Lately, the applications of metal nanomaterials are continuously increasing in numerous areas. However, during production, some of these methods can be hazardous to the environment, plants, and animals. Thus, there is a need for environmental friendly of metal nanomaterials during production. In this research, gold nanoclusters (Au NCs) were synthesized a new natural enzyme protein soybean trypsin inhibitor (STI) as protecting ligand. The results showed that the newly synthesized STI-Au NCs exhibited a very strong red fluorescence emission at 689 nm and excitation 513 nm. Optimum favourable conditions such as STI concentrations, buffers and time for the synthesis of STI-Au NCs were studied for the brightest fluorescence emission. This newly synthesized STI-Au NCs was used for the detection of mercury (Hg2+). The linear ship range for the Hg2+ ions is from 0.01 - 0.05 µM with a detection limit of 0.71 nM. Additionally, this fluorescence STI-Au NCs probe can be easily synthesized and have generated a high fluorescence, hence, broadening its application in various fields of nanoscience.