A high sensitive colorimetric mercury （II） biosensor based on gold nanoparticles （AuNPs） is reported. Thiolated T-rich oligonucleotides were assembled on the surface of AuNPs

which was used as a probe to recognize Hg（II） specifically. The solution was red when the AuNPs were dispersed in the system without target Hg （II）. How- ever

the solution of AuNPs would change to blue when it probed the Hg （II） due to the formation of aggregates of AuNPs. The formation of the T-Hg

2+

-T structure would shorten the distance between the AuNPs

resulting in the for- mation of aggregates and the color change from red to blue that can be observed with naked eyes. This mercury （II） sensor is found to be have high sensitivity and selectivity

as it allows a fast （in minutes） identification of 5μmol/L Hg （II） without any detectable perturbation by a spectrum of non-specific metal ions.