Lead has severe effects on human health,such as delays in physical or mental development for children and the kidney problems in adults.1 The maximum contaminant level (MCL) of lead in public water systems permitted by U.S.Environmental Protection Agency is 15 μg L-1 (74 nM),and the maximum contaminant level goal of lead is zero.Compared with commonly used lead detection techniques,oligonucleotides based fluorescent probe provides sensitive and simple lead detection technology for the well-known interaction between lead and nucleotide.2 In the present study,lanthanides ions were selected as the fluorophore for their stable,sharp fluorescence with large stokes' shift,compared with conventional organic dyes.Moreover,the oligonucleotides could sensitize the fluorescence of lanthanide ions via energy transfer from oligonucleotides to their anchored lanthanide ions was firstly described in 1973.3 A sequence of G-rich oligonucleotides (5'-ATATGGGGGATAT-3',termed GH5,as shown in figure 1) was designed to strongly increase the fluorescence of lanthanide ions,which improved the sensitivity of the detection method for lead.The stoichiometry of the probe influence the fluorescence,which was firstly confirmed.As shown in figure 2a,the result of conductivity titration exhibited two linear segments before and after the equivalence point (the ratio of Tb3+/GH5 was 3.5).Before the equivalence point,the main terbium ions were coordinated with GH5,this slowed the increase of the electrical conductivity of Tb-GH5 complex.After the equivalence point,the increase of the electrical conductivity resulted from the excess free terbium ions (red line),which showed a similar slope with that of free terbium ions (black line).The stoichiometry of the probe was confirmed by fluorescence titration (as shown in figure 2b).The probe (3.5Tb3+-GH5) was used for the detection of lead (as shown in figure 3).The fluorescence intensity of this probe was linear with respect to lead concentration over a range 0.3 nM–2.1 nM (R2=0.99).The limit of detection for lead ions was 0.1 nM at a signal-to-noise ratio of 3.