Highly Sensitive and Selective Chip-Based Fluorescent Sensor for Mercuric Ion: Development and Comparison of Turn-On and Turn-Off Systems

Miniaturization is currently an important trend in environmental and food monitoring because it holds great promise for on-site monitoring and detection. We report here two ready-to-use chip-based fluorescent sensors, compatible with rnicroarray technology for reagentless, one-step, fast, highly sensitive and selective detection of the mercuric ion (Hg2+) in the turn-on and turn-off operation modes. Both operation modes are based on the highly selective T-Hg2+-T coordination between two neighboring polythymine (T) strands at a high probe density and its induced displacement of the complementary polyadenine strand labeled with either fluorophore or quencher, which enables the turn-off and turn-on detection of Hg2+, respectively. The turn-off sensor is slightly more sensitive than the turn-on sensor, and their detection limits are 3.6 and 8.6 nM, respectively, which are both lower than the U.S. Environmental Protection Agency limit of [Hg2+] for drinkable water (10 nM, 2 ppb). Compared to the turn-off sensor with the dynamic Hg2+ detection range from 3.6 nM to 10 mu M (R-2 = 0.99), the turn-on sensor has a broader dynamic Hg2+ detection range, from 8.6 nM to 100 mu M (R-2 = 0.996). Both sensors exhibited superior selectivity over other reported sensors using thymine-rich probes for Hg2+ detection over other common metal ions. In addition, the practical application of the chip-based sensors was demonstrated by detecting spiked Hg2+ in drinking water and fresh milk. The sensor has great potential for on-site practical applications due to its operational convenience, simplicity, speed, and portability.