A conjugated-polymer-based "mix-and-detect" optical sensor for mercury ions is fabricated by using a water-soluble poly[3-(3'-N,N,N-triethylamino-1'-pro-pyloxy)-4-methyl-2,5-thiophene hydrochloride] (PMNT) and a label-free, mercury-specific oligonucleotide (MSO) probe. PMNT binds to the Hg2+-free MSO and the Hg2+-MSO complex in different ways, and exhibits distinguishable and specific optical responses to the target-induced conformation change.