The reduction of tetrachloroaurate(III) ions (AuCl4-) with 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES) was used to detect protein kinase A activity. The phosphorylation of substrate peptides was identified by changes in the solution colors resulting from the aggregation of synthesized gold nanoparticles in HEPES buffer.