Dear Hossam,

Please follow the paper

A Highly Sensitive and Selective Colorimetric Hg2+ Ion Probe Using Gold Nanoparticles Functionalized with Polyethyleneimine

in the link:Article A Highly Sensitive and Selective Colorimetric Hg 2+ Ion Prob...

For the light source you can use LEDs emitting light that can be absorbed by the Hg2+

For using LEDs instead of conventional sources of the spectrophotometer please follow the paper in the link:Article LED Based Spectrophotometer can compete with conventional one

Best wisghes

Dear Hossam,

Please follow the paper

A Highly Sensitive and Selective Colorimetric Hg2+ Ion Probe Using Gold Nanoparticles Functionalized with Polyethyleneimine

in the link:Article A Highly Sensitive and Selective Colorimetric Hg 2+ Ion Prob...

For the light source you can use LEDs emitting light that can be absorbed by the Hg2+

For using LEDs instead of conventional sources of the spectrophotometer please follow the paper in the link:Article LED Based Spectrophotometer can compete with conventional one

Best wisghes

Depending on your lab situation you might consider old fashioned reagents such as dithizone etc