Here, we report a post-synthesis functionalization of the shell of Au nanoclusters (NCs) synthesized using glutathione as a thiolate ligand. The as-synthesized Au NCs are subjected to the post-synthesis functionalization via amidic coupling of dopamine on the cluster shell to tailor photoluminescence (PL) and electrochemiluminescence (ECL) features of the Au NCs. Because the NCs' PL at ca. 610 nm is primarily ascribed to the Au(I)-thiolate (SG) motifs on the cluster shell of the NCs, the post-synthesis functionalization of the cluster shell enhanced the PL intensity of the Au NCs via rigidification of the cluster shell. In contrast to the PL enhancement, the post-synthesis modification of the cluster shell does not enhance the near-infrared (NIR) ECL of the NCs because the NIR ECL at ca. 800 nm is ascribed to the Au(0)-SG motifs in the metallic core of the NCs.