With the tentatively optimal NaOH concentration (70 mM), we investigated the effect of NH4OAc concentration (entry 5 and Figure ). When the NH4OAc concentration was above 100 mM, there was no difference between palladium-free solutions and 30 nM palladium solutions. When the NH4OAc concentration was below100 mM, 30 nM palladium solutions showed increased signals. Importantly, the lower NH4OAc concentrations increased the background signals with no palladium and turned the solutions purple. When the NaOH concentration was increased from 70 mM to 120 mM (entry 6 and Figure ), we observed the same trend.