Detection of Hg2+ using molecular beacon-based fluorescent sensor with high sensitivity and tunable dynamic range

A molecular beacon (MB) sensor was developed for highly sensitive and specific detection of Hg2+ ions with a tunable dynamic range. This method was based on the "turn-on" reaction of a hairpin DNA probe upon binding with mismatched target and Hg2+ ions through the formation of T-Hg2+-T coordination. The conformational change of the MB caused a significant increase in fluorescence intensity, which could be used for Hg2+ sensing. The dynamic range of the sensor could be tuned by rationally controlling the number of T-T mismatches in between the MB loop and mismatched target DNA in the cases where detection of Hg2+ at different concentration ranges is required. With three T-T mismatches in the sequences, the sensor showed higher sensitivity with detection limit of 1.9 nM but narrower dynamic range. Further adding the number of T-T mismatches to seven, the sensor showed wider dynamic range but compromised sensitivity with detection limit of 44.2 nM. By mixing different mismatched targets in the assay, we found that both good sensitivity and wide dynamic range can be achieved. Adding small portion of T3 and T5 to T7 in the assay, the detection limit was significantly lower to 9.5 nM without sacrificing dynamic range. Based on the results, the mechanism of Hg2+ detection based on interaction between MB and mismatched targets was discussed. This sensor showed excellent selectivity toward Hg2+ ions in the presence of other metal ions. The proposed strategy was also able to detect Hg2+ in real water samples. (C) 2014 Elsevier B.V. All rights reserved.