A highly sensitive and selective fluorescence method of quantitative detection for mercury in soil was developed using non-labeled molecular beacon (MB), single-stranded nucleic acid (ssDNA) and fluorescent dye Hoechst 33258. In this analytical method, the loop of MB was designed to be a sequence that was complementary to the ssDNA with multiple T-T mismatches, the stem of MB was completely designed as C-G base pairs, and both ends of the MB are not modified by any fluorophore and quencher. In the absence of Hg
2+, the interaction between Hoechst 33258 and the MB was very weak, and the fluorescence signal of Hoechst 33258 was very low. In the presence of Hg
2+, the MB and ssDNA with multiple T-T mismatches formed a double-stranded nucleic acid (dsDNA)
via the T-Hg
2+-T coordination structure which provided binding sites for Hoechst 33258. Then Hoechst 33258 binded to A-T base pairs of dsDNA, and the fluorescence intensity of Hoechst 33258 was significantly enhanced. Thus, a highly sensitive fluorescence quantitative detection method for Hg
2+ can be realized. In this strategy, the optimal determination conditions for Hg
2+ were a buffer solution pH of 8.2, an incubated temperature of 50°C, an incubated time of 5 min and NaCl of 60 mmol L
−1. Under the optimum conditions, the fluorescence intensity of Hoechst 33258 exhibited a good linear dependence on the concentration of Hg
2+ in the range of 5 × 10
−9 – 400 × 10
−9 mol L
−1. The fitted regression equation was Δ
I = 2.1084
C – 8.9587 with a correlation coefficient of 0.9943 (
R2), and the detection limit of this method was 3 × 10
−9 mol L
−1 (3σ). The proposed method had a high selection; the common substances in soil such as Ca
2+, Mg
2+, Mn
2+, Fe
3+, Cu
2+, Pb
2+, Al
3+, K
+, Na
+, Ni
2+, Cd
2+, Cr
3+, SiO
32−, Cl
−, PO
43−, NH
4+ and S
2− had no interference to the detection of mercury. The proposed method had a high accuracy, and it was applied to detect mercury of ten different types of soil; the recoveries were 97.65 – 103.22%. In addition, the proposed method had a low background emission, fast detection speed and low detection cost.
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