Novel oligothiophene-based dual-mode chemosensor: “Naked-Eye” colorimetric recognition of Hg2+ and sequential off-on fluorescence detection of Fe3+ and Hg2+ in aqueous media and its application in practical samples

•New dual-mode chemosensor A3TA was developed for detection of Fe3+ and Hg2+.•A3TA displayed off-on fluorescence detection of Fe3+ and Hg2+ based on CHEF.•A3TA could selectively sense Hg2+ by color change from khaki to reddish brown.•Sensor A3TA displayed high association constants and low DLs.•Sensor A3TA has been used to quantify Fe3+ and Hg2+ in real water samples.

A novel oligothiophene-based dual-mode chemosensor A3TA was designed and synthesized for Fe3+ and Hg2+ in aqueous media. The sensor A3TA showed high selectivity and sensitivity toward Fe3+ and Hg2+ over other competitive metal ions. Hg2+ was detected by changing color from khaki to reddish brown, could be easily distinguished by bare eyes. Moreover, the sensor also acted as a “turn-on” type fluorescent sensor toward Fe3+ and Hg2+. The detection limits for Fe3+ and Hg2+ were found to be as low as 3.52 × 10−8 and 5.0 × 10−8 M, respectively. The stoichiometric ratio of the sensor toward Fe3+/Hg2+ ions was obtained to be 1:2. Furthermore, the proposed sensor A3TA was also found to have high association constants (105 M−2), wide pH range (4-12) and short response time (3-5 min). In addtion, the sensing mechanism was proposed based on chelation-enhanced fluorescence (CHEF) and investigated in detail by fluorescence spectroscopy, 1H NMR titrations, FT-IR and HRMS spectra analysis. Importantly, the proposed sensor A3TA has been successfully used to quantify trace amounts of Fe3+ and Hg2+ in real samples.

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