, the "naked" (i.e., ligand-free) Au NPs stabilized by excess NaBH4 represent an ideal benchmark for comparative investigations of the effect of the proposed stabilizers on the size, long-term stability, and catalytic activity of Au NPs. This choice is further substantiated by the fact that though also other weakly binding ligands (e.g., citrate) do allow for good accessibility of active sites on the Au nanoparticle surface, they are typically not able to stabilize Au NPs with sizes below 10 nm when using NaBH4 as a reducing agent, prohibiting thus comparative studies in the ultrafine size regime. A 10-fold excess of NaBH4 with respect to HAuCl4 was selected as the optimum molar ratio of NaBH4:HAuCl4 to produce the benchmark ligand-free Au NPs (denoted as Au10) without agglomeration (for details see ESI 2.1 †). For a systematic and realistic comparison, the same optimum NaBH4 concentration was also used to fabricate Au NPs in the presence of K,Na-PHI and its monomeric constituents (KMel, KCya, and KCA). In general, the AuNPs stabilized by different concentrations of NaBH4, K,Na-PHI, KMel, KCya, or KCA exhibit different colours and different electronic absorption properties, which is related to their different sizes and the differently pronounced surface plasmon resonance (SPR) effects (see ESI, Fig. and Fig. , and the detailed discussion of the optimization studies in ESI 2.1 †). For a representative overview, Figure compares the UV-vis electronic absorption spectra and optical images as well as the average hydrodynamic diameters of Au10, K,NaPHI2, KMel0.3, KCya0.3, KCA0.3 (the sample designations reflect the molar stabilizer : HAuCl4 ratios; for details see ESI, Table †) that were obtained at the optimum concentrations for each stabilizer with respect to the smallest achievable size of stable NPs. As presented in Fig. Au10. Moreover, K,NaPHI2 exhibits a considerably broader band with no discernible plasmonic feature at all, suggesting that most Au NPs are below ~3 nm in size. These trends are consistent with the measurements of the hydrodynamic diameter using dynamic light scattering (DLS, Fig. ). In particular, the Au NP samples characterized by a relatively sharp and intense SPR peak show larger hydrodynamic diameters, indicating a larger particle size. More importantly, the inset of Fig. demonstrates that K,NaPHI2 possesses a very narrow hydrodynamic diameter distribution (3.0 ± 0.6 nm) as compared to other samples. These observations suggest that, although all the investigated triazine-and heptazine-based compounds can stabilize Au NPs, the size as well as the size distribution of the particles strongly depend on the polymerization degree of the stabilizer. X-ray photoelectron spectroscopy (XPS) analyses were performed to identify the oxidation state of gold and the chemical composition of Au10, K,NaPHI2, KMel0.3, KCya0.3, and KCA0.3. As shown in Fig. , the two distinct peaks centered at 83.6 and 87.3 eV correlate to the spin-orbit doublet of Au 4f7/2 and 4f5/2, respectively, demonstrating the presence of metallic Au in the samples.