Selectively and sensitively monitoring Hg2+ in aqueous buffer solutions with fluorescent sensors based on unnatural amino acids

Compounds (1–6) bearing two dansyl fluorophores based on unnatural amino acids were synthesized in solid phase synthesis with Fmoc chemistry. Compounds 1–3 showed hypersensitive turn on response to Hg2+ ions in 100% aqueous solution, whereas 4–6 did not show any response to various metal ions. Furthermore, 1–3 detected Hg2+ ions in aqueous solution without interferences of other metal ions and satisfied for monitoring the maximum allowable level (2 ppb, 10 nM) of mercury ions in drinking water demanded by EPA. Among them, 1 showed the most potent binding affinity (Kd = 30 pM) for Hg2+ in 100% aqueous solution, which revealed that the chain length between two dansyl sulfonamide moieties is not critical for selectivity for Hg2+ ions but plays an important role in binding affinity for Hg2+ ions. 1H NMR study reveals that the two sulfonamide and the amide groups of 1 play a critical role for stabilizing the 1-Hg2+ complex.