A simple and sensitive optical sensor for Ag+was developed based on its recognition of C-C mismatched base pair and nucleic acid molecule “light switch” property of Ru(phen)2(7,8-dimethyldipyridophenazine)2+.A self-complementary oligonucleotide containing cytosine-cytosine mismatched base pair，5-TACATACTATACTATCTA-3-(oligo-1)，was designed as the sequence for sensing of silver ion.In the absence of silver ion，the hybridization between the oligo-1 strands would not occur because of the existence of cytosine- cytosine mismatched base pair.However，after adding the silver ion，hybridization occurred between the two oligo-1 strands because of the formation of cytosine-Ag+-cytosine base pair，the formed double-helix DNA prevented the luminescence of nucleic acid molecule “light switch” Ru(phen)2(7,8-dimethyldipyridophenazine)2+ from the quenching of water.As a result，the luminescence of Ru(phen)2(dppx)2+ increased dramatically.Based upon this mechanism，a simple and sensitive method for the detection of silver ion was established.Different factors including concentrations of oligo-1，Ru(phen)2(dppx)2+，NaNO3，pH value and incubation time were investigated.The optimum conditions were as followins:Oligo-1:150 nmol/L，Ru(phen)2(dppx)2+:2.0 μmol/L，NaNO3:120 mmol/L，buffer:20 mmol/L pH 7.5 PBS，incubation time:30 min.Under the optimal conditions，the luminescence intensity was proportional to concentration of Ag+over the range of 10.0-120.0 nmol/L，with a correlation coefficient(r) of 0.995 7.The limit of detection(3δ/Slope) was 8.1 nmol/L.With a good selectivity，the method was applied in the determination of Ag+in practical samples with satisfactory results.