This innovation was developed to ensure fast screening and monitoring of mercury (II) ion in water sources without the need to bring the water samples to the lab. Thus, this innovation would be very helpful to the Environmental Sciencesist and government officer to do the monitoring and analysis of mercury (II) ion. Colorimetric assay of mercury (II) ion using cysteine functionalized gold nanoparticles is a good technique that can provide rapid and efficient for in-situ monitoring of mercury (II) ion as the plasmonic resonance of gold nanoparticles can determine the presence of mercury (II) ion in water sources. The mercury (II) ion detection only takes about 10 minutes to complete. And this technique comes with a smartphone-attached fabricated chamber where it can be connected to the smartphone app that can act as absorbance reader to distinguish the color produced by the system