Design and synthesis of fluorescent peptide-based probes with aggregation-induced emission characteristic for detecting CH3Hg+ and Hg2+ in aqueous environment: Tuning fluorescent detection for CH3Hg+ by replacing peptide receptors

Even though methylmercury (CH3Hg+) is more toxic than Hg2+, it is more difficult to detect methylmercury than Hg2+ because of less charge and less thiophilicity. In the present study, we developed fluorescent probes for detecting CH3Hg+ and Hg2+ from the fluorescent peptide-based probe (1) for Hg2+ with aggregation induced emission (AIE) characteristic by replacing amino acids of the peptide moiety. The fluorescent probes (2–7) based on dipeptide receptors with AIE characteristics were synthesized for sensing CH3Hg+ and Hg2+ in aqueous environment. Probe 2, 4, 6, and 7 showed turn-on responses to CH3Hg+ and Hg2+ in aqueous solutions. Among them, probe 7 had the most desirable sensing features such as high solubility in water (<1% DMSO), sensitive Off-On response toward Hg2+ and CH3Hg+, high selectivity for Hg2+ and CH3Hg + among tested metal ions, reversibility, fast recognition (∼1 min), significant fluorescent signal changes, and low detection limits (1.4 ppb for Hg2+ and 152 ppb for CH3Hg+). Specially, the specific turn-on fluorescent signals to Hg2+ as well as CH3Hg+ were not significantly interference by coexisting Group I and II metal ions (1 mM) and most other heavy metal ions and the detection limit was lower than the maximum allowable levels of CH3Hg+ in food. The mechanism investigation using NMR, ESI mass spectrometer, and DLS measurements revealed that 7 formed nano-sized aggregates after complexation with Hg2+ and CH3Hg+, resulting in the enhanced fluorescent signals. The dipeptide receptor (AlaHis-OH) of 7 provided potent interactions with Hg2+ as well as CH3Hg+ in aqueous solutions. Probe 7 was successfully applied for the quantification of Hg2+ as well as CH3Hg+ in groundwater samples. We first developed fluorescent probes with AIE characteristics for reversibly detecting Hg2+ and CH3Hg+ in aqueous solution. The relationship between the peptide receptor structures and the specific interactions of Hg2+ and CH3Hg+ would provide useful information for further design of fluorescent detection systems for inorganic and organic mercury.