Fig. 2.

Design of a catalytic beacon to detect UO₂²⁺. (A) The secondary structure of an in vitro-selected DNAzyme specific to UO₂²⁺, which contains a substrate (39S) and an enzyme (39E). (B) Secondary structure of a Pb²⁺-specific DNAzyme composed of a substrate (17S) and an enzyme (17E). (C) Design of a catalytic beacon with a fluorophore and two quenchers. Cleavage of the substrate in the presence of UO₂²⁺ enhances the fluorescence. (D) Fluorescence signal in the absence and presence of 400 nM UO₂²⁺ after 10 min. (E) Biochemical assay of the 39E DNAzyme. Lane 1, substrate alone with 1 μM UO₂²⁺; lanes 2–7: time 0 (no UO₂²⁺), 1, 2, 5, 10, and 30 min after addition of 200 nM UO₂²⁺ to the DNAzyme. The upper and lower bands are uncleaved and cleaved substrates, respectively. (F) A sensor array containing both the UO₂²⁺ and Pb²⁺ sensors. The analytes (0.4 μM UO₂²⁺ and/or 2 μM Pb²⁺, or none) added to each well are indicated at the top of the figure. The Pb²⁺ sensor was in Tris acetate buffer (pH 8.2) with 100 mM NaCl, whereas the UO₂²⁺ sensor was in Mes buffer (pH 5.5) with 300 mM NaCl. The image was scanned 10 min after addition of metal ions.