Rational design of fluorescent probe for Hg²⁺ by changing the chemical bond type

Two kinds of fluorescent probes DFBT and DFABT, and their corresponding water-soluble compounds WDFBT and WDFABT, based on the trimers containing a benzo[2,1,3]thiadiazole moiety and two fluorene moieties are synthesized. Their luminescent behavior towards Hg²⁺ ions and other various metal ions in organic and water solutions are studied in detail via absorption and emission spectroscopy. All these probes show a selective "on-off-type" fluorescent response to Hg²⁺ ions in solution over other metal ions with a maximum detection limit of 10^-7 M. Importantly, the probe type can be changed from irreversible to reversible by altering the bridge mode between the functional units from CC triple bond to C-C single bond. Their detection mechanisms towards Hg²⁺ are studied in detail via mass spectrometry and Job plots, which are attributed to irreversible chemical reaction for DFABT and WDFABT and a reversible coordination reaction for DFBT and WDFBT respectively. Our research results about this kind of organic fluorescent probe provide valuable information to the future design of practical Hg²⁺ fluorescent probes.