An improved synthetic method for the synthesis of 3,4-diaminobenzenethiol is presented. The practical application of this reagent as a new ligand for gold nanoparticles for the rapid detection of Cu²⁺ in water is demonstrated. The method is shown to have high sensitivity and selectivity. Well-defined peaks, proportional to the concentration of the corresponding Cu²⁺, were observed from 0.5 μM to 2 μM, and the recovery was in the range of 92–109%. This method provides a facile route for Cu²⁺ analysis.