A) Absorption profile of modified gold nanoparticles before and after the addition of different concentrations of Hg (II) ions, b) Photographic images of color of MPA-PDCA modified gold nanoparticles (13 nM) in the presence of different concentrations of Hg (II) ion, 1) 3 ppm, 2) 6 ppm, 3) 10 ppm, 4) 50 ppm (reprinted with permission from Ref. 71, Copyright 2008, American Chemical Society).