Bifunctionalized silver nanoparticles (triazole-carboxyl Ag NPs) were synthesized through a click reaction and have a cooperative effect on recognition of Co(2+), resulting in appreciable changes in color and absorption properties over other metal ions tested, including Fe(2+), Mn(2+), Hg(2+), Ni(2+), Cd(2+), Cu(2+), Zn(2+), Pb(2+), Li(+), Na(+), K(+), Cs(+). The functionalized silver nanoparticles became aggregated in solution in the present of Co(2+) through cooperative metal-ligand interaction. The colorimetric sensor allows a rapidly quantitive assay of Co(2+) down to the concentration of 7.0 x 10(-6) M. Moreover, the triazole-carboxyl AgNPs can be capable of evaluating the exceeding standard of Co(2+) in drinking water.