A highly sensitive colorimetric sensing platform for the selective trace analysis for Co(2+) ions is reported, based on glutathione (GSH)-modified silver nanoparticles (AgNP). The shape of metallic nanoparticles used in colorimetric detection, using the unique optical properties of plasmonic nanoparticles, is almost spherical. Therefore, in this work we attempted to investigate the selective detection of heavy metal ion (Co(2+)), with the shape of AgNPs (nanosphere, nanoplate, and nanorod). GSH-AgNP with spherical shape shows a high sensitivity for all of the metal ions (Ni(2+), Co(2+), Cd(2+), Pb(2+), and As(3+)) but poor selective recognition for target metal ions. Whereas, AgNPs solution containing rod-type GSH-AgNP has a special response to Co(2+), and its selective detection might be based on the cooperative effect of CTAB and GSH. Therefore, Co(2+) ion could be selectively recognized using rod-type GSH-AgNPs.